Medical device and method for sensing cardiac electrical event signals

Abstract

A medical device includes a sensing circuit configured to sense one or more cardiac electrical signals and sense ventricular event signals from the one or more cardiac electrical signals. The medical device includes control circuit that may be configured to, in response to each ventricular event signal of a plurality of the ventricular event signals sensed by the sensing circuit, store in a medical device memory a suspected R-wave segment from the one or more cardiac electrical signals. The control circuit may determine at least one feature of the suspected R-wave segment and determine that the ventricular event signal of the plurality of ventricular event signals is a confirmed ventricular event signal when the at least one feature meets R-wave confirmation criteria. the control circuit is configured to detect a first threshold number of tachyarrhythmia intervals from the sensed RR intervals; determine a suspected heart rate from the confirmed ventricular event signals; determine that the suspected heart rate meets a heart rate threshold; and detect a tachyarrhythmia in real time in response to at least detecting the first threshold number of tachyarrhythmia intervals from the sensed RR intervals and the suspected heart rate meeting the heart rate threshold; and store data in the memory corresponding to the detected tachyarrhythmia. A telemetry circuit of the medical device is configured to transmit the data corresponding to the detected tachyarrhythmia.

Description

MEDICAL DEVICE AND METHOD FOR SENSING CARDIAC ELECTRICAL EVENT SIGNALS

[0001] This application claims the benefit of U.S. Provisional Patent Application Serial No. 63 / 607,003, filed December 6, 2023, the entire content of which is incorporated herein by reference.TECHNICAL FIELD

[0002] The disclosure relates generally to a medical device and method for sensing cardiac electrical event signals, which may be used for determining a heart rate or detecting a heart rhythm.BACKGROUND

[0003] Medical devices may sense electrophysiological signals from the heart, brain, nerve, muscle or other tissue. Such devices may be implantable, wearable or external devices using implantable and / or surface (skin) electrodes for sensing the electrophysiological signals. In some cases, such devices may be configured to deliver a therapy based on the sensed electrophysiological signals. For example, implantable or external cardiac pacemakers, cardioverter defibrillators, cardiac monitors and the like, sense cardiac electrical signals from a patient’s heart. The medical device may sense cardiac electrical signals from a heart chamber and deliver electrical stimulation therapies to the heart chamber using electrodes carried by a transvenous medical electrical lead that positions electrodes within the patient’s heart.

[0004] A cardiac pacemaker or cardioverter defibrillator may deliver therapeutic electrical stimulation to the heart via electrodes carried by one or more medical electrical leads coupled to the medical device. The electrical stimulation may include electrical pulses such as pacing pulses and / or cardioversion or defibrillation shocks. In some cases, a medical device may sense cardiac electrical signals attendant to the intrinsic depolarizations of the myocardium and control delivery of stimulation pulses to the heart based on sensed cardiac electrical signals. Cardiac signals sensed within a heart chamber using endocardial electrodes carried by transvenous leads, for example, generally have a high signal strength and quality for reliably sensing cardiac electrical events, such asventricular R-waves sensed from within a ventricle. Upon detection of an abnormal rhythm, such as bradycardia, tachycardia or fibrillation, an appropriate electrical stimulation pulse or pulses may be delivered to restore or maintain a more normal rhythm of the heart. For example, an implantable cardioverter defibrillator (ICD) may deliver pacing pulses to the heart of the patient upon detecting bradycardia or tachycardia or deliver cardioversion or defibrillation (CV / DF) shocks to the heart upon detecting tachycardia or fibrillation.SUMMARY

[0005] In general, this disclosure is directed to a medical device and techniques for sensing cardiac electrical event signals, which may be used for determining a heart rate. In some examples, the techniques disclosed herein are used for detecting ventricular tachyarrhythmias and determining when to initiate an anti-tachyarrhythmia therapy. The medical device may be configured to sense ventricular event signals from a first cardiac electrical signal for determining sensed ventricular event intervals, referred to herein as sensed RR intervals or “sensed RRIs.” The medical device may be configured to analyze a cardiac electrical signal segments, which may be obtained from a second cardiac electrical signal different than the first cardiac electrical signal, for confirming sensed ventricular event signals. R-wave segments may be buffered from a cardiac electrical signal when ventricular event signals are sensed from the first cardiac electrical signal. Each R-wave segment may correspond to one suspected R-wave associated with the time of a sensed ventricular event signal. The medical device may confirm a ventricular event signal based on an analysis of an R-wave segment. The medical device may determine a suspected heart rate based on the confirmed ventricular event signals and corresponding R-wave segments.

[0006] In one example, the disclosure provides a medical device including a sensing circuit configured to sense one or more cardiac electrical signals and sense ventricular event signals from the one or more cardiac electrical signals. The medical device may have a memory configured to buffer suspected R-wave segments from the one or more cardiac electrical signals. The medical device includes a control circuit that may be configured to determine sensed RR intervals from the ventricular event signals sensed by the sensing circuit. In response to each ventricular event signal of a plurality of theventricular event signals sensed by the sensing circuit, the control circuit may store, in the memory, a suspected R-wave segment from the one or more cardiac electrical signals. The control circuit may be further configured to determine at least one feature of the suspected R-wave segment and determine that the ventricular event signal of the plurality of ventricular event signals is a confirmed ventricular event signal when the at least one feature meets R-wave confirmation criteria. The control circuit may be further configured to detect a threshold number of tachyarrhythmia intervals from the sensed RR intervals, determine a suspected heart rate from the confirmed ventricular event signals and determine that the suspected heart rate meets a heart rate threshold. The control circuit may be configured to detect a tachyarrhythmia in real time in response to at least detecting the first threshold number of tachyarrhythmia intervals from the sensed RR intervals and the suspected heart rate meeting the heart rate threshold and store data in the memory corresponding to the detected tachyarrhythmia. The medical device may include a telemetry circuit configured to transmit the data corresponding to the detected tachyarrhythmia.

[0007] In another example, the disclosure provides a method including sensing one or more cardiac electrical signals, sensing ventricular event signals from the one or more cardiac electrical signals and determining sensed RR intervals from the sensed ventricular event signals. In response to each ventricular event signal of a plurality of the ventricular event signals, the method may include storing a suspected R-wave segment from the one or more cardiac electrical signals, determining at least one feature of the suspected R-wave segment, and determining that the ventricular event signal of the plurality of ventricular event signals is a confirmed ventricular event signal when the at least one feature meets R- wave confirmation criteria. The method may further include detecting a first threshold number of tachyarrhythmia intervals from the sensed RR intervals, determining a suspected heart rate from the confirmed ventricular event signals, and determining that the suspected heart rate meets a heart rate threshold. The method may include detecting a tachyarrhythmia in real time in response to at least detecting the first threshold number of tachyarrhythmia intervals from the sensed RR intervals and the suspected heart rate meeting the heart rate threshold and storing data corresponding to the detected tachyarrhythmia. The method may include transmitting the data corresponding to the detected tachyarrhythmia.

[0008] In yet another example, the disclosure provides a non-transitory, computer readable medium storing a set of instructions that, when executed by control circuitry of a medical device, cause the medical device to sense one or more cardiac electrical signals, sense ventricular event signals from the one or more cardiac electrical signals, and determine sensed RR intervals from the sensed ventricular event signals. In response to each ventricular event signal of a plurality of the sensed ventricular event signals, the instructions may further cause the medical device to store a suspected R-wave segment from the one or more cardiac electrical signals, determine at least one feature of the suspected R-wave segment; and determine that the ventricular event signal of the plurality of ventricular event signals is a confirmed ventricular event signal when the at least one feature meets R-wave confirmation criteria. The instructions may further cause the medical device to detect a first threshold number of tachyarrhythmia intervals from the sensed RR intervals, detect at least a second threshold number of confirmed ventricular event signals, determine a suspected heart rate from the confirmed ventricular event signals, and determine that the suspected heart rate meets a heart rate threshold. The instructions may further cause the medical device to detect a tachyarrhythmia in response to at least detecting the first threshold number of tachyarrhythmia intervals from the sensed RR intervals and the suspected heart rate meeting the heart rate threshold. The instructions may further cause the medical device to initiate an anti-tachyarrhythmia therapy in response to detecting the tachyarrhythmia.

[0009] This summary is intended to provide an overview of the subject matter described in this disclosure. It is not intended to provide an exclusive or exhaustive explanation of the apparatus and methods described in detail within the accompanying drawings and description below. Further details of one or more examples are set forth in the accompanying drawings and the description below.BRIEF DESCRIPTION OF DRAWINGS

[0010] FIGs. 1A and IB are conceptual diagrams of one example of a medical device system that may be configured to sense cardiac event signals, detect arrhythmia and deliver electrical stimulation therapy according to the techniques disclosed herein.

[0011] FIGs. 2A-2C are conceptual diagrams of a patient implanted with a medical device system in a different implant configuration than the arrangement shown in FIGs. 1A-1B.

[0012] FIG. 3 is a conceptual diagram of an implantable cardioverter defibrillator (ICD) according to some examples.

[0013] FIG. 4 is a conceptual diagram of circuitry that may be included in a sensing circuit of the ICD shown in FIG. 3 according to some examples.

[0014] FIG. 5 is a conceptual diagram of operating states of an ICD for detecting tachyarrhythmia and controlling therapy delivery following a tachyarrhythmia detection according to some examples.

[0015] FIG. 6 is a flow chart of a method that may be performed by a medical device for confirming a sensed heart rate for use in detecting a heart rhythm according to some examples.

[0016] FIG. 7 is a flow chart of a method that may be performed by a medical device for detecting tachyarrhythmia according to some examples.

[0017] FIG. 8 is a flow chart of a method that may be performed by a medical device for determining a suspected RR interval (RRI) according to some examples.

[0018] FIG. 9 is a diagram of cardiac electrical signals that may be sensed by a sensing circuit of a medical device.

[0019] FIG. 10 is a flow chart of a method for detecting tachyarrhythmia according to another example.DETAILED DESCRIPTION

[0020] In general, this disclosure describes a medical device and techniques for sensing cardiac event signals attendant to the electrical depolarization of myocardial tissue and determining a heart rate from sensed cardiac event signals for use in determining a heart rhythm. The determined heart rate may be used for detecting tachyarrhythmia in some examples and controlling anti-tachyarrhythmia therapies. In various examples, the medical device performing the techniques disclosed herein may be included in an ICD system capable of sensing cardiac electrical signals, detecting tachyarrhythmia occurring in real time based on processing and analysis of the sensed cardiac electrical signals, and delivering electrical stimulation therapy for treating the detected tachyarrhythmia. In some examples, the ICD is coupled to an extra-cardiovascular lead. As used herein, the term “extra-cardiovascular” refers to a position outside the blood vessels, heart, and pericardium surrounding the heart of a patient. Implantable electrodes carried by extra-cardiovascular leads may be positioned extra-thoracically (outside the ribcage and sternum) or intra-thoracically (beneath the ribcage or sternum) but generally not in intimate contact with myocardial tissue, e.g., within the heart or within the pericardium. In other examples, the techniques disclosed herein may be performed by an ICD coupled to a transvenous extra-cardiac lead carrying implantable electrodes that can be positioned intravenously but outside the heart in an extra-cardiac location, e.g., within the internal thoracic vein, jugular vein, or other vein, for sensing cardiac electrical signals and delivering electrical stimulation therapies. In still other examples, the techniques disclosed herein may be performed by an ICD coupled to a transvenous lead carrying implantable electrodes that can be positioned within the heart, e.g., within atrial and / or ventricular heart chambers for sensing cardiac electrical signals and delivering electrical stimulation pulses.

[0021] In still other examples, the techniques disclosed herein may be performed by an ICD coupled to a lead carrying implantable electrodes that can be positioned within the pericardium, e.g., in an epicardial location. Furthermore, in some examples, the methods disclosed herein may be performed by a cardiac monitoring device, that does not necessarily include therapy delivery capabilities, for detecting arrhythmia episodes and reporting detected arrhythmia episodes. A medical device configured to perform the methods disclosed herein may be a leadless medical device in some examples, configured to sense cardiac electrical signals using electrodes positioned on the housing of the medical device.

[0022] FIGs. 1A and IB are conceptual diagrams of one example of an ICD system 10 that may be configured to sense cardiac electrical signals, detect tachyarrhythmia and deliver electrical stimulation therapy according to the techniques disclosed herein. FIG. 1A is a front view of ICD system 10 implanted within patient 12. FIG. IB is a side view of ICD system 10 implanted within patient 12. ICD system 10 includes an ICD 14 connected to an electrical stimulation and sensing lead 16, positioned in an extra-cardiovascular location in this example. FIGs. 1A and IB are described in the context of an ICD system 10 capable of providing high voltage CV / DF shocks and / or cardiac pacing pulses in response to detecting a tachyarrhythmia based on processing of sensed cardiac electrical signals. The techniques for sensing and confirming cardiac electrical event signals as disclosed herein may be implemented in a cardiac monitoring device that does not includecardiac pacing and / or CV / DF shock delivery capabilities in some examples. Furthermore, the techniques disclosed herein for sensing and confirming cardiac electrical event signals may be implemented in a variety of medical devices including external or implantable cardiac monitors, pacemakers, and ICDs.

[0023] ICD 14 includes a housing 15 that forms a hermetic seal that protects internal components of ICD 14. The housing 15 of ICD 14 may be formed of a conductive material, such as titanium or titanium alloy. The housing 15 may function as an electrode (sometimes referred to as a “can” electrode). Housing 15 may be used as an active can electrode for use in delivering CV / DF shocks or other high voltage pulses delivered using a high voltage therapy circuit. In other examples, housing 15 may be available for use in delivering unipolar, relatively lower voltage cardiac pacing pulses and / or for sensing cardiac electrical signals in combination with electrodes carried by lead 16. In other instances, the housing 15 of ICD 14 may include a plurality of electrodes on an outer portion of the housing. The outer portion(s) of the housing 15 functioning as an electrode(s) may be coated with a material, such as titanium nitride, e.g., for reducing post- stimulation polarization artifact.

[0024] ICD 14 includes a connector assembly 17 (also referred to as a connector block or header) that includes electrical feedthroughs crossing housing 15 to provide electrical connections between conductors extending within the lead body 18 of lead 16 and electronic components included within the housing 15 of ICD 14. As will be described in further detail herein, housing 15 may house one or more processing circuits, memories, transceivers, cardiac electrical signal sensing circuitry, therapy delivery circuitry, power sources and other components for sensing cardiac electrical signals, detecting a heart rhythm, and controlling and delivering electrical stimulation pulses to treat an abnormal heart rhythm.

[0025] Elongated lead body 18 has a proximal end 27 that includes a lead connector (not shown) configured to be connected to ICD connector assembly 17 and a distal portion 25 that includes one or more electrodes. In the example illustrated in FIGs. 1A and IB, the distal portion 25 of lead body 18 includes defibrillation electrodes 24 and 26 and pace / sense electrodes 28 and 30. In some cases, defibrillation electrodes 24 and 26 may together form a defibrillation electrode in that they may be configured to be activated concurrently. Alternatively, defibrillation electrodes 24 and 26 may form separatedefibrillation electrodes in which case each of the electrodes 24 and 26 may be activated independently.

[0026] Electrodes 24 and 26 (and in some examples housing 15) are referred to herein as “defibrillation electrodes” because they can be utilized, individually or collectively, for delivering high voltage stimulation therapy (e.g., CV / DF shocks). Electrodes 24 and 26 may be elongated coil electrodes and generally have a relatively high surface area for delivering high voltage electrical stimulation pulses compared to pacing and sensing electrodes 28 and 30. However, electrodes 24 and 26 and housing 15 may also be utilized to provide pacing functionality, sensing functionality or both pacing and sensing functionality in addition to or instead of high voltage CV / DF shock therapy. In this sense, the use of the term “defibrillation electrode” herein should not be considered as limiting the electrodes 24 and 26 for use in only high voltage CV / DF shock therapy applications. For example, either of electrodes 24 and 26 may be used as a sensing electrode in a sensing electrode vector for sensing cardiac electrical signals and determining a need for an electrical stimulation therapy.

[0027] Electrodes 28 and 30 are relatively smaller surface area electrodes which are available for use in sensing electrode vectors for sensing cardiac electrical signals and may be used for delivering relatively low voltage pacing pulses in some configurations. Electrodes 28 and 30 are referred to as pace / sense electrodes because they are generally configured for use in low voltage applications, e.g., used as either a cathode or anode for delivery of pacing pulses and / or sensing of cardiac electrical signals, as opposed to delivering high voltage CV / DF shocks. In some instances, electrodes 28 and 30 may provide only pacing functionality, only sensing functionality or both.

[0028] ICD 14 may obtain cardiac electrical signals corresponding to electrical activity of heart 8 via a combination of sensing electrode vectors that include combinations of electrodes 24, 26, 28 and / or 30. In some examples, housing 15 of ICD 14 is used in combination with one or more of electrodes 24, 26, 28 and / or 30 in at least one sensing electrode vector. Various sensing electrode vectors utilizing combinations of electrodes 24, 26, 28, and 30 and housing 15 are described below for sensing one or more cardiac electrical signals. Each cardiac electrical signal that is sensed by ICD 14 may be sensed using a different sensing electrode vector, which may be selected by sensing circuitry included in ICD 14. One or more cardiac electrical signal(s) received via a selectedsensing electrode vector may be used by ICD 14 for sensing cardiac event signals attendant to intrinsic depolarizations of the myocardium, e.g., R- waves attendant to ventricular depolarization and in some cases P-waves attendant to atrial depolarization. Sensed cardiac event signals may be used for determining the heart rate and determining a need for cardiac pacing, e.g., for treating bradycardia or asystole for preventing a long ventricular pause, or for determining a need for anti-tachyarrhythmia therapies, e.g., antitachycardia pacing (ATP) or CV / DF shocks.

[0029] In the example illustrated in FIGs. 1A and IB, electrode 28 is located proximal to defibrillation electrode 24, and electrode 30 is located between defibrillation electrodes 24 and 26. None, one, two or more pace / sense electrodes may be carried by lead body 18. For instance, a third pace / sense electrode may be located distal to defibrillation electrode 26 in some examples. Electrodes 28 and 30 are illustrated as ring electrodes (e.g., circumscribing lead body 18); however, electrodes 28 and 30 may comprise any of a number of different types of electrodes, including ring electrodes, short coil electrodes, hemispherical electrodes, directional electrodes, segmented electrodes, or the like. Electrodes 28 and 30 may be positioned at other locations along lead body 18 and are not limited to the positions shown. In other examples, lead 16 may include fewer or more pace / sense electrodes and / or defibrillation electrodes than the example shown here.

[0030] In the example shown, lead 16 extends subcutaneously or submuscularly over the ribcage 32 medially from the connector assembly 27 of ICD 14 toward a center of the torso of patient 12, e.g., toward xiphoid process 20 of patient 12. At a location near xiphoid process 20, lead 16 bends or turns and extends superiorly, subcutaneously or submuscularly, over the ribcage and / or sternum, substantially parallel to sternum 22. Although illustrated in FIG. 1A as being offset laterally from and extending substantially parallel to sternum 22, the distal portion 25 of lead 16 may be implanted at other locations, such as over sternum 22, offset to the right or left of sternum 22, angled laterally from sternum 22 toward the left or the right, or the like. Alternatively, lead 16 may be placed along other subcutaneous or submuscular paths. The path of extra-cardiovascular lead 16 may depend on the location of ICD 14, the arrangement and position of electrodes carried by the lead body 18, and / or other factors. The techniques disclosed herein are not limited to a particular path of lead 16 or final locations of electrodes 24, 26, 28 and 30.

[0031] Electrical conductors (not illustrated) extend through one or more lumens of the elongated lead body 18 of lead 16 from the lead connector at the proximal lead end 27 to electrodes 24, 26, 28, and 30 located along the distal portion 25 of the lead body 18. The elongated electrical conductors contained within the lead body 18, which may be separate respective insulated conductors within the lead body 18, are each electrically coupled with respective defibrillation electrodes 24 and 26 and pace / sense electrodes 28 and 30. The respective conductors electrically couple the electrodes 24, 26, 28, and 30 to circuitry, such as a therapy delivery circuit and / or a sensing circuit, of ICD 14 via connections in the connector assembly 17, including associated electrical feedthroughs crossing housing 15. The electrical conductors transmit electrical stimulation pulses from a therapy delivery circuit within ICD 14 to one or more of defibrillation electrodes 24 and 26 and / or pace / sense electrodes 28 and 30 and transmit electrical signals produced by the patient’s heart 8 from one or more of defibrillation electrodes 24 and 26 and / or pace / sense electrodes 28 and 30 to the sensing circuit within ICD 14.

[0032] The lead body 18 of lead 16 may be formed from a non-conductive material, including silicone, polyurethane, fluoropolymers, mixtures thereof, and / or other appropriate materials, and shaped to form one or more lumens within which the one or more conductors extend. Lead body 18 may be tubular or cylindrical in shape. In other examples, the distal portion 25 (or all of) the elongated lead body 18 may have a flat, ribbon or paddle shape. Lead body 18 may be formed having a preformed distal portion 25 that is generally straight, curving, bending, serpentine, undulating or zig-zagging.

[0033] In the example shown, lead body 18 includes a curving distal portion 25 having two “C” shaped curves, which together may resemble the Greek letter epsilon, “s.” Defibrillation electrodes 24 and 26 are each carried by one of the two respective C-shaped portions of the lead body distal portion 25. The two C-shaped curves are seen to extend or curve in the same direction away from a central axis of lead body 18, along which pace / sense electrodes 28 and 30 are positioned. Pace / sense electrodes 28 and 30 may, in some instances, be approximately aligned with the central axis of the straight, proximal portion of lead body 18 such that mid-points of defibrillation electrodes 24 and 26 are laterally offset from pace / sense electrodes 28 and 30.

[0034] Other examples of extra-cardiovascular leads including one or more defibrillation electrodes and one or more pacing and sensing electrodes carried by curving, serpentine,undulating or zig-zagging distal portion of the lead body 18 that may be implemented with the techniques described herein are generally disclosed in U.S. Patent No. 10,675,478 (Marshall, et al.), incorporated herein by reference in its entirety. The techniques disclosed herein are not limited to any particular lead body design, however. In other examples, lead body 18 is a flexible elongated lead body without any pre-formed shape, bends or curves.

[0035] ICD 14 analyzes the cardiac electrical signal(s) received from one or more sensing electrode vectors to monitor for abnormal rhythms, such as asystole, bradycardia, ventricular tachycardia (VT) and / or ventricular fibrillation (VF). ICD 14 may analyze the heart rate, e.g., as determined from the rate of sensed cardiac event signals, and / or morphology of the cardiac electrical signals to monitor for ventricular tachyarrhythmia in accordance with techniques disclosed herein. ICD 14 generates and delivers electrical stimulation therapy in response to detecting a tachyarrhythmia, e.g., VT or VF (VT / VF), using a therapy delivery electrode vector which may be selected from any of the available electrodes 24, 26, 28, 30 and / or housing 15. ICD 14 may deliver ATP in response to VT detection and in some cases may deliver ATP prior to a CV / DF shock or during high voltage capacitor charging in an attempt to avert the need for delivering a CV / DF shock. If ATP does not successfully terminate VT or when VF is detected, ICD 14 may deliver one or more CV / DF shocks via one or both of defibrillation electrodes 24 and 26 and / or housing 15.

[0036] In the absence of a sensed ventricular event signal, e.g., when a long pause in ventricular activity or asystole is detected, ICD 14 may generate and deliver a cardiac pacing pulse, such as a post-shock pacing pulse or bradycardia pacing pulse. The cardiac pacing pulses may be delivered using a pacing electrode vector that includes one or more of the electrodes 24, 26, 28, and 30 and the housing 15 of ICD 14.

[0037] ICD 14 is shown implanted subcutaneously on the left side of patient 12 along the ribcage 32. ICD 14 may, in some instances, be implanted between the left posterior axillary line and the left anterior axillary line of patient 12. ICD 14 may, however, be implanted at other subcutaneous or submuscular locations in patient 12. For example, ICD 14 may be implanted in a subcutaneous pocket in the pectoral region. In this case, lead 16 may extend subcutaneously or submuscularly from ICD 14 toward the manubrium of sternum 22 and bend or turn and extend inferiorly from the manubrium to the desired location subcutaneously or submuscularly. In yet another example, ICD 14 may be placedabdominally. Lead 16 may be implanted in other extra-cardiovascular locations as well. For instance, as described with respect to FIGs. 2A-2C, the distal portion 25 of lead 16 may be implanted underneath the sternum / ribcage in the substernal space. FIGs. 1A and IB are illustrative in nature and should not be considered limiting in the practice of the techniques disclosed herein.

[0038] A medical device operating according to techniques disclosed herein may be coupled to a transvenous or non-transvenous lead in various examples for carrying electrodes for sensing cardiac electrical signals and delivering electrical stimulation therapy. For example, the medical device, such as ICD 14, may be coupled to an extra- cardiovascular lead as illustrated in the accompanying drawings, referring to a lead that positions electrodes outside the blood vessels, heart, and pericardium surrounding the heart of a patient. Implantable electrodes carried by extra-cardiovascular leads may be positioned extra- thoracic ally (outside the ribcage and sternum), subcutaneously or submuscularly, or intra-thoracically (beneath the ribcage or sternum, sometimes referred to as a sub-sternal position) and may not necessarily be in intimate contact with myocardial tissue. An extra-cardiovascular lead may also be referred to as a “non-transvenous” lead.

[0039] In other examples, the medical device may be coupled to a transvenous lead that positions electrodes within a blood vessel, which may remain outside the heart in an “extra-cardiac” location or be advanced to position electrodes within a heart chamber. For instance, a transvenous medical lead may be advanced along a venous pathway to position electrodes in an extra-cardiac location within the internal thoracic vein (ITV), an intercostal vein, the superior epigastric vein, or the azygos, hemiazygos, or accessory hemiazygos veins, as examples. In still other examples, a transvenous lead may be advanced to position electrodes within the heart, e.g., within an atrial and / or ventricular heart chambers.

[0040] An external device 40 is shown in telemetric communication with ICD 14 by a wireless communication link 42 in FIG. 1A. External device 40 may include a processor 52, memory 53, display 54, user interface 56 and telemetry unit 58. Processor 52 controls external device operations and processes data and signals received from ICD 14. Display unit 54, which may include a graphical user interface, displays data and other information to a user for reviewing ICD operation and programmed parameters as well as cardiac electrical signals retrieved from ICD 14.

[0041] User interface 56 may include a mouse, touch screen, keypad or the like to enable a user to interact with external device 40 to initiate a telemetry session with ICD 14 for retrieving data from and / or transmitting data to ICD 14, including programmable parameters for controlling cardiac event signal sensing, tachyarrhythmia detection and therapy delivery. Telemetry unit 58 includes a transceiver and antenna configured for bidirectional communication with a telemetry circuit included in ICD 14 and is configured to operate in conjunction with processor 52 for sending and receiving data relating to ICD functions via communication link 42.

[0042] Communication link 42 may be established between ICD 14 and external device 40 using a radio frequency (RF) link such as BLUETOOTH®, Wi-Fi, or Medical Implant Communication Service (MICS) or other RF or communication frequency bandwidth or communication protocols. Data stored or acquired by ICD 14, including physiological signals or associated data derived therefrom, results of device diagnostics, battery status, and histories of detected rhythm episodes and delivered therapies, etc., may be retrieved from ICD 14 by external device 40 following an interrogation command.

[0043] External device 40 may be embodied as a programmer used in a hospital, clinic or physician’s office to retrieve data from ICD 14 and to program operating parameters and algorithms in ICD 14 for controlling ICD functions. External device 40 may alternatively be embodied as a home monitor or handheld device. External device 40 may be used to program cardiac signal sensing parameters, cardiac rhythm detection parameters and therapy control parameters used by ICD 14. At least some control parameters used in detecting tachyarrhythmias and controlling anti-tachyarrhythmia therapy according to the techniques disclosed herein may be programmed into ICD 14 using external device 40 in some examples.

[0044] FIGs. 2A-2C are conceptual diagrams of patient 12 implanted with extra- cardiovascular ICD system 10 in a different implant configuration than the arrangement shown in FIGs. 1A-1B. FIG. 2A is a front view of patient 12 implanted with ICD system 10. FIG. 2B is a side view of patient 12 implanted with ICD system 10. FIG. 2C is a transverse view of patient 12 implanted with ICD system 10. In this arrangement, extra- cardiovascular lead 16 of system 10 is implanted at least partially underneath sternum 22 of patient 12. Lead 16 extends subcutaneously or submuscularly from ICD 14 towardxiphoid process 20 and at a location near xiphoid process 20 bends or turns and extends superiorly within anterior mediastinum 36 (see FIG. 2C) in a substemal position.

[0045] Anterior mediastinum 36 may be viewed as being bounded laterally by pleurae 39, posteriorly by pericardium 38, and anteriorly by sternum 22 (see FIG. 2C). The distal portion 25 of lead 16 may extend along the posterior side of sternum 22 substantially within the loose connective tissue and / or substernal musculature of anterior mediastinum 36. A lead implanted such that the distal portion 25 is substantially within anterior mediastinum 36, may be referred to as a “substemal lead.”

[0046] In the example illustrated in FIGS. 2A-2C, lead 16 is located substantially centered under sternum 22. In other instances, however, lead 16 may be implanted such that it is offset laterally from the center of sternum 22. In some instances, lead 16 may extend laterally such that distal portion 25 of lead 16 is undemeath / below the ribcage 32 in addition to or instead of sternum 22. In other examples, the distal portion 25 of lead 16 may be implanted in other extra-cardiac, intra-thoracic locations, including in the pleural cavity or around the perimeter of and adjacent to the pericardium 38 of heart 8.

[0047] In the various example implant locations of lead 16 and electrodes 24, 26, 28 and 30 shown and described herein, cardiac signals sensed by ICD 14 may have a relatively low and / or variable signal strength, e.g., caused by postural changes, respiration or other body movement. The sensed cardiac signals may be contaminated by skeletal muscle myopotentials or other non-cardiac electrophysiological signals, electromagnetic interference (EMI) or other environmental noise, which could include lead-related noise in some instances (e.g., due to lead fracture or poor lead connection). Oversensing of P- waves, T-waves, skeletal muscle myopotentials, EMI or other non-cardiac noise as false R- waves, may lead to a false tachyarrhythmia detection resulting in unnecessary ATP or CV / DF shock delivery.

[0048] Cardiac signal oversensing occurs when a P-wave or a T-wave is oversensed as an R-wave or a wide QRS is sensed twice as two R-waves instead of one. Non-cardiac event oversensing occurs when non-cardiac signal pulses, e.g., skeletal muscle myopotential signals or EMI, are oversensed as R-waves by ICD 14. An oversensed cardiac event signal or non-cardiac event signal may be sensed as a ventricular event signal. If an oversensed signal falsely sensed as a ventricular event signal is sensed at an RRI from a most recent preceding sensed ventricular event signal that is less than a tachyarrhythmia detectioninterval, the RRI may be counted as a VT / VF interval. Oversensing may therefore lead to a false VT / VF detection. Techniques disclosed herein provide improvements in detecting tachyarrhythmias by a medical device by performing analysis of cardiac signal segments sensed by the medical device to confirm sensed ventricular event signals. Confirmed sensed ventricular event signals may be used in determining a suspected heart rate that can be compared to a sensed heart rate determined from sensed (but not necessarily confirmed) ventricular event signals or another heart rate threshold. The suspected heart rate may be used in determining when VT / VF detection criteria are met and / or when to initiate an antitachyarrhythmia therapy, e.g., by starting ATP and / or starting capacitor charging for delivering a CV / DF shock.

[0049] FIG. 3 is a conceptual diagram of ICD 14 according to one example. The electronic circuitry enclosed within housing 15 (shown conceptually as an electrode in FIG. 3) may include software, firmware and / or hardware that cooperatively monitor cardiac electrical signals, determine when an electrical stimulation therapy is necessary, and deliver therapy as needed according to programmed therapy delivery algorithms and control parameters. ICD 14 may be coupled to a lead, such as lead 16 carrying electrodes 24, 26, 28, and 30, for delivering electrical stimulation pulses to the patient’s heart and for sensing cardiac electrical signals.

[0050] ICD 14 includes a control circuit 80, memory 82, therapy delivery circuit 84, cardiac electrical signal sensing circuit 86, and telemetry circuit 88. A power source 98 provides power to the circuitry of ICD 14, including each of the components 80, 82, 84, 86, and 88 as needed. Power source 98 may include one or more energy storage devices, such as one or more rechargeable or non-rechargeable batteries. The connections between power source 98 and each of the other components 80, 82, 84, 86 and 88 are to be understood from the general block diagram of FIG. 3 but are not shown for the sake of clarity. For example, power source 98 may be coupled to one or more charging circuits included in therapy delivery circuit 84 for charging holding capacitors included in therapy delivery circuit 84 that are discharged at appropriate times under the control of control circuit 80 for producing electrical pulses according to a therapy protocol. Power source 98 is also coupled to components of cardiac electrical signal sensing circuit 86, such as sense amplifiers, analog-to-digital converters, switching circuitry, etc. as needed.

[0051] The circuits shown in FIG. 3 represent functionality included in ICD 14 and may include any discrete and / or integrated electronic circuit components that implement analog and / or digital circuits capable of producing the functions attributed to ICD 14 herein. Functionality associated with one or more circuits may be performed by separate hardware, firmware and / or software components, or integrated within common hardware, firmware and / or software components. For example, cardiac electrical signal sensing and analysis for detecting tachyarrhythmia may be performed cooperatively by sensing circuit 86 and control circuit 80 and may include operations implemented in a processor or other signal processing circuitry included in sensing circuit 86 and / or control circuit 80 executing instructions stored in memory 82 and control signals such as blanking and timing intervals and sensing threshold amplitude signals sent from control circuit 80 to sensing circuit 86.

[0052] Control circuit 80 may include hardware configured to perform subroutines of signal processing and analysis techniques disclosed herein to reduce the processing burden associated with firmware and / or software execution of processing routines. For example hardware subroutines (HSRs) may be implemented in control circuit 80 to perform specific processing functions such as dedicated math operations, which may include any of sum, absolute value, difference, extrema, histogram or memory buffer counts, signal filtering (e.g., biquad filter, difference filter or other filters), etc. These HSRs could be called by control circuit firmware when processing and analyzing a cardiac signal for detecting tachyarrhythmia, which may include a low pass filter, difference filter, gradient filter or other signal processing. HSRs may be called when control circuit 80 is determining various morphology parameters from a cardiac signal for detecting tachyarrhythmia as described herein. These HSRs can unload the processing burden associated with firmware and / or software processing to reduce current drain of power source 98 and thereby extend the useful life of ICD 14.

[0053] The various circuits of ICD 14 may include an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, state machine, HSR, or other suitable components or combinations of components that provide the described functionality. The particular form of software, hardware and / or firmware employed to implement the functionality disclosed herein will be determinedprimarily by the particular system architecture employed in the ICD and by the particular sensing, detection and therapy delivery methodologies employed by the ICD. Providing software, hardware, and / or firmware to accomplish the described functionality in the context of any modem medical device system, given the disclosure herein, is within the abilities of one of skill in the art.

[0054] Memory 82 may include any volatile, non-volatile, magnetic, or electrical non- transitory computer readable storage media, such as random access memory (RAM), readonly memory (ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory, or any other memory device. Furthermore, memory 82 may include non-transitory computer readable media storing instructions that, when executed by one or more processing circuits, cause control circuit 80 and / or other ICD components to perform various functions attributed to ICD 14 or those ICD components. The non-transitory computer-readable media storing the instructions may include any of the media listed above.

[0055] Control circuit 80 communicates, e.g., via a data bus, with therapy delivery circuit 84 and sensing circuit 86 for sensing cardiac electrical signals, detecting cardiac rhythms, and controlling delivery of cardiac electrical stimulation therapies in response to sensed cardiac signals. Therapy delivery circuit 84 and sensing circuit 86 may be electrically coupled to electrodes 24, 26, 28, 30 carried by lead 16 and / or the housing 15, which may function as a common or ground electrode or as an active can electrode for delivering CV / DF shock pulses or cardiac pacing pulses.

[0056] Cardiac electrical signal sensing circuit 86 (also referred to herein as “sensing circuit” 86) may be selectively coupled to electrodes 28, 30 and / or housing 15 in order to monitor electrical activity of the patient’s heart. Sensing circuit 86 may additionally be selectively coupled to defibrillation electrodes 24 and / or 26 for use in a sensing electrode vector together or in combination with one or more of electrodes 28, 30 and / or housing 15. Sensing circuit 86 may be enabled to receive cardiac electrical signals from at least one sensing electrode vector selected from the available electrodes 24, 26, 28, 30, and housing 15 in some examples. At least two, three or more cardiac electrical signals from two, three or more different sensing electrode vectors may be received simultaneously by sensing circuit 86 in some examples. Sensing circuit 86 may monitor one or more cardiac electrical signals for sensing cardiac event signals, e.g., R- waves attendant to intrinsicventricular myocardial depolarizations. In some examples, sensing circuit 86 may be configured to monitor two cardiac electrical signals simultaneously for sensing cardiac event signals. At least one cardiac electrical signal may be sensed by sensing circuit 86 and passed to control circuit 80 for processing and analysis for confirming ventricular sensed event signals as further described below. In some examples, a cardiac electrical signal received over a specified time interval encompassing a triggering Vsense signal may be analyzed for confirming the triggering Vsense signal as likely being a true R-wave or QRS complex. Confirmed Vsense signals may be used in determining a suspected heart rate for use in detecting a tachyarrhythmia as further described below. In the example shown, sensing circuit 86 may include switching circuitry for selecting which of electrodes 24, 26, 28, 30, and housing 15 are coupled as a first sensing electrode vector to a first sensing channel 83 for receiving a first cardiac electrical signal, which electrodes are coupled as a second sensing electrode vector to a second sensing channel 85 of sensing circuit 86 for receiving a second cardiac electrical signal, and which electrodes are coupled as a third sensing electrode vector to a morphology signal channel 87 for receiving a third cardiac electrical signal.

[0057] Each sensing channel 83 and 85, when included, may be configured to amplify, filter and digitize the cardiac electrical signal received from selected electrodes coupled to the respective sensing channel to improve the signal quality for sensing cardiac event signals, such as R-waves. The cardiac event detection circuitry within sensing circuit 86 may include one or more sense amplifiers, filters, rectifiers, threshold detectors, comparators, analog-to-digital converters (ADCs), timers or other analog and / or digital components as described further in conjunction with FIG. 4. A cardiac event sensing threshold may be automatically adjusted by each sensing channel 83 and 85 under the control of control circuit 80, based on sensing threshold control parameters, such as various timing intervals and sensing threshold amplitude values that may be determined by control circuit 80, stored in memory 82, and / or controlled by hardware, firmware and / or software of control circuit 80 and / or sensing circuit 86.

[0058] First sensing channel 83 and second sensing channel 85 may each control a cardiac event sensing threshold, e.g., an R-wave sensing threshold, that is applied to the incoming cardiac electrical signal for sensing cardiac event signals, e.g., R-waves. Upon sensing a cardiac event signal based on a sensing threshold crossing, first sensing channel 83 mayproduce a ventricular sensed event (Vsense) signal that is passed to control circuit 80. For example, upon detecting an R-wave sensing threshold crossing by the cardiac electrical signal received via a first sensing electrode vector, the first sensing channel 83 may generate a Vsense signal that is passed to control circuit 80. Similarly, upon detecting an R-wave sensing threshold crossing by a second cardiac electrical signal received by second sensing channel 85, the second sensing channel 85 may generate a Vsense signal that is passed to control circuit 80. The first and second sensing channels 83 and 85 may be configured to automatically adjust the R-wave sensing threshold used by each respective sensing channel separately. The Vsense signals and relative timing from each other may be used by control circuit 80 for determining sensed event intervals for use in detecting VT / VF and / or controlling pacing pulse delivery.

[0059] For instance, Vsense signals received from sensing circuit 86 by control circuit 80 can be used by control circuit 80 for determining ventricular sensed event intervals, which are referred to herein as sensed RRIs. A sensed RRI can be the time interval between two Vsense signals received consecutively by control circuit 80 from the same sensing channel 83 or 85. Control circuit 80 may include a timing circuit 90 for determining RRIs between consecutive Vsense signals received from a given sensing channel 83 or 85. Based on sensed RRIs, control circuit 80 may determine that rate-based VT / VF detection criteria are met in some examples. In some instances, when a Vsense signal is received following a delivered pacing pulse, the sensed RRI is determined from the pacing pulse to the Vsense signal. As such, sensed RRIs may include time intervals between consecutive Vsense signals and intervals between a delivered pacing pulse and a subsequent Vsense signal.

[0060] Sensing circuit 86 is shown in FIG. 3 as being configured to receive two different cardiac electrical signals by the two cardiac event sensing channels 83 and 85 for sensing R-waves from the two cardiac electrical signals and for receiving a third cardiac electrical signal by morphology signal channel 87 for passing a digitized electrocardiogram (ECG) signal (when the sensing electrodes are located outside the patient’ s heart) or a cardiac electrogram (EGM) signal (e.g., when the sensing electrodes are located inside the patient’s heart) to control circuit 80 for signal analysis. Signal analysis of a received ECG or EGM signal may be performed to detect morphological evidence of VT / VF, morphological evidence of a supraventricular tachycardia (SVT), and / or morphological evidence of non-cardiac noise contamination or P-waves or T-waves being oversensed asfalse R-waves. As described below, signal analysis of R-wave segments of an ECG or EGM signal received from morphology signal channel 87 may be performed for confirming Vsense signals received from one or both of sensing channels 83 and 85.

[0061] The three cardiac electrical signals sensed by sensing circuit 86 may be received using three different sensing electrode vectors selected from the available electrodes 24, 26, 28 and 30 and housing 15. In other examples, two cardiac electrical signals may be received by sensing circuit 86 from two different sensing electrode vectors, with one signal passed to the first sensing channel 83 and the other signal passed to the second sensing channel 85. Either or both of the two signals may be passed to control circuit 80 as a multi-bit digital ECG signal (or EGM signal) used by control circuit 80 for signal analysis according to the techniques disclosed herein. It is to be understood, however, that the techniques disclosed herein can be performed with sensing circuit 86 configured to sense one, two or more cardiac electrical signals via one two or more sensing channels. For instance, sensing of ventricular event signals as described herein could be performed using a single sensing channel 83 or 85 for passing Vsense signals to control circuit 80. Morphology sensing channel 87 may pass cardiac signal segments to control circuit 80 for analysis for confirming Vsense signals. Morphology sensing channel 87 may be optional in some examples when a sensing channel is configured to sense ventricular event signals and pass an ECG signal (or EGM signal) to control circuit 80 for signal analysis.

[0062] Timing circuit 90 may be configured to control various timers and / or counters used in setting various intervals and windows used in sensing ventricular event signals, determining time intervals between received Vsense signals, performing morphology analysis and controlling the delivery timing of electrical stimulation pulses generated by therapy delivery circuit 84. Timing circuit 90 may start a timer in response to receiving Vsense signals from sensing channels 83 and 85 for timing the sensed RRIs between consecutively received in-channel Vsense signals (and in some instances from a delivered pacing pulse to a Vsense signal). Control circuit 80 may pass the sensed RRI to arrhythmia detection circuit 92 for determining and counting VF / VF intervals.

[0063] Control circuit 80 may include an arrhythmia detection circuit 92 configured to analyze sensed RRIs received from timing circuit 90 and cardiac electrical signals received from morphology signal channel 87 for detecting tachyarrhythmia. Arrhythmia detection circuit 92 may be configured to detect a ventricular tachyarrhythmia based onsensed cardiac electrical signals meeting VT / VF detection criteria. Arrhythmia detection circuit 92 may be implemented in control circuit 80 as hardware, software and / or firmware that processes and analyzes signals received from sensing circuit 86 for detecting tachyarrhythmia.

[0064] In some examples, arrhythmia detection circuit 92 may include comparators and counters for counting sensed RRIs determined by timing circuit 90 from Vsense signals received from sensing channel 83 and / or sensing channel 85 that fall into various rate detection zones for determining a ventricular rate or performing other rate- or intervalbased assessment of Vsense signals for detecting and discriminating VT and VF. For example, arrhythmia detection circuit 92 may compare the sensed RRIs determined by timing circuit 90 to one or more tachyarrhythmia detection interval zones, such as a tachycardia detection interval zone and a fibrillation detection interval zone. Sensed RRIs falling into a detection interval zone are counted by a respective VT interval (VTI) counter or VF interval (VFI) counter and in some cases in a combined VT / VF interval counter. The VF detection interval threshold may be set to 300 to 350 milliseconds (ms), as an example. For instance, if the VF detection interval is set to 320 ms, sensed RRIs that are less than 320 ms are counted by the VFI counter. When VT detection is enabled, the VT detection interval may be programmed to be in the range of 350 to 420 ms, or 400 ms as an example. Sensed RRIs that are less than the VT detection interval but greater than or equal to the VF detection interval may be counted by a VTI counter. Criteria for detecting VT or VF may become satisfied when the respective VTI or VFI counter (or a combined VT / VF interval counter) reaches a threshold number of intervals to detect (NID).

[0065] As an example, the NID to detect VT may require that the VTI counter reaches 18 VTIs, 24 VTIs, 32 VTIs or other selected NID. In some examples, the VTIs may be required to be consecutive intervals, e.g., 18 out of 18, 24 out of 24, or 32 out of 32 or 100 out of the most recent 100 consecutive sensed RRIs. The NID required to detect VF may be programmed to a threshold number of X VFIs out of Y consecutive RRIs. For instance, the NID required to detect VF may be 18 VFIs out of the most recent 24 consecutive RRIs, 30 VFIs out 40 consecutive RRIs, or as high as 120 VFIs out of 160 consecutive RRIs as examples (or other percentage of a specified number of sensed RRIs). When a VTI or VFI counter reaches a respective NID, a ventricular tachyarrhythmia may be detected by arrhythmia detection circuit 92. The NID may be programmable and rangefrom as low as 12 to as high as 120, as examples with no limitation intended. A VTI counter or VFI counter may reach a respective NID when VTIs or VFIs are detected consecutively or non-consecutively out of a specified number of most recently sensed RRIs. In some cases, a combined VT / VF interval counter may count both VTIs and VFIs, and control circuit 80 may detect either VT or VF based on the fastest intervals detected when a specified NID is reached.

[0066] In some examples, rate-based VT / VF detection criteria may be met when an NID is reached (or exceeded). However, control circuit 80 may not detect VT / VF based only on the NID being met. For example, as described below, arrhythmia detector circuit 92 may store R-wave segments from a cardiac electrical signal sensed by sensing circuit 86 in memory 82. A Vsense signal received from sensing channel 83 or 85 may trigger storage of an R-wave segment in memory 82. Arrhythmia detector circuit 92 may determine if the R-wave segment meets R-wave confirmation criteria, according to any of the example methods described below. When the R-wave segment meets the R-wave confirmation criteria, the triggering Vsense signal may be determined to be a confirmed Vsense signal. Arrhythmia detector circuit 92 may determine a suspected heart rate from the confirmed Vsense signals and / or associated R-wave segments. When an NID is reached based on sensed RRIs, arrhythmia detector circuit 92 may compare the suspected heart rate to a heart rate threshold for confirming a likely tachyarrhythmia rate before detecting VT / VF.

[0067] Arrhythmia detection circuit 92 may be configured to perform other signal analysis for determining if other detection criteria are satisfied before detecting VT or VF based on an NID being reached. Other signal analysis may include determining if R-wave morphology criteria, onset criteria, stability criteria, noise oversensing criteria and / or cardiac event oversensing criteria are met. To support these additional analyses, sensing circuit 86 may pass a digitized ECG (or EGM) signal to control circuit 80, e.g., from morphology signal channel 87, for morphology analysis performed by arrhythmia detection circuit 92 for detecting and discriminating heart rhythms. A cardiac electrical signal received by the morphology signal channel 87 (and / or sensing channel 83 and / or sensing channel 85) may be passed through a filter and amplifier, provided to a multiplexer and thereafter converted to a multi-bit digital signal by an analog-to-digital converter, all included in sensing circuit 86, for storage in memory 82. Memory 82 may include one or more circulating buffers to temporarily store digital cardiac signal segmentsfor analysis performed by control circuit 80. Control circuit 80 may be a microprocessorbased controller, which may include HSRs, that employs digital signal analysis techniques to characterize the digitized signals stored in memory 82 to recognize and classify the patient’s heart rhythm employing any of numerous signal processing methodologies for analyzing cardiac signals and cardiac event waveforms, e.g., R-waves.

[0068] In some examples, first tachyarrhythmia detection criteria may be met when an NID is reached. When the NID is met, arrhythmia detection circuit 92 may determine if second tachyarrhythmia detection criteria are met. The second tachyarrhythmia detection criteria may require that none of one or more VT / VF rejection rules are met. Each VT / VF rejection rule may include various criteria applied to sensed RRIs and / or morphology features of sensed cardiac signals for detecting evidence of an SVT, oversensing of noise or oversensing of cardiac event signals that would be contradictory to the detection of VT / VF. When a VT / VF rejection rule is met, control circuit 80 may withhold a VT / VF detection based on the NID being reached. When the NID is met, the suspected heart rate determined from R-wave segment analysis meets a heart rate threshold, and no VT / VF rejection rules are met, control circuit 80 may detect VT / VF and control therapy delivery circuit 84 to initiate anti-tachyarrhythmia therapy.

[0069] Various examples of VT / VF rejection rules that control circuit 80 may apply as criteria to interval and / or morphology features determined by control circuit 80 from cardiac electrical signals sensed by the sensing circuit 86 include an SVT rejection rule and an atrial fibrillation rejection rule, for example as generally disclosed in U.S. Patent No. 10,555,684 (Zhang et al., filed February 11, 2020) and in U.S. Patent No. 11,116,981 (Zhang et al., filed December 12, 2018), the entire content of both of which incorporated herein. Other examples of VT / VF rejection rules that control circuit 80 may apply include a TWOS rejection rule, e.g., as generally disclosed in U.S. Patent No. 9,597,525 (Cao, et al., filed May 6, 2015) and in U.S. Patent No. 10,850,113 (Cao, et al., filed July 20, 2017), the entire content of both of which incorporated herein and a PWOS rejection rule, e.g., as generally disclosed in U.S. Patent Application Publication No. 2021 / 0170170 (Mischler et al., filed December 2, 2020), the entire content of which is incorporated herein by reference. Still other VT / VF rejection rules applied by control circuit 80 prior to detecting VT / VF may include non-cardiac noise rejection rules for detecting noise corruption that may be leading to oversensing, e.g., as generally disclosed in U.S. Patent No. 8,095,206(Ghanem, et al., filed May 1, 2007), U.S. Patent No. 9,956,423 (Zhang, et al., filed on April 28, 2016) and U.S. Patent No. 10,470,681 (Greenhut, et al., filed May 26, 2017), all of which are incorporated herein by reference.

[0070] Therapy delivery circuit 84 includes at least one charging circuit 94, including one or more charge storage devices such as one or more high voltage capacitors for generating high voltage shock pulses for treating VT / VF. Charging circuit 94 may include one or more low voltage capacitors for generating relatively lower voltage pulses, e.g., for cardiac pacing therapies. Therapy delivery circuit 84 may include switching circuitry 95 that controls when the charge storage device(s) are discharged through an output circuit 96 across a selected pacing electrode vector or CV / DF shock vector.

[0071] In response to detecting VT / VF, control circuit 80 may schedule a therapy and control therapy delivery circuit 84 to generate and deliver the therapy, such as ATP and / or CV / DF shock(s). Therapy can be generated by initiating charging of high voltage capacitors of charging circuit 94. Charging is controlled by control circuit 80, which monitors the voltage on the high voltage capacitors passed to control circuit 80 as a voltage control signal via a charging control line. When the voltage reaches a predetermined value set by control circuit 80, a logic signal is generated on a capacitor full line and passed to therapy delivery circuit 84, terminating charging. A CV / DF shock pulse is delivered to the heart under the control of the timing circuit 90 by an output circuit 96 of therapy delivery circuit 84 via a control bus. The output circuit 96 may include an output capacitor or other output circuitry through which the charged high voltage capacitor is discharged via switching circuitry, e.g., an H-bridge, which determines the electrodes used for delivering the cardioversion or defibrillation pulse and the pulse wave shape.

[0072] In some instances, therapy delivery circuit 84 may be configured to deliver electrical stimulation pulses for inducing tachyarrhythmia, e.g., T-wave shocks or trains of induction pulses, upon receiving a programming command from external device 40 (FIG. 1A) during ICD implant or follow-up testing procedures. In some examples, the high voltage therapy circuit configured to deliver CV / DF shock pulses can be controlled by control circuit 80 to deliver pacing pulses, e.g., for delivering ATP, post shock pacing pulses, bradycardia pacing pulses or asystole pacing pulses. Therapy delivery circuit 84 may be configured to generate and deliver cardiac pacing pulses using the high voltage capacitor(s) that are chargeable to a shock voltage amplitude by charging the high voltagecapacitor(s) to a relatively lower voltage corresponding to a cardiac pacing pulse amplitude for capturing and pacing the ventricular myocardium.

[0073] Therapy delivery circuit 84 may include a low voltage therapy circuit including one or more separate or shared charging circuits, switch circuits and output circuits for generating and delivering relatively lower voltage pacing pulses for a variety of pacing needs. Charging of one or more capacitors to a programmed pulse amplitude and discharging of the capacitors for a programmed pulse width may be performed by therapy delivery circuit 84 according to control signals received from control circuit 80 for delivering cardiac pacing pulses, including ATP pulses delivered in response to a VT / VF detection. Timing circuit 90 may include various timers or counters that control when cardiac pacing pulses are delivered. The microprocessor of control circuit 80 may set the amplitude, pulse width, polarity or other characteristics of cardiac pacing pulses, which may be based on programmed values stored in memory 82.

[0074] While example therapy delivery circuitry is described here it is to be understood that the methods of the present disclosure for sensing cardiac event signals and determining a heart rate or heart rhythm are not limited to a particular configuration of therapy delivery circuitry. Techniques disclosed herein for determining a suspected heart rate for confirming a rate of Vsense signals for detecting a tachyarrhythmia, for example, may be implemented in a medical device that is not necessarily configured to deliver an anti-tachyarrhythmia therapy. As such, therapy delivery circuit 84 may be optional or may be configured to deliver therapies, e.g., bradycardia pacing, cardiac resynchronization therapy or other pacing therapies, other than anti-tachyarrhythmia therapies.

[0075] Control parameters utilized by control circuit 80 for sensing cardiac event signals, detecting arrhythmias, and controlling therapy delivery may be programmed into memory 82 via telemetry circuit 88. Telemetry circuit 88 includes a transceiver and antenna for communicating with external device 40 (shown in FIG. 1A) using RF communication or other communication protocols as described above. Under the control of control circuit 80, telemetry circuit 88 may receive downlink telemetry from and send uplink telemetry to external device 40. In some cases, VT / VF detections made by control circuit 80 may be logged in memory 82 for reporting to a clinician. Control circuit 80 may store a cardiac signal episode of a detected VT / VF, determine and store the duration of the detected VT / VF, determine and store the detected VT / VF heart rate, update a tracked VT / VFburden, and / or determine and store other VT / VF detection data for transmitting to external device 40 via telemetry circuit 88.

[0076] FIG. 4 is a conceptual diagram of circuitry that may be included in sensing circuit 86 shown in FIG. 3 according to some examples. Sensing circuit 86 may include a first sensing channel 83, second sensing channel 85 and morphology signal channel 87. First sensing channel 83 and second sensing channel 85 may each be selectively coupled via switching circuitry included in sensing circuit 86 to a respective sensing electrode vector including at least one electrode carried by extra-cardiovascular lead 16. First sensing channel 83 may be coupled to a first sensing electrode vector for receiving a first cardiac electrical signal, and second sensing channel 85 may be coupled to a second sensing electrode vector, different than the first sensing electrode vector for receiving a second cardiac electrical signal, different than the first cardiac electrical signal. In some examples, first sensing channel 83 may be coupled to a sensing electrode vector that is a short bipole, having a relatively shorter inter-electrode distance than the sensing electrode vector coupled to the second sensing channel 85 or to morphology signal channel 87. In the example shown, the first sensing channel 83 is coupled to pace / sense electrodes 28 and 30 carried by lead 16. In some examples, first sensing channel 83 may be coupled to a sensing electrode vector that is approximately vertical (when the patient is in an upright position) or approximately aligned with the cardiac axis to increase the likelihood of a relatively high R-wave signal amplitude relative to the P-wave signal amplitude. A relatively short inter-electrode distance, e.g., between electrodes 28 and 30 carried by lead 16, may be relatively less likely to be contaminated by skeletal muscle myopotential noise, EMI or other non-cardiac noise compared to a relatively longer inter-electrode distance but may have greater variability in R-wave signal strength compared to a relatively longer inter-electrode distance.

[0077] The second sensing channel 85 may be coupled to a second sensing electrode vector that is a short bipole or a relatively longer bipole compared to the first sensing electrode vector. The second sensing electrode vector may also be generally vertical or aligned with the cardiac axis. However, the second sensing electrode vector may be orthogonal or transverse relative to the first sensing electrode vector in other examples. In the example shown, the second sensing channel 85 is coupled to pace / sense electrode 30 and housing 15 such that it is a relatively longer bipole that is substantially transverse tothe sensing electrode vector coupled to the first sensing channel 83. In other examples, the first or second sensing channels may be coupled to either of pace / sense electrodes 28 or 30 paired with housing 15, either of pace / sense electrodes 28 or 30 paired with coil electrode 24, or either of pace sense electrodes 28 or 30 paired with coil electrode 26, as long as at least one electrode is different between the two sensing electrode vectors. In further examples, either or both of first or second sensing channels 83 or 85 may be coupled to a sensing electrode vector that does not necessarily include one of pace / sense electrodes 28 or 30. For example, a sensing electrode vector may be coupled to sensing channel 83 or sensing channel 85 that includes one or both of coil electrodes 24 or 26 and / or housing 15.

[0078] Sensing circuit 86 may include a morphology signal channel 87 for sensing a third cardiac electrical signal. For instance, morphology signal channel 87 may receive a raw cardiac electrical signal from a third sensing electrode vector, for example from a vector that includes one electrode 24, 26, 28 or 30 carried by lead 16 paired with housing 15. Morphology signal channel 87 may be selectively coupled to a relatively long bipole having an inter-electrode distance or spacing that is greater than the sensing electrode vector coupled to first sensing channel 83 and / or second sensing channel 85 in some examples. The third sensing electrode vector may be, but not necessarily, approximately orthogonal to at least one of the first channel sensing electrode vector or the second channel sensing electrode vector. In the example shown, coil electrode 24 and housing 15 may be coupled to morphology signal channel 87 to provide the third sensed cardiac electrical signal. The third cardiac electrical signal, also referred to herein as the “morphology signal,” received by morphology signal channel 87 may be used by control circuit 80 for morphology analysis for a variety of sensing and arrhythmia detection purposes. For example, control circuit 80 may perform a morphology analysis of a signal sensed by morphology signal channel 87 to determine when morphology-based tachyarrhythmia classification of a cardiac electrical signal segment occurs for use in controlling when a VT / VF detection is made. According to techniques disclosed herein and described below, control circuit 80 may analyze R-wave segments obtained from the morphology signal for confirming Vsense signals and determining an estimated ventricular rate for corroborating the rate determined from Vsense signals received from a selected sensing channel 83 or 85.

[0079] Control circuit 80 may utilize the morphology signal received from morphology signal channel 87 for performing a variety of morphology-related analyses to support detecting VT / VF. For instance, control circuit 80 may perform a morphology matching analysis involving a wavelet transform for determining a morphology matching score between a sensed signal waveform received from morphology sensing channel 87 and a morphology template established for a normally conducted R-wave. Morphology matching analysis may be performed for discriminating conducted SVT or ST from VT / VF. In some examples, when SVT criteria are met based on morphology matching analysis, a VT / VF rejection rule may be met, preventing control circuit 80 from detecting VT / VF when an NID is reached. Example methods for performing a morphology matching analysis are generally disclosed in U.S. Patent No. 8,521,268 (Zhang et al., filed May 10, 2011), incorporated herein by reference in its entirety.

[0080] Additionally or alternatively, control circuit 80 may perform a morphology analysis of an n-second cardiac signal segment that may be obtained independent of Vsense signals sensed by sensing channel 83 and / or sensing channel 85. Morphology analysis of an n-second signal segment buffered independently of any timing or number of Vsense signals may be performed to detect morphological evidence of VT / VF when a VTI or VFI counter has been increasing but a Vsense signal has not been sensed for a long pause interval, suggesting possible R-wave or fibrillation wave undersensing. When morphological evidence of VT / VF is detected based on analysis of an n-second segment, e.g., a three-second segment, by arrhythmia detection circuit 92 prior to an NID being reached, control circuit 80 may increase the sensitivity for sensing R-waves by the first and / or second sensing channels 83 and 85. Examples of methods for performing morphology analysis of an n-second cardiac signal segment for use in detecting VT / VF and / or controlling the sensitivity of sensing channels 83 and 85 are generally disclosed in PCT International Publication Number WO2023 / 159031(Heinks, et al), incorporated herein by reference in its entirety.

[0081] For these purposes, in some examples, the sensing electrode vector coupled to morphology signal channel 87 may provide a relatively far-field or more global cardiac signal compared to a relatively shorter bipole that may be coupled to the first sensing channel 83 or the second sensing channel 85. In other examples, any vector selected from the available electrodes, e.g., electrodes 24, 26, 28, 30 and / or housing 15, may be includedin a sensing electrode vector coupled to morphology signal channel 87. The sensing electrode vectors coupled to first sensing channel 83 and second sensing channel 85 and, at least in some examples, morphology signal channel 87 may be different sensing electrode vectors, which may have no common electrodes or only one common electrode but not both electrodes in common between the different sensing electrode vectors. In other examples, however, the sensing electrode vector coupled to one of the first sensing channel 83 or the second sensing channel 85 may be the same sensing electrode vector coupled to the morphology signal channel 87. In this case, a sensing channel 83 or 85 and the morphology signal channel 87 may be combined or include shared components such that a morphology signal and Vsense signals may be output to control circuit 80 from one sensing channel.

[0082] The first sensing channel 83 and the second sensing channel 85 may each receive a cardiac electrical signal for sensing ventricular event signals in response to the cardiac electrical signal crossing an R-wave sensing threshold. The morphology signal channel 87 may receive a third cardiac electrical signal for passing a multi-bit digital ECG signal to control circuit 80 for morphology analysis. In the illustrative example shown in FIG. 4, the signals received by first sensing channel 83, second sensing channel 85 and morphology signal channel 87 are provided as differential input signals to a pre-filter and pre-amplifier 62a, 62b, and 72, respectively. Non-physiological high frequency and DC signals may be filtered by a low pass or bandpass filter included in each of pre-filter and pre-amplifiers 62a, 62b and 72, and high voltage signals may be removed by protection diodes included in pre-filter and pre-amplifiers 62a, 62b and 72. Pre-filter and pre-amplifiers 62a, 62b and 72 may amplify the pre-filtered signal by a gain of between 10 and 100, and in one example a gain of 17, though each channel may have a different gain and filter bandwidth. Pre-filter and pre-amplifiers 62a, 62b and 72 may convert the differential input signal to a single-ended output signal passed to an analog-to-digital converter (ADC) 63a, 63b, and 73, respectively. Pre-filter and pre-amplifiers 62a, 62b and 72 may provide anti-alias filtering and noise reduction prior to digitization.

[0083] ADC 63a, ADC 63b and ADC 73, respectively, convert the first cardiac electrical signal, second cardiac electrical signal and third cardiac electrical signal from an analog signal to a digital bit stream, which may be sampled at 128 or 256 Hz, as examples. ADC 63a, ADC 63b and ADC 73 may be sigma-delta converters (SDC), but other types ofADCs may be used. In some examples, the outputs of ADC 63a, ADC 63b and ADC 73 may be provided to decimators (not shown), which function as digital low-pass filters that increase the resolution and reduce the sampling rate of the respective cardiac electrical signals.

[0084] The digital outputs of ADC 63a, ADC 63b and ADC 73 are each passed to respective filters 64a, 64b and 74, which may be digital bandpass filters. The bandpass filters 64a, 64b and 74 may have the same or different bandpass frequencies. For example, filters 64a and 64b may have a bandpass of approximately 10 Hz to 50 Hz, or approximately 13 Hz to 39 Hz, for passing cardiac electrical signals such as R-waves typically occurring in this frequency range. Filter 74 of the morphology signal channel 87 may have a relatively wider bandpass of approximately 2.5 to 100 Hz. In some examples, each of sensing channel 83, sensing channel 85 and morphology signal channel 87 may further include a notch filter 67a, 67b, and 76, respectively, to filter 50 Hz and 60 Hz noise signals. Each notch filter 67a, 67b, and 76 may be individually turned on or off in some examples.

[0085] The narrow bandpass and notch-filtered signal (if notch filter is turned on) in first sensing channel 83 and second sensing channel 85 is passed from respective filter 64a or filter 64b (or 67a or 67b) to rectifier 65a or rectifier 65b to produce a filtered, rectified signal output to respective R-wave detector circuits 66a and 66b. First sensing channel 83 includes an R-wave detector circuit 66a for sensing ventricular event signals in response to the first cardiac electrical signal crossing an R-wave sensing threshold. Second sensing channel 85 includes an R-wave detector circuit 66b for sensing ventricular event signals in response to the second cardiac electrical signal crossing an R-wave sensing threshold, which may be controlled separately from the R-wave sensing threshold controlled by R- wave detector 66a, in some examples. R-wave detector circuits 66a and 66b may each include an auto-adjusting sense amplifier, comparator and / or other detection circuitry that compares the incoming filtered and rectified cardiac electrical signal to an R-wave sensing threshold and produces a Vsense signal 68a or 68b when the respective first or second cardiac electrical signal crosses the respective R-wave sensing threshold outside of a postsense (or post-pace) blanking interval.

[0086] The R -wave sensing threshold may be a multi-level sensing threshold, e.g., as generally disclosed in U.S. Pat. No. 10,252,071 (Cao, et al.), incorporated herein byreference in its entirety. Briefly, the multi-level sensing threshold may have a starting sensing threshold value (e.g., an amplitude in volts or millivolts) held for a first drop time interval, which may be equal to a tachycardia detection interval or an expected R-wave to T-wave interval, then drops to a second sensing threshold value held until a second drop time interval expires, which may be 0.6 to 2.5 seconds long in some examples, or 1 to 2.5 seconds long in other examples, and can be 2.15 seconds (from the Vsense signal) in one example. The R-wave sensing threshold may drop from the first sensing threshold value to the second sensing threshold value in a single step decrement in some examples. After the second drop time interval, the sensing threshold drops to a minimum sensing threshold, which may be equal to a programmed sensitivity or an increased sensitivity based on morphology analysis of an n-second cardiac signal segment (as described in the aboveincorporated PCT International Publication Number WO2023 / 159031(Heinks, et al). The increased sensitivity can be a sensitivity amplitude setting that is lower in amplitude (e.g., in millivolts) than the programmed sensitivity amplitude setting. The sensitivity can be referred to herein as the “sensing floor” because it represents the minimum amplitude of the cardiac electrical signal that may be sensed as a ventricular event signal, e.g., an R- wave or fibrillation wave. The R-wave sensing threshold may drop to the sensing floor e.g., to the programmed sensitivity or to an increased sensitivity (lower amplitude setting than the programmed sensitivity), in a single step decrement in some examples.

[0087] The R-wave sensing thresholds used by R-wave detector circuits 66a and 66b may each be set to a starting value based on a maximum peak amplitude of the respective first or second cardiac electrical signal determined by the R-wave detector circuits 66a or 66b during the most recent post-sense blanking interval. In some examples, an R-wave peak tracking period may be defined as a portion of the post-sense blanking period during which the maximum peak amplitude is determined. The starting R-wave sensing threshold of each sensing channel 83 and 85 may decrease over time according to one or more stepwise drops and / or linear or non-linear decay rates until reaching the minimum sensing threshold, e.g., equal to the sensitivity setting, or until an R-wave sensing threshold crossing by the cardiac electrical signal occurs. In some instances, the R-wave sensing threshold may be adjusted to the minimum sensing threshold before the expiration of the first drop time interval or before the expiration of the second drop time interval depending on the maximum peak amplitude determined during the R-wave peak tracking period.

[0088] The techniques described herein are not limited to a specific behavior of the sensing threshold or specific R-wave sensing techniques. Instead, other decaying, step- wise adjusted or other automatically adjusted sensing thresholds may be utilized for sensing ventricular event signals from the respective first and second cardiac electrical signals. R-wave detector circuits 66a or 66b may produce a Vsense signal 68a or 68b, respectively, in response to the respective first cardiac electrical signal or second cardiac electrical signal crossing the R-wave sensing threshold. The Vsense signal 68a or 68b is passed to control circuit 80.

[0089] The wideband-filtered, digital cardiac electrical signal 78 output from morphology signal channel 87 may be passed to control circuit 80 for performing morphology-based cardiac signal analyses. In some examples, the digital cardiac electrical signal 78 is passed to rectifier 75 and a rectified wideband filtered signal 79 is passed to control circuit 80 for processing and analysis. In some cases, both the filtered, non-rectified signal 78 and the rectified signal 79 are passed to control circuit 80 from morphology signal channel 87 for use in determining morphological features of the ECG signal.

[0090] The configuration of sensing channels 83 and 85 and morphology signal channel 87 as shown in FIG. 4 is illustrative in nature and should not be considered limiting of the techniques described herein. Sensing circuit 86 may include more or fewer components than illustrated and described in FIG. 4 and some components may be shared between sensing channels 83 and 85 and morphology signal channel 87. For example, a common cardiac electrical signal from a selected sensing electrode vector may be received by a prefilter and preamplifier circuit and ADC and subsequently be passed to a narrowband filter in one of sensing channels 83 or 85 and to a wideband filter in morphology signal channel 87. In other examples, sensing circuit 86 may include one or more sensing channels for producing Vsense signals in response to R-wave sensing threshold crossings and may include one or more morphology signal channels. In other examples, a wideband filtered morphology signal may be passed to control circuit 80 from one sensing channel configured to produce Vsense signals and pass a morphology signal to control circuit 80 for performing analysis of cardiac signal segments according to the techniques disclosed herein. Furthermore, the components for filtering, amplifying, digitizing, rectifying, etc. may be arranged in a different order or combination than shown in FIG. 4.

[0091] FIG. 5 is a conceptual diagram 100 of operating states of ICD 14 for performing tachyarrhythmia detection and controlling delivery of anti-tachy arrhythmia therapy according to some examples. At block 102, ICD 14 is operating in an unconcerned sensing state 1 (also referred to herein as “unconcerned state 1” or merely “state 1.” During the unconcerned sensing state 1, control circuit 80 receives Vsense signals from sensing circuit 86 and determines sensed RRIs, which may be used for updating the VTI and VFI counters. In some examples, Vsense signals are received from each of sensing channels 83 and 85. Each sensing channel 83 and 85 may be initially sensing ventricular event signals according to a programmed sensitivity for each respective sensing channel, which may be a user programmed or default sensitivity setting.

[0092] Sensed RRIs can be determined by control circuit 80 between successively received, in-channel Vsense signals for each sensing channel 83 and 85. Control circuit 80 may compare the sensed RRIs to a VT detection interval zone or threshold interval (when VT detection is enabled) and / or to a VF detection interval zone or threshold interval for identifying VTIs and / or VFIs. When a sensed RRI is determined to be a VTI or VFI, a respective VTI counter or VFI counter (and / or combined VT / VF interval counter) designated for counting VTIs and VFIs identified for the respective sensing channel 83 or 85 is incremented. Thus each sensing channel 83 and 85 (when two sensing channels are included) may be associated with a VTI counter, a VFI counter and / or a combined VT / VF interval counter. The VTI counter, VFI counter and a combined VT / VF interval counter, if used, are also referred to herein collectively as VTI / VFI counters. However, it is recognized that in some examples, VT detection may not be enabled in ICD 14 such that only VFI counters may be used for tracking VFIs for each sensing channel 83 and 85. Furthermore, it is to be understood that two sensing channels 83 and 85 are not required for performing the techniques disclosed herein. One sensing channel 83 or 85 in combination with morphology sensing channel may be included to provide control circuit 80 with Vsense signals indicating the timing of ventricular event signals sensed from a first cardiac electrical signal sensed by one sensing channel. Morphology signal channel 87 may provide a cardiac electrical signal that can be analyzed by control circuit 80 for confirming Vsense signals and determining a suspected heart rate in support of a VT / VF detection when an NID is met. FIG. 5 is described in conjunction with the examplesensing circuit 86 shown in FIG. 4 having two sensing channels 83 and 85 passing Vsense signals to control circuit 80 for the sake of illustration.

[0093] When a VTI or VFI counter for at least one sensing channel 83 or 85 reaches an NID required for detecting VT or VF, ICD 14 may transition (as indicated by arrow 103) to the concerned tachyarrhythmia detection state 2 of block 104. ICD 14 may transition to the concerned tachyarrhythmia suspected state 2, also referred to herein as “concerned state 2” or simply “state 2,” at block 104 in response to the NID being reached.

[0094] When two (or more) sensing channels are provided, control circuit 80 may identify one sensing channel 83 or 85 as the reliable sensing channel for VT / VF detection. For example, when the VTI / VFI counter for one sensing channel 83 or 85 reaches an NID, control circuit 80 may evaluate the relative timing of Vsense signals and / or morphology matching scores of sensed signal waveforms for selecting one sensing channel 83 or 85 as the reliable sensing channel for VT / VF detection. The sensing channel 83 or 85 that is selected as the reliable sensing channel can be required to have reached an NID (by an associated VTI / VFI counter) in order to transition from state 1 of block 102 to state 2 of block 104. If the sensing channel 83 or 85 that is identified as the reliable sensing channel for detecting VT / VF is not the sensing channel associated with a VTI / VFI counter that has reached an NID, control circuit 80 may remain in the unconcerned sensing state 1 of block 102. Control circuit 80 may perform methods during the unconcerned sensing state 1 for sensing ventricular event signals using two sensing channels 83 and 85, selecting a reliable sensing channel for VT / VF detection, and determining when criteria are met for transitioning to the concerned state 2 of block 104 as generally disclosed in U.S. Patent Application Publication No. 2023 / 0100431, filed August 29, 2022 (Liu, et al.). Briefly, control circuit 80 may select either sensing channel 83 or 85 when both sensing channels have reached an NID. When one sensing channel reaches the NID (candidate sensing channel) but the other has not (non-candidate sensing channel), control circuit 80 may evaluate if a high percentage of time-matched Vsense signals are received from both sensing channels 83 and 85. If so, and if the longest sensed RRI is relatively short, e.g., less than a threshold interval, the candidate sensing channel may be deemed reliable and selected as the sensing channel for VT / VF detection.

[0095] Control circuit 80 may additionally or alternatively determine if evidence of oversensing is detected from the candidate sensing channel 83 or 85 that has reached anNID and / or if evidence of undersensing is detected from the non-candidate sensing channel 83 or 85 that has not reached an NID. If a sensing channel has reached the NID without evidence of oversensing in the candidate sensing channel or with evidence of undersensing in the non-candidate sensing channel, the candidate sensing channel that has reached the NID may be selected as the sensing channel for reliable VT / VF detection.

[0096] Evidence of oversensing can be detected by performing noise detection algorithms, P-wave oversensing and / or T-wave oversensing detection algorithms. For instance, control circuit 80 may identify and count noisy cycles by counting signal pulses in a cardiac signal segment encompassing a Vsense signal or other noise detection methods. Control circuit 80 may compare peak amplitudes of sensed waveforms, determine amplitude ratios or other amplitude analysis for identifying evidence of noise contamination or amplitude variation, alternating sensed RRIs, or other alternating waveform morphology due to P- wave oversensing and / or T-wave oversensing causing false Vsense signals.

[0097] Evidence of undersensing in the sensing channel 83 or 85 that has not reached an NID (non-candidate sensing channel) when the other sensing channel (candidate sensing channel) has reached an NID can be detected by control circuit 80 by determining that peak amplitudes detected during the R-wave track and peak window are relatively low, e.g., near the sensitivity setting in effect. Low amplitude peaks of sensed waveforms may be an indication that some true R-waves or true fibrillation waves may have a peak amplitude that is less than the R-wave sensing threshold resulting in undersensing in the non-candidate sensing channel. Evidence of undersensing that may be detected by control circuit 80 may include relatively long sensed RRIs, which may correspond to the sum of two true RRIs with one or more undersensed R-waves or fibrillation waves occurring during the relatively long sensed RRI. Evidence of undersensing that control circuit 80 may detect may include a relatively high matched events ratio, e.g., all Vsense signals produced by the non-candidate sensing channel being matched in time to a Vsense signal produced by the candidate sensing channel. Control circuit 80 may select the candidate sensing channel as the reliable sensing channel for VT / VF detection when the non- candidate sensing channel is likely to be undersensing based on the detected undersensing evidence.

[0098] In some examples, the methods disclosed herein may be used for verifying that an NID reached by one (or both) sensing channels 83 or 85 is corroborated by a suspectedheart rate determined from confirmed Vsense signals as further described below. When the NID is met and a suspected heart rate determined according to the presently disclosed methods meets a heart rate threshold, control circuit 80 may transition to the concerned state 2 of block 104 in some examples.

[0099] In response to both sensing channels 83 and 85 reaching the NID, or one sensing channel 83 or 85 reaching the NID and being selected as the reliable sensing channel for VT / VF detection, control circuit 80 may transition to state 2 of block 104 as indicated by arrow 103. Control circuit 80 may use the Vsense signals from the selected sensing channel 83 or 85, identified as being the reliable sensing channel for VT / VF detection or a default sensing channel when both sensing channels 83 and 85 reach the NID, for determining sensed RRIs and triggering the storage of R-wave segments used for confirming Vsense signals as described below. Vsense signals may trigger storage of R- wave segments for verifying Vsense signals and determining a suspected heart rate before and / or after transitioning to state 2 of block 104. Other rate-based criteria may be applied to Vsense signals received from sensing circuit 86 for determining when first criteria for detecting VT / VF is met causing the transition to state 2 of block 104.

[0100] Furthermore, in some examples, control circuit 80 may rely on rate-based and / or morphology-based criteria for causing the transition from state 1 to state 2. When a VTI / VFI counter has started increasing but has not reached an NID, undersensing of R- waves or fibrillation waves may be preventing the NID from being reached. Control circuit 80 may be configured to analyze an n-second cardiac signal segment for detecting morphological evidence of VT / VF that does not rely on the timing of Vsense signals received from a sensing channel 83 or 85. If morphological evidence of VT / VF is detected, control circuit 80 may transition from the unconcerned state 1 to the concerned state 2 in some examples. As such, first criteria for detecting VT / VF may be satisfied based on sensed RRIs (which may be confirmed based on a suspected heart rate as described below) and / or based on a morphological analysis of sensed cardiac signals. Morphology metrics determined for detecting evidence of VT / VF in an n-second cardiac signal segment may include low slope content, spectral width, and mean period, as examples, but do not rely on sensed RRIs. Example methods for performing cardiac electrical signal morphology analysis that could be used for determining when first VT / VF detection criteria are met for causing the transition from state 1 to state 2 are generallydisclosed in U.S. Patent Application Publication No. 2023 / 0148939 (Aranda Hernandez et al., filed October 7, 2022).

[0101] Various criteria may be applied to sensed RRIs determined from Vsense signals received from a sensing channel 83 and / or 85 and / or a morphology signal received from morphology sensing channel 87 for transitioning from the unconcerned sensing state of block 102 to the concerned sensing state of block 104. It is to be understood that a transition from state 1 of block 102 to state 2 of block 104 may not be representative of a VT / VF detection that would cause ATP and / or CV / DF shock therapy to be scheduled or initiated. Rather, first criteria for detecting VT / VF may become met during the unconcerned sensing state 1 such that additional cardiac signal analysis is enabled and performed in the concerned state 2. The additional cardiac signal analysis performed in state 2 may require greater processing power and burden than the first VT / VF detection criteria applied in the unconcerned state 1. During the concerned tachyarrhythmia detection state 2 of block 104, control circuit 80 may suspect that a VT / VF episode may be occurring based on the NID being reached, as one example of first VT / VF detection criteria being met, to cause the transition to state 2 to occur. However, while operating in the concerned state 2, control circuit 80 may perform additional cardiac signal analysis, e.g., using the morphology signal from morphology sensing channel 87, to determine if additional criteria for detecting VT / VF are met.

[0102] For example, control circuit 80 may determine that VT / VF detection criteria are met when none of one or more VT / VF rejection rules are met. Control circuit 80 may apply one or more VT / VF rejection rules that, when satisfied, prevent a VT / VF detection from being made and prevent a transition from state 2 to charging state 3 of block 106. One or more VT / VF rejection rules, also referred to herein as “rejection rules,” may be applied to the Vsense signals and / or the morphology signal received from morphology sensing channel 87 relating to detecting evidence of sinus tachycardia (ST), SVT, EMI, skeletal muscle noise (which can also be referred to as “myopotential noise”), P-wave oversensing (PWOS) and / or T-wave oversensing (TWOS), as examples. If no VT / VF rejection rules are met and the NID is met for detecting VT / VF with a suspected heart rate meeting a threshold heart rate as disclosed herein, control circuit 80 may transition to the charging state 3 of block 106. Examples of VT / VF rejection rules are listed above and described in the above-incorporated references.

[0103] If a VT / VF rejection rule is determined to be met during the concerned state 2 of block 104, ICD 14 may withhold detection of the suspected VT / VF episode and remain in the concerned state 2 of block 104. If the NID is met and no VT / VF rejection rules are met based on the analysis of cardiac electrical signals sensed by sensing circuit 86, control circuit 80 may detect VT / VF. As disclosed herein, however, control circuit 80 may make a final check to confirm the Vsense signals leading up to the NID being reached by analyzing R-wave segments obtained from the morphology signal and determining a suspected heart rate. If the suspected heart rate does not meet a heart rate threshold, the sensed heart rate (based on Vsense signals and sensed RRIs) is not confirmed. Control circuit 80 may remain in the concerned state 2 of block 104 as indicated by arrow 105.

[0104] If the suspected heart rate meets the heart rate threshold, control circuit 80 may confirm the sensed heart rate and the NID being reached as valid evidence for detecting VT / VF. Control circuit 80 may subsequently transition to the charging state 3 of block 106 when VT / VF is detected (as indicated by arrow 107). During charging state 3 of block 106, therapy delivery circuit 84 may begin charging a high voltage capacitor, under the control of control circuit 80, for preparing to deliver a CV / DF shock.

[0105] In some instances, however, while operating in state 2 of block 104, control circuit 80 may be analyzing sensed RRIs and / or cardiac electrical signals for detecting termination of the fast heart rate. Control circuit 80 may determine that termination criteria are met based on analysis of the cardiac electrical signals sensed by sensing circuit 86 before all VT / VF detection criteria are satisfied for making a VT / VF detection. Control circuit 80 may transition back to the unconcerned state 1 as indicated by arrow 101 in response to determining that termination criteria are met. Control circuit 80 may determine that termination criteria are met during state 2 (block 104) when a median sensed RRI or other representative sensed RRI determined from Vsense signals received from sensing circuit 86 after an NID is reached is greater than a slow threshold interval. The median sensed RRI may be determined from the most recent 3 to 30 sensed RRIs or the most recent 12 sensed RRIs for example. In other examples, a mean, trimmed mean, mode or other representative sensed RRI may be determined instead of the median sensed RRI and, if greater than a slow threshold interval, control circuit 80 may transition back to state 1 (block 102). The slow threshold interval may be the VTI, the VTI plus an offset, the VFI, the VFI plus an offset, or a percentage or offset greater than the rhythm cycle length(RCL) determined from sensed RRIs associated with the NID being reached in various examples.

[0106] For instance, the RCL may be determined as the mean or median of a specified number of most recent sensed RRIs counted as VT / VF intervals, e.g., just prior to the NID being reached. The RCL may be computed by control circuit 80 as a trimmed mean, e.g., by determining the mean sensed RRI after dropping the shortest and / or longest sensed RRI of the most recent N sensed RRIs up to the NID being met. In an example, if eight sensed RRIs determined after the NID is reached are longer than the RCL plus an offset (e.g., plus 40, 50, 60, 70, or 80 ms), control circuit 80 may detect termination and transition back to the unconcerned state 1 (block 102). Termination criteria may be met during the concerned sensing state 2 of block 104 when a threshold number (e.g., 6, 8, 10, 12 or other specified number or percentage) of sensed RRIs are greater than the slow threshold interval. Upon returning to the unconcerned sensing state 1, control circuit 80 may reset the VTI / VFI counters to zero when termination criteria are met. A variety of methods may be implemented for determining that a suspected or detected VT / VF episode has terminated prior to therapy being delivered. In some examples, techniques disclosed herein may be used for confirming Vsense signals and for determining when a suspected heart rate determined based on confirmed Vsense signals meet termination criteria for controlling a transition from state 2 back to state 1. Termination criteria may be met when a suspected heart rate is less than a slow rate threshold in some examples. Termination criteria may be satisfied by the suspected heart rate being less than a slow rate threshold alone or in combination with other criteria applied to sensed RRIs and or morphology signal analysis. For example, a representative suspected RRI determined from confirmed Vsense signals (as further described below) may be compared to the median sensed RRI for confirming the median sensed RRI prior to detecting termination based on the RCL.

[0107] In other instances, control circuit 80 may transition back to the unconcerned state 1 by detecting one or more normal sinus rhythm (NSR) beats based on a sensed RRI being greater than an NSR threshold interval and an R-wave morphology matching score associated with the sensed RRI being relatively high (e.g., greater than an NSR match threshold). When a threshold number of NSR beats are detected, control circuit 80 may reset the VTI / VFI counters to zero or to a non-zero value but less value than the current VTI / VFI counter value. Because the NID is no longer met by the reset VTI / VFI counter,control circuit 80 may transition from the concerned state 2 of block 104 back to the unconcerned state 1 of block 102. In another example, control circuit 80 may reset a VTI / VFI counter to a lower value in response to a suspected heart rate being less than a slow heart threshold or less than a heart rate threshold set based on sensed RRIs.

[0108] During the charging state 3 of block 106, after a VT / VF detection is made, control circuit 80 may determine that abort therapy criteria are met based on an analysis of the cardiac electrical signals sensed by sensing circuit 86. Control circuit 80 may abort a CV / DF shock prior to delivery when 60%, 70%, 80% or other threshold number of sensed RRIs are longer than a slow interval threshold. For instance, if 4 out of the most recent 5 sensed RRIs are at least 60 ms (or other offset) longer than the RCL, control circuit 80 may abort a CV / DF shock therapy. In another example, abort therapy criteria may be met when at least 4 out of 5 most recent sensed RRIs are equal to or greater than the VF detection interval threshold plus an offset (e.g., 60 ms). In some examples, when VT is detected, the slow interval threshold is the RCL plus 60 ms. When the detected tachyarrhythmia is a fast VT or VF, the slow interval threshold may be the VF detection interval threshold plus 60 ms or the RCL plus 60 ms, whichever is greater. However, in some examples, when the difference between the minimum sensed RRI and the maximum sensed RRI used to compute the RCL is more than a threshold difference (e.g., more than 50 ms difference), the slow interval threshold may be determined as the VF detection interval threshold plus an offset, e.g., 60 ms.

[0109] In still other examples, control circuit 80 may determine suspected RRIs and / or a suspected heart rate according to the techniques described below and apply abort therapy criteria to the suspected RRIs and / or suspected heart rate. The abort therapy criteria may be met when a threshold number of suspected RRIs are less than a slow interval threshold and / or a suspected heart rate is less than a slow heart rate threshold. Control circuit 80 may apply abort therapy criteria to suspected RRIs and / or sensed RRIs in various examples. It is recognized that a variety of abort therapy criteria may be defined and applied to sensed RRIs and / or suspected RRIs by control circuit 80 up until CV / DF shock delivery for making a determination that the rhythm is a slowing rhythm, justifying aborting the CV / DF shock.

[0110] When abort therapy criteria are met during charging state 3 of block 106, control circuit 80 may terminate charging of the high voltage capacitor by therapy delivery circuit84 (if charging is not already complete), cancel the pending CV / DF shock and advance to the redetection state 5 of block 110. Operations during the redetection state 5 are further described below.

[0111] In some examples, therapy delivery circuit 84 may be configured to deliver one or more sequences of ATP therapy during high voltage capacitor charging of charging state 3 (block 106). Control circuit 80 may determine that abort therapy criteria are met during capacitor charging after ATP has been delivered and advance to the redetection state 5 of block 110. In other examples, control circuit 80 may determine that termination criteria are met (according to any of the examples described above) during or upon completion of capacitor charging. While not explicitly shown in FIG. 5, control circuit 80 may transition back to the unconcerned sensing state 1 of block 102 in response to termination criteria being met during charging state 3 in some examples. The detected VT / VF may spontaneously terminate or a delivered ATP therapy may terminate the VT / VF episode without having to deliver a CV / DF shock.

[0112] When capacitor charging is completed for delivering a CV / DF shock during charging state 3 (block 106) before abort therapy criteria are met (or termination criteria are met), control circuit 80 may transition to the shock delivery state 4 of block 108. Therapy delivery circuit 84 may deliver a CV / DF shock after the transition to state 4. Therapy delivery circuit 84 may be controlled by control circuit 80 to synchronize the CV / DF shock to the patient’s intrinsic heart rhythm, e.g., based on the timing of received Vsense signals. A CV / DF shock may be synchronized to a Vsense signal received outside a refractory period. The CV / DF shock may be delivered upon expiration of a specified time interval after charge completion. In some cases, abort therapy criteria could be met after capacitor charging is completed but before the CV / DF shock is delivered, e.g., while waiting for a synchronizing event during the shock delivery state 4.

[0113] Control circuit 80 may determine that abort therapy criteria are met after capacitor charging is completed when a specified number of Vsense signals are refractory events (e.g., when three or another specified number of consecutive Vsense signals are received during a post-sense ventricular refractory period). Control circuit 80 may determine that abort therapy criteria are met after capacitor charging when a single sensed RRI is greater than the slow interval threshold or when any of the other examples of abort therapy criteria described above are met. It is to be understood that any time after capacitor charging hasstarted, when abort therapy criteria are met before the CV / DF shock is delivered, control circuit 80 may transition to redetection state 5 of block 110 without delivering the CV / DF shock. In other examples, the CV / DF shock can be delivered during state 4 without necessarily being synchronized to the patient’s intrinsic heart rhythm.

[0114] After shock delivery (or when abort therapy criteria are met before shock delivery), control circuit 80 may transition to redetection state 5 at block 110. VT / VF may be redetected by control circuit 80 when a reduced NID is met following shock delivery or after a CV / DF shock is aborted due to the abort therapy criteria being met. Control circuit 80 may determine that the reduced NID is met based on Vsense signals received from the sensing channel 83 or 85 that was selected as the reliable sensing channel for tachyarrhythmia detection when control circuit 80 transitioned to the concerned state 2. In some examples, control circuit 80 may redetect VT / VF when the reduced NID is met and any VT / VF rejection rules applied during redetection state 5 are not met. The redetection NID may be, for example, 25%, 30%, 50%, 60%, or 70% of the NID required to transition from the unconcerned state 1 to the concerned state 2. If VT / VF redetection criteria are met in state 5, control circuit 80 may return to the charging state 3 of block 106.

[0115] In some examples, when the reduced, redetection NID is met, control circuit 80 may confirm Vsense signals and / or determine a suspected heart rate according to the techniques described below for supporting a redetection based on the reduced NID. If a threshold number of Vsense signals leading up to the redetection NID are not confirmed, a threshold number of suspected RRIs determined from confirmed Vsense signals are longer than VT / VF detection intervals and / or sensed RRIs, and / or a suspected heart rate does not meet a heart rate threshold, control circuit 80 may remain in the redetection state 5. Control circuit 80 may transition to the charging state 3 when the redetection NID is met and a suspected heart rate determined from confirmed Vsense signal meets a heart rate threshold in some examples.

[0116] A CV / DF shock can be delivered (one or more times) by therapy delivery circuit 84 in shock delivery state 4 to terminate the VT / VF. Control circuit 80 may be configured to apply redetection criteria and / or termination criteria to cardiac electrical signals sensed by sensing circuit 86 after each CV / DF shock (during redetection state 5) for determining if the detected VT / VF has been terminated or an additional shock is needed based on redetection criteria being met. If termination criteria are met during the redetection state 5before redetection criteria are met, control circuit 80 may transition back to the unconcerned sensing state 1 of block 102. The VTI / VFI counters may be reset to zero upon transitioning to the unconcerned sensing state 1.

[0117] When control circuit 80 transitions back to the unconcerned sensing state 1 of block 102, e.g., from the concerned state 2 of block 104 or from the redetection state 5 of block 110 when termination criteria are met, control circuit 80 may restore the sensitivity of the sensing channel(s) 83 and 85 to the programmed sensitivity (if the sensitivity had previously been increased by setting a lower voltage amplitude used as the R-wave sensing floor in response to an n-second cardiac signal segment being classified as VT / VF based on morphology analysis as generally disclosed in the above-incorporated PCT International Publication Number WO2023 / 159031). When the sensitivity of a sensing channel 83 or 85 has been increased in the unconcerned state 1 of block 102 and criteria are subsequently met for transitioning to the concerned state 2 of block 104 with the increased sensitivity in effect, the increased sensitivity may remain in effect during all subsequently reached tachyarrhythmia operating states of blocks 104, 106, 108 and 110 (when a shock is delivered during the shock delivery state of block 108, sensing circuit 86 may be blanked or disabled). The increased sensitivity of sensing circuit 86 may be applied for sensing ventricular event signals for determining when termination criteria are met (before and / or after shock delivery), determining abort therapy criteria are met, synchronizing a VT / VF shock and determining when redetection criteria are met after shock delivery, for example. Each time ICD 14 enters the unconcerned sensing state 1 of block 102, the sensitivity for sensing channels 83 and 85 may be restored to the programmed sensitivity, e.g., to the user programmed setting or a default sensitivity setting.

[0118] Each time ICD 14 enters the unconcerned sensing state 1, a suspected RRI buffer in memory 82 may optionally be cleared. As described below, control circuit 80 may store suspected RRIs determined from confirmed Vsense signals in memory 82 for use in determining a suspected heart rate. During the unconcerned sensing state 1, control circuit 80 may disable morphology sensing channel 87 or disable at least some aspects of morphology signal analysis performed for confirming Vsense signals according to the techniques disclosed herein. In response to a VTI / VFI counter beginning to increase in the unconcerned sensing state 1, the morphology sensing channel 87 may be enabled,buffering of R-wave segments from the morphology signal in memory 82 may be enabled, and / or analysis of buffered R-wave segments for verifying Vsense signals may be started by control circuit 80. For instance, when a VTI / VFI counter reaches a threshold value that is less than the NID, e.g., a threshold value of 2, 3 5, 8 or other specified value, control circuit 80 may initiate the R-wave segment analysis performed for confirming Vsense signals and determining a suspected heart rate according to the techniques disclosed herein. In this way, when the NID is reached by one or both of sensing channels 83 and / or 85, suspected RRIs may be available in memory 82 for determining the suspected heart rate for corroborating the NID being met without delay.

[0119] While the methods disclosed herein are described in conjunction with an ICD 14 capable of delivering an anti-tachyarrhythmia therapy, it is contemplated that the heart rate confirmation methods disclosed herein may be included in a medical device, implanted or external, configured to monitor a patient’s heart rhythm for accumulating rhythm data, e.g., number of tachyarrhythmia episodes, duration of tachyarrhythmia episodes, tachyarrhythmia burden etc., without necessarily initiating or delivering a therapy in response to a VT / VF detection. For example, the methods disclosed herein may be implemented in a cardiac monitoring device capable of processing, analyzing, and storing data that can be transmitted, e.g., by a telemetry circuit, to an external device, e.g., external device 40 shown in FIG. 1A. For instance, in response to a VT / VF detection, control circuit 80 may control telemetry circuit 88 to transmit a signal to alert the patient, clinician or other caregiver of a detected VT / VF episode. Transmitted data may be used to generate a display of data related to detected VT / VF episodes, e.g., in a graphical user interface, by display unit 54. In some examples, if a VT / VF detection is withheld or delayed due to a suspected heart rate not meeting a heart rate threshold, data relating to confirmed and / or unconfirmed Vsense signals, suspected RRIs, sensed RRIs, and a suspected heart rate determined from confirmed Vsense signals may be stored in memory and reported via transmission to external device 40.

[0120] Using the techniques disclosed herein, accuracy of VT / VF episode data obtained by a cardiac monitoring device can be improved to avoid including false VT / VF episode detections due oversensing, undersensing or other sensing issues and provides improvements in ambulatory monitoring of a patient for collecting vital heart rhythm data that cannot always be captured during an office visit, consultation with a clinician, orrelative short-term monitoring of a patient over a few hours, days or weeks. The techniques of the present disclosure further improve heart rhythm monitoring techniques by reducing human error and burden placed on a clinician or other expert in reviewing collected cardiac signals or data derived therefrom to distinguish and discriminate between episodes of supraventricular rhythms, oversensing, undersensing, true VT / VF, and / or other heart rhythms, which can be impractical or unfeasible based on human visual inspection of recorded cardiac signals.

[0121] FIG. 6 is a flow chart 150 of a method that may be performed by a medical device for confirming a sensed heart rate for use in detecting a heart rhythm according to some examples. With continued reference to FIG. 3, at block 152, control circuit 80 receives a Vsense signal from a sensing channel 83 or 85. At block 154, control circuit 80 may determine a sensed RRI from the currently received Vsense signal to a most-recent preceding Vsense signal received from the same sensing channel 83 or 85 (or a delivered ventricular pacing pulse). In some instances, control circuit 80 may be receiving Vsense signals from both sensing channels 83 and 85. In this case, control circuit 80 may be determining sensed RRIs for both sensing channels and buffering the sensed RRIs in memory 82 for each corresponding sensing channel. In other instances, control circuit 80 may be receiving Vsense signals from a single sensing channel, e.g., when only one sensing channel is provided or when a single sensing channel 83 or 85 is selected as a reliable sensing channel for detecting VT / VF rhythms after transitioning to the concerned state 2 of FIG. 5, for instance, as generally disclosed in the above-incorporated U.S. Patent Application Publication No. 2023 / 0100431, filed August 29, 2022 (Liu, et al.).

[0122] At block 156, control circuit 80 may buffer a suspected R-wave segment in response to the Vsense signal received at block 152. The suspected R-wave segment, also referred to herein as an “R-wave segment,” may be buffered from the morphology signal received from morphology signal channel 87 over a specified time interval relative to the triggering Vsense signal received at block 152. The suspected R-wave segment may be 100 to 250 ms in duration and is 150 to 160 ms in duration in some examples. The suspected R-wave segment may extend before and after the time of the triggering Vsense signal such that, assuming the Vsense signal is due to a true R-wave crossing the R-wave sensing threshold applied by the given sensing channel 83 or 85, the R-wave is encompassed by the suspected R-wave segment. The suspected R-wave segment mayencompass exactly one R-wave when the Vsense signal is caused by a true R-wave. When the Vsense signal is a falsely sensed R-wave, e.g., corresponding to a noise signal pulse or other cardiac event signal such as a P-wave or a T-wave, the waveform that crossed the suspected R-wave segment is encompassed by the suspected R-wave segment. In an example, the suspected R-wave segment is 152 milliseconds (ms) long including 19 sample points prior to the Vsense signal and 19 sample points after the Vsense signal when the sampling rate is 256 Hz.

[0123] At block 158, control circuit 80 determines at least one suspected R-wave segment feature for comparison to R-wave confirmation criteria, e.g., to an R-wave threshold or range, for confirming that the suspected R-wave segment includes a true R-wave, and thereby confirming the associated, triggering Vsense signal. As described below, control circuit 80 may determine the maximum absolute amplitude of the suspected R-wave segment and compare the maximum absolute amplitude to an amplitude threshold for confirming a true R-wave. The amplitude threshold may be different than the R-wave sensing threshold applied by sensing channel 83 or 85. The amplitude threshold may be based on, e.g., a percentage of, the maximum absolute amplitude of one or more most recently confirmed R-wave segments.

[0124] In other examples, one or more R-wave segment features may be determined by control circuit 80 and may include one or more of the maximum absolute amplitude, a suspected R-wave width, a suspected R-wave area, a maximum slope, a minimum slope, a peak-to-peak amplitude, a difference between the maximum and minimum slopes, and / or a morphology matching score determined by performing a wavelet transform or other waveform morphology matching analysis.

[0125] When the R-wave segment is not confirmed as being a likely R-wave based on the comparison of the R-wave segment feature(s) to a respective threshold or range, the associated, triggering Vsense signal is not confirmed. Control circuit 80 may return to block 152 (“no” branch) to wait for the next Vsense signal. In response to confirming the R-wave segment at block 158, and thereby confirming the associated, triggering Vsense signal, control circuit 80 may determine a suspected RRI at block 160. The suspected RRI may be determined as the time interval extending from the current confirmed Vsense signal to the most recent preceding confirmed Vsense signal. In some instances, one or more unconfirmed Vsense signals, each associated with an R-wave segment that does notmeet the R-wave confirmation criteria, may occur between two consecutive confirmed Vsense signals. The suspected RRI may be determined by control circuit 80 at block 160 as the time interval extending from a fiducial point of the current confirmed R-wave segment (e.g., the time of the maximum absolute peak amplitude or other feature determined from the current confirmed R-wave segment) to the analogous fiducial point of the most recent preceding confirmed R-wave segment. Other examples of methods for determining the suspected RRI at block 160 are described below in conjunction with FIG. 9.

[0126] At block 161, control circuit 80 may determine a suspected heart rate based on the suspected RRIs. At block 162, control circuit 80 may compare the suspected heart rate to a heart rate threshold. Control circuit 80 may determine the heart rate threshold based on the sensed RRIs, e.g., as a multiple of a representative sensed RRI. Control circuit 80 may determine the heart rate threshold based on a programmed VT detection interval or VF detection interval in other examples. For instance the heart rate threshold may be a rate interval threshold that is a multiple or offset greater than one of the programmed VT detection interval or VF detection interval. In still other examples, control circuit 80 may determine the heart rate threshold based on a combination of the programmed VT detection interval or VF detection interval and a representative sensed RRI. For example, the heart rate threshold may be determined as the greatest one of a multiple of a representative sensed RRI, e.g., 1.3 times the median sensed RRI, or the VT detection interval or the VF detection interval, whichever is greatest. It is recognized that numerous methods may be conceived for determining a heart rate threshold that is applied by control circuit 80 to a suspected heart rate determined by control circuit 80 from confirmed R- wave segments and corresponding confirmed Vsense signals for verifying or confirming a heart rate, e.g., a tachyarrhythmia rate, of Vsense signals.

[0127] In this way, control circuit 80 may use the suspected RRIs to confirm a heart rate corresponding to the sensed RRIs determined from Vsense signals received from a sensing channel 83 or 85. As further described below, control circuit 80 may buffer RRIs in memory 82 as they are determined from Vsense signals received from sensing channel 83 or 85. Control circuit 80 may buffer suspected RRIs in memory 82 as they are determined from confirmed R-wave segments. The sensed RRIs determined from Vsense signals and the suspected RRIs determined from confirmed R-wave segments (and / or confirmedVsense signals) may each be buffered in a first-in-first-out (FIFO) buffer, for example, storing the most recent 6 to 30 sensed RRIs and suspected RRIs, respectively. Control circuit 80 may determine a median sensed RRI or other representative value of the sensed RRIs (e.g., a mean, nth shortest, trimmed mean or trimmed median, etc.). Control circuit 80 may determine a median suspected RRI or other representative value of the suspected RRIs (e.g., a mean, nth shortest, trimmed mean or trimmed median, etc.). Control circuit 80 may compare the median suspected RRI to the median sensed RRI at block 162 for confirming the sensed heart rate. For example, when the median of the suspected RRIs (determined from confirmed R-wave segments) is not more than a specified multiple greater than the median of the sensed RRIs (determined from confirmed and unconfirmed Vsense signals), the sensed heart rate may be confirmed at block 162.

[0128] In an illustrative example, when the median of 12 (or other specified number) of most recently determined suspected RRIs is not more than 20%, 25%, 30%, or 35% greater than the median of 12 (or other specified number) of most recent sensed RRIs, the sensed heart rate (based on Vsense signals) may be confirmed at block 162. As described below, an NID reached by a sensing circuit 83 or 85 indicating a sensed heart rate in a VT or VF rate zone may be confirmed based on the suspected heart rate meeting a heart rate threshold set based on the sensed RRIs. Here the sensed heart rate and the suspected heart rate may be represented by a representative suspected RRI and a representative sensed RRI, respectively. Alternatively, control circuit 80 may convert the median suspected RRI to a suspected heart rate (e.g., in beats per minute) and convert the median sensed RRI to a sensed heart rate (in beats per minute). Control circuit 80 may determine that the sensed heart rate is confirmed at block 162 when the suspected heart rate is at least a specified percentage of the sensed heart rate. For instance, when the suspected heart rate is at least 70%, 75%, 80%, 85% or 90% of the sensed heart rate, control circuit 80 may confirm the sensed heart rate.

[0129] When the sensed heart rate is confirmed (“yes” branch of block 162), control circuit 80 may perform a response to the confirmed sensed heart rate. For example, control circuit 80 may use sensed heart rate and / or corresponding Vsense signals (and sensed RRIs) for detecting the heart rhythm at block 164. As described below, the sensed RRIs may be relied upon by control circuit 80 for detecting VT / VF when an NID is reached by at least one sensing channel 83 or 85. When the rate of confirmed Vsense signals does notmeet a threshold heart rate at block 162, however, control circuit 80 may return to block 152 to wait for the next Vsense signal without detecting a heart rhythm based on the Vsense signals and sensed RRIs.

[0130] In the example shown, in some instances, control circuit 80 may withhold a response to the sensed heart rate at block 166 when the sensed heart rate is not confirmed based on the suspected heart rate not meeting a threshold heart rate. For instance, if an NID has been reached while control circuit 80 is operating in the unconcerned sensing state 1 (described above in conjunction with FIG. 5), control circuit 80 may not transition to the concerned state 2 if the sensed heart rate is not confirmed based the suspected heart rate not meeting a heart rate threshold at block 162. In other instances, if control circuit 80 is operating in the concerned state 2 (see FIG. 5), control circuit 80 may not transition to the charging state 3 if the sensed heart rate is not confirmed based the suspected heart rate not meeting a heart rate threshold at block 162.

[0131] Additionally or alternatively, control circuit 80 may perform a response to the suspected heart rate not meeting a heart rate threshold for confirming the sensed heart rate at block 166. For example, if the suspected heart rate is slower than the heart rate threshold (or a median suspected RRI is longer than a threshold RRI), control circuit 80 may perform a response to the suspected heart rate, which may represent a slowing heart rate after a threshold number of VT / VF intervals have been counted from sensed RRIs. As described above in conjunction with FIG. 5, example responses by control circuit 80 to a suspected heart rate being slower than a heart rate threshold (e.g., not confirming a sensed heart rate) may include determining that termination criteria are met, determining that abort therapy criteria are met or determining that VTI / VFI counter reset criteria are met. The response performed by control circuit 80 when the suspected heart rate does not meet a heart rate threshold for confirming the sensed heart rate may depend on which operating state 1, 2, 3, or 5 control circuit 80 is operating in as described above in conjunction with FIG. 5. A response to the suspected heart rate may include a state transition in some instances, e.g., if the suspected heart rate is less than a slow threshold rate satisfying termination or abort therapy criteria. A response to the suspected heart rate not meeting a heart rate threshold may include decreasing a value of a VTVVFI counter. In other examples, the response to the suspected heart rate not meeting the heart rate threshold to confirm the sensed rate may be only to withhold a response to the NID being reached (e.g.,withhold a transition to concerned state 2 or withhold a VT / VF detection and a transition to charging state 3).

[0132] FIG. 7 is a flow chart 200 of a method that may be performed by a medical device for detecting VT / VF according to some examples. At block 202, control circuit 80 may determine that a VTI / VFI counter has reached a first threshold, e.g., a value of 1, 2, 3, 5, 8 or other threshold less than the NID. Control circuit 80 may determine when a VTI / VFI counter has begun increasing due to sensed RRIs determined from V-sense signals being VTI / VFIs. When a VTI / VFI counter reaches a first threshold value, control circuit 80 may begin buffering suspected R-wave segments in response to each Vsense signal (e.g., from a selected one or both of sensing channels 83 and 85).

[0133] At block 204, control circuit 80 stores in memory 82 R-wave segments in response to each Vsense signal consecutively received from a given sensing channel 83 or 85. As described above, a Vsense signal received by control circuit 80 may trigger storage of the R-wave segment. The R-wave segment may be stored from the morphology signal. The morphology signal sample points may be buffered in memory 82 until a Vsense signal is received. A specified number of buffered sample points preceding the triggering Vsense signal and following the triggering Vsense signal may be stored in memory 82 so that the stored R-wave segment includes a time interval extending earlier and later than the triggering Vsense signal.

[0134] At block 206, control circuit 80 may update suspected RRIs stored in memory 82 as R-wave segments are received and confirmed according to the method of FIG. 6. Control circuit 80 may identify confirmed and unconfirmed R-wave segments from the buffered suspected R-wave segments by determining at least one feature of the respective suspected R-wave segment and comparing the feature to R-wave confirmation criteria, e.g., to an R-wave threshold or range, as generally described above in conjunction with FIG. 6. Control circuit 80 may determine a suspected RRI using the timing of a feature of a confirmed R-wave segment or the associated confirmed Vsense signal and the timing of a feature of a most recently preceding confirmed R-wave segment or associated confirmed Vsense signal. Methods for determining a suspected RRI are described below in conjunction with FIG. 8 and FIG. 9. Control circuit 80 may buffer suspected RRIs in memory 82 as they are determined at block 206 in response to each confirmed R-wave segment and associated, triggering Vsense signal.

[0135] At block 208, control circuit 80 may determine when an NID is reached by a VTI / VFI counter of at least one of sensing channel 83 or sensing channel 85. In response to at least one sensing channel 83 or 85 reaching an NID based on sensed RRIs determined from consecutively received, in-channel Vsense signals, control circuit 80 may determine a representative sensed RRI and a representative suspected RRI at block 210. Control circuit 80 may determine a median sensed RRI and a median suspected RRI as generally described above in conjunction with FIG. 6.

[0136] At block 212, control circuit 80 may determine if the median suspected RRI is less than a specified multiple, e.g., 120%, 130%, 140% or other selected multiple, of the median sensed RRI. In other words, control circuit 80 may determine if the median suspected RRI is not more than a threshold percentage of the median sensed RRI. Alternatively, control circuit 80 may convert the representative RRIs to respective heart rates and determine if the suspected heart rate corresponding to the median suspected RRI is at least a threshold percentage, e.g., 75%, of the sensed heart rate corresponding to the median sensed RRI. The sensed heart rate may be confirmed by control circuit 80 when the suspected heart rate is at least a specified percentage (e.g., 75%) of the sensed heart rate or meets another specified heart rate threshold.

[0137] If the suspected heart rate is not at least a specified percentage of the sensed heart rate, control circuit 80 may not detect VT / VF based on the NID being reached. Control circuit 80 may advance to block 215 to wait for the next Vsense signal. For example, with reference to FIG. 5, control circuit 80 may not transition from unconcerned sensing state 1 to concerned sensing state 2 or from concerned sensing state 2 to charging state 3 to initiate an anti-tachyarrhythmia therapy when the sensed heart rate is not confirmed by the suspected heart rate meeting a heart rate threshold. Control circuit 80 may not perform the analyses required for determining if VT / VF rejection rules are met when the sensed heart rate is not confirmed by the suspected heart rate. If the NID is still met (block 205) after receiving the next Vsense signal (block 215), control circuit 80 may buffer the next suspected R-wave segment at block 204. If the NID is not met at block 205 after the next Vsense signal is received at block 215, control circuit 80 may return to block 202 and continue the process of flow chart 202, e.g., as long as a VTI / VFI counter is at least a first threshold value (block 202).

[0138] If the sensed heart rate is confirmed based on the suspected heart rate at block 212 (“yes” branch), control circuit 80 may advance to block 214 to confirm that all VT / VF detection criteria are met. For example, control circuit 80 may verify that no VT / VF rejection rules are met. If all VT / VF detection criteria are met at block 214 when the NID is met and the suspected heart rate meets a heart rate threshold (e.g., a threshold percentage of the sensed heart rate), control circuit 80 may detect VT / VF at block 216. Control circuit 80 may control therapy delivery circuit 84 to initiate an antitachyarrhythmia therapy, e.g., by starting ATP and / or initiating charging of high voltage capacitor(s) for generating a CV / DF shock therapy. If any VT / VF detection criteria are not met at block 214 (e.g., if at least one VT / VF rejection rule is met when the NID is met) and the suspected heart rate meets the heart rate threshold for confirming the sensed heart rate, control circuit 80 may withhold a VT / VF detection and advance to block 215 to wait for the next Vsense signal and repeat the process of flow chart 200.

[0139] FIG. 8 is a flow chart 250 of a method that may be performed by a medical device for determining a suspected RRI according to some examples. At block 252, an R-wave confirmation threshold may be determined by control circuit 80. Control circuit 80 may initially set the R-wave confirmation threshold to a default value. The starting default value may be 0.01 to 1.0 millivolts as examples. The default value may be set in ADC units, e.g., 1 or 2 ADC units, as examples, which may correspond to approximately 0.01 to 0.1 millivolts depending on the dynamic input range of the ADC. It is to be understood that when control circuit 80 determines more than one feature of the suspected R-wave segments for confirming a Vsense signal, more than one R-wave confirmation thresholds or ranges may be set at block 252, each corresponding to a respective R-wave segment feature.

[0140] At block 254, control circuit 80 receives a Vsense signal from sensing circuit 86. The Vsense signal may be received from either sensing channel 83 or 85, e.g., when control circuit 80 is operating in the unconcerned sensing state 1 of FIG. 5. After control circuit 80 transitions to the concerned sensing state 2 of FIG. 5, control circuit 80 may receive the Vsense signal from a selected one of sensing channels 83 or 85 determined to be a reliable sensing channel for VT / VF detection, e.g., according to the methods generally disclosed in the above-incorporated U.S. Patent Application Publication No. 2023 / 0100431 (Liu, et al.).

[0141] At block 256, control circuit 80 buffers an R-wave segment in memory 82 to extend earlier and later than the time of the triggering Vsense signal to capture the signal waveform that caused the sensing channel 83 or 85 to produce a Vsense signal. When control circuit 80 is receiving Vsense signals from both sensing channels 83 and 85, if both Vsense signals are received within a threshold time of each other, e.g., within 25 to 75 ms of each other, control circuit 80 may buffer a single R-wave segment at block 256 for use in confirming the time-matched Vsense signals from both sensing channels 83 and 85 as confirmed Vsense signals. In other examples, an R-wave segment may be buffered from the morphology signal received from morphology signal channel 87 in response to each Vsense signal received from sensing channels 83 and 85.

[0142] At block 258, control circuit 80 determines one or more features of the suspected R-wave segment. For the sake of illustration, control circuit 80 may determine at least a maximum absolute amplitude of the R-wave segment. Any of the example features listed herein may be determined, alone or in any combination in other examples.

[0143] Control circuit 80 may determine a suspected R-wave time at block 260, which may be the time of the determined feature, e.g., the time of the maximum absolute amplitude, or the time of another fiducial point of the R-wave segment. In some examples, the R-wave segment feature is the maximum absolute amplitude and the suspected R-wave time is the time of the maximum absolute amplitude sample point. In other examples, the R-wave segment feature is a maximum slope (which may be positive or negative) and the suspected R-wave time is the time of the maximum slope point. In other examples, however, the suspected R-wave time may be determined as the time of a fiducial point of the R-wave segment that is not necessarily the time of the one or more features of the R- wave segment determined at block 258. For instance, the determined feature may be the signal pulse width of a maximum amplitude signal pulse of the rectified R-wave segment and the suspected R-wave time may be the time of the maximum absolute amplitude sample point. In still other examples, the feature may be the maximum absolute amplitude and the suspected R-wave time may be earliest time that the R-wave segment crosses an amplitude threshold, which may or may not be equal to the R-wave confirmation threshold at block 252. When the threshold is not crossed by the R-wave segment, the R-wave segment and associated, triggering Vsense is not confirmed and a suspected R-wave time may be undetermined by control circuit 80.

[0144] At block 262, control circuit 80 may determine if the one or more features of the R-wave segment meet the R-wave confirmation threshold(s) set at block 252. In an illustrative example, if the maximum absolute amplitude of the R-wave segment is greater than or equal to an R-wave confirmation threshold amplitude set at block 252, control circuit 80 may confirm the R-wave segment and associated, triggering Vsense signal as being a true R-wave. If the maximum absolute amplitude is less than the threshold amplitude set at block 252, control circuit 80 does not confirm the R-wave segment and associated triggering Vsense signal (“no” branch of block 262). Control circuit 80 may return to block 254 to wait for the next Vsense signal. No suspected RRI is determined for the current R-wave segment that is unconfirmed.

[0145] When the maximum absolute amplitude meets the R-wave confirmation threshold amplitude threshold set at block 252, control circuit 80 may confirm the R-wave segment at block 262 (and the associated Vsense signal). Control circuit 80 may advance to block 264 to determine a suspected RRI for the confirmed R-wave segment. The suspected RRI may extend from the suspected R-wave time of the currently confirmed R-wave segment to the suspected R-wave time of the most recent preceding confirmed R-wave segment. The suspected RRI may extend from the currently confirmed Vsense signal associated with the confirmed R-wave segment to the most recent, preceding confirmed Vsense signal. Control circuit 80 may determine the suspected RRI as the sum of sensed RRIs occurring between two consecutively confirmed Vsense signals. Control circuit 80 may determine the suspected RRI based on the suspected R-wave times determined from two consecutively confirmed R-wave segments and a sum of sensed RRIs occurring between the associated, consecutively confirmed Vsense signals. Other example methods for determining the suspected RRI at block 264 are described below in conjunction with FIG. 9.

[0146] At block 270, control circuit 80 may buffer the suspected RRI in memory 82, e.g., in a FIFO buffer allocated for storing a specified number of suspected RRIs. The buffered suspected RRIs may be used by control circuit 80 for comparison to buffered sensed RRIs for confirming that the sensed RRIs are reliable for heart rhythm determination. For instance, control circuit 80 may determine corresponding suspected and sensed heart rates from the respective buffered suspected RRIs and buffered sensed RRIs for confirming thesensed heart rate, e.g., according to the methods described above in conjunction with FIGs. 6 and 7.

[0147] In some examples, control circuit 80 may limit the maximum suspected RRI to a specified maximum interval. As shown in FIG. 8, if the suspected R-wave segment feature(s) do not meet R-wave confirmation criteria at block 262 (“no” branch), control circuit 80 may optionally determine if a maximum time interval since the most recent confirmed R-wave segment has elapsed. For instance, control circuit 80 may start a timer to count down a maximum suspected RRI interval in response to a confirmed Vsense signal or suspected R-wave time of a confirmed R-wave segment. If the time from the suspected R-wave time or triggering Vsense signal of the most recently confirmed R-wave segment has reached the maximum interval, control circuit 80 may set the current suspected RRI to the maximum interval at block 268 without waiting for the next confirmed R-wave segment. The suspected RRI buffered as the maximum interval may be used by control circuit 80 in determining a suspected heart rate for confirming a sensed heart rate according to any of the methods disclosed herein. The maximum interval may be buffered as a suspected RRI in memory 82 at block 270. The maximum interval may be set to a specified value, e.g., 1.5 to 4 seconds, 2 to 3 seconds or 2.55 seconds in various examples. If the maximum interval has not expired since the most recent preceding confirmed R-wave segment or associated confirmed Vsense signal (“no” branch of block 266), control circuit 80 may return to block 254 to wait for the next Vsense signal.

[0148] When the suspected RRI is set to the maximum interval at block 268, the R-wave confirmation threshold set at block 252 may be adjusted to a nominal value at block 252. For instance, when the R-wave segment maximum absolute amplitude does not meet the R-wave confirmation criteria at block 262, a suspected RRI is “forced” by setting the suspected RRI to the maximum interval. Because a maximum absolute amplitude that meets the R-wave confirmation threshold is not available from the R-wave segment, the maximum absolute amplitude for the suspected RRI set to the maximum interval may be “forced” to a nominal value, e.g., 1 to 6 ADC units or 4 ADC units in an example. The R- wave confirmation threshold set at block 252 may be set based on the nominal value, e.g., 4 ADC units. In other examples, the R-wave confirmation threshold updated at block 252 to be applied to the next R-wave segment may be kept equal to or set to a percentage of the current R-wave confirmation threshold (that was not met by the current R-wavesegment). In other examples, the R-wave confirmation threshold may be updated at block 252 based on a maximum absolute amplitude of the R-wave segment even though the maximum absolute amplitude did not meet the current R-wave confirmation threshold.

[0149] FIG. 9 is a diagram 300 of cardiac electrical signals that may be sensed by sensing circuit 86 of ICD 14. Cardiac electrical signal 301 may be received by sensing channel 83 or 85. Vsense signals 304 may be produced by the sensing channel 83 or 85 in response to an R-wave sensing threshold crossing by the cardiac electrical signal 301. Morphology signal 302 may be passed to control circuit 80 from morphology signal channel 87. It is to be understood that while non-rectified signals are shown in this example, the signal passed to R-wave detector circuit 66a or 66b in a respective sensing channel 83 or 85 may be a rectified signal that is compared to an auto -adjusting R-wave sensing threshold amplitude as described above. The morphology signal 302 or R-wave segments obtained therefrom may be rectified prior to or after being passed to control circuit 80 for use in in determining features from R-wave segments.

[0150] In response to each Vsense signal 304, control circuit 80 may store an R-wave segment 306, shown as the signal waveform within each rectangular box representing a time window extending over time interval 307 over which the R-wave segment is obtained. Control circuit 80 may obtain an R-wave segment 306 from at least one cardiac electrical signal sensed by sensing circuit 86, e.g., the morphology signal received from morphology sensing channel 87. Each R-wave segment 306 is obtained by control circuit 80 in response to one triggering Vsense signal 304 received from one sensing channel 83 or 85. The one Vsense signal 304 may occur simultaneously or within a short time interval (e.g., within 50 ms) of a second Vsense signal from the second sensing channel in some instances. Each R-wave segment 306 may be obtained, however, over a time interval 307 that is defined relative to one Vsense signal and is expected to include the signal waveform that is sensed as a ventricular event (e.g., one R-wave or QRS complex when the Vsense signal corresponds to a true R-wave crossing the R-wave sensing threshold amplitude). As such, time interval 307 may extend a specified time interval prior to and a specified time interval after the Vsense signal 304 that triggers the storage of the R-wave segment.

[0151] Control circuit 80 may determine the maximum peak amplitude 305 of each R- wave segment 306 as a feature of the R-wave segment. When the R-wave segment is arectified signal segment, the maximum absolute peak amplitude can be determined from the R-wave segment 306. Control circuit 80 may determine the suspected R-wave time 308 as the time of the maximum peak amplitude 305. The suspected R-wave segment feature determined as the maximum peak amplitude 305 and the suspected R-wave time308 determined as the time of the maximum peak amplitude 305 are illustrative, nonlimiting examples of a suspected R-wave segment feature and a suspected R-wave time. Other examples are listed above.

[0152] Each suspected R-wave segment 306 (and corresponding triggering Vsense signal) may be confirmed (C) or unconfirmed (UC) by control circuit 80 by comparing the feature, in this case maximum peak amplitude 305, to R-wave confirmation criteria, in this case a threshold amplitude 309, 310 or 312. The R-wave confirmation threshold amplitude309 may be initially set to a nominal or default value. The R-wave confirmation threshold amplitudes 310 and 312 may be updated to a new value based on one or more confirmed R-wave segments as further described below.

[0153] The first (leftmost) R-wave segment 306 shown in FIG. 9 may be confirmed (C) by control circuit 80 because the maximum peak amplitude 305 is greater than the initial R- wave confirmation threshold amplitude 309. Control circuit 80 may determine an updated R-wave confirmation threshold 310 based on the feature, in this example maximum peak amplitude 305, of the confirmed R-wave segment. R-wave confirmation threshold 310 may be set to a percentage of the maximum peak amplitude 305, e.g., to 50% to 70% or to 55% to 60% of the maximum peak amplitude 305 as examples. In one example, control circuit 80 adjusts the R-wave confirmation threshold from amplitude 309 to amplitude 310 by determining 56% of the maximum peak amplitude 305 of the most recently confirmed R-wave segment. The updated R-wave confirmation threshold amplitude 310 is applied to subsequent R-wave segments 306 until the next R-wave segment is confirmed and the R- wave confirmation threshold amplitude is adjusted again. In other examples, the R-wave confirmation threshold may be determined based one or more most recently confirmed R- wave segments and may be determined using a mathematical or statistical relationship of the feature(s) determined from the one or more R-wave segments.

[0154] The next four R-wave segments after the first confirmed R-wave segment in the example shown in FIG. 9 are unconfirmed (UC) R-wave segments 330 because the maximum peak amplitude of each respective R-wave segment is less than the R-waveconfirmation threshold amplitude 310. The fifth R-wave segment 316 is a confirmed R- wave segment because the maximum peak amplitude 315 of the R-wave segment 316 is greater than the R-wave confirmation threshold amplitude 310.

[0155] Control circuit 80 may determine an updated R-wave confirmation threshold amplitude 312 based on the maximum peak amplitude 315 of the confirmed R-wave segment 316. The updated R-wave confirmation threshold amplitude 312 may be applied to subsequent R-wave segments until the next R-wave segment is confirmed and the R- wave confirmation threshold is adjusted again.

[0156] The R -wave confirmation threshold amplitudes 309, 310 and 312 are shown as fixed values until being updated based on a feature of a confirmed R-wave segment. It is contemplated that the R-wave confirmation threshold amplitude may decrease by step decrements or decay over a given R-wave segment such that the maximum peak amplitude is compared to a value of the R-wave segment confirmation threshold amplitude at a given time point in the R-wave segment. Additionally or alternatively, it is contemplated that in some examples the R-wave confirmation threshold amplitude may decay or decrease from one unconfirmed R-wave segment to the next subsequent R-wave segment until an R- wave segment is confirmed. The maximum peak amplitude of the confirmed R-wave segment may then be used by control circuit 80 to update the R-wave sensing threshold amplitude to a percentage of the maximum peak amplitude.

[0157] Control circuit 80 may determine time difference 322 between the suspected R- wave time 318 (determined as the time of the maximum peak amplitude 315 in this example) and the Vsense signal that triggered storage of the R-wave segment 316. This time difference 322 may be used by control circuit 80 in computing a suspected RRI 314 extending between the currently confirmed R-wave segment 316 and the most recent preceding confirmed R-wave segment 306. The suspected RRI 314 may be determined as the time interval between the suspected R-wave time 308 and the suspected R-wave time 318 of the currently confirmed R-wave segment 316.

[0158] In other examples, control circuit 80 may determine the suspected RRI 314 using sensed RRIs, e.g., RRI 320, determined between consecutively received Vsense signals 304. For example, the suspected RRI 314 may be determined as the sum of all sensed RRIs beginning with the confirmed Vsense signal that triggered storage of the most recentpreceding confirmed R-wave segment 306 and ending on the confirmed Vsense signal that triggered storage of the currently confirmed R-wave segment 316.

[0159] In still other examples, the suspected RRI 314 may be determined as a combination of sensed RRIs and the time differences, e.g., time difference 322, between suspected R- wave times of confirmed R-wave segments and corresponding confirmed V sense signals for the leading confirmed R-wave segment 306 and the ending confirmed R-wave segment 316. For example, time difference 322 may be referred to as “confirmed time difference 2” (CTD2) and is the time interval from the ending suspected R-wave time 318, referred to as confirmation time 2 (CT2) and the time of the Vsense signal that triggered storage of the confirmed R-wave segment 316. A “confirmed time difference 1” (CTD1) may be determined as the time interval from the suspected R-wave time 308, referred to as confirmation time 1 (CT1), of the most recent preceding confirmed R-wave segment 306 and the time of the Vsense signal that triggered storage of the leading, confirmed R-wave segment 306. In the illustrative example shown, CT1 is at the same time as the triggering Vsense signal so that CTD1 is zero. In other instances, however CTD1 and CTD2 may be positive, negative or zero values. The suspected RRI 314 may be determined as the difference between CTD2 and CTD 1 (e.g., CTD2 - CTD1) plus the sensed RRI starting from the leading confirmed Vsense signal (corresponding to confirmed R-wave segment 306) plus the sensed RRI ending on the trailing (next consecutive) confirmed Vsense signal (corresponding the confirmed R-wave segment 316) plus any intervening sensed RRIs beginning or ending with unconfirmed Vsense signals that triggered the storage of unconfirmed R-wave segments. When two consecutive R-wave segments are confirmed, the suspected RRI can be determined by control circuit 80 as the sensed RRI determined from the two consecutively confirmed Vsense signals plus the difference between CTD2 and CTD1. When non-consecutive R-wave segments are confirmed, the suspected RRI 314 can be determined by control circuit 80 as CTD2-CTD1 plus the sum of all intervening sensed RRIs starting from the leading confirmed Vsense signal and ending with the next consecutive confirmed Vsense signal.

[0160] As described above in conjunction with FIG. 7, a representative suspected RRI may be determined from a specified number of suspected RRIs, e.g., the median suspected RRI from 12 most recent suspected RRIs. The representative suspected RRI may be compared to a representative sensed RRI, e.g., the median sensed RRI from 12 most recentsensed RRIs. When the suspected heart rate corresponding to the representative suspected RRI is at least 75% (or another specified percentage) of the sensed heart rate corresponding to the representative sensed RRI, the sensed heart rate is confirmed. If an NID is met and control circuit 80 confirms the sensed heart rate based on the suspected RRIs (and any other VT / VF detection requirements are met), control circuit 80 may detect VT / VF and initiate anti-tachyarrhythmia therapy. If an NID is met but control circuit 80 does not confirm the sensed heart rate based on suspected RRIs, control circuit 80 may withhold or delay initiating an anti-tachyarrhythmia therapy, e.g., by withholding or delaying a VT / VF detection, until the NID is reached and the sensed heart rate is confirmed based on the suspected RRIs. If control circuit 80 is operating in the unconcerned sensing state 1 of FIG. 5, control circuit 80 may withhold a transition to the concerned state 2 when an NID is met but control circuit 80 does not confirm the sensed heart rate based on suspected RRIs.

[0161] FIG. 10 is a flow chart 400 of a method for detecting tachyarrhythmia according to another example. At block 402, control circuit 80 may determine if a VTVVFI counter has started increasing, e.g., reached at least a first threshold value that is less than an NID. In response to a VTI / VFI counter reaching a first threshold value, control circuit 80 may enable buffering and analysis of R-wave segments for confirming Vsense signals. Once r- wave segment analysis is enabled, R-wave segments may be buffered in response to each Vsense signal at block 404.

[0162] At block 406, control circuit 80 may identify confirmed R-wave segments and updated suspected RRIs determined according to any of the examples described above in a suspected RRI buffer of memory 82. If a VTVVFI counter has not reached an NID (“no” branch of block 408), control circuit 80 may continue to analyze R-wave segments buffered at block 404 in response to Vsense signals and updating suspected RRIs in a FIFO buffer of memory 82 at block 406. When a VTVVFI counter reaches an NID, as determined at block 408, control circuit 80 may determine if a threshold number N of buffered suspected RRIs are equal to the maximum interval. As described above in conjunction with FIG. 8, if a confirmed R-wave segment is not identified before a maximum interval from the most recent confirmed R-wave segment, control circuit 80 may buffer a suspected RRI equal to the maximum interval. At block 410 of FIG. 10, control circuit 80 may determine if one, two, three or other threshold number (N) ofsuspected RRIs buffered in memory 82 are equal to the maximum interval. If so, control circuit 80 may skip determining a suspected heart rate for confirming the sensed rate. Control circuit 80 may advance to block 420 to determine if all VT / VF detection criteria are met in response to the NID being reached at block 408.

[0163] If less than a threshold number of suspected RRIs are equal to the maximum interval at block 410 (“no” branch), control circuit 80 may determine if at least a threshold number of suspected RRIs have been buffered in memory 82. If less than a threshold number (X) suspected RRIs are buffered in memory 82, control circuit 80 may skip determining a suspected heart rate based on the suspected RRIs for confirming the sensed rate. Control circuit 80 may respond to the NID being reached by determining if all VT / VF detection criteria are met at block 420.

[0164] The threshold number of suspected RRIs required for determining a suspected heart rate for confirming a sensed heart rate may be between 3 and 20 and may depend on the size of the suspected RRI buffer and / or the programmed NID. For example, when the suspected RRI buffer is allocated to store 12 consecutively determined suspected RRIs, the required number of buffered suspected RRIs may be between 3 and 11 and may be dependent on the programmed NID. Memory 82 may store a look-up table for X based on the programmed NID for a given suspected RRI buffer size. In an illustrative example, for a buffer size of 12, when the NID is 12 or less, X may be 3. When the NID is 18, X may be 5. When the NID is 24, X may be 6. When the NID is 30, X may be 7, and when the NID is 45 or higher, X may be 11. The value of X may be 20% to 50% or about 25% to 30% of the NID with a minimum of 3 and a maximum of 11 for example.

[0165] If fewer than the required number of suspected RRIs have been buffered in memory 82 when the NID is reached, control circuit 80 may advance to block 420. Otherwise, control circuit 80 may advance to block 412 to determine a suspected heart rate. As described above, the suspected heart rate may be determined as a representative, e.g., median, value of the buffered suspected RRIs or converted to the corresponding rate in beats per minute.

[0166] At block 414, control circuit 80 may compare the suspected heart rate to a fast rate threshold for confirming the rate of Vsense signals. As described above, the fast rate threshold may be a percentage of the sensed heart rate (or a corresponding multiple of a representative sensed RRI). In other examples, the fast rate threshold may be based on theVT detection interval (when VT is enabled and a corresponding VT NID is reached at block 408) or based on the VF detection interval (and a VF NID is reached at block 408). If the suspected heart rate meets the fast rate threshold at block 414, the sensed heart rate is confirmed (meaning that the NID being reached is valid evidence of a VT / VF). Control circuit 80 may advance to block 420 to determine if all VT / VF detection criteria are met (e.g., no VT / VF rejection rules met). If all VT / VF detection criteria are met, control circuit 80 may detect VT / VF at block 422 and control therapy delivery circuit 84 to initiate an anti-tachyarrhythmia therapy, e.g., initiate ATP and / or capacitor charging for delivery of a CV / DF shock).

[0167] If all VT / VF detection criteria are not met, control circuit 80 may advance to block 424 to wait for the next Vsense signal. If the NID is still met at block 426, control circuit 80 may continue the process of confirming the Vsense signals and updating suspected RRIs in memory 82. If the NID is no longer met, control circuit 80 may still continue the process of confirming the Vsense signals and updating suspected RRIs in memory 82 as long as a VTVVFI counter still meets at least a first threshold value (block 402).

[0168] Referring again to block 414, if the suspected heart rate does not meet the fast rate threshold for confirming the sensed heart rate at block 414, control circuit 80 may withhold or delay a VT / VF detection and / or initiating an anti-tachyarrhythmia therapy because at least some of the sensed RRIs detected as VT / VF intervals may be unconfirmed Vsense signals, e.g., due to oversensing. Control circuit 80 may advance directly to block 424 to wait for the next Vsense signal when the suspected heart rate does not meet a fast rate threshold for confirming the sensed heart rate at block 414.

[0169] In the example shown, control circuit 80 may optionally compare suspected heart rate to a slow rate threshold at block 416. As described above, in some examples, control circuit 80 may determine if termination criteria, abort therapy criteria, or VTVVFI counter reset criteria are met based on the suspected heart rate. If the suspected heart rate is not less than a slow rate threshold at block 416, control circuit 80 may advance to block 424 to wait for the next Vsense signal. In some examples, however, if the suspected heart rate is less than a slow rate threshold at block 416, control circuit 80 may perform a response to the slow suspected heart rate at block 418.

[0170] The response to a slow suspected heart rate performed by control circuit 80 may be a termination detection and transitioning back to the unconcerned state 1 of FIG. 5 (e.g.,from the concerned state 2). The response to a slow suspected rate performed by control circuit 80 may be decreasing the tracked number of tachyarrhythmia intervals, e.g., by resetting a VTI / VFI counter, which may have reached the NID (at block 408), to zero or to a non-zero value less than the NID. Control circuit 80 may advance from block 418 to block 424 to wait for the next Vsense signal. In some instances, the slow rate response at block 418 may include a transition back to the unconcerned state 1 of FIG. 5, where control circuit resumes analyzing sensing RRIs for detecting tachyarrhythmia.

[0171] As such, the suspected heart rate, that is being updated by control circuit 80 as Vsense signals are received and confirmed, may be used by control circuit 80 to detect a slowing heart rate for detecting termination of a fast rate or VT / VF and / or for determining that abort therapy criteria are met. In some examples, the sensed heart rate determined from sensed RRIs may be confirmed by the suspected heart rate (e.g., being within a multiple of the sensed heart rate) for detecting a slowing heart rate to corroborate termination detection by control circuit 80. In the example shown, determination of a suspected heart rate by control circuit 80 is performed after a VTVVFI reaches an NID. It is to be understood that in other examples, control circuit 80 could be configured to determine the suspected heart rate for confirming a sensed heart rate before an NID is reached, e.g., at any time before and / or after an NID is reached.

[0172] In other examples, the slow rate response performed at block 418 may include adjusting the gain of the morphology sensing channel 87 (e.g., by increasing the gain of ADC 73 shown in FIG. 4). If the suspected rate is slower than the slow rate threshold, the gain of the morphology sensing channel 87 may be too low to allow reliable detection of suspected R-waves from the morphology signal. In still other examples, the slow rate response performed at block 418 may include adjusting a bradycardia pacing parameter. When the suspected rate is slower than the slow rate threshold but an NID is met based on Vsense signals, some Vsense signals may be due noise signals being oversensed on a sensing channel 83 or 85. Noise oversensing may be causing control circuit 80 to inhibit bradycardia pacing pulses. Noise reversion pacing may be initiated by control circuit 80 to enable bradycardia pacing pulses to be delivered by therapy delivery circuit 84. Noise reversion pacing may be initiated by increasing the post-sense ventricular refractory period so that a Vsense during the extended refractory period does not inhibit a scheduled ventricular pacing pulse (e.g., at a bradycardia lower rate interval from a previous non-refractory Vsense). In this way, when noise is suspected of causing frequent Vsense signals that inhibit bradycardia pacing during a true slow heart rate, a scheduled pacing pulse is not inhibited due to noise oversensing causing a Vsense signal in the extended post-sense ventricular refractory period.

[0173] Further disclosed herein is the subject matter of the following examples:

[0174] Example 1. A medical device comprising including a sensing circuit configured to sense one or more cardiac electrical signals and sense ventricular event signals from the one or more cardiac electrical signals. The medical device further comprising a memory configured to buffer suspected R-wave segments from the one or more cardiac electrical signals and a control circuit in communication with the sensing circuit and configured to determine sensed RR intervals from the ventricular event signals sensed by the sensing circuit. The control circuit is further configured to, in response to each ventricular event signal of a plurality of the ventricular event signals sensed by the sensing circuit, store in the memory a suspected R-wave segment from the one or more cardiac electrical signals, determine at least one feature of the suspected R-wave segment and determine that the ventricular event signal of the plurality of ventricular event signals is a confirmed ventricular event signal when the at least one feature meets R-wave confirmation criteria. The control circuit may be configured to detect a first threshold number of tachyarrhythmia intervals from the sensed RR intervals, determine a suspected heart rate from the confirmed ventricular event signals, determine that the suspected heart rate meets a heart rate threshold and detect a tachyarrhythmia in real time in response to at least detecting the first threshold number of tachyarrhythmia intervals from the sensed RR intervals and the suspected heart rate meeting the heart rate threshold. The control circuit may be configured to store data in the memory corresponding to the detected tachyarrhythmia. The medical device may include a telemetry circuit configured to transmit the data corresponding to the detected tachyarrhythmia.

[0175] Example 2. The medical device of example 1 wherein the control circuit is further configured to, for each of at least the confirmed ventricular event signals, determine a suspected R-wave time from the associated suspected R-wave segment and determine the suspected heart rate using at least a portion of the suspected R-wave times.

[0176] Example 3. The medical device of example 2 wherein the control circuit is further configured to determine the suspected heart rate using at least a portion of the suspectedR-wave times by, for each suspected R-wave segment having the at least one feature meeting the R-wave confirmation criteria, determine a suspected RR interval using at least the respective suspected R-wave time and one or more sensed RR intervals and determine the suspected heart rate from the suspected RR intervals.

[0177] Example 4. The medical device of any one of examples 2 — 3 wherein the control circuit is further configured to determine the suspected heart rate using at least a portion of the suspected R-wave times by, for each confirmed ventricular event signal, determining a time interval difference extending from the confirmed ventricular event signal to the suspected R-wave time and determining a suspected RR interval as the difference between two consecutively determined time interval differences determined for two consecutively confirmed ventricular event signals summed with all intervening sensed RR intervals. The control circuit determining the suspected heart rate from the determined suspected RR intervals.

[0178] Example 5. The medical device of any one of examples 1 — 4 wherein the control circuit is further configured to determine the heart rate threshold from the sensed RR intervals.

[0179] Example 6. The medical device of any one of examples 1 — 5 wherein the control circuit is further configured to detect the tachyarrhythmia intervals by comparing the sensed RR intervals to a tachyarrhythmia interval threshold and determine the heart rate threshold from at least the tachyarrhythmia interval threshold.

[0180] Example 7. The medical device of any one of examples 1 — 6 wherein the control circuit is further configured to determine the suspected heart rate from the suspected RR intervals by determining a representative suspected RR interval from the suspected RR intervals. The control circuit may be configured to determine the heart rate threshold by determining a representative sensed RR interval and determining the heart rate threshold as a specified multiple of the representative sensed RR interval. The control circuit may be configured to determine that the suspected heart rate meets the heart rate threshold in response to the representative suspected RR interval being less than the specified multiple of the representative sensed RR interval.

[0181] Example 8. The medical device of any one of examples 1 — 7 wherein the control circuit is further configured to detect a second threshold number of tachyarrhythmia intervals from the sensed RR intervals, the second threshold number of tachyarrhythmiaintervals being less than the first threshold number of tachyarrhythmia intervals. In response to detecting at least the second threshold number of tachyarrhythmia intervals from the sensed RR intervals, the control circuit may be configured to initiate determining the features of the suspected R-wave segments.

[0182] Example 9. The medical device of any one of examples 1 — 8 wherein the control circuit is further configured to determine a third threshold number based on the first threshold number, determine that at least the third threshold number of the plurality of sensed ventricular event signals are confirmed ventricular event signals, and determine the suspected heart rate when at least the third threshold number of the plurality of sensed ventricular event signals are confirmed ventricular event signals.

[0183] Example 10. The medical device of any one of examples 1 — 9 wherein the control circuit is further configured to adjust the R-wave confirmation criteria based on the at least one feature determined from a stored one of the suspected R-wave segments and apply the adjusted R-wave confirmation criteria to a next consecutive suspected R-wave segment.

[0184] Example 11. The medical device of any one of examples 1 — 10 wherein the control circuit is further configured to determine the at least one feature by determining a maximum absolute amplitude of the R-wave segment, compare the maximum absolute amplitude to an R-wave confirmation amplitude threshold of the R-wave confirmation criteria and determine that the ventricular event signal of the plurality of sensed ventricular event signals is a confirmed ventricular event signal in response to at least the maximum absolute amplitude of the R-wave segment meeting the R-wave confirmation amplitude threshold.

[0185] Example 12. The medical device of any one of examples 1 — 11 wherein the control circuit is further configured to, for each suspected R-wave segment, determine a suspected RR interval and determine when at least a specified minimum number of suspected RR intervals are determined. The control circuit may be configured to determine the suspected heart rate when at least the specified minimum number of suspected RR intervals are determined. When less than the specified minimum number of suspected RR intervals are determined, the control circuit may be configured to detect the tachyarrhythmia in response to at least detecting the first threshold number of tachyarrhythmia intervals without determining the suspected heart rate.

[0186] Example 13. The medical device of any one of examples 1 — 12 wherein the control circuit is further configured to determine suspected RR intervals from the suspected R-wave segments, determine that one or more suspected RR intervals are a maximum interval limit and detect the tachyarrhythmia in response to at least detecting the first threshold number of tachyarrhythmia intervals from the sensed RR intervals without determining the suspected heart rate when one or more suspected RR intervals are the maximum interval limit.

[0187] Example 14. The medical device of any one of examples 1 — 13 further comprising a therapy delivery circuit configured to initiate an anti-tachyarrhythmia therapy in response to the control circuit detecting the tachyarrhythmia.

[0188] Example 15. The medical device of example 14 wherein the therapy delivery circuit comprises a high voltage capacitor, and the control circuit is further configured to control the therapy delivery circuit to initiate the anti-tachyarrhythmia therapy by initiating charging of the high voltage capacitor.

[0189] Example 16. The medical device of any one of examples 1 — 15 wherein the control circuit is further configured to determine when a suspected heart rate determined from the confirmed ventricular event signals is less than a slow rate threshold and perform a slow rate response comprising at least one of: decreasing a tracked number of tachyarrhythmia intervals, detecting termination of a tachyarrhythmia, adjusting a gain used by the sensing circuit for sensing the one or more cardiac electrical signals or controlling a therapy delivery circuit to deliver noise reversion pacing.

[0190] Example 17. A method including sensing one or more cardiac electrical signals, sensing ventricular event signals from the one or more cardiac electrical signals and determining sensed RR intervals from the sensed ventricular event signals. The method may include, in response to each ventricular event signal of a plurality of the sensed ventricular event signals, storing a suspected R-wave segment from the one or more cardiac electrical signals, determining at least one feature of the suspected R-wave segment and determining that the ventricular event signal of the plurality of sensed ventricular event signals is a confirmed ventricular event signal when the at least one feature meets R-wave confirmation criteria. The method may include detecting a first threshold number of tachyarrhythmia intervals from the sensed RR intervals, determining a suspected heart rate from the confirmed ventricular event signals, and determining thatthe suspected heart rate meets a heart rate threshold. The method may include detecting a tachyarrhythmia in real time in response to at least detecting the first threshold number of tachyarrhythmia intervals from the sensed RR intervals and the suspected heart rate meeting the heart rate threshold, storing data corresponding to the detected tachyarrhythmia and transmitting the data corresponding to the detected tachyarrhythmia.

[0191] Example 18. The method of example 17 further comprising, for each of at least the confirmed ventricular event signals, determining a suspected R-wave time from the associated suspected R-wave segment and determining the suspected heart rate using at least a portion of the suspected R-wave times.

[0192] Example 19. The method of example 18 wherein determining the suspected heart rate using at least a portion of the suspected R-wave times comprises, for each suspected R-wave segment having the at least one feature meeting the R-wave confirmation criteria, determining a suspected RR interval using at least the respective suspected R-wave time and one or more sensed RR intervals and determining the suspected heart rate from the suspected RR intervals.

[0193] Example 20. The method of any one of examples 18 — 19 wherein determining the suspected heart rate using at least a portion of the suspected R-wave times comprises, for each confirmed ventricular event signal, determining a time interval difference extending from the confirmed ventricular event signal to the suspected R-wave time and determining a suspected RR interval as the difference between two consecutively determined time interval differences determined for two consecutively confirmed ventricular event signals summed with all intervening sensed RR intervals. The method may include determining the suspected heart rate from the determined suspected RR intervals.

[0194] Example 21. The method of any one of examples 17 — 20 further comprising determining the heart rate threshold from the sensed RR intervals.

[0195] Example 22. The method of any one of examples 17 — 21 further comprising detecting the tachyarrhythmia intervals by comparing the sensed RR intervals to a tachyarrhythmia interval threshold and determining the heart rate threshold from at least the tachyarrhythmia interval threshold.

[0196] Example 23. The method of any one of examples 17 — 22 further comprising determining the suspected heart rate from the suspected RR intervals by determining a representative suspected RR interval from the suspected RR intervals. The method mayfurther include determining the heart rate threshold by determining a representative sensed RR interval and determining the heart rate threshold as a specified multiple of the representative sensed RR interval. The method may further include determining that the suspected heart rate meets the heart rate threshold in response to the representative suspected RR interval being less than the specified multiple of the representative sensed RR interval.

[0197] Example 24. The method of any one of examples 17 — 23 further comprising detecting a second threshold number of tachyarrhythmia intervals from the sensed RR intervals, the second threshold number of tachyarrhythmia intervals being less than the first threshold number of tachyarrhythmia intervals. The method may include, in response to detecting at least the second threshold number of tachyarrhythmia intervals from the sensed RR intervals, initiate determining the features of the suspected R-wave segments.

[0198] Example 25. The method of any one of examples 17 — 24 further comprising determining a third threshold number based on the first threshold number, determining that at least the third threshold number of the plurality of sensed ventricular event signals are confirmed ventricular event signals, and determining the suspected heart rate when at least the third threshold number of the plurality of the sensed ventricular event signals are confirmed ventricular event signals.

[0199] Example 26. The method of any one of examples 17 — 25 further comprising adjusting the R-wave confirmation criteria based on the at least one feature determined from a stored one of the suspected R-wave segments and apply the adjusted R-wave confirmation criteria to a next consecutive suspected R-wave segment.

[0200] Example 27. The method of any one of examples 17 — 26 further comprising determining the at least one feature by determining a maximum absolute amplitude of the R-wave segment, comparing the maximum absolute amplitude to an R-wave confirmation amplitude threshold of the R-wave confirmation criteria and determining that the ventricular event signal of the plurality of sensed ventricular event signals is a confirmed ventricular event signal in response to at least the maximum absolute amplitude of the R- wave segment meeting the R-wave confirmation amplitude threshold.

[0201] Example 28. The method of any one of examples 17 — 27 further comprising, for each suspected R-wave segment determining a suspected RR interval, determining when at least a specified minimum number of suspected RR intervals are determined, anddetermining the suspected heart rate when at least the specified minimum number of suspected RR intervals are determined. When less than the specified minimum number of suspected RR intervals are determined, the method may further include detecting the tachyarrhythmia in response to at least detecting the first threshold number of tachyarrhythmia intervals without determining the suspected heart rate.

[0202] Example 29. The method of any one of examples 17 — 28 further comprising determining suspected RR intervals from the suspected R-wave segments, determining that one or more suspected RR intervals are a maximum interval limit and detecting the tachyarrhythmia in response to at least detecting the first threshold number of tachyarrhythmia intervals from the sensed RR intervals without determining the suspected heart rate when one or more suspected RR intervals are the maximum interval limit.

[0203] Example 30. The method of any one of examples 17 — 29 further comprising initiating an anti-tachyarrhythmia therapy in response to detecting the tachyarrhythmia.

[0204] Example 31. The method of example 30 further comprising initiating the antitachyarrhythmia therapy by initiating charging of a high voltage capacitor.

[0205] Example 32. The method of any one of examples 17 — 31 further comprising determining when a suspected heart rate determined from the confirmed ventricular event signals is less than a slow rate threshold and performing a slow rate response. The slow rate response may include at least one of: decreasing a tracked number of tachyarrhythmia intervals, detecting termination of a tachyarrhythmia, adjusting a gain used for sensing the one or more cardiac electrical signals or delivering noise reversion pacing.

[0206] Example 33. A non-transitory, computer readable medium storing a set of instructions that, when executed by control circuitry of a medical device, cause the medical device to sense one or more cardiac electrical signals and sense ventricular event signals from the one or more cardiac electrical signals. The instructions may further cause the medical device to determine sensed RR intervals from the sensed ventricular event signals and, in response to each ventricular event signal of a plurality of the sensed ventricular event signals, store a suspected R-wave segment from the one or more cardiac electrical signals, determine at least one feature of the suspected R-wave segment, and determine that the ventricular event signal of the plurality of sensed ventricular event signals is a confirmed ventricular event signal when the at least one feature meets R-wave confirmation criteria. The instructions may further cause the medical device to detect afirst threshold number of tachyarrhythmia intervals from the sensed RR intervals, detect at least a second threshold number of confirmed ventricular event signals, determine a suspected heart rate from at least the second threshold number of confirmed ventricular event signals, determine that the suspected heart rate meets a heart rate threshold and detect a tachyarrhythmia in response to at least detecting the first threshold number of tachyarrhythmia intervals from the sensed RR intervals and the suspected heart rate meeting the heart rate threshold. The instructions may further cause the medical device to initiate an anti-tachyarrhythmia therapy in response to detecting the tachyarrhythmia.

[0207] It should be understood that, depending on the example, certain acts or events of any of the methods described herein can be performed in a different sequence, may be added, merged, or left out altogether (e.g., not all described acts or events are necessary for the practice of the method). Moreover, in certain examples, acts or events may be performed concurrently, e.g., through multi-threaded processing, interrupt processing, or multiple processors, rather than sequentially. In addition, while certain aspects of this disclosure are described as being performed by a single circuit or unit for purposes of clarity, it should be understood that the techniques of this disclosure may be performed by a combination of units or circuits associated with, for example, a medical device.

[0208] In one or more examples, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored as one or more instructions or code on a computer-readable medium and executed by a hardware -based processing unit. Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media (e.g., RAM, ROM, EEPROM, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer).

[0209] Instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable logic arrays (FPLAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor,” as used herein may refer to any of the foregoing structure or any other structure suitable for implementation of the techniques described herein. Also, the techniques could be fully implemented in one or more circuits or logic elements.

[0210] Thus, a medical device has been presented in the foregoing description with reference to specific examples. It is to be understood that various aspects disclosed herein may be combined in different combinations than the specific combinations presented in the accompanying drawings. It is appreciated that various modifications to the referenced examples may be made without departing from the scope of the disclosure and the following claims.

Claims

WHAT IS CLAIMED IS:

1. A medical device comprising: a sensing circuit configured to: sense one or more cardiac electrical signals; and sense ventricular event signals from the one or more cardiac electrical signals; a memory configured to buffer suspected R-wave segments from the one or more cardiac electrical signals; a control circuit in communication with the sensing circuit and configured to: determine sensed RR intervals from the ventricular event signals sensed by the sensing circuit; in response to each ventricular event signal of a plurality of the sensed ventricular event signals sensed by the sensing circuit: store in the memory a suspected R-wave segment from the one or more cardiac electrical signals; determine at least one feature of the suspected R-wave segment; and determine that the ventricular event signal of the plurality of sensed ventricular event signals is a confirmed ventricular event signal when the at least one feature meets R-wave confirmation criteria; detect a first threshold number of tachyarrhythmia intervals from the sensed RR intervals; determine a suspected heart rate from the confirmed ventricular event signals; determine that the suspected heart rate meets a heart rate threshold; and detect a tachyarrhythmia in real time in response to at least detecting the first threshold number of tachyarrhythmia intervals from the sensed RR intervals and the suspected heart rate meeting the heart rate threshold; and store data in the memory corresponding to the detected tachyarrhythmia; and a telemetry circuit configured to transmit the data corresponding to the detected tachyarrhythmia.

2. The medical device of claim 1 wherein the control circuit is further configured to: for each of at least the confirmed ventricular event signals, determine a suspectedR-wave time from the associated suspected R-wave segment; and determine the suspected heart rate using at least a portion of the suspected R-wave times.

3. The medical device of claim 2 wherein the control circuit is further configured to determine the suspected heart rate using at least a portion of the suspected R-wave times by: for each suspected R-wave segment having the at least one feature meeting the R- wave confirmation criteria, determine a suspected RR interval using at least the respective suspected R-wave time and one or more sensed RR intervals; and determine the suspected heart rate from the suspected RR intervals.

4. The medical device of any one of claims 2 — 3 wherein the control circuit is further configured to determine the suspected heart rate using at least a portion of the suspected R-wave times by: for each confirmed ventricular event signal: determining a time interval difference extending from the confirmed ventricular event signal to the suspected R-wave time; and determining a suspected RR interval as the difference between two consecutively determined time interval differences determined for two consecutively confirmed ventricular event signals summed with all intervening sensed RR intervals; and determining the suspected heart rate from the determined suspected RR intervals.

5. The medical device of any one of claims 1 — 4 wherein the control circuit is further configured to determine the heart rate threshold from the sensed RR intervals.

6. The medical device of any one of claims 1 — 4 wherein the control circuit is further configured to:detect the tachyarrhythmia intervals by comparing the sensed RR intervals to a tachyarrhythmia interval threshold; and determine the heart rate threshold from at least the tachyarrhythmia interval threshold.

7. The medical device of any one of claims 1 — 5 wherein the control circuit is further configured to: determine the suspected heart rate from the suspected RR intervals by determining a representative suspected RR interval from the suspected RR intervals; determine the heart rate threshold by: determining a representative sensed RR interval; and determining the heart rate threshold as a specified multiple of the representative sensed RR interval; and determine that the suspected heart rate meets the heart rate threshold in response to the representative suspected RR interval being less than the specified multiple of the representative sensed RR interval.

8. The medical device of any one of claims 1 — 7 wherein the control circuit is further configured to: detect a second threshold number of tachyarrhythmia intervals from the sensed RR intervals, the second threshold number of tachyarrhythmia intervals being less than the first threshold number of tachyarrhythmia intervals; and in response to detecting at least the second threshold number of tachyarrhythmia intervals from the sensed RR intervals, initiate determining the features of the suspected R-wave segments.

9. The medical device of any one of claims 1 — 8 wherein the control circuit is further configured to: determine a third threshold number based on the first threshold number; determine that at least the third threshold number of the plurality of sensed ventricular event signals are confirmed ventricular event signals; anddetermine the suspected heart rate when at least the third threshold number of the plurality of sensed ventricular event signals are confirmed ventricular event signals.

10. The medical device of any one of claims 1 — 9 wherein the control circuit is further configured to: adjust the R-wave confirmation criteria based on the at least one feature determined from a stored one of the suspected R-wave segments; and apply the adjusted R-wave confirmation criteria to a next consecutive suspected R- wave segment.

11. The medical device of any one of claims 1 — 10 wherein the control circuit is further configured to: determine the at least one feature by determining a maximum absolute amplitude of the R-wave segment; compare the maximum absolute amplitude to an R-wave confirmation amplitude threshold of the R-wave confirmation criteria; and determine that the ventricular event signal of the plurality of sensed ventricular event signals is a confirmed ventricular event signal in response to at least the maximum absolute amplitude of the R-wave segment meeting the R-wave confirmation amplitude threshold.

12. The medical device of any one of claims 1 — 11 wherein the control circuit is further configured to: for each suspected R-wave segment determine a suspected RR interval; determine when at least a specified minimum number of suspected RR intervals are determined; determine the suspected heart rate when at least the specified minimum number of the suspected RR intervals are determined; and when less than the specified minimum number of suspected RR intervals are determined, detect the tachyarrhythmia in response to at least detecting the first threshold number of tachyarrhythmia intervals without determining the suspected heart rate.

13. The medical device of any one of claims 1 — 12 wherein the control circuit is further configured to: determine suspected RR intervals from the suspected R-wave segments; determine that one or more suspected RR intervals meet a maximum interval limit; and detect the tachyarrhythmia in response to at least detecting the first threshold number of tachyarrhythmia intervals from the sensed RR intervals when one or more suspected RR intervals meet the maximum interval limit.

14. The medical device of any one of claims 1 — 13 further comprising a therapy delivery circuit configured to initiate an anti-tachyarrhythmia therapy in response to the control circuit detecting the tachyarrhythmia.

15. The medical device of claim 14 wherein: the therapy delivery circuit comprises a high voltage capacitor; and the control circuit is further configured to control the therapy delivery circuit to initiate the anti-tachyarrhythmia therapy by initiating charging of the high voltage capacitor.

16. The medical device of any one of claims 1 — 15 wherein the control circuit is further configured to: determine when a suspected heart rate determined from the confirmed ventricular event signals is less than a slow rate threshold; and perform a slow rate response comprising at least one of: decreasing a tracked number of tachyarrhythmia intervals; detecting termination of a tachyarrhythmia; adjusting a gain used by the sensing circuit for sensing the one or more cardiac electrical signals; or controlling a therapy delivery circuit to deliver noise reversion pacing.

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