Medical device and method for detecting cardiac event signal variability

Abstract

A medical device is configured to receive a cardiac electrical signal receive a cardiac electrical signal and sense cardiac event signals from the cardiac electrical signal. The medical device can detect a variability condition of waveform morphology features of the sensed cardiac event signals and generate a control parameter output for delaying delivery of a tachyarrhythmia therapy by the medical device in response to detecting the variability condition.

Description

MEDICAL DEVICE AND METHOD FOR DETECTING CARDIAC EVENT SIGNAL VARIABILITY

[0001] This application claims the benefit of U.S. Provisional Patent Application Serial No. 63 / 710,519, filed October 22, 2024, the entire content of which is incorporated herein by reference.TECHNICAL FIELD

[0002] The disclosure relates generally to a medical device and method for detecting cardiac event signal variability.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 (CV / DF) shocks. In some cases, a medical device may sense cardiac event signals attendant to the intrinsic depolarizations of the myocardium and control delivery of stimulation pulses to the heart based on sensed cardiac event 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 event signals, such as ventricular R-waves sensed from within a ventricle or atrial P-waves sensed from within an atrium. Upon detection of an abnormal rhythm, such as bradycardia, tachycardia orfibrillation, an 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 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 event signals attendant to myocardial depolarization, e.g., R-waves, and monitoring cardiac event signal features for detecting a variability condition. Variability in the morphology of cardiac event signal waveforms could lead to oversensing of cardiac event signals in some instances. In some examples, the medical device may be coupled to an extracardiac medical lead carrying electrodes positioned outside of the heart for sensing cardiac electrical signals and delivering electrical stimulation pulses, including pacing pulses and / or CV / DF shocks. A medical device operating according to the techniques disclosed herein is configured to sense at least one cardiac electrical signal and monitor for cardiac event signal variability. When a cardiac event signal variability condition is detected, the medical device may respond by generating a control parameter output for effectively delaying a tachyarrhythmia therapy to avoid delivering a tachyarrhythmia therapy when oversensing due to cardiac event signal variability is occurring.

[0006] In one example, the disclosure provides a medical device including a therapy delivery circuit configured to deliver a tachyarrhythmia therapy and a sensing circuit configured to receive a cardiac electrical signal and sense cardiac event signals from the cardiac electrical signal. The medical device further includes a control circuit configured to detect a variability condition of waveform morphology features of the cardiac event signals sensed by the sensing circuit and generate a control signal output for causing a delay in delivery of the tachyarrhythmia therapy by the therapy delivery circuit in response to detecting the signal variability condition.

[0007] In another example, the disclosure provides a method including receiving a cardiac electrical signal, sensing cardiac event signals from the cardiac electrical signal, detecting a variability condition of waveform morphology features of the sensed cardiac eventsignals and generating a control parameter output for delaying delivery of a tachyarrhythmia therapy in response to detecting the variability condition.

[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 receive a cardiac electrical signal, sense cardiac event signals from the cardiac electrical signal, detect a variability condition of waveform morphology features of the sensed cardiac event signals and generate a control parameter output for delaying delivery of a tachyarrhythmia therapy by the medical device in response to detecting the variability condition.

[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. lAand 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. 1 A-1B.

[0012] FIG. 3 is a conceptual diagram illustrating another example of an IMD that may be configured to sense cardiac signals, monitor for variability of sensed cardiac event signals, and modify IMD operation when a variability condition is met according to the techniques disclosed herein.

[0013] FIG. 4 is a diagram of a medical device configured to sense cardiac electrical signals and deliver cardiac electrical stimulation according to some examples.

[0014] FIG. 5 is a flow chart of a method that may be performed by a medical device for sensing cardiac event signals and monitoring for cardiac event signal variability according to some examples.

[0015] FIG. 6 is a flow chart of a method that may be performed by a medical device for detecting cardiac event signal variability and performing a response to the cardiac eventsignal variability detection for delaying tachyarrhythmia therapy according to some examples.

[0016] FIG. 7 is a flow chart of a method that may be performed by a medical device for detecting cardiac event signal variability and performing a response to the cardiac event signal variability detection by modifying a cardiac event signal sensing control parameter and / or tachyarrhythmia detection criteria.

[0017] FIG. 8 is a flow chart of a method that may be performed by a medical device for controlling tachyarrhythmia therapy and detection when cardiac event signal variability is detected according to some examples.DETAILED DESCRIPTION

[0018] In general, this disclosure describes a medical device and techniques for monitoring variability of cardiac event signals, e.g., R-waves attendant to ventricular depolarizations, for use in controlling cardiac signal analysis, arrhythmia detection, and / or therapy delivery. The medical device may control delivery of electrical stimulation therapies based at least in part on cardiac event intervals, e.g., RR intervals (RRIs) determined between consecutively sensed cardiac event signals, e.g., sensed as being R-waves. In some instances, cardiac event signals can be oversensed or undersensed.Oversensing can occur when a given cardiac event signal, e.g., an R-wave, is falsely sensed due to non-cardiac noise signals (e.g., electromagnetic interference (EMI), skeletal muscle myopotentials, etc.) and / or other cardiac event signals (e.g., P-waves or T-waves) being falsely sensed as the cardiac event signal of interest. Undersensing can occur when the given cardiac event signal of interest, e.g., the R-wave, falls below a sensing threshold amplitude applied to a sensed cardiac electrical signal. Variability in the waveform morphology of the cardiac event signal can lead to oversensing and / or undersensing. For example, variability in the peak amplitude of R-waves can lead to oversensing of P-waves and / or T-waves, which may normally be lower in amplitude than R-waves but may at times become more similar in amplitude to R-waves due to variability in the R-wave morphology. When the amplitude of R-waves decreases, e.g., due to patient posture changes, respiration phase or other factors, P-waves and / or T-waves may be falsely oversensed as R-waves. The techniques disclosed herein provide improvements in cardiac event signal sensing and therapy delivery control that promote reliable arrhythmiadetection even when cardiac event signals such as R-waves are variable in waveform morphology, e.g., in amplitude, slope, width, area, overall waveform shape or other features of the QRS waveform attendant to ventricular depolarization.

[0019] 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 arrhythmia based on an analysis of the sensed cardiac electrical signals, and delivering electrical stimulation therapy for treating the arrhythmia, e.g., to promote or restore a more normal heart rhythm. 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. A non-transvenous medical electrical lead that is advanced to an implant site outside the blood vessels may also be referred to as an “extravascular” lead. 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, a transvenous extra-cardiac lead may carry 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 cardiac pacing pulses from a location away from the heart. In still other examples, electrodes used for sensing cardiac electrical signals by a medical device for sensing cardiac event signals according to the techniques disclosed herein may be carried by leads advanced into a heart chamber or positioned on a heart chamber. In still other examples, electrodes used for sensing cardiac electrical signals by a medical device configured to monitor for cardiac event signal variability according to the techniques disclosed herein may be carried by the medical device housing, when the device is a leadless pacemaker for example, that can be advanced into a heart chamber or positioned on a heart chamber.

[0020] FIGs. 1 A and IB are conceptual diagrams of one example of an IMD system, in this case an ICD system 10, that may be configured to operate according to the techniques disclosed herein for sensing cardiac electrical signals, detecting arrhythmia and delivering electrical stimulation therapy. FIG. lAis 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. ICDsystem 10 includes an ICD 14 connected to an electrical stimulation and sensing lead 16, positioned in an extra-cardiovascular location in this example. FIGs. 1 A 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 cardiac arrhythmia based on processing and analysis of sensed cardiac electrical signals. The techniques for monitoring cardiac event signal features and modifying medical device operation when a variability condition is met by the cardiac event signal features as disclosed herein may be implemented in a cardiac monitoring device configured to detect and record episodes of cardiac arrhythmia that does not necessarily include cardiac pacing and / or CV / DF shock delivery capabilities in some examples. Furthermore, the techniques disclosed herein for monitoring cardiac event signal features and modifying medical device operation when a variability condition is met may be implemented in a variety of medical devices including external or implantable cardiac monitors, pacemakers, and ICDs.

[0021] 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.

[0022] 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 heartrhythm, and controlling and delivering electrical stimulation pulses to treat an abnormal heart rhythm.

[0023] 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. 1 A 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 separate defibrillation electrodes in which case each of the electrodes 24 and 26 may be activated independently.

[0024] 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 stimulation 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.

[0025] 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.

[0026] ICD 14 may obtain cardiac electrical signals corresponding to electrical activity of heart 8 via one or more 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. Each cardiac electrical signal that is sensed by ICD 14 may be received using a different sensing electrode vector, which may be selected by sensing circuitry included in ICD 14. As described herein, in some examples the cardiac electrical signal(s) received via a selected sensing electrode vector may be used by ICD 14 for sensing cardiac event signals attendant to intrinsic depolarizations of the myocardium, e.g., R-waves and / or P-waves. Sensed cardiac event signals may be used for determining the heart rate and determining a need for cardiac pacing (e.g., for treating bradycardia, atrioventricular conduction block, or asystole) or for determining a need for tachyarrhythmia therapies (e.g., anti-tachycardia pacing (ATP) or CV / DF shocks).

[0027] In the example illustrated in FIGs. lAand IB, electrode 28 is located proximal to defibrillation electrode 24, and electrode 30 is located between defibrillation electrodes 24 and 26. In some examples, a third pace / sense electrode (not shown) may be located distal to defibrillation electrode 26, proximate to or on the distal end of lead body 18. Electrodes 28 and 30 are illustrated as ring electrodes; 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 necessarily 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.

[0028] 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. 1 A 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 fromsternum 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.

[0029] 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.

[0030] 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.

[0031] 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, insome 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.

[0032] Other examples of extra-cardiovascular leads that may be implemented with the techniques described herein may include one or more defibrillation electrodes and one or more pacing and sensing electrodes carried by a curving, serpentine, undulating or zigzagging distal portion of the lead body 18. 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.

[0033] ICD 14 may analyze the cardiac electrical signal(s) received from sensing electrode vectors to monitor for abnormal rhythms, such as asystole, bradycardia, supraventricular tachycardia (SVT), ventricular tachycardia (VT) and / or ventricular fibrillation (VF). ICD 14 may analyze the rate of sensed cardiac event signals, the pattern of sensed cardiac event signals, and / or morphology of the cardiac electrical signals to monitor for a long pause, bradycardia, atrial tachyarrhythmia and ventricular tachyarrhythmia, as examples. ICD 14 may generate and deliver 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.

[0034] 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 pacing pulse, such as a post-shock pacing pulse or bradycardia pacing pulse, to capture and pace the ventricles. The 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.

[0035] In some examples, at least one sensing electrode vector including at least two electrodes may be selected for sensing cardiac signal segments corresponding to one sensed R-wave and / or cardiac signal segments that extend over a specified time intervalthat may encompass more than one sensed R-wave (and may be acquired independent of the timing of a sensed R-wave) for use in detecting and classifying cardiac rhythms.Morphology analysis of a sensed cardiac electrical signal may be triggered by a threshold number of R- waves sensed at tachyarrhythmia intervals. According to the techniques disclosed herein, in some examples the ICD 14 may modify its operation when sensed cardiac event signal features meet a variability condition by enabling cardiac signal morphology analysis (which may be in addition to and / or in place of any morphology analysis already being performed) and / or enabling other cardiac signal analysis in order to confirm cardiac event signal sensing for arrhythmia detection. ICD 14 may modify its operation by enabling and performing an enhanced tachyarrhythmia detection algorithm for promoting reliable tachyarrhythmia detection in the presence of variable cardiac event signal morphology.

[0036] 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 placed abdominally. Lead 16 may be implanted in other extra-cardiovascular locations. For instance, as described with respect to FIGs. 2A-2C, the distal portion 25 of lead 16 may be implanted underneath the stemum / ribcage in the substernal space. The ICD and lead configuration shown in FIGs. 1 A and IB are illustrative in nature and should not be considered limiting in the practice of the techniques disclosed herein.

[0037] 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 bepositioned extra-thoracically (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. In some examples, a non-transvenous lead coupled to ICD 14 may be advanced to position the distal portion of the lead carrying one or more electrodes on or within the pericardium.

[0038] 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 chamber. In still other examples, a leadless medical device may be implanted within a heart chamber carrying housing-based electrodes for sensing and analyzing cardiac electrical signals and controlling the delivery of electrical stimulation therapies using at least some aspects of the techniques disclosed herein.

[0039] An external device 40 is shown in telemetric communication with ICD 14 by a wireless communication link 42 in FIG. 1 A. 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.

[0040] 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, arrhythmia 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 inconjunction with processor 52 for sending and receiving data relating to ICD functions via communication link 42.

[0041] 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.

[0042] 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 sensing cardiac event signals, detecting arrhythmias and delivering therapy according to the techniques disclosed herein may be programmed into ICD 14 using external device 40 in some examples.

[0043] 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. 1 A-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 toward xiphoid 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.

[0044] 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 substemal musculature of anterior mediastinum36. A lead implanted such that the distal portion 25 is substantially within anterior mediastinum 36, may be referred to as a “substernal lead.”

[0045] 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 underneath / 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 or within the pericardium 38 of heart 8.

[0046] 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, and / or may be contaminated by skeletal muscle myopotentials and / or environmental EMI. Undersensing of R- waves or fibrillation waves may result in an undetected tachyarrhythmia when ATP or CV / DF therapy may be needed. Oversensing of P-waves, T-waves, skeletal muscle myopotentials, EMI or other noise as false R-waves may lead to a false tachyarrhythmia detection resulting in unnecessary ATP or CV / DF shock delivery. In other instances, oversensing of cardiac signals (e.g., falsely sensing P-waves or T-waves as being R-waves) or non-cardiac noise (skeletal muscle myopotentials, EMI or other electrical noise) may result in withholding of pacing pulses when cardiac pacing is actually needed to prevent a long ventricular pause or asystole. Undersensing of R-waves or fibrillation waves may cause unneeded ventricular pacing pulse delivery that could confound VT / VF detection. Techniques disclosed herein provide improvements in detecting arrhythmia and controlling electrical stimulation therapy delivery by monitoring for cardiac event signal variability and generating a control parameter output for delaying a tachyarrhythmia therapy in the case of a detected tachyarrhythmia in the presence of cardiac event signal variability and / or for performing enhanced cardiac event signal sensing and / or tachyarrhythmia detection in the presence of cardiac event signal variability. The techniques disclosed herein provide specific improvements in medical devices provided for arrhythmia detection and therapy delivery.

[0047] FIG. 3 is a conceptual diagram illustrating another example of an IMD that may be configured to sense cardiac signals, monitor for variability of sensed cardiac event signals,and modify IMD operation when a variability condition is met according to the techniques disclosed herein. In this example, the IMD is shown as a pacemaker 114, implanted within the right atrium (RA) of a patient’s heart. In some examples, pacemaker 114 is a transcatheter, leadless pacemaker that can be implanted wholly within a heart chamber. Pacemaker 114 may be reduced in size compared to subcutaneously implanted pacemakers or ICDs and may be generally cylindrical in shape to facilitate transvenous implantation via a delivery catheter. For example, pacemaker housing 115 may have a generally cylindrical, longitudinal sidewall extending from a distal end 170 to a proximal end 172 of pacemaker 114. Distal end 170 is referred to as “distal” in that it is expected to be the leading end as pacemaker 114 is advanced through a delivery tool, such as a catheter, and placed against a targeted pacing site. In other examples, housing 115 may have a generally prismatic shape. The housing 115 encloses the electronics and a power supply for sensing cardiac signals, producing pacing pulses and controlling therapy delivery and other functions of pacemaker 114 as described herein.

[0048] Pacemaker 114 may be configured to sense atrial and ventricular event signals, e.g., P-waves attendant to atrial depolarizations and R- waves attendant to ventricular depolarizations. Pacemaker 114 may be configured as a dual chamber pacemaker capable of sensing both atrial and ventricular event signals and delivering atrial pacing pulses and ventricular pacing pulses as needed based on the sensed atrial and / or ventricular event signals. In other examples, pacemaker 114 may be configured as a single chamber pacemaker capable of delivering only atrial pacing pulses or capable of delivering only ventricular pacing pulses but may still be capable of dual chamber sensing of both atrial and ventricular event signals. In still other examples, pacemaker 114 may be configured to sense and pace a single heart chamber, atrial or ventricular.

[0049] Pacemaker 114 may be a leadless pacemaker that includes electrodes carried on the pacemaker housing without requiring medical electrical leads extending from pacemaker 114 for sensing cardiac electrical signals and delivering cardiac pacing pulses. Pacemaker 114 is shown including electrodes 162, 164 and 165, spaced apart along the housing 115 of pacemaker 114, for sensing cardiac electrical signals and delivering pacing pulses. In other examples, pacemaker 114 may include more than three electrodes as shown here, but at least three electrodes may be provided for sensing an atrial signal and a ventricular signal using two different sensing electrode vectors in some examples. Electrode 164 is shown asa tip electrode extending from distal end 170 of housing 115. Electrodes 162 and 165 are shown as ring electrodes along the lateral sidewall of housing 115. Electrodes 162 and 165 may be ring electrodes circumscribing the lateral sidewall, for example adjacent proximal end 172 and adjacent distal end 170, respectively.

[0001] In the example shown, pacemaker 114 is implanted in the RA for providing ventricular pacing from an atrial location. Pacemaker 114 may be configured for delivering ventricular pacing pulses via the heart’s native conduction system and / or ventricular myocardium from a RA approach. For example, the distal end of pacemaker 114 may be positioned at the inferior end of the interatrial septum, beneath the atrioventricular (AV) node and near the tricuspid valve annulus, generally in the Triangle of Koch, to position a tip electrode 164 for advancement into the interatrial septum toward the His bundle of the native His-Purkinje conduction system. Tip electrode 164 is shown as a screw-in helical electrode which may provide fixation of pacemaker 114 at an implant site as well as serving as a pacing and sensing electrode. Electrode 164 may be a tissuepiercing electrode that can be advanced from within the right atrial chamber to a ventricular pacing site, e.g., for delivering pacing to the His-Purkinje conduction system and / or for pacing of ventricular septal myocardial tissue.

[0050] A second electrode, e.g., a ring electrode 162 or ring electrode 165, may be spaced proximally from the tip electrode 164 for use with the tip electrode 164 for bipolar pacing of the right and left ventricles via the His-Purkinje system and / or ventricular myocardium. Ventricular pacing pulses delivered by pacemaker 114 may capture at least a portion of the His bundle and / or ventricular myocardium for delivering ventricular pacing to the ventricles, e.g., the right ventricle (RV) and / or left ventricle (LV), from an atrial implant location of pacemaker 114. The techniques disclosed herein are not necessarily limited to a particular implant location of pacemaker 114, however, and may be practiced in a pacemaker implanted in a variety of operative locations for providing cardiac signal sensing of atrial and / or ventricular events and, at least in some examples, delivering cardiac pacing to at least one heart chamber. Pacemaker 114 may receive a first cardiac electrical signal as an atrial signal via the distal ring electrode 165 paired with the proximal ring electrode 162. Pacemaker 114 may receive a second cardiac electrical signal as a ventricular signal via the distal tip electrode 164 paired with the proximal ring electrode 162.

[0051] Electrodes 162, 164 and 165 may be, without limitation, titanium, platinum, iridium or alloys thereof and may include a low polarizing coating, such as titanium nitride, iridium oxide, ruthenium oxide, platinum black, among others. Electrodes 162, 164 and 165 may be positioned at locations along pacemaker 114 other than the locations shown. Electrodes 162, 164, 165 may be provided as other types of electrodes, such as button, fishhook, hemispherical, segmented, and are not necessarily limited to being a combination of one helical screw in electrode and two ring electrodes as shown here.

[0052] Housing 115 is formed from a biocompatible material, such as a stainless steel or titanium alloy. In some examples, the housing 115 may include an insulating coating. Examples of insulating coatings include parylene, urethane, PEEK, or polyimide, among others. The entirety of the housing 115 may be insulated, but only electrodes 162, 164 and 165 are uninsulated. Tip electrode 164 may serve as a cathode electrode and can be coupled to internal circuitry, e.g., a pacing pulse generator and cardiac electrical signal sensing circuitry, enclosed by housing 115 via an electrical feedthrough crossing housing 115. Electrodes 162 and 165 may be formed as a conductive portion of housing 115 defining respective ring electrodes that are electrically isolated from each other and from the other portions of the housing 115 as generally shown in FIG. 3.

[0053] Pacemaker 114 may include features for facilitating deployment to and fixation at an implant site. For example, pacemaker 114 may optionally include a delivery tool interface 171 at the proximal end 172 of pacemaker 114 that is configured to connect to a delivery device, such as a catheter, used to position pacemaker 114 at an implant location during an implantation procedure. The delivery tool interface 171 may receive a tether, clamp or other delivery tool member to enable a clinician to advance, retract and steer pacemaker 114 to an implant site and rotate pacemaker 114 to advance the helical tip electrode 164 into the cardiac tissue. Helical tip electrode 164 in this example provides fixation of pacemaker 114 at the implant site. In other examples, however, pacemaker 114 may include a set of fixation tines or other fixation members to secure pacemaker 114 to cardiac tissue. Numerous types of active and / or passive fixation members may be employed for anchoring or stabilizing pacemaker 114 in an implant position. Pacemaker 114 may be capable of bidirectional wireless communication with an external device 40 for programming sensing and pacing control parameters, e.g., as generally described above in conjunction with FIG. 1A.

[0054] Pacemaker 114 may be configured to operate in one or more pacing modes, such as atrial synchronous ventricular pacing, dual chamber pacing, atrial single chamber pacing and / or atrial asynchronous ventricular single chamber pacing. Pacemaker 114 may be configured to detect one or more types of tachyarrhythmia, such as atrial fibrillation (AF), atrial flutter, sinus tachycardia and / or other SVT rhythms. Pacemaker 114 may sense cardiac event signals for detecting and discriminating SVT and VT / VF. When a detected tachyarrhythmia is determined to be an SVT, pacemaker 114 may deliver atrial ATP to terminate the SVT in some examples. When VT / VF is detected based on an analysis of the rate of sensed P-waves, sensed R-waves and / or patterns of sensed P-waves and sensed R-waves, for example, pacemaker 114 may be configured to withhold ATP delivery. In other examples, pacemaker 114 may be configured to deliver ATP to the ventricular chambers (e.g., using tip electrode 164 paired with ring electrode 162), e.g., when ICD 14 (of FIG.1A) is co-implanted with pacemaker 114 so that CV / DF shock capabilities are available if ATP does not terminate or the VT / VF.

[0055] FIG. 4 is a diagram of a medical device configured to sense cardiac electrical signals and deliver cardiac electrical stimulation according to some examples. The electronic circuitry enclosed within housing 15 (represented as an electrode in FIG. 4) 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. FIG. 4 is described with reference to ICD 14 of FIGs. 1A-2C for the sake of illustration. It is to be understood that the circuitry and functionality generally described in conjunction with FIG. 4 and other flow charts and diagrams presented herein with reference to ICD 14 may be adapted as needed for implementation in the pacemaker 114 shown in FIG. 3 or other cardiac signal sensing medical devices including cardiac monitors, pacemakers and ICDs.

[0056] ICD 14 may be coupled to a lead, such as lead 16 (shown in FIG. 1 A) carrying electrodes 24, 26, 28 and 30, for delivering electrical stimulation pulses to the patient’s heart and for sensing cardiac electrical signals. 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 mayinclude 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. 4 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, anal og-to-digi tai converters, switching circuitry, etc. as needed.

[0057] The circuits shown in FIG. 4 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 arrhythmia 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.

[0058] Control circuit 80 and sensing circuit 86 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 sensing circuit 86 and control circuit 80 to perform specific processing functions such as dedicated math operations, which may include any of sum, absolute value, difference, ratio, product, extrema, histogram 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 sensing cardiac event signals and detecting arrhythmia. These HSRs can unload the processing burden associated with firmwareand / or software processing to reduce current drain of power source 98 and thereby extend the useful life of ICD 14.

[0059] 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 determined primarily 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 modern medical device system, given the disclosure herein, is within the abilities of one of skill in the art.

[0060] 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, sensing circuit 86, therapy delivery circuit 84 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.

[0061] 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 and 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.

[0062] Cardiac electrical signal sensing circuit 86 (also referred to herein as “sensing circuit” 86) may be selectively coupled to electrodes 28 and 30 and / or housing 15 in order to monitor electrical activity of the patient’s heart. Sensing circuit 86 may additionally beselectively 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 and 30 and / or housing 15. Sensing circuit 86 may be enabled to receive cardiac electrical signals from one or more sensing electrode vectors selected from the available electrodes 24, 26, 28, 30 and housing 15 in some examples. In some cases, 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. Sensing circuit 86 may monitor one or more cardiac electrical signals for sensing cardiac event signals, e.g., P-waves attendant to intrinsic atrial myocardial depolarizations and / or R-waves attendant to intrinsic ventricular myocardial depolarizations.

[0063] In some examples, sensing circuit 86 may include multiple channels 83, 85 and 87 for receiving multiple cardiac electrical signals simultaneously for sensing cardiac event signals and / or passing a cardiac electrical signal to control circuit 80 for morphology analysis. Sensing circuit 86 may include switching circuitry for selecting which of electrodes 24, 26, 28, 30 and housing 15 are coupled to each channel 83, 85 and 87. In the example shown, sensing circuit 86 may include a first cardiac event signal sensing channel 83, a second cardiac event sensing channel 85 and a morphology channel 87. It is to be understood, however, that ICD 14 may include fewer channels or only one sensing channel in other examples. For instance, in some examples, ICD 14 may include only one cardiac event signal sensing channel 83 and a morphology channel 87.

[0064] The first and second cardiac event sensing channels 83 and 85, referred to hereafter as “sensing channels” 83 and 85, may be configured to sense cardiac event signals, e.g., R-waves or P-waves, in response to a cardiac event sensing threshold crossing by the respectively received cardiac electrical signal. Depending on the particular lead and electrode arrangement and the sensing electrode pair coupled to the respective sensing channel 83 or 85, the sensing channel 83 or 85 may be configured as an atrial sensing channel for sensing atrial P-waves or a ventricular sensing channel for sensing ventricular R-waves. In the case of ICD 14, sensing channels 83 and 85 may be configured for sensing R-waves from respective cardiac electrical signals and producing sensed ventricular event signals (“Vsense signals”) in response to R-wave sensing threshold crossings by the respective cardiac electrical signal.

[0065] Each sensing channel 83 and 85 may include a prefilter and amplifier circuit for receiving a cardiac electrical signal, an analog to digital convertor (ADC), rectifier, a bandpass filter, notch filter, or combination of low pass and / or high pass filters for providing a filtered, amplified and rectified signal to a cardiac event detector circuit. The cardiac event detector circuit may compare the incoming rectified, amplified and filtered signal to an R-wave sensing threshold. When the amplitude of the incoming signal crosses the R-wave sensing threshold, the cardiac event detector circuit may pass a Vsense signal to control circuit 80. In other examples, a slew rate detector or other method may be used for sensing R-waves from the received cardiac electrical signal by generally determining a cardiac signal feature and comparing the cardiac signal feature to a threshold or range expected for the cardiac signal feature if it is a true R-wave.

[0066] An R-wave sensing threshold applied to a filtered, amplified and rectified signal may be a fixed threshold, e.g., a programmed value or determined by control circuit 80 based on an average or median maximum peak amplitude of previously sensed R-waves. In other examples, the R-wave sensing threshold may be multi-level or decreasing threshold that may decrease from a starting value to a sensing floor. The starting value may be set to a percentage of the maximum peak amplitude of one or more most recently sensed R-waves. Sensing circuit 86 may include a peak track and hold circuit for detecting a maximum peak amplitude during a post-sense blanking period that follows an R-wave sensing threshold crossing that can be used for setting the next starting value of the R-wave sensing threshold. The sensing floor or minimum value may correspond to a programmed ventricular sensitivity. The R-wave sensing threshold may decrease from the starting value to the sensing floor according to one or more step decrements (which may occur at specified drop time intervals) and / or decay rates (over one or more decay intervals). The R-wave sensing thresholds used by R-wave detectors of sensing channels 83 and 85 may be controlled separately according to different R-wave sensing threshold control parameters. 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.

[0067] Morphology channel 87 may include a prefilter and amplifier for receiving a cardiac electrical signal via a selected sensing electrode vector. Morphology channel 87 may include an ADC, notch filter, bandpass filter and / or combinations of low pass and high pass filters for passing a relatively wideband-filtered, digital cardiac electrical signal to control circuit 80 for performing morphology analysis when needed, e.g., for verifying Vsense signals and / or performing arrhythmia detection algorithms by control circuit 80. In some examples, the digital cardiac electrical signal may be rectified by morphology channel 87 for passing a rectified signal to control circuit 80 for processing and analysis. In some cases, both a filtered, non-rectified signal and a rectified signal can be passed to control circuit 80 from morphology channel 87 for use in determining morphology features of the cardiac electrical signal.

[0068] Upon sensing a cardiac event signal based on a sensing threshold crossing, sensing circuit 86 may pass a cardiac sensed event signal, e.g., a Vsense signal, to control circuit 80. Vsense signals received from sensing circuit 86 by control circuit 80 can be used by control circuit 80 for determining sensed event intervals, e.g., RRIs. An RRI is the time interval between two Vsense signals consecutively received by control circuit 80 from the same sensing channel 83 or 85, which may also be referred to as an “in-channel” sensed event interval. In some instances, when a Vsense signal is received following a delivered pacing pulse, the RRI is determined from the pacing pulse to the Vsense signal. As such, cardiac event intervals may include time intervals from a pacing pulse delivered by therapy delivery circuit 84 to a cardiac sensed event signal received from sensing circuit 86. Control circuit 80 may include a timing circuit 90 for determining RRIs. Based on determined RRIs (and in some examples PPIs, RPIs and / or PRIs if sensing circuit 86 includes an atrial sensing channel for sensing P-waves), control circuit 80 may detect ventricular arrhythmias, e.g., bradycardia, asystole, SVT, VT, and / or VF.

[0069] Control circuit 80 may be configured to enable or power on morphology channel 87 to sense a cardiac electrical signal that can be passed to control circuit 80 as a multi-bit digitized electrocardiogram (ECG) signal (when sensed outside the heart or a cardiac electrogram (EGM) when sensed in the heart) to control circuit 80 for processing and analysis. Time segments of the morphology signal received from morphology channel 87 may be buffered in memory 82. In various examples, a relatively short time segment, e.g., 150 to 600 ms or about 180 to 500 ms in duration, may be buffered in response to aVsense signal received from at least one of sensing channels 83 or 85. A morphology analysis, e.g., R-wave morphology matching or R-wave feature analysis, may be performed for verifying a Vsense signal as being a true R-wave or, in some instances, identifying a likely VT / VF waveform (e.g., a non-sinus QRS waveform based on a low R-wave morphology matching score when compared to an R-wave template representative of an R-wave conducted from the atria during normal sinus rhythm).

[0070] In some examples, control circuit 80 may be configured to enable morphology channel 87 to sense and pass a morphology signal for buffering, processing and analyzing relatively longer cardiac signal segments, e.g., 0.5 to 5 second segments or 0.5 to 3 second segments as examples. The relatively longer cardiac signal segments may be obtained independent of the relative timing of Vsense or Asense signals (produced in response to a P-wave sensing threshold crossing by a sensed cardiac electrical signal) received from sensing circuit 85. The relatively longer cardiac signal segments may be analyzed for detecting asystole, for example, when no Asense or Vsense signals are being received. The relatively longer cardiac signal segments may be analyzed for detecting evidence of VT / VF, e.g., when fibrillation waves or low amplitude R-waves may be undersensed by sensing channels 83 and 85. Examples of methods for obtaining a time segment of a morphology signal and performing morphology analysis for detecting arrhythmia that may be implemented in conjunction with the techniques disclosed herein are generally disclosed in U.S. Patent Application Publication No. 2023 / 0148939A1 (Aranda Hernandez et al., filed October 7, 2022), the entire content of which is incorporated herein by reference.

[0071] In this example of three cardiac electrical signals received by sensing channels 83 and 85 and morphology channel 87, the three signals may be received from 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 sensing channel 83 and another signal passed to sensing channel 85. Either or both of the two signals may be passed to control circuit 80 as a multi-bit digital ECG (or EGM) signal used by control circuit 80 for morphology analysis for analysis of a predetermined time segment of the ECG signal for detecting arrhythmias.

[0072] Control circuit 80 may include a timing circuit 90 and an arrhythmia detection circuit 92. Timing circuit 90 may be configured to control various timers and / or counters used in setting various time intervals and windows used in sensing cardiac event signals, determining time intervals between received sensed cardiac event signals, e.g., RRIs between consecutively received Vsense signals, performing morphology analysis and controlling the timing of cardiac pacing pulses and CV / DF shocks generated by therapy delivery circuit 84. Timing circuit 90 may start various timers in response to receiving Vsense signals from sensing circuit 86 for timing cardiac event intervals, e.g., RRIs, between consecutively received Vsense signals. Timing circuit 90 may pass the cardiac event intervals to arrhythmia detection circuit 92 for detecting and counting tachyarrhythmia intervals.

[0073] Arrhythmia detection circuit 92 may be configured to analyze cardiac event intervals received from timing circuit 90 and, in some examples, a cardiac electrical signal (also referred to herein as the “morphology signal”) received from morphology channel 87 for detecting arrhythmia. Arrhythmia detection circuit 92 may be configured to detect a long ventricular pause, asystole, SVT, and / or VT / VF based on sensed cardiac electrical signals meeting respective detection criteria. For example, when a threshold number of Vsense signals from one sensing channel 83 or 85 each occur at a sensed event interval (RRI) that is less than a tachyarrhythmia detection interval, control circuit 80 may detect VT / VF. An RRI that is less than the tachyarrhythmia detection interval can be counted as a tachyarrhythmia interval. In some examples, a tachyarrhythmia detection based on the threshold number of tachyarrhythmia intervals being reached may be confirmed or rejected based on a morphology analysis of the morphology signal received from morphology channel 87. 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 arrhythmia according to a variety of arrhythmia detection algorithms.

[0074] In some examples, arrhythmia detection circuit 92 may include comparators and counters for counting cardiac sensed event intervals determined by timing circuit 90 that fall into various rate detection zones for determining a cardiac rate or performing other rate- or interval-based assessment of sensed cardiac event signals for detecting and discriminating SVT and / or VT / VF. For example, arrhythmia detection circuit 92 maycompare the RRIs determined by timing circuit 90 between Vsense signals to one or more tachyarrhythmia detection interval zones, such as a tachycardia detection interval zone and a fibrillation detection interval zone. 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, 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. 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. VT or VF may be detected when the respective VTI or VFI counter (or a combined VT / VF interval counter) reaches a threshold number of intervals to detect (NID).

[0075] 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 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 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 range from as low as 12 to as high as 120, with no limitation intended. As described below, the programmed value of the NID may be automatically adjustable, e.g., temporarily increased, by control circuit 80 in response to detecting a variability condition of sensed cardiac event signals to delay delivery of a tachyarrhythmia therapy by therapy delivery circuit by delaying detection of the tachyarrhythmia. A VTI counter or VFI counter may reach a respective programmed or temporarily increased NID when VTIs or VFIs are detected consecutively or non-consecutively out of a specified number of most recent RRIs. In some cases, a combined VT / VF interval counter may count both VTIs and VFIs, and control circuit 80may detect a tachyarrhythmia episode based on the fastest intervals detected when a specified NID is reached.

[0076] Arrhythmia detection circuit 92 may be further configured to detect atrial tachyarrhythmia (AT) intervals from PPIs received from timing circuit 90 when sensing circuit 80 is configured to sense P-waves from a received cardiac electrical signal. In this case, arrhythmia detection circuit 92 may count AT intervals for detecting an SVT. In some examples, an SVT, such as atrial fibrillation, may be detected by arrhythmia detection circuit 92 based on an analysis of RRI variability, PRIs, RPIs, ratio of atrial rate to ventricular rate or other rate or interval-based analysis. Detection of SVT by arrhythmia detection circuit 92 may enable control circuit 80 to discriminate between SVT and VT / VF for appropriate control of CV / DF shock delivery for treating potentially lifethreatening tachyarrhythmias.

[0077] Arrhythmia detection circuit 92 may be configured to perform other signal analysis for determining if other tachyarrhythmia detection criteria are satisfied before detecting VT / VF based on an NID being reached, such as R-wave morphology criteria, onset criteria, stability criteria and noise and oversensing rejection criteria. To support these additional analyses, sensing circuit 86 may pass a digitized morphology signal to control circuit 80 from morphology 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 channel 87 (and / or any of sensing channels 83 or 85) may be buffered in memory 82. Memory 82 may include one or more circulating buffers to temporarily store digital cardiac signal segments for analysis performed by control circuit 80. Control circuit 80 may be a microprocessor-based 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 signal waveforms, e.g., P-waves, T-waves and sinus or non-sinus R-waves.

[0078] As further described below, in some examples, control circuit 80 may perform a first tachyarrhythmia detection algorithm for detecting VT / VF when the cardiac event signal variability condition is not met. Performing the first tachyarrhythmia detection algorithm may include performing analysis of the morphology signal and RRIs fordetermining when a first set of tachyarrhythmia detection criteria are met. When control circuit 80 determines that sensed cardiac event signals meet the variability condition, however, control circuit 80 may enable a second, enhanced tachyarrhythmia detection algorithm for detecting VT / VF. Control circuit 80 may perform the enhanced tachyarrhythmia detection algorithm by adding or substituting morphology signal analysis that requires greater or more complex signal processing than the first tachyarrhythmia detection algorithm. The enhanced tachyarrhythmia detection algorithm may include requiring that a count of VT / VF intervals reaches the programmed or a temporarily increased NID in addition to enhanced morphology signal analysis compared to the morphology signal analysis that is performed according to the first tachyarrhythmia detection algorithm. The enhanced tachyarrhythmia detection algorithm may be enabled by control circuit 80 to confirm a tachyarrhythmia detection based on the programmed or temporarily increased NID being reached and may effectively delay a tachyarrhythmia therapy by requiring a longer time to detect the tachyarrhythmia than the first tachyarrhythmia detection algorithm. When control circuit 80 detects tachyarrhythmia based on the first tachyarrhythmia detection algorithm or based on the enhanced tachyarrhythmia detection algorithm, therapy delivery circuit 84 may deliver a tachyarrhythmia therapy, e.g., one or more sequences of ATP and / or CV / DF shock(s).

[0079] 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.

[0080] 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, which is passed to control circuit 80 via a charging control line. When the voltage reaches a predetermined value set by control circuit 80, a logic signal can be generated on a capacitor full line and passed to therapydelivery circuit 84, terminating charging. A CV / DF shock pulse can be 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. 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. 1 A) during ICD implant or followup testing procedures.

[0081] 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, atrial synchronous ventricular 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 voltage capacitor(s) to a relatively lower voltage corresponding to a cardiac pacing pulse amplitude for capturing and pacing the ventricular myocardium.

[0082] 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 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. Timing circuit 90 may include various timers or counters that control when cardiac pacing pulses are delivered, e.g., by timing out various pacing escape intervals. 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. Circuitry included in a therapy delivery circuit 84 for generating and delivering electrical stimulation pulses according to a therapy delivery protocol can vary between devices and is not limited to any particular therapy delivery circuitry configuration for use in conjunction with the techniques disclosed herein for detecting avariability condition of sensed cardiac event signals and generating a control parameter output to delay a tachyarrhythmia therapy, enable sensed cardiac event signal verification and / or enable an enhanced tachyarrhythmia detection algorithm when the variability condition is detected.

[0083] In some examples, ICD 14 may include one or more sensors 97 for sensing physiological signals from the patient. Sensor(s) 97 may include an accelerometer, a pressure sensor, a temperature sensor, an impedance sensor, an oxygen saturation sensor or the like. ICD 14 may include an accelerometer, for example, for sensing acceleration signals passed to control circuit 80 for determining patient physical activity metrics. In some examples, control circuit 80 may be configured to detect a variability condition of the sensed cardiac event signals based on criteria that is different when the patient is at rest than when the patient is physically active. In some examples, control circuit 80 may be configured to detect the variability condition when the patient is at rest as determined based on the acceleration signals. In other examples, control circuit 80 may be configured to detect the variability condition when the patient physical activity is greater than a threshold activity level as determined based on the acceleration signals (or another sensor signal that is correlated to patient physical activity). In some examples, the response to detecting the variability condition can be different when the patient is at rest than when a non-resting patient activity level is detected as further described below.

[0084] 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. 1 A) 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.

[0085] FIG. 5 is a flow chart 200 of a method that may be performed by a medical device for sensing cardiac event signals and monitoring for cardiac event signal variability according to some examples. The process of flow chart 200 is described in conjunction with the ICD 14 of FIG. 4 for the sake of illustration but may be performed by any of the example medical device systems described herein. With continued reference to ICD 14 of FIG. 4, at block 202 sensing circuit 86 senses cardiac event signals from a cardiacelectrical signal received by a sensing channel, e.g., sensing channel 83 and / or sensing channel 85. For the sake of illustration, the flow chart 200 of FIG. 5 and other flow charts presented herein refer to the cardiac event signal that is being sensed as an R-wave attendant to ventricular depolarization. In this case, sensing circuit 86 may produce a Vsense signal at block 202 each time the cardiac electrical signal received by a sensing channel 83 or 85 meets R-wave sensing criteria, e.g., crosses an R-wave sensing threshold amplitude applied to the cardiac electrical signal. In other examples, however, the cardiac event signals being sensed at block 202 could be P-waves that are sensed when the cardiac electrical signal, e.g., an atrial signal received by an atrial sensing channel, meets P-wave sensing criteria, e.g., crosses a P-wave sensing threshold amplitude.

[0086] For the sake of example, the cardiac event signals sensed at block 202 may be sensed by sensing circuit 86 in response to the amplitude of a rectified, filtered and amplified cardiac electrical signal crossing a sensing threshold amplitude. The sensing threshold amplitude, e.g., the R-wave sensing threshold amplitude, may be an autoadjusting threshold that is set to a starting value at the expiration of a post-sense (or postpace) blanking period. During the post-sense blanking period that may be started at the R-wave sensing threshold crossing, sensing circuit 86 may determine a maximum peak amplitude of the sensed cardiac event signal, e.g., by a peak-track-and-hold circuit of sensing circuit 86. The starting value of the R-wave sensing threshold at the expiration of the post-sense blanking period may be applied to the cardiac electrical signal by sensing circuit 86 as a percentage of the maximum peak amplitude determined during the postsense blanking period. The percentage of the maximum peak amplitude used by sensing circuit 86 for determining the starting value may be between 40% and 80% of the maximum peak amplitude or between 50% and 60% of the maximum peak amplitude, as examples.

[0087] As described above in conjunction with FIG. 4, the R-wave sensing threshold may decrease according to one or more decay rates and or step drops until a programmed ventricular sensitivity is reached. The programmed ventricular sensitivity is the lowest amplitude of the cardiac electrical signal that is sensed as cardiac event signal and is sometimes referred to as the “sensing floor.” When the R-wave peak amplitude is variable, the starting value of the R-wave sensing threshold is variable and can be as low as the programmed ventricular sensitivity if the R-waves (or fibrillation waves) become small,relatively low amplitude signals. During periods of low amplitude and / or high variability of the R-wave morphology, which may result in low or variable maximum peak amplitudes of sensed event signals, changes in the starting value of the R-wave sensing threshold applied to the cardiac electrical signal from beat to beat could lead to false Vsense signals being passed to control circuit 80, e.g., due to oversensing of P-waves, T-waves or non-cardiac noise sensed as false R-waves.

[0088] As such, control circuit 80 may monitor one or more sensed event signal features for determining when a variability condition is met by the sensed cardiac event signals. At block 204, control circuit 80 may determine at least one feature of sensed cardiac event signals. When R-waves are sensed based on the amplitude of the cardiac electrical signal crossing an R-wave sensing threshold (that is set based on the maximum peak amplitude of the most recent sensed cardiac event signal), control circuit 80 may monitor at least the maximum peak amplitudes of the sensed cardiac event signals. In this case, control circuit 80 may determine the cardiac event signal features at block 204 as the maximum peak amplitudes of sensed cardiac event signals received from sensing circuit 86. The maximum peak amplitudes may be received by control circuit 80 with each associated Vsense signal produced by sensing circuit 86. Control circuit 80 may buffer the maximum peak amplitudes in memory 82 to perform a cardiac event signal variability analysis.Maximum peak amplitudes (and / or other cardiac event signal features) may be buffered for a specified number of most recent Vsense signals, e.g., on a first-in-first-out basis, to enable analysis of the maximum peak amplitude for detecting a variability condition.

[0089] In addition or alternatively to determining and buffering maximum peak amplitudes for detecting a variability condition, control circuit 80 may determine a peak to peak amplitude, a signal width, a signal area, a maximum slope, signal width normalized by the maximum peak amplitude, signal area normalized by the maximum peak amplitude, and / or an overall morphology match score or correlation to a known true R-wave template that may be stored in memory 82. The morphology match score may be determined by determining differences between wavelet transform coefficients of the sensed cardiac electrical signal and the stored R-wave template, for example. In other examples, instead of (or in addition to) using a stored R-wave template, morphology match scores may be determined between consecutively sensed cardiac electrical signal waveforms or between each of a series of sensed cardiac electrical signal waveforms and a selected one of thesensed cardiac event signals. The morphology match scores may be determined as cardiac event signal features for detecting a change in cardiac event signal morphology, e.g., as evidenced by a decrease or relatively low match score between two or more of the sensed cardiac electrical signal waveforms.

[0090] At block 206, control circuit 80 may determine if the cardiac event signal features meet a variability condition. For example, control circuit 80 may determine if the maximum peak amplitude determined for the most recent Vsense signal is less than a threshold percentage of the most recent preceding Vsense signal maximum peak amplitude. In another example, control circuit 80 may determine a moving average, median or other representative value of the most recent preceding 2, 3, 5, 8 or other specified number of maximum peak amplitudes determined for corresponding Vsense signals. Control circuit 80 may compare the maximum peak amplitude determined for the most recent Vsense signal to a specified percentage of the representative value of the specified number of preceding maximum peak amplitudes. In some examples, control circuit 80 may classify Vsense signals as suspected true R-waves or suspected false R-waves based on cardiac event signal features and use only the suspected true R-waves for determining a representative value of the maximum peak amplitudes.

[0091] In other examples, control circuit 80 may apply Kalman filtering, Autoregressive Integrated Moving Average (ARIMA), or other statistical analysis algorithms to the sensed cardiac electrical signal sample point amplitudes to determine a representative amplitude value to be compared with the peak amplitude corresponding to the latest Vsense event for determining if a variability condition is met. These methods for determining the representative amplitude value can be applied to a time window comprising the x most recent peak amplitudes, where x is an integer number.

[0092] In some examples, control circuit 80 only detects a variability condition that represents a decrease in maximum peak amplitude relative to preceding maximum peak amplitudes. If the maximum peak amplitude increases compared to preceding maximum peak amplitudes, control circuit 80 may not detect the variability condition. However, in some examples, as further described below, an increase in maximum peak amplitude after detecting the variability condition representing a decrease in maximum peak amplitude may be detected by control circuit 80 for determining when the detected variability condition is unmet or no longer being detected so that any response to the detectedvariability condition can be reversed. In other examples, control circuit 80 may determine that a variability condition is met when the maximum peak amplitude determined for at least the most recent Vsense signal represents a threshold change, which can be a threshold increase or a threshold decrease in amplitude, compared to one or more most recent preceding maximum peak amplitudes.

[0093] In some examples, control circuit 80 may determine that the variability condition is met when the change in the cardiac event signal features persists for a specified number of cardiac cycles. For example, a single Vsense signal associated with a threshold percentage decrease in maximum peak amplitude may not be determined to meet the variability condition. However, when at least two, three or other specified number of maximum peak amplitudes, occurring consecutively or occurring within a specified number of consecutive cardiac cycles (e.g., 3 out 5, 4 out of 6, 5 out of 8, or other percentage of a specified number of consecutive cardiac cycles) are less than a representative maximum peak amplitude determined from preceding maximum peak amplitudes, control circuit 80 may detect a variability condition. A single decrease in maximum peak amplitude for one cardiac cycle may not be sufficient for determining that the variability condition is met.

[0094] In an illustrative example, a median (or other representative) maximum peak amplitude may be determined from the most recent preceding n maximum peak amplitudes where n may be between 3 and 30 as non-limiting examples. When the current ith maximum peak amplitude is a threshold percentage less than the representative maximum peak amplitude, control circuit 80 may count the ith maximum peak amplitude as a decreased amplitude. Without updating the median maximum peak amplitude (or other representative value) using the ith maximum peak amplitude that is counted as a decreased maximum peak amplitude, control circuit 80 may compare the next i+m maximum peak amplitudes to the representative, where m may be between 1 and 20 or between 2 and 10 as non-limiting examples. Each maximum peak amplitude associated with the ith and i+m Vsense signals that is less than the median maximum peak amplitude determined prior to the ith maximum peak amplitude is counted as a decreased amplitude by control circuit 80. When the count of decreased amplitudes reaches a threshold value, control circuit 80 may detect the variability condition. If the threshold value is not reached, or if a specified number of the i+m maximum peak amplitudes are within a percentage of the representative maximum peak amplitude, the decreased amplitudecounter may be reset and the representative maximum peak amplitude may be updated. The criteria applied by control circuit 80 for determining when the variability condition is met may be established based on individual patient need. For example, some patients might demonstrate significant R-wave amplitude variability during a respiration cycle, which may warrant detecting relatively short term fluctuations in R-wave morphology, and other patients may demonstrate R-wave amplitude variability primarily due to postural changes, which may warrant detecting relatively longer term, sustained R-wave morphology changes.

[0095] In other examples, control circuit 80 may determine consecutive absolute differences between maximum peak amplitudes for a specified number of consecutively received Vsense signals or between maximum peak amplitudes and representative peak amplitude value. The consecutive differences may be summed to determine a variability metric that may be compared to a threshold. When the sum of consecutive differences in the maximum peak amplitudes meets or exceeds the threshold, control circuit 80 may determine that the variability condition is met at block 206.

[0096] Additionally or alternatively, control circuit 80 may determine that an ith maximum peak amplitude is less than a threshold percentage of the i-1 maximum peak amplitude. Control circuit 80 may determine the differences between each of the next i+m (where m may be 1 to 10 in a non-limiting example) maximum peak amplitudes and the i-1 maximum peak amplitude. If the differences each meet a threshold and / or the sum of the differences meet a threshold, control circuit 80 may determine that the variability condition is met.

[0097] Additionally or alternatively, control circuit 80 may detect the variability condition when a threshold decrease in the peak amplitude followed by a return to a higher peak amplitude is detected repeatedly a threshold number of times within a specified time window, e.g., fluctuation between a low and high peak amplitude three or more times within 30 to 120 seconds. In this case, intermittent short intervals of decreased R-wave peak amplitude can be determined as meeting the variability condition by control circuit 80. In addition to or instead of comparing the maximum peak amplitudes associated with Vsense signals to the variability condition, any of the cardiac event signal features listed above may be compared to each other and / or a reference value for detecting when the cardiac event signal feature(s) represent a threshold change indicative of R-wavevariability to detect the onset of a change in R-wave morphology and / or a sustained or intermittent change in R-wave morphology satisfying the variability condition.

[0098] It is noted that, in each of the examples presented herein, control circuit 80 may be configured to determine that the variability condition is met when one or more other specified patient conditions are also met. For example, control circuit 80 may determine that the variability condition is met when the time of day is within a specified range and / or the patient activity level is within a specified range, greater than a specified activity threshold, or less than a specified activity threshold. For instance, the variability condition may be determined to be met by control circuit 80 when the cardiac event signal features meet the variability condition and the time of day is during daytime hours. During nighttime hours, the patient may be expected to be asleep, and, at least in some patients, variation of the cardiac event signal morphology may not be expected and / or the patient may not respond to a communication signal requesting approval to delay a therapy as further described below. In other patients, greater variability may be expected at night due to sleeping posture(s) such that the variability condition can be determined at night but not during daytime hours. In still other examples, control circuit 80 may be configured to detect the variability condition when the patient physical activity is determined to be a resting state. In still other examples, control circuit 80 may be configured to detect the variability condition when the patient physical activity is at least a specified threshold (non-resting). Criteria relating to the time of day and / or patient activity level, as examples, may be specified and stored in memory 92 for use by control circuit 80 for determining when the variability condition is met in addition to criteria applied to the cardiac event signal features. These other criteria applied to patient-related conditions such as time of day or patient physical activity are optional but, when used, may be tailored to the needs of a given patient.

[0099] In some examples, different criteria that are applied by control circuit 80 to the sensed cardiac event signal features can be stored in memory 82 for different patient-related conditions, such as the time of day and / or the patient activity level. For example, the criteria applied by control circuit 80 to the cardiac event signal features for detecting the variability condition during daytime hours may be different than the criteria applied during nighttime hours. Additionally or alternatively, the criteria applied by control circuit 80 to the cardiac event signal features for detecting the variability condition during aresting activity level may be different than the criteria applied during a non-resting activity level. Control circuit 80 may select from different sets of criteria stored in memory 82 corresponding to a detected patient-related condition (e.g., time or day and / or patient activity level) for determining if the variability condition is met

[0100] When the variability condition is not met (“no” branch of block 206), control circuit 80 may wait for the next Vsense signal from sensing circuit 86 and determine the next sensed cardiac event signal feature (blocks 202 and 204). When control circuit 80 determines that the cardiac event signal features meet the variability condition at block 206, control circuit 80 may generate a control signal output at block 208. The control signal output generated in response to detecting the variability condition of the cardiac event signal features may cause a delay of a tachyarrhythmia therapy, e.g., at least later than the time that the programmed NID is reached. The control signal output may cause the ICD telemetry circuit 88 to transmit a communication signal requesting a user confirmation or approval to delay therapy. The control signal output may increase the NID from the programmed value to a temporary higher value. The control signal output may cause the control circuit 80 to enable cardiac event signal verification for verifying that Vsense signals likely correspond to true R-waves. The control signal output may additionally or alternatively cause the control circuit 80 to perform an enhanced tachyarrhythmia detection algorithm that includes more complex or increased processing and analysis of cardiac electrical signal morphology.

[0101] At block 210, a response to the cardiac event signal variability condition is performed according to the control signal output, which may include modifying a tachyarrhythmia detection criteria, delaying a tachyarrhythmia detection and therapy, transmitting a communication signal, and / or adjusting a cardiac event signal sensing control parameter, as examples. Example responses that may be performed by control circuit 80 according to the control signal output generated by control circuit 80 when the variability condition is detected are further described below.

[0102] It is noted that, in each of the examples presented herein, control circuit 80 may generate the control signal output based on one or more patient-related conditions, such as the time of day, patient physical activity level, patient body temperature, an impedance signal, a pressure signal, an oxygen saturation signal, or other physiological signal sensed by sensor(s) 97. For example, control circuit 80 may transmit a communication signalrequesting approval for a therapy delay when the time of day is daytime but may start a therapy delay timer without transmitting a communication signal when the time of day is nighttime. Control circuit 80 may modify tachyarrhythmia detection criteria (e.g., increase the NID) or enable an enhanced tachyarrhythmia detection algorithm when the patient activity level is greater than a specified threshold, in another illustrative example. Control circuit 80 may generate one control signal output during a first patient-related condition and a different control signal output during a second patient related condition that is different than the first patient related condition. In this way, the response performed by ICD 14 at block 210 when the variability condition is met can be different depending on one or more other conditions that can be monitored by control circuit 80, e.g., time of day based on an internal clock of control circuit 80 and / or a physiological condition determined by control circuit 80 based on one or more physiological signals received from sensors 97.

[0103] FIG. 6 is a flow chart 300 of a method that may be performed by a medical device for detecting cardiac event signal variability and performing a response to the cardiac event signal variability detection for delaying tachyarrhythmia therapy according to some examples. At block 302, sensing circuit 86 senses cardiac event signals, e.g., ventricular event signals, in response to sensing threshold crossings, e.g., R-wave sensing threshold crossings, by a sensed cardiac electrical signal. Sensing circuit 86 may determine the maximum peak amplitude of the sensed cardiac signal at block 304. Control circuit 80 receives the maximum peak amplitude that follows a Vsense signal, produced by sensing circuit 86 when the cardiac electrical signal crosses the R-wave sensing threshold. The maximum peak amplitude received from sensing circuit 86 is determined by control circuit 80 as the sensed cardiac event signal feature at block 304 in this example. The maximum peak amplitude may be buffered in memory 82 by control circuit 80. Other examples of cardiac event signal features that could be determined in addition to or instead of the maximum peak amplitude are listed above. It is noted that the cardiac event signal feature(s) determined for a given Vsense signal can be a waveform morphology feature that is determined from one sensed cardiac event signal waveform, not between two sensed cardiac event signals such as the time interval, e.g., an RRI, between two different sensed cardiac event signals. As such, in some examples, each cardiac event signal feature that is determined for detecting a variability condition is determined from a single sensedcardiac event signal waveform. It is noted that, depending on the cardiac event signal feature being determined, the cardiac event signal feature may be determined from the same cardiac electrical signal that crosses the R-wave sensing threshold amplitude or a different cardiac electrical signal sensed by sensing circuit 86, e.g., the morphology signal sensed by morphology channel 87.

[0104] At block 306, control circuit 80 may determine if tachyarrhythmia is being detected. Control circuit 80 may determine if tachyarrhythmia detection criteria are met based on at least a programmed NID being reached by sensed event intervals, e.g., RRIs determined between consecutively received Vsense signals. A variety of tachyarrhythmia detection algorithms may be performed by ICD 14 for detecting tachyarrhythmia, which may include cardiac signal morphology analysis and can include applying one or more rejection rules that analyze cardiac signal features, sensed event intervals, and / or waveform morphology of a morphology signal time segment to detect evidence of oversensing, supraventricular tachycardia, cardiac electrical signal noise or evidence of a shockable rhythm. Control circuit 80 may determine that tachyarrhythmia detection criteria are met based on a first tachyarrhythmia detection algorithm, which may include a programmed NID that is reached by a VTI counter, VFI counter or combined VT / VF interval counter. If the tachyarrhythmia detection criteria are not met at block 306 according to the first tachyarrhythmia detection algorithm, the process may return to block 302 and continue sensing cardiac event signals without determining whether or not the cardiac event signal features meet the variability condition.

[0105] If control circuit 80 determines that the tachyarrhythmia detection criteria are met at block 306, control circuit 80 may determine if the sensed cardiac event signal features meet a variability condition according to any of the examples described above in conjunction with FIG. 5. If not, control circuit 80 may advance to block 314 to deliver tachyarrhythmia therapy in response to detecting tachyarrhythmia according to the tachyarrhythmia detection criteria being met at block 306, e.g., according to a first tachyarrhythmia detection algorithm. ATP and / or a CV / DF shock may be delivered at block 314 to terminate the detected tachyarrhythmia.

[0106] However, if the cardiac event signal variability condition is met at block 308, control circuit 80 may generate a control signal output at block 310 to cause a delay in the tachyarrhythmia therapy delivered by therapy delivery circuit 84 that would otherwise bedelivered without delay at block 314 in response to the tachyarrhythmia detection criteria being met (at block 306).

[0107] In response to the variability condition being met, control circuit 80 may generate the control signal output at block 310 as a communication signal and control telemetry circuit 88 to transmit the communication signal. The communication signal may be a therapy delivery request signal that can be received by an external device, e.g., external device 50 shown in FIG. 1, which may be a handheld device such as a mobile phone, tablet or other personal electronic device. The patient or another caregiver may respond to the communication signal received by external device 50 by transmitting a response signal. The response signal may be an “approve therapy” (or “deliver therapy now”) signal to approve immediate therapy delivery by therapy delivery circuit 84 or a “disapprove therapy” (or “delay therapy”) signal to approve continued delay of the therapy delivery. When an “approve therapy” (or “deliver therapy now”) signal is received by telemetry circuit 88, control circuit 80 may confirm the tachyarrhythmia detection at block 310 and cancel any further delay of the therapy delivery. Therapy delivery circuit 84 may deliver the tachyarrhythmia therapy at block 314.

[0108] In some instances, however, the patient may not be experiencing symptoms or may be in a position or engaged in an activity (e.g., driving) that is unsafe for receiving a CV / DF shock. Control circuit 80 may receive a “disapprove therapy” (or “delay therapy”) signal via telemetry circuit 88 that allows the continued delay in delivering the tachyarrhythmia therapy. In this case, when telemetry circuit 88 receives the “disapprove therapy” signal, control circuit 80 may not confirm the tachyarrhythmia detection at block 312 an continue to delay the tachyarrhythmia therapy delivery. Control circuit 80 may advance to block 316 to determine if a therapy delay termination condition is met, as further described below.

[0109] Additionally or alternatively, at block 310, control circuit 80 may generate the control signal output to delay tachyarrhythmia therapy by starting a timer of control circuit 80 to time out a therapy delay time interval. When the cardiac event signal variability condition is met (“yes” branch of block 308), control circuit 80 may delay tachyarrhythmia therapy delivery for up to a specified time interval, e.g., 10 to 30 seconds, to allow more time for the variability in the cardiac event signal morphology to subside and / or for further cardiac signal sensing and analysis to be performed for determining ifthe tachyarrhythmia detection criteria are still met after the delay time interval. In some examples, control circuit 80 may control telemetry circuit 88 to transmit the communication signal to wait for an “approve therapy” or disapprove therapy” signal as described above and concomitantly start a therapy delay time interval.

[0110] At block 316, control circuit 80 may determine that a therapy delay termination condition is met when the delay time interval expires. If control circuit 80 determines that the tachyarrhythmia detection criteria are still being met at the expiration of the delay time interval, e.g., based on the same, non-modified cardiac event signal sensing performed at block 302 and the same, non-modified tachyarrhythmia detection criteria (e.g., a first tachyarrhythmia detection algorithm) as applied at block 306, control circuit 80 may advance to block 314 to control therapy delivery circuit 84 to deliver tachyarrhythmia therapy. In other examples, as further described below in conjunction with FIGs. 7 and 8, control circuit 80 may modify cardiac event signal sensing and / or tachyarrhythmia detection criteria in response to detecting the variability condition, which may be in addition to or instead of the techniques described herein in conjunction with FIG. 6 for delaying the tachyarrhythmia therapy.[oni] In some instances, control circuit 80 may determine that a therapy delay termination condition is detected at block 316 because a communication signal is received via telemetry circuit 88, e.g., transmitted by an external device by a patient or other user. The received communication signal may be an “approve therapy” signal transmitted in reply to the communication signal transmitted by telemetry circuit 88 at block 310. The signal received from the external device may cancel a previously received “disapprove therapy” or “delay therapy” signal transmitted by the patient or another user, e.g., if the patient subsequently becomes symptomatic or has assumed a safe position for receiving a shock therapy. If tachyarrhythmia detection criteria are still met when the “approve therapy” signal is received, control circuit 80 can confirm tachyarrhythmia detection and control therapy delivery circuit 84 to deliver the tachyarrhythmia therapy at block 314, which may be according to a programmed menu of tachyarrhythmia therapies.

[0112] When a communication signal has been transmitted by telemetry circuit 88 and a reply signal is not received by telemetry circuit 88 before the therapy delay time interval expires, the patient may be symptomatic or unable to respond. Control circuit 80 may determine that the therapy delay termination condition is met at block 316 if thetachyarrhythmia detection criteria are still being met, no reply signal has been received and the therapy delay time interval is expired. Therapy delivery circuit 84 may deliver the delayed tachyarrhythmia therapy at block 314.

[0113] At any time during the delay time interval (or upon expiration of the delay time interval), control circuit 80 may determine that the tachyarrhythmia detection criteria are no longer met. For example, the NID may become unmet during the delay time interval or other tachyarrhythmia detection criteria may be become unmet (e.g., a VT / VF rejection rule may become met). If control circuit 80 determines that the therapy delay termination condition is not met (“no” branch of block 316) and the tachyarrhythmia detection criteria are no longer satisfied (“no” branch of block 306), control circuit 80 may return to block 302 to continue cardiac event signal sensing (by sensing circuit 86), monitoring for tachyarrhythmia and monitoring for cardiac event signal variability.

[0114] FIG. 7 is a flow chart 400 of a method that may be performed by a medical device for detecting cardiac event signal variability and performing a response to the cardiac event signal variability detection by modifying a cardiac event signal sensing control parameter and / or tachyarrhythmia detection criteria. With continued reference to ICD 14 of FIG. 4, at block 402, sensing circuit 86 senses cardiac event signals in response to sensing threshold crossings, e.g., R-wave sensing threshold crossings, by a sensed cardiac electrical signal. Sensing circuit 86 may determine the maximum peak amplitude of the sensed cardiac event signal at block 404. Control circuit 80 receives the maximum peak amplitude for a Vsense signal received from sensing circuit 86. The received maximum peak amplitude can be determined by control circuit 80 as the sensed cardiac event signal feature in this example, which may be buffered in memory 82 by control circuit 80 for determining if a variability condition is met.

[0115] When sensing circuit 86 includes two sensing channels 83 and 85 (as shown in FIG. 4), each sensing channel 83 and 85 may pass Vsense signals and associated maximum peak amplitudes to control circuit 80. The maximum peak amplitudes may be buffered for each sensing channel 83 and 85 in memory 82. In other examples, one sensing channel may be selected from the two available sensing channels 83 and 85 as the default or nominal sensing channel that is used for sensing cardiac event signals until a variability condition is detected.

[0116] At block 406, control circuit 80 may determine if a triggering event is detected. The triggering event causes control circuit 80 to check if the variability condition is met. Control circuit 80 may determine that a triggering event is detected by determining that a threshold number of short (fast) cardiac event intervals are detected, e.g., a threshold numbers of VT / VF intervals determined as RRIs between consecutively received Vsense signals. In the example of FIG. 6 described above, control circuit 80 may determine if cardiac event signal features meet the variability condition when tachyarrhythmia detection criteria are met and respond accordingly. In other examples, however, control circuit 80 may determine if the cardiac event signal variability condition is met when at least a threshold number, less than the NID, of short cardiac event intervals are detected as a triggering event. For example, control circuit 80 may determine when a count of RRIs that are less than a specified short interval threshold reaches a threshold value. The short interval threshold can be a VT or VF interval threshold, an SVT threshold, a VT or VF interval threshold plus an offset, or another specified short interval threshold that corresponds to a relatively fast ventricular rate. When the short interval threshold is a VT or VF interval, control circuit 80 may determine when a VTI counter or VFI counter (or combined VT / VF interval counter) reaches a threshold number that is less than the NID at block 406. Control circuit 80 may determine that the threshold number of fast cardiac event intervals is reached at block 406 when 3, 5, 8, 10, 12, 15, 20 or other specified number of fast RRIs are counted. In an example, when a VTI or VFI counter reaches a specified percentage of the NID, e.g., 10%, 15%, 20%, 30%, or 50% as non-limiting examples, control circuit 80 may determine that the threshold number of fast cardiac event intervals has been reached at block 406. If the result at decision block 406 is negative, the process may return to block 402.

[0117] When control circuit 80 determines that the threshold number of fast intervals is reached at block 406, this can be a triggering event for control circuit 80 to determine if cardiac event signal features buffered in memory 82 and / or subsequently determined cardiac event signal features meet the variability condition at block 408. In other examples, control circuit 80 may be configured to detect a long RRI as a triggering event for determining if a variability condition is met. A long RRI, e.g., greater than a long pause threshold between two consecutive Vsense signals may be detected as a triggering event for analysis of the variability condition because one or more low amplitude R-wavesmarking the onset of a drop in R-wave amplitude may be undersensed by sensing circuit 86. The long pause threshold may be a specified value, e.g., 1 to 3 seconds, a multiple of a VTI or VFI, or a multiple of a recent RRI as examples.

[0118] After detecting the triggering event, control circuit 80 determines if the cardiac event signal features meet the variability condition. When the variability condition is not met as determined at block 408, the process may advance to block 412 to determine if tachyarrhythmia detection criteria are met. If tachyarrhythmia is not detected at block 412, and a sensing control parameter or a tachyarrhythmia detection control parameter has not been previously modified (“no” branch of block 415), as further described below, the process may return to block 402. If tachyarrhythmia detection criteria are met at block 412, therapy delivery circuit 84 may deliver tachyarrhythmia therapy at block 414. After the therapy delivery, the process can return to block 402.

[0119] If the variability condition is met at block 408, according to any of the examples described above in conjunction with FIG. 5, control circuit 80 may generate a control signal output at block 410 that modifies cardiac event signal sensing and / or tachyarrhythmia detection criteria. The control signal output may cause control circuit 80 to adjust a sensing control parameter, enable cardiac event signal verification, and / or adjust a tachyarrhythmia detection control parameter or enable an enhanced tachyarrhythmia detection algorithm. In response to at least a threshold number of fast intervals and cardiac event signal variability, for example, control circuit 80 may generate the control signal output at block 410 that modifies cardiac event signal sensing and / or tachyarrhythmia detection criteria, which may have the effect of delaying tachyarrhythmia therapy when the modification results in longer tachyarrhythmia detection times than when unmodified cardiac event signal sensing and / or unmodified tachyarrhythmia detection is performed.

[0120] In some examples, cardiac event signal sensing is modified at block 410 according to the control signal output generated by control circuit 80. Cardiac event signal sensing may be modified by switching the sensing channel 83 or 85 that is being used by sensing circuit 86 for sensing cardiac event signals. For instance, when control circuit 80 is determining RRIs from Vsense signals received from sensing channel 83, and the variability condition is detected for cardiac event signal features associated with Vsense signals from sensing channel 83, the control signal output may cause control circuit 80 touse Vsense signals received from the other sensing channel 85 for determining RRIs for tachyarrhythmia detection. In some cases, control circuit 80 may determine if the variability condition is met for the cardiac event signal features determined for Vsense signals received from each of sensing channel 83 and sensing channel 85. When the variability condition is detected for one sensing channel but not the other, the other sensing channel may be selected for as the sensing channel from which Vsense signals are used for determining RRIs.

[0121] In other examples, when the variability condition is detected, control circuit 80 may enable sensed cardiac event signal verification by analyzing one or more cardiac event signal features corresponding to each Vsense signal received from a given sensing channel 83 or 85 for verifying a Vsense signal as a true R-wave. The maximum peak amplitudes and RRIs, for example, may be compared to detect alternating peak amplitudes and patterns of short and long RRIs that may indicate cardiac event oversensing, e.g., P-wave oversensing or T-wave oversensing, so that suspected true R-waves can be identified and used in determining RRIs. Other cardiac signal features, e.g., signal width, signal area, maximum slope, overall signal waveform morphology, etc., may be used in addition to or alternatively to the maximum peak amplitude and / or RRIs for verifying a Vsense signal as being a true R-wave. In other examples, the primary frequency component of sensed event signals may be determined by control circuit 80 and used in discriminating Vsense signals corresponding to true R-waves from P-waves, T-waves, or non-cardiac noise signals.

[0122] In some examples, control circuit 80 may generate a control signal output at block 410 to enable the morphology channel 87 to pass cardiac signal segments that each encompass the time of a single one Vsense signal from a given sensing channel 83 or 85 (or one Vsense signal from both sensing channels 83 and 85 that occur near each other in time). The cardiac signal segment may be received from morphology channel 87 to enable a morphology analysis of the signal waveform in the morphology signal coinciding in time with the Vsense signal. A waveform morphology matching score may be determined between the morphology signal segment and an R-wave template, for example, to discriminate between true R-waves and other waveforms that may be falsely sensed as R-waves due to variability in the true QRS waveform morphology. A waveform morphology matching score, e.g., determined by control circuit 80 using wavelet transform, correlation or other waveform matching methods, can be compared to a match threshold when an R-wave sensing threshold crossing is detected so that true R-waves can be verified and used by control circuit 80 in determining RRIs.

[0123] Additionally or alternatively, control circuit 80 may generate a control signal output at block 410 that changes the sensing electrode vector used by sensing circuit 86 for sensing cardiac event signals. A different sensing electrode vector may be available for sensing a cardiac electrical signal having R-waves that are of higher amplitude and / or less variable than the sensing electrode vector that resulted in the variability condition being met. When a different sensing electrode vector is available, control circuit 80 may select a different sensing electrode vector that is switchably coupled to a sensing channel of sensing circuit 86, for instance.

[0124] In other examples, when two sensing channels 83 and 85 are available, a second sensing channel may be enabled or turned on in response to the control signal output generated at block 410. A second sensing channel of sensing circuit 86, for example sensing channel 85, may be powered off until the variability condition is detected at block 408. Control circuit 80 may power on the second sensing channel 85 when the variability condition is met at block 408 based on cardiac event signal features corresponding to Vsense signals from the first sensing channel 83. Control circuit 80 may use both sensing channels 83 and 85 for verifying true R-waves when Vsense signals are received from both ventricular sensing channels 83 and 85 within a threshold time interval of each other. If a Vsense signal is received from only one sensing channel 83 or 85 but not the other sensing channel 85 or 83 within a threshold time interval, other cardiac signal feature analysis may be performed for verifying the single Vsense signal received from the single sensing channel 83 or 85. The verified R-waves may be used by control circuit 80 for determining RRIs used for tachyarrhythmia detection.

[0125] In addition to or alternatively to enabling cardiac event signal verification as described above, control circuit 80 may generate the output control signal at block 410 in response to the variability condition being met to modify the tachyarrhythmia detection algorithm. Control circuit 80 may enable an enhanced tachyarrhythmia detection algorithm. For example, the criteria for detecting VT / VF may be modified. Control circuit 80 may increase the NID required for detecting VT / VF from a programmed NID to a temporary higher NID. The temporary higher NID may result in longer tachyarrhythmiadetection times, effectively delaying a tachyarrhythmia therapy compared to when the non-modified tachyarrhythmia detection algorithm is being used.

[0126] Control circuit 80 may enable an enhanced tachyarrhythmia detection algorithm at block 410 by enabling a more complex or sophisticated cardiac signal morphology analysis. The enhanced tachyarrhythmia detection algorithm may include turning “on” application of one or more VT / VF rejection rules that are not used if the variability condition is not detected. If met, a VT / VF rejection rule may cause withholding of a VT / VF detection for at least one cardiac cycle so that tachyarrhythmia detection criteria are met when the NID is met an no VT / VF rejection rules are satisfied.

[0127] VT / VF rejection rules may include various criteria applied to the features or morphology of a cardiac electrical signal segment that indicate the presence of electrical noise, cardiac event signal oversensing, and / or SVT such that a tachyarrhythmia detection and VT / VF therapy should be delayed or withheld at least until the VT / VF rejection rule is not met. A VT / VF rejection rule and associated methods for detecting non-cardiac noise and rejecting a VT / VF detection based on evidence of non-cardiac noise that may be enabled in an enhanced tachyarrhythmia detection algorithm in the presently disclosed techniques is generally described in U.S. Patent No. 10,470,681 (Greenhut, et al.) and in U.S. Patent No. 10,561,332 (Zhang, et al.), both of which patents are incorporated herein by reference in their entirety. A VT / VF rejection rule and associated methods for detecting T-wave oversensing and rejecting a VT / VF detection based on evidence of T-wave oversensing that may be enabled in an enhanced tachyarrhythmia detection algorithm in the presently disclosed techniques is generally described in U.S. Patent No. 10, 850,113 (Cao, et al.), incorporated herein by reference in its entirety, and in the above-incorporated U.S. Patent No. 10,470,681 (Greenhut, et al.). A VT / VF rejection rule and associated methods for detecting P-wave oversensing and rejecting a VT / VF detection based on evidence of P-wave oversensing that may be enabled in an enhanced tachyarrhythmia detection algorithm in the presently disclosed techniques is generally described in U.S. Patent No. 11,931,585 (Mischler, et al.). A VT / VF rejection rule and associated methods for detecting evidence of an SVT and rejecting a VT / VF detection based on the evidence of the SVT that may be enabled in an enhanced tachyarrhythmia detection algorithm in the presently disclosed techniques is generally described in U.S. Patent No. 10,555,684 (Zhang, et al.), incorporated herein by reference in its entirety.

[0128] Applying a VT / VF rejection rule may require determination of morphology signal features from morphology signal segments received from morphology channel 87 in some examples. For instance, morphology signal segments may be acquired that encompass a single one sensed cardiac signal waveform for determining if the morphology signal segment includes a waveform that matches a sinus R-wave template or has features that are evidence of a VT / VF beat that is considered to be a shockable or treatable heart rhythm. Additionally or alternatively, control circuit 80 may enable buffering of n-second morphology signal segments that begin and end independent of the timing of Vsense signals received from the sensing channels 83 and / or 85. Control circuit 80 may perform morphology signal analysis of the n-second segments to determine if evidence of tachycardia or fibrillation waves are present in the signal, indicating a potentially shockable or treatable heart rhythm (e.g., VT or VF). It is contemplated that numerous tachyarrhythmia detection algorithm modifications or enhancements may be conceived that may increase the number of cardiac cycles evaluated and / or the cardiac signal analysis that is performed for reliably detecting VT / VF in the presence of R-wave morphology variability.

[0129] If control circuit 80 detects tachyarrhythmia at block 412 according to the modified cardiac event signal sensing (with sensed cardiac event signal verification enabled) and / or modified tachyarrhythmia detection criteria (e.g., enhanced tachyarrhythmia detection algorithm which may include an increased NID), control circuit 80 may advance to block 314 to control therapy delivery circuit 84 to deliver tachyarrhythmia therapy. If control circuit 80 does not detect tachyarrhythmia at block 412 and the modified cardiac event signal sensing and / or modified tachyarrhythmia detection algorithm is / are in effect (“yes” branch of block 415), control circuit 80 may determine if the variability condition is no longer met at block 416.

[0130] In some examples, the modified cardiac event signal sensing and / or modified tachyarrhythmia detection criteria may be applied for up to a specified time interval. When control circuit 80 does not detect tachyarrhythmia at block 412 before the expiration of the specified time interval, control circuit 80 may determine that the variability condition is unmet at block 416 in response to the expiration of the specified time interval and no tachyarrhythmia detection. The specified time interval may correspond to the therapy delay time interval that can be started upon detecting the variability condition in someexamples, as described above in conjunction with FIG. 6. Control circuit 80 may reverse the control signal output generated in response to detecting the variability condition to restore the non-modified cardiac event signal sensing and / or tachyarrhythmia detection criteria and algorithm at block 418 when the variability condition is unmet. The process may return to block 402 to continue sensing cardiac event signal sensing according to nonmodified sensing control parameters and monitoring for fast cardiac event intervals (or another triggering event) at block 406 and cardiac event signal variability (block 408).

[0131] Additionally or alternatively, if tachyarrhythmia is not yet detected at block 412, control circuit 80 may evaluate sensed cardiac event signal features for determining if the variability condition is unmet at block 416. This process may occur in parallel with tachyarrhythmia detection processing and analysis and may be combined with the tachyarrhythmia detection algorithm when cardiac event signal features being used for the cardiac event signal sensing verification and or enhanced tachyarrhythmia detection algorithm can also be used for detecting a return of relatively higher amplitude and / or nonvariable QRS morphology waveforms.

[0132] Control circuit 80 may determine that the variability condition is unmet at block 416 if the maximum peak amplitudes associated with Vsense signals are increased to be consistently within a specified percentage of previous maximum peak amplitudes determined prior to the variability condition being met. As an illustrative example, control circuit 80 may store a representative maximum peak amplitude of cardiac event signals sensed prior to the variability condition being met. The representative maximum peak amplitude may be the most recent maximum peak amplitude determined before a threshold decrease in maximum peak amplitude is detected and may be the recent preceding or representative maximum peak amplitude that subsequent maximum peak amplitude(s) were compared to for detecting the variability condition. For instance, as described above, the representative maximum peak amplitude may be an average or median maximum peak amplitude of a specified number (e.g., 3 to 12) maximum peak amplitudes determined before the variability condition became met. When at least 5, 8, 12, 16, 20 or other specified number of maximum peak amplitudes are determined to be within a threshold percentage of the pre-variability condition representative maximum peak amplitude, control circuit 80 may determine that the variability condition is unmet at block 416.

[0133] If the decision at block 416 is negative (“no” branch), sensing circuit 86 and control circuit 80 may continue to operate according to the modified sensing and / or tachyarrhythmia detection control parameters for detecting tachyarrhythmia at block 412. If the decision at block 416 is affirmative (“yes” branch), control circuit 80 may restore the cardiac event signal sensing and / or tachyarrhythmia detection control parameters to the non-modified settings. For instance, sensed cardiac event signal verification using a second sensing channel, analysis of morphology signal segments, and / or comparative analysis of sensed cardiac event signal features can be disabled at block 418. An enhanced tachyarrhythmia algorithm that may include an increased NID, enabled VT / VF rejection rules, and / or increased morphology signal analysis may be disabled at block 418 with the programmed NID and non-modified tachyarrhythmia detection algorithm being restored. Generally, any modifications or adjustments made at block 410 in response to the variability condition being met may be reversed at block 418. The process may return to block 402. In some patients, the QRS waveform may be consistently variable such that if the sensed cardiac event signal verification and / or enhanced tachyarrhythmia detection algorithm are enabled a threshold number of time in response to the variability condition being detected, at least some aspects of the modified cardiac event signal sensing and / or modified tachyarrhythmia detection may remain permanently enabled in some examples.

[0134] FIG. 8 is a flow chart 500 of a method that may be performed by a medical device for controlling tachyarrhythmia therapy and detection when cardiac event signal variability is detected according to some examples. At block 502, control circuit 80 receives a Vsense signal from sensing circuit 86. As described above, control circuit 80 may receive Vsense signals from one or more sensing channels 83 and 85 of sensing circuit 86. At block 504, control circuit 80 may buffer at least one sensed cardiac event signal feature associated with the Vsense signal and an RRI determined from the Vsense signal and the most recent preceding ventricular event, e.g., pacing pulse or in-channel Vsense signal. Control circuit 80 may determine the signal feature(s) from a cardiac electrical signal received from sensing circuit 86 (e.g., from the sensing channel that passed the Vsense signal or from the morphology channel) and / or determine the signal feature of the cardiac event signal as a feature value received from sensing circuit 86, e.g., the maximum peak amplitude determined by sensing circuit 86 following an R-wave sensing threshold crossing during a post-sense blanking period.

[0135] At block 506, control circuit 80 may determine if a threshold number of VT / VF intervals is reached by a VTI counter, VFI counter or combined VTI / VFI counter. The threshold number may be a specified value that can be compared to the value of the VTI counter (if VT detection is enabled), the VF interval counter, and / or the combined VT / VF interval counter (if used). In some examples, the threshold number of VT / VF intervals is the NID required to detect VT (if VT detection is enabled) or the NID required to detect VF, which may be different than the NID required to detect VT. In some cases, the threshold number that is reached at block 506 can be an NID applied to a combined VT / VF interval counter. In other examples, the threshold number of VT / VF intervals is a number that is less than a programmed NID. In some examples, control circuit 80 may apply a first threshold number to the VTI count and a second threshold number different than the first threshold number to the VFI count. In some examples, a third threshold number may be applied to a combined VT / VF interval count. The threshold number applied to a VTI counter, VFI counter or a combined VT / VF interval counter may be a percentage of the corresponding programmed NID applied to the respective counter for detecting VT / VF.

[0136] If a threshold number of VT / VF intervals is not detected, control circuit 80 may continue to receive Vsense signals (block 502) and buffer corresponding cardiac event signal features and RRIs at block 504. When a threshold number of VT / VF intervals is detected by control circuit 80 at block 506, control circuit 80 may determine if the buffered cardiac event signal features meet a variability condition at block 508, according to any of the examples given above, e.g., in conjunction with FIG. 5. If the variability condition is not met, control circuit 80 may advance to block 514 to determine if tachyarrhythmia is detected, e.g., by determining if the NID is reached in combination with any other VT / VF detection criteria according to a first tachyarrhythmia detection algorithm. Therapy delivery circuit 84 may deliver tachyarrhythmia therapy at block 516 in response to the a tachyarrhythmia detection made by control circuit 80.

[0137] When control circuit 80 determines that the variability condition is met (block 508) and the threshold number of VT / VF intervals is detected (block 506), control circuit 80 may generate a control signal output at block 510 that causes a tachyarrhythmia therapy delay. The control signal output may be a signal to start a delay timer that may be set to a specified maximum therapy delay time interval, e.g., 15 to 120 seconds or 20 to 60seconds or about 30 seconds as examples. The control signal output may be a communication signal that is transmitted by telemetry circuit 88 requesting a response from the patient or another caregiver to confirm that a therapy is needed or confirming a therapy delay (e.g., indicating that the patient is either not symptomatic or in an unsafe position for shock delivery and that the therapy can be delayed).

[0138] At block 512, control circuit 80 may additionally or alternatively generate a control signal output that modifies the cardiac event signal sensing and / or tachyarrhythmia detection algorithm according to any of the examples described above in conjunction with FIG. 7. At block 514, control circuit 80 may detect a tachyarrhythmia when detection criteria are met according to the modified detection algorithm and any tachyarrhythmia therapy delay has expired or been manually terminated (e.g., based on a communication reply signal received by telemetry circuit 88). A non-response to a communication signal transmitted at block 510, meaning no reply signal is received back after the communication signal is transmitted, can be determined by control circuit 80 as a confirmation of the tachyarrhythmia at block 514 when the NID is reached and any other tachyarrhythmia detection criteria are met.

[0139] Therapy delivery circuit 84 can deliver tachyarrhythmia therapy (e.g., ATP and / or CV / DF shock) at block 516 in response to the control circuit 80 detecting the tachyarrhythmia, according to any modifications in the cardiac event signal sensing and / or tachyarrhythmia detection algorithm and / or after any tachyarrhythmia therapy delay is determined to be terminated according to a termination condition, e.g., as described above in conjunction with FIG. 6.

[0140] After the therapy is delivered at bock 516, any modifications made in response to a control signal output generated by control circuit 80 when the variability condition is detected may be reversed. For instance, sensed cardiac event signal verification, if enabled, may be disabled. An enhanced tachyarrhythmia detection algorithm, if enabled, may be disabled.

[0141] If tachyarrhythmia is not detected at block 514, control circuit 80 may return to block 502. In some examples, any modifications to cardiac event signal sensing and / or the tachyarrhythmia detection algorithm may be terminated when a therapy delay time interval expires or is terminated manually (e.g., in response to a user transmitted communication signal received by telemetry circuit 88). Control circuit 80 may return to block 502 andresume storing signal features and RRIs (block 504) in response to received Vsense signals (block 502). The tachyarrhythmia therapy delay and modified detection algorithm may be re-enabled if the threshold number of VT / VF intervals is reached again (or is still being reached) and the variability condition is detected again at block 508.

[0142] In other examples, if tachyarrhythmia is not detected at block 514 and modifications to cardiac event signal sensing and / or the tachyarrhythmia detection algorithm have been made at block 510, the modifications may remain in effect until the variability condition becomes unmet and / or the VTI count, VFI count or a combined VT / VF interval count falls below a threshold number (which may be the same or different than the threshold number applied at block 506).

[0143] Further disclosed herein is the subject matter of the following examples:

[0144] Example 1. A medical device including a therapy delivery circuit configured to deliver a tachyarrhythmia therapy and a sensing circuit configured to receive at least a first cardiac electrical signal and sense cardiac event signals from the first cardiac electrical signal. The medical device may include a control circuit configured to determine cardiac event signal features corresponding to the sensed cardiac event signals, detect a variability condition of the cardiac event signal features and generate a control signal output for causing a delay in delivery of the tachyarrhythmia therapy by the therapy delivery circuit in response to detecting the signal variability condition.

[0145] Example 2. The medical device of example 1 wherein the control circuit is further configured to determine cardiac event intervals between consecutively sensed cardiac event signals, detect a threshold number of cardiac event intervals that are tachyarrhythmia intervals and, in response to detecting the threshold number of cardiac event intervals that are tachyarrhythmia intervals, analyze the cardiac event signal features for detecting the variability condition.

[0146] Example 3. The medical device of example 2 wherein the control circuit is further configured to detect the threshold number of tachyarrhythmia intervals as a number of tachyarrhythmia intervals required to detect tachyarrhythmia.

[0147] Example 4. The medical device of any one of examples 1 — 3 further comprising a communication circuit configured to transmit a communication signal. The control circuit may be further configured to detect a tachyarrhythmia based on at least the sensed cardiac event signals and, in response to detecting the tachyarrhythmia and detecting thevariability condition, generate the control parameter output by generating a therapy delay request signal for transmission as the communication signal by the communication circuit. The communication circuit may be further configured to receive a user reply signal for confirming delaying delivery of the tachyarrhythmia therapy.

[0148] Example 5. The medical device of example 4 wherein the control circuit is further configured to determine that the user reply signal is not received before a maximum reply time expires and terminate the delay of the delivery of the tachyarrhythmia therapy in response to not receiving the user reply signal. The therapy delivery circuit is further configured to deliver the delayed tachyarrhythmia therapy in response to the control circuit terminating the delay.

[0149] Example 6. The medical device of any one of examples 1 — 5 wherein the control circuit is further configured to generate the control parameter output by starting a delay time interval for delaying delivery of the tachyarrhythmia therapy.

[0150] Example 7. The medical device of example 6 wherein the control circuit is further configured to detect a tachyarrhythmia based on at least the sensed cardiac event signals and detect an expiration of the delay time interval. The therapy delivery circuit is further configured to deliver the delayed tachyarrhythmia therapy in response to the tachyarrhythmia being detected and the delay time interval being expired.

[0151] Example 8. The medical device of any one of examples 1 — 7 wherein the control circuit is further configured to determine sensed cardiac event intervals from consecutively sensed cardiac event signals, determine that a threshold number of tachyarrhythmia intervals required to detect tachyarrhythmia is reached by the sensed cardiac event intervals, generate the control parameter output for delaying delivery of the tachyarrhythmia therapy by increasing the number of tachyarrhythmia intervals required to detect tachyarrhythmia and determine that that the increased number of tachyarrhythmia intervals is reached by the sensed cardiac event intervals. The therapy delivery circuit being further configured to deliver the delayed tachyarrhythmia therapy in response to at least the increased number of tachyarrhythmia intervals being reached.

[0152] Example 9. The medical device system of any one of examples 1 — 8 wherein the control circuit is further configured to determine sensed cardiac event intervals from consecutively sensed cardiac event signals, determine that a threshold number of tachyarrhythmia intervals required to detect tachyarrhythmia is reached by the sensedcardiac event interval and generate the control parameter output for delaying delivery of the tachyarrhythmia therapy by enabling an enhanced tachyarrhythmia detection algorithm. The control circuit may be configured to detect tachyarrhythmia according to the enhanced tachyarrhythmia detection algorithm. The therapy delivery circuit being further configured to deliver the delayed tachyarrhythmia therapy in response to the control circuit detecting the tachyarrhythmia according to the enhanced tachyarrhythmia detection algorithm.

[0153] Example 10. The medical device system of example 9 wherein the control circuit is further configured to enable the enhanced tachyarrhythmia detection algorithm by enabling a cardiac signal morphology analysis for determining if a tachyarrhythmia detection rejection rule is met.

[0154] Example 11. The medical device of any one of examples 1 — 10 wherein the control circuit is further configured to enable sensed cardiac event signal verification in response to detecting the variability condition.

[0155] Example 12. The medical device of example 11 wherein the control circuit is further configured to enable sensed cardiac event signal verification by enabling determining a primary frequency component of sensed cardiac event signals.

[0156] Example 13. The medical device of any one of examples 11 — 12 wherein the sensing circuit is further configured to sense a second cardiac electrical signal in response to the control circuit enabling sensed cardiac event signal verification.

[0157] Example 14. The medical device of any one of examples 1 — 13 wherein the sensing circuit is further configured to determine the cardiac event signal features as maximum peak amplitudes of the sensed cardiac event signals, and the control circuit is further configured to detect the variability condition by at least detecting a threshold decrease in the maximum peak amplitudes of the sensed cardiac event signals.

[0158] Example 15. The medical device of example 14 wherein the control circuit is further configured to detect the variability condition by detecting the threshold decrease in the maximum peak amplitudes for at least a specified number of sensed cardiac event signals.

[0159] Example 16. The medical device of any one of examples 1 — 15 wherein the control circuit is further configured to determine that the variability condition of cardiac event signal features of cardiac event signals sensed by the sensing circuit is unmet and, inresponse to determining that the variability condition is unmet, reverse the control signal output that causes the delay in delivery of the tachyarrhythmia therapy by the therapy delivery circuit.

[0160] Example 17. A method including receiving at least a first cardiac electrical signal, sensing cardiac event signals from the first cardiac electrical signal, determining cardiac event signal features corresponding to the sensed cardiac event signals and detecting a variability condition of the cardiac event signal features. The method may include generating a control parameter output for delaying delivery of a tachyarrhythmia therapy in response to detecting the variability condition.

[0161] Example 18. The method of example 17 further comprising determining cardiac event intervals between consecutively sensed cardiac event signals, detecting a threshold number of cardiac event intervals that are tachyarrhythmia intervals and, in response to detecting the threshold number of cardiac event intervals that are tachyarrhythmia intervals, analyzing the cardiac event signal features for detecting the variability condition.

[0162] Example 19. The method of example 18 further comprising detecting the threshold number of tachyarrhythmia intervals as a number of tachyarrhythmia intervals required to detect tachyarrhythmia.

[0163] Example 20. The method of any one of examples 17 — 19 further comprising detecting a tachyarrhythmia based on at least the sensed cardiac event signals and, in response to detecting the tachyarrhythmia and detecting the variability condition, generating the control parameter output by generating a therapy delay request signal for transmission as a communication signal. The method may further include confirming delaying delivery of the tachyarrhythmia therapy when a user reply signal to the transmitted therapy delay request signal is received.

[0164] Example 21. The method of example 20 further comprising determining that the user reply signal is not received before a maximum reply time expires, terminating the delay of the delivery of the tachyarrhythmia therapy in response to not receiving the user reply signal and delivering the delayed tachyarrhythmia therapy in response to terminating the delay.

[0165] Example 22. The method of any one of examples 17 — 21 further comprising generating the control parameter output by starting a delay time interval for delaying delivery of the tachyarrhythmia therapy.

[0166] Example 23. The method of example 22 further comprising detecting a tachyarrhythmia based on at least the sensed cardiac event signals, detecting an expiration of the delay time interval and delivering the delayed tachyarrhythmia therapy in response to the tachyarrhythmia being detected and the delay time interval being expired.

[0167] Example 24. The method of any one of examples 17 — 23 further comprising determining sensed cardiac event intervals from consecutively sensed cardiac event signals, determining that a threshold number of tachyarrhythmia intervals required to detect tachyarrhythmia is reached by the sensed cardiac event intervals and generating the control parameter output for delaying delivery of the tachyarrhythmia therapy by increasing the number of tachyarrhythmia intervals required to detect tachyarrhythmia. The method may further include determining that that the increased number of tachyarrhythmia intervals is reached by the sensed cardiac event intervals and delivering the delayed tachyarrhythmia therapy in response to at least the increased number of tachyarrhythmia intervals being reached.

[0168] Example 25. The method of any one of examples 17 — 24 further comprising determining sensed cardiac event intervals from consecutively sensed cardiac event signals, determining that a threshold number of tachyarrhythmia intervals required to detect tachyarrhythmia is reached by the sensed cardiac event intervals and generating the control parameter output for delaying delivery of the tachyarrhythmia therapy by enabling an enhanced tachyarrhythmia detection algorithm. The method may further include detecting tachyarrhythmia according to the enhanced tachyarrhythmia detection algorithm and delivering the delayed tachyarrhythmia therapy in response to detecting the tachyarrhythmia according to the enhanced tachyarrhythmia detection algorithm.

[0169] Example 26. The method of example 25 further comprising enabling the enhanced tachyarrhythmia detection algorithm by enabling cardiac signal morphology analysis for determining if a tachyarrhythmia detection rejection rule is met.

[0170] Example 27. The method of any one of examples 17 — 26 further comprising enabling sensed cardiac event signal verification in response to detecting the variability condition.

[0171] Example 28. The method of example 27 further comprising enabling the sensed cardiac event signal verification by enabling determining a primary frequency component of sensed cardiac event signals.

[0172] Example 29. The method of any one of examples 27 — 28 further comprising sensing a second cardiac electrical signal in response to sensed cardiac event signal verification being enabled.

[0173] Example 30. The method of any one of examples 17 — 29 further comprising determining the cardiac event signal features as maximum peak amplitudes of the sensed cardiac event signals and detecting the variability condition by detecting at least a threshold decrease in the maximum peak amplitudes of the sensed cardiac event signals.

[0174] Example 31. The method of example 30 further comprising detecting the variability condition by detecting the threshold decrease in the maximum peak amplitudes for at least a specified number of sensed cardiac event signals.

[0175] Example 32. The method of any one of examples 17 — 31 further comprising determining that the variability condition of cardiac event signal features of sensed cardiac event signals is unmet and, in response to determining that the variability condition is unmet, reversing the control signal output that causes the delay in delivery of the tachyarrhythmia therapy.

[0176] 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 receive a cardiac electrical signal, sense cardiac event signals from the cardiac electrical signal, determine cardiac event signal features corresponding to the sensed cardiac event signals and detect a variability condition of the cardiac event signal features. The instructions may further cause the medical device to generate a control parameter output for delaying delivery of a tachyarrhythmia therapy in response to detecting the variability condition.

[0177] 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.

[0178] 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).

[0179] 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.

[0180] 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

1. WHAT IS CLAIMED IS:

1. A medical device comprising:3.a therapy delivery circuit configured to deliver a tachyarrhythmia therapy;4.a sensing circuit configured to:5.receive at least a first cardiac electrical signal; and6.sense cardiac event signals from the first cardiac electrical signal; and a control circuit configured to:7.determine cardiac event signal features corresponding to the sensed cardiac event signals;8.detect a variability condition of the cardiac event signal features; and generate a control signal output for causing a delay in delivery of the tachyarrhythmia therapy by the therapy delivery circuit in response to detecting the signal variability condition.

2. The medical device of claim 1 wherein the control circuit is further configured to:10.determine cardiac event intervals between consecutively sensed cardiac event signals;11.detect a threshold number of cardiac event intervals that are tachyarrhythmia intervals; and12.in response to detecting the threshold number of cardiac event intervals that are tachyarrhythmia intervals, analyze the cardiac event signal features for detecting the variability condition.

3. The medical device of any one of claims 1 — 2 further comprising a communication circuit configured to transmit a communication signal; and wherein14.the control circuit is further configured to:15.detect a tachyarrhythmia based on at least the sensed cardiac event signals; and16.in response to detecting the tachyarrhythmia and detecting the variability condition, generate the control parameter output by generating a therapy delay request signal for transmission as the communication signal by the communication circuit; and the communication circuit being further configured to receive a user reply signal for confirming delaying delivery of the tachyarrhythmia therapy.

4. The medical device of claim 3 wherein:18.the control circuit is further configured to:19.determine that the user reply signal is not received before a maximum reply time expires; and20.terminate the delay of the delivery of the tachyarrhythmia therapy in response to not receiving the user reply signal; and21.the therapy delivery circuit is further configured to deliver the delayed tachyarrhythmia therapy in response to the control circuit terminating the delay.

5. The medical device of any one of claims 1 — 4 wherein the control circuit is further configured to generate the control parameter output by starting a delay time interval for delaying delivery of the tachyarrhythmia therapy.

6. The medical device of claim 5 wherein:24.the control circuit is further configured to:25.detect a tachyarrhythmia based on at least the sensed cardiac event signals; and26.detect an expiration of the delay time interval; and27.the therapy delivery circuit is further configured to deliver the delayed tachyarrhythmia therapy in response to the tachyarrhythmia being detected and the delay time interval being expired.

7. The medical device of any one of claims 1 — 6 wherein the control circuit is further configured to:29.determine sensed cardiac event intervals from consecutively sensed cardiac event signals;30.determine that a threshold number of tachyarrhythmia intervals required to detect tachyarrhythmia is reached by the sensed cardiac event intervals; generate the control parameter output for delaying delivery of the tachyarrhythmia therapy by increasing the number of tachyarrhythmia intervals required to detect tachyarrhythmia;31.determine that that the increased number of tachyarrhythmia intervals is reached by the sensed cardiac event intervals; and32.the therapy delivery circuit being further configured to deliver the delayed tachyarrhythmia therapy in response to at least the increased number of tachyarrhythmia intervals being reached.

8. The medical device system of any one of claims 1 — 7 wherein the control circuit is further configured to:34.generate the control parameter output for delaying delivery of the tachyarrhythmia therapy by enabling an enhanced tachyarrhythmia detection algorithm;35.detect tachyarrhythmia according to the enhanced tachyarrhythmia detection algorithm; and36.the therapy delivery circuit being further configured to deliver the delayed tachyarrhythmia therapy in response to the control circuit detecting the tachyarrhythmia according to the enhanced tachyarrhythmia detection algorithm.

9. The medical device system of claim 8 wherein the control circuit is further configured to enable the enhanced tachyarrhythmia detection algorithm by enabling a cardiac signal morphology analysis for determining if a tachyarrhythmia detection rejection rule is met.

10. The medical device of any one of claims 1 — 9 wherein the control circuit is further configured to enable sensed cardiac event signal verification in response to detecting the variability condition.

11. The medical device of claim 10 wherein the control circuit is further configured to enable sensed cardiac event signal verification by enabling determining a primary frequency component of sensed cardiac event signals.

12. The medical device of any one of claims 10 — 11 wherein the sensing circuit is further configured to sense a second cardiac electrical signal in response to the control circuit enabling sensed cardiac event signal verification.

13. The medical device of any one of claims 1 — 12 wherein:41.the sensing circuit is further configured to determine the cardiac event signal features as maximum peak amplitudes of the sensed cardiac event signals; and42.the control circuit is further configured to detect the variability condition by at least detecting a threshold decrease in the maximum peak amplitudes of the sensed cardiac event signals.

14. The medical device of any one of claims 1 — 13 wherein the control circuit is further configured to:44.determine a patient-related condition as one of a first patient-related condition or a second patient-related condition that is different than the first patient-related condition, the patient-related condition being determined by determining at least one of a time of day or a patient physical activity level; and45.based on the determined patient-related condition, generate the control signal output by generating one of a first control signal output in response to determining the first patient-related condition or a second control signal output that is different than the first control signal output in response to determining the second patient-related condition.

15. The medical device of any one of claims 1 — 14 wherein the control circuit is further configured to:47.determine that the variability condition of cardiac event signal features of cardiac event signals sensed by the sensing circuit is unmet; and48.in response to determining that the variability condition is unmet, reverse the control signal output that causes the delay in delivery of the tachyarrhythmia therapy by the therapy delivery circuit.

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