Systems and methods for sensing and detection in an implantable cardioverter defibrillator for extracardiac use

Abstract

The present disclosure relates to systems and methods for sensing and detection in an extra - cardiac implantable cardioverter - defibrillator. An extra - cardiac implantable cardioverter - defibrillator senses an R - wave from a first cardiac electrical signal through a first sensing channel and stores a time segment of a second cardiac electrical signal in response to each sensed R - wave. The ICD determines the intervals between successive sensed R - waves and, in response to at least a predetermined number of the intervals being less than a tachyarrhythmia detection interval, analyzes at least a portion of the time segment of the second cardiac electrical signal corresponding to the most recent R - wave among the sensed R - waves to confirm the most recent R - wave among the R - waves. The ICD updates an unconfirmed beat count in response to the most recent R - wave among the R - waves not being confirmed and stops detecting a tachyarrhythmia episode in response to the unconfirmed beat count being equal to or greater than a suppression threshold.

Description

[0001] This application is a divisional application of the invention patent application entitled "Systems and Methods for Sensing and Detection in an Implantable Cardioverter Defibrillator External to the Heart", with an international filing date of March 29, 2017, an international application number of PCT / US2017 / 024668, and a national stage entry application number in China of 201780026417.7. Technical Field

[0002] The present disclosure generally relates to an implantable cardioverter defibrillator (ICD) and method for sensing cardiac electrical signals and detecting tachyarrhythmias using extracardiac electrodes. Background Art

[0003] Medical devices such as cardiac pacemakers and ICDs provide therapeutic electrical stimulation to a patient's heart through electrodes carried by one or more medical electrical leads and / or electrodes on the housing of the medical device. The electrical stimulation can include signals such as pacing pulses, or cardioversion shocks or defibrillation shocks. In some cases, the medical device can sense cardiac electrical signals associated with the heart's intrinsic depolarization or pacing-induced depolarization, and control the delivery of stimulation signals to the heart based on sensing the cardiac electrical signals. Upon detection of an abnormal rhythm such as bradycardia, tachycardia, or fibrillation, one or more appropriate electrical stimulation signals can be delivered to restore or maintain a more normal rhythm of the heart. For example, an ICD can deliver pacing pulses to a patient's heart upon detection of bradycardia or tachycardia, or deliver cardioversion shocks or defibrillation shocks to the heart upon detection of tachycardia or fibrillation. An ICD can sense cardiac electrical signals in a cardiac chamber and use electrodes carried by transvenous medical electrical leads to deliver electrotherapy treatment to the cardiac chamber. Cardiac signals sensed within the heart typically have high signal strength and quality for reliably sensing cardiac electrical events such as R waves. In other examples, non-transvenous leads can be coupled to an ICD, in which case cardiac signal sensing presents new challenges for accurately sensing cardiac electrical events. Summary of the Invention

[0004] In general, the present disclosure relates to techniques for sensing cardiac electrical signals via an ICD to detect the onset of tachyarrhythmia. An ICD operating in accordance with the techniques disclosed herein senses R waves from a first cardiac electrical signal and triggers the storage of time segments of a second cardiac electrical signal in response to each sensed R wave. When a sensed R wave is determined to occur at an interval less than a tachyarrhythmia detection interval, the ICD analyzes the stored time segments of the second cardiac electrical signal to confirm the corresponding sensed R wave that triggered the storage of these time segments. If a required number of tachyarrhythmia intervals for detecting the onset of tachyarrhythmia are reached, but a threshold number of the sensed R waves are not confirmed based on the analysis of the time segments of the second cardiac electrical signal, the detection of tachyarrhythmia may be stopped.

[0005] In one example, the present disclosure provides an extracardiac ICD including a sensing circuit, a memory, and a control circuit. The sensing circuit has a first sensing channel and a second sensing channel. The first sensing channel is configured to receive a first cardiac electrical signal via an extracardiac sensing electrode vector coupled to the extracardiac ICD and sense an R wave in response to the first cardiac electrical signal crossing an R wave sensing threshold. The second sensing channel is configured to receive a second cardiac electrical signal via a second extracardiac sensing electrode vector coupled to the extracardiac ICD and different from the first extracardiac sensing electrode vector. The control circuit is coupled to the sensing circuit and the memory and is configured to: store time segments of the second cardiac electrical signal in the memory in response to each of the R waves sensed by the first sensing channel; determine the intervals between consecutive R waves sensed by the first sensing channel; and in response to at least a first predetermined number of the intervals being less than a tachyarrhythmia detection interval, analyze at least a portion of the time segment of the second cardiac electrical signal corresponding to the most recent R wave sensed by the first sensing channel to confirm the most recent R wave among the R waves. The control circuit updates an unconfirmed beat count in response to the most recent R wave among the R waves not being confirmed based on the analysis of at least the portion of the corresponding time segment; and compares the unconfirmed beat count with an inhibition threshold in response to a second predetermined number of the intervals being less than a tachyarrhythmia detection interval. The control circuit stops detecting the onset of tachyarrhythmia in response to the unconfirmed beat count being equal to or greater than the inhibition threshold.

[0006] In another example, the present disclosure provides a method performed by an extracardiac ICD, the method comprising: sensing an R wave by a first sensing channel of the extracardiac ICD in response to a first cardiac electrical signal crossing an R wave sensing threshold, the first cardiac electrical signal being received by the first sensing channel via a first extracardiac sensing electrode vector coupled to the extracardiac ICD; storing time segments of a second cardiac electrical signal in response to each of the R waves sensed by the first sensing channel, the second cardiac electrical signal being received by a second sensing channel of the extracardiac ICD via a second extracardiac sensing electrode vector; and determining, by a control circuit of the extracardiac ICD, an interval between consecutive R waves sensed by the first sensing channel. In response to at least a first predetermined number of the intervals being less than a tachyarrhythmia detection interval, the method further comprises: analyzing at least a portion of the time segment of the second cardiac electrical signal corresponding to the most recent R wave among the R waves sensed by the first sensing channel to confirm the most recent R wave among the R waves; and updating an unconfirmed beat count in response to the most recent R wave among the R waves not being confirmed based on at least the portion of the corresponding time segment. In response to a second predetermined number of the intervals being less than the tachyarrhythmia detection interval, the method comprises: comparing the unconfirmed beat count with a suppression threshold; and stopping detection of a tachyarrhythmia episode in response to the unconfirmed beat count being equal to or greater than the suppression threshold.

[0007] In another example, the present disclosure provides a non-transitory computer-readable storage medium including an instruction set that, when executed by a processor of an extracardiac ICD, causes the extracardiac ICD to: sense an R wave via a first sensing channel of a sensing circuit of the extracardiac ICD in response to a first cardiac electrical signal crossing an R wave sensing threshold, the first cardiac electrical signal being received via the first extracardiac sensing electrode vector coupled to the ICD through the first sensing channel; store in a memory of the extracardiac ICD a time segment of a second cardiac electrical signal that is received via a second sensing electrode vector by a second sensing channel of the ICD for each of the R waves sensed by the first sensing channel; determine an interval between consecutive R waves sensed by the first sensing channel; in response to at least a first predetermined number of intervals among the intervals being less than a tachyarrhythmia detection interval, analyze at least a portion of the time segment of the second cardiac electrical signal corresponding to the latest R wave among the R waves sensed by the first sensing channel to confirm the latest R wave among the R waves; in response to the latest R wave among the R waves not being confirmed based on the analysis of at least the portion of the corresponding time segment, update an unconfirmed beat count; in response to a second predetermined number of intervals among the intervals being less than the tachyarrhythmia detection interval, compare the unconfirmed beat count with a suppression threshold; and in response to the unconfirmed beat count being equal to or greater than the suppression threshold, stop detecting a tachyarrhythmia episode.

[0008] This Summary is intended to provide an overview of the subject matter described in this disclosure. This Summary is not intended to provide an exclusive or exhaustive explanation of the devices and methods described in the following drawings and specification. Further details of one or more examples are set forth in the following drawings and specification. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1A and 1B is a conceptual diagram of an extracardiac ICD system according to one example.

[0010] Figures 2A to 2C is a conceptual diagram of a patient with an extracardiac ICD system implanted with Figure 1A in different implantation configurations.

[0011] Figure 3 is a conceptual diagram of a distal portion of an extracardiac lead having an electrode configuration according to another example.

[0012] Figure 4 is a schematic diagram of an ICD according to one example of Figures 1A to 2C

[0013] Figure 5is according to an example of Figure 4 A diagram of the circuitry included in the sensing circuit of

[0014] Figure 6 is Figure 5 A graph of the attenuation characteristics of a notch filter that may be included in the sensing circuit of

[0015] Figure 7 is according to an example of a method performed by an Figures 1A to 2C ICD for sensing and validating R waves used in tachyarrhythmia detection.

[0016] Figure 8 is a diagram of a filtered cardiac electrical signal and an amplitude ratio threshold applicable to validating an R-wave sensing event.

[0017] Figure 9 is an example of an amplitude ratio threshold lookup table for validating an R-wave sensing event that may be stored in the memory of an Figures 1A to 2C ICD.

[0018] Figure 10 is a flowchart of a method for detecting tachyarrhythmia by an ICD according to an example.

[0019] Figure 11 is a flowchart of a method for detecting tachyarrhythmia by an ICD according to another example.

[0020] Figure 12 is according to another example of Figure 4 A diagram of the circuitry included in the sensing circuit of

[0021] Figure 13 is a flowchart of a method for detecting tachyarrhythmia by an ICD according to yet another example. DETAILED DESCRIPTION

[0022] In general, the present disclosure describes techniques for using extra-cardiovascular electrodes to sense cardiac electrical signals. As used herein, the term "extra-cardiovascular" refers to a location external to the blood vessels, the heart, and the pericardium surrounding a patient's heart. Implantable electrodes carried by extra-cardiovascular leads may be positioned outside the thorax (outside the chest cavity and sternum) or inside the thorax (below the chest cavity or sternum), but are generally not in close contact with myocardial tissue. The techniques disclosed herein provide a method for reliably sensing R waves associated with ventricular depolarization using extra-cardiovascular electrodes by obtaining two cardiac electrical signals using two sensing electrode vectors and validating an event sensed from the first cardiac electrical signal using the second cardiac electrical signal.

[0023] These techniques have been described in connection with an implantable medical lead carrying an extracardiac electrode, but aspects disclosed herein can be used in conjunction with other cardiac inductive sensing lead and electrode systems. For example, techniques for using a second cardiac electrical signal to confirm sensed R waves, as described in connection with the figures, can be implemented using any implantable or external medical device configured to sense cardiac electrical signals, including: implantable pacemakers, ICDs, or cardiac monitors coupled to transvenous or epicardial leads carrying sensing electrodes; leadless pacemakers, ICDs, or cardiac monitors having housing-based sensing electrodes; and external pacemakers, defibrillators, or cardiac monitors coupled to external electrodes, body surface electrodes, or skin electrodes.

[0024] Figure 1A and Figure 1B is a conceptual diagram of an extracardiac ICD system 10 according to one example. Figure 1A is a front view of the ICD system 10 implanted within a patient 12. Figure 1B is a side view of the ICD system 10 implanted within a patient 12. The ICD system 10 includes an ICD 14 connected to an extracardiac electrical stimulation and sensing lead 16. The description is in the context of an ICD system 10 capable of delivering defibrillation shocks and / or cardioversion shocks, as well as pacing pulses. Figure 1A and Figure 1B .

[0025] The ICD 14 includes a housing 15 that forms an airtight seal protecting the internal components of the ICD 14. The housing 15 of the ICD 14 can be formed of a conductive material such as titanium or a titanium alloy. The housing 15 can act as an electrode (sometimes referred to as a can electrode). The housing 15 can be used as an active can electrode when delivering cardioversion / defibrillation (CV / DF) shocks or other high-voltage pulses delivered using a high-voltage therapy circuit. In other examples, the housing 15 can be used to deliver unipolar low-voltage cardiac pacing pulses in combination with a lead-based cathode electrode and to sense cardiac electrical signals in combination with a lead-based electrode. In other instances, the housing 15 of the ICD 14 can include a plurality of electrodes on an external portion of the housing. The external portion(s) of the housing 15 that act as the electrode(s) can be coated with a material such as titanium nitride.

[0026] The ICD 14 includes a connector assembly 17 (also referred to as a connector block or header), the connector assembly including electrical feedthroughs that intersect the housing 15 to provide an electrical connection between conductors extending within the lead body 18 of the lead 16 and electronic components included within the housing 15 of the ICD 14. As will be described in more detail herein, the housing 15 may house one or more processors, memories, transceivers, electrical heart signal sensing circuitry, therapy delivery circuitry, a power source, and other components for sensing cardiac electrical signals, detecting cardiac rhythms, and controlling and delivering electrical stimulation pulses to treat abnormal cardiac rhythms.

[0027] The lead 16 includes an elongate lead body 18 having a proximal end 27 and a distal portion 25, the proximal end including a lead connector (not shown) configured to connect to the ICD connector assembly 17, and the distal portion including one or more electrodes. In Figure 1A and Figure 1B the example shown, the distal portion 25 of the lead 16 includes defibrillation electrodes 24 and 26 and pacing / sensing electrodes 28, 30, and 31. In some cases, the defibrillation electrodes 24 and 26 may together form a defibrillation electrode as they may be configured to be activated simultaneously. Alternatively, the defibrillation electrodes 24 and 26 may form separate defibrillation electrodes, in which case each of the electrodes 24 and 26 may be independently activated. In some instances, the defibrillation electrodes 24 and 26 are coupled to electrically isolated conductors, and the ICD 14 may include a switching mechanism to allow the electrodes 24 and 26 to be used as a single defibrillation electrode (e.g., activated simultaneously to form a common cathode or anode) or as separate defibrillation electrodes (e.g., activated separately, one as a cathode and one as an anode; or one activated at a time, one as an anode or cathode and the other remaining inactive, with the housing 15 serving as the active electrode).

[0028] The electrodes 24 and 26 (and in some examples, the housing 15) are referred to herein as defibrillation electrodes because they are used, either alone or in combination, to deliver high-voltage stimulation therapy (e.g., cardioversion or defibrillation shocks). The electrodes 24 and 26 may be elongate coil electrodes and generally have a relatively high surface area for delivering high-voltage electrical stimulation pulses as compared to the low-voltage pacing and sensing electrodes 28, 30, and 31. However, in addition to or instead of high-voltage stimulation therapy, the electrodes 24 and 26 and the housing 15 may also be used to provide a pacing function, a sensing function, or both a pacing and sensing function. In this sense, the use of the term "defibrillation electrode" herein should not be construed as limiting the electrodes 24 and 26 to only high-voltage cardioversion / defibrillation shock therapy applications. For example, the electrodes 24 and 26 may be used in a pacing electrode vector to deliver extracardiac pacing pulses such as ATP pulses, and / or in a sensing vector to sense cardiac electrical signals and detect ventricular tachycardia (VT) and ventricular fibrillation (VF).

[0029] Electrodes 28, 30, and 31 are relatively small surface area electrodes for delivering low voltage pacing pulses and for sensing cardiac electrical signals. Electrodes 28, 30, and 31 are referred to as pacing / sensing electrodes because they are generally configured for low voltage applications, e.g., as a cathode or anode for delivering pacing pulses and / or sensing cardiac electrical signals. In some instances, electrodes 28, 30, and 31 may provide only a pacing function, only a sensing function, or both.

[0030] In Figure 1A and Figure 1B the example shown in Figure 1A and Figure 1B electrode 28 is positioned proximal to defibrillation electrode 24, and electrode 30 is positioned between defibrillation electrodes 24 and 26. A third pacing / sensing electrode 31 may be positioned distal to defibrillation electrode 26. In

[0031] Lead 16 extends subcutaneously or submuscularly above chest cavity 32 from intermediate of connector assembly 17 of ICD 14 towards the center of the patient 12's torso (e.g., towards xiphoid process 20 of patient 12). At a location near xiphoid process 20, lead 16 bends or turns and extends subcutaneously or submuscularly above the chest cavity and / or sternum, substantially parallel to sternum 22. Although shown in Figure 1A and Figure 1B as being laterally offset from sternum 22 and extending substantially parallel thereto, lead 16 may also be implanted at other locations, such as above sternum 22, offset to the right or left of sternum 22, angled laterally towards the left or right with respect to sternum 22, etc. Alternatively, lead 16 may be placed along other subcutaneous or submuscular paths. The path of extracardiac lead 16 may depend on the location of ICD 14, the arrangement and location of the electrodes carried by distal portion 25 of the lead, and / or other factors.

[0032] A conductor (not shown) extends from a lead connector at the proximal lead end 27 through one or more lumens of the elongated lead body 18 of the lead 16 to electrodes 24, 26, 28, 30, and 31 that are positioned along the distal portion 25 of the lead body 18. The lead body 18 may have a tubular or cylindrical shape. In other examples, the distal portion 25 (or all) of the elongated lead body 18 may have a flat, ribbon, or paddle shape. The lead body 18 of the lead 16 may be formed of a non-conductive material (including, silicone, polyurethane, fluoropolymer, mixtures thereof, and other suitable materials) and shaped to form one or more lumens within which one or more conductors extend. However, the techniques disclosed herein are not limited to this configuration or to any particular lead body design.

[0033] The elongated conductive bodies contained within the lead body 18 are each electrically coupled to corresponding defibrillation electrodes 24 and 26 and pacing / sensing electrodes 28, 30, and 31 that may be separate corresponding insulated conductors within the lead body. The corresponding conductors electrically couple the electrodes 24, 26, 28, 30, and 31 to the circuitry of the ICD 14, such as a therapy delivery circuit and / or a sensing circuit, via connections in the connector assembly 17 (including associated feedthroughs that intersect the housing 15). The conductive bodies transmit therapy from the therapy delivery circuit within the ICD 14 to one or more of the defibrillation electrodes 24 and 26 and / or the pacing / sensing electrodes 28, 30, and 31, and transmit sensed electrical signals from one or more of the defibrillation electrodes 24 and 26 and / or the pacing / sensing electrodes 28, 30, and 31 to the sensing circuit within the ICD 14.

[0034] The ICD 14 may obtain electrical signals corresponding to the electrical activity of the heart 8 via a sensing vector combination that includes a combination of the electrodes 28, 30, and / or 31. In some examples, the housing 15 of the ICD 14 is used in combination with one or more of the electrodes 28, 30, and / or 31 in the sensing electrode vector. The ICD 14 may even use a sensing vector that includes one or both of the defibrillation electrodes 24 and / or 26, such as between the electrodes 24 and 26, or one of the electrodes 24 or 26 in combination with one or more of the electrodes 28, 30, 31 and / or the housing 15.

[0035] The ICD 14 analyzes the cardiac electrical signals received from one or more of the sensing vectors to monitor for abnormal rhythms, such as bradycardia, VT, or VF. The ICD 14 may analyze the heart rate and / or the morphology of the cardiac electrical signals to monitor for tachyarrhythmias according to any one of a variety of tachyarrhythmia detection techniques. An example technique for detecting tachyarrhythmias is described in U.S. Patent No. 7,761,150 (Ghanem et al.).

[0036] The ICD 14 generates and delivers electrical stimulation therapy in response to detecting a tachyarrhythmia (e.g., VT or VF). The ICD 14 can deliver ATP in response to VT detection and, in some cases, can deliver ATP before a CV / DF shock or during charging of the high-voltage capacitor to attempt to avoid the need to deliver a CV / DF shock. The ATP can be delivered using an extracardiac pacing electrode vector selected from any one of electrodes 24, 26, 28, 30, 31, and / or the can 15. The pacing electrode vector can be different from the sensing electrode vector. In one example, cardiac electrical signals are sensed between pacing / sensing electrodes 28 and 30 and between one of pacing / sensing electrodes 28 or 30 and the can 15, and an ATP pulse is delivered between the pacing / sensing electrode 30 used as the cathode electrode and the defibrillation electrode 24 used as the return anode electrode. In other examples, pacing pulses can be delivered between pacing / sensing electrode 28 and either (or both) of defibrillation electrodes 24 or 26, or between defibrillation electrodes 24 and 26. These examples are not intended to be limiting, and it should be recognized that other sensing electrode vectors and pacing electrode vectors can be selected according to individual patient needs.

[0037] If ATP does not successfully terminate VT, or when VF is detected, the ICD 14 can deliver one or more cardioversion or defibrillation (CV / DF) shocks via one or both of defibrillation electrodes 24 and 26 and / or the can 15. The ICD 14 can use electrodes 24 and 26 alone or together as the cathode (or anode) and the can 15 as the anode (or cathode) to deliver CV / DF shocks. The ICD 14 can use a pacing electrode vector including one or more of electrodes 24, 26, 28, 30, 31 of the ICD 14 and the can 15 to generate and deliver other types of electrical stimulation pulses, such as post-shock pacing pulses or bradycardia pacing pulses.

[0038] Figure 1A and Figure 1Bis illustrative in nature and should not be construed as limiting the practice of the techniques disclosed herein. In other examples, lead 16 may include fewer than three pacing / sensing electrodes or more than three pacing / sensing electrodes and / or a single defibrillation electrode or more than two electrically isolated or electrically coupled defibrillation electrodes or electrode segments. The pacing / sensing electrodes 28, 30, and / or 31 may be located elsewhere along the length of lead 16. For example, lead 16 may include a single pacing / sensing electrode 30 between defibrillation electrodes 24 and 26 and no pacing / sensing electrodes distal to defibrillation electrode 26 or proximal to defibrillation electrode 24. Various example configurations of extracardiac leads and electrodes and dimensions that may be implemented in conjunction with the extracardiac pacing techniques disclosed herein are described in U.S. Publication No. 2015 / 0306375 (Marshall et al.) and U.S. Publication No. 2015 / 0306410 (Marshall et al.).

[0039] ICD 14 is shown implanted subcutaneously along the left side of the body of patient 12 within chest cavity 32. In some instances, ICD 14 may be implanted between the patient's left posterior axillary line and left anterior axillary line. However, ICD 14 may be implanted at other subcutaneous or submuscular locations within patient 12. For example, ICD 14 may be implanted in a subcutaneous pocket within the pectoral muscle region. In such a case, lead 16 may extend subcutaneously or submuscularly from ICD 14 toward the manubrium of sternum 22 and curve or turn downward or away subcutaneously or submuscularly from the manubrium and extend to a desired location. In yet another example, ICD 14 may be placed in the abdomen. Lead 16 may likewise be implanted in other extracardiac locations. For example, as described with respect to Figures 2A to 2C the distal portion 25 of lead 16 may be implanted beneath the sternum / chest cavity in the subxiphoid space.

[0040] External device 40 is shown communicating telemetrically with ICD 14 via communication link 42. External device 40 may include a processor, a display, a user interface, a telemetry unit, and other components for communicating with ICD 14 to transmit and receive data via communication link 42. A radio frequency (RF) link, such as Wi-Fi or Medical Implant Communication Service (MICS) or other RF or communication band, may be used to establish communication link 42 between ICD 14 and external device 40.

[0041] The external device 40 can be implemented as a programmer for use in a hospital, clinic, or doctor's office to retrieve data from the ICD 14 and program the operating parameters and algorithms in the ICD 14 to control ICD functions. The external device 40 can be used to program the heart rhythm detection parameters and treatment control parameters used by the ICD 14. Data stored or acquired by the ICD 14, including physiological signals or associated data derived therefrom, device diagnostic results, and the history of detected rhythm events and delivered treatments, can be retrieved by the external device 40 from the ICD 14 after an interrogation command. The external device 40 can alternatively be implemented as a home monitor or a handheld device.

[0042] Figures 2A to 2C is a conceptual diagram of a patient 12 implanted with an extracardiac ICD system 10 that employs an implantation configuration different from the arrangement shown in Figures 1A to 1B Figure 2A is a front view of a patient 12 implanted with an ICD system 10. Figure 2B is a side view of a patient 12 implanted with an ICD system 10. Figure 2C is a transverse view of a patient 12 implanted with an ICD system 10. In this arrangement, the extracardiac lead 16 of the system 10 is at least partially implanted under the sternum 22 of the patient 12. The lead 16 extends subcutaneously or submuscularly from the ICD 14 toward the xiphoid process 20 and bends or turns at a location near the xiphoid process 20 and extends upward in a sub-sternal position within the anterior mediastinum 36.

[0043] The anterior mediastinum 36 can be considered to be bounded laterally by the pleura 39, posteriorly by the pericardium 38, and anteriorly by the sternum 22. In some instances, the anterior wall of the anterior mediastinum 36 can also be formed by the transversus thoracis muscle and one or more costal cartilages. The anterior mediastinum 36 includes a certain amount of loose connective tissue (e.g., areolar tissue), adipose tissue, some lymphatic vessels, lymph nodes, sub-sternal muscle tissue, small lateral branches of the internal thoracic artery or vein, and the thymus. In one example, the distal portion 25 of the lead 16 extends substantially along the posterior side of the sternum 22 within the loose connective tissue and / or sub-sternal muscle tissue of the anterior mediastinum 36.

[0044] A lead implanted such that the distal portion 25 is substantially within the anterior mediastinum 36 can be referred to as a "sub-sternal lead". In Figures 2A to 2C ​In the example shown, lead 16 is positioned generally centered beneath sternum 22. However, in other instances, lead 16 may be implanted such that it is laterally offset from the center of sternum 22. In some instances, lead 16 may extend laterally such that the distal portion 25 of lead 16 is beneath (in addition to or instead of) sternum 22 within thoracic cavity 32. In other examples, the distal portion 25 of lead 16 may be implanted into other extra - cardiac intrathoracic locations (including the pleural cavity) or around and adjacent to but generally not within the perimeter of pericardium 38 of heart 8. Other implant locations and lead and electrode arrangements that may be used in conjunction with the cardiac pacing techniques described herein are generally disclosed in the above - cited patent applications.

[0045] Figure 3 is a conceptual diagram showing Figures 1A to 2C the distal portion 25' of another example of an extra - cardiac lead 16 having a curved distal portion 25' of lead body 18'. The lead body 18' may be formed to have a curved, bent, serpentine, or zig - zag shape along the distal portion 25'. In the example shown, defibrillation electrodes 24' and 26' are carried along the curved portion of lead body 18'. The pacing / sensing electrode 30' is carried between defibrillation electrodes 24' and 26'. The pacing / sensing electrode 28' is carried proximal to the proximal defibrillation electrode 24'. In this example, no electrode is provided distal to defibrillation electrode 26'.

[0046] As Figure 3 shown, the lead body 18' may be formed to have a curved distal portion 25' including two "C" - shaped curves that together may resemble the Greek letter epsilon "ε". Defibrillation electrodes 24' and 26' are each carried by one of these two corresponding C - shaped portions of the distal portion 25' of the lead body, which extend or bend away from the central axis 33 of lead body 18' in the same direction. In the example shown, the pacing / sensing electrode 28' is proximal to the C - shaped portion carrying electrode 24', and the pacing / sensing electrode 30' is proximal to the C - shaped portion carrying electrode 26'. In some instances, the pacing / sensing electrodes 28' and 30' may be generally aligned with the central axis 33 of the straight proximal portion of lead body 18' such that the mid - points of defibrillation electrodes 24' and 26' are laterally offset from electrodes 28' and 30'. Other examples of extra - cardiac leads that may utilize the pacing techniques described herein, including one or more defibrillation electrodes and one or more pacing and sensing electrodes carried by a curved, serpentine, wavy, or zig - zag distal portion of a lead body, are generally disclosed in U.S. Patent Application No. 14 / 963,303.

[0047] Figure 4Schematic diagram of an example ICD 14. The electronic circuitry enclosed within a housing 15 (schematically shown as electrodes in Figure 4 ) includes software, firmware, and hardware that cooperate to monitor cardiac electrical signals, determine when electrical stimulation therapy is needed, and deliver therapy as needed in accordance with a programmed therapy delivery algorithm and control parameters. The software, firmware, and hardware are configured to detect tachyarrhythmias and deliver anti-tachyarrhythmia therapy, e.g., detect ventricular tachyarrhythmias and in some cases distinguish VT from VF to determine when ATP or CV / DF shocks are needed. The ICD 14 is coupled to cardiovascular leads, such as lead 16 carrying cardiovascular electrodes 24, 26, 28, 30, and 31 (if present), for delivering electrical stimulation pulses to a patient's heart and for sensing cardiac electrical signals.

[0048] The ICD 14 includes a control circuit 80, a memory 82, a therapy delivery circuit 84, a sensing circuit 86, and a telemetry circuit 88. A power source 98 provides power to the circuitry of the ICD 14 (including each of components 80, 82, 84, 86, and 88) as needed. The power source 98 may include one or more energy storage devices, such as one or more rechargeable or non-rechargeable batteries. The connection between the power source 98 and each of the other components 80, 82, 84, 86, and 88 is understood in accordance with the overall block diagram of Figure 4 but is not shown for clarity. For example, the power source 98 may be coupled to a low voltage (LV) charging circuit and a high voltage (HV) charging circuit included in the therapy delivery circuit 84 to charge a low voltage capacitor and a high voltage capacitor included in the therapy delivery circuit 84, respectively, to generate corresponding low voltage pacing pulses such as bradycardia pacing pulses, post-shock pacing pulses, or ATP pulses, or to generate high voltage pulses such as CV / DF shock pulses. In some examples, the high voltage capacitor rather than the low voltage capacitor is charged and used to deliver ATP pulses, post-shock pacing pulses, or other pacing pulses. As needed, the power source 98 is also coupled to components of the sensing circuit 86, such as a sensing amplifier, an analog-to-digital converter, switching circuitry, etc.

[0049] Figure 4The functional blocks shown represent functions included in the ICD 14 and may include any discrete and / or integrated electronic circuit components implementing analog and / or digital circuits capable of producing the functions attributable to the ICD 14 herein. The various components may include application specific integrated circuits (ASICs), electronic circuits, processors (shared, dedicated, or grouped) executing one or more software or firmware programs and memories, combinational logic circuits, state machines, or other suitable components or combinations of components providing the described functions. The specific form of software, hardware, and / or firmware for implementing the functions disclosed herein will be primarily determined by the specific system architecture employed in the ICD and by the specific detection and treatment delivery methods employed by the ICD. Providing software, hardware, and / or firmware to implement the described functions within the context of any modern ICD system is within the capabilities of those skilled in the art, given the disclosures herein.

[0050] Memory 82 may include any volatile, non-volatile, magnetic, or electrical non-transitory computer-readable storage medium, such as random access memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM), electrically erasable programmable ROM (EEPROM), flash memory, or any other storage device. Additionally, memory 82 may include a non-transitory computer-readable medium storing instructions which, when executed by one or more processing circuits, cause the control circuit 80 or other ICD components to perform the various functions attributable to the ICD 14 or those ICD components. The non-transitory computer-readable medium storing these instructions may include any of the media listed above.

[0051] The functions attributable to the ICD 14 herein may be implemented as one or more integrated circuits. Depicting different features as components is intended to highlight different functional aspects and does not necessarily imply that such components must be implemented by separate hardware or software components. Rather, the functions associated with one or more components may be performed by separate hardware, firmware, or software components, or integrated within a common hardware, firmware, or software component. For example, cardiac event sensing operations and tachyarrhythmia detection operations may be performed by the sensing circuit 86 under the control of the control circuit 80 and may include operations implemented in a processor or other signal processing circuitry included in the control circuit 80, which execute instructions stored in the memory 82 and control signals sent from the control circuit 80 to the sensing circuit 86, such as blanking intervals and timing intervals and sensing threshold amplitude signals.

[0052] The control circuit 80 communicates with the therapy delivery circuit 84 and the sensing circuit 86 via a data bus, for example, to sense cardiac electrical activity, detect cardiac rhythm, and control the delivery of cardiac electrical stimulation therapy in response to sensed cardiac signals. The therapy delivery circuit 84 and the sensing circuit 86 are electrically coupled to electrodes 24, 26, 28, 30, and 31 (if present, as shown in Figures 1A to 3 ), carried by lead 16 (e.g., as shown in Figures 1A to 2A ), and the housing 15, which may act as a common or ground electrode or as an active metal can electrode for delivering CV / DF shock pulses or cardiac pacing pulses.

[0053] The sensing circuit 86 can be selectively coupled to electrodes 28, 30, 31, and / or the housing 15 to monitor the electrical activity of the patient's heart. The sensing circuit 86 can additionally be selectively coupled to defibrillation electrodes 24 and / or 26 for sensing electrode vectors. The sensing circuit 86 is enabled to selectively receive cardiac electrical signals from at least two sensing electrode vectors from among the available electrodes 24, 26, 28, 30, 31, and the housing 15. At least two cardiac electrical signals from two different sensing electrode vectors can be received simultaneously by the sensing circuit 86, and the sensing circuit 86 can monitor one or both of these cardiac electrical signals at a time to sense cardiac electrical signals. For example, the sensing circuit 86 can include switching circuitry for selecting which of electrodes 24, 26, 28, 30, 31, and the housing 15 are coupled to sensing channels 83 or 85 that include cardiac event detection circuitry, e.g., as described in connection with Figure 5 and Figure 12 . The switching circuitry can include a switch array, a switch matrix, a multiplexer, or any other type of switching device suitable for selectively coupling components of the sensing circuit 86 to the selected electrodes. The cardiac event detection circuitry within the sensing circuit 86 can include one or more sensing amplifiers, filters, rectifiers, threshold detectors, comparators, analog-to-digital converters (ADCs), or other analog or digital components as further described in connection with Figure 5 and Figure 12 . The cardiac event sensing threshold can be automatically adjusted by the sensing circuit 86 under the control of the control circuit 80 based on timing periods and sensing thresholds determined by the control circuit 80, stored in the memory 82, and / or controlled by the hardware of the control circuit 80 and / or the sensing circuit 86.

[0054] In some examples, the sensing circuit 86 includes a plurality of sensing channels 83 and 85 for obtaining cardiac electrical signals from a plurality of sensing vectors selected from electrodes 24, 26, 28, 30, 31, and the housing 15. Each of the sensing channels 83 and 85 can be configured to amplify, filter, and digitize the cardiac electrical signals received from the selected electrodes coupled to the corresponding sensing channels to improve the signal quality for detecting cardiac events such as R waves. For example, each of the sensing channels 83 and 85 can include a pre-filter and an amplifier for filtering and amplifying the signals received from the selected electrodes. The resulting raw cardiac electrical signals can be transmitted from the pre-filter and the amplifier to a cardiac event detection circuitry in at least one of the sensing channels 83 to sense cardiac events in real time from the received cardiac electrical signals. As disclosed herein, the sensing channel 83 can be configured to sense cardiac events such as R waves based on a cardiac event sensing threshold, and the second sensing channel 85 can be configured to transmit the digitized cardiac electrical signals obtained from different sensing electrode vectors to the control circuit 80 for confirming the cardiac events sensed by the first sensing channel 83.

[0055] When a cardiac event is detected based on a sensing threshold crossing, the first sensing channel 83 can generate a sensed event signal, such as an R wave sensed event signal, which is transmitted to the control circuit 80. The sensed event signal is used by the control circuit 80 to trigger the storing of a time segment of the second cardiac electrical signal for post-processing and analysis to confirm, for example, the R wave sensed event signal as described hereinafter in connection with Figures 7 to 9 The memory 82 can be configured to store a predetermined number of cardiac electrical signal segments, e.g., at least one, two, or other number of cardiac electrical signal segments, in a circular buffer under the control of the control circuit 80. Each segment can be written to the memory 82 over a time interval that extends before and after the R wave sensed event signal generated by the first sensing channel 83. When confirmation of the R wave sensed by the first sensing channel 83 is required based on the detection of a predetermined number of tachyarrhythmia intervals, the control circuit 80 can access the stored cardiac electrical signal segments, which can be done prior to tachyarrhythmia detection and in some cases can cause the tachyarrhythmia detection to be stopped.

[0056] The R-wave sensing event signal is also used by control circuit 80 to determine the RR interval (RRI) in order to detect tachyarrhythmias and to determine the need for therapy. The RRI is the time interval between successive sensed R-waves and can be ascertained between successive R-wave sensing event signals received from sensing circuit 86. For example, control circuit 80 can include timing circuit 90 which is operative to determine the RRI between successive R-wave sensing event signals received from sensing circuit 86 and to control various timers and / or counters used to time the delivery of therapy by therapy delivery circuit 84. Timing circuit 90 can additionally set time windows such as a morphology template window, a morphology analysis window, etc., or perform other timing-related functions of ICD 14, including synchronizing cardioversion shocks or other therapies delivered by therapy delivery circuit 84 with sensed cardiac events.

[0057] Control circuit 80 is also shown as including tachyarrhythmia detector 92 which is configured to analyze the signals received from sensing circuit 86 in order to detect the onset of a tachyarrhythmia. Tachyarrhythmia detector 92 can be implemented in control circuit 80 as hardware and / or firmware that processes and analyzes the signals received from sensing circuit 86 to detect VT and / or VF. In some examples, the timing of the R-wave sensing event signals received from sensing circuit 86 is used by timing circuit 90 to determine the RRI between the sensing event signals. Tachyarrhythmia detector 92 can include comparators and counters for counting the RRIs determined by timing circuit 90 that fall within respective frequency detection zones in order to determine the ventricular rate, or perform other frequency- or interval-based evaluations to detect and distinguish VT and VF.

[0058] For example, tachyarrhythmia detector 92 can compare the RRIs determined by timing circuit 90 to one or more tachyarrhythmia detection interval zones such as a tachycardia detection interval zone and a fibrillation detection interval zone. The RRIs that fall into the detection interval zones are counted by corresponding VT interval counters or VF interval counters and, in some instances, in a combined VT / VF interval counter included in tachyarrhythmia detector 92. When the interval counter reaches a detection threshold, ventricular tachyarrhythmia can be detected by tachyarrhythmia detector 92. Tachyarrhythmia detector 92 can be configured to perform other signal analyses to determine if other detection criteria are met prior to detecting VT or VF, such as R-wave morphology criteria, onset criteria, and noise and oversensing suppression criteria. In combination Figure 11 and Figure 13Examples of other parameters that can be determined from the cardiac electrical signals received by the sensing circuit 86 are described, which are used to determine the status of a tachyarrhythmia detection inhibition rule that may cause the cessation of VT or VF detection.

[0059] To support these additional analyses, the sensing circuit 86 can transmit digitized electrocardiogram (ECG) signals to the control circuit 80 for performing morphological analysis by the tachyarrhythmia detector 92 to detect and distinguish heart rhythms. Cardiac electrical signals from selected sensing vectors (e.g., from the first sensing channel 83 and / or the second sensing channel 85) can be passed through filters and amplifiers, provided to a multiplexer, and thereafter converted to a multi-bit digital signal by an analog-to-digital converter for storage in the memory 82, where the filters, amplifiers, multiplexer, and analog-to-digital converter are all included in the sensing circuit 86. The memory 82 can include one or more circular buffers for temporarily storing segments of digital cardiac electrical signals for analysis by the control circuit 80 to confirm R waves sensed by the sensing channel 83, determine morphological match scores, detect T-wave oversensing, detect noise contamination, and more functions as further described below.

[0060] The control circuit 80 can be a microprocessor-based controller that uses digital signal analysis techniques to characterize the digitized signals stored in the memory 82 in order to identify and classify the patient's heart rhythm using any of a number of signal processing methods for analyzing cardiac signals and cardiac event waveforms (e.g., R waves). Examples of devices and algorithms suitable for utilizing the techniques for performing R-wave sensing and confirmation and tachyarrhythmia detection described herein are generally disclosed in the following patents: U.S. Patent No. 5,354,316 (Keimel); U.S. Patent No. 5,545,186 (Olson et al.); U.S. Patent No. 6,393,316 (Gillberg et al.); U.S. Patent No. 7,031,771 (Brown et al.); U.S. Patent No. 8,160,684 (Ghanem et al.); and U.S. Patent No. 8,437,842 (Zhang et al.).

[0061] The therapy delivery circuit 84 includes: a charging circuitry; one or more charge storage devices, such as one or more high-voltage capacitors and / or low-voltage capacitors; and a switching circuitry that controls when to discharge the (multiple) capacitors across a selected pacing electrode vector or a CV / DF shock vector. Charging the capacitors to a certain programmed pulse amplitude and discharging the capacitors for a certain programmed pulse width can be performed by the therapy delivery circuit 84 in accordance with control signals received from the control circuit 80. The timing circuit 90 of the control circuit 80 can include various timers or counters that control when to deliver ATP or other cardiac pacing pulses. For example, the timing circuit 90 can include a programmable digital counter that is set by the microprocessor of the control circuit 80 for controlling the basic time intervals associated with the various pacing modes or ATP sequences delivered by the ICD 14. The microprocessor of the control circuit 80 can also set the amplitude, pulse width, polarity, or other characteristics of the cardiac pacing pulses, which can be based on the programmed values stored in the memory 82.

[0062] During pacing, the escape interval counter within the timing circuit 90 is reset upon sensing an R wave as indicated by a signal from the sensing circuit 86. Depending on the selected pacing mode, when the escape interval counter expires, a pacing pulse is generated by the pulse output circuit of the therapy delivery circuit 84. The pacing output circuit is coupled via a switch matrix to the desired pacing electrodes in order to discharge one or more capacitors across the pacing load. The escape interval counter is reset upon pacing pulse generation and thus controls the basic timing of the cardiac pacing function including ATP. The duration of the escape interval is determined by the control circuit 80 via the data / address bus. When the escape interval counter is reset by sensing an R wave, the count value present in the escape interval counter can be used by the timing circuit 90 to measure the RRI as described above for detecting the occurrence of various arrhythmias by the tachyarrhythmia detector 92.

[0063] The memory 82 can include a read-only memory (ROM) in which a program for controlling the operation of the control circuit 80 resides. The memory 82 can further include a random access memory (RAM) that is configured to hold a series of recirculation buffers for measuring RRI, counting, or other data for analysis by the tachyarrhythmia detector 92 to predict or diagnose arrhythmias.

[0064] In response to detecting ventricular tachycardia, ATP therapy can be delivered by loading a protocol from a microprocessor included in control circuit 80 to timing circuit 90 based on the type and frequency of the detected tachycardia. In cases where higher voltage cardioversion or defibrillation pulses are needed, e.g., the tachyarrhythmia is VF or VT and is not terminated via ATP therapy, control circuit 80 activates the cardioversion and defibrillation control circuitry system included in control circuit 80 to initiate charging of a high-voltage capacitor via a charging circuit under the control of a high-voltage charging control line, where the high-voltage capacitor and the charging circuit are both included in therapy delivery circuit 84. The voltage on the high-voltage capacitor is monitored via a voltage capacitor line that leads to control circuit 80. When the voltage reaches a predetermined value set by control circuit 80, a logic signal is generated on a capacitor full line that leads to therapy delivery circuit 84, thereby terminating the charging. The defibrillation or cardioversion pulse is delivered to the heart via a control bus by an output circuit of therapy delivery circuit 84 under the control of timing circuit 90. The output circuit determines the electrodes for delivering the cardioversion or defibrillation pulse and the pulse waveform. The therapy delivery and control circuitry system generally disclosed in any of the patents cited above can be implemented in ICD 14.

[0065] The control parameters used by control circuit 80 to detect heart rate and control therapy delivery can be programmed into memory 82 via telemetry circuit 88. Telemetry circuit 88 includes a transceiver and an antenna for communicating with an external device 40 ( Figure 1A as shown) using RF communication as described above. Under the control of control circuit 80, telemetry circuit 88 can receive downlink telemetry from external device 40 and send uplink telemetry to the external device. In some cases, telemetry circuit 88 can be used to send and receive communication signals to / from another medical device implanted in patient 12.

[0066] Figure 5 is a diagram of the circuitry included in a first sensing channel 83 and a second sensing channel 85 of sensing circuit 86 according to one example. The first sensing channel 83 can be selectively coupled via a switching circuitry system (not shown) to a first sensing electrode vector for receiving a first cardiac electrical signal, where the first sensing electrode vector includes Figures 1A to 2CThe electrodes carried by the cardiovascular lead 16 are shown. The first sensing channel 83 may be coupled to a sensing electrode vector that is a short dipole having a relatively short interelectrode distance or spacing compared to a second electrode vector coupled to the second sensing channel 85. In the example shown, the first sensing electrode vector may include pacing / sensing electrodes 28 and 30. In other examples, depending on the interelectrode spacing and location of the distal portion 25 of the lead 16, the first sensing electrode vector coupled to the sensing channel 83 may include pacing / sensing electrodes 30 and 31 and in some cases pacing / sensing electrodes 28 and 31. In other examples, for instance, the first sensing channel 83 may be selectively coupled to a sensing electrode vector including defibrillation electrodes 24 and / or 26, such as a sensing electrode vector between pacing / sensing electrode 28 and defibrillation electrode 24, between pacing / sensing electrode 30 and either defibrillation electrode 24 or 26, or between pacing / sensing electrodes 26 and 31. In some examples, the first sensing electrode vector may be between defibrillation electrodes 24 and 26.

[0067] The sensing circuit 86 includes a second sensing channel 85 that receives a second cardiac electrical signal from a second sensing vector, such as from a vector including electrodes 30 and the housing 15 as shown, or a vector including electrodes 28 and the housing 15. The second sensing channel 85 may be selectively coupled to other sensing electrode vectors that may form a long dipole having a greater interelectrode distance or spacing than the sensing electrode vector coupled to the first sensing channel 83. As described below, the second cardiac electrical signal received via the long dipole by the second sensing channel 85 may be used by the control circuit 80 for morphological analysis (including, for example, combined Figure 10 pulse morphology analysis, noise suppression, and other analyses as described). In other examples, any vector selected from available electrodes such as electrodes 24, 26, 28, 30, and / or 31 and / or the housing 15 may be included in the sensing electrode vector coupled to the second sensing channel 85.

[0068] The electrical signals generated by input electrodes 28 and 30 across sensing channel 83 and the input electrodes across sensing channel 85 (i.e., electrode 30 and housing 15) are provided as differential input signals to pre-filters and pre-amplifiers 62 and 72, respectively. Non-physiological high-frequency and DC signals can be filtered by low-pass or band-pass filters included in each of pre-filters and pre-amplifiers 62 and 72, and high-voltage signals can be removed by protection diodes included in pre-filters and pre-amplifiers 62 and 72. Pre-filters and pre-amplifiers 62 and 72 can amplify the pre-filtered signals with a gain between 10 and 100, and in one example, the gain is 17.5, and can convert the differential signal to a single-ended output signal, which is transmitted to analog-to-digital converter (ADC) 63 in the first sensing channel 83 and ADC 73 in the second sensing channel 85. Pre-filters and pre-amplifiers 62 and 72 can provide anti-aliasing filtering and noise reduction before digitization.

[0069] ADC 63 and ADC 73 respectively convert the first cardiac electrical signal from an analog signal to a first digital bitstream and the second cardiac electrical signal to a second digital bitstream. In one example, ADC 63 and ADC 73 can be Sigma-Delta Converters (SDCs), but other types of ADCs can be used. In some examples, the outputs of ADC63 and ADC 73 can be provided to a decimator (not shown), which acts as a digital low-pass filter to increase the resolution and reduce the sampling rate of the corresponding first and second cardiac electrical signals.

[0070] In sensing channel 83, the digital output of ADC 63 is transmitted to filter 64, which can be a digital band-pass filter having a band-pass of approximately 10 Hz to 30 Hz for transmitting cardiac electrical signals such as R waves that typically occur in this frequency range. The band-pass filtered signal is transmitted from filter 64 to rectifier 65 and then to R-wave detector 66. R-wave detector 66 can include an auto-adjusting sensing amplifier, comparator, and / or other detection circuitry that compares the filtered and rectified first cardiac electrical signal with an R-wave sensing threshold in real time and generates an R-wave sensing event signal 68 when the cardiac electrical signal crosses the R-wave sensing threshold.

[0071] As disclosed in U.S. Patent Application No. 15 / 142,171, granted to Cao et al., the R-wave sensing threshold can be controlled as a multi-level sensing threshold by the sensing circuit 86 and / or the control circuit 80. Briefly, the multi-level sensing threshold can have an initial sensing threshold that is maintained over a time interval equal to the tachycardia detection interval, and then drops to a second sensing threshold and is maintained until the drop time interval expires, which can be 1 second to 2 seconds long. The sensing threshold drops to a minimum sensing threshold after the drop time interval. The initial sensing threshold can be the lower of a predetermined percentage of the most recent, previously sensed R-wave peak amplitude and the maximum sensing threshold limit determined using a sensitivity-based gain and a programmed sensitivity setting. In other examples, the R-wave sensing threshold used by the R-wave detector 66 can be set to an initial value based on the previous R-wave peak amplitude and decay linearly or exponentially over time until the minimum sensing threshold is reached. However, the techniques of the present application are not limited to the specific behavior of the sensing threshold. Rather, other automatically adjusted sensing thresholds can be utilized.

[0072] In other examples, the filtered and digitized cardiac electrical signal (output of the filter 64) from the sensing channel 83 can be stored in the memory 82 for signal processing by the control circuit 80 for detecting the onset of tachyarrhythmia. In one example, the output of the rectifier 65 is transmitted to the differentiator 67, which determines the Nth order difference signal 69 that is transmitted to the memory 82. The control circuit 80 can retrieve the stored signal from the memory 82 for signal analysis by the tachyarrhythmia detector 92 according to the implemented tachyarrhythmia detection algorithm. For example, the T-wave over-sensing algorithm implemented in the tachyarrhythmia detector 92 can detect evidence of T-wave over-sensing from the first order difference signal 69 generated by the differentiator 67. As generally disclosed in U.S. Patent No. 7,831,304 (Cao et al.), a method for detecting T-wave over-sensing using a difference signal can be performed by the tachyarrhythmia detector 92.

[0073] The second cardiac electrical signal digitized by the ADC 73 can be transmitted to the filter 74 for band-pass filtering, for example, from 10 Hz to 30 Hz. In some examples, the sensing channel 85 includes a notch filter 76. The notch filter 76 can be implemented as firmware or hardware and is provided to attenuate 50 Hz - 60 Hz electrical noise, muscle noise, and other electromagnetic interference (EMI) or electrical noise / artifacts in the second cardiac electrical signal. Using, for example Figures 1A to 3The cardiac electrical signals obtained by the extracardiac electrodes shown in [Figure] are more likely to be contaminated by 50 Hz - 60 Hz electrical noise, muscle noise, and other EMI electrical noise / artifacts compared to intracardiac electrodes. Thus, a notch filter 76 can be provided to significantly attenuate the amplitude of signals in the 50 Hz - 60 Hz range while minimally attenuating signals in the approximately 1 Hz - 30 Hz range corresponding to the frequencies of typical cardiac electrical signals. In combination with Figure 6 describes an example of a notch filter designed with minimal computational requirements and its filtering characteristics.

[0074] The output signal 78 of the notch filter 76 can be transmitted from the sensing circuit 86 to the memory 82 under the control of the control circuit 80 so that segments of the second cardiac electrical signal 78 are stored in a temporary buffer of the memory 82. For example, the timing circuit 90 of the control circuit 80 can set a time interval or a certain number of sampling points relative to the R-wave sensing event signal 68 received from the first sensing channel 83, at which the second cardiac electrical signal 78 is stored in the memory 82. As described in combination with Figures 7 to 13 the buffered segments of the second cardiac electrical signal are analyzed by the control circuit 80 on an as-needed trigger basis to confirm the R-wave sensed by the first sensing channel 83.

[0075] The notch filter 76 can be implemented as a digital filter for real-time filtering performed by firmware that is part of the sensing channel 85 or by the control circuit 80 to filter the buffered digital output of the filter 74. In some examples, the output of the filter 74 of the sensing channel 85 can be stored in the memory 82 in time segments defined with respect to the R-wave sensing event signal 68. When the control circuit 80 is triggered to analyze the stored second cardiac electrical signal to confirm the R-wave sensing event signal, for example as described in combination with Figure 7 、 Figure 10 、 Figure 11 and Figure 13 before further processing and analysis of the stored segments of the second cardiac electrical signal, the notch filter 76 can be applied to the stored segments. In this way, if confirmation of the R-wave sensed by the first sensing channel 83 does not require analysis of the stored signal segments, the firmware implemented to perform the operation of the notch filter 76 does not need to be executed.

[0076] The configurations of the sensing channels 83 and 85 are illustrative in nature and should not be considered limiting of the techniques described herein. With Figure 5Compared with that shown and described herein, the sensing channels 83 and 85 of the sensing circuit 86 may include more or fewer components. However, the first sensing channel 83 is configured to detect an R wave in real time from the first cardiac electrical signal in, for example, hardware-implemented components based on the first cardiac electrical signal crossing an R wave sensing threshold, and the second sensing channel 85 is configured to provide a second cardiac electrical signal for storage in the memory 82 for post-processing and analysis by the control circuit 80 to confirm an R wave sensing event signal generated by the first sensing channel 83.

[0077] Figure 6 FIG. 50 is a graph of the attenuation characteristics of the notch filter 76 of the second sensing channel 85. In one example, the notch filter 76 is implemented as a digital filter in firmware. The output of the digital notch filter can be determined by the firmware implemented in the second sensing channel 85 according to the following equation:

[0078] Y(n) = (x(n) + 2x(n - 2) + x(n - 4)) / 4

[0079] where x(n) is the amplitude of the nth sample point of the digital signal received by the notch filter 76 at a sampling rate of 256 Hz, x(n - 2) is the amplitude of the (n - 2)th sample point, and x(n - 4) is the amplitude of the (n - 4)th sample point. Y(n) is the amplitude of the nth sample point of the notch-filtered, digital second cardiac electrical signal. Figure 6 Graph 50 of FIG. represents the attenuation of the amplitude Y(n) as a function of frequency. At a frequency of 60 Hz, the attenuation of the amplitude of Y(n) is -40 decibels (dB). At a frequency of 50 Hz, the attenuation is -20 dB, and at 23 Hz, which may be typical for the R wave of a cardiac electrical signal, the attenuation is limited to -3 dB. Thus, the notch filter 76 can provide highly attenuated 50 Hz and 60 Hz noise, muscle noise, other EMI, and other electrical noise / artifacts while passing the lower frequency cardiac signals in the second cardiac electrical signal output of the sensing channel 85. Although the notch filter 76 may not attenuate frequencies close to the maximum frequency of 128 Hz, the filter 74 of the second sensing channel 85, which may be a band-pass filter, can sufficiently reduce the signal content in the higher frequency range above 60 Hz.

[0080] When using a different sampling rate other than 256 Hz, the number of sample points indicated in the above equation for determining the notch-filtered signal can be modified as needed. However, the resulting frequency response may or may not be the same as Figure 6The same as that shown. The notch filter 76 uses a minimum computation with only the required two additions and three shifts. In other examples, other digital filters can be used for the attenuation of 50 Hz and 60 Hz. For example, for a sampling rate of 256 Hz, the filtered signal Y(n) can be determined as Y(n) = (x(n) + x(n - 1) + x(n - 2) + x(n - 3)) / 4, which has less attenuation at 50 Hz and 60 Hz than Figure 6 the frequency response shown, but serves as a low-pass, notch filter with greater attenuation at higher frequencies (greater than 60 Hz) than Figure 6 the frequency response shown.

[0081] Figure 7 FIG. 100 is a flowchart of a method performed by the ICD 14 according to one example to sense and confirm an R wave for tachyarrhythmia detection. At blocks 102 and 104, two different sensing electrode vectors are selected by the sensing circuit 86 for receiving a first cardiac electrical signal through a first sensing channel 83 and a second cardiac electrical signal through a second sensing channel 85. These two sensing electrode vectors can be selected under the control of the control circuit 80 by a switching circuitry included in the sensing circuit 86. In some examples, these two sensing electrode vectors are programmed by the user and retrieved by the control circuit 80 from the memory 82 and transmitted to the sensing circuit 86 as a vector selection control signal.

[0082] The first sensing vector selected at block 102 for obtaining the first cardiac electrical signal can be a relatively short dipole, e.g., between electrodes 28 and 30 of the lead 16, or between electrodes 28 and 24, or other electrode combinations as described above. The relatively short dipole can include electrodes that are very close to each other and very close to the ventricular heart chamber compared to other available sensing electrode pairs. The first sensing vector can be a vertical sensing vector (relative to the patient's upright or standing position) or substantially aligned with the cardiac axis to maximize the amplitude of the R wave in the first cardiac electrical signal for reliable R wave sensing.

[0083] The second sensing electrode vector for obtaining the second cardiac electrical signal at block 104 can be a relatively long dipole having an interelectrode distance greater than that of the first sensing electrode vector. For example, the second sensing electrode vector can be selected as the vector between one of the pacing sensing electrodes 28 or 30 and the ICD housing 15, between one of the defibrillation electrodes 24 or 26 and the housing 15, or other combinations between one electrode along the distal portion of the lead 16 and the housing 15. In some examples, this sensing vector can be orthogonal or nearly orthogonal to the first sensing vector, but it is not required that the first and second sensing vectors be orthogonal vectors. Compared to the first sensing electrode vector, the second sensing electrode vector can provide a relatively more global or far-field cardiac electrical signal. The second cardiac electrical signal obtained at block 104 can be used for waveform morphology analysis by the tachyarrhythmia detector 92 of the control circuit 80 and for cardiac signal analysis to confirm the R-wave sensing event signal generated by the first sensing channel 83 of the sensing circuit 86.

[0084] At block 106, the sensing circuit 86 can generate an R-wave sensing event signal in response to the first sensing channel 83 detecting that the first cardiac electrical signal crosses the R-wave sensing threshold. The R-wave sensing event signal can be transmitted to the control circuit 80. In response to the R-wave sensing event signal, following the "Yes" branch of block 106, at block 108, the control circuit 80 is triggered to store a segment of the second cardiac electrical signal (sensing vector 2, block 104) received from the second sensing channel 85 in the circular buffer of the memory 82. The digitized segment of the second cardiac electrical signal can be 100 ms to 500 ms long, for example, including sample points before and after the time of the R-wave sensing event signal, which may or may not be centered in time on the R-wave sensing event signal received from the sensing circuit 86. For example, the segment can extend 100 ms after the R-wave sensing event signal and have a duration of 200 ms to 500 ms, such that the segment extends from approximately 100 ms to 400 ms before the R-wave sensing event signal to 100 ms after the R-wave sensing event signal. In other examples, the segment can be centered on the R-wave sensing event signal or extend more sample points after the R-wave sensing event signal than before it. In one example, the buffered segment of the cardiac electrical signal is at least 50 sample points obtained at a sampling rate of 256 Hz, or approximately 200 ms. In another example, the buffered segment is at least 92 sample points sampled at 256 Hz, or approximately 360 ms, and can be used for morphology analysis, noise analysis, T-wave oversensing, and / or other analysis by the tachyarrhythmia detector 92 to detect VT or VF. In combination Figure 10Describes additional analysis of the buffered second cardiac electrical signal that can be performed by the tachyarrhythmia detector 92 for detecting VT or VF, or for ceasing to detect VT or VF. The memory 82 can be configured to store a predetermined number of second cardiac electrical segments in a circular buffer, e.g., at least 1 cardiac electrical signal segment and in some cases two or more cardiac electrical signal segments, such that the oldest segment is overwritten by the newest segment. However, if the R-wave confirmation threshold is not reached as described below, the previously stored segment may never be analyzed for R-wave confirmation before being overwritten. In some examples, a single segment of the second cardiac electrical signal can be stored and, if not needed for confirming the R-wave sensed by the first channel, the segment is overwritten by the next segment corresponding to the next R-wave sensing event signal.

[0085] In addition to buffering segments of the second cardiac electrical signal, the control circuit 80 also responds to the R-wave sensing event signal generated at block 106 by determining at block 110 the RRI that ends with the current R-wave sensing event signal and begins with the most recent previous R-wave sensing event signal. The timing circuit 90 of the control circuit 80 can convey RRI timing information to the tachyarrhythmia detection circuit 92, which adjusts a tachyarrhythmia interval counter at block 112. If the RRI is longer than the tachycardia detection interval (TDI), the tachyarrhythmia interval counter remains unchanged. If the RRI is shorter than the TDI but longer than the fibrillation detection interval (FDI), i.e., if the RRI is in the tachycardia detection interval region, the VT interval counter is incremented at block 112. If the RRI is shorter than or equal to the FDI, the VF interval counter is incremented at block 112. In some examples, if the RRI is less than the TDI, the combined VT / VF interval counter is incremented.

[0086] After updating the tachyarrhythmia interval counter at block 112, the tachyarrhythmia detector 92 compares the counter value with the R-sense confirmation threshold at block 114 and compares the counter value with the VT and VF detection thresholds at block 132. If the VT or VF detection interval counter has reached the R-sense confirmation threshold, the "Yes" branch of block 114, the second cardiac electrical signal from the sensing channel 85 is analyzed to confirm the R-wave sensed by the first sensing channel 83 at block 106. The R-sense confirmation threshold can be a VT or VF interval count greater than or equal to a count with one or the other higher count value. Different R-sense confirmation thresholds can be applied to the VT interval counter and the VF interval counter. For example, the R-sense confirmation threshold can be a count of two on the VT interval counter and a count of three on the VF interval counter. In other examples, the R-sense confirmation threshold is a higher number, e.g., five or higher, but can be less than the number of intervals required to detect VT or VF. In addition to or instead of applying the R-sense confirmation threshold to separate VT or VF counters, the R-sense confirmation threshold can also be applied to a combined VT / VF interval counter.

[0087] If none of the tachyarrhythmia interval counters reach the R-sense confirmation threshold at block 114, the control circuit 80 waits at block 108 for the next R-wave sensing event signal to buffer the next segment of the second cardiac electrical signal. If the R-sense confirmation threshold is reached at block 114, the control circuit 80 determines at block 116 the maximum amplitude of the buffered signal segment stored for the most recent R-wave sensing event signal. The maximum amplitude can be determined based on the differential signal determined from the buffered signal segment. For example, the nth-order differential signal can be determined from the buffered signal segment by determining the difference between the ith and the i - nth signal sample points of the buffered signal segment. In one example, a 4th-order differential signal is determined.

[0088] The maximum absolute value of the differential signal is estimated as the amplitude of the event in the second cardiac electrical signal sensed as the R-wave from the first cardiac electrical signal. The time of the maximum absolute value of the signal is identified as the time of the event in the second cardiac electrical signal. When the R-wave is not the first R-wave to be confirmed since reaching the R-sense confirmation threshold, the control circuit 80 determines at block 118 the amplitude ratio as the ratio of the maximum absolute value determined at block 116 to the event amplitude for the most recently confirmed R-wave sensing event determined from the second cardiac electrical signal. At block 120, the control circuit determines the time interval from the most recent event in the second cardiac electrical signal confirmed as an R-wave sensing event to the event determined at block 116.

[0089] When the R wave is the first R wave to be confirmed after reaching the R sensing confirmation threshold, it can be assumed that the first confirmation event on the second cardiac electrical signal occurs simultaneously with an R wave sensing event signal having a default maximum amplitude. The default maximum amplitude can be set to be equal to the amplitude of the R wave sensed by the first sensing channel 83, a nominal value (e.g., 1 millivolt), or a previously determined R wave amplitude or an average R wave amplitude determined from the second cardiac electrical signal. Alternatively, the maximum absolute amplitude of the differential signal and its time can be identified and stored as the initial values used to determine the amplitude ratio and time for the next R wave to be confirmed at blocks 118 and 120. In other examples, a previously determined R wave amplitude (e.g., from a previous time when the R sensing confirmation threshold was reached) or a default R wave amplitude can be used to determine the amplitude ratio for the first R wave to be confirmed after reaching the R sensing confirmation threshold. The first R wave can be confirmed based on this amplitude ratio and / or the time since the previous R wave sensing event signal.

[0090] At block 122, the control circuit 80 determines a ratio threshold to be applied to the amplitude ratio based on the time interval determined at block 120. In one example, the ratio threshold is retrieved from a look-up table stored in the memory. In other examples, the ratio threshold can be calculated based on the time interval determined at block 120. The ratio threshold can be a variable threshold that decreases as the time interval since the most recently confirmed R wave increases. Thus, the time interval determined at block 120 is used to determine what ratio threshold should be applied to the amplitude ratio determined at block 118 to confirm the R wave sensed by the first sensing channel 83. The ratio threshold can decrease in a linear manner, an exponential manner, or a stepwise manner, or a combination thereof. For example, the ratio threshold can decrease with a continuous slope or decay rate over some time portion since the most recently confirmed R wave and can remain constant over other time portions since the most recently confirmed R wave. Examples of time-varying ratio thresholds and methods for determining the ratio threshold at block 122 are described in conjunction with Figure 8 and Figure 9 describes examples of time-varying ratio thresholds and methods for determining the ratio threshold at block 122.

[0091] At block 124, the control circuit 80 compares the ratio threshold determined at block 122 with the amplitude ratio determined at block 118. If the amplitude ratio is equal to or greater than the ratio threshold, the R wave sensing event is confirmed at block 126. If the amplitude ratio is less than the ratio threshold, the R wave sensing event is not confirmed at block 128. The event can be an over-sensed T wave, P wave, muscle noise, electromagnetic interference, or other non-cardiac electrical noise that has been over-sensed by the first sensing channel 83.

[0092] At block 130, the control circuit 80 adjusts an unconfirmed beat counter. If an R-wave sensing event is not confirmed, the unconfirmed beat counter is incremented by one count. If the R-wave sensing event is confirmed at block 126, the unconfirmed beat counter may maintain its current value or be decremented. In some examples, the unconfirmed beat counter keeps track of how many of the most recent predetermined number of consecutive R-wave sensing event signals generated by the first sensing channel 83 are unconfirmed in an x out of y fashion. For example, the unconfirmed beat counter may keep track of how many of the most recent 12 R-wave sensing event signals are not confirmed as R-waves based on the amplitude ratio comparison performed at block 124.

[0093] In addition to counting how many beats are unconfirmed at block 130, data related to the most recent n events analyzed by the control circuit 80 may be stored in a rolling buffer. For example, data may be stored for the most recent twelve events analyzed for unconfirmed R-wave sensing event signals. The stored data may include event amplitude, amplitude ratio, event timing, the time interval since the most recent confirmed event, and whether the event was confirmed.

[0094] Although in the examples according to Figure 7 an R-wave sensing event signal is confirmed or not confirmed based on the amplitude ratio confirmed from the second cardiac electrical signal, it should be appreciated that in addition to or instead of comparing with the event amplitude as described above, other characteristics of the second cardiac signal may be compared with the R-wave confirmation criteria. For example, the peak slew rate, event area, event signal width, or other characteristics of a buffered cardiac electrical signal segment may be compared with corresponding thresholds to confirm the event as an R-wave. The threshold may be defined as the minimum ratio of the characteristic relative to the similar characteristic of the most recent previous event that was confirmed as an R-wave, or may be a threshold directly compared with a characteristic determined from the buffered cardiac electrical signal segment that is independent of previous events.

[0095] If any of the tachyarrhythmia interval counters adjusted at block 112 reaches the interval count for detecting (NID) tachyarrhythmia, as determined at block 132, the tachyarrhythmia detector 92 of the control circuit 80 determines at block 134 whether a suppression rule is met prior to detecting a tachyarrhythmia. In one example, the NID required to detect VT may be a count of 16 VT intervals. The NID for detecting VF may be a count of 30 VF intervals out of the last 40 RRIs. If the NID is reached, one or more suppression rules may be applied to suppress VT or VF detection based on the RRI count meeting the NID. Below, for example, in connection with Figure 10Describes various inhibition rules. At least one inhibition rule is related to the number of R waves sensed by the first sensing channel 83 that are not confirmed by analysis of the second cardiac electrical signal.

[0096] For example, at block 134, the unconfirmed beat counter updated at block 130 can be compared with an inhibition rule criterion. The inhibition rule criterion can be an inhibition threshold that requires at least x out of y events to not be confirmed R waves. For example, if at least 3, at least 4, at least 6, or some other predetermined number of events out of the latest 12 events (or some other predetermined number of events) analyzed to confirm an R wave sensing event signal are not confirmed R waves, the inhibition rule is satisfied, the "Yes" branch of block 134. Stop the upcoming VT or VF detection based on reaching the NID at block 132, and do not deliver anti - tachyarrhythmia therapy.

[0097] If all inhibition rules are not satisfied, the "No" branch of block 134, then do not stop the upcoming detection of a VT or VF episode. Detect VT or VF at block 136 based on the corresponding VT or VF interval counter reaching the respective NID. The control circuit 80 controls the therapy delivery circuit 84 to deliver an appropriate anti - tachyarrhythmia therapy, such as ATP or cardioversion / defibrillation shock, according to the programmed therapy control parameters.

[0098] Figure 8 Is a diagram of the filtered second cardiac electrical signal 200 and the amplitude ratio threshold 210, which can be applied to the amplitude ratio determined from the second cardiac electrical signal at Figure 7 block 118 to confirm an R wave sensing event signal from the first sensing channel 83. The amplitude ratio is not a sensing threshold that is compared in real - time with the second cardiac electrical signal 200. The first sensing channel 83 can operate by sensing an R wave when the first cardiac electrical signal crosses an R wave amplitude sensing threshold defined in mV. However, the second cardiac electrical signal provided to the control circuit 80 by the second sensing channel 85 is not compared in real - time with an amplitude sensing threshold when it is acquired. Instead, as described in connection with Figure 7 if the first sensing channel 83 generates an R wave sensing event signal, the second cardiac electrical signal is buffered in the memory 82, and if the tachyarrhythmia interval counter reaches an R wave confirmation threshold, the buffered signal is post - processed to determine whether the ratio of the maximum amplitude determined from the buffered signal segment to the maximum amplitude determined from the previously confirmed R waves of the second cardiac electrical signal reaches or exceeds the ratio threshold 210.

[0099] Ratio threshold 210 is shown with respect to the second cardiac electrical signal 200 because the ratio threshold 210 is not a fixed value but varies over time. The ratio threshold 210 decreases as the time increases since the R wave 202 was recognized. This time-varying ratio threshold is why the control circuit 80 determines, at Figure 7 box 120 the time since the previous R wave sensing event was recognized in order to determine, at box 122, the value of the ratio threshold that is applied to the amplitude ratio to confirm or not confirm the R wave sensing event signal.

[0100] The cardiac electrical signal 200 can be generated by the second sensing channel 85 by filtering, amplifying, and digitizing the cardiac electrical signals received by the second sensing electrode vector. Although the signal 200 is conceptually shown as having only a positive waveform, it should be understood that the signal 200 can have positive and negative portions and need not be a rectified signal. The absolute value of the maximum peak amplitude (positive or negative) can be determined at Figure 7 box 116 based on the stored second cardiac electrical signal segments. The cardiac electrical signal 200 includes an R wave 202, a T wave 204, a P wave 206, and a subsequent R wave 240. The R wave 202 represents a recognition event that occurs at the time point 205. The time point 205 is the sampling point of the maximum absolute value of the differential signal determined in response to the R wave sensing event signal 250 as described above in connection with Figure 7 what was described.

[0101] If an R wave sensing event signal occurs during the blanking interval 214 after the time point 205 of the previous recognized R wave 202, a new R wave sensing event is not confirmed. The sensing channel 83 may have sensed the same R wave 202 twice, or may have sensed non-cardiac electrical noise as an R wave.

[0102] After the blanking interval, at the time point corresponding to the expiration of the blanking interval 214, the ratio threshold 210 is equal to the starting value 220, which can be set to 0.6 in one example, but can be in the range between 0.4 and 0.7 in other examples. In one embodiment, the ratio threshold 210 is stored in a look-up table and retrieved by the control circuit 80 from the memory 82 for comparison with the amplitude ratio determined in response to the R wave sensing event signal from the first sensing channel.

[0103] Figure 9This is an example of a look-up table 300 for ratio thresholds 304 that can be stored in a memory 82 for corresponding event time intervals, and these event time intervals can be stored as corresponding sample point numbers 302. If the blanking interval 214 is approximately 150 ms, then when the sampling rate is 256 Hz, the maximum amplitude can be determined such that the first sample point of the event time point is at sample point 38. In other examples, the blanking interval 214 can be longer or shorter than 150 ms, and the first sample point number stored in the look-up table 300 will correspond to the sample point number at which the blanking interval 214 expires after the confirmation event time point 205 (considered the "zero" sample point).

[0104] The ratio threshold is stored as the starting ratio threshold 220 for the first sample point number entry, which is 0.6 in this example. If the control circuit 80 receives an R-wave sensing event signal from the first sensing channel 83 and the detection interval counter is equal to or greater than the R-wave confirmation threshold, then the maximum event amplitude and event time are determined from the buffered second cardiac electrical signal. The event time can be determined as the sample point number since the event time 205 of the most recent confirmed R-wave 202. If the event time is determined to be sample point number 38, then the control circuit 80 retrieves the ratio threshold stored in the look-up table 300 for sample point number 38, which is 0.6 in this example. This ratio threshold is applied to the amplitude ratio determined from the maximum amplitude of the buffered second cardiac electrical signal and the maximum amplitude determined from the most recent confirmed R-wave 202.

[0105] Referring again to Figure 8 , the ratio threshold 210 is shown to decrease at a constant decay rate 222 until the first time interval 216 expires. The time interval 216 can be defined as starting at the time point 205 of the confirmation event 202 or starting after the blanking interval 205 expires. The time interval 216 can extend up to 1 second from the time point 205 of the most recent confirmed R-wave 202. In one example, the decay rate 222 can be approximately 0.3 / second, such that if the time interval 216 is approximately 1 second, the ratio threshold 224 is 0.3 when the starting ratio threshold 220 is 0.6.

[0106] Starting at the expiration of time interval 216, ratio threshold 210 remains at a constant value 224 until the expiration of a second time interval 218. The constant value 224 is, in one example, a ratio of approximately 1 / 3 (0.3), but in other examples can be between 1 / 5 (0.2) and 1 / 2 (0.5). After the expiration of time interval 216, the value 224 can be maintained for up to 500 ms (for a total time interval 218 that lasts up to 1.5 seconds). This change from decay rate 222 to constant value 224 is reflected in look-up table 300 as ratio threshold 0.3 starting at sample point number 256 and extending to sample point number 383.

[0107] At the expiration of time interval 218, ratio threshold 210 steps down to an intermediate ratio threshold 226 and then decays at a constant rate 228 until the ratio threshold reaches a minimum ratio threshold 230. The step down from the constant value 224 can be to a ratio threshold of approximately 1 / 6 to 1 / 4. In one example, the ratio threshold steps down from approximately 1 / 3 (0.3) to an intermediate ratio threshold of 1 / 5 (0.2) at 0.5 seconds after the expiration of time interval 216. This change is reflected in look-up table 300 as ratio threshold 0.2 at sample point 384 (0.5 seconds after sample point 256).

[0108] The second decay rate 228 can be the same as decay rate 222 or a slower decay rate such that ratio threshold 210 reaches minimum ratio threshold 230 (e.g., 1 / 32 (0.03), 1 / 64 (0.015)) or other predetermined minimum ratio at approximately 2.5 seconds (sample point number 640) after the time point 205 of the previously identified R wave 202. The behavior of ratio threshold 210 moving forward in time from the identified R wave 202 is captured in look-up table 300 ( Figure 9 ). For example, at an exemplary decay rate 222 of 0.3 / second, the ratio threshold is 0.587 at sample point number 48, and so on.

[0109] These values enumerated herein and reflected in look-up table 300 for ratio thresholds 220, 224, 226, and 230 and time intervals 216 and 218 are illustrative in nature; other values less than or greater than the enumerated values can be used to implement a time-varying ratio magnitude for confirming an R wave sensing event. The values of ratio thresholds and time intervals or decay rates and total decay intervals used to control the change from one ratio threshold to another will depend in part on the sampling rate, which is 256 Hz in the provided examples but can be greater than or less than 256 Hz in other examples.

[0110] Refer again to Figure 8, if the R-wave sensing event signal 252 is generated by the first sensing channel 83, the control circuit 80 is triggered to store a time segment 254 of the second cardiac electrical signal 200 in the memory 80. The time segment 254 can be 360 ms in one example and can be between 300 ms and 500 ms in other examples. If the VT interval counter or the VF interval counter or the combined VT / VF interval counter has reached the R-wave confirmation threshold, the control circuit 80 determines the maximum amplitude based on the buffered cardiac signal time segment. As described above, the maximum amplitude can be the maximum absolute value of the x-order differential signal determined from the second cardiac electrical signal 200. The maximum amplitude can be determined from a portion of the stored cardiac signal time segment. For example, the maximum amplitude can be determined from a segment 255 that is a sub-segment or a portion of the stored time segment 254. When the total time segment 254 is 360 ms to 500 ms long, the segment 255 can be about 50 ms to 300 ms long, such as 200 ms long. The segment 255 can be defined relative to the time when the R-wave sensing event signal 252 is received.

[0111] The sample point number 197 at which the maximum amplitude occurs within the time segment 255 represents the number of sample points since the event time 205 (sample point number zero) of the latest confirmed R-wave 202. The sample point number 197 is determined as the event time of the maximum amplitude of the cardiac signal time segment 255. The control circuit 80 uses this sample point number to look up the corresponding ratio threshold 304 in the look-up table 300. For example, the maximum amplitude during the time segment 255 obtained in response to the R-wave sensing event signal 252 can occur at the sample point number 197, which is about 0.77 seconds after the event time point 205. The stored ratio threshold for the sample point number 197 can be about 0.4 starting from an initial value 220 at an attenuation rate 222 of about 0.3 / second (or 0.0012 per sample point), and the initial value in this example is 0.6 starting at the sample point number 38. If the amplitude ratio of the maximum amplitude determined at the sample point number 197 during the time segment 255 to the maximum amplitude determined for the confirmed R-wave 202 is greater than or equal to 0.4, the R-wave sensing event 252 is confirmed. In this example, the cardiac electrical signal has a lower baseline amplitude during the interval 255, and thus, the R-wave sensing event signal 252 is not confirmed. The control circuit 80 increases the unconfirmed event counter as described in Figure 7 described.

[0112] Similarly, the control circuit 80 may receive an R-wave sensing event signal 256 and determine a maximum amplitude during a time segment 259 of the buffered cardiac electrical signal segment 258 (defined relative to the R-wave sensing event signal 256). The event time sample point number 403 at which the maximum amplitude has occurred since the event time 205 is used to look up a ratio threshold from the look-up table 300. In this case, the amplitude ratio determined from the buffered second cardiac electrical signal during the time segment 259 exceeds the ratio threshold 210 at the event time sample point number 403 of the maximum amplitude during the time segment 259, and the event time sample point number corresponds to the R-wave 240. The R-wave sensing event signal 256 is confirmed by the control circuit 80. In this way, the second cardiac electrical signal from the sensing channel 85 is analyzed only when the R-wave sensing event confirmation condition is met, for example, when the tachyarrhythmia interval counter is active and has reached a threshold count, which may be less than the number of intervals required to detect the onset of VT or VF. The R-wave sensing event of the first sensing channel is confirmed based on post-processing of the buffered second cardiac electrical signal.

[0113] Figure 10 FIG. 400 is a flow chart of a method for detecting tachyarrhythmia by an ICD 14 according to another example. The operations performed at blocks 102-114, 132, 136, 138, and 140 in the flow chart 400 generally may correspond to Figure 7 the blocks with the same numbers shown and described above. At blocks 102 and 104, two different sensing electrode vectors are selected by the sensing circuit 86 for receiving a first cardiac electrical signal through the first sensing channel 83 and a second cardiac electrical signal through the second sensing channel 85, as described above in connection with Figure 5 and Figure 7 described.

[0114] At block 106, sensing circuit 86 may generate an R-wave sensing event signal in response to the first sensing channel 83 detecting that a first cardiac electrical signal crosses the R-wave sensing threshold. The R-wave sensing event signal may be transmitted to control circuit 80. In response to the R-wave sensing event signal, control circuit 80 is triggered at block 108 to store a segment of the second cardiac electrical signal received from the second sensing channel 85 in a circular buffer of memory 82. The digitized segment of the second cardiac electrical signal, which may be defined in time relative to the time of the R-wave sensing event signal received from sensing circuit 86, may be, for example, 100 ms to 500 ms long. In one example, the buffered segment of the cardiac electrical signal is at least 92 sample points obtained at a sampling rate of 256 Hz, or approximately 360 ms, 68 of these sample points may be before and include the sample point at which the R-wave sensing event signal is received, and 24 of these sample points may extend after the sample point at which the R-wave sensing event signal is received.

[0115] In addition to buffering the segment of the second cardiac electrical signal, control circuit 80 also responds to the R-wave sensing event signal generated at block 106 by determining at block 110 the RRI that ends with the current R-wave sensing event signal and begins with the most recent previous R-wave sensing event signal. The timing circuit 90 of control circuit 80 may transmit RRI timing information to the tachyarrhythmia detection circuit 92, which adjusts the tachyarrhythmia detection counter at block 112, as described above in connection with Figure 7 that which has been described.

[0116] After updating the VT interval counter and the VF interval counter at block 112, the tachyarrhythmia detector 92 compares the interval counter values with an R-sensing confirmation threshold at block 114 and compares the counter values with VT and VF NID detection thresholds at block 132. If the VT or VF interval counter has reached the R-sensing confirmation threshold, the "yes" branch of block 114, then the second cardiac electrical signal from sensing channel 85 is analyzed to confirm the R-wave sensed by the first sensing channel 83 at block 106. In one example, the R-sensing confirmation threshold is a count of two on the VT interval counter and a count of 3 on the VF interval counter. Other examples are given above in connection with Figure 7 that which has been given.

[0117] If none of the interval counters reaches the R-sense confirmation threshold at block 114, the control circuit 80 waits at block 108 for the next R-wave sensing event signal to buffer the next segment of the second cardiac electrical signal. In some cases, the oldest buffered cardiac signal segment may be overwritten by the next cardiac signal segment and never be analyzed for R-wave confirmation or for any other purpose because analysis of the buffered cardiac signal segments is not required when the VT and VF interval counters are inactive (at count zero) or remain below the R-sense confirmation threshold.

[0118] If the R-sense confirmation threshold is reached at block 114, the control circuit 80 applies a notch filter to the stored segment of the second cardiac electrical signal at block 416. The notch filter applied at block 416 can correspond to the filter described in connection with Figure 6 The notch filter significantly attenuates 50 Hz - 60 Hz electrical noise, muscle noise, other EMI, and other noise / artifacts in the stored segment of the second cardiac electrical signal. Using the notch-filtered segment, the control circuit 80 performs multiple analyses on the segment to determine if any suppression rules are met. As described below, if the suppression rules are met, the upcoming VT or VF episode detection based on reaching the NID threshold can be stopped at block 132.

[0119] As described in connection with Figure 7 An amplitude ratio can be determined at block 418 to confirm the R-wave sensing event signal that triggered buffering of the currently stored segment of the second cardiac electrical signal. The determination performed at block 418 can include operations performed at blocks 116, 118, and 120 of Figure 7 At block 428, the amplitude ratio is used to update the R-wave confirmation suppression rules.

[0120] At block 418, the maximum peak amplitude used to determine the amplitude ratio can be determined from a portion of the stored cardiac signal segment. For example, if a 360 ms or 500 ms segment is stored at block 108, only a 200 ms segment (e.g., approximately 52 samples at 256 Hz sampling) centered in time on the R-wave sensing event signal can be analyzed to determine the amplitude ratio at block 418. A longer signal segment can be stored at block 108 than is required to determine the amplitude ratio at block 418, such that the longer segment can be used for other signal analysis procedures performed by the tachyarrhythmia detector 92 as described below.

[0121] In the case where the current R-wave sensing event signal is not the first confirmed R-wave sensing event signal since reaching the R-sensing confirmation threshold at block 114, the control circuit 80 can determine, at block 418, the event interval as the time interval or number of sampling points from the maximum peak amplitude to the previously confirmed R-wave sensing event. At block 428, the control circuit 80 can compare the amplitude ratio with a ratio threshold, which can be retrieved from a look-up table stored in the memory 82 using the determined event interval as described in connection with Figure 8 and Figure 9 . If the amplitude ratio is greater than the ratio threshold, the sensed R-wave is confirmed. If the amplitude ratio is less than the ratio threshold, the sensed R-wave is not confirmed.

[0122] At block 428, the X-of-Y unconfirmed beat counter in Y can be updated by the tachycardia detector 92 to reflect the number of unconfirmed R-wave sensing event signals among the latest Y R-wave sensing event signals. For example, the X-of-Y counter can count how many of the latest 12 R-wave sensing event signals are not confirmed as R-waves. If the X-of-Y count reaches a suppression threshold, e.g., if at least 3, 4, 5, or another predetermined number of R-wave sensing event signals out of 12 R-wave sensing event signals are not confirmed as R-waves, then the R-wave suppression rule for stopping tachycardia detection is satisfied. A flag or logical value can be set by the control circuit 80 to indicate that the R-wave suppression rule is satisfied. Updating the R-wave suppression rule at block 428 can include operations described in connection with Figure 7 for blocks 122, 124, 126, 128, 130, and 134.

[0123] At blocks 420, 422, 424, and 426, other cardiac signal parameters can be determined from the notch-filtered cardiac signal segments to update the status of other tachycardia detection suppression rules at corresponding blocks 430, 432, 434, and 436. In some examples, the digitized cardiac electrical signals from the first sensing channel 83 can be analyzed and used to update the status of the tachycardia detection stop rule. For example, the notch-filtered cardiac electrical signals from the second sensing channel 85 can be analyzed at blocks 420, 422, and 426 to update the overall morphology suppression rule, the beat morphology suppression rule, and the noise suppression rule correspondingly at blocks 430, 432, and 436. The differential signal 60 from the first sensing channel 83 (see Figure 5 ) can be analyzed at block 424 to update the T-wave oversensing (TWOS) rule at block 434.

[0124] At block 420, one or more overall morphological parameters can be determined from the notch-filtered second cardiac signal segment. The overall morphological parameters can include, but are not limited to: low slope content, noise pulse count, normalized rectified amplitude, or other noise metrics. Examples of overall morphological parameters that can be determined are generally disclosed in U.S. Patent Nos. 7,761,150 (Ghanem et al.) and 8,437,842 (Zhang et al.) cited above. These overall morphological parameters can be determined using the entire second cardiac signal segment stored at block 108 or a portion of the stored segment. In one example, at least 92 sample points, approximately 360 ms, which can be a portion of the stored segment or the entire stored segment, are analyzed to determine the overall morphological parameters.

[0125] At block 430, the overall morphological parameters are used to update the state of the overall morphological suppression rule. A criterion or threshold can be applied to each of the determined overall morphological parameters, and when a desired number of these overall morphological parameters meet the criterion or threshold applied to the corresponding parameter, the overall morphological suppression rule can be satisfied. For example, if at least two of three overall morphological parameters meet the noise detection criterion, the overall morphological suppression rule is satisfied. At block 430, the control circuit 80 can set a flag or logic signal indicating this.

[0126] At block 422, a morphological match score is determined from the stored second cardiac electrical signal segment. The morphological match score can be determined by performing a wavelet transform or other morphological matching analysis on a portion of the stored segment, e.g., on at least 48 signal sample points or approximately 190 ms, and can be performed using the notch-filtered signal generated at block 416. The morphological matching analysis can include: aligning a selected portion of the stored segment with a previously determined known R-wave template; and determining the morphological match score. The morphological match score can have a possible value range from 0 to 100 and indicates how well the morphology of the second cardiac signal segment matches the known R-wave template. The wavelet transform method generally disclosed in U.S. Patent No. 6,393,316 (Gillberg et al.) is an example of a morphological matching method that can be performed at block 422 for determining the match score. Other morphological matching methods that can be implemented by the tachyarrhythmia detector 92 can compare the waveform, amplitude, slope, time inflection points, number of peaks, or other features of the stored second cardiac electrical signal with the known R-wave template. More specifically, the waveform duration or width, waveform polarity, waveform positive slope, waveform negative slope, and / or other waveform features can be used alone or in combination to characterize the similarity between the unknown waveform and the known R-wave template. The morphological matching method can use one morphological feature of the stored second cardiac electrical signal or a combination of two or more morphological features to determine the match with the known R-wave template. A posture-independent method for determining the morphological match score generally disclosed in the pre-grant U.S. Patent Application No. 2016 / 0022166 (Stadler et al.) can be performed, which includes generating a posture-independent R-wave template for template matching. Other beat morphological matching techniques that can be used at block 422 are generally disclosed in U.S. Patent No. 8,825,145 (Zhang et al.) and U.S. Patent No. 8,983,586 (Zhang et al.).

[0127] At block 432, the tachyarrhythmia detector 92 uses the morphological match scores to update the beat morphology suppression rule. In one example, the beat morphology suppression rule can be satisfied when a minimum number of the morphological match scores among a predetermined number of the most recent morphological match scores exceed a match score threshold. For example, if at least three out of eight of the most recent morphological match scores exceed a match score threshold of 50, 60, 70, or some other score threshold, the beat morphology suppression rule is satisfied. A relatively high match score exceeding the selected match score threshold indicates that the unknown beat matches the known R-wave template and is thus a normal R-wave rather than a VT or VF beat. Accordingly, when a threshold number of the morphological match scores among the most recent morphological match scores are determined to be normal R-waves, the beat morphology suppression rule is satisfied, and the control circuit 80 can set a flag or logic signal indicating this.

[0128] At block 424, one or more TWOS parameters are determined from the stored digitized cardiac electrical signals. In some cases, these TWOS parameters are determined from the first-order difference signal 69 received from the first sensing channel 83 as described above in connection with Figure 5 The first-order difference signal is determined by subtracting the amplitude of the n - 1th sample point from the nth sample point. Alternatively, the second cardiac electrical signal from the sensing channel 85 can be used to determine the TWOS parameters, either before or after notch filtering. The tachyarrhythmia detector 92 can be configured to perform the T-wave oversensing suppression algorithm by determining the differential filtered cardiac electrical signal and the TWOS parameters as generally disclosed in U.S. Patent No. 7,831,304 (Cao et al.) cited above. Other aspects of detecting TWOS that can be used to determine the TWOS parameters from the first or second cardiac electrical signals are generally disclosed in U.S. Patent No. 8,886,296 (Patel et al.) and U.S. Patent No. 8,914,106 (Charlton et al.).

[0129] At block 434, the tachyarrhythmia detector 92 uses the (multiple) TWOS parameters determined for the currently stored cardiac signal segment to update the status of the TWOS suppression rule to satisfied or not satisfied. For example, if one or more TWOS parameters indicate that an R-wave sensing event signal generated by the first sensing channel 83 is likely an oversensed T-wave, the TWOS event counter can be updated at block 434. If the TWOS event counter reaches a threshold, the TWOS suppression rule is satisfied. The control circuit 80 can set a flag or logic signal indicating when the TWOS suppression rule is satisfied.

[0130] Other noise parameters may be determined at block 426 to identify oversensing due to noise artifacts. These noise parameters determined at block 426 may include determining peak amplitudes from the notch-filtered cardiac electrical signals. All or a portion of the stored signal segments may be used to determine one or more amplitude peaks. The peak amplitudes determined at block 426 may include the maximum peak amplitude determined at block 418 for determining the amplitude ratio. The maximum peak amplitude of one or more of the stored cardiac signal segments is compared to a noise detection criterion to determine whether the noise suppression rule is satisfied at block 436. The control circuit 80 sets a flag or logic signal to indicate the status of the noise suppression rule at block 436.

[0131] After adjusting the VT interval counter and the VF interval counter at block 112, the tachyarrhythmia detector 92 compares the interval counters to the VT and VF NID detection thresholds at block 132. If the VT or VF interval counter has reached the NID, the tachyarrhythmia detector 92 checks the status of the suppression rule at block 440. If the suppression criterion is satisfied at block 440 based on the status of one or more suppression rules, the "yes" branch of block 440, then the VT or VF detection based on the RRI analysis at blocks 110, 112, and 132 is stopped at block 140. No VT or VF therapy is delivered. The process returns to block 110 to determine the next RRI upon receipt of the next R-wave sensing event signal from the sensing channel 83.

[0132] If the suppression criterion is not satisfied, the "no" branch of block 440, then a VT or VF episode is detected at block 136 based on which of the VT or VF interval counters reached its corresponding NID threshold. The control circuit 80 controls the therapy delivery circuit 84 to deliver therapy at block 138 based on the type of the detected episode and the programmed therapy delivery control parameters.

[0133] In some examples, the suppression criterion applied at block 440 only requires a single suppression rule to be satisfied for the tachyarrhythmia detector 92 to stop VT or VF detection. In other examples, two or more suppression rules may be required to be satisfied before stopping the RRI-based VT or VF detection. In still other examples, one suppression rule may be associated with another suppression rule such that the suppression criterion is satisfied at block 440. For example, the R-wave confirmation suppression rule may only be used to satisfy the suppression criterion if the overall morphology suppression rule is also satisfied. In this case, the R-wave confirmation suppression rule alone cannot be used to satisfy the suppression criterion at block 440. The overall morphology suppression rule may be used alone, in combination with the R-wave confirmation suppression rule, or in combination with another rule to satisfy the suppression criterion.

[0134] These suppression rules updated at blocks 428 through 436 can be programmatically enabled or disabled by a user using the external device 40. The control circuit 80 can determine which parameters are needed at blocks 418 through 426 to update the status of only the suppression rules that are enabled or programmed as "on".

[0135] Figure 11 is a flowchart 500 of a method for detecting tachyarrhythmias by an ICD 14 according to another example. In Figure 7 and Figure 10 example, the number of RRIs falling within the corresponding VT or VF interval range or region determined from the first sensing electrode vector 102 is tracked by corresponding VT and VF interval counters. The counts of the VT and VF interval counters are compared with corresponding VT and VF NID thresholds at block 132. Figure 11 Blocks with the same numbers in Figure 7 and Figure 10 correspond to blocks with similar numbers shown in

[0136] In other examples, the second cardiac electrical signal received from the second electrode vector at block 104 through the second sensing channel 85 can also be used to determine RRIs and determine at decision block 438 whether the NID threshold is reached. The tachyarrhythmia detector 92 can include a second VT and VF interval counter for counting the RRIs determined from the second cardiac electrical signal received through the second sensing channel 85. The second VT and VF interval counter can be updated at block 415 based on the RRIs determined from the second cardiac electrical signal received via the second sensing electrode vector 104.

[0137] In one instance, after the R sensing confirmation threshold is reached at block 114, the tachyarrhythmia detector 92 can start updating the second VT and VF interval counter at block 415. The process of updating the second VT and VF interval counter starting from an initial zero count can include: confirming the R wave at block 428 based on comparing the amplitude ratio with a ratio threshold as described in connection with Figure 7 blocks 122, 124, 126, and 128. If the R wave is confirmed at block 428, the tachyarrhythmia detector 92 compares the event interval determined at block 418 with the VT and VF interval regions at block 415. The event interval determined at block 418 is the time interval from the time of the latest confirmed R wave event to the event time of the maximum absolute amplitude of the time segment stored for the latest R wave sensing event signal.

[0138] If the most recent R-wave sensing event signal is confirmed at block 428, then at block 415 the event interval can be compared to VT and VF interval regions defined to be the same as the interval regions applied to the RRIs determined from the R-wave sensing event signals generated by the first sensing channel 83 at block 112. If the event interval determined for the confirmed R-wave at block 418 falls within the VT interval region, then at block 415 the second VT interval counter is incremented. If the event interval falls within the VF interval region, then at block 415 the second VF interval counter is incremented. In some examples, if the event interval falls within the VT or VF interval region, then the combined VT / VF interval counter is incremented.

[0139] If one of the first VT or VF interval counters (or combined VT / VF interval counter) applied to the RRIs determined from the first sensing channel 83 reaches the NID at block 132, then the tachyarrhythmia detector 92 can compare the second VT and VF interval counters to a second NID requirement at block 438. The second VT NID and second VF NID used by the tachyarrhythmia detector 92 can be less than the VT NID and VF NID applied to the first VT and VF interval counters at block 132. In some examples, after the R-sensing confirmation threshold is reached at block 114, updating of the second VT and VF interval counters begins. Thus, the counts of the second VT and VF interval counters can be less than the first VT and VF interval counters (adjusted at block 112). The counts of the second VT and VF interval counters can lag behind the first VT and VF interval counters by the number of intervals required to reach the R-sensing confirmation threshold. For example, if the first VT interval counter is required to have a count of at least 2 or the first VF interval counter is required to have a count of at least 3 to reach the R-sensing confirmation threshold at block 114, then the second VT or VF interval counter can have a count of at least 2 or 3, respectively, less than the corresponding first VT or VF interval counter.

[0140] If one of the second VT or VF interval counters reaches the second NID, the "yes" branch of block 438, then the tachyarrhythmia detector 92 is based on as described above in connection with Figure 10The updated suppression rule status at blocks 428 to 436 is described to determine at block 440 whether the suppression criteria are met. If neither the second VT or VF interval counter reaches the second NID at block 438, the "No" branch of block 438, the tachyarrhythmia detector 92 does not proceed to check the suppression criteria at block 440. Instead, the tachyarrhythmia detector 92 may wait for the corresponding first VT or VF interval counter to reach the first VT or VF NID at block 132 and wait for the corresponding second VT or VF interval counter to reach the corresponding second NID at block 438. For example, to proceed to block 440 to determine whether the suppression criteria are met and then either detect VT at block 136 or stop VT detection at block 140, it is required that the first VT interval counter reaches the first VT NID at block 132 and that the second VT interval counter reaches the second VT NID at block 438. Similarly, to proceed to block 440 to determine whether the suppression criteria are met and then either detect VF at block 136 or stop VF detection at block 140, it is required that the first VF interval counter reaches the first VF NID at block 132 and that the second VF interval counter reaches the second VF NID at block 438.

[0141] Figure 12 is according to another example of Figure 4 the circuit diagram of the circuitry included in the sensing circuit. In Figure 12 the components with the same numbers in the sensing channels 83 and 85 correspond to the components with similar numbers described and shown therein in conjunction with Figure 5 In the example of Figure 5 the first sensing channel 83 is configured to sense R waves in real time through the R wave detector 66 and generate an R wave sensing event signal 68, which is transmitted to the timing circuit 90 when an R wave is sensed. The second sensing channel 85 is configured to transmit the filtered digitized output signal 78 to the memory 82 to store a second cardiac electrical signal segment when triggered by the R wave sensing event signal 68 from the first sensing channel 83 without performing real-time R wave sensing from the second cardiac electrical signal.

[0142] In Figure 12In the example, the second sensing channel 85 is configured to transmit the digitized filtered output signal 78 to the memory 82 for storing the second cardiac electrical signal segment as described above. The sensing channel 85 is additionally configured to perform real-time R-wave sensing from the second cardiac electrical signal. In this case, the second sensing channel 85 includes a rectifier 75 for rectifying the digitized and band-pass filtered signal output of the filter 74. The rectified signal is transmitted from the rectifier 75 to the R-wave detector 77. The R-wave detector may include a sense amplifier, a comparator, or other R-wave detection circuitry configured to apply an automatically adjusted R-wave sensing threshold to the rectified signal to sense an R-wave in response to a positive R-wave sensing threshold crossing.

[0143] The second sensing channel 85 may generate R-wave sensing event signals 79, which are transmitted in real time to the timing circuit 90 for determining the RRI based on the second cardiac electrical signal. The RRI may be determined as the time interval or sample point count between consecutively received R-wave sensing event signals 79. The timing circuit 90 may transmit the RRI determined from the R-wave sensing event signals 79 from the second sensing channel 85 to the tachyarrhythmia detector 92 for updating the second VT and VF interval counters based on the RRI determined from real-time R-wave sensing through the second sensing channel 85.

[0144] In Figure 11 Flowchart 500, the second VT and VF interval counters are updated by the tachyarrhythmia detector 92 at block 415 based on the R-waves confirmed at block 428 due to post-processing of the stored second cardiac electrical signal segments. In Figure 11 the example, the second sensing channel 85 does not sense R-waves in real time (the signal segments that can be recorded by the R-wave sensing event signals are not generated in real time as an R-wave is sensed based on the R-wave sensing threshold). In Figure 12 it, the second sensing channel 85 is configured to sense R-waves in real time from the second cardiac electrical signal received by the second sensing vector 104, and thus, the tachyarrhythmia detector 92 may update the second VT and VF interval counters based on real-time R-wave sensing through the second sensing channel 85.

[0145] Figure 13 is Flowchart 500 of a method for detecting tachyarrhythmias by the ICD 14 according to another example, in which the second sensing channel 85 is configured to sense R-waves from the second cardiac electrical signal in real time, and additionally, the control circuit 80 is configured to confirm the R-waves sensed by the first sensing channel 83 by post-processing the second cardiac electrical signal. Figure 13 The blocks with the same numbers in Figure 7 and / or Figure 11correspond to the boxes with similar numbers shown and described in conjunction therewith.

[0146] In Figure 13 the example of, at box 105, the R-wave detector 77 of the sensing channel 85 generates an R-wave sensing event signal 79 ( Figure 12 as shown in ) in response to the second cardiac electrical signal crossing a second R-wave sensing threshold. The second R-wave sensing threshold can be an auto-adjusting threshold and can be different from the R-wave sensing threshold used by the R-wave detector 66 of the first sensing channel 83. The timing circuit 90 determines the RRI between consecutive R-wave sensing event signals 79 received from the second sensing channel 85 at box 105 and transmits the determined RRI to the tachyarrhythmia detector 92. At box 419, the tachyarrhythmia detector 92 adjusts a second VT interval counter or a second VF interval counter in response to each RRI determined at box 105, and both of these interval counters can be X-in-Y type counters. In this example, the second VT and VF interval counters of the tachyarrhythmia detector 92 can be updated in real time, similar to the first VT and VF interval counters used to count the RRI determined from the first cardiac electrical signal. The second VT and VF interval counters can be updated on a beat-by-beat basis without requiring an R-sensing confirmation threshold (box 114) to be reached first.

[0147] If the tachyarrhythmia detector 92 determines at box 132 that a first VT NID or a first VF NID is reached, the tachyarrhythmia detector 92 compares the second VT and VF interval counters with a second VT NID or a second VF NID, respectively, at box 439. In this case, the second VT NID and the second VF NID can be the same as the first VT NID and the first VF NID because the first and second VT interval counters and the first and second VF interval counters are all updated in response to real-time sensed R-waves. If the second VT or VF NID has not been reached (the "no" branch of box 430), the tachyarrhythmia detector 92 can return to box 132 to wait for the VT or VF NID threshold to be reached based on the R-waves sensed in real time by both the corresponding first and second sensing channels 83 and 85.

[0148] If the corresponding second VT or VF NID is reached at box 439 when the first VT or VF NID is reached at box 132, the tachyarrhythmia detector 92 is as previously described in conjunction with Figure 10Determine whether the inhibition criterion is satisfied at block 440 as described. If the inhibition criterion is not satisfied, stop any upcoming VT or VF detection based on the real-time detection of tachyarrhythmia intervals from both the first and second cardiac electrical signals at block 140 in response to the inhibition criterion being satisfied or confirmed at block 135. Control circuit 80 controls the therapy delivery circuit to deliver anti-tachyarrhythmia therapy at block 138 in response to VT or VF detection.

[0149] Accordingly, a method and apparatus for confirming R-wave sensing events and detecting ventricular tachyarrhythmias in an extracardiac ICD system have been presented in the foregoing description in conjunction with specific examples. In other examples, the various methods described herein may include steps performed in different orders or combinations than the illustrative examples shown and described herein. It should be understood that various modifications may be made to the reference examples without departing from the scope of the present disclosure and the following claims.

Claims

1. A medical device, comprising: a sensing circuit, the sensing circuit comprising: a first sensing channel configured to receive a first cardiac electrical signal via a first sensing electrode vector coupled to the medical device and to sense a first plurality of R waves in response to the first cardiac electrical signal crossing a first R wave sensing threshold, and a second sensing channel configured to receive a second cardiac electrical signal via a second sensing electrode vector coupled to the medical device and to sense a second plurality of R waves in response to the second cardiac electrical signal crossing a second R wave sensing threshold, wherein the second sensing electrode vector is different from the first sensing electrode vector; and a control circuit coupled to the sensing circuit and configured to: determine a first plurality of intervals between consecutive R waves in the first plurality of R waves sensed by the first sensing channel; adjust at least one of a first VT interval counter or a first VF interval counter based on a comparison of each interval in the first plurality of intervals with a corresponding VT or VF detection interval region; and determine that at least one of the first VT interval counter or the first VF interval counter exceeds a corresponding VT or VF detection interval quantity threshold; determine a second plurality of intervals between consecutive R waves in the second plurality of R waves sensed by the second sensing channel; adjust at least one of a second VT interval counter or a second VF interval counter based on a comparison of each interval in the second plurality of intervals with a corresponding VT and VF detection interval region; and determine that at least one of the second VT interval counter or the second VF interval counter exceeds a corresponding VT or VF detection interval quantity threshold; determine whether a suppression criterion is met in response to determining both: at least one of the first VT interval counter or the first VF interval counter exceeds the corresponding VT or VF detection interval quantity threshold, and at least one of the second VT interval counter or the second VF interval counter has reached the corresponding VT or VF detection interval quantity threshold; and detect a VT episode or a VF episode when the suppression criterion is not met, wherein the first plurality of R waves will be sensed by the sensing circuit through real-time R wave sensing, and the second plurality of R waves will be sensed by the sensing circuit without performing real-time R wave sensing.

2. The medical device according to claim 1, characterized in that, After the R waves sensed from the first cardiac electrical signal are confirmed based on the second cardiac electrical signal, R waves will be sensed from the second cardiac electrical signal.

3. The medical device according to claim 1, wherein Further comprising a therapy delivery circuit, wherein the control circuit is configured to: control the therapy delivery circuit to deliver an anti-tachyarrhythmia therapy in response to detecting the VT episode or the VF episode.

4. The medical device according to claim 1, characterized in that, The control circuit is configured to determine whether the suppression criterion is met by determining whether at least one of the following occurs: an amplitude ratio is greater than a ratio threshold; a required quantity of overall morphological parameters meets a criterion or threshold applied to the corresponding parameter; a T wave over-sensing event counter has reached a threshold; and The maximum peak amplitude of one or more stored segments of the cardiac signal satisfies a noise suppression rule.

5. The medical device according to claim 1, characterized in that, Further comprising a memory, wherein the control circuit is coupled to the sensing circuit and the memory, and the control circuit is configured to: Store a time segment of the second cardiac electrical signal in the memory in response to each of the first plurality of R waves sensed by the first sensing channel; Determine whether a suppression criterion is satisfied in response to at least one of the first VT interval counter or the first VF interval counter reaching an R wave confirmation threshold, based at least on an analysis of at least a portion of the time segment of the second cardiac electrical signal.

6. The medical device according to claim 5, characterized in that, The control circuit is further configured to: pass the time segment of the second cardiac electrical signal through a notch filter prior to analyzing at least the portion of the time segment of the second cardiac electrical signal.

7. The medical device according to claim 5, characterized in that, The control circuit is further configured to: when neither the first VT interval counter nor the first VF interval counter reaches the R wave confirmation threshold, control the memory to overwrite the time segment of the second cardiac electrical signal without analyzing the time segment.

8. The medical device according to claim 5, wherein The control circuit is configured to analyze at least the portion of the time segment of the second cardiac electrical signal in response to at least one of the following: The VT interval counter reaches a count of at least 2, or The VF interval counter reaches a count of at least 3.

9. The medical device according to claim 1, wherein The control circuit is further configured to: stop detecting the VT episode or the VF episode when the suppression criterion is satisfied.

10. The medical device according to claim 1, characterized in that, The control circuit is configured to determine whether the suppression criterion is satisfied by determining whether at least one of the following occurs: The noise suppression criterion is satisfied; The T wave over-sensing suppression criterion is satisfied; The overall morphology suppression criterion is satisfied; The beat morphology suppression criterion is satisfied; and The R wave suppression criterion is satisfied.

11. The medical device according to claim 1, characterized in that, Further comprising a lead coupled to the medical device and including a plurality of electrodes, wherein the first sensing electrode vector has a first inter-electrode spacing, and the second sensing electrode vector has a second inter-electrode spacing, the second inter-electrode spacing being greater than the first inter-electrode spacing.

12. The medical device according to claim 1, wherein, The medical device includes a housing that encloses the sensing circuit, the medical device further includes a lead coupled to the medical device and including a plurality of electrodes, wherein the first sensing electrode vector is a short dipole between the electrodes of the lead, and the second sensing electrode vector is a relatively long dipole between the electrodes along the distal portion of the lead and the housing of the medical device.

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