Medical device and method for delivering atrial arrhythmia therapy

Abstract

A medical device system is configured to sense one or more cardiac signals and deliver a first therapy by delivering atrioventricular nodal stimulation (AVNS) pulse trains generated according to first pulse parameters. The medical device system may detect an atrial tachyarrhythmia from the one or more cardiac signals and, in response to detecting the atrial tachycardia, terminate the first therapy. The medical device system may, in response to at least the atrial tachyarrhythmia being detected, deliver a second therapy by delivering AVNS pulse trains generated according to second pulse parameters during the detected atrial tachycardia, the second pulse parameters being different than the first pulse parameters.

Description

Ref. No. A0013103 WO01MEDICAL DEVICE AND METHOD FOR DELIVERING ATRIAL ARRHYTHMIA THERAPYCROSS-RELATED REFERENCES

[0001] This application is a PCT application that claims priority to, and the benefit of, U.S. Provisional Patent Application No. 63 / 750,180, filed January 27, 2025, the entire contents of which is incorporated herein by reference.TECHNICAL FIELD

[0002] The disclosure relates generally to a medical device and method for delivering electrical stimulation therapies for treating atrial arrhythmias.BACKGROUND

[0003] Medical devices may sense electrophysiological signals from the heart, brain, nerve, muscle or other tissue. Such devices may be implantable, wearable or external devices using implantable and / or surface (skin) electrodes for sensing the electrophysiological signals. In some cases, such devices may be configured to deliver a therapy based on the sensed electrophysiological signals. For example, implantable or external cardiac pacemakers, cardioverter defibrillators, cardiac monitors and the like, sense cardiac electrical signals from a patient’s heart. Such devices may deliver electrical stimulation therapies, such as cardiac pacing pulses and / or cardioversion or defibrillation (CV / DF) shocks, to the heart using electrodes, which may be carried by one or more medical electrical leads coupled to the medical device. Cardiac signals sensed from the heart may be analyzed for detecting an abnormal rhythm. Upon detection of an abnormal rhythm, such as bradycardia, tachycardia or fibrillation, an appropriate electrical stimulation pulse or pulses may be delivered to restore or maintain a more normal rhythm of the heart. For example, an implantable cardioverter defibrillator (ICD) may deliver bradycardia pacing pulses to the heart of the patient in the absence of sensed intrinsic myocardial depolarization signals, e.g., R-waves, deliver antitachycardia pacing pulses in response to detecting tachycardia, or deliver CV / DF shocks to the heart upon detecting tachycardia or fibrillation.

[0004] During normal sinus rhythm (NSR), the heartbeat is regulated by electrical signals produced by the sino-atrial (SA) node located in the right atrial wall. Each intrinsic atrialRef. No. A0013103 WO01depolarization signal produced by the SA node spreads across the atria, causing the depolarization and contraction of the atria, and arrives at the atrioventricular (AV) node. The AV node responds by propagating a ventricular depolarization signal through the Bundle of His (or “His bundle”) of the ventricular septum and thereafter to the Purkinje branches and the Purkinje muscle fibers of the right and left ventricles. This native conduction system including the His bundle, right and left branches (sometimes referred to as the right and left bundle branches) and the Purkinje fibers may be referred to as the “His-Purkinje conduction system” or “His-Purkinje system.”

[0005] Patients with a conduction system abnormality, e.g., poor AV node conduction, poor SA node function, or other conduction abnormalities, may receive a pacemaker to restore a more normal heart rhythm and AV synchrony. In patients having intact intrinsic atrioventricular (AV) conduction, atrial depolarizations occurring during an atrial tachyarrhythmia such as atrial flutter or atrial fibrillation, can be conducted to the ventricles at a fast and / or irregular rate, which can be symptomatic. Some patients experience chronic or persistent atrial fibrillation (AF) that can result a rapid and / or irregular ventricular rate.Patients diagnosed with chronic or persistent AF may undergo AV nodal ablation with implantation of a pacemaker to provide ventricular pacing. AV nodal ablation that causes permanent AV conduction block can prevent the AF depolarizations from conducting to the ventricles, but a pacemaker can be provided to deliver ventricular pacing to sustain the heartbeat after AV nodal ablation. The patient becomes pacemaker dependent when AV nodal ablation is used to address conducted atrial arrhythmias.SUMMARY

[0006] In general, the disclosure is directed to a medical device and techniques for controlling and delivering cardiac electrical stimulation therapy for inhibiting atrial tachyarrhythmias and for regulating the ventricular rate during an atrial tachyarrhythmia. The medical device may be configured to sense cardiac electrical signals and deliver AV nodal stimulation (AVNS) therapies comprising pulse trains, which may be delivered in the area of the AV node or a parasympathetic nerve during cardiac refractory periods, e.g., atrial refractory periods and / or ventricular refractory periods. The AVNS therapy may be delivered according to a first set of pulse parameters to deliver atrial tachyarrhythmia prevention therapy (“AT / AF prevention therapy”) at times when atrial tachyarrhythmia is not being detected. The AT / AF prevention therapy is delivered according to the first set of pulseRef. No. A0013103 WO01parameters selected to inhibit future atrial tachyarrhythmia. When atrial tachyarrhythmia is detected, the AVNS pulse trains may be delivered according to a second set of pulse parameters to deliver ventricular (V) rate control therapy during the atrial tachyarrhythmia.

[0007] The V rate control therapy can be delivered by the medical device to reduce the ventricular rate and / or promote a regular ventricular rate during a conducted AT / AF. The medical device may be configured to detect atrial tachyarrhythmia, e.g., atrial tachycardia (AT) or atrial fibrillation (AF), and may detect a fast and / or irregular ventricular rate when AT or AF is detected. The medical device may initiate the V rate control therapy to suppress intrinsic AV conduction, e.g., by lengthening the refractory period of the AV node. The AVNS pulse trains delivered for V rate control therapy can be delivered at a pulse frequency, pulse amplitude, pulse width, and pulse number that causes a ventricular rate response by blocking conduction of at least some atrial depolarizations to the ventricles, effectively slowing or regulating the ventricular rate.

[0008] At times when AT / AF is not being detected and the V rate control therapy is not being delivered, the medical device may be configured to deliver AVNS pulse trains according to the second set of pulse parameters for inhibiting the recurrence AT / AF. This AT / AF prevention therapy is delivered to the AV node or a parasympathetic nerve branch as pulse trains having relatively lower delivered energy than the pulse trains delivered for V rate control therapy. For instance, the AT / AF prevention therapy may be delivered using a pulse amplitude and / or pulse width that is that is less than the pulse amplitude and / or pulse width used to deliver the V rate control therapy in some examples.

[0009] In one example, the disclosure provides a medical device system including sensing circuitry configured to sense one or more cardiac signals and control circuitry configured to detect an atrial tachyarrhythmia from the one or more cardiac signals. The medical device system includes therapy delivery circuitry configured to deliver a first therapy by delivering first AVNS pulse trains generated according to first pulse parameters. The therapy delivery circuitry may be further configured to, in response to the control circuitry detecting the atrial tachyarrhythmia, terminate the first therapy. In response to at least the atrial tachyarrhythmia being detected by the control circuitry, the therapy delivery circuitry may deliver a second therapy by delivering second AVNS pulse trains generated according to second pulse parameters different than the first pulse parameters.

[0010] In another example, the disclosure provides a method including sensing one or more cardiac signals, delivering a first therapy by delivering first AVNS pulse trains generatedRef. No. A0013103 WO01according to first pulse parameters, detecting an atrial tachyarrhythmia from the one or more cardiac signals, and, in response to detecting the atrial tachyarrhythmia, terminating the first therapy. The method may further include, in response to at least the atrial tachyarrhythmia being detected, delivering a second therapy by delivering second AVNS pulse trains generated according to second pulse parameters, the second pulse parameters different than the first pulse parameters.

[0011] In yet another example, the disclosure provides a non-transitory computer readable medium storing a set of instructions that, when executed by control circuitry of a medical device system, cause the medical device system to sense one or more cardiac signals, deliver a first therapy by delivering first AVNS pulse trains generated according to first pulse parameters, detect an atrial tachyarrhythmia from the one or more cardiac signals, and, in response to detecting the atrial tachyarrhythmia, terminate the first therapy. The instructions may further cause the medical device system to, in response to at least the atrial tachyarrhythmia being detected, deliver a second therapy by delivering second AVNS pulse trains generated according to second pulse parameters, the second pulse parameters different than the first pulse parameters.

[0012] This summary is intended to provide an overview of the subject matter described in this disclosure. It is not intended to provide an exclusive or exhaustive explanation of the apparatus and methods described in detail within the accompanying drawings and description below. Further details of one or more examples are set forth in the accompanying drawings and the description below.BRIEF DESCRIPTION OF DRAWINGS

[0013] FIG. l is a conceptual diagram of a medical device system including an implantable medical device coupled to transvenous electrical leads extending to operative positions relative to a patient’s heart.

[0014] FIG. 2 is a conceptual diagram of a medical device system for sensing cardiac electrical signals and delivering AVNS according to some examples.

[0015] FIG. 3 is a conceptual diagram of a medical device system that may include multiple implantable medical devices for sensing cardiac electrical signals, delivering AVNS and delivering ventricular pacing according to another example.

[0016] FIG. 4 is a conceptual diagram of an implantable medical device (IMD) configured to sense cardiac electrical signals and deliver AVNS according to some examples.Ref. No. A0013103 WO01

[0017] FIG. 5 is a flow chart of a method for controlling AVNS therapy by an IMD system according to some examples.

[0018] FIG. 6 is a flow chart of a method for controlling AVNS by an IMD system according to another example.

[0019] FIG. 7 depicts a diagram of an AVNS pulse train that may be delivered during the V rate control therapy and a diagram of an AVNS pulse train that may be delivered during the AT / AF prevention therapy according to some examples.

[0020] FIG. 8 is a flow chart of a method that may be performed by a medical device for selecting pulse parameters used for delivering the V rate control therapy and the AT / AF prevention therapy according to some examples.DETAILED DESCRIPTION

[0021] In general, this disclosure describes a medical device and techniques for delivering AVNS for V rate control during AT / AF and, at times when AT / AF is not occurring, delivering AVNS for inhibiting the recurrence of AT / AF. As used herein, “AVNS” refers to stimulation of the AV node directly or indirectly, e.g., via a branch of the vagal nerve that innervates the AV node. AVNS can also be referred to as AV node vagal stimulation (AVNVS) when the vagal nerve or a branch thereof is stimulated to indirectly stimulate the AV node. AVNS can be delivered at an epicardial site, endocardial site, AV nodal fat pad, cardiac nerve plexus, or within the myocardium (e.g., by an electrode advanced into the myocardium from an epicardial or endocardial approach), as examples, or at any other operative location that results in suppressed conduction of atrial depolarizations to the ventricles via the AV node.

[0022] The suppressed AV node conduction due to AVNS may be the result of an increased duration of the refractory period of the AV node following a conducted atrial depolarization. The suppressed conduction can have the effect of altering the rate of intrinsically conducted atrial depolarizations to the ventricles such that the ventricular rate can be decreased and / or regulated to provide ventricular rate stability during AT / AF. The AVNS delivered during AT / AF is therefore referred to as “V rate control therapy” and is delivered according to pulse parameters that result in effective suppression of the AV nodal conduction.

[0023] At times when the V rate control therapy is not being delivered, the medical device may be configured to deliver AVNS at a relatively lower energy level to inhibit theRef. No. A0013103 WO01recurrence of AT / AF. The AVNS delivered when AT / AF is not occurring is referred to herein as “AT / AF prevention therapy” and is delivered according to pulse parameters that result in a relatively lower delivered energy during each AVNS pulse train than the AVNS pulse trains delivered for the V rate control therapy. The lower energy AVNS pulse trains may act to stimulate parasympathetic pathways, which might prevent the recurrence of sympathetic driven AT / AF, or at least prevent an increasing trend in the incidence of AT / AF. In some cases, as a patient begins to experience AT / AF, the incidence of AT / AF episodes may increase over time. By delivering AT / AF prevention therapy, the incidence of AT / AF may be reduced or at least not increase over time.

[0024] As further described below, AVNS can be delivered as pulse trains that may be delivered during cardiac refractory periods. Cardiac refractory periods may include any of the SA node refractory period, AV node refractory period, atrial myocardial refractory period, ventricular myocardial refractory period or other physiological refractory periods of cardiac tissue. Each pulse train is delivered at a pulse frequency and individual pulse energy controlled by the AVNS pulse parameters. The AVNS may be delivered according to multiple pulse parameters (e.g., start time during a cardiac cycle (e.g., after a sensed P-wave or sensed R-wave), pulse frequency, pulse amplitude, pulse width, and pulse number) to promote effective V rate control during an AT / AF episode when atrial depolarizations may be conducted to the ventricles causing a rapid and / or irregular ventricular rate. The medical device may adjust one or more of the pulse parameters, e.g., by generally lowering or decreasing one or more of the pulse parameters, to deliver the AT / AF prevention therapy at times when AT / AF is not being detected.

[0025] FIG. 1 is a conceptual diagram of a medical device system 10 including an implantable medical device (IMD) 14 coupled to transvenous electrical leads 16, 18, and 21. IMD 14 may be configured to deliver electrical stimulation pulses and sense cardiac electrical signals in the right atrium (RA), the right ventricle (RV) and / or the left ventricle (LV). IMD housing 15 encloses internal circuitry corresponding to the various circuits and components described in conjunction with FIG. 4 below, for performing the functionality of IMD 14 as disclosed herein, including delivering AVNS.

[0026] IMD housing 15 may form a hermetic seal that protects internal components of IMD 14. Housing 15 may be formed of a conductive material, such as titanium or titanium alloy. Housing 15 may function as an electrode (sometimes referred to as a “can” electrode).Housing 15 may be used as an active can electrode for use in delivering high voltage CV / DFRef. No. A0013103 WO01shock pulses to heart 8 for terminating a tachyarrhythmia, e.g., atrial tachycardia, atrial fibrillation, ventricular tachycardia or ventricular fibrillation. In other examples, housing 15 may be available for use in delivering unipolar, relatively lower voltage cardiac pacing pulses and / or for sensing cardiac electrical signals in combination with electrodes carried by a lead coupled to IMD 14. In other instances, the housing 15 of IMD 14 may include multiple electrodes on an outer portion of the housing. The outer portion(s) of the housing 15 functioning as an electrode(s) may be coated with a material, such as titanium nitride, e.g., for reducing post-stimulation polarization artifact.

[0027] IMD 14 includes a connector assembly (or “connector block”) 17 that includes insulated electrical feedthroughs crossing housing 15 to provide electrical connections between conductors (not shown in FIG. 1) extending within the leads 16, 18 and 21 to the electronic components enclosed by housing 15. An antenna (not shown in FIG. 1) may be carried in connector assembly 17 for coupling RF signals transmitted to / from an external device 50 to a telemetry circuit enclosed by housing 15. As described below, housing 15 may enclose one or more processing circuits, memories, transceivers, cardiac electrical signal sensing circuitry, therapy delivery circuitry, power sources and other components for sensing cardiac electrical signals, processing and analyzing sensed cardiac electrical signals, and delivering electrical stimulation pulses to the patient’s heart 8 as needed.

[0028] In the example shown, connector assembly 17 is configured to receive a proximal lead connector 40, 42 and 44 of each of RA lead 16, RV lead 18 and LV lead 21, respectively. Each lead 16, 18, and 21 can be advanced transvenously for positioning electrodes for sensing and stimulation in the atria or ventricles of heart 8. The proximal portion of each lead 16, 18, and 21 may be configured as an industry standard or custom lead connector 40, 42 and 44, respectively. Connector assembly 17 includes connector bores that are appropriately sized for receiving the proximal portion of each lead 16, 18 and 21, e.g., lead connectors 40, 42 or 44. Each connector bore includes electrical contacts that become aligned with and physically mate with a corresponding electrical contact of the respective lead connector 40, 42 or 44 providing physical and electrical connection of each lead 16, 18 and 21 to IMD 14.

[0029] RAlead 16 includes an elongated lead body 41, proximal lead connector 40 and distal electrodes 20 and 22 in the example shown. RAlead 16 may be advanced transvenously for positioning its distal end, carrying electrodes 20 and 22, into the RA. RAlead 16 is equipped with pacing and sensing electrodes 20 and 22, shown as a tip electrode 20 and a ring electrode 22 spaced proximally from tip electrode 20, along RAlead body 41. Tip electrodeRef. No. A0013103 WO0120 may be used as a cathode electrode with ring electrode 22 serving as an anode electrode for bipolar pacing and bipolar sensing in the RA. Furthermore, tip electrode 20 and ring electrode 22 may be used for delivering AVNS in the area of the cardiac nerve plexus, e.g., along the posterior wall of the RA, adjacent to the coronary sinus ostium 9. The electrodes 20 and 22 may be positioned along an inferior portion of the posterior RA endocardial wall, adjacent the coronary sinus ostium 9, to provide high frequency bursts of pulses for suppressing conduction of the AV node during AT / AF and for providing relatively lower energy bursts of pulses for inhibiting the recurrence of AT / AF according to the techniques described herein. Electrodes 20 and 22 may be implanted in operative proximity to cardiac nerves, AV nodal fat pad, or other locations that enable electrical pulse trains to be delivered to effectively provide the V rate control therapy and the AT / AF prevention therapy as further described below. When AT / AF is not being detected, atrial pacing pulses may be delivered by electrodes 20 and 22 for pacing the RAby capturing the atrial myocardial tissue. Unipolar atrial pacing pulses may be delivered by one of electrodes 20 or 22 and IMD housing 15 in some examples. Atrial pacing pulses may be delivered to provide bradycardia pacing as needed, and AVNS pulse trains may be delivered during myocardial (atrial and / or ventricular) refractory periods following atrial pacing pulses for providing AT / AF prevention therapy using electrodes carried by RAlead 16.

[0030] The electrodes 20 and 22 are each connected to a respective insulated conductor extending within the elongated body 41. Each insulated conductor is coupled at its proximal end to an electrical connector of the proximal lead connector 40, which becomes electrically connected to internal IMD circuitry via respective electrical feedthroughs in IMD connector assembly 17. While RAlead 16 is shown carrying two electrodes 20 and 22, RAlead 16 may be provided with more electrodes for providing different electrode pairs positioned for delivering different types of therapeutic electrical pulses. Furthermore, while a single RAlead 16 is shown, in some examples, a second RAlead may be included in medical device system 10 to provide dedicated electrodes pairs positioned at optimal therapy delivery sites for delivering different types of therapeutic electrical pulses. For example, AVNS pulse trains for AT / AF prevention therapy, AVNS pulse trains for V rate control therapy and bradycardia pacing pulses may be delivered using different or shared electrode pairs carried by one or more leads in various lead and electrode configurations.

[0031] RV lead 18 includes an elongated lead body 43 having a proximal connector 42 at its proximal end for coupling lead 18 to IMD connector assembly 17 and electrodes 24, 26, 28Ref. No. A0013103 WO01and 30 carried along a distal portion of lead body 43. RV lead 18 may be advanced transvenously through the RA and into the RV to position electrodes 24, 28 and 30 in the RV. RV lead 18 is shown carrying a distal tip electrode 28 and ring electrode 30 spaced proximally from tip electrode 28 for bipolar sensing of cardiac electrical signals in the RV and delivering ventricular pacing pulses. Tip electrode 28 may be used as a cathode electrode for pacing and sensing with ring electrode 30 serving as an anode electrode for delivering bipolar ventricular pacing pulses. In other examples, tip electrode 28 may be paired with IMD housing 15 or one of coil electrodes 24 or 26 for delivering ventricular pacing pulses.

[0032] RV lead tip electrode 28 is shown implanted in the RV apex for delivering ventricular myocardial pacing. It is to be understood, however, that the RV lead electrode locations and other electrode locations shown herein are illustrative in nature and not intended to be limiting. For example, RV lead tip electrode 28 may be implanted in the interventricular septum to deliver pacing, which may include delivering ventricular pacing to an inferior portion of the His bundle, or in the area of the left bundle branch and / or right bundle branch to deliver ventricular pacing pulses via at least a portion of the native conduction system of heart 8.

[0033] RV lead 18 is shown carrying an RV coil electrode 24 spaced proximally from ring electrode 30 and a superior vena cava (SVC) coil electrode 26 spaced proximally from RV coil electrode 24. SVC coil electrode 26 may be carried along the length of RV lead body 43 such that it is positioned at least partially within the RA and / or SVC when the distal end of RV lead 18 is advanced within the RV. Coil electrodes 24 and 26 are elongated electrodes having a relatively high surface area compared to electrodes 20, 22, 28 and 30. Coil electrodes 24 and 26 may have a surface area ranging from 50 to 100 times greater than the surface area of electrodes 20, 22, 28 and 30, for example. For the sake of convenience, electrodes 24 and 26 are referred to herein as “coil electrodes” because they may take the form of a coiled electrode, which may include a single wire or filar or multiple wires or filars (e.g., a braided multi-filar wire, a stranded multi-filar wire, etc.) that winds helically around a longitudinal portion of lead body 43 to provide a relatively high surface area electrode for delivering high voltage CV / DF shocks. However, it is to be understood that electrodes 24 and 26 may be configured as other types of high surface area electrodes that can be used for delivering CV / DF shocks, which may include ribbon electrodes, plate electrodes, serpentine electrodes, zig-zagging electrodes, segmented electrodes or other types of physical electrodeRef. No. A0013103 WO01configurations that provide a relatively large surface area and low impedance that do not necessarily include a coiled wire.

[0034] Coil electrodes 24 and 26 (and in some examples housing 15) are sometimes referred to as “defibrillation electrodes” or “CV / DF electrodes” because they can be utilized, individually or collectively, for delivering high voltage CV / DF shocks. However, in some examples, depending in part on the implant location, a coil electrode available for delivering CV / DF shocks may be utilized in a cardiac sensing electrode vector to sense cardiac electrical signals or in a pacing electrode vector for delivering pacing pulses. In this sense, the use of the term “defibrillation electrode” or “CV / DF electrode” herein should not be considered as limiting the coil electrodes 24 and 26 for use in only high voltage CV / DF shock therapy applications. While two coil electrodes 24 and 26 are shown along lead body 43 of RV lead 18, in other examples only one coil electrode, e.g., RV coil electrode 24 or SVC coil electrode 26 (either of which may be used in combination with housing 15 for delivering high voltage shock pulses), or more than two coil electrodes may be carried by lead body 43. In still other examples, two or more coil electrodes may be carried by two or more different lead bodies extending from IMD 14. In still other examples, IMD 14 may not be configured to deliver high voltage CV / DF shocks in which case coil electrodes 24 and 26 are optional and may not be included on RV lead 18.

[0035] Each of electrodes 24, 26, 28 and 30 carried by RV lead body 43 are connected to a respective insulated conductor extending within lead body 43 of RV lead 18. Lead body 43 may be a multi-lumen lead body in some examples to accommodate multiple, insulated conductors. The proximal ends of the insulated conductors are coupled to corresponding electrical connectors (not illustrated in FIG. 1) of proximal lead connector 42 for providing electrical connection to IMD 14 via electrical feedthroughs in connector assembly 17.

[0036] The RV lead tip electrode 28 and the RA lead tip electrode 20 can be active fixation electrodes providing fixation of the distal ends of leads 18 and 16, respectively, at an implant site in addition to providing cardiac electrical signal sensing and cardiac electrical stimulation functionality. In FIG. 1, RAtip electrode 20 (which can be used in delivering atrial pacing and AVNS) and RV tip electrode 28 (for delivering ventricular pacing) are each shown as a helical, screw-in electrode that can be rotatably advanced into cardiac tissue to provide lead fixation. In other examples, tip electrode 20 and / or tip electrode 28 may be configured as fishhook electrodes, hemispherical electrodes, button electrodes or other types of electrodes. When the tip electrode 20 or 28 of the medical lead 16 or 18 does not provide fixation of theRef. No. A0013103 WO01distal end of the elongated lead body, the respective RA lead 16 or RV lead 18 may be equipped with other fixation mechanisms, such as tines or hooks, that may engage with cardiac tissue at an implant site for promoting stable fixation of the tip electrodes 20 and 28 at a desired therapy delivery site.

[0037] The proximal ring electrode 22 of RA lead 16 and the proximal ring electrode 30 of RV lead 18 may each be ring electrodes that fully or partially circumscribe the respective lead body 41 or 43. In various examples, the relatively low surface area electrodes 20, 22, 28 and 30 may be implemented as ring electrodes, short coil electrodes, button electrodes, hemispherical electrodes, directional electrodes, segmented electrodes, helical electrodes, fishhook electrodes, or other shaped electrodes and are not limited to being exclusively ring electrodes and helical screw-in electrodes as shown here.

[0038] RA lead electrodes 20 and 22 and RV lead electrodes 28 and 30, being relatively small surface area electrodes, are available for use in sensing cardiac electrical signals and may be used for delivering relatively low voltage cardiac electrical pulses, e.g., for delivering AVNS according to techniques disclosed herein, bradycardia pacing, post-shock pacing, antitachycardia pacing (ATP) therapy or other therapeutic cardiac electrical stimulation pulses. In some cases, RV lead electrodes 28 and 30 may be used to deliver high frequency induction pulses delivered to induce a tachyarrhythmia, e.g., during CV / DF threshold testing.Electrodes 20, 22, 28 and 30 are sometimes referred to as “pace / sense electrodes” because they are generally configured for use in low voltage applications, e.g., used as either a cathode or anode for delivery of pacing pulses and / or sensing of cardiac electrical signals, as opposed to delivering high voltage CV / DF shocks. In some instances, electrodes 20, 22, 28 and 30 may provide only pacing functionality, only sensing functionality or both. As indicated above, atrial lead 16 may carry multiple electrodes such that an electrode pair for delivering AVNS may be different than an electrode pair used for delivering atrial pacing pulses and / or an electrode pair used for sensing atrial electrical signals or a second atrial lead may be provided for positioning different, selectable electrode pairs for sensing atrial signals, delivering AVNS for V rate control, delivering AVNS for AT / AF prevention therapy and / or delivering atrial pacing pulses.

[0039] LV lead 21 includes elongated lead body 45 having a proximal connector 44 and distal electrodes 34 and 36. LV lead 21 may be advanced transvenously into the RA, and further into a cardiac vein 32 via the coronary sinus ostium 9 to position electrodes 34 and 36 along the lateral free wall of the left ventricle. Electrodes 34 and 36 may be used as a bipolarRef. No. A0013103 WO01sensing and pacing electrode pair for sensing cardiac electrical signals from the LV and for delivering LV pacing pulses. LV lead 21 may be coupled to IMD 14 for providing ventricular pacing pulses to a left ventricular pacing site. LV lead 21 may be used to deliver ventricular pacing pulses during V rate control therapy when a ventricular pacing interval expires to prevent ventricular asystole when AV node conduction is blocked by the AVNS. When the RV lead 18 is present for providing ventricular pacing, LV lead 21 is optional and may be excluded in some medical device systems that employ the AVNS techniques disclosed herein. In other examples LV lead 21 may be provided and RV lead 18 may be excluded. When both LV lead 21 and RV lead 18 are included in IMD system 10, IMD 14 may be configured to deliver cardiac resynchronization therapy (CRT) for promoting biventricular synchrony as well as atrioventricular synchrony.

[0040] RV lead 18 and LV lead 21 are both shown to illustrate different ventricular pacing sites that could be utilized in conjunction with an AVNS therapy. A ventricular pacing site utilized for providing ventricular pacing in conjunction with AVNS therapy is not limited to a particular pacing site and may be provided in the RV, LV, interventricular septum, along the His-Purkinje conduction system, or any other operative location for pacing and capturing the ventricles to prevent ventricular asystole during AVNS therapy. While LV lead 21 is shown as a bipolar lead having two electrodes 34 and 36 for the sake of convenience, LV lead 21 may be a unipolar lead having one electrode or a multi-polar lead, e.g., having three or four electrodes. In some examples, LV lead 21 is a quadripolar lead having four electrodes, e.g., four ring electrodes or one tip electrode and three ring electrodes, with proximal connector 44 configured as an industry standard IS-4 connector.

[0041] Electrodes 20, 22, 24, 26, 28, 30, 34 and 36 may be formed from titanium, platinum, iridium or alloys thereof, as examples with no limitation intended, and may include a low polarizing coating, such as titanium nitride, iridium oxide, ruthenium oxide, platinum black, among others. Lead bodies 41, 43 and 45 may each be formed from a non-conductive material, including silicone, polyurethane, fluoropolymers, mixtures thereof, and / or other appropriate materials. Each lead body may be shaped to form one or more lumens within which one or more insulated electrical conductors extend between the electrical connectors of the proximal lead connectors 40, 42 and 44 and the respective electrodes carried by the lead body. The lead bodies 41, 43, and 45 may be generally tubular or cylindrical in shape but may have a flattened or ribbon shape in some examples. Any of the lead bodies 41, 43 and 45 may have a pre-formed shape such as a curve or bend, which may be along a distal portion of theRef. No. A0013103 WO01lead body, to facilitate guidance and implantation of the lead body distal end at a targeted implant site. In other examples, the lead bodies 41, 43 and 45 may be elongated flexible bodies without any preformed shapes or curves.

[0042] In the example shown in FIG. 1, RA lead 16 and RV lead 18 are configured as “true bipolar” leads in that a cardiac electrical signal can be sensed between tip electrode 20 and ring electrode 22 in the RA, and a cardiac electrical signal can be sensed between tip electrode 28 and ring electrode 30 in the RV. In other examples, a lead coupled to IMD 14 may be an “integrated bipolar” lead, referring to a lead that is configured to sense cardiac electrical signals using a tip electrode and a coil electrode, e.g., RV tip electrode 28 and RV coil electrode 24, omitting the need for a ring electrode 30. In an “integrated bipolar” lead, for example, the RV coil electrode 24 may serve the purposes of bipolar sensing of cardiac electrical signals and delivering ventricular pacing pulses when paired with tip electrode 28 and delivering high voltage CV / DF shocks in combination with SVC coil electrode 26 and / or housing 15.

[0043] It is to be understood that although IMD 14 is described as a multi-chamber device capable of sensing and pacing in the RA, RV and LV, in other examples, IMD 14 may be a dual chamber device, e.g., coupled to RA lead 16 and RV lead 18. In still other examples, IMD 14 may be a single chamber device, e.g., coupled only to RA lead 16. IMD 14 provided as a single chamber device coupled to RA lead 16 may sense far field R- waves for detecting fast and / or irregular ventricular rates for use in controlling AVNS therapy. While IMD 14 is described above as being capable of delivering both low voltage cardiac pacing and AVNS therapies as well as high voltage CV / DF shocks, an IMD operating according to techniques disclosed herein may be configured to deliver AVNS and dual chamber cardiac pacing (e.g., atrial and ventricular pacing) without high voltage CV / DF shock therapy capabilities.

[0044] An external device 50 is shown in telemetric communication with IMD 14 by a wireless communication link 51 in FIG. 1. External device 50 may be embodied as a programmer used in a hospital, clinic or physician’s office to retrieve data from IMD 14 and to program operating parameters and algorithms in IMD 14 for controlling IMD functions. External device 50 may alternatively be embodied as a home monitor or handheld device for retrieving data from IMD 14. External device 50 may be used to program cardiac signal sensing parameters, cardiac rhythm detection parameters, therapy delivery control parameters including AVNS control parameters and other operating and control parameters used by IMD 14.Ref. No. A0013103 WO01

[0045] External device 50 may include a processor 52, memory 53, display unit 54, user interface 56 and telemetry unit 58. Processor 52 executes instructions stored in memory 53. Processor 52 may include any one or more of a microprocessor, a controller, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or equivalent discrete or analog logic circuitry. In some examples, processor 52 may include multiple components, such as any combination of one or more microprocessors, one or more controllers, one or more DSPs, one or more ASICs, or one or more FPGAs, as well as other discrete or integrated logic circuitry. The functions attributed to processor 52 herein may be embodied as software, firmware, hardware or any combination thereof.

[0046] Memory 53 may include any volatile, non-volatile, magnetic, optical, or electrical media, such as a random-access memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory, or any other digital or analog media. Memory 53 may be configured to store instructions executed by processor 52 for obtaining data received from IMD 14 and for generating a GUI on display unit 54. Memory 53 may store various operating parameter settings of IMD 14 that may be used in generating various GUI windows, menus, reports, etc. by processor 52.

[0047] Display unit 54 may generate a display of cardiac electrical signals, programmed operating settings of IMD 14 and other device and patient related data received from processor 52 (which may be received from IMD 14 via telemetry unit 58). Display unit 54 may be configured to generate a GUI including various windows, icons, user selectable menus, etc. to facilitate interaction by a user with the external device 50, e.g., for programming AVNS control parameters described herein. Display unit 54 may function as an input and / or output device using technologies including liquid crystal displays (LCD), quantum dot display, dot matrix displays, light emitting diode (LED) displays, organic lightemitting diode (OLED) displays, cathode ray tube displays, e-ink, or monochrome, color, or any other type of display capable of generating tactile, audio, and / or visual output. In some examples, display unit 54 is a presence-sensitive display that may serve as a user interface device that operates both as one or more input devices and one or more output devices.

[0048] User interface unit 56 may include a mouse, touch screen, keypad or the like to enable a user to interact with external device 50, e.g., to initiate and terminate an interrogation session for retrieving data from IMD 14, adjust settings of display unit 54, enter programming commands or selections or make other user requests. Telemetry unit 58Ref. No. A0013103 WO01includes a transceiver and antenna configured for bidirectional communication with a telemetry circuit included in an IMD 14, e.g., in response to user requests.

[0049] Telemetry unit 58 is configured to operate in conjunction with processor 52 for sending and receiving data relating to IMD functions via a wireless communication link 51 with IMD 14. Communication link 51 may be established using a radio frequency (RF) link such as BLUETOOTH®, Wi-Fi, Medical Implant Communication Service (MICS) or other communication bandwidth. In some examples, external device 50 may include a programming head that is placed proximate IMD 14 to establish and maintain a communication link 51, and in other examples external device 50 and IMD 14 may be configured to communicate using a distance telemetry algorithm and circuitry that does not require the use of a programming head and does not require user intervention to maintain a communication link.

[0050] It is contemplated that external device 50 may be in wired or wireless connection to a communications network via telemetry circuit 58 that includes a transceiver and antenna or via a hardwired communication line for transferring data to a centralized database or computer to allow remote management of the patient. One example of a remote patient management system is the CARELINK® Network (Medtronic, Inc. Dublin, Ireland). Review of operating parameter settings and other data collected from IMD 14 may be performed remotely by a clinician who may authorize programming of operating parameters in IMD 14, e.g., after viewing reports and cardiac electrical signals and other device related data, such as marker channel data, therapy delivery history, IMD generated alerts or the like.

[0051] FIG. 2 is a conceptual diagram of an IMD 114 for sensing cardiac electrical signals and delivering AVNS according to some examples. In this example, a leadless IMD 114 is implanted in the RA for providing AVNS (and atrial pacing as needed). IMD 114 may be configured to provide ventricular pacing as well from the atrial location. IMD 114 may be a transcatheter device that can be delivered to the RA via a catheter or other delivery device for being wholly implanted within the RA. In some examples, IMD 114 may be positioned for delivering ventricular pacing pulses via the heart’s native conduction system and / or ventricular myocardium from a right atrial approach. The distal end 102 of IMD 114 may be positioned at the inferior end of the interatrial septum, beneath the AV node and near the tricuspid valve annulus to position tip electrode 129 for advancement into the interatrial septum toward the His bundle of the native His-Purkinje conduction system. Tip electrode 129 is shown as an electrically conductive surface at the tip of a helical member 128Ref. No. A0013103 WO01extending from distal end 102 in this example. Ring electrode 130, spaced proximally from tip electrode 129, may be used as the return electrode with the cathode tip electrode 129 for pacing the right and left ventricles via the His-Purkinje system and / or ventricular myocardium. Tip electrode 129 may be positioned to capture at least a portion of the His bundle and / or ventricular myocardium for delivering ventricular pacing from an atrial implant location of IMD 114.

[0052] IMD 114 may be capable of dual chamber sensing and pacing in some examples. For instance, a distal ring electrode 120 may be included on pacemaker housing 115 and can be used in combination with the proximal ring electrode 130 for sensing atrial P-waves and, in some examples, delivering atrial pacing pulses. Distal ring electrode 120 may be referred to as an “atrial electrode” in some examples because it can be used for atrial sensing and pacing when IMD 114 is implanted in the atrium. Distal ring electrode 120, however, may additionally or alternatively be used in delivering AVNS. In other examples, one or more electrodes on or extending from distal end 102 of IMD 114 may be provided for delivering AVNS in accordance with the techniques disclosed herein. Examples of various medical device and electrode configurations that may be used in delivering the AVNS therapies as disclosed herein are generally disclosed in U.S. Patent No. 11,813,466 (Kornet, et al., filed January 25, 2021), U.S. Patent No. 11,426,578, (Yang, et al., filed September 13, 2018), the entire content of both incorporated herein by reference.

[0053] In the example of FIG. 2, the cathode tip electrode 129 is as an uninsulated distal tip of a screw-in helical member 128, which may provide fixation of IMD 114 at an implant site as well as serving as a pacing and sensing electrode. In other examples, tip electrode 129 may be other types of electrodes that may or may not provide fixation of IMD 114 at the implant site. Other fixation members, such as tines, hooks, barbs or the like may be provided along distal end 102 for providing fixation of IMD 114 at an implant site that enables AVNS therapy (and in some cases atrial and / or ventricular pacing) to be delivered by electrodes carried by IMD 114.

[0054] IMD 114 may include one or more ring electrodes (e.g., ring electrodes 120 and 130) circumscribing the housing 115. In other examples, IMD 114 may include other types of electrodes such as hook electrodes, button electrodes, hemispherical electrodes, segmented electrodes or other types of electrodes arranged along housing 115 for providing at least cardiac electrical signal sensing and AVNS and in some examples ventricular pacing and / or atrial pacing. Various electrode arrangements provided on a leadless IMD such as IMD 114Ref. No. A0013103 WO01may include one or more electrodes located along the longitudinal sidewall of housing 115, one or more electrodes on distal end 102, one or more electrodes on proximal end 103 and / or one or more electrodes exposed along a member that extends from distal end 102. The member extending from distal end 102 may be straight or helical (e.g., helical member 128 as shown) with one or more portions of the extending member 128 being insulated, non-conductive surfaces to provide one or more electrically conductive surfaces (e.g., tip electrode 129) along the extending member 128 to serve as one or more electrodes.

[0055] Housing 115 encloses circuitry for performing cardiac signal sensing and delivery of cardiac electrical stimulation pulses according to the functionality described herein. The internal device circuitry includes sensing circuitry and pulse generating circuitry as well as control circuitry for controlling IMD 114 functions, e.g., as generally described below in conjunction with FIG. 4.

[0056] FIG. 3 is a conceptual diagram illustrating an IMD system 110 that may be used to sense cardiac electrical signals, deliver AVNS and deliver atrial and / or ventricular pacing if needed according to another example. IMD system 110 is a multi-device system including IMD 114’ implanted within the RA and an IMD 116 implanted in the RV. IMD 114’ can provide atrial signal sensing, atrial pacing if needed and AVNS therapies. IMD 116 can provide ventricular signal sensing and ventricular pacing.

[0057] In some examples, IMD 114’ and IMD 116 are transcatheter leadless pacemakers that can be implanted wholly within a heart chamber. IMDs 114’ and 116 may be reduced in size compared to subcutaneously implanted pacemakers and may be generally cylindrical in shape to enable transvenous implantation via a delivery catheter.

[0058] IMD 114’ may be wholly implanted within the right atrium (RA) and may be implanted along the posterior wall of the RA, adjacent the coronary sinus 9, in operative proximity to the cardiac nerve plexus and AV node for delivering AVNS. IMD 114’ may include a distal tip electrode 123 for delivering atrial pacing pulses if needed, delivering AVNS, and for sensing atrial electrical signals. IMD 114’ may include at least one proximal electrode 122, which may be a ring electrode circumscribing housing 115, to be used in a sensing and therapy delivery electrode vector in combination with electrode 123 for delivering atrial pacing pulses, AVNS pulse trains, and for sensing atrial electrical signals. In some examples, IMD 114’ may be implanted at an epicardial location, outside of the heart 8, e.g., with distal tip electrode 123 implanted in the posterior RA and / or in the atrial septum in an operative location for delivering the AVNS therapy, e.g., targeting a vagal branchRef. No. A0013103 WO01innervating the AV node. IMD 114’ may be provided with a variety of electrode arrangements for providing electrode pairs for sensing atrial signals, delivering AVNS pulse trains and delivering atrial pacing pulses as generally described above in conjunction with FIG. 2 and in the incorporated references. A sensing electrode pair, an AVNS electrode pair, and an atrial pacing electrode pair may each be provided as dedicated electrode pairs that do not share electrodes with each other or be electrode pairs that share one or both electrodes.

[0059] IMD 116 may be wholly implanted within the right ventricle (RV) as shown or implanted on a ventricular chamber, e.g., at an epicardial location. Ventricular pacemaker 116 may be positioned within the RV along the interventricular septum as shown for delivering ventricular pacing pulses to a portion of the native conduction system, e.g., the right bundle branch, left bundle branch, or an inferior portion of the His Bundle. Other operative locations for IMD 116 are possible, such as near the RV apex.

[0060] IMD 116 may include a distal tip electrode 139 and a proximal ring electrode 140 carried on the housing of IMD 116 for sensing ventricular electrical signals and delivering ventricular pacing pulses. The ventricular pacing pulses may be delivered by electrodes 139 and 140 for capturing the ventricular myocardium, a portion of the native conduction system or both. Ventricular pacing may be delivered by IMD 116 for promoting a regular, stable ventricular rhythm, e.g., as needed during AVNS V rate control therapy delivered to suppress AV node conduction. Ventricular pacing may be delivered by IMD 116 at AV pacing intervals from atrial event signals (corresponding to atrial systole) sensed by IMD 116 (e.g., from a cardiac electrical or mechanical signal sensed by IMD 116) or in response to communication or pacing trigger signals transmitted by IMD 114’ to provide atrial synchronous ventricular pacing. Thus IMD 116 may be included in a medical device system 110 to promote AV synchrony at times when AV conduction is impaired or blocked.

[0061] As generally described herein, an IMD system 110 may include cardiac electrical signal sensing circuitry. For example, IMD 114’ may include atrial electrical signal sensing circuitry configured to sense atrial P-waves (via electrodes 122 and 123) attendant to the depolarizations of the atrial myocardium. IMD 116 may include ventricular electrical signal sensing circuitry configured to sense ventricular R- waves (via electrodes 139 and 140) attendant to the depolarizations of the ventricular myocardium.

[0062] The IMD system 110 may further include therapy delivery circuitry configured to deliver AVNS and cardiac pacing pulses. For example, IMD 114’ may include therapy delivery circuitry configured to generate and deliver atrial pacing pulses via electrodes 122Ref. No. A0013103 WO01and 123 in the absence of sensed intrinsic atrial P-waves. The therapy delivery circuitry of IMD 114’ may be further configured to generate AVNS pulse trains according to the techniques disclosed herein. IMD 116 may include therapy delivery circuitry configured to deliver ventricular pacing pulses in the absence of sensed intrinsic ventricular R-waves.

[0063] IMD 114’ and / or IMD 116 may include one or more fixation members, e.g., fixation tines, a fixation helix, or other fixation members for engaging with cardiac tissue at a respective implant site. In the example shown, IMD 114’ is provided with a distal tip electrode 123 in the form of a button or hemispherical electrode. IMD 114’ may have fixation member 113 including one or more tines configured to engage with cardiac tissue at the implant site. In the example shown, IMD 116 is provided with a distal tip electrode 139 that is a helical electrode that can provide fixation of IMD 116 at the implant site. It is recognized that IMD 114’ and IMD 116 may be provided with other types of electrodes and / or fixation members than the example shown in FIG. 3, e.g., any of the example electrodes or fixation members listed herein.

[0064] IMDs 114’ and 116 of medical device system 110 may be capable of bidirectional wireless communication with an external device 50 (shown in FIG. 1) for programming sensing and therapy delivery control parameters as generally described above. IMD 114’ and IMD 116 may be configured to communicate with each other via radio frequency communication, tissue conductance communication (TCC) or other communication methods. IMD 116 may transmit a communication signal to IMD 114’ each time an R-wave is sensed by sensing circuitry of IMD 114’ to enable IMD 114’ to determine various cardiac event intervals for detecting the ventricular rate and determining ventricular rate regularity. The cardiac event intervals, e.g., RR intervals (RRIs), may be used by IMD 114’ in controlling and adjusting V rate control therapy. In other examples, IMD 114’ may be configured to sense far field R-waves for determining cardiac event intervals, e.g., RRIs, for use in controlling and adjusting V rate control therapy.

[0065] In some examples, IMD 116 may transmit a communication signal to IMD 114’ each time a ventricular pacing pulse is delivered or after a threshold number of pacing pulses are delivered out of a specified number of ventricular electrical events (e.g., sensed intrinsic R-waves and delivered ventricular pacing pulses). In this way, IMD system 110 may be configured to detect when a threshold number of ventricular pacing pulses are delivered during V rate control therapy. IMD 114’ may adjust AVNS control parameters in response to the IMD system 110 detecting the threshold number of delivered ventricular pacing pulses.Ref. No. A0013103 WO01Additionally or alternatively, IMD 116 may be configured to determine RRIs between consecutively delivered ventricular pacing pulses and / or sensed intrinsic R-waves and transmit communication signals to IMD 114’ indicating when RRIs do not meet stability criteria (e.g., too short and / or irregular RRIs). IMD 114’ may adjust AVNS control parameters in response to determining that the RRIs do not meet stability criteria, which may be based at least in part on communication signals received from IMD 116 relating to the timing of delivered ventricular pacing pulses and / or sensed ventricular event signals (e.g., corresponding to R-waves) in some examples.

[0066] FIG. 4 is a conceptual diagram of an IMD configured to sense cardiac electrical signals and deliver AVNS therapies for regulating the ventricular rate and inhibiting the recurrence of AT / AF according to some examples. FIG. 4 depicts IMD 14 coupled to electrodes 20, 22, 24, 26, 28, 34, and 36, carried by leads 16, 18 and 21 as shown in FIG. 1. However, it is to be understood that the circuitry, components and functionality described in conjunction with FIG. 4 may generally correspond to circuitry, components and functionality of an IMD adapted to receive a different number of medical electrical leads and associated electrodes for sensing cardiac electrical signals and delivering at least AVNS to a patient’s heart. Furthermore, circuitry, components and functionality described in conjunction with FIG. 4 may be included in a leadless IMD configured to deliver AVNS therapies as described herein, e.g., IMD 114 shown in FIG. 2. In still other examples, circuitry, components and functionality described in conjunction with FIG. 4 and other flow charts and diagrams presented herein may be distributed across multiple IMDs in a multi-device system, such as the two device system shown in FIG. 3. For the sake of convenience, FIG. 4 is described with reference to IMD 14 shown in FIG 1.

[0067] Electrodes 20, 22, 24, 26, 28, 30, 34, 36 and / or housing 15 shown in the system 10 of FIG. 1 may be connected to therapy delivery circuit 84 and / or cardiac electrical signal sensing circuit 86 (also referred to herein as “sensing circuit 86”) as shown in FIG. 4, e.g., via switching circuitry included in therapy delivery circuit 84 and sensing circuit 86. The electronic circuitry enclosed within housing 15 (shown conceptually in FIG. 4 as an electrode, sometimes referred to as a “can electrode”) includes software, firmware and hardware that cooperatively monitor cardiac electrical signals, determine when an electrical stimulation therapy is necessary, and deliver therapy as needed according to programmed therapy delivery algorithms and control parameters. IMD 14 may include a control circuit 80, memory 82, therapy delivery circuit 84, cardiac electrical signal sensing circuit 86, telemetryRef. No. A0013103 WO01circuit 88 and one or more physiological sensors 95. A power source 98 provides power to the circuitry of IMD 14, including each of the components 80, 82, 84, 86, 88 and 95 as needed. Power source 98 may include one or more energy storage devices, such as one or more rechargeable or non-rechargeable batteries. The connections between power source 98 and each of the other components 80, 82, 84, 86, 88 and 95 are to be understood from the general block diagram of FIG. 4 but are not shown for the sake of clarity. For example, power source 98 may be coupled to one or more charging circuits included in therapy delivery circuit 84 for charging holding capacitors included in therapy delivery circuit 84 and operating output circuitry for discharging the holding capacitor(s) at appropriate times under the control of control circuit 80 for producing electrical pulses according to a therapy protocol. Power source 98 is also coupled to components of cardiac electrical signal sensing circuit 86 (such as sense amplifiers, analog-to-digital converters, switching circuitry, etc.), memory 82, telemetry circuit 88 and sensors 95 as needed.

[0068] The various operating circuits shown in FIG. 4 represent functionality included in IMD 14 and may include any discrete and / or integrated electronic circuit components that implement analog and / or digital circuits capable of producing the functions attributed to the IMD herein. Functionality associated with one or more circuits may be performed by separate hardware, firmware and / or software components, or integrated within common hardware, firmware and / or software components. For example, cardiac electrical signal sensing and analysis for detecting tachyarrhythmias, such as AT / AF, ventricular tachycardia (VT) and / or ventricular fibrillation (VF), may be performed cooperatively by sensing circuit 86 and control circuit 80 and may include operations implemented in a processor or other signal processing circuitry included in control circuit 80 executing instructions stored in memory 82. Therapy delivery may be performed cooperatively by therapy delivery circuit 84 under the control of signals received from control circuit 80 for controlling the timing, pulse amplitude, pulse width, polarity, rate, electrode vector and other therapy delivery control parameters used to generate and deliver electrical stimulation pulses by therapy delivery circuit 84, which may include AVNS therapies, cardiac pacing pulses, tachyarrhythmia induction pulses, CV / DF shocks or any other electrical pulses delivered via electrodes 20, 22, 24, 26, 28, 30, 34, 36 and / or housing 15 shown in the system of FIG. 1.

[0069] The various circuits of IMD 14 may include an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, stateRef. No. A0013103 WO01machine, hardware subroutine, or other suitable components or combinations of components that provide the described functionality. The particular form of software, hardware and / or firmware employed to implement the functionality disclosed herein will be determined primarily by the particular system architecture employed in the IMD and by the particular sensing, detection and therapy delivery methodologies employed by the IMD. Providing software, hardware, and / or firmware to accomplish the described functionality in the context of any modern medical device system, given the disclosure herein, is within the abilities of one of skill in the art.

[0070] Memory 82 may include any volatile, non-volatile, magnetic, or electrical non-transitory computer readable storage media, such as random access memory (RAM), readonly memory (ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory, or any other memory device. Furthermore, memory 82 may include non-transitory computer readable media storing instructions that, when executed by one or more processing circuits, cause control circuit 80 and / or other IMD components to perform various functions attributed to IMD 14 (or IMD 114 or the combination of IMD 114’and IMD 116 in system 110) orthose IMD components. The non-transitory computer-readable media storing the instructions may include any of the media listed above.

[0071] Therapy delivery circuit 84 and sensing circuit 86 can be electrically coupled to electrodes 20, 22, 24, 26, 28, 30, 34, 36 and / or housing 15, which may function as a common or ground electrode for sensing electrical signals or delivering therapy or as an active can electrode for delivering CV / DF shock pulses. As such, housing 15 is shown conceptually as an electrode that may be coupled to therapy delivery circuit 84 and / or sensing circuit 86 in FIG. 4. Control circuit 80 communicates, e.g., via a data bus, with therapy delivery circuit 84 and sensing circuit 86 for sensing cardiac electrical signals, detecting cardiac rhythms, and controlling delivery of cardiac electrical stimulation therapies in response to sensed cardiac signals (or the absence thereof).

[0072] Control circuit 80 may include a tachyarrhythmia detection circuit 92, timing circuit 96, and therapy control circuit 94. Tachyarrhythmia detection circuit 92 may be configured to process and analyze signals received from sensing circuit 86, which may be in conjunction with time intervals and / or timing related signals received from timing circuit 96. Timing circuit 96 may generate clock signals and include various timers and / or counters for use in determining time intervals between sensed cardiac event signals attendant to intrinsic myocardial depolarizations, e.g., sensed intrinsic P-waves and / or R-waves, and / or deliveredRef. No. A0013103 WO01pacing pulses. Timing circuit 96 may include various timers and / or counters for controlling the timing of delivered AVNS, pacing pulses and CV / DF shocks. Control circuit 80 may further include a therapy control circuit 94 configured to pass signals to and receive signals from therapy delivery circuit 84 for controlling and monitoring electrical stimulation therapies delivered by therapy delivery circuit 84 according to therapy control parameters, such as AVNS pulse parameters (also referred to herein “pulse parameters”) and therapy protocol parameters as described below.

[0073] Sensing circuit 86 may be selectively coupled to electrodes 20, 22, 24, 26, 28, 30, 34, 36 and / or housing 15 in order to monitor electrical activity of the patient’s heart. Sensing circuit 86 may be enabled to receive cardiac electrical signals from at least one sensing electrode vector selected from the available electrodes. For example, an atrial electrical signal may be sensed via RA lead electrodes 20 and 22 and a ventricular electrical signal may be sensed via RV lead electrodes 28 or 30 or LV lead electrodes 34 or 36. Sensing circuit 86 may include switching circuitry for selecting which electrodes are coupled to sensing circuit 86 for sensing atrial and ventricular electrical signals.

[0074] IMD 14 may include an atrial (A) sensing channel 87 for receiving signals from electrodes carried by RA lead 16 and a ventricular (V) sensing channel 89 for receiving signals from electrodes carried by RV lead 18 and / or LV lead 21 (all shown in FIG. 1). In some examples, two, three or more cardiac electrical signals from two, three or more different sensing electrode vectors may be received simultaneously by respective sensing channels of sensing circuit 86, e.g., atrial sensing channel 87 and ventricular sensing channel 89. Sensing circuit 86 may monitor cardiac electrical signals for sensing cardiac event signals, e.g., P-waves attendant to intrinsic atrial myocardial depolarizations and R-waves attendant to intrinsic ventricular myocardial depolarizations.

[0075] Each sensing channel 87 and 89 may be configured to amplify, filter and digitize the cardiac electrical signal received from selected electrodes coupled to the respective sensing channel 87 and 89 to improve the signal quality for sensing cardiac event signals, such as P-waves and R-waves. The cardiac event sensing circuitry within sensing circuit 86 may include one or more sense amplifiers, filters, anal og-to-digi tai converters (ADCs), rectifiers, threshold detectors, comparators, timers or other analog and / or digital components. For instance, an amplified, filtered and rectified signal sensed using RA lead electrodes 20 and / or 22 may be passed to a P-wave detector included in atrial sensing circuit 87 for sensing P-waves. The P-wave detector may include a sense amplifier, comparator and / or otherRef. No. A0013103 WO01electronic circuitry for applying a P-wave sensing threshold to the atrial electrical signal. In response to the atrial electrical signal crossing the P-wave sensing threshold, sensing circuit 86 may pass an atrial sensed event signal, “Asense signal,” to control circuit 80 indicating that an atrial event signal has been sensed.

[0076] An amplified, filtered and rectified signal sensed using RV lead electrodes 28 and / or 30 may be passed to an R-wave detector included in ventricular sensing channel 89 for sensing R-waves. The R-wave detector may include a sense amplifier, comparator and / or other electronic circuitry for applying an R-wave sensing threshold to the ventricular electrical signal. In response to the ventricular electrical signal crossing the R-wave sensing threshold, sensing circuit 86 may pass a ventricular sensed event signal, “Vsense signal,” to control circuit 80 indicating that a ventricular event signal has been sensed.

[0077] The P-wave and R-wave sensing thresholds may each be automatically adjusted by sensing circuit 86 under the control of control circuit 80, based on sensing threshold control parameters, such as various timing intervals and sensing threshold amplitude values. The sensing threshold control parameters and threshold amplitude values determined by control circuit 80 may be stored in memory 82, and / or controlled by hardware, firmware and / or software of control circuit 80 and / or sensing circuit 86. Starting sensing threshold values may be set to a percentage of the maximum peak amplitude of the most recently sensed event signal. The starting sensing threshold value may be decreased according to one or more decay rates and / or step drops toward a programmed atrial sensitivity or ventricular sensitivity until a threshold crossing is detected or a pacing lower rate interval expires, whichever occurs first. The programmed atrial sensitivity and ventricular sensitivity may be stored in memory 82 as the minimum amplitude or “sensing floor” at which a cardiac event signal, e.g., P-wave or R-wave respectively, is detected.

[0078] Asense and Vsense signals received from sensing circuit 86 by control circuit 80 can be used by control circuit 80 for determining sensed event intervals, which can be PP intervals (PPIs) between consecutively received Asense signals from atrial sensing channel 87 (or from a delivered atrial pacing pulse to an Asense signal) and RRIs extending between consecutively received Vsense signals from ventricular sensing channel 87 (or between two consecutively delivered ventricular pacing pulses or between a delivered ventricular pacing pulse and a Vsense signal). In some examples, PR intervals (PRIs) and / or RP intervals (RPIs) may be determined between consecutively received Asense and Vsense signals and / or consecutively received Vsense and Asense signals, respectively. RRIs may be determinedRef. No. A0013103 WO01and used by control circuit 80 to control (e.g., turn on and adjust if needed) V rate control therapy delivery to promote a regular ventricular rate at a target rate or within a target rate range, which may be programmable by a user, e.g., using external device 50. Control circuit 80 may include timing circuit 96 for determining various cardiac sensed event intervals, such as any of the foregoing example intervals, for use in detecting the heart rhythm and controlling therapy delivery.

[0079] In some examples, sensing circuit 86 passes a digitized cardiac electrogram (EGM) signal to control circuit 80 for P-wave and / or R-wave morphology analysis for use in detecting cardiac tachyarrhythmias, e.g., AT / AF or VT / VF. P-wave and / or R-wave morphology analysis may be performed in combination with cardiac event interval analysis according to an implemented tachyarrhythmia detection algorithm. The AVNS therapy delivery techniques disclosed herein may be implemented in conjunction with a variety of cardiac event signal sensing and tachyarrhythmia detection methods and are not limited to any particular method for sensing P-waves and R-waves or for detecting tachyarrhythmias based on an analysis of cardiac event intervals and / or cardiac signal waveform morphology.

[0080] Timing circuit 96 may be configured to control various timers and / or counters used in setting various blanking periods, refractory periods or other time intervals used in sensing atrial and ventricular event signals by sensing circuit 86. The various timers and / or counters may be used in determining time intervals between received Asense and Vsense signals received from sensing circuit 86 and in controlling the timing of AVNS, cardiac pacing pulses and other electrical pulses generated by therapy delivery circuit 84. Timing circuit 96 may start one or more timers or counters in response to receiving Asense and Vsense signals from sensing circuit 86 and in response to therapy delivery circuit 84 delivering an atrial pacing pulse or a ventricular pacing pulse for scheduling subsequent pacing pulses and for determining various cardiac event intervals for use in controlling atrial pacing, ventricular pacing, AVNS, and / or detecting tachyarrhythmias.

[0081] For example, timing circuit 96 may pass sensed event intervals determined from received Asense signals and Vsense signals (and delivered ventricular pacing pulses) to tachyarrhythmia detection circuit 92 for determining and counting tachyarrhythmia intervals for use in detecting AT / AF and VT / VF. Timing circuit 96 may start pacing escape intervals in response to Asense and Vsense signals received from sensing circuit 86 for controlling the timing of atrial pacing pulses and ventricular pacing pulses according to a pacing mode and pacing rate. As further described below, timing circuit 96 and therapy control circuit 94 mayRef. No. A0013103 WO01control therapy delivery circuit 84 to deliver AVNS pulse trains according to AVNS control parameters in response to tachyarrhythmia detection circuit 92 detecting AT / AF and an unstable ventricular rate. An unstable ventricular rate may refer to a ventricular rate that is faster than a rate threshold and / or includes variable RRIs that meet RRI variability criteria. The control circuit 80 may trigger the onset of V rate control therapy delivery by therapy delivery circuit 84 when an unstable ventricular rate is detected.

[0082] Tachyarrhythmia detection circuit 92 may be implemented in control circuit 80 as hardware, software and / or firmware that processes and analyzes signals received from sensing circuit 86 and / or timing circuit 96 for detecting atrial and ventricular tachyarrhythmias. In some examples, tachyarrhythmia detection circuit 92 may include comparators and counters for counting PPIs and RRIs determined by timing circuit 96 that are tachyarrhythmia intervals. Tachyarrhythmia detection circuit 92 may compare PPIs to atrial tachyarrhythmia detection interval zones, e.g., an AF interval zone and / or an AT interval zone, to detect and count atrial tachyarrhythmia intervals toward detecting AT / AF, for example. Tachyarrhythmia detection circuit 92 may compare RRIs determined by timing circuit 96 to a VT interval zone and / or a VF interval zone. PPIs and RRIs falling into a respective AT / AF or VT / VF detection interval zone can be counted by a respective AT interval counter, AF interval counter, VT interval counter or VF interval counter. In some cases in a combined AT / AF and a combined VT / VF interval counter may be provided to count both AT and AF intervals and VT and VF intervals, respectively. When a threshold number of atrial or ventricular tachyarrhythmia intervals is reached, control circuit 80 may detect AT / AF or VT / VF, respectively. In some examples, a tachyarrhythmia detection based on the threshold number of tachyarrhythmia intervals being reached may be confirmed or rejected based on morphology analysis of a cardiac electrical signal. Morphology and / or interval analysis may be performed for discriminating supraventricular tachyarrhythmia (SVT) from VT / VF. Any of a number of tachyarrhythmia detection methods may be implemented in an IMD performing the AVNS therapy delivery methods disclosed herein.

[0083] Therapy delivery circuit 84 may include at least one charging circuit and one or more charge storage devices such as one or more holding capacitors for generating electrical stimulation pulses for delivery to the patient’s heart via a selected electrode vector. Therapy delivery circuit 84 may include a low voltage therapy delivery circuit for generating relatively low voltage cardiac pacing pulses and AVNS pulse trains. Therapy delivery circuit 84 may include a high voltage therapy delivery circuit for generating high voltage CV / DFRef. No. A0013103 WO01shock pulses. The low voltage therapy delivery circuit may include a low voltage charging circuit, one or more low voltage holding capacitors and a low voltage output circuit for generating and delivering cardiac pacing pulses and AVNS pulse trains, which may have pulse amplitudes up to 12 volts, up to 10 V, up to 8 volts, or up to 5 volts, as examples with no limitation intended. Cardiac pacing pulses may be delivered by the low voltage therapy circuit in response to a pacing escape interval or other pacing timing interval expiring, as determined by control circuit 80, or in response to detection of a triggering event detected by control circuit 80. Cardiac pacing pulses may be delivered for providing bradycardia pacing, asystole pacing, ATP, post-shock pacing, etc.

[0084] The low voltage charging circuit may include a charge pump for charging a low voltage holding capacitor to a pacing voltage amplitude up to a multiple of the battery voltage of power source 98, e.g., up to three or four times the battery voltage. A state machine of control circuit 80 may control charging of a low voltage holding capacitor to a programmed pacing voltage amplitude using a multiple of the battery voltage of power source 98. The low voltage output circuit that may include one or more switching devices and an output or “tip” capacitor through which the low voltage holding capacitor(s) may be discharged for delivering a pacing pulse. A charged low voltage holding capacitor may be discharged via a tip capacitor by switching on an electrode selection switch after charge completion to deliver a pacing pulse to a selected cathode electrode with a return path via a selected anode electrode. The cardiac pacing pulses can be delivered as bipolar pacing pulses via a “tip-to-ring” pacing electrode vector, e.g., in the RV via RV tip electrode 28 to RV ring electrode 30 and / or in the RA via RA tip electrode 20 to RA ring electrode 22, for successfully capturing and pacing the heart. In some instances, ATP may be delivered in response to detecting an atrial tachyarrhythmia or a ventricular tachyarrhythmia in an attempt to terminate the tachyarrhythmia.

[0085] The low voltage therapy circuit may generate trains of pulses for delivering AVNS pulse trains according to the V rate control therapy pulse parameters and, at other times, according to AT / AF prevention therapy pulse parameters. The AVNS pulses delivered during each respective therapy may each have a respective pulse amplitude, e.g., up to 10 volts, and pulse width, e.g., up to 200 microseconds, that is less than the myocardial capture threshold of the atria. As described below, the pulse amplitude, pulse width, pulse number and frequency of the pulses in a pulse train are controlled to suppress AV conduction, e.g., by prolonging the physiological refractory period of the AV node, during the V rate controlRef. No. A0013103 WO01therapy. The pulse amplitude, pulse width, pulse number and pulse frequency of the pulses in the pulse train are controlled to inhibit recurrence of AT / AF during the AT / AF prevention therapy. The pulse energy delivered during the AT / AF prevention therapy can be less than the pulse energy delivered during the V rate control therapy. Various AVNS pulse parameters that may be used by control circuit 80 in controlling therapy delivery circuit 84 to deliver AVNS therapies are described below.

[0086] In some examples, therapy delivery circuit 84 may include a high voltage (HV) therapy circuit, which may include a HV charging circuit, HV holding capacitor(s), and HV output circuit that are operatively controlled by signals from control circuit 80 for charging and subsequently discharging the high voltage capacitor(s) for CV / DF shock delivery when control circuit 80 detects VT / VF. In some examples, CV / DF shocks may be delivered by therapy delivery circuit 84 in response to detecting a sustained episode of AT / AF.

[0087] In some examples, the circuitry included in an IMD system operating according to the techniques disclosed herein may include one or more sensors 95 for sensing various physiological signals, such as an acceleration signal, pressure signal, heart sound signals, temperature signal, or the like. Sensors 95 may include an accelerometer, for instance, for sensing acceleration signals correlated to cardiac motion, patient physical activity or other body motion. When IMD 114 or 114’ (shown in FIGs. 2 and 3, respectively) includes an accelerometer in sensors 95, control circuit 80 may process and analyze the acceleration signal received from sensors 95 for sensing ventricular mechanical event signals and / or atrial mechanical event signals. Control circuit 80 may detect atrial fibrillation, for instance, based on a disappearance of atrial systolic event signals present in the acceleration signal. Control circuit 80 may determine that atrial fibrillation is no longer detected based on detection of atrial systolic event signals and corresponding atrial event intervals determined from the acceleration signal. Control circuit 80 may determine ventricular event intervals between consecutively sensed ventricular mechanical event signals for determining a ventricular rate and / or ventricular event interval variability. Accordingly, determination of an atrial rate, atrial tachyarrhythmia detection, ventricular rate and / or RRI regularity by control circuit 80 is not necessarily limited to processing and analysis of cardiac electrical signals. During delivery of AVNS, electrical signal noise received by sensing circuit 86 may confound sensing of atrial P-waves and ventricular R-waves in some instances. Other physiological sensor signals that include atrial and / or ventricular event signals may be processed and analyzed for detecting atrial and / or ventricular events attendant to atrial depolarization and atrial systole andRef. No. A0013103 WO01ventricular depolarization and ventricular systole, respectively, for determining the heart rhythm and controlling AVNS therapies according to the techniques disclosed herein.

[0088] Telemetry circuit 88 includes a transceiver and antenna for communicating with external device 50 (shown in FIG. 1) using RF communication or other communication protocols as described above. Control parameters utilized by control circuit 80 for sensing cardiac event signals, detecting arrhythmias, and controlling therapy delivery, including AVNS therapy delivery, may be programmed into memory 82 via telemetry circuit 88. Under the control of control circuit 80, telemetry circuit 88 may receive downlink telemetry from and send uplink telemetry to external device 50.

[0089] FIG. 5 is a flow chart 200 of a method for controlling AVNS by an IMD system according to some examples. FIG. 5 and other flow charts herein are generally described in conjunction with IMD 14 shown in FIG. 1 and FIG. 4 for the sake of illustration. However, it is to be understood that the techniques for detecting atrial tachyarrhythmia, detecting an unstable ventricular rate, and controlling AVNS therapies as disclosed herein may be implemented in other single, dual or multi-chamber IMDs having different lead and electrode configurations or in a leadless IMD, such as IMD 114 shown in FIG. 2, or in a multi -device leadless system such as the IMD system 110 shown in FIG. 3.

[0090] At block 202, with continued reference to FIG. 4, control circuit 80 of IMD 14 may determine that therapy start criteria are met for delivering AT / AF prevention therapy. In some instances, one or more conditions may be present during which AVNS therapy may interfere with other IMD sensing, therapy delivery or device testing functions or be otherwise undesirable. For example, at block 202, control circuit 80 may verify that AT / AF is not being detected, VT / VF is not being detected, a programming session with external device 50 is not underway, and that any of a number of device diagnostic or other tests are not being imminent or underway, such as a lead impedance test, a pacing capture threshold test or the like.

[0091] In some examples, control circuit 80 may control the IMD 14 to perform a ventricular refractory period (VRP) and / or an atrial refractory period (ARP) test to determine that therapy start criteria are met at block 202. The VRP and / or ARP may be determined to avoid delivering AVNS pulse trains having a total pulse train duration that is greater than the VRP or ARP. In various examples, the AT / AF prevention therapy AVNS pulse trains may be delivered during the ARP and / or the VRP. The AVNS pulse trains delivered for the V rate control therapy may be delivered during the ARP and / or the VRP. If the ARP and the VRP have different durations, the AVNS pulse trains delivered during the ARP and the VRP mayRef. No. A0013103 WO01be delivered according to different pulse parameters that result in AVNS pulse trains that end no later than the respective ARP or VRP. For instance, the AT / AF prevention therapy AVNS pulse trains may be delivered during ARPs and the AVNS pulse trains for V rate control therapy may be delivered during VRPs or vice versa. When AVNS pulse trains for either of the AT / AF prevention therapy or the V rate control therapy are delivered during both of the ARP and the VRP, a first AVNS pulse train having a first duration that is not longer than the ARP and a second AVNS pulse train having a second duration that is not longer than the VRP may be delivered during the same cardiac cycle. The first duration and the second duration of the ARP AVNS pulse train and the VRP AVNS pulse train, respectively, may be the same or different. It is contemplated that the myocardial refractory period, atrial or ventricular, during which the AVNS pulse trains are delivered may change or be alternating during delivery of the AT / AF prevention therapy and / or during delivery of the V rate control therapy in some examples.

[0092] In some examples, a VRP may be determined by delivering multiple series of ventricular pacing pulses with the final pacing pulse of each series of pacing pulses being delivered at progressively shorter ventricular pacing intervals. Control circuit 80 may determine if the final pacing pulse of each series captures the ventricles or not, e.g., based on sensing an evoked response signal from the cardiac electrical signal by sensing circuit 86. When the final pacing pulse reaches a shortened pacing interval that results in loss of ventricular capture (absence of the pacing evoked R-wave), the shortened pacing interval is determined to be equal to or less than the effective VRP. The longest pacing interval from the second to last to the last, final pacing pulse in the series of pacing pulses that results in loss of ventricular capture may be determined as the VRP. In a similar manner, an ARP may be determined by delivering multiple series of atrial pacing pulses with the final atrial pacing pulse of each series of atrial pacing pulses being delivered at progressively shorter atrial pacing intervals until the last pulse of the series fails to capture the atria. The longest interval to the final atrial pacing pulse that results in loss of atrial capture can be determined by control circuit 80 as the ARP.

[0093] Control circuit 80 may verify that the AVNS pulse train duration for AT / AF prevention therapy is not greater than the VRP and / or ARP period determined at block 202. If the AVNS pulse train duration programmed for controlling the AT / AF prevention therapy according to first pulse parameters is longer than the cardiac refractory period during which the AVNS pulse trains are to be delivered, control circuit 80 may determine that the therapyRef. No. A0013103 WO01start criteria are not met. In other examples, control circuit 80 may adjust one or more pulse parameters used for controlling the AVNS pulse trains to adjust the AVNS pulse train duration to be equal to or less than a cardiac refractory period for both AT / AF prevention therapy and V rate control therapy. For instance, the AVNS pulse train duration can be adjusted by control circuit 80 to be less than the VRP or ARP by decreasing the pulse number, decreasing the pulse width, and / or increasing the pulse frequency of the AVNS pulse trains. In some examples, the start time of the AVNS pulse train during a cardiac cycle may be scheduled earlier after an atrial pacing pulse, Asense signal, ventricular pacing pulse or Vsense signal to promote termination of the pulse train within a myocardial refractory period. The VRP and / or ARP may be determined periodically by control circuit 80 based on heart rate or by performing a pacing interval test as described above, and the AVNS pulse train duration, particularly the ending time, can be adjusted based on the VRP or ARP by adjusting one or more AVNS pulse parameters.

[0094] At block 204, when the therapy start criteria are determined to be met, control circuit 80 may control therapy delivery circuit 84 to deliver AVNS according to AT / AF prevention therapy pulse parameters and therapy protocol parameters, which may be stored in memory 82. The AT / AF prevention therapy is delivered by therapy delivery circuit as trains of pulses scheduled during the VRP and / or ARP every n cardiac cycles, where n is a positive integer, as specified by the therapy protocol parameters. In some examples, n is 1 such that an AVNS pulse train is delivered on every cardiac cycle, e.g., during the ventricular refractory period. However, the AT / AF prevention therapy delivered for inhibiting recurrence of AT / AF may be effective when the AVNS pulse trains are delivered less often than every cardiac cycle. As such, in other examples, n is greater than 1 so that an AVNS pulse train is delivered less often than every cardiac cycle. During the AT / AF prevention therapy delivery, the ratio of cardiac cycles to AVNS pulse trains may be 1:1 or up to 60: 1 as examples, with no limitation intended.

[0095] The AVNS pulse trains may be delivered every n cardiac cycles 24 hours per day as long as a therapy suspend condition is not detected (as further described below) and when AT / AF is not being detected and / or the V rate control therapy is not being delivered. In some examples, AT / AF may be detected, but the ventricular rate may be stable (e.g., less than a rate threshold and / or regular RRIs that do not meet variability criteria). The V rate control therapy may not be triggered when the ventricular rate is stable during AT / AF. It is contemplated that AT / AF prevention therapy could continue to be delivered according to theRef. No. A0013103 WO01therapy protocol parameters during a detected AT / AF episode if the ventricular rate is not unstable and the V rate control therapy is not being delivered. In some examples, the therapy protocol parameters for the AT / AF prevention therapy may specify times of day and / or therapy delivery time intervals (e.g., duty cycle on time and duty cycle off time) during which the AVNS pulse trains are delivered according to the AT / AF prevention therapy pulse parameters. The pulse parameters used for controlling the AT / AF prevention therapy control each individual AVNS pulse train. The therapy protocol parameters used to control the AT / AF prevention therapy are used to control how often and / or when the AVNS pulse trains are delivered.

[0096] For example, the AT / AF prevention therapy may be duty cycled on and off to control when the AVNS pulse trains are delivered. The AT / AF prevention therapy may be duty cycled on for delivery of AVNS pulse trains every n cardiac cycles for 1 minute, 10 minutes, 15 minutes, 30 minutes, 1 hour, 2 hours, 4 hours, 8 hours, 12 hours, 24 hours, or one week as examples. The AT / AF prevention therapy may be duty cycled off for a time interval that is equal to the on time of the duty cycle or different (greater or less) than the on time of the duty cycle. In an illustrative example, the AVNS pulse trains may be delivered every cardiac cycle for four hours then be turned off for four hours. In another illustrative example, the AVNS pulse trains may be delivered during the day, e.g., from 8:00 am until 12:00 midnight and turned off from 12:00 midnight until 8:00 am. It is recognized that numerous combinations of protocol parameters may be selected and programmed in memory 82, which may be tailored to patient need, for controlling the frequency of AVNS pulse train delivery over a given time period for the AT / AF prevention therapy.

[0097] It is contemplated that AT / AF prevention therapy protocol parameters that define the duty cycle on and off times, scheduled times of day for turning on and off the therapy, and / or the number n defining the ratio (n: 1) of cardiac cycles to delivered AVNS pulse trains can be adjusted for a given patient based on the history of AT / AF episodes. For example, a patient having a history of relatively high AT / AF burden (percentage of time that AT / AF is present) or high incidence of symptomatic AT / AF episodes may have a higher duty cycle on time and / or lower number of n so that a greater number of AVNS pulse trains for AT / AF prevention therapy are delivered over a given time period than a patient having a relatively lower historical AT / AF burden. Furthermore, as described below in conjunction with FIG. 6, the protocol parameters and / or pulse parameters used to control the AT / AF preventionRef. No. A0013103 WO01therapy may be adjusted over time as changes in the AT / AF burden, frequency of episodes, episode duration and / or atrial rate during AT / AF changes.

[0098] Cycling on and off AT / AF prevention therapy and / or delivering AT / AF prevention therapy pulse trains less often than every cardiac cycle can reduce the likelihood of the AVNS pulse trains from interfering with or delaying detection of an AT / AF episode. During the AVNS pulse train, the sensing circuit 86 may blank the atrial sensing channel 87 to avoid interference of the AVNS pulses with sensing of intrinsic P-waves. When AVNS pulse trains for AT / AF prevention therapy are delivered relatively frequently, e.g., for longer on times and / or on every cardiac cycle, the AVNS pulse parameters may be selected so that the pulse train is relatively short. For example, the AVNS pulse trains may be delivered with a relatively low pulse number, e.g., two or three pulses, so that blanking of the atrial sensing channel 87 during AVNS pulse train delivery can be reduced to promote sensing of fast atrial rates associated with AT / AF.

[0099] The therapy control parameters may further include one or more therapy delivery limits which may include a maximum limit and / or minimum limit of the delivered therapy in terms of time and / or in terms of delivered energy. For instance, a maximum and / or minimum therapy delivery time, e.g., specified in units of time or as a percentage out of a specified time period, may be stored in memory 82 as a therapy delivery limit. The therapy delivery time may be determined as the product of the pulse train duration and the total number of pulse trains delivered for a given therapy over a specified time period and may be determined in units of time or as a percentage of the specified time period.

[0100] Additionally or alternatively, a maximum and / or minimum therapy delivery energy may be stored in memory 82 as a therapy delivery limit. The therapy delivery energy for a specified time period may be determined as the product of the energy of an individual pulse train (or waveform) and the total number of pulse trains delivered for a given therapy over the specified time period.

[0101] One or more therapy delivery limits may be used by control circuit 80 to control the total time that AVNS pulse trains are being delivered and / or to control the total energy of AVNS pulse trains delivered for a given therapy over a specified time period. Control circuit 80 may be configured to adjust a therapy protocol parameter, such as duty cycle on time and / or off time, to meet a maximum and / or minimum therapy delivery limit. Additionally or alternatively, control circuit 80 may be configured to adjust a pulse parameter, such as pulseRef. No. A0013103 WO01amplitude, pulse width, pulse train duration, pulse frequency or ratio of cardiac cycles to delivered AVNS pulse trains, to meet a maximum and / or minimum therapy delivery limit.

[0102] For example, if the AVNS pulse trains are delivered at a 1 : 1 ratio with cardiac cycles, a faster heart rate will result in a total therapy delivery time and total delivered energy that is greater than when the heart rate is relatively slower. By including a maximum limit and / or minimum limit of the therapy delivery time and / or therapy delivery energy in the therapy protocol parameters for the AT / AF prevention therapy and / or V rate control therapy, control circuit 80 may be enabled to adjust one or more of the protocol parameters and / or pulse parameters for a given therapy to meet those limits. For instance, control circuit 80 may terminate a therapy earlier than a scheduled time or before AT / AF is detected (in the case of AT / AF prevention therapy) or before AT / AF termination is detected (in the case of V rate control therapy) if a maximum therapy delivery time during a specified time period (e.g., one minute, one hour, one day or other specified time period) is reached. Control circuit 80 may adjust a protocol parameter by adjusting the duty cycle on time (increased or decreased), the duty cycle off time (decreased or increased) and / or by adjusting scheduled times of day for turning on or off the AT / AF prevention therapy, as examples, for meeting a maximum or minimum therapy delivery time and / or a maximum or minimum therapy delivery energy during a specified time period.

[0103] Additionally or alternatively, control circuit 80 may adjust one or more of the pulse parameters to meet a maximum and / or minimum therapy delivery time and / or therapy delivery energy. Control circuit 80 may adjust (e.g., increase or decrease) the pulse train duration, pulse number, pulse amplitude, pulse width, pulse frequency and / or ratio of the number of cardiac cycles to delivered AVNS pulse trains as needed to meet a specified maximum or minimum therapy delivery limit.

[0104] In some examples, the therapy protocol parameters may include a minimum limit such that if the heart rate is low, for example, control circuit 80 may adjust a therapy protocol parameter and / or pulse parameter to increase the total time that AVNS pulse trains are delivered and / or the total energy delivered in a specified time period for a given therapy, e.g., the AT / AF prevention therapy. For instance, control circuit 206 may adjust the ratio of cardiac cycles to AVNS pulse trains to a 1 : 1 ratio or other ratio that increases the rate of AVNS pulse train delivery during the specified time period. Control circuit 80 may increase the pulse train duration. Control circuit 206 may increase the on time of the scheduled dutyRef. No. A0013103 WO01cycle and / or decrease the off time of the scheduled duty cycle so that a minimum limit is reached.

[0105] By including a therapy protocol parameter that specifies a maximum limit of therapy delivery time or therapy delivery energy, nerve fatigue can be reduced or avoided and / or the useful life of power source 98 (FIG. 4) can be conserved or extended. Enabling control circuit 80 to control the protocol parameters and / or pulse parameters according to a minimum limit of therapy delivery time and / or total delivered energy may promote the effectiveness of the delivered therapy, e.g., in inhibiting the recurrence of AT / AF and / or in regulating the ventricular rate.

[0106] In some examples, the maximum and / or minimum limit specified for a first portion of a specified time period may be different than the maximum and / or minimum limit specified for a second portion of the same time period. For example, a first minimum or maximum limit may be programmed into memory 82 for daytime hours and a second minimum or maximum limit may be programmed into memory 82 for nighttime hours. In some examples, control circuit 80 may be configured to adjust the therapy protocol parameters and / or pulse parameters used at night to meet a minimum and / or maximum limit that was not met during the day or vice versa so that a total maximum or minimum limit for the 24 hour period can be met. In an illustrative example, a minimum 25% therapy delivery time may be specified for the AT / AF prevention therapy during daytime hours (e.g., from 08:00 to 20:00), but the AT / AF prevention therapy may only be delivered for 15% of the 12 hours (e.g., due to lower than expected heart rate, competing therapies or other conditions preventing AT / AF prevention therapy from being delivered). Control circuit 80 may adjust one or more therapy protocol parameters and / or pulse parameters for controlling the AT / AF prevention therapy during nighttime hours (e.g., from 20:00 to 08:00) so that a minimum limit for the entire 24 hour period can be reached. If a maximum nighttime limit is exceeded, control circuit 80 may adjust a protocol parameter and / or pulse parameter used during the next daytime hours to stay within the maximum limit for the total 24 hour period. A minimum and / or maximum limit may be specified for a given therapy (AT / AF prevention therapy and / or V rate control therapy) to limit the percentage of time that the therapy is delivered and / or to limit the total energy delivered over one or more specified time periods. The specified time periods may range from one minute, one hour, one week or one month in various examples.

[0107] AVNS pulse parameters may be stored in memory 82 for controlling aspects of the AVNS pulse trains that are being scheduled for delivery according to the therapy protocolRef. No. A0013103 WO01parameters. These pulse parameters can include the pulse number, pulse frequency, pulse amplitude, pulse width and pulse train start time (following a triggering atrial or ventricular event, which may be an Asense signal, atrial pacing pulse, Vsense signal or ventricular pacing pulse). The pulse number and / or pulse frequency may be determined based at least in part on the length of the VRP and / or ARP. As described above, the VRP and / or ARP may be determined by control circuit 80. In other examples, the physiological refractory period of the atria or the ventricles may be based on the patient’s heart rate and / or clinical data from a population of patients. In some examples, the pulse frequency is between 25 and 75 Hertz (Hz) or between 40 and 50 Hz and may be selected as an optimal frequency known to suppress AV nodal conduction to have an effective response of decreasing the ventricular rate and / or ventricular cycle variability. The pulse number may be selected based on a cardiac refractory period and / or the pulse frequency. For instance, if the pulse frequency is 50 Hz and the VRP is 70 ms or less, a pulse number of 2 is selected by control circuit 80. If the VRP is greater than 70 but less than or equal to 90 ms, a pulse number of 3 may be selected by control circuit 80. If the VRP is greater than 90 ms but less than or equal to 110 ms, a pulse number of 4 may be selected by control circuit 80. In this example, for each 20 ms increase in VRP, the pulse number can be increased by 1, up to a maximum pulse number, e.g., up to a maximum of 7 or 8 pulses per AVNS pulse train.

[0108] The pulse width may be between 0.05 and 2 ms or between 0.6 and 1.5 ms as examples. The pulse amplitude may be between 0.5 and 5 volts or between 1 and 4 volts as examples. As further described below, the pulse width and / or the pulse amplitude of the AT / AF prevention therapy pulse parameters may be determined based on the pulse width and / or pulse amplitude of the V rate control therapy pulse parameters in some examples. For instance, the pulse width may be set to a nominal pulse width of 1 ms and the pulse amplitude may be set to a percentage of (e.g., 50%) or specified offset less than (e.g., 1 or 2 volts less than) the pulse amplitude used for delivering AVNS pulse trains during the V rate control therapy.

[0109] At block 206, control circuit 80 may determine if AT / AF is detected. If not (“no” branch of block 206), control circuit 80 may determine if another therapy suspend condition is detected at block 212. The AT / AF prevention therapy may be delivered according to the protocol parameters and pulse parameters as described in the various examples above until AT / AF is detected (“yes” branch of block 206) or another therapy suspend condition is detected at block 212. If AT / AF is not detected (“no” branch of block 206), other therapyRef. No. A0013103 WO01suspend conditions that may be detected by control circuit 80 at block 212 that cause therapy delivery circuit 84 to suspend the AT / AF prevention therapy may include a VT / VF detection, a scheduled device test such as a capture threshold test, a lead / electrode impedance test or the like, detecting an RRI that is associated with a VRP that is shorter than the pulse train duration (e.g., a premature ventricular contraction or “PVC”), or a user intervention recognized by control circuit 80 when a command or communication is received via telemetry circuit 88. A short RRI, e.g., associated with a PVC or a run of PVCs, could cause the AVNS pulse train to extend beyond the VRP, which may be undesirable. In other instances, a short RRI may be the onset of a ventricular tachyarrhythmia in which case delivery of AVNS pulse trains may be undesirable to avoid interference with sensing R-waves and detecting the ventricular tachyarrhythmia. As such, a short RRI may be detected as a therapy suspend condition at block 212 in some examples.

[0110] When a therapy suspend condition is detected at block 212, the therapy delivery circuit 84 terminates the AT / AF prevention therapy at block 218. Control circuit 80 may return to block 202 to wait for therapy start criteria to be met as described above before restarting the AT / AF prevention therapy at block 204.

[0111] Referring again to block 206, if control circuit 80 detects AT / AF at block 206, the AT / AF prevention therapy is terminated at block 208 in some examples. As indicated above, AT / AF prevention therapy may continue according to the therapy protocol parameters when AT / AF is being detected if the ventricular rate remains stable and no other therapy is being delivered, e.g., atrial ATP, CV / DF shock, or V rate control therapy. In the illustrative example of flow chart 200, however, AT / AF prevention therapy is terminated when AT / AF is detected to enable monitoring of the AT / AF episode and / or for allowing other therapies to be delivered by IMD 14.

[0112] In some examples, as indicated at block 210, control circuit 80 may determine if atrial anti-tachyarrhythmia therapy criteria are met in response to detecting AT / AF. IMD 14 may be programmed to deliver atrial ATP and / or one or CV / DF shocks if AT / AF is detected. In some examples, the atrial rate, ventricular rate, and / or AT / AF episode duration may be compared to atrial anti -tachyarrhythmia therapy criteria at block 210 to determine if atrial ATP and / or a CV / DF shock should be delivered. If atrial anti -tachyarrhythmia therapy criteria are met (“yes” branch of block 210), therapy delivery circuit 84 may deliver one or more sequences of atrial ATP and / or one or more CV / DF shocks at block 211 in an attemptRef. No. A0013103 WO01to terminate the AT / AF. If the AT / AF is still being detected at block 216, control circuit 80 may control therapy delivery circuit 84 to initiate the V rate control therapy at block 214.

[0113] In some examples, therapy delivery circuit 84 may deliver V rate control therapy before and / or after atrial ATP and / or CV / DF shocks is / are delivered. For example, control circuit 80 may control therapy delivery circuit 84 to deliver V rate control therapy at block 214 and, if the AT / AF is still being detected after a specified time interval, control circuit 80 may control therapy delivery circuit 84 to terminate the V rate control therapy and deliver one or more atrial ATP sequences and / or CV / DF shocks at block 210. V rate control therapy may be delivered after an initial anti-arrhythmia therapy sequence fails to terminate the AT / AF and after one or more additional atrial anti-tachyarrhythmia therapy sequences. As such, it is to be understood from the flow chart 200 of FIG. 5 that V rate control therapy may be delivered in an alternating manner with atrial anti-tachyarrhythmia therapy, e.g., for specified time intervals between atrial anti-tachyarrhythmia therapy attempts, until the AT / AF is no longer detected or a maximum number of atrial ATP and / or CV / DF shocks for terminating the AT / AF have been delivered. In other instances atrial anti-tachyarrhythmia therapies may not be enabled in IMD 14 or atrial anti-arrhythmia therapy delivery criteria may not be met at block 210. Control circuit 80 may advance to block 214 to initiate V rate control therapy after terminating the AT / AF prevention therapy in response to detecting AT / AF.

[0114] Therapy delivery circuit 84 may deliver the V rate control therapy at block 214 to suppress conduction of the AT / AF to the ventricles to reduce the likelihood of a fast and / or irregular ventricular rate. The V rate control therapy may be delivered by adjusting at least one pulse parameter that is used to control delivery of the AVNS pulse trains. For example, the pulse amplitude, pulse width, pulse number and / or pulse frequency of AVNS pulse trains delivered for the V rate control therapy may be increased compared to the corresponding pulse parameter value of the AVNS pulse trains delivered for the AT / AF prevention therapy. The energy of an AVNS pulse train delivered during the V rate control may be greater than the delivered energy of an AVNS pulse train delivered during the AT / AF prevention therapy.

[0115] The AVNS pulse trains during the V rate control therapy may be delivered at a 1:1 ratio with the atrial or ventricular rate, e.g., during a cardiac refractory period following every atrial event (Asense signal or atrial pacing pulse) and / or following every ventricular event (Vsense signal or ventricular pacing pulse in some instances). It is recognized that during AT / AF, the V rate control therapy AVNS pulse trains may be delivered during VRPsRef. No. A0013103 WO01following Vsense signals or ventricular pacing pulses because the atrial rate may be too fast to allow AVNS pulse trains to be delivered during ARPs. The V rate control therapy may be delivered until AT / AF is no longer detected as determined at block 216. As described below in conjunction with FIG. 6, during the V rate control therapy, adjustments to the pulse parameters may be made as needed if the ventricular rate is increased and not decreasing and / or if RRIs are irregular. V rate control pulse parameters may be adjusted as needed to promote effective V rate control by the AVNS pulse trains. Examples of pulse parameters used to control the V rate control therapy are described below in conjunction with FIG. 7. The pulse amplitude of pulses in the AVNS pulse trains delivered at block 214 may be at least 4 volts and may be as high as 12 volts as examples. The pulse amplitude of the pulses in the AVNS pulse trains delivered at block 204 for AT / AF prevention therapy may be 50% or less of the pulse amplitude used for delivering the V rate control therapy in some examples and / or below 4 volts.

[0116] When control circuit 80 no longer detects AT / AF (“no” branch of block 216), therapy delivery circuit 84 may terminate the V rate control therapy at block 216. Control circuit 80 may return to block 202 to wait for the therapy start criteria to be met prior to restarting the AT / AF prevention therapy again at block 204. In some examples, the therapy start criteria may include a specified therapy start delay interval after an AT / AF episode termination is detected at block 216. AT / AF episode termination may be detected by control circuit 80 when at least a threshold number of PPIs are longer than the AT / AF interval zone(s), for example. In some patients, an AT / AF episode may not be expected to recur early after a terminated AT / AF episode. As such, restarting the AT / AF prevention therapy after an AT / AF episode may be delayed for 2, 10, 15, 30, 60 or 120 minutes or up to one day after AT / AF termination is detected. Control circuit 80 may determine that the therapy start criteria are met at block 202 when the therapy start delay interval is expired and any other therapy start criteria are met according to any of the examples given above.

[0117] FIG. 6 is a flow chart 250 of a method for controlling AVNS by an IMD system according to another example. Identically numbered blocks in flow chart 250 correspond to like-numbered blocks in FIG. 5 and are described above. In FIG. 5, V rate control therapy may be started by therapy delivery circuit 84 in response to control circuit 80 detecting AT / AF to prophylactically treat an unstable V rate that could arise during the AT / AF episode. In some cases, the V rate control therapy is started after an atrial ATP therapy and / or CV / DF shock fails to terminate the AT / AF or atrial anti-tachyarrhythmia therapies are disabled. InRef. No. A0013103 WO01other examples, when an AT / AF is detected (block 206), control circuit 80 may terminate the AT / AF prevention therapy at block 208 then analyze sensed cardiac signals for detecting an unstable V rate at block 209 as shown in FIG. 6. In FIG. 6, blocks 210 and 211 of FIG. 5 are not shown, but it is to be understood from the flow chart of FIG. 5 and the foregoing description that, in some examples, atrial anti-tachycardia therapies may be enabled and delivered before and / or after time intervals of V rate control therapy. In the example of FIG.6, control circuit 80 may be configured to detect an unstable V rate by comparing sensed ventricular event intervals, e.g., RRIs, to a rate threshold and / or a variability threshold or variability criteria for detecting a relatively fast and / or irregular V rate as an unstable V rate due to the conducted AT / AF. For instance, a V rate that is faster than 90, 100, 110 or 120 beats per minute may be detected as an unstable V rate. As an example, control circuit 80 may determine a mean, median or shortest RRI or other representative RRI from a specified number of Vsense signals received from sensing circuit 80. The representative RRI may be compared to a threshold interval corresponding to a rate threshold indicative of a relatively fast V rate.

[0118] Additionally or alternatively, control circuit 80 may determine a V event interval variability metric that is representative of RRI spread, dispersion or change between consecutive RRIs. The variability metric can be compared to variability criteria for triggering the onset of the V rate control therapy when AT / AF is being detected. An RRI range, difference between the shortest and longest RRIs, standard deviation of the RRIs, sum of differences between consecutive RRIs, average difference between consecutive RRIs, or other metric(s) representative of the spread, dispersion or change between consecutive RRIs in a series of RRIs may be compared to a variability threshold and / or other variability criteria for detecting an unstable V rate.

[0119] If the V rate is determined to be stable at block 209, e.g., the rate threshold and / or variability threshold are not met so that an unstable V rate is not detected, control circuit 80 may continue to monitor the atrial rhythm at block 216. As long as AT / AF is being detected, control circuit 80 may monitor the V rate for detecting an unstable V rate at block 209. If an unstable rate is not detected and the AT / AF is no longer being detected (“no” branch of block 216), control circuit 80 may return to block 202 without starting or delivering the V rate control therapy (block 218 may be skipped if V rate control therapy not started). Control circuit 80 may wait for therapy start criteria to be met, including waiting for any post- AT / AF therapy delay interval to expire before restarting the AT / AF prevention therapy at block 204.Ref. No. A0013103 WO01

[0120] Referring again to block 209, if an unstable rate is detected at any time after detecting AT / AF and before detecting AT / AF termination, therapy delivery circuit 84 may start the V rate control therapy at block 210. Control circuit 80 continues to monitor the atrial rhythm for detecting termination of AT / AF at block 216. In some examples, control circuit 80 may continue to monitor the V rate when the V rate control therapy is being delivered. If AT / AF is still being detected (“yes” branch of block 216), and the V rate control therapy is being delivered but the unstable V rate criteria are still met (“yes” branch of block 217), control circuit 80 may adjust pulse parameters at block 220. Control circuit 80 may determine that the V rate control therapy is effectively controlling or stabilizing the V rate during AT / AF when RRIs (or a representative RRI such as an average, median, mode, etc.) are longer than a threshold RRI and / or an RRI variability metric does not meet a variability threshold. In some cases, control circuit 80 may determine that the V rate control therapy is effectively stabilizing or controlling the V rate if the V rate decreases (e.g., by 20% or other threshold percentage) during the V rate control therapy compared to the V rate determined during the detected AT / AF prior to starting the V rate control therapy. In some cases, control circuit 80 may determine that the V rate control therapy is effectively stabilizing or controlling the V rate if the RRI variability metric decreases (e.g., by 20% or other threshold percentage) during the V rate control therapy compared to the RRI variability metric determined during the detected AT / AF prior to starting the V rate control therapy. In other examples, control circuit 80 may determine that the V rate control therapy is effective when the ventricular rate is less than a rate threshold that may be programmable by a clinician. Additionally or alternatively, control circuit 80 may determine that the V rate control therapy is effective when an RRI variability metric is less than a variability threshold that may be programmable by a clinician.

[0121] When unstable V rate criteria are still met at block 217, or when control circuit 80 determines that the V rate control therapy is not effectively stabilizing the V rate, control circuit 80 may increase the pulse amplitude and / or pulse width, in some examples, for delivering higher energy AVNS pulse trains that may be more effective in suppressing the AV node conduction. In some examples, the pulse frequency and / or pulse number may be adjusted. A frequency of 40 to 60 Hz or about 50 Hz may be optimal for suppressing AV nodal conduction, however. As such, pulse frequency adjustments may not be performed at block 220 in some examples. In some instances, the pulse number may be increased at block 220. The pulse number may not be increased, however, when the total pulse train durationRef. No. A0013103 WO01would exceed a cardiac refractory period, e.g., the VRP. If the V rate control therapy is not already started or has been started and an unstable V rate is no longer detected (“no” branch of block 217), control circuit 80 may continue to monitor for an unstable V rate and for termination of the AT / AF episode by returning to block 209.

[0122] In some examples, however, control circuit 80 may decrease one or more V rate control pulse parameters at block 219 when the V rate is determined to be stable and / or when ventricular pacing pulses are being delivered due to over suppression of AV nodal conduction. In an effort to conserve power source 98, control circuit 80 may decrease the pulse number, pulse amplitude and / or pulse width, for example, at block 219 to determine if lower energy pulse trains can be effective in maintaining a stable V rate during the AT / AF. If an unstable V rate is again detected at block 209 (e.g., the V rate increases and / or RRI variability increases) and AT / AF is still being detected at block 216, control circuit 80 can increase the pulse parameter(s) at block 220 to the last known values that resulted in effective V rate control.

[0123] While not shown explicitly in FIG. 5 discussed above, it is to be understood that monitoring the V rate after starting the V rate control therapy may be performed for adjusting the pulse parameters of the V rate control therapy as described here in conjunction with blocks 217, 219 and 220 of FIG. 6. An unstable V rate may not be detected prior to starting the V rate control therapy, as described in conjunction with FIG. 5 where the V rate control therapy is started in response to AT / AF detection. V rate monitoring, however, may be performed during the V rate control therapy to promote effective therapy delivery by adjusting pulse parameters as needed during the therapy delivery, e.g., by increasing one or more pulse parameters in response to determining that an unstable V rate is occurring or by decreasing one or more pulse parameters to conserve power source 98 when a stable V rate can be achieved with a lower AVNS pulse train energy.

[0124] When AT / AF is no longer detected at block 216, therapy delivery circuit 84 terminates the V rate control therapy at block 218. In this example, control circuit 80 may update AT / AF episode data at block 222, which may be stored in memory 82, to determine if the AT / AF prevention therapy is effective in reducing the incidence of AT / AF, e.g., reducing the AT / AF burden, AT / AF episode duration, number of AT / AF episodes per day or per week, and / or the atrial rate during detected AT / AF episodes. For instance, control circuit 80 may determine an updated AT / AF burden at block 222 by summing the most recent AT / AF episode duration (time from AT / AF detection to time of AT / AF termination detection) withRef. No. A0013103 WO01any previously detected AT / AF episodes over a 24 hour period. Control circuit 80 may additionally or alternatively determine the episode duration or an average episode duration, the number of AT / AF episodes per day, and / or the highest atrial rate detected during an AT / AF episode as examples of AT / AF episode data.

[0125] At block 224, control circuit 80 may determine if AT / AF inhibition criteria are met. For example, control circuit 80 may determine if AT / AF inhibition criteria are met by comparing the AT / AF burden updated at block 222 to one or more previous daily AT / AF burdens determined and stored in memory 82 for determining if the AT / AF burden is decreasing or acceptable as evidence of effective inhibition of AT / AF by the AT / AF prevention therapy. For example, an average daily AT / AF burden determined from one week or one month of daily AT / AF burdens may be compared to a previously determined average daily AT / AF burden from a preceding week or month. If the AT / AF burden is increasing or has not decreased to at least an acceptable level (“no” branch of block 224), control circuit 80 may determine that the AT / AF inhibition criteria are not met. Control circuit 80 may adjust one or more protocol parameters and / or one or more pulse parameters used in controlling the delivery of the AT / AF prevention therapy at block 226.

[0126] In other examples, control circuit 80 may additionally or alternatively compare the number of AT / AF episodes detected over a given time period to the number of AT / AF episodes detected over one or more preceding time periods to determine if the AT / AF inhibition criteria are met at block 224. The AT / AF inhibition criteria may be met when the number of AT / AF episodes is decreased or is at least not increasing. Additionally or alternatively, control circuit 80 may determine if the AT / AF episode duration is exhibiting a decreasing trend or has reached an acceptable level. Additionally or alternatively, control circuit 80 may determine if the atrial rate during detected AT / AF episodes exhibits a decreasing trend or has reached an acceptable level and / or whether or not an unstable V rate was detected triggering the V rate control therapy to be started. If the V rate control therapy is being delivered for a decreasing number of detected AT / AF episodes because an unstable V rate is not being detected, the AT / AF prevention therapy may be deemed effective in inhibiting AT / AF, by reducing the incidence or severity of AT / AF episodes. The severity of an AT / AF episode may refer to the atrial rate (faster being more severe), ventricular rate (faster and / or greater variability being more severe), and duration (longer being more severe) or any combination thereof, as examples. Any one or combination of metrics determined at block 222 representing the frequency or severity of AT / AF episodes may be used forRef. No. A0013103 WO01determining if the AT / AF prevention therapy is effective in inhibiting the recurrence of AT / AF episodes, compared to a history of AT / AF episodes for the patient and / or compared to acceptable or target thresholds of AT / AF episode metrics established for the patient by a clinician and programmed in memory 82.

[0127] When the AT / AF inhibition criteria are not met as determined by control circuit 80 at block 224, the AVNS pulse train energy and / or the number of pulse trains delivered over a given time period may be adjusted at block 226. The pulse amplitude, pulse width, pulse number and / or pulse frequency may be increased or decreased to test different AVNS pulse train energies for AT / AF prevention effectiveness. If AT / AF episodes are occurring, occurring with increased episode duration and / or occurring with increased frequency, the AT / AF prevention therapy protocol parameters may be adjusted to increase or decrease the total number of AVNS pulse trains being delivered over a given time period, e.g., one day, one week or one month, in an attempt to more effectively inhibit the recurrence of AT / AF.

[0128] If the AT / AF inhibition criteria are met at block 224, e.g., if the AT / AF burden is decreasing, is not increasing, or is considered to be at a clinically acceptable level for the given patient (as determined at block 224), control circuit 80 may advance to block 202 to wait for the therapy start criteria to be met. The current protocol parameters and pulse parameters used to control the AT / AF prevention therapy may be deemed effective in inhibiting the recurrence of AT / AF when the AT / AF incidence and / or severity is determined to be decreased, not increasing or otherwise meeting acceptable thresholds at block 224.

[0129] In some examples, when the AT / AF inhibition criteria are met for a sufficiently long period of time or no AT / AF has been detected for at least a specified time period, control circuit 80 may adjust the AT / AF prevention therapy protocol parameters and / or pulse parameters. For the sake of example, in the flow chart 250 of FIG. 6, when AT / AF is terminated at block 213 because of a therapy suspend condition being detected at block 212, control circuit 80 may determine if no AT / AF episodes have been detected for at least a threshold time period at block 230. The threshold time period may be one week, one month, or three months as examples with no limitation intended. If AT / AF episodes have been detected in the threshold time period (“no” branch of block 230), control circuit 80 may return to block 202 to wait for the therapy start criteria to be met.

[0130] If no AT / AF episodes have been detected over the specified time period (“yes” branch of block 230), the AT / AF prevention therapy may be adjusted at block 232. In some instances, control circuit 80 may decrease the AT / AF prevention therapy at block 232 byRef. No. A0013103 WO01decreasing the duty cycle on time, by increasing the duty cycle off time, and / or by increasing the ratio of cardiac cycles to AVNS pulse trains to thereby reduce the total number of AVNS pulse trains delivered for AT / AF prevention therapy, e.g., over the course of one day or one week. The longevity of the IMD power source 98 may be increased by decreasing the energy requirements for delivering the AVNS pulse trains for AT / AF prevention therapy. When AT / AF is sufficiently inhibited, e.g., as evidenced by updated AT / AF burden and / or other AT / AF episode metrics as described above, the AT / AF prevention therapy may be adjusted, e.g., by decreasing a duty cycle on time, increasing the ratio of cardiac cycles to AVNS pulse trains, or increasing the duty cycle off time. In some cases, AT / AF prevention therapy pulse parameters may be adjusted, e.g., by adjusting the pulse amplitude, the pulse width and / or the pulse number. An optimal pulse amplitude, pulse width, pulse number and / or pulse frequency for inhibiting AT / AF recurrence (e.g., as evidenced by a decreased AT / AF burden) may be identified, however, such that therapy protocol parameters may be adjusted to decrease how often AVNS pulse trains are delivered for AT / AF prevention therapy without adjusting the pulse amplitude, pulse width, pulse number, and / or pulse frequency.

[0131] In various examples, control circuit 80 may determine that AT / AF inhibition criteria are met when no AT / AF episode is detected for a threshold time interval, when AT / AF burden is decreased or at least not increased compared to a previous AT / AF burden, when an AT / AF episode duration is decreased or at least not increased compared to a previous AT / AF episode duration, when the atrial rate is decreased or at least not increased compared to a previous AT / AF episode atrial rate, when the V rate control therapy is not triggered, or any combination thereof. Additionally or alternatively, control circuit 80 may determine that AT / AF inhibition criteria are met when one or more AT / AF episode metrics determined at block 222 fall below an acceptable threshold stored in memory 82, which may be programmable to meet individual patient needs. Control circuit 80 may adjust the AT / AF prevention therapy protocol parameters and / or pulse parameters according to whether the AT / AF inhibition criteria are met or not.

[0132] FIG. 7 depicts a diagram 300 of AVNS pulse trains that may be delivered during the V rate control therapy and a diagram 350 of AVNS pulse trains 352 that may be delivered during the AT / AF prevention therapy according to some examples. Therapy delivery circuit 84 may start the V rate control therapy including AVNS pulse trains such as pulse train 302 in response to control circuit 80 detecting AT / AF and, at least in some examples, an unstable V rate, according to any of the examples described above in conjunction with FIGs. 5 and 6.Ref. No. A0013103 WO01

[0133] An atrial electrical signal 304 that may be sensed by sensing circuit 86 during AT / AF is shown. Sensing circuit 86 may sense AT / AF waves and generate Asense signals 306 that are passed to control circuit 80 in response to P-wave sensing threshold crossings by atrial electrical signal 304. A ventricular electrical signal 320 that may be sensed by sensing circuit 86 is shown. Sensing circuit 86 may sense an R-wave 322 and generate a Vsense signal 308 that is passed to control circuit 80 in response to an R-wave sensing threshold crossing by ventricular electrical signal 320. It is noted that Vsense signals may be produced by a sensing circuit of IMD 114 (of FIG. 2) in response to sensing far-field R-waves. It is further noted that ventricular event signals may be sensed from other cardiac signals sensed using other sensors 95 (see FIG. 4), such as cardiac acceleration signals. As such, sensing ventricular event signals and determining the ventricular rate and / or ventricular event interval variability (corresponding to RRI variability) may be performed by an IMD operating according to the techniques disclosed herein using sensed cardiac signals other than or in addition to cardiac electrical signals, e.g., cardiac signals sensed using a sensor responsive to cardiac mechanical events,.

[0134] During the V rate control therapy, AVNS pulse trains can be delivered after every nth Asense or Vsense signal to suppress conduction of atrial depolarizations to the ventricles at a fast rate, e.g., by extending the AV node refractory period. Each pulse train 302 may include multiple pulses having a pulse amplitude 312 and pulse width 314. The pulse energy may be increased by increasing the pulse amplitude 312 and / or pulse width 314. Pulse amplitude 312 may be between 3.0 and 12.0 volts, between 1 volt and 10 volts, or between 4 volts and 8 volts as non-limiting examples. The pulse width may be 0.1 to 10 milliseconds (ms) as nonlimiting examples. The pulses of each pulse train 302 are shown as monophasic pulses in FIG. 7. In other examples, the pulses in each pulse train 302 may be biphasic or multi-phasic trains of pulses having both positive-going and negative-going trains of pulses which may be discreet pulses or generally more continuous pulse waveforms. The pulses of each pulse train 302 are shown as positive polarity pulses in FIG. 7. In other examples, the pulses may be negative polarity pulses or a combination of positive and negative polarity pulses, e.g., alternating positive and negative pulses or n positive pulses followed by n negative pulses or positive and negative portion(s) of a more continuous waveform, etc. In still other examples, therapy delivery circuit 84 may control the polarity of the pulse trains delivered during AVNS therapies to alternate, e.g., one pulse train may be all first polarity pulses and the next pulse train may be all second polarity pulses, opposite the first polarity. It is recognized that anyRef. No. A0013103 WO01convention for specifying the polarity of pulses as being “positive” or “negative” in a train of discreet or continuous waveforms of the AVNS pulse train may be adopted where the polarity may be defined according to the direction of current or the voltage relative to a reference.

[0135] Each pulse within the pulse train 302 is separated from the next pulse by an interpulse interval 318. The inter-pulse interval may be 5 to 100 ms as non-limiting examples. The inter-pulse interval 318 may be selected in combination with the pulse width 314 to obtain a desired pulse frequency. The pulse frequency, i.e., the frequency of the pulses within the pulse train 302, is defined by the inverse of the pulse period defined by the sum of the pulse width 314 and the inter-pulse interval 318. The pulse frequency can be increased by decreasing the pulse width 314 and / or decreasing the inter-pulse interval 318. The pulse frequency may be 20 to 100 Hz, 30 to 80 Hz or 40 to 60 Hz in various examples. For instance, to achieve a pulse frequency of approximately 40 to 60 Hz when the pulse width is about 0.2 ms in duration, the inter-pulse interval 318 may be between approximately 16 and 25 ms. In the example of FIG. 7, each pulse train 302 is represented as a series of discreet pulses. It is to be understood, however, that in other examples an AVNS pulse train may be delivered as other types of waveforms such as a continuous, e.g., oscillating, pulse waveform which is controlled by pulse parameters such as amplitude, frequency, period and pulse train duration without necessarily having an inter-pulse interval separating discrete pulses of the pulse train as shown in FIG. 7.

[0136] The pulse train duration 316 is defined by the number of pulses in the pulse train 302 and the pulse period (pulse width 314 plus inter-pulse interval 318). The pulse train duration 316 may be 30 ms to 500 ms long or between 40 and 200 ms long as examples, with no limitation intended. The pulse train duration 316 can be adjusted by increasing or decreasing the pulse number and / or increasing or decreasing the pulse frequency. Greater suppression of AV conduction (e.g., longer refractory period of the AV node) can generally be achieved by a pulse train 302 having relatively higher pulse energy. Less suppression of the AV conduction (e.g., shorter AV node refractory period) can be achieved by relatively lower pulse energy.

[0137] The pulse train 302 may be delivered during a cardiac refractory period, e.g., during the ARP 301 or the VRP 321 (as shown), to block conduction of atrial depolarizations to the ventricles during the extended refractory period of the AV node. Each AVNS pulse train 302 may be started upon receiving an Asense signal 306 or a Vsense signal 308, which may be after a start interval 310 following the respective triggering Asense signal 306 or, as shown in FIG. 7, the triggering Vsense signal 310. The rate of the pulse trains delivered during the VRef. No. A0013103 WO01rate control therapy can be controlled to control the ventricular rate. Control circuit 80 may control therapy delivery circuit 84 to deliver an AVNS pulse train 302 after every n Asense or Vsense signals (or atrial pacing pulse or ventricular pacing pulse), where n is a positive integer of 1 or more. In some examples, control circuit 80 delivers an AVNS pulse train 302 on every Asense or Vsense signal during a detected AT / AF episode (which may be determined to be associated with an unstable V rate) but may be delivered less often in other examples.

[0138] The rate at which intrinsic AV conduction is allowed to occur during AT / AF may be controlled by selecting the ratio of pulse trains 302 to cardiac cycles, selecting the pulse amplitude 312, pulse width 314, pulse number and pulse train frequency. For example, control circuit 80 may increase the rate or energy of delivered pulse trains during V rate control therapy due to detection of an unstable rate during AT / AF by decreasing the ratio of cardiac cycles to AVNS pulse trains (delivering AVNS pulse trains on more cardiac cycles) or by increasing the pulse amplitude 312 or pulse width 314. In some examples, the pulse number (shown as 4 pulses in FIG. 7) may be increased if the pulse train duration 316 does not exceed the VRP 321.

[0139] When AV conduction is under-suppressed, by the V rate control therapy, one or more atrial depolarizations may be conducted to the ventricles during a delivered pulse train.Accordingly, when Vsense signals are received during or early after a delivered pulse train or Vsense signals are received at an RRI that is shorter than an RRI lower limit (resulting in a V rate faster than a rate threshold) and / or meet RRI variability criteria, one or more AVNS pulse parameters may be adjusted as described above.

[0140] In some examples, the start interval 310 may be too long following a Vsense signal resulting in the atrial depolarization conducting through to the ventricles before AV conduction is blocked by the pulse train 302. As such, in some examples, the start interval 310 may be shortened when Vsense signals are occurring at RRIs that are shorter than a lower limit associated with V rate threshold and / or are occurring at variable RRIs.

[0141] In diagram 350, an AVNS pulse train 352 is shown having a lower pulse amplitude 362 than the pulse amplitude 312. When AT / AF prevention therapy is delivered, one or more pulse parameters may be adjusted to deliver lower energy AVNS pulse trains than when the V rate control therapy is delivered. In some examples, the pulse amplitude 362 is reduced to a percentage of the pulse amplitude 312 when therapy delivery circuit 84 switches to AT / AF prevention therapy after delivering V rate control therapy. The pulse amplitude 362 may beRef. No. A0013103 WO01between 30% and 70% of the pulse amplitude 312 delivered during V rate control therapy. In an example, the pulse amplitude 362 is 50% of the pulse amplitude 312.

[0142] Additionally or alternatively, the pulse width of pulses in pulse train 352 may be reduced to a percentage of the pulse width 314 of the pulse train 302 delivered for V rate control therapy. Additionally or alternatively, the number of pulses per pulse train may be less in pulse train 352 than in pulse train 302. The pulse frequency of the pulse train 352 delivered during the AT / AF prevention therapy may be the same as the pulse frequency of the pulse train 302 delivered during V rate control therapy. However, the optimal frequency for effectively inhibiting the recurrence of AT / AF may be different than an optimal frequency of pulses within the AVNS pulse trains delivered to suppress AV nodal conduction and control V rate. For example, the pulse frequency of AVNS pulse trains delivered for AT / AF prevention therapy may be 20 to 40 Hz and the pulse frequency of AVNS pulse trains delivered for V rate control therapy may be 40 to 60 Hz, as examples with no limitation intended. When pulse train 352 is delivered having a lower frequency than pulse train 302, the number of pulses in the pulse train 352 may be fewer than the number of pulses in pulse train 302.

[0143] Control circuit 80 may control therapy delivery circuit 84 to deliver pulse train 352 starting at the expiration of the start interval 310 following a Vsense signal 358 received from sensing circuit 86. In other examples, during a normal sinus rhythm, control circuit 80 may control therapy delivery circuit 84 to start the pulse train 352 after a start time interval 357 following an Asense signal 358 received from sensing circuit 86. The R-wave 372 following the conducted P-wave 355 may occur at a predictable AV conduction time during normal sinus rhythm. As such, the pulse train 352 may be delivered during a cardiac refractory period, e.g., during the ARP 301 or the VRP 321(as shown), by starting the pulse train 352 upon expiration of a start time interval 357 that is started upon an Asense signal 356. As described above in conjunction with FIG. 5, the AVNS pulse trains, e.g., pulse train 352, delivered for AT / AF prevention therapy may be delivered according to therapy protocol parameters. The therapy protocol parameters may include a ratio of cardiac cycles to AVNS pulse trains and / or duty cycle on and off times and / or scheduled times of day in some examples.

[0144] FIG. 8 is a flow chart 400 of a method that may be performed by a medical device for selecting pulse parameters used for delivering the V rate control therapy and the AT / AF prevention therapy according to some examples. At block 402, control circuit 80 may determine that pulse parameter test start criteria are met. The test start criteria may be metRef. No. A0013103 WO01when VT / VF is not being detected, AT / AF is not being detected, and it is a scheduled time (e.g. time of day), for example. The method of flow chart 400 may be performed once a day, bi-daily, once a week, bi-weekly or at other scheduled intervals.

[0145] At block 404, control circuit 80 determines baseline V rate data when no AVNS therapy is being delivered. Generally, no cardiac pacing is being delivered so that the intrinsic V rate can be assessed. Control circuit 80 may determine the V rate, e.g., by determining a representative RRI such as a mean or median RRI determined from a specified number of Vsense signals (e.g., 8 to 100 V sense signals) received from sensing circuit 86. In some examples, control circuit 80 may determine a variability metric from the RRIs, e.g., any of the variability metrics described above. The baseline V rate data may be stored in memory 82.

[0146] At block 406, control circuit 80 controls therapy delivery circuit 84 to deliver a test AVNS pulse train beginning at a first set of test pulse parameters. For the sake of example, the first test AVNS pulse train may be delivered starting at a relatively high pulse amplitude, e.g., 12 volts, 10 volts or 8 volts, with a pulse width, pulse number and pulse frequency that is expected to result in an AVNS pulse train that is effective in suppressing AV nodal conduction and decrease the V rate. A nominal pulse width, e.g., 1 ms, nominal frequency of 50 Hz and a pulse number selected to result in a total pulse train duration that does not exceed the ARP or the VRP may be used with the starting pulse amplitude. For the sake of the pulse parameter testing, the AVNS pulse trains delivered according to the set of test pulse parameters may be delivered at a 1 : 1 ratio with Vsense signals received from sensing circuit 86.

[0147] At block 408, control circuit 80 redetermines the V rate data, e.g., a median RRI, as the response V rate data to determine if the test AVNS pulse trains have caused a V rate response. The V rate data may be redetermined after the test AVNS pulse trains have been delivered for a specified time interval or number of cardiac cycles, e.g., after at least 3, 5, 10 or 20 AVNS pulse trains or after at least 2, 5, 10, 30, or 60 seconds. At block 410, the V rate data determined at block 408 may be compared to the baseline V rate data. For example, a response median RRI determined at block 408 may be compared to the baseline median RRI. If the response median RRI is greater than the baseline median RRI, control circuit 80 can determine that rate control is effective at block 410. Control circuit 80 may decrement the pulse parameter undergoing testing at block 412 and return to block 406. When the pulse parameter being tested is pulse amplitude, which is started at a relatively high value, rate control is expected to be determined to be effective at block 410 for the first test AVNS pulseRef. No. A0013103 WO01trains. Control circuit 80 may decrement the pulse amplitude by a specified step, e.g., 0.25, 0.5, 0.75 or 1.0 volt at block 412.

[0148] The process of decrementing the pulse parameter (block 412), in this case pulse amplitude, delivering test AVNS pulse trains (block 406) and redetermining the response V rate data (block 408) may be repeated for determining when the AVNS pulse train is no longer effective in V rate control at block 410, as evidenced by response V rate data that matches the baseline V rate data. For example, the response median RRI determined at block 408 may be approximately equal to the baseline RRI, e.g., within a difference threshold or within about 10% or within 20% of the baseline RRI).

[0149] When control circuit 80 determines that rate control is not effective at block 410, control circuit 80 may, at block 414, restore the previous pulse parameter setting that was found to be effective. For the sake of illustration, the response median RRI determined during delivery of AVNS pulse trains having a pulse amplitude of 5.0 volts may be longer than (e.g., at least 20% longer than) the baseline median RRI indicating effective V rate control. If AVNS pulse trains having a pulse amplitude of 4.5 volts results in a response median RRI that substantially matches the baseline median RRI, control circuit 80 can determine that 4.5 volts is not effective for V rate control at block 410. At block 414, control circuit 80 may restore the 5.0 volts pulse amplitude, the last pulse amplitude setting found to be effective.

[0150] At block 416, control circuit 80 may determine if another pulse parameter remains to be tested. In some examples, the pulse width, the pulse number and / or the pulse frequency may be tested, e.g., within the bounds of the resulting total pulse train duration being no greater than the ARP or VRP. For the sake of example, after determining the lowest pulse amplitude having a rate control effect, control circuit 80 may test one or more pulse widths. In other examples different pulse waveforms and / or a modification to the amplitude(s) and / or widths of successive pulses within the AVNS pulse train delivered as a continuous waveform or a series of discreet pulses may be individually applied.

[0151] Control circuit 80 returns to block 406 to deliver test AVNS pulse trains according to the first test setting of a second pulse parameter, in this case pulse width. In some examples, control circuit 80 may return to block 404 to redetermine the baseline V rate data. For instance, if the intrinsic atrial rate has changed, if the test of flow chart 400 has been paused due to an AT / AF detection, VT / VF detection, user intervention or other time delay in the testing has occurred, the V rate data may be redetermined at block 404. The first test pulse width may be equal to the nominal pulse width used during pulse amplitude testing alreadyRef. No. A0013103 WO01performed or may be decremented one step, e.g., by decreasing the pulse width by 0.1 to 0.2 ms.

[0152] Control circuit 80 determines the response V rate data at block 408 as generally described above and compares the response V rate data, e.g., a response median RRI, to the baseline V rate data, e.g., a baseline median RRI, at block 410. If the rate control is effective, for example as evidenced by a response median RRI that is greater than (e.g., at least a 20% longer than) the baseline median RRI, control circuit 80 may decrement the pulse width at block 412. The pulse width may be decremented by a 0.1 to 0.2 ms step as examples. The process of decrementing the pulse width, determining response V rate data and comparing the response V rate data to the baseline V rate data may continue until a pulse width is reached at which the rate control is deemed ineffective by control circuit 80, e.g., as evidenced by a response median RRI that matches the baseline median RRI.

[0153] At block 414, control circuit 80 restores the last pulse width that was determined to result in effective V rate control. If no additional pulse parameters are to be tested (“no” branch of block 416), control circuit 80 may store the results of the test at block 418. Control circuit 80 may select pulse parameters for controlling AVNS therapy delivery based on the test results. The selected pulse parameters may be stored in memory 82. In some examples, control circuit 80 may test pulse number in addition to or alternatively to testing pulse amplitude and / or pulse width. For example, control circuit 80 may start a pulse number test by controlling therapy delivery circuit 84 to deliver the maximum number of pulses for a selected pulse frequency that results in a pulse train duration that is not longer than the ARP or VRP. The pulse number may be decreased progressively until the rate control is determined to be ineffective. The lowest pulse number determined to be effective, for a given pulse amplitude and pulse width, may be stored in memory 82. In this case, the lowest pulse amplitude resulting in effective V rate control when the nominal pulse width, pulse number and pulse frequency are used may first be determined by the process of flow chart 400. The lowest pulse width resulting in effective V rate control when the lowest pulse amplitude and nominal pulse number and pulse frequency are used may be determined. Next the lowest pulse number resulting in effective V rate control when the lowest pulse amplitude and lowest pulse width may be determined. In some examples the lowest frequency may be determined when the lowest pulse amplitude, pulse width and pulse number are used. The pulse frequency, however, may be set to a nominal frequency, e.g., 50 Hz, and not adjusted in some examples.Ref. No. A0013103 WO01

[0154] The pulse parameters stored at block 418 may be the lowest pulse amplitude, lowest pulse width, lowest pulse number and / or lowest pulse frequency determined to be effective in slowing the V rate from a baseline V rate. Additionally or alternatively, control circuit 80 may select the pulse parameters for delivering the V rate control therapy based on the test results. In some examples, the pulse parameters for V rate control therapy delivery are selected as the lowest pulse parameter(s) found to result in effective V rate control during the pulse parameter testing. In other examples, the pulse parameters for V rate control therapy delivery may be selected as being a safety margin or offset greater than the lowest pulse parameter value(s) found to be effective.

[0155] For example, if the lowest effective pulse amplitude is 5.0 volts, control circuit 80 may select 5.25, 5.5, or 6.0 volts as the V rate control therapy pulse amplitude (using a 0.25, 0.5 or 1.0 volt safety margin as examples). The lowest effective pulse width may be selected as the pulse width for V rate control therapy delivery. In other examples, a pulse width that is a safety margin greater than the lowest pulse width may be selected. For example, if 0.1 ms pulse width is found to be the lowest effective pulse width, control circuit 80 may select a pulse width of 0.15 or 0.2 for delivering V rate control therapy. When pulse number and / or pulse frequency are tested, the lowest effective value of the respective pulse parameter may be selected for delivering V rate control therapy or an increment greater than the lowest effective value may be selected (which may be within a maximum pulse train duration limit that is not greater than the ARP or VRP).

[0156] In addition to storing the V rate control therapy pulse parameters at block 418, control circuit 80 may store AT / AF prevention therapy pulse parameters that are based on the V rate control therapy pulse parameters. For example, the pulse amplitude, pulse width, pulse frequency and / or pulse number used for AT / AF prevention therapy may be a specified percentage of the respective V rate control therapy pulse parameter. After storing the pulse parameters for the V rate control therapy and the AT / AF prevention therapy, control circuit 80 may resume delivering AT / AF prevention therapy according to the stored parameters when therapy start criteria are met, e.g., according to the methods described above in conjunction with FIGs. 5 or 6.

[0157] Further disclosed herein is the subject matter of the following examples:

[0158] Example 1. A medical device system including sensing circuitry configured to sense one or more cardiac signals and control circuitry configured to detect an atrial tachyarrhythmia from the one or more cardiac signals. The medical device system includes aRef. No. A0013103 WO01therapy delivery circuitry configured to deliver a first therapy by delivering first atrioventricular nodal stimulation (AVNS) pulse trains generated according to first pulse parameters and, in response to the control circuitry detecting the atrial tachyarrhythmia, terminate the first therapy. The therapy delivery circuitry may be further configured to, in response to at least the atrial tachyarrhythmia being detected by the control circuitry, deliver a second therapy by delivering second AVNS pulse trains generated according to second pulse parameters different than the first pulse parameters.

[0159] Example 2. The medical device system of example 1 wherein the control circuitry is further configured to detect an unstable ventricular rate from the one or more cardiac signals sensed during the detected atrial tachyarrhythmia. The therapy delivery circuit can be further configured to start the second therapy in response to the control circuitry detecting the unstable ventricular rate.

[0160] Example 3. The medical device system of example 2 wherein the control circuitry is further configured to detect the unstable ventricular rate by determining at least one of a ventricular rate metric determined from the one or more cardiac signals that is greater than a rate threshold or a ventricular event interval variability metric determined from the one or more cardiac signals that is greater than a variability threshold.

[0161] Example 4. The medical device system of any one of examples 1 — 3 wherein the control circuitry is further configured to detect an unstable ventricular rate from the one or more cardiac signals after the therapy delivery circuit starts the second therapy. The therapy delivery circuit being further configured to adjust at least one of the second pulse parameters in response to the control circuitry detecting the unstable ventricular rate after the second therapy is started.

[0162] Example 5. The medical device system of any one of examples 1 — 4 wherein the control circuitry is further configured to detect termination of the atrial tachyarrhythmia from the one or more cardiac signals. The therapy delivery circuit may be further configured to terminate the second therapy in response to the control circuitry detecting termination of the atrial tachyarrhythmia and restart the first therapy after terminating the second therapy.

[0163] Example 6. The medical device system of example 5 wherein the control circuit is further configured to start a therapy delay time interval upon detecting termination of the atrial tachyarrhythmia. The therapy delivery circuit may be further configured to restart the first therapy after an expiration of the therapy delay time interval.Ref. No. A0013103 WO01

[0164] Example 7. The medical device system of any one of examples 1 — 6 wherein the therapy delivery circuit is further configured to deliver the first therapy according to the first pulse parameters by delivering the first AVNS pulse trains according to one or more of a first start time during a cardiac cycle, a first pulse amplitude, a first pulse width, a first pulse number and a first pulse frequency and deliver the second therapy according to the second pulse parameters by delivering the second AVNS pulse trains according to one or more of a second start time during a cardiac cycle, a second pulse amplitude, a second pulse width, a second pulse number or a second pulse frequency different than a respective one of the first start time during a cardiac cycle, the first pulse amplitude, the first pulse width, the first pulse number or the first pulse frequency.

[0165] Example 8. The medical device system of example 7 wherein the therapy delivery circuit is further configured to deliver the second AVNS pulse trains according to the second pulse parameters comprising a value of at least one of the second pulse amplitude, the second pulse width, the second pulse number or the second pulse frequency that is greater than a corresponding value of a respective one of the first pulse amplitude, the first pulse width, the first pulse number or the first pulse frequency.

[0166] Example 9. The medical device system of any one of examples 1 — 8 further comprising a memory configured to store one or more atrial tachyarrhythmia episode metrics. The control circuitry may be further configured to determine an updated value of the atrial tachyarrhythmia episode metric in response to detecting the atrial tachyarrhythmia. The control circuit may compare the updated atrial tachyarrhythmia episode metric to at least one previous atrial tachyarrhythmia episode metric stored in the memory and determine that atrial tachyarrhythmia inhibition criteria are not met based on the comparison. The control circuit may adjust at least one of the first pulse parameters in response to determining that the atrial tachyarrhythmia inhibition criteria are not met.

[0167] Example 10. The medical device system of any one of examples 1 — 9 wherein the control circuitry is further configured to determine from the one or more cardiac signals that an incidence of atrial tachyarrhythmia is decreased and adjust at least one of the first pulse parameters in response to determining that the incidence of atrial tachyarrhythmia is decreased.

[0168] Example 11. The medical device system of any one of examples 1 — 10 wherein the therapy delivery circuit is further configured to deliver the first therapy according to one or more protocol parameters comprising at least one of: a ratio of cardiac cycles to AVNS pulseRef. No. A0013103 WO01trains; a duty cycle on time; a duty cycle off time; a scheduled start time of day; a duty cycle; a maximum therapy delivery limit or a minimum therapy delivery limit.

[0169] Example 12. The medical device system of example 11 wherein the control circuit is further configured to determine from the one or more cardiac signals if atrial tachyarrhythmia inhibition criteria are met. In response to determining that the atrial tachyarrhythmia inhibition criteria are met, the control circuit may adjust at least one of the protocol parameters to decrease a number of first AVNS pulse trains delivered by the therapy delivery circuit. In response to determining that the atrial tachyarrhythmia inhibition criteria are not met, the control circuit may adjust at least one of the protocol parameters to increase the number of first AVNS pulse trains delivered by the therapy delivery circuit.

[0170] Example 13. The medical device system of any one of examples 1 — 12 wherein the control circuitry is further configured to determine a baseline ventricular rate metric from the one or more cardiac signals sensed when the second therapy is not being delivered. The control circuit may select a value of at least one of the second pulse parameters by controlling the therapy delivery circuit to deliver test AVNS pulse trains according to each of a plurality of values for the at least one of the second pulse parameters, determining a response ventricular rate metric from the one or more cardiac signals during delivery of test AVNS pulse trains delivered according to each of the plurality of values of the at least one of the second pulse parameters, determining a lowest value of the plurality of values corresponding to a response ventricular rate metric that is less than the baseline ventricular rate metric and selecting the value of the at least one of the second pulse parameters as being equal to or greater than the lowest value. The therapy delivery circuit is further configured to deliver the second therapy according to the selected value of the at least one of the second pulse parameters.

[0171] Example 14. The medical device system of example 13 wherein the control circuit is further configured to select at least one of the first pulse parameters based on the selected value of the second pulse parameter.

[0172] Example 15. The medical device system of any one of examples 1 — 14 wherein the control circuit is further configured to control the therapy delivery circuit to deliver the first AVNS pulse trains during cardiac refractory periods and deliver the second AVNS pulse trains during cardiac refractory periods.

[0173] Example 16. The medical device system of example 15 wherein the control circuit is further configured to determine a cardiac refractory period duration from the one or moreRef. No. A0013103 WO01cardiac signals and adjust at least one of the first pulse parameters or the second pulse parameters based on the cardiac refractory period duration.

[0174] Example 17. The medical device system of any one of examples 1 — 16 wherein the therapy delivery circuitry is further configured to deliver the first AVNS pulse trains during atrial refractory periods and deliver the second AVNS pulse trains during ventricular refractory periods.

[0175] Example 18. The medical device system of any one of examples 1 — 17 wherein the control circuit is further configured to deliver the first AVNS pulse trains during ventricular refractory periods and deliver the second AVNS pulse trains during atrial refractory periods.

[0176] Example 19. The medical device system of any one of examples 1 — 18 wherein the control circuitry is further configured to detect an atrial tachyarrhythmia during delivery of the first therapy by the therapy delivery circuitry and determine a stable ventricular rate from the one or more sensed cardiac signals sensed during the detected atrial tachyarrhythmia detected during delivery of the first therapy. The therapy delivery circuitry may be further configured to continue delivering the first therapy without starting the second therapy in response to the control circuit determining the stable ventricular rate.

[0177] Example 20. The medical device system of any one of examples 1 — 19 further including a memory storing a therapy delivery limit. The control circuit may be further configured to determine that one of the first therapy or the second therapy delivered during a specified time period is not meeting the therapy delivery limit and, in response to determining that the therapy delivery limit is not being met, adjust at least one therapy control parameter from among a plurality of protocol parameters, the first pulse parameters or the second pulse parameters. The therapy delivery circuit may be further configured to deliver the one of the first therapy or the second therapy not meeting the therapy delivery limit according to the at least one adjusted therapy control parameter.

[0178] Example 21. A method comprising sensing one or more cardiac signals and delivering a first therapy by delivering first atrioventricular nodal stimulation (AVNS) pulse trains generated according to first pulse parameters. The method may include detecting an atrial tachyarrhythmia from the one or more cardiac signals and, in response to detecting the atrial tachyarrhythmia, terminating the first therapy. The method may further include, in response to at least the atrial tachyarrhythmia being detected, delivering a second therapy by delivering second AVNS pulse trains generated according to second pulse parameters where at the second pulse parameters are different than the first pulse parameters.Ref. No. A0013103 WO01

[0179] Example 22. The method of example 21 further including detecting an unstable ventricular rate from the one or more cardiac signals sensed during the detected atrial tachyarrhythmia and starting the second therapy in response to the control circuitry detecting the unstable ventricular rate.

[0180] Example 23. The method of example 22 wherein detecting the unstable ventricular rate may include determining at least one of a ventricular rate metric determined from the one or more cardiac signals that is greater than a rate threshold or a ventricular event interval variability metric determined from the one or more cardiac signals that is greater than a variability threshold.

[0181] Example 24. The method of any one of examples 21 — 23 further including detecting an unstable ventricular rate from the one or more cardiac signals after starting the second therapy; and adjusting at least one of the second pulse parameters in response to detecting the unstable ventricular rate after the second therapy is started.

[0182] Example 25. The method of any one of examples 21 — 24 further including detecting termination of the atrial tachyarrhythmia from the one or more cardiac signals, terminating the second therapy in response to detecting termination of the atrial tachyarrhythmia and restarting the first therapy after terminating the second therapy.

[0183] Example 26. The method of example 25 further comprising starting a therapy delay time interval upon detecting termination of the atrial tachyarrhythmia and restarting the first therapy after an expiration of the therapy delay time interval.

[0184] Example 27. The method of any one of examples 21 — 26 further including delivering the first therapy according to the first pulse parameters by delivering the first AVNS pulse trains having a first start time during a cardiac cycle, a first pulse amplitude, a first pulse width, a first pulse number and a first pulse frequency and delivering the second therapy according to the second pulse parameters by delivering the second AVNS pulse trains having at least one of a second pulse amplitude, a second pulse width, a second pulse number or a second pulse frequency different than a respective one of the first pulse amplitude, the first pulse width, the first pulse number and the first pulse frequency.

[0185] Example 28. The method of example 27 further including delivering the second AVNS pulse trains according to the second pulse parameters comprising a value of at least one of the second pulse amplitude, the second pulse width, the second pulse number or the second pulse frequency that is greater than a corresponding value of a respective one of the first pulse amplitude, the first pulse width, the first pulse number or the first pulse frequency.Ref. No. A0013103 WO01

[0186] Example 29. The method of any one of examples 21 — 28 further including storing an atrial tachyarrhythmia episode metric in a memory, determining an updated atrial tachyarrhythmia episode metric in response to detecting the atrial tachyarrhythmia and comparing the updated atrial tachyarrhythmia episode metric to at least one previous atrial tachyarrhythmia episode metric stored in the memory. The method may include determining that atrial tachyarrhythmia inhibition criteria are not met based on the comparing and adjusting at least one of the first pulse parameters in response to determining that the atrial tachyarrhythmia inhibition criteria are not met.

[0187] Example 30. The method of any one of examples 21 — 29 further including determining from the one or more cardiac signals that an incidence of atrial tachyarrhythmia is decreased and adjusting at least one of the first pulse parameters in response to determining that the incidence of atrial tachyarrhythmia is decreased.

[0188] Example 31. The method of any one of examples 21 — 30 further including delivering the first therapy according to one or more protocol parameters comprising at least one of: a ratio of cardiac cycles to AVNS pulse trains, a duty cycle on time, a duty cycle off time, a scheduled start time of day, a maximum therapy delivery limit of a duty cycle or a minimum therapy delivery limit.

[0189] Example 32. The method of example 31 further including determining from the one or more cardiac signals if atrial tachyarrhythmia inhibition criteria are met. In response to determining that the atrial tachyarrhythmia inhibition criteria are met, the method may include adjusting at least one of the protocol parameters to decrease a number of first AVNS pulse trains delivered over a specified time interval. In response to determining that the atrial tachyarrhythmia inhibition criteria are not met, the method may include adjusting at least one of the protocol parameters to increase the number of first AVNS pulse trains delivered over the specified time interval.

[0190] Example 33. The method of any one of examples 21 — 32 further including determining a baseline ventricular rate metric from the one or more cardiac signals sensed when the second therapy is not being delivered and selecting a value of at least one of the second pulse parameters by delivering test AVNS pulse trains according to each of a plurality of values for the at least one of the second pulse parameters. The method may further include determining a response ventricular rate metric from the one or more cardiac signals sensed during delivery of test AVNS pulse trains delivered according to each of the plurality of values of the at least one of the second pulse parameters, determining a lowest value of theRef. No. A0013103 WO01plurality of values corresponding to a response ventricular rate metric that is less than the baseline ventricular rate metric, and selecting the value of the at least one of the second pulse parameters as being equal to or greater than the lowest value. The method may further include delivering the second therapy according to the selected value of the at least one of the second pulse parameters.

[0191] Example 34. The method of example 33 further including selecting at least one of the first pulse parameters based on the selected value of the second pulse parameter.

[0192] Example 35. The method of any one of examples 21 — 34 further including delivering the first AVNS pulse trains during a cardiac refractory period and delivering the second AVNS pulse trains during a cardiac refractory period.

[0193] Example 36. The method of example 35 further including determining a cardiac refractory period duration from the one or more cardiac signals and adjusting at least one of the first pulse parameters or the second pulse parameters based on the cardiac refractory period duration.

[0194] Example 37. The method of any one of examples 21 — 36 further including delivering the first AVNS pulse trains during atrial refractory periods and delivering the second AVNS pulse trains during ventricular refractory periods.

[0195] Example 38. The method of any one of examples 21 — 37 further including delivering the first AVNS pulse trains during ventricular refractory periods and delivering the second AVNS pulse trains during atrial refractory periods.

[0196] Example 39. The method of any one of examples 21 — 38 further including detecting an atrial tachyarrhythmia during delivery of the first therapy, determining a stable ventricular rate from the one or more sensed cardiac signals sensed during the atrial tachyarrhythmia detected during delivery of the first therapy and continuing delivering the first therapy without starting the second therapy in response to determining the stable ventricular rate.

[0197] Example 40. The method of any one of examples 21 — 39 further comprising storing a therapy delivery limit, determining that one of the first therapy or the second therapy delivered during a specified time period is not meeting the therapy delivery limit and, in response to determining that the therapy delivery limit is not being met, adjusting at least one therapy control parameter from among a plurality of protocol parameters, the first pulse parameters or the second pulse parameters. The method may further include delivering the one of the first therapy or the second therapy not meeting the therapy delivery limit according to the at least one adjusted therapy control parameter.Ref. No. A0013103 WO01

[0198] Example 41. A non-transitory computer readable medium storing a set of instructions which, when executed by control circuitry of a medical device system, cause the medical device system to sense one or more cardiac signals and deliver a first therapy by delivering first atrioventricular nodal stimulation (AVNS) pulse trains generated according to first pulse parameters. The instructions may further cause the medical device system to detect an atrial tachyarrhythmia from the one or more cardiac signals and, in response to detecting the atrial tachyarrhythmia, terminate the first therapy. In response to at least the atrial tachyarrhythmia being detected, the instruction may further cause the medical device system to deliver a second therapy by delivering second AVNS pulse trains generated according to second pulse parameters, the second pulse parameters different than the first pulse parameters.

[0199] It should be understood that, depending on the example, certain acts or events of any of the methods described herein can be performed in a different sequence, may be added, merged, or left out altogether (e.g., not all described acts or events are necessary for the practice of the method). Moreover, in certain examples, acts or events may be performed concurrently, e.g., through multi -threaded processing, interrupt processing, or multiple processors, rather than sequentially. In addition, while certain aspects of this disclosure are described as being performed by a single circuit or unit for purposes of clarity, it should be understood that the techniques of this disclosure may be performed by a combination of units or circuits associated with, for example, a medical device.

[0200] In one or more examples, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored as one or more instructions or code on a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media (e.g., RAM, ROM, EEPROM, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer).

[0201] Instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable logic arrays (FPLAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor,” as used herein may refer to any of the foregoing structure or any other structure suitable for implementation of the techniquesRef. No. A0013103 WOOldescribed herein. Also, the techniques could be fully implemented in one or more circuits or logic elements.

[0202] Thus, a medical device has been presented in the foregoing description with reference to specific examples. It is to be understood that various aspects disclosed herein may be combined in different combinations than the specific combinations presented in the accompanying drawings. It is appreciated that various modifications to the referenced examples may be made without departing from the scope of the disclosure and the following claims.

Claims

Ref. No. A0013103 WOOlWHAT IS CLAIMED IS:

1. A medical device system comprising:sensing circuitry configured to sense one or more cardiac signals;control circuitry configured to:detect an atrial tachyarrhythmia from the one or more cardiac signals; and therapy delivery circuitry configured to:deliver a first therapy by delivering first atrioventricular nodal stimulation (AVNS) pulse trains generated according to first pulse parameters;in response to the control circuitry detecting the atrial tachyarrhythmia, terminate the first therapy; andin response to at least the atrial tachyarrhythmia being detected by the control circuitry, deliver a second therapy by delivering second AVNS pulse trains generated according to second pulse parameters different than the first pulse parameters.

2. The medical device system of claim 1 wherein:the control circuitry is further configured to detect an unstable ventricular rate from the one or more cardiac signals sensed during the detected atrial tachyarrhythmia; and the therapy delivery circuit being further configured to start the second therapy in response to the control circuitry detecting the unstable ventricular rate.

3. The medical device system of claim 2 wherein the control circuitry is further configured to detect the unstable ventricular rate by determining at least one of:a ventricular rate metric determined from the one or more cardiac signals that is greater than a rate threshold; ora ventricular event interval variability metric determined from the one or more cardiac signals that is greater than a variability threshold.

4. The medical device system of any one of claims 1 — 3 wherein:the control circuitry is further configured to detect an unstable ventricular rate from the one or more cardiac signals after the therapy delivery circuit starts the second therapy; andRef. No. A0013103 WOOlthe therapy delivery circuit is further configured to adjust at least one of the second pulse parameters in response to the control circuitry detecting the unstable ventricular rate after the second therapy is started.

5. The medical device system of any one of claims 1 — 4 wherein:the control circuitry is further configured to detect termination of the atrial tachyarrhythmia from the one or more cardiac signals; andthe therapy delivery circuit is further configured to:terminate the second therapy in response to the control circuitry detecting termination of the atrial tachyarrhythmia; andrestart the first therapy after terminating the second therapy.

6. The medical device system of claim 5 wherein:the control circuit is further configured to start a therapy delay time interval upon detecting termination of the atrial tachyarrhythmia; andthe therapy delivery circuit is further configured to restart the first therapy after an expiration of the therapy delay time interval.

7. The medical device system of any one of claims 1 — 6 wherein the therapy delivery circuit is further configured to:deliver the first therapy according to the first pulse parameters by delivering the first AVNS pulse trains according to one or more of a first start time during a cardiac cycle, a first pulse amplitude, a first pulse width, a first pulse number and a first pulse frequency; and deliver the second therapy according to the second pulse parameters by delivering the second AVNS pulse trains according to one or more of a second start time during a cardiac cycle, a second pulse amplitude, a second pulse width, a second pulse number or a second pulse frequency different than a respective one of the first start time during a cardiac cycle, the first pulse amplitude, the first pulse width, the first pulse number or the first pulse frequency.

8. The medical device system of claim 7 wherein the therapy delivery circuit is further configured to deliver the second AVNS pulse trains according to the second pulse parameters comprising a value of at least one of the second pulse amplitude, the second pulse width, theRef. No. A0013103 WOOlsecond pulse number or the second pulse frequency that is greater than a corresponding value of a respective one of the first pulse amplitude, the first pulse width, the first pulse number or the first pulse frequency.

9. The medical device system of any one of claims 1 — 8 further comprising a memory configured to store one or more atrial tachyarrhythmia episode metrics and wherein:the control circuitry is further configured to:determine an updated value of an atrial tachyarrhythmia episode metric in response to detecting the atrial tachyarrhythmia;compare the updated atrial tachyarrhythmia episode metric to at least one previous atrial tachyarrhythmia episode metric stored in the memory;determine that atrial tachyarrhythmia inhibition criteria are not met based on the comparison; andadjust at least one of the first pulse parameters in response to determining that the atrial tachyarrhythmia inhibition criteria are not met.

10. The medical device system of any one of claims 1 — 9 wherein the therapy delivery circuit is further configured to deliver the first therapy according to one or more protocol parameters comprising at least one of:a ratio of cardiac cycles to AVNS pulse trains;a duty cycle on time;a duty cycle off time; anda scheduled start time of day;a maximum therapy delivery limit; ora minimum therapy delivery limit.

11. The medical device system of claim 10 wherein the control circuit is further configured to:determine from the one or more cardiac signals if atrial tachyarrhythmia inhibition criteria are met; andone of:Ref. No. A0013103 WOOlin response to determining that the atrial tachyarrhythmia inhibition criteria are met, adjust at least one of the protocol parameters to decrease a number of first AVNS pulse trains delivered by the therapy delivery circuit; orin response to determining that the atrial tachyarrhythmia inhibition criteria are not met, adjust at least one of the protocol parameters to increase the number of first AVNS pulse trains delivered by the therapy delivery circuit.

12. The medical device system of any one of claims 1 — 11 wherein the control circuitry is further configured to:determine a baseline ventricular rate metric from the one or more cardiac signals sensed when the second therapy is not being delivered; andselect a value of at least one of the second pulse parameters by:controlling the therapy delivery circuit to deliver test AVNS pulse trains according to each of a plurality of values for the at least one of the second pulse parameters;determining a response ventricular rate metric from the one or more cardiac signals during delivery of test AVNS pulse trains delivered according to each of the plurality of values of the at least one of the second pulse parameters;determining a lowest value of the plurality of values corresponding to a response ventricular rate metric that is less than the baseline ventricular rate metric; andselecting the value of the at least one of the second pulse parameters as being equal to or greater than the lowest value; andthe therapy delivery circuit is further configured to deliver the second therapy according to the selected value of the at least one of the second pulse parameters.

13. The medical device system of claim 12 wherein the control circuit is further configured to select at least one of the first pulse parameters based on the selected value of the second pulse parameter.

14. The medical device system of any one of claims 1 — 13 wherein:the control circuitry is further configured to:Ref. No. A0013103 WOOldetect an atrial tachyarrhythmia during delivery of the first therapy by the therapy delivery circuitry; anddetermine a stable ventricular rate from the one or more sensed cardiac signals sensed during the detected atrial tachyarrhythmia detected during delivery of the first therapy; andthe therapy delivery circuitry is further configured to continue delivering the first therapy without starting the second therapy in response to the control circuit determining the stable ventricular rate.

15. The medical device system of any one of claims 1 — 14 further comprising:a memory storing a therapy delivery limit; andwherein the control circuit is further configured to:determine that one of the first therapy or the second therapy delivered during a specified time period is not meeting the therapy delivery limit; andin response to determining that the therapy delivery limit is not being met, adjust at least one therapy control parameter from among a plurality of protocol parameters, the first pulse parameters or the second pulse parameters; andthe therapy delivery circuit is further configured to deliver the one of the first therapy or the second therapy not meeting the therapy delivery limit according to the at least one adjusted therapy control parameter.

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