Medical device and method for delivering backup cardiac pacing

Abstract

A medical device system is configured to sense ventricular event signals attendant to ventricular depolarizations, deliver an atrial pacing pulse and start a backup atrial pacing interval to schedule a backup atrial pacing pulse in response to delivering the atrial pacing pulse. The medical device system may be configured to determine if a ventricular event signal is sensed during the backup atrial pacing interval. In response to a ventricular event signal being sensed during the backup atrial pacing interval, the medical device system may determine an atrial capture result of the delivered atrial pacing pulse based on at least a time of the sensed ventricular event signal and cancel the scheduled backup atrial pacing pulse.

Description

MEDICAL DEVICE AND METHOD FOR DELIVERING BACKUP CARDIACPACING

[0001] This application claims the benefit of U.S. Provisional Patent Application Serial No. 63 / 613,009, filed December 20, 2023, the entire content of which is incorporated herein by reference.TECHNICAL FIELD

[0002] This disclosure relates to a medical device and method for delivering backup cardiac pacing pulses.BACKGROUND

[0003] 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 atrial depolarization 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 of the ventricular septum and thereafter to the bundle branches and the Purkinje muscle fibers of the right and left ventricles, sometimes referred to as the “His- Purkinje system.”

[0004] Patients with a conduction system abnormality, e.g., SA node dysfunction or poor AV node conduction, bundle branch block, or other conduction abnormalities, may receive a pacemaker to restore a more normal heart rhythm. A single chamber pacemaker coupled to a transvenous lead carrying electrodes positioned in the right atrium may provide atrial pacing to treat a patient having SA node dysfunction. When the AV node is functioning normally, single chamber atrial pacing may sufficiently correct the heart rhythm. The pacing-evoked atrial depolarizations may be conducted normally to the ventricles via the AV node and the His-Purkinje system maintaining normal AV synchrony. Some patients, however, may experience conduction abnormalities of the AV node, e.g., partial or complete AV block. AV block may be intermittent and may evolve over time. In the presence of high-degree AV block, atrial depolarizations may not be conducted to the ventricles on every atrial cycle or may be conducted but at a prolonged AV conductiontime resulting in poor AV synchrony in the native heart rhythm. In this case, the patient may benefit from a single chamber ventricular pacemaker or a dual chamber pacemaker.

[0005] A dual chamber pacemaker may be implanted in some patients to sense atrial and ventricular electrical signals attendant to atrial and ventricular depolarizations and pace both the atrial and ventricular chambers as needed to promote and maintain AV synchrony. The dual chamber pacemaker may be coupled to a transvenous atrial lead and a transvenous ventricular lead, for placing electrodes for sensing and pacing in both the atrial and ventricular chambers. The pacemaker itself can be implanted in a subcutaneous pocket with the transvenous leads tunneled to the subcutaneous pocket.

[0006] Intracardiac pacemakers have been introduced or proposed for implantation entirely within a patient’s heart eliminating the need for transvenous leads. For example, an intracardiac pacemaker may provide sensing and pacing from within a heart chamber of a patient having a conduction abnormality to promote a more normal heart rhythm.SUMMARY

[0007] The techniques of this disclosure generally relate to a medical device configured to sense cardiac event signals attendant to myocardial depolarizations, e.g., P-waves and R- waves, deliver cardiac pacing pulses and, in some instances, deliver a backup cardiac pacing pulse to promote a regular heart rhythm. The medical device may receive a cardiac electrical signal from electrodes implanted in or on a heart chamber and sense the cardiac event signals from the cardiac electrical signal. The medical device may be a pacemaker configured to sense P-waves attendant to atrial depolarizations and R-waves attendant to ventricular depolarizations. The medical device may be a dual chamber pacemaker configured to deliver both atrial pacing pulses and ventricular pacing pulses. The timing of generated cardiac pacing pulses may be controlled by the medical device based on the sensed cardiac event signals.

[0008] A medical device operating according to the techniques disclosed herein may be configured to deliver atrial pacing pulses for verifying atrial capture by the atrial pacing pulses. The medical device may deliver an atrial pacing pulse and schedule a backup atrial pacing pulse by starting a backup atrial pacing interval. The medical device may determine an atrial pacing capture result based on if and when a ventricular R-wave is sensed during the backup atrial pacing interval. The medical device may apply a captureverification window during the backup atrial pacing interval. The capture verification window may have an end time that occurs earlier after the delivered atrial pacing pulse than an expiration time of the backup atrial pacing interval.

[0009] In one example, the disclosure provides a medical device including sensing circuitry configured to sense ventricular event signals attendant to ventricular depolarizations, pulse generating circuitry configured to generate pacing pulses and control circuitry configured to control the pulse generating circuitry to deliver an atrial pacing pulse. In response to the atrial pacing pulse being generated, start a backup atrial pacing interval to schedule a backup atrial pacing pulse, the backup atrial pacing interval having an expiration time. The control circuitry may be further configured to determine if the sensing circuitry senses a ventricular event signal during the backup atrial pacing interval and, in response to a ventricular event signal being sensed by the sensing circuitry during the backup atrial pacing interval, determine an atrial capture result of the delivered atrial pacing pulse based on at least a time of the sensed ventricular event signal and cancel the scheduled backup atrial pacing pulse. The control circuitry may be further configured to, in response to a ventricular event signal not being sensed by the sensing circuitry during the backup atrial pacing interval, determine the atrial capture result of the delivered atrial pacing pulse as loss of capture and control the pulse generating circuitry to deliver the scheduled backup atrial pacing pulse at the expiration time.

[0010] In another example, the disclosure provides a method including sensing ventricular event signals attendant to ventricular depolarizations, delivering an atrial pacing pulse, and starting a backup atrial pacing interval to schedule a backup atrial pacing pulse in response to delivering the atrial pacing pulse, the backup atrial pacing interval having an expiration time. The method may further include determining if a ventricular event signal is sensed during the backup atrial pacing interval. The method may include, in response to a ventricular event signal being sensed during the backup atrial pacing interval, determining an atrial capture result of the delivered atrial pacing pulse based on at least a time of the sensed ventricular event signal and cancelling the scheduled backup atrial pacing pulse. The method may further include, in response to a ventricular event signal not being sensed during the backup atrial pacing interval, determining the atrial capture result of the delivered atrial pacing pulse as loss of capture and delivering the scheduled backup atrial pacing pulse at the expiration time.

[0011] In another example, the disclosure provides a non-transitory, computer-readable storage medium comprising a set of instructions which, when executed by a control circuit of a medical device, cause the medical device to sense ventricular event signals attendant to ventricular depolarizations, deliver an atrial pacing pulse, start a backup atrial pacing interval to schedule a backup atrial pacing pulse in response to delivering the atrial pacing pulse, the backup atrial pacing interval having an expiration time. The instructions may further cause the medical device to determine if a ventricular event signal is sensed during the backup atrial pacing interval, in response to a ventricular event signal being sensed during the backup atrial pacing interval the instructions may cause the medical device to determine an atrial capture result of the delivered atrial pacing pulse based on at least a time of the sensed ventricular event signal and cancel the scheduled backup atrial pacing pulse. The instructions may further cause the medical device to, in response to a ventricular event signal not being sensed during the backup atrial pacing interval, determine the atrial capture result of the delivered atrial pacing pulse as loss of capture and deliver the scheduled backup atrial pacing pulse at the expiration time.

[0012] The details of one or more aspects of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the techniques described in this disclosure will be apparent from the description and drawings, and from the claims.BRIEF DESCRIPTION OF DRAWINGS

[0013] FIG. 1 is a conceptual diagram illustrating an implantable medical device (IMD) system that may be configured to sense cardiac signals and provide cardiac pacing according to the techniques disclosed herein.

[0014] FIG. 2 is a conceptual diagram of the pacemaker shown in FIG. 1 according to one example.

[0015] FIG. 3 is an example of a medical device system including two leadless pacemakers for providing dual chamber pacing and sensing, which may be configured to perform the techniques disclosed herein.

[0016] FIG. 4 is a conceptual diagram of another example of a medical device system that may be configured to determine atrial capture and control backup atrial pacing pulse delivery according to techniques disclosed herein.

[0017] FIG. 5 is another example of a medical device system in which the presently disclosed methods for delivering atrial pacing pulses, determining an atrial capture result and controlling backup pacing pulses may be implemented.

[0018] FIG. 6 is a conceptual diagram of an example configuration of a medical device configured to perform the atrial pacing, capture determination and backup pacing methods disclosed herein.

[0019] FIG. 7 is a flow chart of a method that can be performed by a medical device system for determining atrial pacing capture and controlling delivery of backup atrial pacing pulses and ventricular pacing pulses according to some examples.

[0020] FIG. 8 is a flow chart of another method that can be performed by a medical device system for determining atrial pacing capture and controlling delivery of backup atrial pacing pulses and ventricular pacing pulses according to some examples.

[0021] FIG. 9 is a timing diagram of atrial and ventricular pacing pulses and various time intervals that may be applied by control circuitry of a medical device system performing the method of FIG. 8.

[0022] FIG. 10 is a timing diagram of timing intervals that may be controlled by a medical device system for scheduling a backup atrial pacing pulse, scheduling a ventricular pacing pulse and determining an atrial pacing capture result after a delivered atrial pacing pulse according to some examples.

[0023] FIG. 11 is a flow chart of a method for performing an atrial capture threshold search according to some examples.

[0024] FIG . 12 is a timing diagram of time intervals that may be controlled by control circuit 206 for scheduling a backup atrial pacing pulse, scheduling a ventricular pacing pulse and determining an atrial pacing capture result according to yet another example.

[0025] FIG. 13 is a flow chart of a method that may be performed by a medical device for determining an atrial capture result and controlling atrial and ventricular pacing pulses according to yet another example.DETAILED DESCRIPTION

[0026] In general, this disclosure describes a medical device and method for determining cardiac pacing capture and for controlling the delivery of backup cardiac pacing pulses. Examples disclosed herein provide a medical device and method for delivering atrialpacing pulses, determining an atrial pacing capture result (e.g., atrial capture, loss of capture, or an indeterminate capture result) following delivery of an atrial pacing pulse, and controlling delivery of backup atrial pacing pulses and ventricular pacing pulses.

[0027] FIG. 1 is a conceptual diagram illustrating an implantable medical device (IMD) system 10 that may be configured to sense cardiac signals and provide cardiac pacing according to the techniques disclosed herein. IMD system 10 is shown including a pacemaker 14, implanted within the right atrium (RA) of a patient’s heart 8. In some examples, pacemaker 14 is a transcatheter, leadless pacemaker that can be implanted wholly within a heart chamber. Pacemaker 14 may be reduced in size compared to subcutaneously implanted pacemakers and may be generally cylindrical in shape to facilitate transvenous implantation via a delivery catheter. Pacemaker 14 may be a leadless pacemaker that includes electrodes carried on the pacemaker housing without requiring medical electrical leads extending from pacemaker 14 for sensing cardiac electrical signals and delivering cardiac pacing pulses.

[0028] Pacemaker 14 may be capable of sensing atrial and ventricular event signals, e.g., P-waves attendant to atrial depolarizations and R-waves attendant to ventricular depolarizations. Pacemaker 14 may be configured as a dual chamber pacemaker capable of sensing both atrial and ventricular event signals and delivering atrial pacing pulses and ventricular pacing pulses as needed based on the sensed atrial and / or ventricular event signals. In other examples, pacemaker 14 may be configured as a single chamber pacemaker capable of delivering only atrial pacing pulses but may still be capable of dual chamber sensing of both atrial and ventricular event signals.

[0029] In the example shown, pacemaker 14 is implanted in the RA for providing ventricular pacing from an atrial location. Pacemaker 14 may be configured for delivering ventricular pacing pulses via the heart’s native conduction system and / or ventricular myocardium from a right atrial approach. For example, the distal end 12 of pacemaker 14 may be positioned at the inferior end of the interatrial septum, beneath the AV node and near the tricuspid valve annulus to position a tip electrode 32 for advancement into the interatrial septum toward the His bundle of the native His-Purkinje conduction system. A second electrode, e.g., a ring electrode 34 or ring electrode 36, may be spaced proximally from the tip electrode 32 for use with the tip electrode 32 for bipolar pacing of the right and left ventricles via the His-Purkinje system and / or ventricular myocardium. Ventricularpacing pulses delivered by pacemaker 14 may capture at least a portion of the His bundle and / or ventricular myocardium for delivering ventricular pacing to the ventricles, e.g., the right ventricle (RV) and / or left ventricle (LV), from an atrial implant location of pacemaker 14. The techniques disclosed herein are not necessarily limited to a particular implant location of pacemaker 14, however, and may be practiced in a pacemaker implanted in a variety of operative locations for providing cardiac signal sensing of atrial and ventricular electrical event signals and delivering cardiac pacing to at least an atrial chamber.

[0030] Pacemaker 14 may be capable of bidirectional wireless communication with an external device 50 for programming sensing and pacing control parameters. External device 50 can be referred to as a “programmer” used by a physician, technician, nurse, clinician or other qualified user for programming operating parameters in pacemaker 14. External device 50 may be located in a clinic, hospital or other medical facility. External device 50 may alternatively be embodied as a home monitor or a handheld device that may be used in a medical facility, in the patient’s home, or another location. Operating parameters, including sensing and therapy delivery control parameters, may be programmed into pacemaker 14 by a user interacting with external device 50.

[0031] External device 50 may include a processor 52, memory 53, display unit 54, user interface 56 and telemetry unit 58. Processor 52 controls external device operations and processes data and signals received from pacemaker 14. Display unit 54 may generate a display, which may include a graphical user interface, of data and information relating to pacemaker functions to a user for reviewing pacemaker operation and programmed parameters. Display unit 54 may generate a display that includes cardiac signals and / or data derived therefrom, cardiac pacing timing markers, cardiac pacing history and / or other physiological data, patient data or device-related data that may be stored by pacemaker 14 and transmitted to external device 50 during an interrogation session. For example, pacemaker 14 may generate an output for transmission to external device 50 including, for example, pacing and sensing event histories, capture test results, capture threshold test results, device operating parameters and device diagnostic data.

[0032] User interface 56 may include a mouse, touch screen, keypad or the like to enable a user to interact with external device 50 to initiate a telemetry session with pacemaker 14 for retrieving data from and / or transmitting data to the pacemaker 14, includingprogrammable parameters for controlling sensing and pacing functions. Telemetry unit 58 includes a transceiver and antenna configured for bidirectional communication with a telemetry circuit included in pacemaker 14 and is configured to operate in conjunction with processor 52 for sending and receiving data relating to pacemaker functions via communication link 48.

[0033] Telemetry unit 58 may establish a wireless bidirectional communication link 48 with pacemaker 14. Communication link 48 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 pacemaker 14 to establish and maintain a communication link 48, and in other examples external device 50 and pacemaker 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.

[0034] It is contemplated that external device 50 may be in wired or wireless connection to a communications network via a telemetry circuit 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. Remote patient management systems including a centralized patient database may enable a clinician to view data relating to sensing and pacing functions performed by pacemaker 14.

[0035] FIG. 2 is a conceptual diagram of the pacemaker 14 shown in FIG. 1 according to one example. Pacemaker 14 includes a housing 15 having a distal end 12 and a proximal end 16. The lateral sidewall 17 of housing 15 extending from distal end 12 to proximal end 16 may be generally cylindrical to facilitate transvenous delivery, e.g., via a catheter. Distal end 12 is referred to as “distal” in that it is expected to be the leading end as pacemaker 14 is advanced through a delivery tool, such as a catheter, and placed against a targeted pacing site. In other examples, housing 15 may have a generally prismatic shape. The housing 15 encloses the electronics and a power supply for sensing cardiac signals, producing pacing pulses and controlling therapy delivery and other functions of pacemaker 14 as described herein.

[0036] Pacemaker 14 is shown including electrodes 32, 34 and 36 spaced apart along the housing 15 of pacemaker 14 for sensing cardiac electrical signals and delivering pacingpulses. Electrode 32 is shown as a tip electrode extending from distal end 12 of housing 15. Electrodes 34 and 36 are shown as ring electrodes along the lateral sidewall 17 of housing 15. Electrodes 34 and 36 may be ring electrodes circumscribing the lateral sidewall 17, for example adjacent proximal end 16 and adjacent distal end 12, respectively.

[0037] As shown in FIG. 1, tip electrode 32 can be advanced from within the right atrial chamber to a ventricular pacing site, e.g., for delivering pacing to the His-Purkinje conduction system and / or for pacing of ventricular septal myocardial tissue. Tip electrode 32 is shown as a screw-in helical electrode which may provide fixation of pacemaker 14 at an implant site as well as serving as a pacing and sensing electrode. In other examples, tip electrode 32 may have a straight, fishhook or other shape and may have a tissue -piercing distal tip to facilitate advancement to a ventricular pacing site, e.g., within the septum of the heart 8, when pacemaker 14 is implanted in the RA. In some examples, pacemaker 14 may include fixation member(s), such as one or more curved or angled tines, configured to engage atrial endocardial tissue for anchoring pacemaker 14 at an implant site. Numerous types of active and / or passive fixation members may be employed for anchoring or securely stabilizing pacemaker 14 at implant site.

[0038] Tip electrode 32 may serve as a cathode electrode with ring electrode 34 serving as a return anode for delivering ventricular pacing pulses. Tip electrode 32 and ring electrode 34 may be used as a bipolar pair for ventricular pacing and for receiving a ventricular electrical signal from which R-waves can be sensed by sensing circuitry enclosed by housing 15. Ring electrodes 34 and 36 may form a second anode and cathode pair for bipolar atrial pacing and sensing an atrial electrical signal from which P-waves can be sensed by the sensing circuitry enclosed by housing 15.

[0039] While electrodes 34 and 36 are shown as ring electrodes circumscribing the cylindrical sidewall 17 of housing 15, in other examples, an atrial cathode electrode may be located on distal end 12 and / or along longitudinal sidewall 17 as one or more button, hemispherical, fishhook, segmented, short coil or other type of electrodes or combinations thereof. The proximal ring electrode 34 serving as a return anode electrode for ventricular and atrial pacing electrode pairs may alternatively be positioned along the proximal end 16 of housing 15 and / or longitudinal sidewall 17 as one or more button, hemispherical, fishhook, segmented, short coil or other type of electrodes or combinations thereof.Electrodes 32, 34 and 36 may be, without limitation, titanium, platinum, iridium or alloys thereof and may include a low polarizing coating, such as titanium nitride, iridium oxide, ruthenium oxide, platinum black, among others.

[0040] Electrodes 32, 34, and 36 may be positioned at locations along pacemaker 14 other than the locations shown. While pacemaker 14 is shown as a leadless pacemaker including three housing-based electrodes, pacemaker 14 may be provided with a different number of electrodes, for example more than three electrodes, in other examples. For instance, a second anode electrode may be carried by pacemaker 14 to provide a dedicated ventricular pacing and sensing cathode and anode pair and a dedicated atrial pacing and sensing cathode and anode pair that does not necessarily share a common anode between the ventricular and atrial pacing and sensing electrode pairs. Examples of various leadless dual chamber pacemakers and pacing electrode arrangements in which the presently disclosed techniques may be implemented are generally disclosed in U.S. Patent No. 11,426,578 (Yang, et al.), incorporated herein by reference in its entirety.

[0041] Housing 15 is formed from a biocompatible material, such as a stainless steel or titanium alloy. In some examples, the housing 15 may include an insulating coating. Examples of insulating coatings include parylene, urethane, PEEK, or polyimide, among others. The entirety of the housing 15 may be insulated, but only electrodes 32 34, and 36 uninsulated. Tip electrode 32 can be coupled to internal circuitry, e.g., a pacing pulse generator and cardiac electrical signal sensing circuitry, enclosed by housing 15 via an electrical feedthrough crossing housing 15. Electrodes 34 and 36 may be formed as a conductive portion of housing 15 defining respective ring electrodes that are electrically isolated from each other and from the other portions of the housing 15 as generally shown in FIG. 2.

[0042] Pacemaker 14 may include features for facilitating deployment to and fixation at an implant site. For example, pacemaker 14 may optionally include a delivery tool interface 18. Delivery tool interface 18 may be located at the proximal end 16 of pacemaker 14 and is configured to connect to a delivery device, such as a catheter, used to position pacemaker 14 at an implant location during an implantation procedure. The delivery tool interface 18 may enable a clinician to advance, retract and steer pacemaker 14 to an implant site and rotate pacemaker 14 to advance the helical tip electrode 32 intothe cardiac tissue (and / or deploy other fixation members when included on pacemaker housing 15).

[0043] The example techniques disclosed herein for determining atrial pacing capture and controlling backup atrial pacing pulses during a capture test are described as being implemented in and performed by the leadless pacemaker 14 as an illustrative example. It is to be understood, however, that aspects of the methods disclosed herein may be implemented in a variety of leadless pacemaker configurations, including two device systems, as well as pacemaker or implantable cardioverter defibrillator systems that include one or more medical electrical leads for positioning electrodes in or on the heart 8, e.g., transvenous and / or epicardial lead and electrode systems. Other example medical device systems that may be configured to practice the techniques disclosed herein are described in conjunction with FIGs. 3-5.

[0044] FIG. 3 is an example of a medical device system 110 including two leadless pacemakers 114 and 116 for providing dual chamber pacing and sensing. Pacemaker 114 may be configured to be implanted in the RA for pacing and sensing in the atrial chamber. Pacemaker 114 may include a distal tip electrode 132 and a proximal ring electrode 134 for use as a bipolar atrial pacing and sensing electrode pair. In other examples, pacemaker 114 may include more than two electrodes for sensing cardiac electrical signals and delivering atrial pacing. The electrodes 132 and 134 are carried on pacemaker housing 115. For example, electrode 134 may be a ring electrode circumscribing the cylindrical, longitudinal sidewall of housing 115. Pacemaker 114 may include one or more fixation tines 118 for engaging with atrial tissue for securely anchoring pacemaker 114 at an implant site. Pacemaker 114 is shown implanted at an implant site near the interatrial septum above the tricuspid valve as an example, but pacemaker 114 may be positioned at other locations on or in the RA.

[0045] Atrial pacemaker housing 115 may enclose sensing circuitry, pulse generating circuitry, communication circuitry, and control circuitry, as well as a power supply and in some cases other physiological sensors, as generally described below in conjunction with FIG. 6, for performing sensing and pacing functions. Pacemaker 114 may be configured to deliver atrial pacing pulses, determine a capture result following an atrial pacing pulse (e.g., atrial capture, indeterminate capture or loss of capture) and control delivery of backup atrial pacing pulses according to the techniques disclosed herein. As describedbelow, the timing of a sensed ventricular event signal following a delivered atrial pacing pulse may be used by control circuitry of pacemaker 114 for determining the capture result. In some examples, pacemaker 114 may be configured to sense far- field R-waves from a cardiac electrical signal sensed by electrodes 132 and 134. In other examples, pacemaker 114 may receive a communication signal from pacemaker 116, implanted in the right ventricle (RV) indicating the timing of a sensed R-wave for use in determining an atrial capture result and for use in controlling delivery of a backup atrial pacing pulse according to the techniques described below.

[0046] Atrial pacemaker 114 may be configured to wirelessly communicate with ventricular pacemaker 116, as shown by arrow 148, to coordinate AV synchronous dual chamber pacing in some examples and to communicate the timing of cardiac events for performing atrial capture management and backup atrial pacing according to the methods disclosed herein. Atrial pacemaker 114 and ventricular pacemaker 116 may each include communication circuitry for transmitting and / or receiving communication signals, which may be radiofrequency signals, tissue conductance communication signals or other forms of communication signals. In various examples, atrial pacemaker 114 may transmit a communication signal to ventricular pacemaker 116 to indicate the timing of a sensed P- wave or a delivered atrial pacing pulse. Ventricular pacemaker 116 may be triggered to deliver a ventricular pacing pulse at an AV pacing interval in response to receiving the communication signal. In some examples, ventricular pacemaker 116 may transmit a signal to atrial pacemaker 114 indicating the timing of a sensed R-wave or a delivered ventricular pacing pulse. As indicated above, the timing of a sensed R-wave may be used by atrial pacemaker 114 for determining an atrial pacing capture result and for controlling delivery of backup atrial pacing pulses. When a backup atrial pacing pulse is delivered by atrial pacemaker 114, a communication signal transmitted from atrial pacemaker 114 to ventricular pacemaker 116 can trigger ventricular pacemaker 116 to deliver a ventricular pacing pulse in synchrony with the backup atrial pacing pulse.

[0047] Ventricular pacemaker 116 is shown including a housing 125 carrying at least two housing based electrodes 136 and 138 for providing a bipolar electrode pair for sensing ventricular electrical signals and delivering ventricular pacing pulses. In other examples, ventricular pacemaker 116 may include another ring electrode, e.g., positioned near the distal end from which tip electrode 136 extends, for providing a second pacing and / orsensing electrode pair. Tip electrode 136 may be a helical electrode that may serve as a cathode electrode and a fixation member for anchoring pacemaker 116 at an implant site. In other examples, electrodes 136 and 138 may be other types of electrodes, e.g., according to any of the examples listed herein. Ventricular pacemaker housing 125 may enclose sensing circuitry, pulse generating circuitry, communication circuitry, and control circuitry, as well as a power supply and in some cases other physiological sensors, as generally described below in conjunction with FIG. 6 for performing sensing and pacing functions.

[0048] Ventricular pacemaker 116 is shown implanted along the interventricular septum 9 so that tip electrode 136 may be advanced into the septum to a conduction system pacing site for delivering ventricular pacing via the His-Purkinje system. For example, tip electrode 136 may be advanced to a pacing site in the area of the left bundle branch, right bundle branch or His bundle. In other examples, tip electrode 136 may be positioned at other pacing site locations, e.g., along the septum 9 for pacing the septal myocardium or in an apical location for myocardial pacing.

[0049] The methods disclosed herein are advantageously practiced in a medical device having a housing of reduced size and volume capacity, such as in the leadless intracardiac pacemaker 14 of FIG. 1 or the leadless atrial pacemaker 114 of FIG. 2, which may operate in a dual chamber leadless pacemaker system including both atrial pacemaker 114 and leadless ventricular pacemaker 116. The methods disclosed herein may be employed in a device capable of delivering atrial pacing using a reduced number of holding capacitors included in pulse generating circuitry for delivering cardiac pacing pulses. As further described below in conjunction with FIG. 6, pulse generating circuitry included in pacemaker 14 or pacemaker 114 may include holding capacitors that are charged to a pacing pulse voltage amplitude and discharged for a pacing pulse width for delivering a pacing pulse. By reducing the number of holding capacitors required for delivering a primary pacing pulse and a backup pacing pulse, the overall size of the medical device can be reduced and / or space for a larger power supply or other electronic circuitry may be made available within the medical device housing, e.g., housing 15 or 115.

[0050] In past practice, a backup atrial pacing pulse may be delivered within the atrial absolute refractory period following a primary or test atrial pacing pulse. This way, if the primary or test atrial pacing pulse captures the atrial myocardium, the backup atrial pacingpulse has no effect (e.g., fails to capture the atrial myocardium) because the tissue is in an absolute refractory state. When the primary or test atrial pacing pulse fails to capture the atrial myocardium, however, the backup atrial pacing pulse delivered within a time interval that is less than the atrial absolute refractory period from the primary or test atrial pacing pulse can capture the atrial myocardium and promote a regular atrial rate (and ventricular rate).

[0051] In order to deliver a backup atrial pacing pulse within the physiological atrial refractory period after a primary or test atrial pacing pulse, a different capacitor is generally needed to generate the backup atrial pacing pulse than the capacitor that is discharged to deliver the primary atrial pacing pulse. Recharging the same capacitor(s) used to deliver the primary atrial pacing pulse may not be fast enough to deliver the backup atrial pacing pulse within the physiological absolute refractory period of the atria, e.g., within 50 to 100 milliseconds (ms) after the primary atrial pacing pulse. By scheduling a backup atrial pacing pulse at a longer backup pacing interval, e.g., at least 300 ms or more or at least 400 ms or more, the same capacitor(s) charged and discharged for delivering the primary atrial pacing pulse may be recharged for delivering the backup atrial pacing pulse. A single holding capacitor (or a combination of holding capacitors) may be charged to an atrial pacing pulse voltage amplitude for generating and delivering an atrial pacing pulse. Using the techniques disclosed herein, the same single capacitor or combination of capacitors may be recharged to deliver a backup atrial pacing pulse that is scheduled at a backup atrial pacing interval that is longer than the atrial absolute refractory period and the atrial vulnerable period (e.g., during the atrial relative refractory period or atrial repolarization phase).

[0052] As such, by implementing the methods disclosed herein, at least one capacitor that may normally be included in pulse generating circuitry for generating backup atrial pacing pulses may be eliminated, allowing the overall size of the pacemaker to be reduced. While this size reduction is particularly advantageous in a pacemaker that is wholly implanted within the RA, the size reduction of any implantable medical device can be advantageous, for example to promote greater patient comfort and ease of implant and explant. Additionally or alternatively, eliminating at least one capacitor that may normally be included for generating a backup pacing pulse can allow for an increased size of the power supply or other electronic circuitry of the pacemaker within the same or smaller medicaldevice housing size. As such, the techniques disclosed herein may be implemented a medical device system including a pacemaker or implantable cardioverter defibrillator (ICD) that is implanted inside or outside the heart, e.g., subcutaneously or submuscularly, and can be a leadless pacemaker or connected to medical electrical leads for positioning pacing and sensing electrodes in operative locations relative to the patient’s heart.

[0053] FIG. 4 is a conceptual diagram of another example of a medical device system 150 that may be configured to determine atrial capture and control backup atrial pacing pulse delivery according to techniques disclosed herein. In this example, pacemaker 154 includes housing 155 enclosing interval device circuitry and a connector assembly 153, sometimes referred to as a “connector block” or “header,” connected to housing 155.Connector assembly 153 can be provided with one or more connector bores configured to receive the proximal lead connector(s) (not shown in FIG. 4) of one or more medical electrical leads, e.g., atrial pacing and sensing lead 156 and ventricular pacing and sensing lead 158.

[0054] Pacemaker 154 may be a dual chamber pacemaker configured to receive atrial electrical signals and deliver atrial pacing pulses via electrodes 166 and 168 of atrial pacing and sensing lead 156. Electrical conductors extending through the atrial lead body 157 electrically couple respective electrodes 166 and 168 to the sensing and pacing circuitry enclosed by housing 155, e.g., via contacts within connector assembly 153 and electrical feedthroughs crossing housing 155.

[0055] Pacemaker 154 may be configured to receive ventricular electrical signals and deliver ventricular pacing pulses via electrodes 162 and 164 of ventricular pacing and sensing lead 158. Electrical conductors extending through the ventricular lead body 159 electrically couple respective electrodes 162 and 164 to the sensing and pacing circuitry enclosed by housing 155, e.g., via contacts within connector assembly 153 and electrical feedthroughs crossing housing 155.

[0056] In the example shown, atrial pacing and sensing lead 156 is advanced transvenously into the RA, and atrial tip electrode 166 may be anchored at an atrial pacing and sensing site, e.g., along the interatrial septum or another atrial endocardial location. Atrial tip electrode 166 may be a helical electrode serving as a cathode electrode and a fixation member for anchoring lead 156 at the atrial pacing and sensing site. In other examples, atrial lead 156 may include a non-tissue piercing distal tip electrode and one ormore fixation tines or other fixation members for anchoring lead 156 at the atrial pacing and sensing site. Electrode 168 may be a ring electrode circumscribing the lead body 157 and spaced proximally along lead body 157 from distal tip electrode 166.

[0057] Ventricular pacing and sensing lead 158 is advanced transvenously into the RA where ventricular tip electrode 162 may be advanced to a conduction system pacing site from a right atrial approach. For example, tip electrode 162 may be advanced into the interventricular septum 9 from an insertion point in the Triangle of Koch to position ventricular tip electrode 162 along or near the His bundle for delivering ventricular pacing pulses via the native conduction system. Ventricular tip electrode 162 may be a helical electrode serving as a cathode electrode and a fixation member for anchoring ventricular pacing and sensing lead 158 at the desired implant site. Electrode 164 may be a ring electrode circumscribing the lead body 159 and spaced proximally along lead body 159 from distal tip electrode 162.

[0058] Pacemaker 154 may be configured to deliver atrial pacing pulses via atrial pacing and sensing lead 156, sense ventricular R-waves via ventricular pacing and sensing lead 158 for determining an atrial pacing capture result and for controlling delivery of backup atrial pacing pulses according to the techniques disclosed herein, as further described below. The lead and electrode configuration of medical device system 150 is one example of a transvenous, dual chamber medical device system. As indicated above, the techniques disclosed herein are not limited to being practiced in conjunction with a particular medical device system or any particular lead and electrode configuration or electrode implant locations. A variety of lead and / or electrode configurations and electrode implant sites may be utilized in combination with the techniques disclosed herein. For example, instead of the two lead system shown in FIG. 4, a single lead carrying multiple electrodes could be connected to pacemaker 154 to provide an atrial pacing and sensing electrode pair and a ventricular pacing and sensing electrode pair carried by the same lead, e.g., lead 158. A single lead may carry three or more electrodes that can be selectively coupled to sensing and pacing circuitry of pacemaker 154, e.g., via switching circuitry, for selecting atrial sensing electrodes, atrial pacing electrodes, ventricular sensing electrodes and / or ventricular pacing electrodes.

[0059] FIG. 5 is another example of a medical device system 170 in which the presently disclosed methods for delivering atrial pacing pulses, determining an atrial capture resultand controlling backup atrial pacing pulses may be implemented. Medical device system 170 includes a pacemaker 174 coupled to an atrial lead 176 and a ventricular lead 178 via connector assembly 173 that provides connection of atrial pacing and sensing electrodes 186 and 188 and ventricular pacing and sensing electrodes 182 and 184 to pacing and sensing circuitry enclosed by housing 175.

[0060] In this example, pacemaker 174 is capable of dual chamber sensing and pacing and delivery of high voltage cardioversion / defibrillation (CV / DF) shocks and may be referred to as an ICD. As such, at least one relatively high surface area electrode for delivering high voltage CV / DF shocks may be carried by at least one of leads 176 and / or 178. In the example shown, ventricular lead 178 includes an RV coil electrode 190 and a superior vena cava (SVC) coil electrode 192 provided for delivering high voltage CV / DF shocks, together and / or in combination with pacemaker housing 175 serving as an active can electrode. Electrodes 190 and 192 may be provided as elongated coil electrodes and may be referred to as “defibrillation electrodes” but may also be used, individually or in combination, in a sensing electrode vector in some examples for sensing cardiac electrical signals.

[0061] In this example, ventricular lead 178 is shown advanced into the RV of the patient’s heart 8 for positioning ventricular pacing and sensing electrodes 182 and 184 at a desired ventricular pacing and sensing site. As shown, tip electrode 182 may be advanced into the interventricular septum 12 for positioning tip electrode 182 at a location for pacing the ventricles via the His-Purkinje conduction system. For example, tip electrode 182 may be advanced into the interventricular septum 12 to a left bundle branch area pacing site, to a right bundle branch area pacing site or, from an insertion point near the base of interventricular septum 9, to a His bundle pacing site, as examples. In other examples, tip electrode 182 may be advanced to and anchored at a ventricular myocardial pacing site, e.g., in or along the interventricular septum or at or near the apex of the RV.

[0062] Pacemaker 174 may be configured to perform the techniques described herein for delivering atrial pacing pulses via atrial electrodes 186 and 188, sensing atrial P-waves via atrial electrodes 186 and 188, deliver ventricular pacing pulses via ventricular electrodes 182 and 184, sense ventricular R-waves via ventricular electrodes 182 and 184 and determine an atrial capture result based on if and when a ventricular R-wave is sensed after delivery of an atrial pacing pulse. Pacemaker 174 may schedule a backup atrialpacing pulse at a backup atrial pacing interval that is longer than the physiological refractory period of the atria. As further described below, control circuitry of pacemaker 174 or any of the other example pacemakers described herein may determine whether to deliver or cancel the backup atrial pacing pulse based on whether an atrial P-wave or a ventricular R-wave is sensed during the backup atrial pacing interval. The control circuitry may determine whether to deliver a ventricular pacing pulse during or after the backup atrial pacing interval based on if and when an atrial P-wave is sensed or the backup atrial pacing interval is delivered when no ventricular R-wave is sensed by sensing circuitry of the pacemaker.

[0063] FIG. 6 is a conceptual diagram of an example configuration of a medical device configured to perform the atrial pacing, capture determination and backup pacing methods disclosed herein. For the sake of convenience, FIG. 6 is described with reference to the leadless intracardiac pacemaker 14 of FIG. 1. It is to be understood, however, that the circuitry and functionality attributed to pacemaker 14 for practicing the techniques disclosed herein may be implemented in other medical device systems including any of the example medical device systems described in conjunction with FIGs. 3-5.

[0064] Pacemaker 14 may include a pulse generator 202, a cardiac electrical signal sensing circuit 204, a control circuit 206, telemetry circuit 208, memory 210, sensor(s) 212 and a power source 214. The various circuits represented in FIG. 6 may be combined on one or more integrated circuit boards which include an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, state machine or other suitable components that provide the described functionality.

[0065] Cardiac electrical signal sensing circuit 204, referred to hereafter as “sensing circuit” 204, is configured to receive at least one cardiac electrical signal via electrodes coupled to pacemaker 14, e.g., via tip electrode 32 and proximal ring electrode 34. A second cardiac electrical signal may be received via ring electrodes 34 and 36 (and / or electrode tip 32 and distal ring electrode 36). As such, sensing circuit 204 may have multiple sensing channels, e.g., an atrial sensing channel 203 and a ventricular sensing channel 205. While pacemaker 14 is shown having three electrodes in the examples shown herein, pacemaker 14 may be provided with two electrodes or more than three electrodes in other examples, which may be coupled to sensing circuit 204 (and / or pulse generator202) in selected sensing (and / or pacing) electrode vectors. Sensing circuit 204 may include switching circuitry for coupling a sensing electrode pair to a respective sensing channel 203 or 205. For example, an atrial signal may be received by sensing circuit 204 by switchably coupling distal ring electrode 36 and proximal ring electrode 34 to atrial sensing channel 203 in a bipolar atrial sensing electrode pair. A ventricular signal may be received by sensing circuit 204 by switchably coupling tip electrode 32 and proximal ring electrode 34 to ventricular sensing channel 205 in a bipolar ventricular sensing electrode pair.

[0066] Sensing channels 203 and 205 may include filters, amplifiers, analog-to-digital converters (ADCs), rectifiers, sense amplifiers, comparators, and / or other circuitry for sensing cardiac event signals, e.g., P-waves and R-waves respectively, and producing sensed cardiac event signals, e.g., atrial sensed event signals (Asense signals) and ventricular sensed event signals (Vsense signals), that are passed to control circuit 206 from the respective sensing channels 203 and 205. Sensing circuit 204 may be configured to pass a filtered and amplified multi-bit digital cardiac electrogram (EGM) signal to control circuit 206, e.g., from one or both of atrial and ventricular sensing channels 203 and 205. The EGM signal(s) may be processed and analyzed by control circuit 206 for determining a heart rhythm and / or stored in memory 210 as cardiac signal episodes that can be transmitted by telemetry circuit 208, e.g., to external device 50 (shown in FIG. 1).

[0067] Each of the atrial sensing channel 203 and the ventricular sensing channel 205 may receive raw cardiac electrical signals from the selected sensing electrode vector as input to a pre-filter and amplifier circuit for filtering and amplifying the raw signal. The pre-filter and amplifier circuits each include a high pass filter to remove DC offset, e.g., a 2.5 to 5 Hz high pass filter, or a wideband filter having a bandpass of 2.5 Hz to 100 Hz or narrower to remove DC offset and high frequency noise. The pre-filter and amplifier circuits may pass the filtered and amplified signal to an ADC of the respective sensing channel 203 or 205.

[0068] The ADCs may each pass a rectified digital signal to a narrowband filter of the respective sensing channel 203 or 205. Atrial sensing channel 203 may include a bandpass filter having bandpass cutoff frequencies for passing P-wave signals and attenuating other cardiac event signals, e.g., R-waves and T-waves. The low pass cutoff frequency of the narrow bandpass filter of atrial sensing channel 203 may be between 25 Hz and 100 Hz orbetween 50 and 80 Hz and the high pass cutoff frequency may be between 5 Hz and 25 Hz or between 15 and 20 Hz, as examples. In an example, the atrial sensing channel 203 includes bandpass filter having a low pass frequency of 70 Hz and a high pass frequency of 17 Hz.

[0069] Atrial sensing channel 203 may include an atrial event detector circuit, which may include a sense amplifier, comparator or other event detection circuitry, that compares the incoming rectified, filtered and amplified atrial EGM signal to a P-wave sensing threshold. For example, when the incoming signal crosses a P-wave sensing threshold, the atrial sensing channel 203 may produce Asense signal that can be passed to control circuit 206 indicating the timing of a sensed P-wave. The P-wave sensing threshold may be an autoadjusting threshold that is automatically decreased by sensing circuit 204 from a starting value to a minimum value or until a P-wave sensing threshold crossing occurs. The P- wave sensing threshold amplitude is initially set to a starting value applied to the atrial EGM signal upon expiration of a post-atrial blanking period and can be adjusted to a minimum sensing threshold or “sensing floor” that may be equal to a programmed atrial sensitivity.

[0070] Atrial sensing channel 203 may include a peak track and hold circuit or other circuitry for detecting the maximum peak amplitude of the atrial EGM signal during a portion of the post-atrial blanking period. The P-wave sensing threshold starting value may be set based on the maximum peak amplitude, e.g., to a percentage of the maximum peak amplitude following a P-wave sensing threshold crossing. In some examples, the P- wave sensing threshold may be set to 50% to 80% of the maximum peak amplitude. The P-wave sensing threshold may be decreased according to one or more decay rates and corresponding decay time intervals until the atrial EGM signal crosses the P-wave sensing threshold or the atrial sensitivity is reached. The atrial sensitivity defines the minimum atrial EGM signal amplitude that can be sensed as a P-wave. In other examples, the P- wave sensing threshold may be a fixed value that does not decay over time and may be applied as a percentage of the maximum peak amplitude of the atrial EGM signal during a post-atrial blanking period or based on the programmed atrial sensitivity. If an atrial pacing interval, which may be set to a programmed LRI expires before an Asense signal is received by control circuit 206, pulse generator 202 may deliver an atrial pacing pulse.

[0071] Ventricular sensing channel 205 may include an ADC for receiving the filtered and amplified input signal. The ADC of ventricular sensing channel 205 may pass a rectified signal to a narrow bandpass filter having bandpass cutoff frequencies for passing R-wave signals and attenuating other cardiac event signals, e.g., P-waves and T-waves. The low pass cutoff frequency of a bandpass filter included in ventricular sensing channel 205 may be between 25 Hz and 100 Hz or between 40 and 80 Hz and the high pass cutoff frequency may be between 5 Hz and 25 Hz or between 15 and 20 Hz, as examples. In an example, ventricular sensing channel 205 is provided with a bandpass filter having a low pass frequency of 70 Hz and a high pass frequency of 17 Hz. The bandpass cutoff frequencies of atrial sensing channel 203 and ventricular sensing channel 205 may be the same or different cutoff frequencies. In some examples, the cutoff frequencies are user programmable.

[0072] Ventricular sensing channel 205 may include a ventricular event detector circuit, which may include a sense amplifier, comparator or other event detection circuitry, that compares the incoming rectified, filtered and amplified ventricular EGM signal to an R- wave sensing threshold. For example, when the incoming signal crosses an R-wave sensing threshold, the sensing circuit 204 may produce a Vsense signal that can be passed to control circuit 206. The R-wave sensing threshold may be an auto-adjusting threshold that is automatically decreased by sensing circuit 204 from a starting value until the threshold is crossed by the ventricular EGM signal. The R-wave sensing threshold amplitude is initially set to a starting value applied to the ventricular EGM signal upon expiration of a post- ventricular blanking period and can be adjusted to a minimum sensing threshold or “sensing floor” that may be equal to a programmed ventricular sensitivity.

[0073] Sensing circuit 204 may include a peak track and hold circuit or other circuitry for detecting the maximum peak amplitude of the ventricular EGM signal following an R- wave sensing threshold crossing during a peak tracking portion of the post-ventricular blanking period. The R-wave sensing threshold starting value may be set based on the maximum peak amplitude, e.g., to a percentage of the maximum peak amplitude. In some examples, the R-wave sensing threshold may be set to 50 to 80% of the maximum peak amplitude. The R-wave sensing threshold may be decreased according to one or more decay rates and corresponding decay time intervals until the ventricular EGM signal crosses the R-wave sensing threshold or the ventricular sensitivity is reached. Theventricular sensitivity defines the minimum ventricular EGM signal amplitude that can be sensed as an R-wave. In other examples, the R-wave sensing threshold may be a fixed value that does not decay, e.g., a fixed percentage of the maximum peak amplitude or a fixed value based on a programmed ventricular sensitivity. If a ventricular pacing interval or an AV pacing interval expires before a Vsense signal is received by control circuit 206, pulse generator 202 may deliver a ventricular pacing pulse.

[0074] Control circuit 206 may provide sensing control signals to sensing circuit 204. Sensing control parameters may include R-wave sensing threshold adjustment parameters, e.g., the percentage of the maximum peak amplitude used for setting the starting R-wave sensing threshold and the ventricular sensitivity, and P-wave sensing threshold adjustment parameters, e.g., the percentage of the maximum peak amplitude used for setting the starting P-wave sensing threshold and the atrial sensitivity. Sensing control parameters may include various blanking and refractory intervals applied to the atrial EGM signal, e.g., a post-sense atrial blanking period, a post-pace atrial blanking period, an atrial refractory period and a post-ventricular atrial blanking period. Sensing control parameters may include various blanking and refractory intervals applied to the ventricular EGM signal, e.g., a post-sense ventricular blanking period, a post-pace ventricular blanking period, a ventricular refractory period and a post-atrial ventricular blanking period.

[0075] When sensing circuit 204 is configured to receive a raw atrial electrical signal and a raw ventricular electrical signal, components included in an atrial sensing channel 203 and in a ventricular sensing channel 205 may be separate or shared between both sensing channels 203 and 205 in various examples. For example, pre-filter / amplifiers and / or ADCs may be shared by both atrial sensing channel 203 and ventricular sensing channel 205 with separate outputs being passed to an atrial channel bandpass filter and atrial event detector circuit and to a ventricular channel bandpass filter and ventricular event detector circuit. Different filtering and amplification may be applied to the output of an ADC before passing separate signals to the respective atrial event detector circuit and ventricular event detector circuit.

[0076] Control circuit 206 may include a pace timing circuit 242 and processor 244. Control circuit 206 may receive Vsense signals and Asense signals from sensing circuit 204 for use in controlling the timing of cardiac pacing pulses. Vsense signals may be passed from sensing circuit 204 to control circuit 206 in response to ventricular sensingchannel 205 sensing a ventricular event signal to indicate the timing of a sensed R-wave. Asense signals may be passed from sensing circuit 204 to control circuit 206 in response to atrial sensing channel 203 sensing an atrial event signal to indicate the timing of a sensed P-wave.

[0077] Processor 244 may pass sensing control parameters to sensing circuit 204 for use in sensing cardiac event signals from the cardiac electrical signal(s). Processor 244 may include one or more clocks for generating clock signals that are used by pace timing circuit 242 to time out various pacing intervals for providing atrial and / or ventricular pacing according to an operating pacing mode. Depending on the operating pacing mode of control circuit 206, pace timing circuit 242 may start various pacing intervals to schedule pacing pulses. Control circuit 206 may be configured to operate in a variety of programmable and / or automatically switchable pacing modes. During an atrial synchronous ventricular pacing mode, which may be denoted as a DDD or VDD pacing mode for example, ventricular pacing pulses may be delivered synchronously with atrial pacing pulses and received Asense signals. For example, in response to receiving an Asense signal, pace timing circuit 242 may start an AV pacing interval to control the timing of an atrial synchronous ventricular pacing pulse. When a ventricular pacing pulse is delivered by pulse generator 202 upon expiration of the AV pacing interval, pace timing circuit 242 may start a ventricular pacing interval to schedule a ventricular pacing pulse according to a programmed ventricular lower rate. During atrial synchronous ventricular pacing, if an Asense signal is not received or an atrial pacing pulse is not delivered prior to the expiration of a ventricular pacing interval, pulse generator 202 may deliver an asynchronous ventricular pacing pulse to avoid ventricular asystole and restart the ventricular pacing interval. The scheduled ventricular pacing pulse can be inhibited if an Asense signal is received (or an atrial pacing pulse is delivered) before the ventricular pacing interval expires. The pending pacing pulse may be cancelled and an atrial synchronous triggered ventricular pacing pulse can be delivered at the AV pacing interval from the Asense signal (or delivered atrial pacing pulse).

[0078] In response to receiving a Vsense signal from sensing circuit 204, pace timing circuit 242 may inhibit a pending ventricular pacing pulse scheduled at the ventricular pacing interval (or scheduled at an AV pacing interval) and restart the ventricular pacing interval. The ventricular pacing interval may be a lower rate interval (LRI) correspondingto a programmed minimum or base ventricular lower pacing rate. In other instances, the ventricular pacing interval may be a temporary ventricular pacing interval set to a rate smoothing interval to avoid an abrupt change in ventricular rate. In other instances, the ventricular pacing interval may be a temporary rate response pacing interval set to provide rate response pacing during increased patient physical activity, which may be determined from a signal from sensor(s) 212 as further described below.

[0079] Pace timing circuit 242 may start a pacing escape interval timer upon receiving an Asense or Vsense signal from sensing circuit 204. The value reached by an escape interval timer between two consecutive Asense signals or between an Asense signal and a preceding atrial pacing pulse can be determined as a PP interval (PPI) for use in determining an atrial rate. The value reached by an escape interval timer between two consecutive Vsense signals or between a Vsense signal and a preceding ventricular pacing pulse can be determined as an RR interval (RRI) for use in determining a ventricular rate. The atrial rate and / or ventricular rate may be determined by control circuit 206 for storing cardiac data in memory 210, controlling pacing mode switching, or other pacemaker functions in some examples.

[0080] Pace timing circuit 242 may include a timer or counter for determining the time from an atrial pacing pulse delivered by pulse generator 202 to a Vsense signal received from sensing circuit 204. This time interval may be referred to as a paced AV interval or “Apace- Vsense interval” and may be measured by control circuit 206 for use in determining a capture result of a delivered atrial pacing pulse according to the techniques disclosed herein, as further described below. Pace timing circuit 242 may include a timer or counter for determining the time from an Asense signal received from sensing circuit 204 to a Vsense signal received from sensing circuit 204. This time interval may be referred to as a sensed AV interval or “Asense- Vsense interval” and may be measured by control circuit 206 for use in determining a capture result of a delivered atrial pacing pulse when an Asense is received during a backup atrial pacing interval according to the techniques disclosed herein.

[0081] As further described below, control circuit 206 may perform atrial capture management by determining a capture result following an atrial pacing pulse delivered by pulse generator 202. The capture result may be determined based on the timing of any Asense and / or Vsense signals received following the atrial pacing pulse. Pace timingcircuit 242 may start a backup atrial pacing interval upon delivery of an atrial pacing pulse and control circuit 206 may determine a capture result based on any Asense and / or Vsense signals received during the backup atrial pacing interval and cancel delivery of the backup atrial pacing pulse when an Asense or Vsense signal is received during the backup atrial pacing interval. As described below, control circuit 206 may control pulse generator 202 to deliver a ventricular pacing pulse when a Vsense signal is not received during the backup atrial pacing interval or during an AV pacing interval extending from an expiration of the backup atrial pacing interval.

[0082] Pulse generator 202 generates electrical pacing pulses that can be delivered to pace the ventricles of the patient’s heart via cathode electrode 32 and return anode electrode 34. Pulse generator 202 may generate electrical pacing pulses for pacing the atria, e.g., using electrodes 36 and 34. In addition to providing control signals to pace timing circuit 242 and pulse generator 202 for controlling the timing and delivery of pacing pulses, processor 244 may retrieve programmable pacing control parameters from memory 210, such as pacing pulse amplitude and pacing pulse width, which are passed to pulse generator 202 for controlling pacing pulse delivery.

[0083] Pulse generator 202 may include charging circuit 230, switching circuit 232 and an output circuit 234. Charging circuit 230 is configured to receive current from power source 214 and may include at least one holding capacitor that may be charged to a pacing pulse amplitude, e.g., under the control of a voltage regulator included in charging circuit 230. The pacing pulse amplitude may be set based on a control signal from control circuit 206. In the example shown, charging circuit 230 includes an atrial holding capacitor 235 that may be charged to an atrial pacing pulse amplitude and discharged via switch 232 and output circuit 234 through the atrial pacing electrode vector including electrodes 34 and 36. Charging circuit 230 includes a ventricular holding capacitor 236 that may be charged to a ventricular pacing pulse amplitude and discharged via switch 232 and output circuit 234 through the ventricular pacing electrode vector including electrodes 32 and 34. It is to be understood that in some examples, atrial holding capacitor 235 and ventricular holding capacitor 236 may each be a single capacitor or one or both may be a combination of holding capacitors configured for being charged to respective atrial pacing and ventricular pacing pulse amplitudes. While some pacemakers described as examples in conjunction with FIGs. 1-5 above may include at least one additional holding capacitor that can becharged for delivering backup pacing pulses, in some examples, a backup pacing holding capacitor is omitted in pacemaker 14 to facilitate miniaturization of pacemaker 14 for ease of implantation, for example within an atrial heart chamber.

[0084] Switching circuit 232 may control when a holding capacitor 235 or 236 of charging circuit 230 is coupled to the output circuit 234 for delivering an atrial or ventricular pacing pulse, respectively. For example, switching circuit 232 may include a switch that is activated by a timing signal received from pace timing circuit 242 upon expiration of a pacing escape interval and kept closed for a programmed pacing pulse width to enable discharging of a holding capacitor of charging circuit 230. The ventricular or atrial holding capacitor 236 or 235, previously charged to the ventricular or atrial pacing pulse voltage amplitude, respectively, can be discharged across respective ventricular pacing electrodes 32 and 34 or across atrial pacing electrodes 36 and 34 through an output capacitor of output circuit 234 for the programmed pacing pulse duration. While switching circuit 232 and output circuit 234 are shown as being shared circuitry for an atrial pacing channel that includes atrial holding capacitor 235 and a ventricular pacing channel that includes ventricular holding capacitor 236, in other examples each atrial pacing channel and ventricular pacing channel may include separate charging circuitry, switching circuitry and output circuitry that are operated under the control of control circuit 206 for delivering atrial and ventricular pacing pulses, respectively.

[0085] Pacemaker 14 may include one or more sensor(s) 212 for sensing physiological signals which may include cardiac mechanical signal sensors. For example, sensor(s) 212 may include an accelerometer for sensing patient and / or cardiac motion. Sensor(s) 212 may include a single-axis or multi-axis accelerometer for producing acceleration signals in one or more dimensions, which can be used for determining a relative level of patient physical activity. In some examples, pacemaker 14 may be capable of delivering rate response pacing based on a patient physical activity metric determined from an acceleration signal produced by sensor(s) 212. Control circuit 206 may receive a rectified acceleration signal from sensor(s) 212 and determine a patient physical activity metric from the acceleration signal, e.g., by summing acceleration signal sample point amplitudes over an activity metric time interval. The activity metric may be converted to a target heart rate to meet the patient’s metabolic demand. The target heart rate may be converted to a sensor indicated rate (SIR) based on an SIR transfer function that includes a lower rate setpoint and an activities of daily living (ADL) range and a maximum upper rate, for example. During a rate response pacing mode, pulse generator 202 may be controlled by control circuit 206 to deliver atrial or ventricular pacing pulses at a rate response pacing rate determined based on the SIR.

[0086] Additionally or alternatively, sensor(s) 212 may include a pressure sensor, impedance sensor, heart sound sensor, accelerometer or gyroscope or other sensor for sensing cardiac mechanical signals attendant to contraction and / or relaxation of the atria and / or ventricles and / or opening and closing of heart valves. While the illustrative examples described herein refer to cardiac electrical signals being sensed by sensing circuit 204 for pacing Asense and Vsense signals to control circuit 206, it is contemplated that control circuit 206 may be configured to receive one or more signals from sensor(s) 212 producing a signal responsive to cardiac mechanical function and detect ventricular event signals and / or atrial event signals corresponding to ventricular systole or atrial systole, respectively. The ventricular and / or atrial event signals detected from one or more cardiac mechanical signals by control circuit 206 during an atrial backup pacing interval may be used in determining an atrial capture result and controlling backup pacing pulses according to the methods described herein. For example, control circuit 206 may be configured to detect a ventricular event signal from an accelerometer signal and determine an atrial capture result based on the timing of at least the ventricular event signal according to the methods described below.

[0087] Memory 210 may include computer-readable instructions that, when executed by control circuit 206, cause control circuit 206 to perform various functions attributed throughout this disclosure to pacemaker 14. The computer-readable instructions may be encoded within memory 210. Memory 210 may include any non-transitory, computer- readable storage media including 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 other digital media with the sole exception being a transitory propagating signal.

[0088] Memory 210 may store sensed event data based on Vsense signals and Asense signals received from sensing circuit 204 for use in determining an atrial pacing capture result and controlling delivery of backup atrial pacing pulses and ventricular pacing pulses according to the techniques disclosed herein. In some examples, memory 210 includes abuffer that stores one or more episodes of EGM signals received from sensing circuit 204. Memory 210 may store one or more episodes of EGM signals for transmission via telemetry circuit 208 when control circuit 206 determines atrial loss of capture or performs an atrial pacing threshold search.

[0089] Telemetry circuit 208 includes a transceiver 209 and antenna 211 for transferring and receiving data via a radio frequency (RF) communication link. Telemetry circuit 208 may be capable of bi-directional communication with external device 50 (FIG. 1) as described above. Cardiac electrical signals and / or data derived therefrom, atrial and ventricular pacing histories, capture thresholds and the like may be transmitted by telemetry circuit 208 to external device 50. Programmable control parameters and algorithms for sensing cardiac event signals and controlling pacing therapies delivered by pulse generator 202 may be received by telemetry circuit 208 and stored in memory 210 for access by control circuit 206. In some examples, pacemaker 14 may be configured to communicate with a second implantable medical device via telemetry circuit 208 or via tissue conduction communication signals that may be transmitted by pulse generator via electrodes 32, 34 and / or 36. For instance, in the example shown in FIG. 3, two leadless pacemakers 114 and 116 may be configured to communicate with each other via respective telemetry circuits, e.g., via RF communication or tissue conduction communication, for coordinating dual chamber pacing and for communicating the timing of Vsense signals for use in determining an atrial capture result and controlling delivery of a backup atrial pacing pulse according to the techniques disclosed herein.

[0090] Power source 214 provides power to each of the other circuits and components of pacemaker 14 as required. Power source 214 may include one or more energy storage devices, such as one or more rechargeable or non-rechargeable batteries. The connections between power source 214 and other pacemaker circuits and components are not shown in FIG. 6 for the sake of clarity but are to be understood from the general block diagram of FIG. 6. Power source 214 may provide power as needed to pulse generator 202, sensing circuit 204, telemetry circuit 208, memory 210 and sensor(s) 212.

[0091] The functions attributed to pacemaker 14 herein may be embodied as one or more processors, controllers, hardware, firmware, software, or any combination thereof. Depiction of different features as specific circuitry is intended to highlight different functional aspects and does not necessarily imply that such functions must be realized byseparate hardware, firmware or software components or by any particular circuit architecture. Rather, functionality associated with one or more circuits described herein may be performed by separate hardware, firmware or software components, or integrated within common hardware, firmware or software components. For example, a process for confirming a cardiac event signal may be implemented in control circuit 206 executing instructions stored in memory 210 and relying on input from sensing circuit 204. Providing software, hardware, and / or firmware to accomplish the described functionality in the context of any modem medical device, given the disclosure herein, is within the abilities of one of skill in the art.

[0092] FIG. 7 is a flow chart 250 of a method for determining an atrial capture result and controlling backup pacing pulses by a medical device according to some examples. The method of flow chart 250 and other flow charts and timing diagrams described below are described with reference to pacemaker 14 shown in FIG. 1 and FIG. 6 for the sake of convenience. It is to be understood that the process of the flow charts and timing diagrams presented here could be implemented in any of the example medical device systems described above.

[0093] At block 252, pulse generator 202 delivers an atrial pacing pulse (Apace). As described below, the Apace may be a test Apace during a capture management test or a capture threshold search. In other examples, the Apace may be delivered according to a pacing mode and pacing pulse parameters currently in effect for which control circuit 206 may verify capture of the Apace. In response to the delivered Apace, control circuit 206 may schedule a backup Apace at block 254 by starting a backup atrial pacing interval.

[0094] At block 256, control circuit 206 determines if a Vsense signal is received prior to the backup atrial pacing interval expiration. If not, pulse generator 202 may deliver the backup Apace at block 258 and determine loss of capture at block 260. Control circuit 206 may control pulse generator to deliver a ventricular pacing pulse at an AV pacing interval following the Apace at block 258.

[0095] If a Vsense signal is received by control circuit 206 prior to expiration of the backup atrial pacing interval at block 256, control circuit 206 may cancel the scheduled backup Apace at block 262. Control circuit 206 may determine an atrial capture result at block 264 based on at least the time of the Vsense signal, e.g., the time interval from the delivered Apace to the Vsense signal. As further described below control circuit 206 maydetermine the capture result by applying at least a capture verification window that expires prior to the backup atrial pacing interval to the Apace- Vsense interval for verifying capture of the Apace. In some examples, if an Asense signal is received by control circuit 206 prior to the Vsense signal during the backup atrial pacing interval, control circuit 206 may determine the capture result based on at least the time of the Vsense signal relative to the Apace (e.g., Apace-Vsense interval) and the time of the Vsense signal relative to the Asense signal (e.g., Asense-Vsense interval). Control circuit 206 may perform a response to the capture result as further described below in conjunction with the flow charts of FIGs. 8 and 11.

[0096] FIG. 8 is a flow chart 300 of a method that can be performed by pacemaker 14 for determining an atrial capture result and controlling delivery of backup atrial pacing pulses. FIG. 9 is a timing diagram 400 of atrial and ventricular pacing pulses and various time intervals that may be applied by control circuitry of pacemaker 14 for performing the method of FIG. 8.

[0097] Referring to FIG. 8, at block 302, control circuit 206 controls pulse generator 202 to generate and deliver an Apace. The Apace may be a test pulse delivered to confirm atrial capture during a capture test. In other instances, the Apace may be delivered during an atrial capture threshold search. In some cases, the Apace may be delivered according to a programmed pacing mode, e.g., as a bradycardia pacing pulse delivered at programmed atrial lower rate interval. The Apace may be delivered at a shortened pacing interval, in some cases, e.g., shorter than a sensed intrinsic atrial rate, to promote delivery of the Apace earlier than an intrinsic atrial depolarization. Control circuit 206 may be configured to verify capture of a delivered Apace and schedule a backup Apace according to the method of FIG. 8 on a beat-by-beat or less frequent basis in various examples. For instance, a capture test may be performed once per minute, once per hour, once per day or other scheduled frequency. An atrial capture threshold search may be performed daily or according to a specified schedule and may be performed in response to control circuit 206 detecting loss of capture during a capture test. An atrial capture threshold search may be performed upon receiving a user-entered command via telemetry circuit 208, e.g., during a telemetry session with external device 50.

[0098] At block 304, control circuit 206 may schedule a backup Apace by starting a backup atrial pacing interval in response to the pulse generator 202 delivering the atrialpacing pulse at block 302. Control circuit 206 may determine an atrial capture result based on if and when a Vsense signal is received after the Apace during the backup atrial pacing interval. It is noted that when the current pacing mode is an AV synchronous dual chamber mode, in order to determine a capture result of the Apace delivered at block 302 based on when and if a Vsense signal is received from sensing circuit 204 after the Apace, a ventricular pacing pulse is not scheduled at an AV pacing interval following the Apace. Control circuit 206 may not start an AV pacing interval when the Apace is delivered at block 302, the pacing mode may be temporarily changed from the AV synchronous dual chamber mode to prevent delivery of an AV synchronous ventricular pacing pulse, or control circuit 206 may start an extended AV pacing interval upon delivery of the Apace at block 202 that is longer than the backup atrial pacing interval (as described below in conjunction with FIG. 9).

[0099] Control circuit 206 may determine a time interval from the Apace delivered at block 302 to a Vsense received during the backup atrial pacing interval and compare this Apace- Vsense time interval to a capture verification window that expires earlier than the backup atrial pacing interval. With reference to FIG. 9, the backup atrial pacing interval 404 may be started by control circuit 206, e.g., by setting a pacing escape interval timer or counter, upon delivery of Apace 402. Control circuit 206 may apply the capture verification window 416 during the backup atrial pacing interval 404.

[0100] Capture verification window 416 may have a start time 414 and an end time 420, that are both earlier than the expiration time 406 of backup atrial pacing interval 404. As such, in some examples, control circuit 206 may start multiple timers or counters (for counting clock cycles) simultaneously upon delivery of the Apace 402 (or sequentially in the order of the start / end times of the various timing intervals) to control delivery (or withholding) of the backup atrial pacing pulse 430 and for determining if Apace 402 captured the atria. For example, a capture verification start time interval 412, also referred to herein as “indeterminate time interval” 412, a capture verification end time interval 418 and the backup atrial pacing interval 404 may each be started by control circuit 206 upon delivery of Apace 402.

[0101] In other examples, control circuit 206 may start a timer or counter upon delivering Apace 402 to measure the paced AV time interval from Apace 402 until a Vsense signal is received. Control circuit 206 may start the backup atrial pacing interval 404 that may beterminated upon receiving an Asense or a Vsense. The backup Apace 430 may be canceled if an Asense or Vsense is received before the backup atrial pacing interval 404 expires. If an Asense is received by control circuit 206 from sensing circuit 204 before the backup atrial pacing interval 404 expires and before a Vsense is received, the time expired of the backup atrial pacing interval 404 may be determined as the PPI from the Apace 402 to the Asense. If a Vsense is received before an Asense and before the backup Apace 430 is delivered, the time expired of the backup atrial pacing interval 404 may be determined by control circuit 206 as the paced AV interval from the Apace to the Vsense. This paced AV interval may be compared to various timing thresholds, e.g., the start time 414 and end time 420 of capture verification window 416 for determining the atrial capture result.

[0102] The backup atrial pacing interval 404 may be at least 350 ms, at least 400 ms, or at least 450 ms as illustrative examples. In one example, the backup atrial pacing interval 404 is 420 ms such that if no Asense or Vsense signal is received prior to the expiration time 406 of backup atrial pacing interval 404, the backup Apace (BAP) 430 is delivered by pulse generator 202.

[0103] The capture verification window 416 may have a start time 414 that is between 80 ms and 180 ms after Apace 402, as examples with no limitation intended. In some examples, control circuit 206 may determine the capture verification window start time 414 based on a previously measured AV conduction time in the patient (e.g., a measured Apace- Vsense interval or an Asense- Vsense interval). The capture verification window start time 414 may be, for example, a measured AV conduction time less an offset, e.g., minus 20 to 50 ms. In one example, the start time 414 is about 24 ms less than a measured AV conduction time.

[0104] The capture verification window 416 may have an end time 420 that is between 180 and 330 ms as examples, with no limitation intended. The capture verification window end time 420 may be determined by control circuit 206 based on a previously measured AV conduction time. For example, the end time 420 may equal the previously measured AV conduction time plus an offset of 10 to 80 ms or about 24 ms greater than the measured AV conduction time in an example. Using the example of the start time 414 being 24 ms less than the measured AV conduction time and the end time 420 being 24 ms greater than the measured AV conduction time, the capture verification window 416 is 48 ms in duration. In some examples, the capture verification window 416 may have anadjustable start time 414 based on the pacing rate that Apace 402 is delivered at because the AV conduction time can be heart rate dependent.

[0105] In some examples, control circuit 206 may start a post-atrial ventricular blanking period 410 upon delivery of Apace 402 by pulse generator 202. Post-atrial ventricular blanking period 410 may be 20 to 100 ms or about 30 to 60 ms as examples. The ventricular sensing channel 205 of sensing circuit 204 may be disabled during post-atrial ventricular blanking period 410 or any Vsense signals received during the post-atrial ventricular blanking period 410 may be ignored by control circuit 206. This post-atrial ventricular blanking period 410 may prevent oversensing of atrial pacing artifact in the sensed cardiac electrical signal from being falsely sensed as an R-wave.

[0106] Control circuit 206 may start post-atrial pacing blanking period 408 in response to the delivered Apace 402. The atrial sensing channel 203 of sensing channel 204 may be disabled or blanked during the post-atrial pacing blanking period 408 to avoid oversensing of post-atrial pacing signal artifact. In some examples, an atrial refractory period may be applied by control circuit 206, which may be equal to or greater than the post-atrial pacing blanking period. Any Asense signal received by control circuit 206 during the atrial refractory period may be ignored by control circuit 206, at least for purposes of scheduling an atrial or ventricular pacing pulse. In some examples, an Asense signal received by control circuit 206 during the atrial refractory period may cause control circuit 206 to cancel the backup Apace 430. As further described below, control circuit 206 may still wait for a Vsense signal, however, for determining the atrial capture result.

[0107] In some examples, control circuit 206 may start a ventricular pacing interval 432 in response to the delivered Apace 402 to schedule a ventricular pacing pulse (Vpace) 434 for delivery by pulse generator 202 if a Vsense signal is not received before the expiration time 406 of backup atrial pacing interval 404. As will be described below, in some instances, an Asense signal may be received by control circuit 206 during the backup atrial pacing interval 404, but a Vsense signal may not be received during the backup atrial pacing interval 404. Control circuit 206 may schedule the Vpace 434 at the expiration of the ventricular pacing interval 432 to avoid ventricular asystole. In other examples, control circuit 206 may start an AV pacing interval 436 upon expiration of the backup atrial pacing interval 404 if a Vsense signal has not been received during the backup atrial pacing interval, whether or not the backup Apace 430 is delivered.

[0108] Referring again to FIG. 8 with continued reference to FIG. 9, control circuit 206 may wait for the backup atrial pacing interval 404 to expire at block 308 while monitoring for a Vsense signal (block 312) or an Asense signal (block 310) from sensing circuit 204 prior to the backup atrial pacing interval 404 expiring. If a Vsense is received at block 312 before the backup atrial pacing interval 404 expires, control circuit 206 may cancel the scheduled backup Apace 430 at block 330. At block 332, control circuit 206 may determine the capture result of the Apace delivered at block 302 based on the timing of the Vsense signal. For example, if the Vsense signal is received during the capture verification window 416 (see FIG. 9), control circuit 206 may determine that the Apace 402 successfully captured the atria as evidenced by the conducted depolarization to the ventricles, resulting in the Vsense signal during the expected AV conduction time range represented by capture verification window 416. Control circuit 206 may respond to the capture determination at block 334 by continuing delivery of atrial pacing at the current atrial pacing pulse output. If control circuit 206 is performing an atrial capture threshold search, however, control circuit 206 may adjust the atrial pacing pulse output and deliver another Apace at block 302 at the adjusted atrial pacing pulse output until the lowest atrial pacing pulse output at which atrial capture is still detected is identified by control circuit 206 as the atrial pacing capture threshold.

[0109] If the Vsense signal is received at block 312 earlier than the capture verification window 416, e.g., during the indeterminate time interval 412 that expires upon the start time 414 of capture verification window 416, the Vsense signal is received too early to be caused by conduction of a pacing evoked atrial depolarization associated with Apace 402. In this case, control circuit 206 may determine that the capture result is indeterminate at block 332 of FIG. 8. Control circuit 206 may respond to the indeterminate capture result at block 334 by delivering another Apace at the same atrial pacing output at block 302 and repeating the process of flow chart 300. In some examples, control circuit 206 may respond to the indeterminate capture result by aborting or delaying subsequent capture determinations, e.g., by aborting or delaying an atrial capture management test or atrial capture threshold search, when the indeterminate capture result is the nth indeterminate capture result for the same Apace output, where n may be a specified threshold value, e.g., 1, 2, 3, 4, or 5 as examples.

[0110] If the Vsense signal is received at block 308 after the capture verification window 416, e.g., an Apace-Vsense interval that is longer than the capture verification window end time 420, control circuit 206 may determine that the Apace 402 delivered at block 302 failed to capture the atria. Control circuit 206 may determine a loss of capture at block 332. In some examples, a second indeterminate time window may be included after the capture verification window 416 as further described in conjunction with FIG. 10 below. In the example shown in FIG. 9, if the Vsense signal is received after the end time 420 of capture verification window 416, control circuit 206 determines loss of capture at block 332 of FIG. 8. The time interval 422 from the capture verification window end time 420 to the expiration of the ventricular pacing interval 432 or AV pacing interval 436, whichever is used by control circuit 206 for scheduling Vpace 434, may be referred to as a “loss of capture” window. A Vsense signal received by control circuit 206 during the loss of capture window 422 may be received too late after Apace 402 to be a conducted, pacing- evoked atrial depolarization associated with Apace 402.

[0111] If a Vsense signal is received by control circuit 206 prior to the expiration time 406 of the backup atrial pacing interval 404, however, control circuit 206 may cancel the scheduled backup Apace 430 and does not schedule the Vpace 434 by not starting AV pacing interval 436. In other examples, if control circuit 206 starts ventricular pacing interval 432 upon Apace 402 delivery, control circuit 206 may terminate the ventricular pacing interval 432 and cancel the scheduled Vpace 434.

[0112] If the backup atrial pacing interval 404 expires (“yes” branch of block 308 in FIG. 8) without a Vsense signal (“no” branch of block 312) or an Asense signal (“no” branch of block 310) being received by control circuit 206, pulse generator 202 may deliver the backup Apace 430 (block 314) and schedule Vpace 434 at the AV pacing interval 436 from the backup Apace 430 (or allow the ventricular pacing interval 432 to continue running if previously started upon delivery of Apace 402). It is noted that control circuit 206 may control charging of an atrial holding capacitor 235 of pulse generator 202 (see FIG. 6) to an atrial pacing pulse amplitude to deliver the Apace 402 and recharging of the atrial holding capacitor 235 by pulse generator 202 to a backup atrial pacing pulse amplitude during the backup atrial pacing interval 404. The backup atrial pacing pulse amplitude (of backup Apace 430) may be greater than the pacing pulse amplitude of Apace 402. Theatrial holding capacitor 235 may be discharged to deliver backup Apace 430 at the expiration time 406 of backup atrial pacing interval 404.

[0113] The backup Apace 430 may be delivered by pulse generator 202 at a maximum programmable atrial pacing pulse output or at the currently programmed atrial pacing pulse output plus an offset (e.g., plus 2, 2.5, or 3.0 volts) up to the maximum available atrial pacing pulse output. In still other examples, the backup Apace 430 may be delivered by pulse generator 202 at a user programmable backup atrial pacing pulse output. In still other examples, the back Apace 430 may be delivered by pulse generator 202 at the lower one of: a user programmed backup atrial pacing pulse output or the currently programmed atrial pacing pulse output plus a specified offset. The atrial holding capacitor 235 may be recharged for delivering a next Apace (which may be at a lower pulse amplitude than the backup Apace 430) after the backup Apace 430 is delivered at block 314 of FIG. 8.

[0114] The loss of capture window 422 may extend from the end time 420 of the capture verification window 416 until a Vsense signal is received or the Vpace 434 is delivered, whichever comes first. If a Vsense signal is received during the loss of capture window 422 (“yes” branch of block 308 of FIG. 8) before the backup atrial pacing interval 404 expires, the backup Apace may be cancelled (block 330) and loss of capture is detected at block 332 based on the Vsense signal timing in the loss of capture window 422. If a Vsense signal is received during the loss of capture window 422 after the backup Apace 430 is delivered, e.g., during the AV pacing interval 436 (or prior to expiration of the ventricular pacing interval 432 if running), control circuit 206 may detect loss of capture by Apace 402 and cancel delivery of the scheduled Vpace 434.

[0115] Referring again to FIG. 8 with continued reference to FIG. 9, if no Asense or Vsense is received from sensing circuit 204 prior to the backup atrial pacing interval expiring at block 308, pulse generator 202 delivers the scheduled backup atrial pacing pulse 430 at block 314. At block 318, control circuit 206 may schedule Vpace 434. Control circuit 206 may schedule Vpace 434 by starting AV pacing interval 436 upon delivery of the backup Apace 430 at block 314. In other examples, ventricular pacing interval 432, set to be a desired AV pacing interval longer than the backup atrial pacing interval 404, may continue to run. The AV pacing interval 436 or that added to the backup atrial pacing interval 404 for scheduling a Vpace 434 may be 50 to 150 ms or 60 to 100 ms and can be 80 ms as examples. If no Vsense signal is received at block 320 prior to expiration of theAV pacing interval 436 (or ventricular pacing interval 432), pulse generator 202 may deliver the scheduled Vpace 434 (block 324), upon expiration of the AV pacing interval 436 (or ventricular pacing interval 432). In other examples, instead of cancelling the Vpace at block 322 in response to a Vsense signal received at block 320 during the AV pacing interval 436, pulse generator 202 may be controlled to deliver the scheduled Vpace at the AV pacing interval 436 (regardless of whether a Vsense signal is received during the AV pacing interval 436). In this instance, the Vpace may be delivered during the absolute refractory period of the ventricular tissue following the intrinsic R-wave sensed by sensing circuit 204 and is unlikely to capture the ventricles.

[0116] When no Asense or Vsense is received by control circuit 206 from sensing circuit 204 prior to expiration of the backup atrial pacing interval 404, control circuit 206 determines loss of capture at block 326. When the backup Apace is delivered at block 314 due to no Asense signal or Vsense signal during the backup atrial pacing interval 404, and a Vpace is delivered at block 324 (whether or not a Vsense signal is received during the AV pacing interval 436), control circuit 206 may determine loss of capture at block 326. Control circuit 206 may perform a response to determining loss of capture at block 334.

[0117] The response may be to increase the atrial pacing pulse output. The response may be to trigger an atrial pacing capture threshold search. If the process of flow chart 300 is being performed as part of an atrial pacing capture threshold search, control circuit 206 may increase the atrial pacing pulse output and deliver another Apace at block 302 to search for the lowest atrial pacing pulse output for which capture is verified. In some instances, control circuit 206 may perform the atrial pacing capture threshold search by starting with a relatively high atrial pacing pulse output and decreasing the atrial pacing pulse output until loss of capture is detected. In this case, when loss of capture is detected at block 326 (or block 332 based on the timing of a received Vsense signal), control circuit 206 may determine the atrial pacing capture threshold to be the preceding, higher atrial pacing pulse output that resulted in atrial capture. Control circuit 206 may set the atrial pacing pulse output to a safety margin (e.g., 0.25 to 2.0 V) greater than the atrial pacing capture threshold at block 334. Control circuit 206 may control pulse generator 202 to deliver atrial pacing according to a programmed pacing mode using the atrial pacing pulse output set based on the atrial pacing capture threshold at block 334.

[0118] In some instances, an Asense signal may be received by control circuit 206 during the backup atrial pacing interval 404 without ever receiving a Vsense signal. The Asense signal can be evidence of atrial loss of capture or could be a premature atrial contraction. If no Vsense is received (“no” branch of block 312) prior to an Asense signal received at block 310, control circuit 206 may cancel the backup Apace at block 316 in response to the Asense signal and schedule a Vpace at block 318 (or allow ventricular pacing interval 432 to continue running if previously started). In some examples, control circuit 206 may schedule the Vpace to occur at an AV pacing interval 436 after expiration of the backup atrial pacing interval 404 as shown in FIG. 9 even though the backup Apace is cancelled. In other examples, control circuit 206 may schedule the Vpace to occur at the expiration time 406 of the backup atrial pacing interval 404. In still other examples, the Vpace could be scheduled at an AV pacing interval from the Asense signal.

[0119] If control circuit 206 does not receive a Vsense signal at block 320 prior to the scheduled time of the Vpace, pulse generator 202 may deliver the scheduled Vpace 434 (at block 324). Control circuit 206 may determine loss of capture at block 326 when no Vsense signal is received and perform a loss of capture response at block 334 as described above.

[0120] If a Vsense signal is received by control circuit 206 at block 320 before the scheduled time of the Vpace, however, control circuit 206 may cancel the scheduled Vpace at block 322. Control circuit 206 may determine the capture result at block 332 based on the time of the Vsense signal and, at least in some examples, based on the relative timing of the Asense and Vsense signals (received at block 310 and 320, respectively).

[0121] Control circuit 206 may determine the Asense- Vsense time interval and the Apace- Vsense time interval at block 332. If the Asense-Vsense time interval is less than a minimum AV conduction time threshold, control circuit 206 may determine the capture result based on the Vsense time interval. The Asense signal may be an oversensed far-field R-wave. In this situation, the Vsense signal may be associated with a conducted depolarization following the Apace 402 when the Asense-Vsense time interval is shorter than the minimum AV conduction time threshold. If the Vsense signal is received during the capture verification window 416 and less than the minimum AV conduction time from an Asense signal, control circuit 206 may determine atrial pacing capture. If the Vsensesignal is received before the capture verification window 416, the capture result may be indeterminate. If the Vsense signal is received after the capture verification window 416, control circuit 206 may determine loss of capture. Control circuit 206 may perform a response to the determined atrial capture result at block 334 according to any of the examples given above.

[0122] If the Asense- Vsense time interval is at least the minimum AV conduction time threshold, however, the Vsense signal may be associated with the Asense signal, as a conducted depolarization following the intrinsic atrial P-wave, as opposed to being associated with the Apace 402. The capture result may be determined based on the timing of both the Asense and Vsense signals. Control circuit 206 may determine the capture result to be indeterminate when the Vsense signal is received before the loss of capture window 422 (or loss of capture window 452 as shown in FIG. 10 and described below) and at least the minimum AV conduction time threshold after an Asense signal. A Vsense signal received any time before the loss of capture window 422 that follows an Asense signal by at least the minimum AV conduction time may be a premature atrial contraction that is conducted to the ventricles, for example. As such, the capture result can be indeterminate at block 328.

[0123] If the Asense- Vsense time interval is at least the minimum AV conduction time threshold and the Vsense signal is received by control circuit 206 during the loss of capture window 422 (or loss of capture window 452 shown in FIG. 10), control circuit 206 may determine the atrial capture result as being loss of capture at block 328. In this case, the relatively late Asense signal followed by a Vsense signal that meets the minimum AV conduction time may be an indication that the Apace 402 failed to capture and an intrinsic beat has occurred. Control circuit 206 may perform the response to the determined capture result at block 334 according to any of the examples described above.

[0124] FIG. 10 is a diagram 450 of timing intervals that may be controlled by control circuit 206 for scheduling a backup atrial pacing pulse, scheduling a ventricular pacing pulse and determining an atrial pacing capture result after pacemaker 14 delivers an Apace according to another example. Identically numbered elements shown in FIG. 10 correspond to like-numbered elements shown in FIG. 9 and described above. In this example, however, control circuit 206 may set a loss of capture window start time interval 440 upon delivery of Apace 402. The expiration time 456 of the loss of capture windowstart time interval 440 marks the start time of loss of capture window 452. In other examples, control circuit 206 starts one or more timers upon delivery of Apace 402 to time out an Apace- Vsense interval and an Apace- Asense interval and / or an Asense-Vsense interval (as a difference between the former two intervals). The loss of capture start time interval 440 may be the threshold time interval after which control circuit 206 may determine a loss of capture result in response to a Vsense signal received after the threshold time interval (equal to start time 456 of the loss of capture window 452). Instead of the loss of capture window 452 starting at the end time 420 of capture verification window 416 as in the example of FIG. 9, the capture verification window 416 and the loss of capture window 452 are separated by a second indeterminate window 454 in this example. The loss of capture window 452 may extend from the loss of capture window start time 456 until a Vsense is received or until a Vpace 434 is delivered (e.g., at an AV pacing interval 436 after the expiration time 406 of the backup atrial pacing interval 404), whichever comes first.

[0125] The time interval 454 between the end time 420 of capture verification window 416 and the loss of capture window start time 456 may referred to as a second “ignore” window or second “indeterminate” window. When a Vsense signal is received during second indeterminate time interval 454, control circuit 206 may ignore the Vsense signal for the purposes of determining the capture result. The timing of a Vsense signal in second indeterminate time interval 454 may be non-specific to confidently determining capture or loss of capture by the Apace 402. In some cases, heart rate dependent AV block can result in varying AV conduction times. A Vsense signal in the indeterminate window 454 may be a conducted depolarization resulting from the Apace 402 but at a relatively long AV conduction time due to heart rate dependent AV block. In other instances, a Vsense signal in the second indeterminate time interval 454 may be unrelated to the Apace 402. As such, to account for instances of rate dependent AV block, the indeterminate time interval 454 may be applied by control circuit 206 to the time of a Vsense signal to prevent a false loss of capture determination due to a prolonged AV conduction time. A false loss of capture determination in this situation may lead to an unnecessary increase in atrial pacing pulse output.

[0126] As such, in the example of FIG. 10, two windows of time, first indeterminate time interval 412 and second indeterminate time interval 454, may be applied by control circuit206 as indeterminate windows in that a Vsense signal received during either of these time intervals 412 or 454 may result in an unknown or indeterminate capture result. Control circuit 206 may control the pulse generator 202 to repeat delivery of the Apace at the same pacing pulse output when the capture result is indeterminate as a second attempt at determining capture or loss of capture based on the timing of a Vsense signal in either the capture verification window 416 or the loss of capture window 452. A Vsense signal received by control circuit 206 at any time during the backup atrial pacing interval 404 may result in cancellation of the scheduled backup Apace 430 and the Vpace 434.

[0127] As described above, if an Asense signal and a Vsense signal are received during the backup atrial pacing interval 404 and the Asense-Vsense interval is less than a minimum AV conduction time, control circuit 206 may determine the capture result based on the time of the Vsense signal according to the indeterminate time intervals 412 and 454, capture verification window 416 and loss of capture window 452. The Asense signal that precedes the Vsense signal by less than the minimum AV conduction time may be a far- field R-wave that is oversensed by the sensing circuit 204 as a false P-wave. If the Asense- Vsense interval is at least the minimum AV conduction time, control circuit 206 may determine the capture result as indeterminate if the Vsense signal is received before the loss of capture window start time 456 and determine loss of capture if the Vsense signal is received in the loss of capture window 452.

[0128] FIG. 11 is a flow chart 600 of a method for performing an atrial capture threshold search according to some examples. At block 602, control circuit 206 may confirm a stable atrial rate by performing an atrial rate stability check. Control circuit 206 may verify that the atrial rate is stable prior to delivering the first test Apace of the atrial pacing capture threshold search. For example, control circuit 206 may determine if a specified number of consecutive atrial cycles are within a threshold interval of each other and that no premature atrial contractions and no premature ventricular contractions are sensed. Control circuit may determine a premature atrial contraction is sensed, for example, when an Asense signal is received at a short PPI interval from a preceding Asense signal or from a delivered Apace with no intervening Vpace or Vsense signal (in the short PPI). Control circuit 206 may detect a premature ventricular contraction when a Vsense signal is received at a short RRI interval from a preceding Vsense signal or delivered Vpace with no intervening Apace or Asense signal in the short RRI. Control circuit 206 may verifythat the atrial rate is a stable rate for at least 6 to 12 atrial cycles or 8 atrial cycles as examples. It is to be understood that if the atrial rate stability check fails, e.g., irregular atrial intervals and / or premature beats are detected, the atrial pacing capture threshold search may be aborted or delayed until the atrial rate stability check does not fail at block 602.

[0129] At block 604, control circuit 206 may confirm that AV conduction is intact by performing an AV conduction stability check. For example, control circuit 206 may withhold ventricular pacing (e.g., by extending the AV pacing interval or changing pacing mode) and measure the AV conduction time from an Asense signal or delivered Apace to the subsequent Vsense for a specified number of atrial cycles. In some examples, control circuit 206 may control the pulse generator 202 to deliver one or more “support” Apaces prior to the test Apace to promote a stable paced rate and confirmed, stable AV conduction prior to a test Apace being delivered. The one or more support Apaces may be delivered at a high pacing output (e.g., higher pulse amplitude and / or pulse width than the subsequent test Apace) to promote capture and rate support with AV conduction stability being confirmed prior to the test Apace delivered at block 605. If the AV conduction times determined following a specified number of consecutive atrial events (e.g., Apaces), are within a threshold range of each other and / or meet other stability criteria, control circuit 206 may confirm AV conduction stability at block 604. It is to be understood that if AV conduction stability is not confirmed at block 604, control circuit 206 may abort or delay the atrial pacing capture threshold search until AV conduction stability criteria are met at block 604 (and atrial rate stability criteria are met at block 602).

[0130] When atrial rate stability and AV conduction stability criteria are met at blocks 602 and 604 respectively, control circuit 206 may control pulse generator 202 to deliver the first test Apace of the pacing threshold search at block 605. At block 606, control circuit 206 schedules the backup Apace by starting a backup atrial pacing interval as described in the examples given above. The backup atrial pacing interval is set to expire later than a capture verification window that may be applied to the time of a Vsense signal received by control circuit 206 during the backup atrial pacing interval for determining an atrial capture result according to any of the examples described above. Control circuit 206 may schedule a Vpace at block 606 when the test Apace is delivered, e.g., by starting aventricular pacing interval that is an AV pacing interval, e.g., 80 ms, longer than the backup atrial pacing interval.

[0131] At block 608, control circuit 206 waits to receive a Vsense signal from sensing circuit 204. If a Vsense signal is received prior to the expiration of the backup atrial pacing interval as determined at block 608, control circuit 206 may cancel the scheduled backup Apace at block 610. If a Vpace is already scheduled by starting the ventricular pacing interval that is an AV pacing interval longer than the backup atrial pacing interval, the Vpace may be cancelled by control circuit 206. If a Vpace has not been scheduled, control circuit 206 does not schedule a Vpace because the Vsense signal was received, inhibiting delivery of a Vpace. Control circuit 206 advances to block 630 to determine the atrial capture result based on at least the time of the Vsense signal according to any of the examples given above. For example, control circuit 206 may start a timer upon delivering the test Apace at block 605. The value that the timer has reached when the Vsense signal is received may be compared to the capture verification window start time, the capture verification window end time, and the loss of capture window start time. If the Vsense signal occurs before the capture verification window start time or between the capture verification window end time and the loss of capture start time, the capture result may be indeterminate at block 630. Control circuit 206 may repeat the test Apace at the same pulse output by advancing to block 632 (determining that another test Apace is needed) and returning to block 604 (to reconfirm AV conduction stability by delivering a specified number of support Apaces at a higher pulse output) without performing a test Apace pulse amplitude or pulse width adjustment. In other examples, control circuit 206 may return to block 602 to reconfirm a stable heart rhythm and confirm AV conduction stability (block 604) before the next test Apace. Another test Apace may be delivered at block 605 at the same pulse output when the capture result is indeterminate.

[0132] If the Vsense signal received at block 608 occurs on or after the capture verification window start time and before or on the capture verification window end time, control circuit 206 may determine atrial capture at block 630. Control circuit 206 may advance to block 632 to determine if another test pulse is needed. Control circuit 206 may control pulse generator 202 to successively decrease the pacing pulse amplitude or pulse width as every n test Apaces are delivered (where n may be 1, 2, 3, 5 or other specified number). In some cases, the capture result is determined for at least 2 Apaces delivered atthe same pacing pulse output until at least a threshold number of either capture or loss of capture results match. When loss of capture is detected for a given Apace pulse output, the preceding higher pulse output is identified as the atrial pacing capture threshold. In other examples, control circuit 206 may control pulse generator 202 to successively increase the pacing pulse amplitude or width as test Apaces are delivered so that when capture is detected the first time (for at least a specified number of test Apaces at the same pulse output), the corresponding pacing pulse output can be identified as the atrial pacing capture threshold by control circuit 206. Control circuit 206 may determine that another test Apace is needed at block 630 until the lowest test Apace pulse output is determined to capture the atria for at least a specified threshold number of test Apaces (which may be 1 or more test Apaces).

[0133] If a Vsense is not received before the expiration of the backup atrial pacing interval (“no” branch of block 608), and no Asense is received during the backup atrial pacing interval (“no” branch of block 612), pulse generator 202 may deliver the backup Apace at block 614. If an Asense is received during the backup atrial pacing interval (“yes” branch of block 612), the Apace may be cancelled at block 615. Control circuit 206 may schedule a Vpace at block 616 (if not already scheduled). In some examples, the Vpace is scheduled at an AV pacing interval after the scheduled (but cancelled) backup Apace. The Vpace may be scheduled at the AV pacing interval, e.g., 80 ms, after the expiration of the backup atrial pacing interval. If a Vsense signal is not received before the scheduled time of the Vpace (“no” branch of block 618), pulse generator 202 may deliver the scheduled Vpace at block 620. Control circuit 206 may determine atrial loss of capture at block 622 when no Vsense signal is received. Control circuit 202 may advance to block 632 to determine if another test Apace is needed according to the capture threshold test protocol.

[0134] If a Vsense signal is received at block 618 before the expiration of the AV pacing interval that may be started upon expiration of the backup atrial pacing interval (even though no Apace is delivered due to the Asense signal), the scheduled Vpace may be cancelled at block 624. In other examples, the scheduled Vpace may still be delivered because it will be delivered during the absolute refractory period from the Vsense signal without capturing the ventricles. The scheduled time of the Vpace may define the expiration of the loss of capture window as described above in conjunction with FIGs. 9 and 10. As such, when a Vsense occurs prior to the scheduled Vpace during the loss ofcapture window, control circuit 206 determines atrial loss of capture at block 622. As described above in conjunction with FIG. 8, if the Vsense signal is received at block 618 earlier than the loss of capture window, control circuit 206 may determine the capture result at block 630 based on the time of the Vsense signal and / or the Asense- Vsense interval. For example, if the Vsense signal time is before the loss of capture window and is less than a minimum AV conduction time, the Asense signal may be an oversensed far- field R-wave and the Vsense signal may be a valid R-wave conducted in response to the test Apace. Thus the Vsense signal time relative to the Apace (Apace- Vsense interval) and relative to any Asense signal sensed prior to the Vsense signal (Asense- Vsense signal) may be used by control circuit 206 at block 630 to determine the capture result.

[0135] After determining the capture result at block 630 as one of capture, loss of capture or indeterminate, control circuit 206 determines whether another test Apace is needed at block 632. If not, control circuit 206 can identify the lowest test Apace output that resulted in atrial capture as the atrial pacing capture threshold at block 634. Control circuit 206 may set the atrial pacing pulse output to a safety margin greater than the capture threshold at block 634 (e.g., 0.25 to 2.0 Volts greater than the capture threshold). After completing the atrial pacing capture threshold search, control circuit 206 may control pulse generator 202 to deliver atrial (and ventricular) pacing according to the programmed (or temporary) pacing mode for delivering a cardiac pacing therapy using the atrial pacing pulse output selected at block 634.

[0136] FIG. 12 is a timing diagram 700 of time intervals that may be controlled by control circuit 206 for scheduling a backup atrial pacing pulse, scheduling a ventricular pacing pulse and determining an atrial pacing capture result after pacemaker 14 delivers an Apace according to yet another example. Identically numbered elements shown in FIG. 12 correspond to like-numbered elements shown in FIG. 10 as described above. In this example, however, control circuit 206 may start a ventricular safety pace (VSP) window 460 upon delivery of Apace 402. The VSP window 460 may be longer than the post-atrial ventricular blanking period 410. The VSP window 460 may be shorter than or equal to the indeterminate time interval 412.

[0137] If control circuit 206 receives a Vsense signal from sensing circuit 204 during the VSP window 460, control circuit 206 may deliver a VSP pulse 464 upon the expiration of the VSP window 460. The backup Apace 430 and Vpace 434 (scheduled at an AV pacinginterval 436 after the scheduled time of backup Apace 430) may both be cancelled by control circuit 206 in response to pulse generator 202 delivering the VSP pulse 464. Because the Vsense received during the VSP window 460 falls within the indeterminate time interval 412, control circuit 206 may determine the atrial pacing capture result as being indeterminate. If a Vsense is not received by control circuit 206 during the VSP window 460, control circuit 206 may cancel or withhold delivery of the VSP pulse 464 by pulse generator 202. Control circuit 206 may monitor for any Asense and Vsense signals received during the backup atrial pacing interval 404 for determining the atrial capture result and controlling delivery of backup Apace 430 and Vpace 434 according to any of the methods described above.

[0138] FIG. 13 is a flow chart 750 of a method that may be performed by pacemaker 14 for determining an atrial capture result and controlling atrial and ventricular pacing pulses according to another example. At block 752, pulse generator 202 delivers an atrial pacing pulse (Apace). As described above, the Apace may be a test Apace during a capture management test or a capture threshold search. In other examples, the Apace may be delivered according to a pacing mode and pacing pulse parameters currently in effect, for which control circuit 206 may verify capture of the Apace. In response to the delivered Apace, control circuit 206 may schedule a backup Apace at block 754 by starting a backup atrial pacing interval.

[0139] At block 756, control circuit 206 may start a VSP window, e.g., VSP window 460 shown in FIG. 12. At block 758, control circuit 206 may wait for the VSP window to expire while monitoring for a Vsense signal (and Asense signal if the post- atrial pacing blanking period is expired). If a Vsense signal is received during the VSP window, control circuit 206 may control pulse generator 202 to deliver the VSP pulse at block 760 upon expiration of the VSP window. In this way, pacemaker 14 may reduce the likelihood of ventricular asystole in the case of the Vsense signal being an oversensed (false) signal.The VSP pulse will capture the ventricles if the Vsense signal is a false signal. If, however, the Vsense signal received during the VSP window is a true R-wave, the VSP pulse will be delivered during the physiological absolute refractory period of the ventricles and fail to capture the ventricles. When the VSP pulse is delivered at block 760, control circuit 206 may cancel the backup Apace at block 762 and withhold or cancel a Vpace that could be scheduled at an AV pacing interval after the backup Apace.

[0140] At block 764, control circuit 206 may determine the atrial capture result as being indeterminate when the Vsense signal is received during the VSP window and the VSP pulse is delivered. The Vsense signal received during the VSP window is received during the indeterminate time interval 412 (see FIG. 12) when the VSP window 460 is equal to or shorter than the indeterminate time interval 412. Control circuit 206 may perform a response to the indeterminate capture determination at block 764 according to any of the examples described above.

[0141] Referring again to block 758, if the VSP window expires without a Vsense signal being received from sensing circuit 204, control circuit 206 may control pulse generator 202 to withhold the VSP pulse at block 759. Control circuit 206 may advance to block 766 for determining if a Vsense signal is received during the backup atrial pacing interval (after the VSP window). If a Vsense signal is not received during the backup atrial pacing interval, control circuit 206 may control pulse generator 202 to deliver the backup Apace at block 768 upon expiration of the backup atrial pacing interval, which may be followed by a Vpace at an AV pacing interval after the backup Apace as described above. At block 770, control circuit 206 determines the atrial capture result as a loss of capture.

[0142] If a Vsense signal is received by control circuit 206 at block 766 prior to expiration of the backup atrial pacing interval, control circuit 206 may cancel the scheduled backup Apace at block 762. Control circuit 206 may determine an atrial capture result at block 764 based on at least the time of the Vsense signal, e.g., the time interval from the delivered Apace to the Vsense signal. As described above control circuit 206 may determine the capture result by applying at least a capture verification window that expires prior to the backup atrial pacing interval to the time of the Vsense signal for verifying capture of the Apace. As described above, if an Asense signal is received by control circuit 206 prior to the Vsense signal during the backup atrial pacing interval, control circuit 206 may determine the capture result based on at least the time of the Vsense signal relative to the Apace and the time of the Vsense signal relative to the Asense signal. Control circuit 206 may determine the atrial capture result according to any of the timing diagrams and flow charts described above. Control circuit 206 may perform a response to the capture result according to any of the examples described above.

[0143] Further disclosed herein are the following examples:

[0144] Example 1. A medical device system comprising sensing circuitry configured to sense ventricular event signals attendant to ventricular depolarizations, pulse generating circuitry configured to generate pacing pulses, and control circuitry configured to control the pulse generating circuitry to deliver an atrial pacing pulse. In response to the atrial pacing pulse being generated, the control circuitry may be configured to start a backup atrial pacing interval to schedule a backup atrial pacing pulse, the backup atrial pacing interval having an expiration time. The control circuitry may be further configured to determine if the sensing circuitry senses a ventricular event signal during the backup atrial pacing interval. In response to a ventricular event signal being sensed by the sensing circuitry during the backup atrial pacing interval, the control circuitry may be configured to determine an atrial capture result of the delivered atrial pacing pulse based on at least a time of the sensed ventricular event signal and cancel the scheduled backup atrial pacing pulse. The control circuitry may be further configured to, in response to a ventricular event signal not being sensed by the sensing circuitry during the backup atrial pacing interval, determine the atrial capture result of the delivered atrial pacing pulse as loss of capture and control the pulse generating circuitry to deliver the scheduled backup atrial pacing pulse at the expiration time.

[0145] Example 2. The medical device system of example 1 wherein the control circuitry is further configured to determine the atrial capture result as one of indeterminate, capture or loss of capture.

[0146] Example 3. The medical device system of any one of examples 1 — 2 wherein the control circuitry is further configured to apply a capture verification window to the time of the sensed ventricular event signal, the capture verification window having an end time that is earlier after the delivered atrial pacing pulse than the expiration time of the backup atrial pacing interval, and determine the atrial capture result as atrial capture based on the time of the sensed ventricular event signal when the time of the sensed ventricular event signal is during the capture verification window.

[0147] Example 4. The medical device system of example 3 wherein the control circuit is further configured to apply the capture verification window having a start time after the delivered atrial pacing pulse and determine the atrial capture result as indeterminate based on the time of the sensed ventricular event signal when the time of the sensed ventricular event signal is earlier than the start time of the capture verification window.

[0148] Example 5. The medical device system of any one of examples 2 — 3 wherein the control circuitry is further configured to apply a loss of capture threshold time to the time of the sensed ventricular event signal, the loss of capture threshold time being at least as long as the capture verification window end time after the delivered atrial pacing pulse, and determine the capture result as loss of capture in response to the time of the sensed ventricular event signal meeting the loss of capture threshold time.

[0149] Example 6. The medical device system of example 5 wherein the control circuitry is further configured to determine the atrial capture result of the delivered atrial pacing pulse based on the time of the sensed ventricular event signal by: applying the loss of capture threshold time as a threshold time greater than the end time of capture verification window and determining the capture result as indeterminate in response to the time of the sensed ventricular event signal being between the end time of the capture verification window and the loss of capture threshold time.

[0150] Example 7. The medical device system of any one of examples 1 — 6 wherein the control circuitry is further configured to schedule a ventricular pacing pulse at an atrioventricular pacing interval from the expiration time of the backup atrial pacing interval and cancel the scheduled ventricular pacing pulse in response to the sensing circuitry sensing the ventricular event signal during the backup atrial pacing interval.

[0151] Example 8. The medical device system of any one of examples 1 — 7 wherein the sensing circuitry is further configured to sense atrial event signals attendant to atrial depolarizations. The control circuitry may be further configured to determine that the sensing circuitry senses an atrial event signal during the backup atrial pacing interval, determine that a ventricular event signal is sensed by the sensing circuitry after the atrial event signal and during the backup atrial pacing interval and determine an atrioventricular time interval from the sensed atrial event signal to the time of the sensed ventricular event signal. The control circuitry may be further configured to determine the atrial capture result of the delivered atrial pacing pulse based on the time of the sensed ventricular event signal from the delivered atrial pacing pulse and the atrioventricular time interval.

[0152] Example 9. The medical device system of example 8 wherein the control circuitry is further configured to compare the atrioventricular time interval to a minimum atrioventricular conduction time threshold and, when the atrioventricular time interval does not meet the minimum conduction time interval, determine the atrial capture resultbased on the time of the sensed ventricular event signal from the atrial pacing pulse. When the atrioventricular time interval meets the minimum conduction time interval, the control circuitry may be configured to determine the atrial capture result as one of indeterminate or loss of capture.

[0153] Example 10. The medical device system of example 9 wherein the control circuitry is further configured to determine the atrial capture result as loss of capture when the time of the sensed ventricular event signal from the delivered atrial pacing pulse meets a loss of capture threshold time.

[0154] Example 11. The medical device system of any one of examples 1 — 10 wherein the sensing circuitry is further configured to sense atrial event signals attendant to atrial depolarizations, and the control circuitry is further configured to schedule a pending ventricular pacing pulse at an atrioventricular pacing interval from the expiration time of the backup atrial pacing interval. The control circuitry may be further configured to determine that the sensing circuitry senses an atrial event signal before the expiration time of the backup atrial pacing interval, cancel the scheduled backup atrial pacing pulse in response to the atrial event signal sensed by the sensing circuitry, determine that a ventricular event signal is not sensed by the sensing circuitry prior to an expiration of the atrioventricular pacing interval and determine the atrial capture result as loss of capture in response to the ventricular event signal not being sensed prior to the expiration of the atrioventricular pacing interval. The pulse generating circuitry may be further configured to deliver the pending ventricular pacing pulse in response to a ventricular event signal not being sensed prior to the expiration of the backup atrial pacing interval.

[0155] Example 12. The medical device system of any one of examples 1 — 11 wherein the control circuitry is further configured to start a ventricular safety pace window in response to the pulse generating circuitry delivering the atrial pacing pulse, the ventricular safety pace window extending during the backup atrial pacing interval and expiring earlier than the backup atrial pacing interval, and determine if the sensing circuitry senses a ventricular event signal during the ventricular safety pace window. The control circuitry may be further configured to, in response to the sensing circuitry sensing a ventricular event signal during the ventricular safety pace window, control the pulse generator to deliver a ventricular safety pace pulse upon expiration of the ventricular safety pace window.

[0156] Example 13. The medical device system of any one of examples 1 — 12 wherein the pulse generating circuitry comprises a charging circuit and an atrial holding capacitor. The pulse generating circuitry may be configured to charge the atrial holding capacitor for delivering the atrial pacing pulse and recharge the atrial holding capacitor during the backup atrial pacing interval for generating the scheduled backup atrial pacing pulse.

[0157] Example 14. The medical device system of any one of examples 1 — 13 further comprising a housing enclosing the sensing circuitry, the pulse generating circuitry and the control circuitry.

[0158] Example 15. The medical device system of example 14 further comprising an atrial pacing electrode pair and a ventricular pacing electrode pair on the housing, the ventricular pacing electrode pair comprising a tissue piercing electrode.

[0159] Example 16. A method comprising sensing ventricular event signals attendant to ventricular depolarizations, delivering an atrial pacing pulse, starting a backup atrial pacing interval to schedule a backup atrial pacing pulse in response to delivering the atrial pacing pulse, the backup atrial pacing interval having an expiration time. The method further comprising determining if a ventricular event signal is sensed during the backup atrial pacing interval and, in response to a ventricular event signal being sensed during the backup atrial pacing interval, determining an atrial capture result of the delivered atrial pacing pulse based on at least a time of the sensed ventricular event signal and cancelling the scheduled backup atrial pacing pulse. The method may further include, in response to a ventricular event signal not being sensed during the backup atrial pacing interval, determining the atrial capture result of the delivered atrial pacing pulse as loss of capture and delivering the scheduled backup atrial pacing pulse at the expiration time.

[0160] Example 17. The method of example 16 further comprising determining the atrial capture result as one of indeterminate, capture or loss of capture.

[0161] Example 18. The method of any one of examples 16 — 17 further comprising applying a capture verification window to the time of the sensed ventricular event signal, the capture verification window having an end time that is earlier after the delivered atrial pacing pulse than the expiration time of the backup atrial pacing interval. The method may further include determining the atrial capture result as atrial capture based on the time of the sensed ventricular event signal when the time of the sensed ventricular event signal is during the capture verification window.

[0162] Example 19. The method of example 18 further comprising applying the capture verification window having a start time after the delivered atrial pacing pulse and determining the atrial capture result as indeterminate based on the time of the sensed ventricular event signal when the time of the sensed ventricular event signal is earlier than the start time of the capture verification window.

[0163] Example 20. The method of any one of examples 17 — 19 further comprising applying a loss of capture threshold time to the time of the sensed ventricular event signal, the loss of capture threshold time being at least as long as the capture verification window end time after the delivered atrial pacing pulse. The method further comprising determining the capture result as loss of capture in response to the time of the sensed ventricular event signal meeting the loss of capture threshold time.

[0164] Example 21. The method of example 20 wherein determining the atrial capture result of the delivered atrial pacing pulse based on the time of the sensed ventricular event signal comprises applying the loss of capture threshold time as a threshold time greater than the end time of capture verification window and determining the capture result as indeterminate in response to the time of the sensed ventricular event signal being between the end time of the capture verification window and the loss of capture threshold time.

[0165] Example 22. The method of any one of examples 16 — 21 further comprising scheduling a ventricular pacing pulse at an atrioventricular pacing interval from the expiration time of the backup atrial pacing interval and cancelling the scheduled ventricular pacing pulse in response to the sensing circuitry sensing the ventricular event signal during the backup atrial pacing interval.

[0166] Example 23. The method of any one of examples 16 — 22 further comprising sensing atrial event signals attendant to atrial depolarizations, determining that an atrial event signal is sensed during the backup atrial pacing interval, determining that a ventricular event signal is sensed after the atrial event signal and during the backup atrial pacing interval, determining an atrioventricular time interval from the sensed atrial event signal to the sensed ventricular event signal and determining the atrial capture result of the delivered atrial pacing pulse based on the time of the sensed ventricular event signal from the delivered atrial pacing pulse and the atrioventricular time interval.

[0167] Example 24. The method of example 23 further comprising comparing the atrioventricular time interval to a minimum atrioventricular conduction time threshold.When the atrioventricular time interval does not meet the minimum conduction time interval, the method may further include determining the atrial capture result based on the time of the sensed ventricular event signal from the atrial pacing pulse. When the atrioventricular time interval meets the minimum conduction time interval, the method may further include determining the atrial capture result as one of indeterminate or loss of capture.

[0168] Example 25. The method of example 24 further comprising determining the atrial capture result as loss of capture when the time of the sensed ventricular event signal from the delivered atrial pacing pulse meets a loss of capture threshold time.

[0169] Example 26. The method of any one of examples 16 — 25 further comprising sensing atrial event signals attendant to atrial depolarizations, scheduling a pending ventricular pacing pulse at an atrioventricular pacing interval from the expiration time of the backup atrial pacing interval, determining that the sensing circuitry senses an atrial event signal before the expiration time of the backup atrial pacing interval and cancelling the scheduled backup atrial pacing pulse in response to the atrial event signal sensed by the sensing circuitry. The method may further include determining that a ventricular event signal is not sensed by the sensing circuitry prior to an expiration of the atrioventricular pacing interval and determining the atrial capture result as loss of capture in response to the ventricular event signal not being sensed prior to the expiration of the atrioventricular pacing interval. The method may further include delivering the pending ventricular pacing pulse in response to a ventricular event signal not being sensed prior to the expiration of the backup atrial pacing interval.

[0170] Example 27. The method of any one of examples 16 — 26 further comprising starting a ventricular safety pace window in response to the delivered atrial pacing pulse, the ventricular safety pace window extending during the backup atrial pacing interval and expiring earlier than the backup atrial pacing interval. The method may further include determining if a ventricular event signal is sensed during the ventricular safety pace window and, in response to a ventricular event signal sensed during the ventricular safety pace window, delivering a ventricular safety pace pulse upon expiration of the ventricular safety pace interval.

[0171] Example 28. The method of any one of examples 16 — 27 further comprising charging an atrial holding capacitor for delivering the atrial pacing pulse and rechargingthe atrial holding capacitor during the backup atrial pacing interval for generating the scheduled backup atrial pacing pulse.

[0172] Example 29. A non-transitory computer readable medium storing a set of instructions that, when executed by a medical device system, cause the medical device system to sense ventricular event signals attendant to ventricular depolarizations, deliver an atrial pacing pulse, start a backup atrial pacing interval to schedule a backup atrial pacing pulse in response to delivering the atrial pacing pulse, the backup atrial pacing interval having an expiration time and determine if a ventricular event signal is sensed during the backup atrial pacing interval. The instructions may further cause the medical device system to, in response to a ventricular event signal being sensed during the backup atrial pacing interval, determine an atrial capture result of the delivered atrial pacing pulse based on at least a time of the sensed ventricular event signal and cancel the scheduled backup atrial pacing pulse. The instructions may further cause the medical device to, in response to a ventricular event signal not being sensed during the backup atrial pacing interval, determine the atrial capture result of the delivered atrial pacing pulse as loss of capture and deliver the scheduled backup atrial pacing pulse at the expiration time.

[0173] 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.

[0174] 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 othermedium 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).

[0175] Instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable logic arrays (FPLAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor,” as used herein may refer to any of the foregoing structure or any other structure suitable for implementation of the techniques described herein. Also, the techniques could be fully implemented in one or more circuits or logic elements.

[0176] Thus, a medical device system 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

CLAIMS:

1. A medical device system comprising: sensing circuitry configured to sense ventricular event signals attendant to ventricular depolarizations; pulse generating circuitry configured to generate pacing pulses; and control circuitry configured to: control the pulse generating circuitry to deliver an atrial pacing pulse; in response to the atrial pacing pulse being generated, start a backup atrial pacing interval to schedule a backup atrial pacing pulse, the backup atrial pacing interval having an expiration time; determine if the sensing circuitry senses a ventricular event signal during the backup atrial pacing interval; in response to a ventricular event signal being sensed by the sensing circuitry during the backup atrial pacing interval: determine an atrial capture result of the delivered atrial pacing pulse based on at least a time of the sensed ventricular event signal; and cancel the scheduled backup atrial pacing pulse; and in response to a ventricular event signal not being sensed by the sensing circuitry during the backup atrial pacing interval: determine the atrial capture result of the delivered atrial pacing pulse as loss of capture; and control the pulse generating circuitry to deliver the scheduled backup atrial pacing pulse at the expiration time.

2. The medical device system of claim 1 wherein the control circuitry is further configured to: determine that the ventricular event signal is sensed by the sensing circuitry during the backup atrial pacing interval; and in response to the ventricular event signal being sensed by the sensing circuitry during the backup atrial pacing interval, determine the atrial capture result as one of indeterminate, capture or loss of capture.

3. The medical device system of any one of claims 1 — 2 wherein the control circuitry is further configured to: determine that the ventricular event signal is sensed by the sensing circuitry during the backup atrial pacing interval; in response to the ventricular event signal being sensed by the sensing circuitry during the backup atrial pacing interval, apply a capture verification window to the time of the sensed ventricular event signal, the capture verification window having an end time that is earlier after the delivered atrial pacing pulse than the expiration time of the backup atrial pacing interval; and determine the atrial capture result as atrial capture when the time of the sensed ventricular event signal is during the capture verification window.

4. The medical device system of claim 3 wherein the control circuit is further configured to: apply the capture verification window having a start time after the delivered atrial pacing pulse; and determine the atrial capture result as indeterminate when the time of the sensed ventricular event signal is earlier than the start time of the capture verification window.

5. The medical device system of any one of claims 3—4 wherein the control circuitry is further configured to: apply a loss of capture threshold time to the time of the sensed ventricular event signal, the loss of capture threshold time being at least as long as the capture verification window end time after the delivered atrial pacing pulse; and determine the capture result as loss of capture when the time of the sensed ventricular event signal meets the loss of capture threshold time.

6. The medical device system of claim 5 wherein the control circuitry is further configured to: apply the loss of capture threshold time as a threshold time greater than the end time of capture verification window; anddetermine the capture result as indeterminate when the time of the sensed ventricular event signal is between the end time of the capture verification window and the loss of capture threshold time.

7. The medical device system of any one of claims 1 — 6 wherein the control circuitry is further configured to: schedule a ventricular pacing pulse at an atrioventricular pacing interval from the expiration time of the backup atrial pacing interval; and cancel the scheduled ventricular pacing pulse in response to the sensing circuitry sensing the ventricular event signal during the backup atrial pacing interval.

8. The medical device system of any one of claims 1 — 7 wherein: the sensing circuitry is further configured to sense atrial event signals attendant to atrial depolarizations; and the control circuitry is further configured to: determine that the sensing circuitry senses an atrial event signal during the backup atrial pacing interval; determine that a ventricular event signal is sensed by the sensing circuitry after the atrial event signal and during the backup atrial pacing interval; determine an atrioventricular time interval from the sensed atrial event signal to the time of the sensed ventricular event signal; and determine the atrial capture result of the delivered atrial pacing pulse based on the time of the sensed ventricular event signal from the delivered atrial pacing pulse and the atrioventricular time interval.

9. The medical device system of claim 8 wherein the control circuitry is further configured to: compare the atrioventricular time interval to a minimum atrioventricular conduction time threshold; when the atrioventricular time interval does not meet the minimum conduction time interval, determine the atrial capture result based on the time of the sensed ventricular event signal from the atrial pacing pulse; andwhen the atrioventricular time interval meets the minimum conduction time interval, determine the atrial capture result as one of indeterminate or loss of capture.

10. The medical device system of any one of claims 1 — 9 wherein: the sensing circuitry is further configured to sense atrial event signals attendant to atrial depolarizations; the control circuitry is further configured to: schedule a pending ventricular pacing pulse at an atrioventricular pacing interval from the expiration time of the backup atrial pacing interval; determine that the sensing circuitry senses an atrial event signal before the expiration time of the backup atrial pacing interval; cancel the scheduled backup atrial pacing pulse in response to the atrial event signal sensed by the sensing circuitry; determine that a ventricular event signal is not sensed by the sensing circuitry prior to an expiration of the atrioventricular pacing interval; and determine the atrial capture result as loss of capture in response to the ventricular event signal not being sensed prior to the expiration of the atrioventricular pacing interval; and the pulse generating circuitry is further configured to deliver the pending ventricular pacing pulse in response to a ventricular event signal not being sensed prior to the expiration of the backup atrial pacing interval.

11. The medical device system of any one of claims 1 — 10 wherein the control circuitry is further configured to: start a ventricular safety pace window in response to the pulse generating circuitry delivering the atrial pacing pulse, the ventricular safety pace window extending during the backup atrial pacing interval and expiring earlier than the backup atrial pacing interval; determine if the sensing circuitry senses a ventricular event signal during the ventricular safety pace window; and in response to the sensing circuitry sensing a ventricular event signal during the ventricular safety pace window, control the pulse generator to deliver a ventricular safety pace pulse upon expiration of the ventricular safety pace window.

12. The medical device system of any one of claims 1 — 11 wherein the pulse generating circuitry comprises a charging circuit and an atrial holding capacitor, the pulse generating circuitry being configured to: charge the atrial holding capacitor for delivering the atrial pacing pulse; and recharge the atrial holding capacitor during the backup atrial pacing interval for generating the scheduled backup atrial pacing pulse.

13. The medical device system of any one of claims 1 — 12 further comprising a housing enclosing the sensing circuitry, the pulse generating circuitry and the control circuitry.

14. The medical device system of claim 13 further comprising an atrial pacing electrode pair and a ventricular pacing electrode pair on the housing, the ventricular pacing electrode pair comprising a tissue piercing electrode.

15. The medical device system of any one of claims 1 — 14 wherein the control circuit is further configured to: determine an atrial capture threshold based on at least the determined atrial capture result; and control the pulse generating circuitry to deliver atrial pacing pulses according to the atrial capture threshold.

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