Patent Publication Number: US-RE48197-E

Title: Atrial contraction detection by a ventricular leadless pacing device for atrio-synchronous ventricular pacing

Description:
This application claims the benefit of U.S. Provisional Application No. 62/028,957, filed Jul. 25, 2014, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The disclosure relates to cardiac pacing, and more particularly, to cardiac pacing using a leadless pacing device. 
     BACKGROUND 
     An implantable pacemaker may deliver pacing pulses to a patient&#39;s heart and monitor conditions of the patient&#39;s heart. In some examples, the implantable pacemaker comprises a pulse generator and one or more electrical leads. The pulse generator may, for example, be implanted in a small pocket in the patient&#39;s chest. The electrical leads may be coupled to the pulse generator, which may contain circuitry that generates pacing pulses and/or senses cardiac electrical activity. The electrical leads may extend from the pulse generator to a target site (e.g., an atrium and/or a ventricle) such that electrodes at the distal ends of the electrical leads are positioned at a target site. The pulse generator may provide electrical stimulation to the target site and/or monitor cardiac electrical activity at the target site via the electrodes. 
     A leadless pacing device has also been proposed for sensing electrical activity and/or delivering therapeutic electrical signals to the heart. The leadless pacing device may include one or more electrodes on its outer housing to deliver therapeutic electrical signals and/or sense intrinsic depolarizations of the heart. The leadless pacing device may be positioned within or outside of the heart and, in some examples, may be anchored to a wall of the heart via a fixation mechanism. 
     SUMMARY 
     The disclosure describes a leadless pacing device (hereinafter, “LPD”) that is configured for implantation in a ventricle of a heart of a patient, and is configured to deliver atrio-synchronous ventricular pacing based on detection of atrial contraction. More particularly, the LPD includes a motion sensor configured to generate a motion signal as a function of heart movement. The motion sensor may include one or more accelerometers, which may have a single axis, or multiple axes. The LPD is configured to analyze the motion signal within an atrial contraction detection window. The atrial contraction detection window begins upon completion of an atrial contraction detection delay period, which begins upon detection of activation of the ventricle. The LPD is configured to detect a contraction of an atrium of the heart based on the analysis of the motion signal within the atrial contraction detection window. If the LPD does not detect a ventricular depolarization subsequent to the atrial contraction, e.g., within an atrioventricular (AV) interval beginning when the atrial contraction was detected, the LPD delivers a ventricular pacing pulse. In some examples, the LPD is configured to deliver atrio-synchronous ventricular pacing using an electrical AV interval based on detection of atrial depolarizations via a plurality of electrodes of the LPD and, if the LPD is unable to detect atrial depolarizations, switch to delivering atrio-synchronous ventricular pacing using a mechanical AV interval, which may be shorter than the electrical AV interval, based on detection of atrial contractions. 
     In one example, a leadless pacing device is configured to provide atrio-synchronous ventricular pacing. The leadless pacing device comprises a plurality of electrodes, a motion sensor configured to generate a motion signal as a function of movement of a heart of a patient, a stimulation module coupled to the plurality of electrodes, wherein the stimulation module is configured to generate pacing pulses and deliver the pacing pulses to a ventricle of the heart via the plurality of electrodes, and an electrical sensing module coupled to the plurality of electrodes, wherein the electrical sensing module is configured to detect depolarizations of the ventricle within a cardiac electrogram sensed via the plurality of electrodes. The leadless pacing device further comprises a mechanical sensing module coupled to the motion sensor. The mechanical sensing module is configured to receive the motion signal from the motion sensor, identify an activation of the ventricle and, upon identification of the activation of the ventricle, initiate an atrial contraction detection delay period. The mechanical sensing module is further configured to analyze the motion signal within an atrial contraction detection window that begins upon completion of the atrial contraction detection delay period, and detect a contraction of an atrium of the heart based on the analysis of the motion signal within the atrial contraction detection window. The leadless pacing device further comprises a processing module configured to control the stimulation module to generate a pacing pulse and deliver the pacing pulse to the ventricle via the plurality of electrodes in response to the detection of the contraction of the atrium by the mechanical sensing module. The leadless pacing device further comprises a housing configured to be implanted within the ventricle, wherein the housing encloses the motion sensor, the stimulation module, the electrical sensing module, the mechanical sensing module, and the processing module. 
     In another example, a method for providing atrio-synchronous ventricular pacing by a leadless pacing device implanted within a ventricle of a heart of a patient comprises identifying an activation of the ventricle, upon identification of the activation of the ventricle, initiating an atrial contraction detection delay period, and analyzing a motion signal within an atrial contraction detection window that begins upon completion of the atrial contraction detection delay period. The motion signal is generated by a motion sensor of the leadless pacing device as a function of movement of the heart. The method further comprises detecting a contraction of an atrium of the heart based on the analysis of the motion signal within the atrial contraction detection window, and delivering a pacing pulse to the ventricle in response to the detection of the contraction of the atrium. 
     In another example, a leadless pacing device is configured to provide atrio-synchronous ventricular pacing. The leadless pacing device comprises means for generating a motion signal as a function of movement of a heart of a patient, means for identifying an activation of a ventricle of the heart, means for initiating an atrial contraction detection delay period upon identification of the activation of the ventricle, and means for analyzing the motion signal within an atrial contraction detection window that begins upon completion of the atrial contraction detection delay period. The leadless pacing device further comprises means for detecting a contraction of an atrium of the heart based on the analysis of the motion signal within the atrial contraction detection window, and means for delivering a pacing pulse to the ventricle in response to the detection of the contraction of the atrium. 
     In another example, a computer-readable storage medium comprises instructions stored thereon that, when executed by one or more programmable processors of a leadless pacing device configured to provide atrio-synchronous ventricular pacing, cause the one or more processors to identify an activation of the ventricle, upon identification of the activation of the ventricle, initiate an atrial contraction detection delay period, and analyze a motion signal within an atrial contraction detection window that begins upon completion of the atrial contraction detection delay period. The motion signal is generated by a motion sensor of the leadless pacing device as a function of movement of the heart. The instructions further cause the one or more processors to detect a contraction of an atrium of the heart based on the analysis of the motion signal within the atrial contraction detection window, and control delivery of a pacing pulse to the ventricle in response to the detection of the contraction of the atrium. 
     The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a conceptual diagram illustrating an example leadless pacing system that comprises an example leadless pacing device configured to deliver atrio-synchronous ventricular pacing based on atrial contraction detection implanted within a patient. 
         FIG. 2  is a conceptual diagram further illustrating the example leadless pacing device of  FIG. 1 . 
         FIG. 3  is a conceptual diagram illustrating another example leadless pacing system that comprises another example leadless pacing device configured to deliver atrio-synchronous ventricular pacing based on atrial contraction detection implanted within a patient. 
         FIG. 4  is a functional block diagram illustrating an example configuration of a leadless pacing device configured to deliver atrio-synchronous ventricular pacing based on atrial contraction detection. 
         FIG. 5  is a graph illustrating a cardiac electrogram and a corresponding motion signal. 
         FIG. 6  is a timing diagram illustrating an example technique for delivering atrio-synchronous ventricular pacing based on atrial contraction detection. 
         FIG. 7  is a flow diagram of an example technique for delivering atrio-synchronous ventricular pacing based on atrial contraction detection that may be performed by a leadless pacing device implanted within a ventricle. 
         FIG. 8  is a flow diagram illustrating an example technique for detecting an atrial contraction based on analysis of a motion signal that may be performed by a leadless pacing device implanted within a ventricle. 
         FIG. 9  is a flow diagram illustrating an example technique for verifying efficacy of atrio-synchronous ventricular pacing based on atrial contraction detection that may be performed by a leadless pacing device implanted within a ventricle. 
         FIG. 10  is a flow diagram illustrating an example technique for switching between an atrio-synchronous ventricular pacing mode and an asynchronous pacing mode that may be performed by a leadless pacing device implanted within a ventricle. 
         FIG. 11  is a flow diagram illustrating an example technique for switching between atrio-synchronous ventricular pacing in response to atrial depolarizations and atrio-synchronous ventricular pacing in response to atrial contractions that may be performed by a leadless pacing device implanted within a ventricle. 
     
    
    
     DETAILED DESCRIPTION 
     Typically, dual-chamber implantable pacemakers are implanted within a pocket within the patient&#39;s chest, and coupled to a right-atrial lead and a right-ventricular lead. The right-atrial lead extends from the implantable pacemaker in the pocket to the right atrium of the patient&#39;s heart, and positions one or more electrodes within the right atrium. The right-ventricular lead extends from the implantable pacemaker in the pocket to the right ventricle of the patient&#39;s heart, and positions one or more electrodes within the right ventricle. 
     Such dual-chamber implantable pacemakers sense respective cardiac electrical activity, e.g., respective cardiac electrograms, via the one or more electrodes implanted within the right atrium and the one or more electrodes implanted within the right ventricle. In particular, such dual-chamber implantable pacemakers detect intrinsic atrial depolarizations via the one or more electrodes implanted within the right atrium, and intrinsic ventricular depolarizations via the one or more electrodes implanted within the right ventricle. The implantable pacemakers may also deliver pacing pulses to the right atrium and the right ventricle via the one or more electrodes in the right atrium and the right ventricle, respectively. Due to the ability to sense both atrial and ventricular electrical activity, such dual-chamber implantable pacemakers may be able to provide atrio-synchronous ventricular pacing. For patients with intermittent AV node conduction, it may be preferable to inhibit ventricular pacing and allow an intrinsic ventricular depolarization to occur for a time, referred to as the AV interval, after an intrinsic atrial depolarization or atrial pace. Such atrio-synchronous pacing in dual-chamber implantable pacemakers may be according to the VDD or DDD programming modes, which have been used to treat patients with various degrees of AV block. 
     Implantable cardiac leads and the pocket in which pacemakers are implanted may be associated with complications. To avoid such complications leadless pacing devices sized to be implanted entirely within one chamber, e.g., the right ventricle, of the heart have been proposed. Some proposed leadless pacemakers include a plurality of electrodes that are affixed to, or are a portion of, the housing of the leadless pacing device. 
     Some proposed leadless pacing devices are capable of sensing intrinsic depolarizations of, and delivering pacing pulses to, the chamber of the heart in which they are implanted via the plurality of electrodes. However, because they are not coupled to electrodes in any other chamber, some proposed leadless pacing devices are incapable of sensing intrinsic depolarizations of, and delivering pacing pulses to, another chamber of the heart. Consequently, when implanted in the right ventricle, for example, such proposed leadless pacing devices may be unable to sense intrinsic atrial depolarizations of the atria, and may be limited to delivery of ventricular pacing according to an asynchronous ventricular pacing, e.g., according to a VVI or VVIR mode. 
       FIG. 1  is a conceptual diagram illustrating an example leadless pacing system  10 A that comprises an example leadless pacing device (LPD)  12 A that is configured to deliver atrio-synchronous ventricular pacing based on atrial contraction detection. In the example of  FIG. 1 , LPD  12 A is implanted within right ventricle  18  of heart  16  of patient  14 . More particularly, LPD  12 A is fixed or attached to the inner wall of the right ventricle  18  proximate to the apex of the right ventricle in the example of  FIG. 1 . In other examples, LPD  12 A may be fixed to the inner wall of right ventricle  18  at another location, e.g., on the intraventricular septum or free-wall of the right ventricle, or may be fixed to the outside of heart  16 , i.e., epicardially, proximate to right ventricle  18 . In other examples, LPD may be fixed within, on, or near the left-ventricle of heart  16 . 
     LPD  12 A includes a plurality of electrodes that are affixed to, or are a portion of, the housing of LPD  12 A. LPD  12 A senses electrical signals associated with depolarization and repolarization of heart  16 , i.e., a cardiac electrogram signal, via the electrodes. LPD  12 A also delivers cardiac pacing pulses to right ventricle  18  via the electrodes. 
     LPD  12 A detects depolarizations of right ventricle  18  within the cardiac electrogram. In some examples, LPD  12 A is not configured to detect intrinsic depolarizations of an atrium, e.g., right atrium  20 , or the atria of heart  16  generally, within the cardiac electrogram signal. In other examples, LPD  12 A is configured to detect atrial depolarizations within the cardiac electrogram signal. In some examples, LDP  12 A is configured to detect atrial depolarizations with the cardiac electrogram signal, but may, at times, be unable to reliably detect atrial depolarizations, e.g., due to the quality of the cardiac electrogram signal, or the relatively small magnitude of the atrial depolarizations within a cardiac electrogram signal sensed via electrodes disposed within right ventricle  18 . LPD  12 A is configured to detect mechanical contractions of an atrium, e.g., right atrium  20 , or the atria of heart  16  generally, e.g., as an alternative to sensing electrical depolarizations of right atrium  20 . In this manner, LPD  12 A may be configured to deliver atrio-synchronous ventricular pacing to right ventricle  18  even when not configured, or unable, to detect atrial depolarizations. 
     As described in greater detail below, LPD  12 A includes a motion sensor configured to generate a motion signal as a function of movement of a heart of a patient. LPD  12 A is configured to identify an activation event of right ventricle  18 , and analyze the motion signal within an atrial contraction detection window that begins upon completion of an atrial contraction detection delay period that is initiated upon detection of the activation of the ventricle. The activation of the ventricle may be an intrinsic depolarization of the ventricle or delivery of a pacing pulse to the ventricle. In some examples, LPD  12 A may be configured to detect contraction of right ventricle  18  based on the motion signal, and identify activation of the ventricle based on the detected ventricular contraction. 
     LPD  12 A is configured to detect an atrial contraction based on the analysis of the motion signal within the atrial contraction detection window. If a subsequent intrinsic depolarization of right ventricle  18  is not detected, e.g., within an AV interval beginning when the atrial contraction was detected, LPD  12 A is further configured to deliver the pacing pulse to right ventricle  18 . In this manner, LPD  12 A is configured to deliver atrio-synchronous pacing to right ventricle  18  based on detection of atrial contractions. 
     In some examples, LPD  12 A is configured to assess the efficacy of the delivery of atrio-synchronous pacing to right ventricle  18 . For example, LPD  12 A may detect a resulting contraction of right ventricle  18  based on the motion signal after delivery of a pacing pulse to the right ventricle, and determine whether the delivery of the pacing pulse to the right ventricle was effective based on the detection of the contraction of the right ventricle. In some examples, LPD  12 A may determine one or more metrics of the ventricular contraction, such as a timing or amplitude of the ventricular contraction, and adjust the delivery of the ventricular pacing based on the one or more metrics. LPD  12 A may adjust the AV interval, which begins upon detection of atrial contraction, based on the one or more metrics, as one example. 
     In addition to the motion of the heart, a motion signal generated by the motion sensor of LPD  12 A may include more general motion of patient  14  due to patient activity or experienced by patient, e.g., driving in a car. Such motion of patient  14  may interfere with the ability of LPD  12 A to detect atrial contractions. In some examples, LPD  12 A is configured to determine an amount of motion of patient  14  based on the motion signal, and change from delivery of ventricular pacing according to an atrio-synchronous pacing mode to delivery of ventricular pacing according to an asynchronous pacing mode in response to determining that the amount of patient motion exceeds a threshold. In some examples, LPD  12 A is additionally or alternatively configured to change from delivery of ventricular pacing according to an atrio-synchronous pacing mode to delivery of ventricular pacing according to an asynchronous pacing mode in response to determining that the heart rate is relatively high and/or irregular, e.g., based on intervals between successive intrinsic ventricular depolarizations and a stored threshold value, such as approximately 100 beats-per-minute (bpm). In some examples, LPD  12 A is additionally or alternatively configured to change from delivery of ventricular pacing according to an atrio-synchronous pacing mode to delivery of ventricular pacing according to an asynchronous pacing mode in response to determining that an atrial contraction was not detected during a predetermined number of cardiac cycles. According to an asynchronous ventricular pacing mode, e.g., VVI or VVIR, LPD  12 A delivers a ventricular pacing pulse if an intrinsic ventricular depolarization is not detected within a VV interval that begins when a previous intrinsic ventricular depolarization was detected, or a previous ventricular pacing pulse was delivered. 
     As illustrated in  FIG. 1 , leadless pacing system  10 A also includes a medical device programmer  22 , which is configured to program LPD  12 A and retrieve data from LPD  12 A. Programmer  22  may be a handheld computing device, desktop computing device, a networked computing device, or any other type of computing device, as examples. Programmer  22  may include a computer-readable storage medium having instructions that cause a processor of programmer  22  to provide the functions attributed to programmer  22  in the present disclosure. LPD  12 A may wirelessly communicate with programmer  22 . For example, LPD  12 A may transfer data to programmer  22  and may receive data from programmer  22 . Programmer  22  may also wirelessly program and/or wirelessly charge LPD  12 A. 
     Data retrieved from LPD  12 A using programmer  22  may include cardiac electrograms and motion signals stored by LPD  12 A that indicate the electrical and mechanical activity of heart  16 , and marker channel data that indicates the occurrence and timing of sensing, diagnosis, and therapy events associated with LPD  12 A, e.g., detection of atrial and ventricular depolarizations, atrial and ventricular contractions, and delivery of pacing pulses. Data transferred to LPD  12 A using programmer  22  may include, for example, operational programs for LPD  12 A that causes LPD  12 A to operate as described herein. As examples, data transferred to LPD  12 A using programmer  22  may include lengths of any AV intervals, atrial contraction detection delay periods, and atrial contraction detection windows described herein, any threshold values, such as for detecting atrial and/or ventricular contractions, or programming used by LPD  12 A to determine such values based on determined parameters of heart  16 , patient  14 , or LPD  12 A. 
       FIG. 2  is a conceptual diagram further illustrating LPD  12 A. As illustrated in  FIG. 2 , LPD  12 A includes an outer housing  30 , fixation times  32 A- 32 D (collectively “fixation tines  32 ”), and electrodes  34  and  36 . Outer housing  30  is configured to allow, e.g., has a size and form factor that allows, LPD  12 A to be entirely implanted within a chamber of heart  16 , such as right ventricle  18 . As illustrated in  FIG. 2 , housing  30  may have a cylindrical (e.g., pill-shaped) form factor in some examples. Housing  30  may be hermetically sealed to prevent ingress of fluids into the interior of housing  30 . 
     Fixation tines  32  extend from outer housing  30 , and are configured to engage with cardiac tissue to substantially fix a position of housing  30  within a chamber of heart  16 , e.g., at or near an apex of right ventricle  18 . Fixation tines  32  are configured to anchor housing  30  to the cardiac tissue such that LPD  12 A moves along with the cardiac tissue during cardiac contractions. Fixation tines  32  may be fabricated from any suitable material, such as a shape memory material (e.g., Nitinol). The number and configuration of fixation tines  32  illustrated in  FIG. 2  is merely one example, and other numbers and configurations of fixation tines for anchoring an LPD housing to cardiac tissue are contemplated. Additionally, although LPD  12 A includes a plurality of fixation tines  32  that are configured to anchor LPD  12 A to cardiac tissue in a chamber of a heart, in other examples, LPD  12 A may be fixed to cardiac tissue using other types of fixation mechanisms, such as, but not limited to, barbs, coils, and the like. 
     LPD  12 A is configured to sense electrical activity of heart  16 , i.e., a cardiac electrogram, and deliver pacing pulses to right ventricle  18 , via electrodes  34  and  36 . Electrodes  34  and  36  may be mechanically connected to housing  30 , or may be defined by a portion of housing  30  that is electrically conductive. In either case, electrodes are electrically isolated from each other. Electrode  34  may be referred to as a tip electrode, and fixation tines  32  may be configured to anchor LPD  12 A to cardiac tissue such that electrode  34  maintains contact with the cardiac tissue. Electrode  36  may be defined by a conductive portion of housing  30  and, in some examples, may define at least part of a power source case that houses a power source (e.g., a battery) of LPD  12 A. In some examples, a portion of housing  30  may be covered by, or formed from, an insulative material to isolate electrodes  34  and  36  from each other and/or to provide a desired size and shape for one or both of electrodes  34  and  36 . 
     Outer housing  30  houses electronic components of LPD  12 A, e.g., an electrical sensing module for sensing cardiac electrical activity via electrodes  34  and  36 , a motion sensor, a mechanical sensing module for detecting cardiac contractions, and an electrical stimulation module for delivering pacing pulses via electrodes  34  and  36 . Electronic components may include any discrete and/or integrated electronic circuit components that implement analog and/or digital circuits capable of producing the functions attributed to an LPD described herein. Additionally, housing  30  may house a memory that includes instructions that, when executed by one or more processors housed within housing  30 , cause LPD  12 A to perform various functions attributed to LPD  12 A herein. In some examples, housing  30  may house a communication module that enables LPD  12 A to communicate with other electronic devices, such as medical device programmer  22 . In some examples, housing  30  may house an antenna for wireless communication. Housing  30  may also house a power source, such as a battery. 
       FIG. 3  is a conceptual diagram illustrating another example leadless pacing system  10 B that comprises another example LPD  12 B configured to deliver atrio-synchronous ventricular pacing based on atrial contraction detection. Leadless pacing system  10 B and LPD  12 B may be substantially the same as leadless pacing system  10 A and LPD  12 A described above with respect to  FIGS. 1 and 2 . Unlike LPD  12 A, however, LPD  12 B includes a sensing extension  40  that includes an electrode  42 . In some examples, sensing extension  40  may include one or more additional electrodes having the same polarity as electrode  42 . Although not illustrated in  FIG. 3 , LPD  12 B may include an electrode  34 , but may not include electrode  36 , as described above with respect to LPD  12 A and  FIG. 2 . 
     Electrode  42  is electrically connected to electronics within a housing of LPD  12 B (e.g., an electrical sensing module and a stimulation module) via an electrical conductor of sensing extension  40 . In some examples, the electrical conductor of sensing extension  40  is connected to the electronics via an electrically conductive portion of the housing of LPD  12 B, which may correspond to electrode  36  of LPD  12 A ( FIG. 2 ), but may be substantially completely insulated (e.g., completely electrically insulated or nearly completely electrically insulated). Substantially completely electrically insulating the conductive portion of the housing may allow an electrical sensing module of LPD  12 B to sense electrical cardiac activity with electrode  42  of sensing extension  40 , rather than the conductive portion of the housing. 
     Additionally, as shown in  FIG. 3 , sensing extension  40  extends away from LPD  12 , which enables electrode  42  to be positioned relatively close to right atrium  20 . As a result, a cardiac electrogram sensed by LPD  12 B via electrodes  34  ( FIGS. 2 ) and  42  may include a higher amplitude far-field atrial depolarization signal than a cardiac electrogram sensed by LPB  12 A via electrodes  34  and  36  ( FIG. 2 ). In this way, sensing extension  40  may facilitate detection of atrial depolarizations when LPD  12 B is implanted in right ventricle  18 . In some examples, sensing extension  40  is sized to be entirely implanted within right ventricle  18 . In other examples, sensing extension  40  is sized to extend into right atrium  20 . 
     LPD  12 B is configured to detect atrial depolarizations within a cardiac electrogram signal. Accordingly, LPD  12 B may be configured to deliver atrio-synchronous ventricular pacing based on detection of atrial depolarizations. For example, LPD  12 B may be configured to deliver a pacing pulse to right ventricle  18  if an intrinsic depolarization of right ventricle  18  is not detected within an AV interval after detection of a depolarization of right atrium  20 . 
     However, despite sensing extension  40 , LPD  12 B may, at times, be unable to detect depolarizations of right atrium  20 , e.g., due to reduced cardiac electrogram signal quality. Reduced cardiac electrogram signal quality may include reduced amplitude of the atrial component of the cardiac electrogram signal and/or increased noise. Reduced cardiac electrogram signal quality may be caused by, for example, movement of sensing extension  40  relative to right atrium  20 , which may be caused by posture or activity of patient  14 , or other conditions of patient  14 , heart  16 , and/or LPD  12 B. Consequently, LPD  12 B is also configured to detect atrial contractions, and deliver atrio-synchronous ventricular pacing based on the atrial contractions, as described with respect to LPD  12 A. 
     In some examples, LPD  12 B is configured to determine that an atrial depolarization was not detected during a cardiac cycle. For example, LPD  12 B may be configured to determine that an atrial depolarization was not detected between consecutive ventricular depolarizations. In some examples, in response to determining that a depolarization of the atrium was not detected during a predetermined number of cardiac cycles, LPD  12 B is configured to switch from delivering atrio-synchronous ventricular pacing based on detection of atrial depolarization and using an electrical AV interval, to delivering atrio-synchronous ventricular pacing based on detection of atrial contractions and using a mechanical AV interval. Because mechanical contraction of the atrium occurs after electrical depolarization of the atrium, the mechanical AV interval may be shorter than the electrical AV interval. 
       FIG. 4  is a functional block diagram illustrating an example configuration of an LPD  12 A to deliver atrio-synchronous ventricular pacing based on atrial contraction detection. LPD  12 B of  FIG. 3  may have a similar configuration. However, electrode  36  of LPD  12 A may be replaced by electrode  42  of LPD  12 B, which may be positioned a greater distance away from electrode  34  and LPD  12 B, as described above with respect to  FIG. 3 . 
     LPD  12 A includes a processing module  50 , memory  52 , stimulation module  54 , electrical sensing module  56 , motion sensor  58 , mechanical sensing module  60 , communication module  62 , and power source  64 . Power source  64  may include a battery, e.g., a rechargeable or non-rechargeable battery. 
     Modules included in LPD  12 A represent functionality that may be included in LPD  12 A of the present disclosure. Modules of the present disclosure may include any discrete and/or integrated electronic circuit components that implement analog and/or digital circuits capable of producing the functions attributed to the modules herein. For example, the modules may include analog circuits, e.g., amplification circuits, filtering circuits, and/or other signal conditioning circuits. The modules may also include digital circuits, e.g., combinational or sequential logic circuits, memory devices, and the like. The functions attributed to the modules herein may be embodied as one or more processors, hardware, firmware, software, or any combination thereof. Depiction of different features as modules is intended to highlight different functional aspects, and does not necessarily imply that such modules must be realized by separate hardware or software components. Rather, functionality associated with one or more modules may be performed by separate hardware or software components, or integrated within common or separate hardware or software components. 
     Processing module  50  may include any one or more of a microprocessor, a controller, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or equivalent discrete or integrated logic circuitry. In some examples, processing module  50  includes multiple components, such as any combination of one or more microprocessors, one or more controllers, one or more DSPs, one or more ASICs, or one or more FPGAs, as well as other discrete or integrated logic circuitry. Additionally, although illustrated as separate functional components in  FIG. 4 , some or all of the functionality attributed to stimulation module  54 , electrical sensing module  56 , mechanical sensing module  60 , and communication module  62  may implemented in the one or more combination of one or more microprocessors, one or more controllers, one or more DSPs, one or more ASICs, one or more FPGAs, and/or other discrete or integrated logic circuitry that implements processing module  50 . 
     Processing module  50  may communicate with memory  52 . Memory  52  may include computer-readable instructions that, when executed by processing module  50 , cause processing module  50  and any other modules of LPD  12 A to perform the various functions attributed to them herein. Memory  52  may include any volatile, non-volatile, magnetic, or electrical media, such as a random access memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM (EEPROM), Flash memory, or any other memory device. 
     Stimulation module  54  and electrical sensing module  56  are electrically coupled to electrodes  34 ,  36 . Processing module  50  is configured to control stimulation module  54  to generate and deliver pacing pulses to heart  16  (e.g., right ventricle  18  in the example shown in  FIG. 1 ) via electrodes  34 ,  36 . In addition, processing module  50  is configured to control electrical sensing module  56  monitor a signal from electrodes  34 ,  36  in order to monitor electrical activity of heart  16 . Electrical sensing module  56  may include circuits that acquire an electrical signal from electrodes  34 ,  36 , as well as circuits to filter, amplify, and otherwise process the electrical signal. The electrical signal includes intrinsic cardiac electrical activity, such as depolarizations and repolarizations of the ventricles and, in some cases, depolarizations of the atria, and may be referred to as a cardiac electrogram signal. Electrical sensing module  56  detects ventricular depolarizations within the cardiac electrogram signal and, in some examples, detects atrial depolarizations within the cardiac electrogram signal. 
     LPD  12 A also includes motion sensor  58 . In some examples, motion sensor  58  comprises one or more accelerometers. In some examples, motion sensor  58  comprises a plurality of accelerometers, e.g., three accelerometers, each of which is oriented to detect motion in the direction of a respective axis or vector. The axes or vectors may be orthogonal. In other examples, motion sensor  58  may comprises one or more different sensors that generate a signal as a function of motion, instead of or in addition to the one or more accelerometers, such as gyros, mercury switches, or bonded piezoelectric crystals. 
     Mechanical sensing module  60  includes circuitry to receive the motion signal from motion sensor  58 , as well as circuits to filter, amplify, and otherwise process the motion signal. Because LPD  12 A is affixed to heart  16 , motion sensor  60  generates a motion signal that varies as a function of motion of the heart, including motion associated with the contraction of the atria, and motion associated with the subsequent contraction of the ventricles. Because LPD  12 A is implanted within patient  14 , the motion signal generated by motion sensor  58  also varies as a function of any motion of (or experienced by) the patient, e.g., due to patient activity. 
     As described in greater detail below, mechanical sensing module  60  analyzes the motion signal generated by motion sensor  58  to detect contraction of an atrium. Mechanical sensing module  60  may also analyze the motion signal to detect ventricular contraction. To detect atrial or ventricular contractions, mechanical sensing module  60  may filter the motion signal to exclude components other than cardiac motion, e.g., components of the motion signal associated with motion engaged in or experienced by patient  14 . For example, to detect contraction of an atrium, mechanical sensing module  60  may high-pass filter the motion signal, e.g., to exclude frequencies lower than about 40 Hz. As another example, to detect contraction of a ventricle, mechanical sensing module  60  may high-pass filter the motion signal, e.g., to exclude frequencies lower than about 60 Hz. 
     Mechanical sensing module  60  may also analyze the motion signal to detect other parameters of patient  14 , such as patient activity level. To detect patient activity level, mechanical sensing module  60  may filter the motion signal to exclude components other than those resulting from patient activity, such as components associated with cardiac contraction. For example, mechanical sensing module  60  may low-pass filter the motion signal generated by motion sensor  58 , e.g., to exclude frequencies above about 40 Hz. Processing module  50  may control stimulation module  54  to deliver rate responsive ventricular pacing based on the activity level determined by motion sensing module  60 . For example, processing module  50  may adjust an AV interval based on the activity level. 
     In examples in which motion sensor  58  includes a plurality of accelerometers or other sensors, a motion signal generated by motion sensor  58  may include one or more of the signals generated by the sensors, respectively, or a combination of one or more of the respective signals, which may be referred to as component signals of the motion signal. Mechanical sensing module  60  may derive the motion signal based on one or more of the component signals according to a sensing vector, where different sensing vectors specify a different one or more of the component signals. In some examples, mechanical sensing module  60  is configured to derive the motion signal according to a variety of different sensing vectors. In some examples, mechanical sensing module  60  may be configured to sense different parameters or events, e.g., atrial contractions, ventricular contractions, and patient activity, using different sensing vectors. In some examples, mechanical sensing module  60  is configured to detect an event or parameter, e.g., atrial contraction, according to a plurality of sensing vectors, and identify one or more sensing vectors that provide adequate detection of the event. 
     Communication module  62  may include any suitable hardware (e.g., an antenna), firmware, software, or any combination thereof for communicating with another device, such as programmer  22  ( FIGS. 1 and 3 ) or a patient monitor. Under the control of processing module  50 , communication module  62  may receive downlink telemetry from and send uplink telemetry to other devices, such as programmer  22  or a patient monitor, with the aid of an antenna included in communication module  62 . 
     Memory  52  may include data recorded by LPD  12 A, e.g., cardiac electrograms, motion signals, heart rates, information regarding detection of atrial contractions, ventricular pacing efficacy, etc. Under the direction of processing module  50 , communication module  62  may transfer data recorded by LDP  12 A to another device, such as programmer  22 . Memory  52  may also store programming data received by processing module  50  from another device, such as programmer  22 , via communication module  62 . The programming data stored in memory  52  may include, as examples, lengths of any AV intervals, atrial contraction detection delay periods, and atrial contraction detection windows described herein. The programming data stored in memory  52  may additionally or alternatively include any threshold values described herein, such as for detecting atrial and/or ventricular contractions, determining whether pacing is efficacious, or determining whether atrio-synchronous ventricular pacing should by suspended in favor of asynchronous pacing. The programming data stored in memory  52  may additionally or alternatively include data used by processing module  50  to determine any values described herein, e.g., based determined parameters of heart  16 , patient  14 , or LPD  12 A. 
       FIG. 5  is a graph illustrating a cardiac electrogram signal  70  and a corresponding motion signal  72  generated by one or more accelerometers. Cardiac electrogram signal  70  includes ventricular depolarizations (R-waves)  74 A and  74 B, and corresponding ventricular repolarizations (T-waves)  76 A and  76 B. A cardiac cycle  78  may be defined as the period from one ventricular depolarization  74 A to the next ventricular depolarization  74 B, or the period between any repeating fiducial features of cardiac electrogram signal  70  or motion signal  72 . 
     As illustrated by  FIG. 5 , cardiac cycle  78  includes an ejection phase, which may also be referred to as systole. During the ejection phase a ventricular contraction  80 A occurs as a result of ventricular depolarization  74 A. The S1 and S2 heart sounds, which are associated with ventricular contraction, occur at the beginning and end, respectively, of the ejection phase. The S1 and S2 heart sounds are produced by closing of the atrioventricular values and semilunar valves of heart  16 , respectively. 
     After the ejection phase, cardiac cycle  78  includes a passive filing stage during diastole, during which passive filling of the ventricles may produce the S3 heart sound. Additionally, near the end of diastole, an atrial contraction  82  occurs, actively filling of the ventricles. The active filing of the ventricles may produce the S4 heart sound. The atrial depolarization that resulted in atrial contraction  82  is not present in cardiac electrogram  70 . Another cardiac cycle begins with ventricular depolarization  74 B, and the resulting ventricular contraction  80 B. 
     Mechanical sensing module  60  detects atrial contractions, and may also detect ventricular contractions, based on an analysis of a motion signal generated by motion sensor  58 . The motion signal generated by motion sensor  58  may vary based on the movement of tissue of heart  16 , as well as any associated mechanical perturbations or vibrations, during contraction of heart  16 . Mechanical perturbations or vibrations may include those associated with the S1-S4 hearts sounds discussed above. For example, mechanical sensing module  60  may detect an atrial contraction based on features in motion signal  72  that are indicative of movement of cardiac tissue during atrial contraction, and/or the presence of mechanical perturbations associated with the S4 heart sound. As another example, mechanical sensing module  60  may detect a ventricular contraction based on features in motion signal  72  that are indicative of movement of cardiac tissue during ventricular contraction, and/or the presence of mechanical perturbations associated with the S1 heart sound. 
       FIG. 6  is a timing diagram illustrating an example technique for delivering atrio-synchronous ventricular pacing based on atrial contraction detection. The timing diagram of  FIG. 6  includes a ventricular marker channel, and a corresponding motion signal. According to the example technique for delivering atrio-synchronous ventricular pacing based on atrial contraction detection, mechanical sensing module  60  identifies an activation of a ventricle, e.g., right ventricle  18 . An activation of a ventricle may be an intrinsic or paced depolarization of the ventricle, or a mechanical contraction of the ventricle. Mechanical sensing module  60  may identify activation of a ventricle by determining that electrical sensing module  56  detected an intrinsic depolarization  90 A of the ventricle, by determining that stimulation module  54  delivered a pacing pulse to the ventricle, or by detecting mechanical contraction  92 A of ventricle. 
     In response to identifying activation of the ventricle, mechanical sensing module  60  waits for an atrial contraction detection delay period  94 , and then analyzes the motion signal generated by motion sensor  58  within an atrial contraction detection window  96  that begins the atrial contraction detection delay period  94  after the activation of the ventricle, i.e., that begins upon completion of the atrial contraction detection delay period  94 . In the example of  FIG. 6 , mechanical sensing module  60  determined that electrical sensing module detected ventricular depolarization  90 A, and analyzes the motion signal within atrial contraction detection window  96  that begins atrial contraction detection delay period  94  after detection of ventricular depolarization  90 A. 
     Starting atrial contraction detection window  96  upon completion of atrial contraction delay period  94  may allow mechanical sensing module  60  to avoid misidentifying ventricular contraction  92 A, or other motion of heart during the cardiac cycle prior to atrial depolarization and contraction, as an atrial contraction. In some examples, atrial contraction delay period  94  is at least approximately 300 milliseconds. In some examples, atrial contraction delay period  94  is at least approximately 400 milliseconds, or is approximately 400 milliseconds. In some examples, atrial contraction detection delay period  94  is at least approximately 600 milliseconds. In some examples, processing module  50  and/or mechanical sensing module  60  adjusts atrial contraction detection delay period  94  based on a heart rate of patient  14 , e.g., based on one or more intervals between consecutive intrinsic ventricular depolarizations detected by electrical sensing module  56 . For example, processing module  50  and/or mechanical sensing module  60  may increase atrial contraction detection delay period  94  as heart rate decreases, and decrease atrial contraction detection delay period  94  as heart rate increases. In some examples, a clinician or other user may program a length of atrial contraction delay period  94 , e.g., using programmer  22 . The user may select the length of atrial contraction delay period  94  based on individual patient characteristics. 
     Based on the analysis of the motion signal within atrial contraction detection window  96 , mechanical sensing module  60  may detect atrial contraction  98 . Mechanical sensing module  60  may extend atrial contraction detection window  96 , and the associated analysis of the motion signal, until detection of atrial contraction  98 , or until a subsequent intrinsic ventricular depolarization  90 B is detected by electrical sensing module  56 , or a subsequent ventricular pacing pulse  104  is delivered by stimulation module  54 . In some examples, as described above, mechanical sensing module  60  filters the motion signal within atrial contraction detection window  96 . Mechanical sensing module  60  may also rectify the motion signal within atrial contraction detection window  96 . In some examples, mechanical sensing module  60  detects atrial contraction  98  by comparing an amplitude of the motion signal within atrial contraction detection window  96  to a threshold  100 . In some examples, mechanical sensing module  60  determines a derivative signal of the motion signal, e.g., the filtered and/or rectified motion signal, and compares an amplitude of the derivative signal, which represents the rate of change of the motion signal, to threshold  100 . In some examples, mechanical sensing module  60  detects the time of atrial contraction  98  as the earliest time point at which the amplitude of the motion signal, or it derivative signal, exceeds threshold  100 . 
     In some examples, threshold  100  is a constant value, which may be determined by a manufacturer of an LPD  12 A, or programmed by a clinician using programmer  22 . In other examples, mechanical sensing module  60  and/or processing module  50  determines threshold  100  based on a peak amplitude of the motion signal during one or more previously detected atrial contractions. For example, mechanical sensing module  60  and/or processing module  50  may determine that threshold  100  is a value within a range from approximately 20 percent to approximately 80 percent, such as approximately 50 percent, of the peak amplitude of the motion signal during the most recently detected atrial contraction, or of an average peak amplitude of the motion signal during a plurality of previously detected atrial contractions. 
     In some examples, instead of or in addition to detection of atrial contraction  98  based on a comparison of the motion signal to threshold  100 , mechanical sensing module  60  may detect atrial contraction  98  using morphological comparison techniques. For example, mechanical sensing module  60  may compare the motion signal within atrial contraction detection window  96  to one or more templates representing one or more features of a motion signal during atrial contraction. Mechanical sensing module  60  may detect atrial contraction  98  at the point when a statistic resulting from the comparison indicates a sufficient level of similarity between the motion signal and the one or more templates. 
     In some examples, processing module  50  determines whether electrical sensing module  56  detects an intrinsic ventricular depolarization  90 B resulting from the atrial depolarization that led to atrial contraction  98 . For example, processing module  50  may determine whether electrical sensing module  56  detects intrinsic ventricular depolarization  90 B within an AV interval  102  that begins upon detection of atrial contraction  98  by mechanical sensing module  60 . If electrical sensing module  56  does not detect intrinsic depolarization  90 B within AV interval  102 , e.g., because it did not occur due to AV nodal block, then processing module  50  controls electrical stimulation module  54  to generate and deliver ventricular pacing pulse  104  at the expiration of AV interval  102 . In this manner, LPD  12 A delivers atrio-synchronous ventricular pacing based on detection of atrial contractions. 
     Due to the delay between atrial depolarization and atrial contraction  98 , and the resulting temporal proximity between atrial contraction  98  and the time at which a paced or intrinsic ventricular depolarization should occur, AV interval  102 , which may be referred to as a mechanical AV interval, may be shorter than an (electrical) AV interval employed by a pacemaker that provides atrio-synchronous ventricular pacing based on detection of atrial depolarizations. In some examples, AV interval  102  is less than approximately 100 milliseconds. In some examples, AV interval  102  is less than approximately 50 milliseconds. In some examples, AV interval  102  is approximately 30 milliseconds. In some examples, mechanical AV interval  102  is approximately 20 to 30 milliseconds shorter than an electrical AV interval for the patient. 
     In some examples, processing module  50  does not employ an AV interval. In such examples, upon detection of atrial contraction  98  by mechanical sensing module  60 , processing module determines whether electrical sensing module  56  has detected intrinsic ventricular depolarization  90 B. If electrical sensing module  56  has not detected intrinsic ventricular depolarization  90 B, then processing module  50  controls stimulation module  54  to generate and deliver a ventricular pacing pulse. 
     In some examples, LPD  12 A determines whether the delivery of ventricular pacing pulse  104  was effective based on detection of the ventricular contraction  92 B resulting from the delivery of pacing pulse  104 . In such examples, mechanical sensing module  60  detects ventricular contraction  92 B based on the motion signal, e.g., based on a comparison of the motion signal to a threshold  106  in a manner similar to that employed for detection of atrial contraction  98  based on threshold  100 , or based on a morphological analysis. In some examples, mechanical sensing module  60  detects the time of ventricular contraction  110  to be the first time-point after delivery of pacing pulse  104  when the amplitude of the motion signal exceeds threshold  106 . Mechanical sensing module  60  and/or processing module  50  may determine an interval  108  from delivery of pacing pulse  104  to a time of detection of ventricular contraction  92 B. Mechanical sensing module  60  may also determine a peak amplitude  110  of the motion signal during ventricular contraction  92 B. 
     In some examples, processing module  50  adjusts AV interval  102  based on the determination of whether the delivery of pacing pulse  104  to the ventricle was effective. For example, processing module  50  may decrease AV interval  102  in response to determining that interval  108  is less than a threshold. Additionally or alternatively, processing module  50  may increase AV interval  102  in response to determining that peak amplitude  110  is greater than a threshold. 
       FIG. 7  is a flow diagram of an example technique for delivering atrio-synchronous ventricular pacing based on atrial contraction detection that may be performed by a LPD implanted within a ventricle, such as LPD  12 A or LPD  12 B implanted within right ventricle  18  of heart  16 . The example technique of  FIG. 7  may be performed, at least in part, by a processing module  50  of such an LPD. According to the example technique of  FIG. 7 , the LPD identifies ventricular activation ( 120 ), and detects a subsequent atrial contraction based on a motion signal generated by a motion sensor of the LPD ( 122 ). The LPD then determines whether an intrinsic ventricular depolarization resulting from the atrial depolarization that caused the detected atrial contraction has been detected, e.g., within an AV interval beginning upon detection of the atrial contraction ( 124 ). 
     If the LPD detects an intrinsic ventricular depolarization resulting from the atrial depolarization that caused the detected atrial contraction (YES of  124 ), then the LPD identifies the intrinsic ventricular depolarization as a ventricular activation that begins the next cardiac cycle ( 120 ). If the LPD does not detect an intrinsic ventricular depolarization resulting from the atrial depolarization that caused the detected atrial contraction (NO of  124 ), then the LPD delivers a ventricular pacing pulse ( 126 ). For example, the LPD may deliver a ventricular pacing pulse upon expiration of the AV interval without detecting an intrinsic ventricular depolarization. The LPD identifies delivery of the ventricular pacing pulse as a ventricular activation that begins the next cardiac cycle ( 120 ). The LPD may also determine whether the delivery of the cardiac pacing pulse was effective, e.g., as described above with respect to  FIG. 6  ( 128 ). 
       FIG. 8  is a flow diagram illustrating an example technique for detecting an atrial contraction based on analysis of a motion signal (e.g.,  122  of  FIG. 7 ) that may be performed by a LPD implanted within a ventricle, such as LPD  12 A or LPD  12 B implanted within right ventricle  18  of heart  16 . The example technique of  FIG. 8  may be performed, at least in part, by a processing module  50  of such an LPD. According to the example technique of  FIG. 8 , the LPD begins an atrial contraction detection delay period upon identification of a ventricular activation event ( 130 ). The LPD begins an atrial contraction detection window upon expiration of the atrial contraction delay period ( 132 ). The LPD analyzes the motion signal generated by the motion sensor of the LPD within the atrial contraction detection window. 
     The LPD filters the motion signal within the atrial contraction detection window, rectifies the filtered signal, and generates a derivative signal of the filtered and rectified motion signal within the atrial contraction detection window ( 134 ). The LPD determines whether an amplitude of the derivative signal within the atrial contraction detection window exceeds a threshold ( 136 ). In response to determining that the amplitude of the derivative signal within the atrial contraction detection window exceeds the threshold (YES of  136 ), the LPD detects an atrial contraction ( 138 ). 
       FIG. 9  is a flow diagram illustrating an example technique for verifying efficacy of atrio-synchronous ventricular pacing based on atrial contraction detection that may be performed by a LPD implanted within a ventricle, such as LPD  12 A or LPD  12 B implanted within right ventricle  18  of heart  16 . According to the example technique of  FIG. 9 , the LPD detects a ventricular contraction resulting from a ventricular pacing pulse based on the motion signal generated by a motion sensor of the LPD after delivery of the ventricular pacing pulse ( 140 ). For example, the LPD may detect a time of the ventricular contraction as a time when an amplitude of the motion signal, e.g., an amplitude of a derivative signal generated from a filtered and rectified motion signal, exceeds a threshold. 
     The LPD determines an interval from the delivery of the ventricular pacing pulse to the time of detection of the ventricular contraction ( 142 ). The LPD determines whether the interval is less than a threshold ( 144 ). If the interval is less than the threshold (YES of  144 ), then the LPD decreases an AV interval used for delivery of atrio-synchronous ventricular pacing pulses after detection of an atrial contraction ( 146 ). 
     If the interval is not less than the threshold, e.g., is greater than the threshold (NO of  144 ), then the LPD determines a peak amplitude of the motion signal during the detected ventricular contraction ( 148 ). The LPD determines whether the peak amplitude of the motion signal during the detected ventricular contraction is greater than a threshold ( 150 ). If the peak amplitude is greater than the threshold (YES of  150 ), then the LPD increases an AV interval used for delivery of atrio-synchronous ventricular pacing pulses after detection of an atrial contraction ( 152 ). If the peak amplitude is not greater than the threshold, e.g., is less than the threshold (NO of  150 ), then the LPD maintains the AV interval at its current value ( 154 ). 
       FIG. 10  is a flow diagram illustrating an example technique for switching between an atrio-synchronous ventricular pacing mode and an asynchronous pacing mode that may be performed by a LPD implanted within a ventricle, such as LPD  12 A or LPD  12 B implanted within right ventricle  18 . The example technique of  FIG. 10  may be performed, at least in part, by a processing module  50  of such an LPD. According to the example technique of  FIG. 10 , the LPD operates in an atrio-synchronous ventricular pacing mode in which the LPD delivers atrio-synchronous ventricular pacing based detection of atrial contractions, as described herein ( 160 ). The atrio-synchronous ventricular pacing mode in which the LPD delivers atrio-synchronous ventricular pacing based detection of atrial contractions may be similar to a conventional VDD pacing mode, and may be referred to as a VDD pacing mode. 
     The LPD determines whether a patient activity level, or a level of motion experienced by the patient, exceeds a threshold ( 162 ). The LPD may determine the patient activity or motion level based on the motion signal generated by the motion sensor of the LPD. If the activity or motion level exceeds the threshold (YES of  162 ), then the LPD switches to an asynchronous ventricular pacing mode ( 164 ). In the asynchronous ventricular pacing mode, the LDP may deliver pacing pulses to the ventricle if an intrinsic ventricular depolarization is not detected within a VV interval from the last paced or intrinsic ventricular depolarization. The asynchronous ventricular pacing mode of the LPD may be similar to a conventional VVI or VVIR pacing mode, and may be referred to as a WI or VVIR pacing mode. 
     If the activity or motion level does not exceed the threshold, e.g., is less than the threshold (NO of  162 ), then the LPD determines whether the heart rate is greater than a threshold, e.g., greater than approximately 80 beats-per-minute or approximately 100 beats-per-minute, and/or irregular ( 166 ). The LPD may determine the heart rate and its regularity based on intervals between previous ventricular depolarizations. If the heart rate is greater than the threshold and/or irregular (YES of  166 ), then the LPD switches to the asynchronous ventricular pacing mode ( 164 ). 
     If the heart rate is not greater than the threshold and/or not irregular (NO of  166 ), then the LPD determines whether it is able to detect atrial contractions based on an analysis of the motion signal generated by a motion sensor of the LPD ( 168 ). For example, the LPD may determine that it is unable to detect atrial contractions if it determines that it has not detected atrial contractions for a predetermined number of cardiac cycles. The predetermined number of cardiac cycles may be any number of one or more cardiac cycles, which may be consecutive or non-consecutive. For example, the predetermined number of cardiac cycles may be three. If LPD determines that it is unable to detect atrial contraction (NO of  168 ), then the LPD switches to the asynchronous ventricular pacing mode ( 170 ). If the LPD determines that it is unable to detect atrial contractions (NO of  168 ), then the LPD may also change a motion signal sensing vector according to which the LPD derives the motion signal from one or more of a plurality of signals generated by the motion sensor, e.g., the plurality accelerometers of the motion sensor ( 172 ). 
     If the LPD determines that it is able to detect atrial contractions (YES of  168 ), then LPD may continue to deliver ventricular pacing according to the atrio-synchronous ventricular pacing mode ( 160 ). Further, after delivering pacing according to the asynchronous pacing mode ( 164 ,  170 ) for a period of time, or until a condition that led to the switch to the asynchronous mode has abated, the LPD may switch to delivery of ventricular pacing according to the atrio-synchronous ventricular pacing mode. 
       FIG. 11  is a flow diagram illustrating an example technique for switching between atrio-synchronous ventricular pacing in response to atrial depolarizations and atrio-synchronous ventricular pacing in response to atrial contractions that may be performed by a LPD implanted within a ventricle, such as right ventricle  18 , that is able to detect depolarizations of an atrium, such as right atrium  20 . LPD  12 B that is coupled to sensing extension  40  is one example of such an LPD, although LPD  12 A may also be configured to detect depolarizations of the atrium. The example technique of  FIG. 11  may be performed by a processing module  50  of such an LPD. 
     According to the example technique of  FIG. 11 , the LPD delivers atrio-synchronous pacing a first, electrical AV interval after detection of atrial depolarizations ( 180 ). The LPD determines whether it is unable to detect atrial depolarizations ( 182 ). For example, the LPD may determine that it is unable to detect atrial depolarizations if it determines that it has not detected atrial depolarizations for a predetermined number of cardiac cycles, e.g., has not detected an atrial depolarization between consecutive ventricular depolarizations of a predetermined number of cardiac cycles. The predetermined number of cardiac cycles may be any number of one or more cardiac cycles, which may be consecutive or non-consecutive. If LPD determines that it is unable to detect atrial depolarizations (YES of  182 ), then the LPD may activate atrial contraction detection, and switch to delivery of atrio-synchronous pacing a second, mechanical AV interval after detection of atrial contractions ( 184 ). If LPD determines that it is able to detect atrial depolarizations (NO of  182 ), or some time delivering atrio-synchronous ventricular pacing based on atrial contraction detection ( 184 ), then the LPD may continue or switch back to delivery of atrio-synchronous ventricular pacing based on atrial depolarization detection ( 180 ). 
     The techniques described in this disclosure, including those attributed to LPDs  12 , programmer  22 , or various constituent components, may be implemented, at least in part, in hardware, software, firmware or any combination thereof. For example, various aspects of the techniques may be implemented within one or more processors, including one or more microprocessors, DSPs, ASICs, FPGAs, or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components, embodied in programmers, such as physician or patient programmers, stimulators, image processing devices or other devices. The term “processor” or “processing circuitry” may generally refer to any of the foregoing logic circuitry, alone or in combination with other logic circuitry, or any other equivalent circuitry. 
     Such hardware, software, firmware may be implemented within the same device or within separate devices to support the various operations and functions described in this disclosure. In addition, any of the described units, modules or components may be implemented together or separately as discrete but interoperable logic devices. Depiction of different features as modules or units is intended to highlight different functional aspects and does not necessarily imply that such modules or units must be realized by separate hardware or software components. Rather, functionality associated with one or more modules or units may be performed by separate hardware or software components, or integrated within common or separate hardware or software components. 
     When implemented in software, the functionality ascribed to the systems, devices and techniques described in this disclosure may be embodied as instructions on a computer-readable medium such as RAM, ROM, NVRAM, EEPROM, FLASH memory, magnetic data storage media, optical data storage media, or the like. The instructions may be executed to support one or more aspects of the functionality described in this disclosure. 
     Various examples have been described. These and other examples are within the scope of the following claims.