Patent Publication Number: US-2019167991-A1

Title: Leadless cardiac pacemaker with reversionary behavior

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/593,662 filed on Dec. 1, 2017, the disclosure of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally relates to implantable medical devices, and more particularly, to systems that use a leadless cardiac pacemaker for monitoring, pacing and/or defibrillating a patient&#39;s heart. 
     BACKGROUND 
     Implantable medical devices are commonly used today to monitor a patient and/or deliver therapy to a patient. For example, and in some instances, pacing devices are used to treat patients suffering from various heart conditions that may result in a reduced ability of the heart to deliver sufficient amounts of blood to a patient&#39;s body. Such heart conditions may lead to slow, rapid, irregular, and/or inefficient heart contractions. To help alleviate some of these conditions, various medical devices (e.g., pacemakers, defibrillators, etc.) can be implanted in a patient&#39;s body. Such devices may monitor and in some cases provide electrical stimulation (e.g. pacing, defibrillation, etc.) to the heart to help the heart operate in a more normal, efficient and/or safe manner. In some cases, it may be beneficial to detect cardiac events occurring in multiple chambers of the heart. In some cases, this may be used to enhance the effectiveness of the cardiac pacing therapy and/or may allow different types of cardiac pacing therapy to be delivered. 
     SUMMARY 
     This disclosure generally relates to implantable medical devices, and more particularly, to systems that use a leadless cardiac pacemaker for monitoring, pacing and/or defibrillating a patient&#39;s heart. In a first example, a leadless cardiac pacemaker (LCP) may be configured to sense cardiac activity and to deliver pacing therapy to a patient&#39;s heart. The LCP may comprise a housing, a first electrode secured relative to the housing and exposed to the environment outside of the housing, a second electrode secured relative to the housing and responsive to the environment outside of the housing, a sensing module disposed within the housing and exposed to the environment outside of the housing, the sensing module configured to detect one or more atrial fiducials that are indicative of an atrial contraction, and a control module operatively coupled to the first electrode, the second electrode, and the sensing module. The control module may be configured to track the one or more atrial fiducials over a plurality of cardiac cycles and deliver a plurality of different ventricular pacing therapies to the patient&#39;s heart via the first electrode and the second electrode, wherein the control module selects which ventricular pacing therapy to deliver based, at least in part, on one or more of the tracked atrial fiducials. 
     Alternatively or additionally to any of the examples above, in another example, the plurality of different ventricular pacing therapies may comprise two or more of VDD, VDDR, VVI, VVIR, VOO and VOOR. 
     Alternatively or additionally to any of the examples above, in another example, the plurality of different ventricular pacing therapies may comprise a first ventricular pacing therapy having a first pacing rate and a second ventricular pacing therapy having a second pacing rate different from the first pacing rate. 
     Alternatively or additionally to any of the examples above, in another example, the control module may be configured to dynamically select which ventricular pacing therapy to deliver based, at least in part, on one or more of the tracked atrial fiducials. 
     Alternatively or additionally to any of the examples above, in another example, the sensing module may be further configured to detect one or more ventricle fiducials that are indicative of a ventricle contraction, and the control module is configured to track the one or more ventricle fiducials over a plurality of cardiac cycles. 
     Alternatively or additionally to any of the examples above, in another example, the control module may be configured to select which ventricular pacing therapy to deliver based, at least in part, on one or more of the tracked atrial fiducials and one or more of the tracked ventricle fiducials. 
     Alternatively or additionally to any of the examples above, in another example, the control module may be configured to change from a first ventricular pacing therapy to a second ventricular pacing therapy when a predetermined one of the tracked atrial fiducials comes too close in time to a predetermined one of the tracked ventricle fiducials. 
     Alternatively or additionally to any of the examples above, in another example, the one or more atrial fiducials may comprise one or more of an electrical p-wave, a pressure a-wave, an acoustic S1 sound, an acoustic S2 sound, an acoustic S3 sound, an acoustic S4 sound, an indication of ventricular volume, an indication of ventricular wall thickness, an indication of cardiac tissue vibration, an indication of mitral valve position, and an indication of tricuspid valve position. 
     Alternatively or additionally to any of the examples above, in another example, the sensing module may be configured to detect two or more atrial fiducials that are indicative of an atrial contraction, and wherein the control module is configured to select which ventricular pacing therapy to deliver based, at least in part, on a selected one of the tracked atrial fiducials. 
     Alternatively or additionally to any of the examples above, in another example, the sensing module may be configured to detect two or more atrial fiducials that are indicative of an atrial contraction, and wherein the control module is configured to select which ventricular pacing therapy to deliver based, at least in part, on two or more of the tracked atrial fiducials. 
     Alternatively or additionally to any of the examples above, in another example, the control module may continuously track the one or more atrial fiducials over a plurality of cardiac cycles. 
     Alternatively or additionally to any of the examples above, in another example, the control module may intermittently track the one or more atrial fiducials over a plurality of cardiac cycles. 
     Alternatively or additionally to any of the examples above, in another example, the control module may be configured to switch from delivering a first ventricular pacing therapy to a delivering second ventricular pacing therapy when the control module can no longer track one or more of the atrial fiducials. 
     Alternatively or additionally to any of the examples above, in another example, the control module may enter a search mode to attempt to re-acquire the one or more of the atrial fiducials that can no longer be tracked. 
     Alternatively or additionally to any of the examples above, in another example, the sensing module may comprise one or more of a pressure measurement module and an acoustic measurement module. 
     In another example, a leadless cardiac pacemaker (LCP) may be configured to sense cardiac activity and to deliver pacing therapy to a patient&#39;s heart. The LCP may comprise a housing, a first electrode secured relative to the housing and exposed to the environment outside of the housing, a second electrode secured relative to the housing and exposed to the environment outside of the housing, a sensing module disposed within the housing and responsive to the environment outside of the housing, the sensing module including one or more of a pressure measurement module and an acoustic measurement module, the sensing module configured to detect one or more atrial fiducials that are indicative of an atrial contraction and one or more ventricle fiducials that are indicative of a ventricle contraction, and a control module operatively coupled to the first electrode, the second electrode, and the sensing module. The control module may be configured to track the one or more atrial fiducials and one or more ventricle fiducials over a plurality of cardiac cycles and deliver a plurality of different ventricular pacing therapies to the patient&#39;s heart via the first electrode and the second electrode, wherein the control module transitions from delivery of a first one of the different ventricular pacing therapies to a second one of the different ventricular pacing therapies based, at least in part, on one or more of the tracked atrial fiducials. The plurality of different ventricular pacing therapies may comprise two or more of VDD, VDDR, VVI, VVIR, VOO and VOOR. 
     Alternatively or additionally to any of the examples above, in another example, the control module may transition from delivery of a first one of the different ventricular pacing therapies to a second one of the different ventricular pacing therapies based, at least in part, on one or more of the tracked atrial fiducials and one or more of the tracked ventricle fiducials. 
     Alternatively or additionally to any of the examples above, in another example, the control module may be configured to switch from delivering a first ventricular pacing therapy to a delivering second ventricular pacing therapy when the control module can no longer track one or more of the atrial fiducials. 
     Alternatively or additionally to any of the examples above, in another example, the plurality of different ventricular pacing therapies may comprise a ventricular pacing therapy having a first pacing rate and a ventricular pacing therapy having a second pacing rate different from the first pacing rate. 
     In another example, a leadless cardiac pacemaker (LCP) may be configured to sense cardiac activity and to deliver pacing therapy to a patient&#39;s heart. The LCP may comprise a housing, a first electrode secured relative to the housing and exposed to the environment outside of the housing, a second electrode secured relative to the housing and exposed to the environment outside of the housing, a sensing module for sensing a hemodynamic state of the patient, and a control module operatively coupled to the first electrode, the second electrode, and the sensing module. The control module may be configured to deliver a plurality of different ventricular pacing therapies to the patient&#39;s heart via the first electrode and the second electrode, wherein the control module dynamically selects which ventricular pacing therapy to deliver based, at least in part, on the sensed hemodynamic state of the patient. The plurality of different ventricular pacing therapies may comprise two or more of VDD, VDDR, VVI, VVIR, VOO and VOOR. 
     Alternatively or additionally to any of the examples above, in another example, the sensing module comprises one or more of a pressure measurement module and an acoustic measurement module. 
     The above summary is not intended to describe each embodiment or every implementation of the present disclosure. Advantages and attainments, together with a more complete understanding of the disclosure, will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure may be more completely understood in consideration of the following description of various illustrative embodiments in connection with the accompanying drawings, in which: 
         FIG. 1  is a schematic block diagram of an illustrative leadless cardiac pacemaker (LCP) according to one example of the present disclosure; 
         FIG. 2  a schematic block diagram of another medical device (MD), which may be used in conjunction with an LCP  100  ( FIG. 1 ) in order to detect and/or treat cardiac arrhythmias and other heart conditions; 
         FIG. 3  is a schematic diagram of an exemplary medical system that includes an LCP and another medical device, in accordance with yet another example of the present disclosure; 
         FIG. 4  is a graphical representation of an illustrative electrocardiogram (ECG) showing a temporal relationship between electrical signals of the heart and mechanical indications of contraction of the heart; 
         FIG. 5  is a graph showing example pressures and volumes within the heart over time; 
         FIG. 6  is an illustrative table of various artifacts occurring during the cardiac cycle and different ways to detect them; 
         FIG. 7  is an illustrative table of various artifacts occurring during the cardiac cycle and during which cardiac phase each occur; 
         FIG. 8  is a side view of an illustrative LCP; 
         FIG. 9A  is a partial cross-sectional plan view of an example LCP implanted within a heart during ventricular filling; 
         FIG. 9B  is a partial cross-sectional plan view of an example LCP implanted within a heart during ventricular contraction; 
         FIG. 10  is a flow diagram showing an illustrative method of detecting atrial activity from an LCP implanted in a ventricle of the heart and generating and delivering a ventricular pacing pulse using the same; 
         FIG. 11  is a schematic diagram of an illustrative signal averaging method that can be used by an LCP implanted in the ventricle to help identify atrial timing fiducials; 
         FIG. 12  shows a portion of an illustrative ventricle pressure signal; 
         FIG. 13  shows a graph of illustrative cardiac signals including heart sounds, right ventricular pressure, and an electrocardiogram, along with various intervals between detectable characteristics of such signals; 
         FIG. 14  shows a graph of illustrative cardiac signals including heart sounds, right ventricular pressure, and an electrocardiogram, along with various timing delays (AV intervals) from detectable characteristics of such signals to a desired ventricle pacing pulse; 
         FIG. 15  is an illustrative method for determining when a medical device should utilize reversion; 
         FIG. 16  illustrates a comparison of pacing intervals on an electrocardiogram when the device is operating in a normal VDD tracking mode and pacing intervals on an electrocardiogram when the device is operating in a VDD pseudo tracking mode; and 
         FIG. 17  is a graphic representation of higher order derivatives that can be used by an LCP implanted in the ventricle to help identify atrial timing fiducials. 
     
    
    
     While the disclosure is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit aspects of the disclosure to the particular illustrative embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure. 
     DESCRIPTION 
     The following description should be read with reference to the drawings in which similar elements in different drawings are numbered the same. The description and the drawings, which are not necessarily to scale, depict illustrative embodiments and are not intended to limit the scope of the disclosure. While the present disclosure is applicable to any suitable implantable medical device (IMD), the description below uses pacemakers and more particularly leadless cardiac pacemakers (LCP) as particular examples. 
     All numbers are herein assumed to be modified by the term “about”, unless the content clearly dictates otherwise. The recitation of numerical ranges by endpoints includes all numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, and 5). 
     As used in this specification and the appended claims, the singular forms “a”, “an”, and “the” include the plural referents unless the content clearly dictates otherwise. As used in this specification and the appended claims, the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise. 
     It is noted that references in the specification to “an embodiment”, “some embodiments”, “other embodiments”, etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is contemplated that the feature, structure, or characteristic may be applied to other embodiments whether or not explicitly described unless clearly stated to the contrary. 
     A normal, healthy heart induces contraction by conducting intrinsically generated electrical signals throughout the heart. These intrinsic signals cause the muscle cells or tissue of the heart to contract in a coordinated manner. These contractions force blood out of and into the heart, providing circulation of the blood throughout the rest of the body. Many patients suffer from cardiac conditions that affect the efficient operation of their hearts. For example, some hearts develop diseased tissue that no longer generate or efficiently conduct intrinsic electrical signals. In some examples, diseased cardiac tissue may conduct electrical signals at differing rates, thereby causing an unsynchronized and inefficient contraction of the heart. In other examples, a heart may generate intrinsic signals at such a low rate that the heart rate becomes dangerously low. In still other examples, a heart may generate electrical signals at an unusually high rate, even resulting in cardiac fibrillation. In some cases such an abnormality can develop into a fibrillation state, where the contraction of the patient&#39;s heart chambers are almost completely de-synchronized and the heart pumps very little to no blood. Implantable medical devices, which may be configured to determine occurrences of such cardiac abnormalities or arrhythmias and deliver one or more types of electrical stimulation therapy to patient&#39;s hearts, may help to terminate or alleviate these and other cardiac conditions. 
     It is contemplated that atrial events or artifacts indicative of an atrial event may be used by a device implanted in the right (or left) ventricle to time a pacing pulse for the ventricle in support of treating bradycardia events. In some cases, the timing of the ventricle pacing pulse may be adjusted to maximize the amount of blood entering the right ventricle through passive filling. In some instances, this may include adjusting an AV delay relative to an atrial fiducial (e.g., atrial kick). In some cases, a measured pressure change (or other atrial fiducial) over time may be used to support management of a CRT cardiac therapy (e.g. if placed in the left ventricle), patient health status monitoring and/or any other suitable goal. It is contemplated that measuring events in one of or both of the ventricle and atrium using a single leadless cardiac pacemaker may replicate a dual chamber system using only a single device. For example, such a system may enable a device to be positioned in a ventricle and capable of sensing intrinsic ventricular and atrial events and pacing the ventricle when appropriate (e.g., a VDD pacemaker). 
       FIG. 1  depicts an illustrative leadless cardiac pacemaker (LCP) that may be implanted into a patient to provide bradycardia therapy, cardiac resynchronization therapy (CRT), anti-tachycardia pacing (ATP) therapy, defibrillation therapy, and/or the like. As can be seen in  FIG. 1 , the illustrative LCP  100  may be a compact device with all components housed within and/or on the LCP housing  120 . In the example shown in  FIG. 1 , the LCP  100  includes a communication module  102 , a pulse generator module  104 , an electrical sensing module  106 , a mechanical sensing module  108 , a processing module  110 , a battery  112 , and electrodes  114 . It is contemplated that the LCP  100  may include more or less modules, depending on the application. 
     The communication module  102  may be configured to communicate with remote devices such as sensors, other devices, and/or the like, that are located externally and/or internally to the patient&#39;s body. The other devices may be device primarily functioning as a medical device (e.g. a LCP programmer, an implanted sensor) or a device primarily functioning as a non-medical device (e.g. a personal computer, tablet computer, smart phone, laptop computer or the like). Irrespective of the location or primary function, the remote devices (i.e., external to the LCP  100  but not necessarily external to the patient&#39;s body) may communicate with the LCP  100  via the communication module  102  to accomplish one or more desired functions. For example, the LCP  100  may communicate information, such as sensed signals, data, instructions, messages, etc., to a remote medical device through the communication module  102 . The remote medical device may then use the communicated signals, data, instructions and/or messages to perform various functions, such as determining occurrences of arrhythmias, delivering electrical stimulation therapy, storing received data, analyzing received data, transmitting the received data to an external programmer or server or the like for review by a physician, and/or performing any other suitable function. The LCP  100  may additionally receive information such as signals, data, instructions and/or messages from the remote medical device through the communication module  102 , and the LCP  100  may use the received signals, data, instructions and/or messages to perform various functions, such as determining occurrences of arrhythmias, delivering electrical stimulation therapy, storing received data, analyzing received data, and/or performing any other suitable function. The communication module  102  may be configured to use one or more methods for communicating with remote devices. For example, the communication module  102  may communicate via radiofrequency (RF) signals, inductive coupling, optical signals, acoustic signals, conducted communication signals, and/or any other signals suitable for communication. 
     In the example shown in  FIG. 1 , the pulse generator module  104  may be electrically connected to the electrodes  114 . In some examples, the LCP  100  may include one or more additional electrodes  114 ′. In such examples, the pulse generator  104  may also be electrically connected to the additional electrodes  114 ′. The pulse generator module  104  may be configured to generate electrical stimulation signals. For example, the pulse generator module  104  may generate electrical stimulation signals by using energy stored in a battery  112  within the LCP  100  and deliver the generated electrical stimulation signals via the electrodes  114  and/or  114 ′. Alternatively, or additionally, the pulse generator  104  may include one or more capacitors, and the pulse generator  104  may charge the one or more capacitors by drawing energy from the battery  112 . The pulse generator  104  may then use the energy of the one or more capacitors to deliver the generated electrical stimulation signals via the electrodes  114  and/or  114 ′. In at least some examples, the pulse generator  104  of the LCP  100  may include switching circuitry to selectively connect one or more of the electrodes  114  and/or  114 ′ to the pulse generator  104  in order to select which of the electrodes  114 / 114 ′ (and/or other electrodes) that the pulse generator  104  uses to deliver the electrical stimulation therapy. The pulse generator module  104  may generate electrical stimulation signals with particular features or in particular sequences in order to provide one or multiple of a number of different stimulation therapies. For example, the pulse generator module  104  may be configured to generate electrical stimulation signals to provide electrical stimulation therapy to combat bradycardia, tachycardia, cardiac dyssynchrony, bradycardia arrhythmias, tachycardia arrhythmias, fibrillation arrhythmias, cardiac synchronization arrhythmias and/or to produce any other suitable electrical stimulation therapy. Some more common electrical stimulation therapies include bradycardia therapy, anti-tachycardia pacing (ATP) therapy, cardiac resynchronization therapy (CRT), and cardioversion/defibrillation therapy. 
     In some examples, the LCP  100  may not include a pulse generator  104  or may turn off the pulse generator  104 . When so provided, the LCP  100  may be a diagnostic only device. In such examples, the LCP  100  may not deliver electrical stimulation therapy to a patient. Rather, the LCP  100  may collect data about cardiac electrical activity and/or other physiological parameters of the patient and communicate such data and/or determinations to one or more other medical devices via the communication module  102 . 
     In some examples, the LCP  100  may include an electrical sensing module  106 , and in some cases, a mechanical sensing module  108 . The electrical sensing module  106  may be configured to sense the cardiac electrical activity of the heart. For example, the electrical sensing module  106  may be connected to the electrodes  114 / 114 ′, and the electrical sensing module  106  may be configured to receive cardiac electrical signals conducted through the electrodes  114 / 114 ′. The cardiac electrical signals may represent local information from the chamber (e.g. near field) in which the LCP  100  is implanted. For instance, if the LCP  100  is implanted within a ventricle of the heart, cardiac electrical signals sensed by the LCP  100  through the electrodes  114 / 114 ′ may represent ventricular cardiac electrical signals, and possibly some weaker atrial electrical signals. The electrical sensing module  106  may be configured to detect voltage, current and/or impedance. An electrogram sensing module may be provided as a part of the electrical sensing module. 
     The mechanical sensing module  108  may include one or more sensors, such as an accelerometer, a gyroscope, a microphone, a hydrophone, a blood pressure sensor, a heart sound sensor, a blood-oxygen sensor, a temperature sensor, a flow sensor, a strain sensor, and/or any other suitable sensors that are configured to measure one or more mechanical and/or chemical parameters of the patient. In some cases, the mechanical sensing module  108  may include two or more of a pressure measurement module, an acoustic measurement module, an acceleration measurement module. 
     Both the electrical sensing module  106  and the mechanical sensing module  108  may be connected to a processing module  110 , which may provide signals representative of the sensed mechanical parameters. Although described with respect to  FIG. 1  as separate sensing modules, in some cases, the electrical sensing module  106  and the mechanical sensing module  108  may be combined into a single sensing module, as desired. 
     The electrodes  114 / 114 ′ can be secured relative to the housing  120  but exposed to the tissue and/or blood surrounding the LCP  100 . In some cases, the electrodes  114  may be generally disposed on or near either end of the LCP  100  and may be in electrical communication with one or more of the modules  102 ,  104 ,  106 ,  108 , and  110 . The electrodes  114 / 114 ′ may be supported by the housing  120 , although in some examples, the electrodes  114 / 114 ′ may be secured relative to the housing  120  through short connecting wires (e.g. tail) such that one or more of the electrodes  114 / 114 ′ may be spaced from the housing  120 . In examples where the LCP  100  includes one or more electrodes  114 ′, the electrodes  114 ′ may in some cases be disposed on the sides of the housing  120  of the LCP  100 , which may increase the number of electrodes by which the LCP  100  may sense cardiac electrical activity, deliver electrical stimulation and/or communicate with an external medical device. The electrodes  114 / 114 ′ can be made up of one or more biocompatible conductive materials such as various metals or alloys that are known to be safe for implantation within a human body. In some instances, the electrodes  114 / 114 ′ connected to LCP  100  may have an insulative portion that electrically isolates the electrodes  114 / 114 ′ from adjacent electrodes, the housing  120 , and/or other parts of the LCP  100 . 
     The processing module  110  can be configured to control the operation of the LCP  100 . For example, the processing module  110  may be configured to receive electrical signals from the electrical sensing module  106  and/or the mechanical sensing module  108 . Based on the received signals, the processing module  110  may determine, for example, a need for pacing therapy such as bradycardia therapy, cardiac resynchronization therapy (CRT), anti-tachycardia pacing (ATP) therapy, defibrillation therapy, and/or the like. The processing module  110  may control the pulse generator module  104  to generate electrical stimulation in accordance with one or more pacing therapies. The processing module  110  may further receive information from the communication module  102 . In some examples, the processing module  110  may use such received information to help determine the need for pacing therapy and/or what type of pacing therapy. The processing module  110  may additionally control the communication module  102  to send/receive information to/from other devices. 
     In some examples, the processing module  110  may include a pre-programmed chip, such as a very-large-scale integration (VLSI) chip and/or an application specific integrated circuit (ASIC). In such embodiments, the chip may be pre-programmed with control logic in order to control the operation of the LCP  100 . By using a pre-programmed chip, the processing module  110  may use less power than other programmable circuits (e.g., general purpose programmable microprocessors) while still being able to maintain basic functionality, thereby potentially increasing the battery life of the LCP  100 . In other examples, the processing module  110  may include a programmable microprocessor. Such a programmable microprocessor may allow a user to modify the control logic of the LCP  100  even after implantation, thereby allowing for greater flexibility of the LCP  100  than when using a pre-programmed ASIC. In some examples, the processing module  110  may further include a memory, and the processing module  110  may store information on and read information from the memory. In other examples, the LCP  100  may include a separate memory (not shown) that is in communication with the processing module  110 , such that the processing module  110  may read and write information to and from the separate memory. 
     The battery  112  may provide power to the LCP  100  for its operations. In some examples, the battery  112  may be a non-rechargeable lithium-based battery. In other examples, a non-rechargeable battery may be made from other suitable materials, as desired. Because the LCP  100  is an implantable device, access to the LCP  100  may be limited after implantation. Accordingly, it is desirable to have sufficient battery capacity to deliver therapy over a period of treatment such as days, weeks, months, years or even decades. In some instances, the battery  112  may a rechargeable battery, which may help increase the useable lifespan of the LCP  100 . In still other examples, the battery  112  may be some other type of power source, as desired. 
     To implant the LCP  100  inside a patient&#39;s body, an operator (e.g., a physician, clinician, etc.), may fix the LCP  100  to the cardiac tissue of the patient&#39;s heart. To facilitate fixation, the LCP  100  may include one or more anchors  116 . The anchor  116  may include any one of a number of fixation or anchoring mechanisms. For example, the anchor  116  may include one or more pins, staples, threads, screws, helix, tines, and/or the like. In some examples, although not shown, the anchor  116  may include threads on its external surface that may run along at least a partial length of the anchor  116 . The threads may provide friction between the cardiac tissue and the anchor to help fix the anchor  116  within the cardiac tissue. In other examples, the anchor  116  may include other structures such as barbs, spikes, or the like to facilitate engagement with the surrounding cardiac tissue. 
       FIG. 2  depicts an example of another medical device (MD)  200 , which may be used in conjunction with an LCP  100  ( FIG. 1 ) in order to detect and/or treat cardiac arrhythmias and other heart conditions. In the example shown, the MD  200  may include a communication module  202 , a pulse generator module  204 , an electrical sensing module  206 , a mechanical sensing module  208 , a processing module  210 , and a battery  218 . Each of these modules may be similar to the modules  102 ,  104 ,  106 ,  108 , and  110  of the LCP  100 . Additionally, the battery  218  may be similar to the battery  112  of the LCP  100 . In some examples, the MD  200  may have a larger volume within the housing  220  than LCP  100 . In such examples, the MD  200  may include a larger battery and/or a larger processing module  210  capable of handling more complex operations than the processing module  110  of the LCP  100 . 
     While it is contemplated that the MD  200  may be another leadless device such as shown in  FIG. 1 , in some instances the MD  200  may include leads such as leads  212 . The leads  212  may include electrical wires that conduct electrical signals between the electrodes  214  and one or more modules located within the housing  220 . In some cases, the leads  212  may be connected to and extend away from the housing  220  of the MD  200 . In some examples, the leads  212  are implanted on, within, or adjacent to a heart of a patient. The leads  212  may contain one or more electrodes  214  positioned at various locations on the leads  212 , and in some cases at various distances from the housing  220 . Some of the leads  212  may only include a single electrode  214 , while other leads  212  may include multiple electrodes  214 . Generally, the electrodes  214  are positioned on the leads  212  such that when the leads  212  are implanted within the patient, one or more of the electrodes  214  are positioned to perform a desired function. In some cases, the one or more of the electrodes  214  may be in contact with the patient&#39;s cardiac tissue. In some cases, the one or more of the electrodes  214  may be positioned substernally or subcutaneously and spaced from but adjacent to the patient&#39;s heart. In some cases, the electrodes  214  may conduct intrinsically generated electrical signals to the leads  212 , e.g., signals representative of intrinsic cardiac electrical activity. The leads  212  may, in turn, conduct the received electrical signals to one or more of the modules  202 ,  204 ,  206 , and  208  of the MD  200 . In some cases, the MD  200  may generate electrical stimulation signals, and the leads  212  may conduct the generated electrical stimulation signals to the electrodes  214 . The electrodes  214  may then conduct the electrical signals and deliver the signals to the patient&#39;s heart (either directly or indirectly). 
     The mechanical sensing module  208 , as with the mechanical sensing module  108 , may contain or be electrically connected to one or more sensors, such as microphones, hydrophones, accelerometers, gyroscopes, blood pressure sensors, heart sound sensors, blood-oxygen sensors, acoustic sensors, ultrasonic sensors, strain sensors, and/or other sensors which are configured to measure one or more mechanical/chemical parameters of the heart and/or patient. In some examples, one or more of the sensors may be located on the leads  212 , but this is not required. In some examples, one or more of the sensors may be located in the housing  220 . 
     While not required, in some examples, the MD  200  may be an implantable medical device. In such examples, the housing  220  of the MD  200  may be implanted in, for example, a transthoracic region of the patient. The housing  220  may generally include any of a number of known materials that are safe for implantation in a human body and may, when implanted, hermetically seal the various components of the MD  200  from fluids and tissues of the patient&#39;s body. 
     In some cases, the MD  200  may be an implantable cardiac pacemaker (ICP). In this example, the MD  200  may have one or more leads, for example leads  212 , which are implanted on or within the patient&#39;s heart. The one or more leads  212  may include one or more electrodes  214  that are in contact with cardiac tissue and/or blood of the patient&#39;s heart. The MD  200  may be configured to sense intrinsically generated cardiac electrical signals and determine, for example, one or more cardiac arrhythmias based on analysis of the sensed signals. The MD  200  may be configured to deliver CRT, ATP therapy, bradycardia therapy, and/or other therapy types via the leads  212  implanted within the heart or in concert with the LCP by commanding the LCP to pace. In some examples, the MD  200  may additionally be configured to provide defibrillation therapy. 
     In some instances, the MD  200  may be an implantable cardioverter-defibrillator (ICD). In such examples, the MD  200  may include one or more leads implanted within a patient&#39;s heart. The MD  200  may also be configured to sense cardiac electrical signals, determine occurrences of tachyarrhythmias based on the sensed signals, and may be configured to deliver defibrillation therapy in response to determining an occurrence of a tachyarrhythmia. In some instances, the MD  200  may be a subcutaneous implantable cardioverter-defibrillator (S-ICD). In examples where the MD  200  is an S-ICD, one of the leads  212  may be a subcutaneously or substernally implanted lead that is spaced from the heart. In at least some examples where the MD  200  is an S-ICD, the MD  200  may include only a single lead which is implanted subcutaneously or substernally, but this is not required. In some cases, the S-ICD lead may extend subcutaneously from the S-ICD can, around the sternum and may terminate adjacent the interior surface of the sternum and spaced from the heart. 
     In some examples, the MD  200  may not be an implantable medical device. Rather, the MD  200  may be a device external to the patient&#39;s body, and may include skin-electrodes that are placed on a patient&#39;s body. In such examples, the MD  200  may be able to sense surface electrical signals (e.g., cardiac electrical signals that are generated by the heart or electrical signals generated by a device implanted within a patient&#39;s body and conducted through the body to the skin). In such examples, the MD  200  may be configured to deliver various types of electrical stimulation therapy, including, for example, defibrillation therapy. The MD  200  may be further configured to deliver electrical stimulation via the LCP by commanding the LCP to deliver the therapy. 
     It is contemplated that one or more LCPs  100  and/or one or more MDs  200  may be used in combination as an example medical device system. The various devices  100 ,  200  may communicate through various communication pathways including using RF signals, inductive coupling, conductive coupling optical signals, acoustic signals, or any other signals suitable for communication. The system may further include and be in communication with a display. The display may be a personal computer, tablet computer, smart phone, laptop computer, or other display as desired. In some instances, the display may include input means for receiving an input from a user. For example, the display may also include a keyboard, mouse, actuatable (e.g., pushable) buttons, or a touchscreen display. These are just examples. Some illustrative medical device systems are described in commonly assigned Patent Application No. 62/547,458, entitled IMPLANTABLE MEDICAL DEVICE WITH PRESSURE SENSOR and filed on Aug. 18, 2017, which is hereby incorporated by reference. 
       FIG. 3  shows an example system  250  incorporating an LCP  100  and a MD  200 . In  FIG. 3 , an LCP  100  is shown fixed to the interior of the right ventricle of the heart H, and MD  200  including a pulse generator is shown coupled to a lead  212  having one or more electrodes  214   a,    214   b,    214   c.  In some cases, the MD  200  may be part of a subcutaneous implantable cardioverter-defibrillator (S-ICD), and the one or more electrodes  214   a,    214   b,    214   c  may be positioned subcutaneously or substernally adjacent the heart. In some cases, the S-ICD lead may extend subcutaneously from the S-ICD can, around the sternum and one or more electrodes  214   a,    214   b,    214   c  may be positioned adjacent the interior surface of the sternum but spaced from the heart H. In some cases, the LCP  100  may communicate with the subcutaneous implantable cardioverter-defibrillator (S-ICD). 
     In some cases, the LCP  100  may be in the left ventricle, right atrium or left atrium of the heart, as desired. In some cases, more than one LCP  100  may be implanted. For example, one LCP may be implanted in the right ventricle and another may be implanted in the right atrium. In another example, one LCP may be implanted in the right ventricle and another may be implanted in the left ventricle. In yet another example, one LCP may be implanted in each of the chambers of the heart. Further, the LCP  100  may be used without the second MD  200 . 
     The medical device system  250  may also include an external support device, such as external support device  260 . The external support device  260  can be used to perform functions such as device identification, device programming and/or transfer of real-time and/or stored data between devices using one or more of the communication techniques described herein. As one example, communication between the external support device  260  and the MD  200  is performed via a wireless mode (e.g. RF, Bluetooth, inductive communication, etc.), and communication between the MD  200  and the LCP  100  is performed via a conducted mode (e.g. conducted communication). In some examples, communication between the LCP  100  and the external support device  260  is accomplished by sending communication information through the MD  200 . However, in other examples, communication between the LCP  100  and the external support device  260  may be direct. In some embodiments, the external support device  260  may be provided with or be in communication with a display  262 . The display  262  may be a personal computer, tablet computer, smart phone, laptop computer, or other display as desired. In some instances, the display  262  may include input means for receiving an input from a user. For example, the display  262  may also include a keyboard, mouse, actuatable buttons, or be a touchscreen display. These are just examples. 
     With reference to  FIG. 4 , it will be appreciated that the heart is controlled via electrical signals that pass through the cardiac tissue and that can be detected by implanted devices such as but not limited to the LCP  100  and/or MD  200  of  FIG. 1  or  2 .  FIG. 4  is a graphical representation of an illustrative electrocardiogram (ECG)  300  showing a temporal relationship between electrical signals of the heart and mechanical indications  302  (e.g. heart sounds) of contraction of the heart. As can be seen in the illustrative ECG  300 , a heartbeat includes a P-wave that indicates atrial depolarization associated with an atrial contraction to load the ventricles. A QRS complex, including a Q-wave, an R-wave and an S-wave, represents a ventricular depolarization associated with the ventricles contracting to pump blood to the body and lungs. A T-wave shows the repolarization of the ventricles in preparation for a next heart beat. With heart disease, the timing of these individual events may be anomalous or abnormal, and the shape, amplitude and/or timing of the various waves can be different from that shown. It will be appreciated that the ECG  300  may be detected by implanted devices such as but not limited to the LCP  100  and/or MD  200  of  FIG. 1 or 2 . 
     The electrical signal  300  typically instructs a portion of the heart to contract, which then results in a corresponding mechanical contraction. There is a correspondence between a characteristic in the electrical signal (e.g. ECG  300 ) and a corresponding mechanical response. The mechanical response is typically delayed because it takes some time for the heart to respond to the electrical signal. 
     It will be appreciated that heart sounds may be considered as one example of mechanical indications of the heart beating. Other illustrative mechanical indications may include, for example, endocardial acceleration or movement of a heart wall detected by an accelerometer in the LCP, acceleration or movement of a heart wall detected by an accelerometer in the SICD, a pressure, pressure change, or pressure change rate in a chamber of the heart detected by a pressure sensor of the LCP, acoustic signals caused by heart movements detected by an acoustic sensor (e.g. accelerometer, microphone, etc.), twisting of the heart detected by a gyroscope in the LCP and/or any other suitable indication of a heart chamber beating. 
     In some cases, there may be a first heart sound denoted S1 that is produced by vibrations generated by closure of the mitral and tricuspid valves during a ventricular contraction, a second heart sound denoted S2 that is produced by closure of the aortic and pulmonary valves, a third heart sound denoted S3 that is an early diastolic sound caused by the rapid entry of blood from the right atrium into the right ventricle and from the left atrium into the left ventricle, and a fourth heart sound denoted S4 that is a late diastolic sound corresponding to late ventricular filling during an active atrial contraction. These are mechanical responses that can be detected using various sensors (e.g. microphone, hydrophone, accelerometer, etc.). 
     Because the heart sounds are a result of cardiac muscle contracting or relaxing in response to an electrical signal, it will be appreciated that there is a delay between the electrical signal, indicated by the ECG  300 , and the corresponding mechanical indication, indicated in the example shown by the heart sounds trace  302 . For example, the P-wave of the ECG  300  is an electrical signal triggering an atrial contraction. The S4 heart sound is the mechanical signal caused by the atrial contraction. In some cases, it may be possible to use this relationship between the P-wave and the S4 heart sound. For example, if one of these signals can be detected, their expected timing relationship can be used as a mechanism to search for the other. For example, if the P-wave can be detected, a window following the P-wave can be defined and searched in order to help find and/or isolate the corresponding S4 heart sound. In some cases, detection of both signals may be an indication of an increased confidence level in a detected atrial contraction. In some cases, detection of either signal may be sufficient to identify an atrial contraction. The identification of an atrial contraction may be used to identify an atrial contraction timing fiducial (e.g. a timing marker of the atrial contraction). 
     With traditional systems having transvenous leads, the intracardiac electrodes are placed to detect the atrial depolarization while also delivering pacing therapy to one or both ventricles. As a result, the circuitry of a single device would receive, directly, information for the P-wave allowing delivery at a timed interval of a pacing pulse to properly coordinate the ventricular pace with the atrial contraction and improve pumping efficiency. However, with a system only having an LCP implanted within a ventricle, it may be difficult to detect the relatively small P-wave from within the ventricle, and as such, it is contemplated that the LCP may be configured to detect atrial activity without relying on the P-wave (e.g. using S4). The detected atrial activity may be used to identify an atrial timing fiducial that can be used as a basis for timing a pacing pulse in the ventricle (e.g. after an AV delay). 
     In some examples, a time window for atrial artifact detection is defined during which the LCP  100  may specifically look for atrial artifacts (such as, but not limited to, atrial contraction) to determine an atrial timing fiducial. Such windows may be defined by analysis of the cardiac signals obtained from a patient using, for example, a detected ventricular event such as the R-wave/QRS complex or the T-wave of a previous heart beat as the starting point for timing delays  304 ,  306 , as shown in  FIG. 4 . Timing delays  304 ,  306  may be dynamic based on the overall heart beat rate of the patient using data gathered from a patient or using a formula or accepted relationship. Other windows may be determined based on detected atrial artifacts and/or determined atrial events, as described in more detail herein. 
     In some cases, the relationship of certain electrical signals and/or mechanical indications may be used to predict the timing of other electrical signals and/or mechanical indications within the same heartbeat. Alternatively, or in addition, the timing of certain electrical signals and/or mechanical indications corresponding to a particular heartbeat may be used to predict the timing of other electrical signals and/or mechanical indications within a subsequent heartbeat. 
     It will be appreciated that as the heart undergoes a cardiac cycle, the blood pressures and blood volumes within the heart vary over time.  FIG. 5  illustrates how these parameters correlate with the electrical signals and corresponding mechanical indications.  FIG. 5  shows an illustrative example of the aortic pressure, left ventricular pressure, left atrial pressure, left ventricular volume, an electrocardiogram (ECG or egram), and heart sounds of the heart over two consecutive heart beats. A cardiac cycle may begin with diastole, and the mitral valve opens. The ventricular pressure falls below the atrial pressure, resulting in the ventricle filling with blood. During ventricular filling, the aortic pressure slowly decreases as shown. During systole, the ventricle contracts. When ventricular pressure exceeds the atrial pressure, the mitral valve closes, generating the S1 heart sound. Before the aortic valve opens, an isovolumetric contraction phase occurs where the ventricle pressure rapidly increases but the ventricle volume does not significantly change. Once the ventricular pressure equals the aortic pressure, the aortic valve opens and the ejection phase begins where blood is ejected from the left ventricle into the aorta. The ejection phase continues until the ventricular pressure falls below the aortic pressure, at which point the aortic valve closes, generating the S2 heart sound. At this point, the isovolumetric relaxation phase begins and ventricular pressure falls rapidly until it is exceeded by the atrial pressure, at which point the mitral valve opens and the cycle repeats. 
     Contractions of the atria are initiated near the end of ventricular diastole. The active atrial contraction pushes or forces additional volumes of blood into the ventricles (often referred to as “atrial kick”) in addition to the volumes associated with passive filling. In some cases, the atrial kick contributes in the range of about 20% of the volume of blood toward ventricular preload. At normal heart rates, the atrial contractions are considered highly desirable for adequate ventricular filling. However, as heart rates increase, atrial filling becomes increasingly important for ventricular filling because the time interval between contractions for active filling becomes progressively shorter. Cardiac pressure curves for the pulmonary artery, the right atrium, and the right ventricle, and the cardiac volume curve for the right ventricle, may be similar to those illustrated in  FIG. 5 . Typically, the cardiac pressure in the right ventricle is lower than the cardiac pressure in the left ventricle. 
     The heart sound signals shown in  FIG. 5  can be recorded using acoustic sensors, for example a microphone, which may capture the acoustic waves resulted from such heart sounds. In another example, the heart sounds can be recorded using accelerometers or pressure sensors that capture the vibrations or pressure waves caused by the heart sounds. The heart sound signals can be recorded within or outside the heart. These are just examples. 
     In some cases, sensing atrial events or artifacts indicative of an atrial event may allow a device, such as LCP  100  implanted in the ventricle, to detect an atrial contraction, resulting in, for example, an atrial kick. In some cases, signals that provide an indication of an atrial contraction may include one or more of an S3 heart sound signal, an S4 heart sound signal, an A-wave signal (pressure wave) and a P-wave signal. In some cases, signals that can provide an indication of a ventricular contraction may include one or more of an R-wave, a ventricle pressure signal, a ventricle change in pressure signal (dP/dt), a ventricle wall acceleration signal, a ventricle twist signal, a blood flow rate signal, and a ventricle volume signal. These are just some examples. 
     Some other events or artifacts detected may include, but are not limited to, S1 heart sounds, S2 heart sounds, ventricular volume, ventricular wall dimension, cardiac tissue and/or blood vibration, atrium to ventricle blood movement, ventricular wall and/or atrioventricular (AV) valve position, akinetic pressure, ventricular twist, and any other event or artifact suitable for identifying an atrial event, and/or combinations thereof. 
     It is contemplated that a number of different sensor modalities may be used to help detect atrial events or artifacts indicative of an atrial event from the ventricle.  FIG. 6  shows a table  320  that includes a column for each of various illustrative artifact(s), and a row for each illustrative sensor modality. An “X” indicates the sensor modalities that may be used to detect the corresponding artifact. 
     In  FIG. 6 , it can be seen that voltage may be used to detect P-waves, such as via an electrogram or an electrocardiogram (ECG). It is contemplated that, in some cases, an LCP implanted in the right ventricle may have a free end (e.g. end that is not affixed to the tissue) pointed towards the tricuspid valve. Due to their anatomical proximity, the electrodes of the LCP may be used to detect atrial depolarization (e.g., the p-wave). From the ventricle, the p-wave may be relatively small and difficult to detect. In some cases, the LCP may identify a time window around when the p-wave is expected, and the LCP may increase amplification and/or add special filtering and/or signal averaging (e.g. see  FIG. 11 ) to help identify the p-wave during the window. Alternatively, or in addition, the p-wave may be detected along with one or more other artifacts to help confirm an atrial contraction and to develop an atrial timing fiducial therefrom. 
     As shown in  FIG. 6 , pressure may be used to identify a number of different atrial artifacts. For example DC and/or near DC type pressure measurements (e.g. 0-10 Hz range) may be used to identify passive filling of the ventricle (e.g., akinetic pressure). Low frequency (e.g. 1-5 Hz range) AC type pressure measurements may be used to detect the A-wave (atrial pressure wave in the ventricle), while higher frequency (e.g. 15-30 Hz range) AC type pressure measurements may be used to detect heart sounds. These are just examples. In some cases, pressure may be used to identify the transition between passive and active filling modes. This transition may be used as an indicator of atrial contraction. Other suitable methods for measuring or detecting pressure in one or more heart chambers may also be used, as desired. Some illustrative but non-limiting pressure sensors and configurations for sensing pressure using an LCP are described in commonly assigned Patent Application No. 62/413,766 entitled “IMPLANTABLE MEDICAL DEVICE WITH PRESSURE SENSOR and filed on Oct. 27, 2016, and Patent Application No. 62/547,458, entitled IMPLANTABLE MEDICAL DEVICE WITH PRESSURE SENSOR and filed on Aug. 18, 2017, which are hereby incorporated by reference. 
     As shown in  FIG. 6 , impedance measurements may be used to determine ventricular volume changes which may then be used to infer a pressure wave (e.g. A-wave) due to an atrial contraction. In one example, as the volume of blood in the ventricle changes, the impedance between the electrodes of the LCP changes. It is contemplated that the rate of change in the volume (e.g., an increase in the rate of blood entering the ventricle and hence a faster change in volume of the ventricle) may be used to identify the start of active filling and thus an atrial contraction. Some illustrative uses of impedance measurements in the heart are described in commonly assigned patent application Ser. No. 15/630,677 entitled LEADLESS CARDIAC PACEMAKER FOR GENERATING CARDIAC PRESSURE-VOLUME LOOP and filed on Jun. 22, 2017, which is hereby incorporated by reference. 
     As blood enters the ventricle as a result of an atrial contraction, the ventricle may stretch. The stretching of the ventricle may be measure with a strain sensor. A strain sensor may require two or more points of fixation. Acceleration may be used to measure contractility of the heart H, as well as sounds. In some cases, cardiac output can be determined when acceleration measurements are combined with ventricle pressure, cardiac volume and/or other sensed parameters. 
     It should be understood that the table  320  shown in  FIG. 6  is not intended to include every possible artifact or sensor modality for detecting each artifact. Those of skill in the art will recognize that other artifacts, sensor modalities and/or combinations thereof may be used to identify an atrial event from the ventricle. In one additional example, a respiratory phase sensor may be used with other atrial artifacts described herein or by itself to help identify an atrial artifact. 
     The atrial event and/or artifacts indicative of an atrial event may occur during either or both passive ventricular filling or active ventricular filling.  FIG. 7  illustrates a table  330  of the cardiac phases, and the artifact(s) that may occur during that phases of the cardiac cycle, where an “X” is used to denote that the corresponding artifact occurs during the identified cardiac phase. Due to an electromechanical delay, the initial portion of the P-wave may fall into the passive filling phase while the later portion may fall into the active filling phase, and that is why an “X” is in both rows of the table  330 . Although not required, it is contemplated that force per unit area type measurements may be provided as a DC voltage or current and/or a low frequency pressure signal linearly proportional to pressure. Sound type pressure measurements (e.g., infrasonic and sonic) may be provided as an AC pressure. 
     In some instances, ultrasound may use a combined ultrasound source and sensor, although this is not required. The source and sensor may be separately provided, as desired. It is contemplated that ultrasound imaging may be used in a device implanted in the ventricle to see the atrial wall (e.g., through the tricuspid valve), tricuspid closing, and/or a flow increase due to an atrial contraction to help identify an A-wave. In some cases, ultrasound sensor may detect an atrial arrhythmia (e.g. atrial flutter or atrial fibrillation). During normal sinus rhythm (NSR) atrial blood flow into the ventricle is comprised of two sequential components, an E (early) wave followed by an A (atrial) wave. During atrial arrhythmias the E wave is largely unchanged from that in NSR, however the A wave is either missing (atrial fibrillation) or smaller and much faster (atrial flutter). During a detected atrial arrhythmia an LCP with an ultrasound sensor may modify its behavior (e.g. revert from VVD mode to VVI mode). 
     It should be noted that while the heart sounds are indicated as capable of being identified with an accelerometer, the accelerometer actually measures or detects mechanical vibration associated with the heart sound and not the pressure of the sound waves. In some cases, the measured artifact may not occur distinctly within one cardiac phase or another. For example, ventricular twist may be used to identify the end of active ventricular filling (e.g., ejection). Further, the S1 heart sound may occur at the end of active ventricular filling, while the S2 heart sound may occur shortly before the beginning of passive ventricular filling. These are just some examples. 
     In some cases, the LCP  100  may be configured to determine an atrial contraction timing fiducial based at least in part upon a sensed indication of an atrial contraction in a first heartbeat and/or a sensed indication of a ventricular contraction in the first heartbeat and/or one or more immediately preceding heartbeat(s). In some cases, the processing module  110  of the LCP  100  may be configured to generate and deliver a ventricle pacing pulse using the determined atrial contraction timing fiducial (e.g. after an A-V delay). 
     As described above, atrial events or artifacts indicative of an atrial event may be used by an LCP in the ventricle (e.g. right ventricle) to time a pacing pulse for the ventricle in support of treating bradycardia events. In some cases, the timing of the ventricle pacing pulse may be adjusted to improve the amount of blood entering the right ventricle through active filling. In some instances, this may include adjusting an AV delay relative to an atrial fiducial (e.g., atrial kick). In some cases, a measured pressure change (or other atrial fiducial) over time may be used to support management of a CRT cardiac therapy (if placed in the left ventricle), patient health status monitoring and/or any other suitable goal. It is contemplated that detecting events in one of or both of the ventricle and atrium using a single LCP implanted in the ventricle may replicate a dual chamber system with only a single device. That is, a single device positioned in the ventricle may listening to both the ventricle and the atrium and pacing accordingly (e.g., a VDD device). 
       FIG. 8  is a side view of an illustrative implantable leadless cardiac pacemaker (LCP)  400  which may be positioned within the ventricle and configured to listen to both the ventricle and the atrium. The LCP  400  may be similar in form and function to the LCP  100  described above. The LCP  400  may include any of the sensing, electrical, control, and/or pacing modules and/or structural features described herein. The LCP  400  may include a shell or housing  402  having a proximal end  404  and a distal end  406 . The illustrative LCP  400  includes a first electrode  410  secured relative to the housing  402  and positioned adjacent to the distal end  406  of the housing  402  and a second electrode  412  secured relative to the housing  402  and positioned adjacent to the proximal end  404  of the housing  402 . In some cases, the housing  402  may include a conductive material and may be insulated along a portion of its length. A section along the proximal end  404  may be free of insulation so as to define the second electrode  412 . The electrodes  410 ,  412  may be sensing and/or pacing electrodes to provide electro-therapy and/or sensing capabilities. The first electrode  410  may be capable of being positioned against or otherwise in contact with the cardiac tissue of the heart, while the second electrode  412  may be spaced away from the first electrode  410 . The first and/or second electrodes  410 ,  412  may be exposed to the environment outside the housing  402  (e.g., to blood and/or tissue). 
     It is contemplated that the housing  402  may take a variety of different shapes. For example, in some cases, the housing  402  may have a generally cylindrical shape. In other cases, the housing  402  may have a half-dome shape. In yet other embodiments, the housing  402  may be a rectangular prism. It is contemplated that the housing may take any cross sectional shape desired, including but not limited to annular, polygonal, oblong, square, etc. 
     In some cases, the LCP  400  may include a pulse generator (e.g., electrical circuitry) and a power source (e.g., a battery) within the housing  402  to provide electrical signals to the electrodes  410 ,  412  to control the pacing/sensing electrodes  410 ,  412 . While not explicitly shown, the LCP  400  may also include a communications module, an electrical sensing module, a mechanical sensing module, and/or a processing module, and the associated circuitry, similar in form and function to the modules  102 ,  106 ,  108 ,  110  described above. The various modules and electrical circuitry may be disposed within the housing  402 . Electrical communication between the pulse generator and the electrodes  410 ,  412  may provide electrical stimulation to heart tissue and/or sense a physiological condition. 
     In the example shown, the LCP  400  includes a fixation mechanism  414  proximate the distal end  406  of the housing  402 . The fixation mechanism  414  is configured to attach the LCP  400  to a wall of the heart H, or otherwise anchor the LCP  400  to the anatomy of the patient. As shown in  FIG. 8 , in some instances, the fixation mechanism  414  may include one or more, or a plurality of hooks or tines  416  anchored into the cardiac tissue of the heart H to attach the LCP  400  to a tissue wall. In other instances, the fixation mechanism  414  may include one or more, or a plurality of passive tines, configured to entangle with trabeculae within the chamber of the heart H and/or a helical fixation anchor configured to be screwed into a tissue wall to anchor the LCP  400  to the heart H. These are just examples. 
     The LCP  400  may further include a docking member  420  proximate the proximal end  404  of the housing  402 . The docking member  420  may be configured to facilitate delivery and/or retrieval of the LCP  400 . For example, the docking member  420  may extend from the proximal end  404  of the housing  402  along a longitudinal axis of the housing  402 . The docking member  420  may include a head portion  422  and a neck portion  424  extending between the housing  402  and the head portion  422 . The head portion  422  may be an enlarged portion relative to the neck portion  424 . For example, the head portion  422  may have a radial dimension from the longitudinal axis of the LCP  400  that is greater than a radial dimension of the neck portion  424  from the longitudinal axis of the LCP  400 . In some cases, the docking member  420  may further include a tether retention structure  426  extending from or recessed within the head portion  422 . The tether retention structure may define an opening  428  configured to receive a tether or other anchoring mechanism therethrough. The retention structure may take any shape that provides an enclosed perimeter surrounding the opening such that a tether may be securably and releasably passed (e.g., looped) through the opening  428 . In some cases, the retention structure may extend though the head portion  422 , along the neck portion  424 , and to or into the proximal end  404  of the housing  402 . The docking member  420  may be configured to facilitate delivery of the LCP  400  to the intracardiac site and/or retrieval of the LCP  400  from the intracardiac site. While this describes one example docking member  420 , it is contemplated that the docking member  420 , when provided, can have any suitable configuration. 
     It is contemplated that the LCP  400  may include one or more sensors  430  coupled to or formed within the housing  402  such that the sensor(s) is exposed to and/or otherwise operationally coupled with (e.g., responsive to) the environment outside the housing  402  to measure or detect various artifacts within the heart. The one or more sensors  430  may be of a same modality or a combination of two or more different sensing modalities, as desired. For example, the one or more sensors  430  may be use voltage, pressure, sound, ultrasound, impedance, strain, acceleration, flow, and/or rotation to detect P-waves, A-waves, S1-S4 heart sounds, ventricular volume, ventricular wall dimensions, cardiac tissue and/or blood vibration, atrium to ventricle blood movement, ventricular wall and/or atrioventricular valve position, akinetic pressure, and/or ventricular twist, such as described with respect to  FIGS. 6 and 7 . The sensors may be a part of, coupled to, and/or in electrical communication with a sensing module disposed within the housing  402 . In addition to sensing artifacts within the heart, the sensing module may be further configured to detect physiological conditions that may impact the LCP&#39;s ability to detect artifacts including, but not limited to posture, activity and/or respiration. The use of two or more sensors in combination may allow for the removal of some common mode noise (e.g., may eliminate gross body motion). 
     In some cases, the one or more sensors  430  may be coupled to an exterior surface of the housing  402 . In other cases, the one or more sensors  430  may be positioned within the housing  402  with an artifact acting on the housing and/or a port on the housing  402  to affect the sensor  430 . In one illustrative example, if the LCP  400  is placed in the right ventricle, the sensor(s)  430  may be a pressure sensor configured to measure a pressure within the right ventricle. If the LCP  400  is placed in another portion of the heart (such as one of the atriums or the left ventricle), the pressures sensor(s) may measure the pressure within that portion of the heart. In some cases, the sensor(s)  430  may be sensitive enough to detect an artifact in a heart chamber different from the chamber in which the LCP  400  is positioned. For example, in some instances a sensor  430  may detect a pressure change caused by an atrial contraction (e.g., atrial kick) when the LCP  400  is placed in the right ventricle. Some illustrative sensor configurations will be described in more detail herein. 
       FIG. 9A  is a plan view of the example leadless cardiac pacing device  400  implanted within a right ventricle RV of the heart H during ventricular filling. The right atrium RA, left ventricle LV, left atrium LA, and aorta A are also illustrated.  FIG. 9B  is a plan view of the leadless cardiac pacing device  610  implanted within a right ventricle of the heart H during ventricular contraction. These figures illustrate how the volume of the right ventricle may change over a cardiac cycle. As can be seen in  FIGS. 9A and 9B , the volume of the right ventricle during ventricular filling is larger than the volume of the right ventricle of the heart after ventricular contraction. 
     While it is desirable to identify an atrial contraction often associated with the A-wave, the A-wave can be difficult to detect as it may be very small in magnitude and detection of it may come and go. It is contemplated that a combination of sensor modalities and/or measured atrial artifacts may be used to identify an atrial timing fiducial. For example, it is contemplated that any of the sensor modalities identified with respect to  FIGS. 6 and 7  may be combined with any other sensor modality to identify an atrial timing fiducial. In some cases, a pressure signal may be used to determine a number of parameters. For example, a pressure signal may be used to determine or detect an A-wave (atrial kick). In another example, the pressure signal may be used to determine or detect a pressure pulse or pressure vibrations associated with S4, which may, for example, be in the 15-30 Hz range. In some cases, the S4 heart sound may be easier to detect using a pressure signal from a pressure sensor than from an accelerometer signal from accelerometer or using an acoustic signal from an acoustic sensor, particularly since the ventricular pressure is not changing substantially during this time period (ventricle is filling) and since there may be a great deal of unwanted signal (i.e. noise) in the accelerometer signal due to patient activity. In another example, a pressure signal may be used to determine a change in ventricle pressure relative to time (dP/dt). 
     In some cases, the circuitry and/or processing module of the LCP  400  may also be configured determine an atrial contraction timing fiducial based at least in part upon two or more of a signal received from the electrical sensing module, mechanical sensing module, and/or communication module. In some cases, the electrical cardiac signal received via the electrode arrangement  410 ,  412  may include at least a portion of an electrocardiogram (ECG). In some cases, the electrical cardiac signal received via electrode arrangement  410 ,  412  may include a P-wave. In some instances, the electrical cardiac signal received via the electrode arrangement  410 ,  412  may include a QRS complex, from which a QRS width can be determined. In some cases, the electrical cardiac signal received via electrode arrangement  410 ,  412  may include two consecutive R waves, from which an R-wave to R-wave interval can be determined. In some cases, the electrical cardiac signal may include a conducted or other communicated electrical signal from another device (e.g. SICD device) that includes an indication of an atrial or other contraction of the heart H. In some cases, the processing module and/or circuity may be configured to generate and deliver a ventricle pacing pulse using the atrial contraction timing fiducial. 
     It is contemplated that the use of sensors to determine an atrial contraction timing fiducial without having to detect the A-wave may allow the LCP  100 ,  400  to predict or recognize when an A-wave likely occurred, even when the A-wave itself was not detected. The predicted time of the A-wave may then be used as an atrial contraction timing fiducial for pacing the ventricle. The A-wave may be particularly difficult to detect when, for example, the heart is experiencing atrial fibrillation, a patient is in certain postures, the respiration rate is high, the patient activity is high, the heart rate is high, the atria are hypocontractile or akinetic, and/or during periods of high heart rate variability (HRV). 
     In the cardiac cycle, the ventricles receive blood from the atria first through passive filling and then through active filling. Discussion of passive and active filling will be described with reference to a right side of the heart, however, it should be understood that a similar process is occurring in the left side of the heart. Passive filling of the right ventricle begins when the there is a pressure gradient between the chambers causing the tricuspid valve to open and blood accumulated in the right atrium to flow into the right ventricle. Both the right atrium and the right ventricle continue to fill as blood returns to the heart. The right atrium contracts near the end of ventricular diastole. Atrial depolarization begins at the P-wave of the electrocardiogram. As a result of the P-wave, atrial cells develop tension and shortening causing the atrial pressure to increase (e.g. A-wave). These active contraction forces force additional volumes of blood into the ventricle (often referred to as the “atrial kick”). The active contraction forces begin the active filling phase. At normal heart rates, the atrial contractions are considered desirable for adequate ventricular filling. As heart rate increases, atrial filling becomes increasingly important for ventricular filling because the time interval between contractions for filling becomes progressively shorter. Atrial fibrillation and/or asynchronized atrial-ventricular contractions can result in a minimal contribution to preload via atrial contraction. 
     As described above, the fourth heart sound (e.g., S4) is typically a gallop sound that results from a forceful atrial contraction during presystole that ejected blood into a ventricle which cannot expand further. The fourth heart sound occurs during the last one-third of diastole about 90 milliseconds before S1. The frequency of S4 may be in the range of about 15 Hertz (Hz) to about 30 Hz, although the frequency may sometimes be outside this range. Due to the low pitch, S4 (and sometimes S3) are usually not audible with a typical stethoscope. It is contemplated that the S4 heart sound may be used to identify the start of active filling of the ventricle. In some cases, the processing module  110  and/or circuitry may be programmed to begin looking for the S4 heart sound just before the S1 heart sound is expected (projected from one or more previous heart beats). 
     The heart sounds may be time dependent on the heart rate in manner that changes linearly with the heart rate. For example, as the heart rate increases, the time between the heart sounds (e.g. S1 to S1; S4 to S1, etc.) may decrease in a linear and predicable manner. This may allow the S4 heart sound to be used to identify a reliable atrial event and/or as an atrial timing fiducial over a range of heart rates. 
     As noted above, the S4 heart sound may be identified and/or detected using a variety of different sensors, including but not limited to a higher frequency pressure sensor (e.g. 15 to 30 Hz), a hydrophone, a microphone, and/or an accelerometer. These are just some examples of how the LCP  400  can detect an artifact during active ventricular filling and identify an atrial timing fiducial based on the detected artifact. 
     While the above example was described with respect to active filling, it is contemplated that an artifact identified during passive filling may also be used to identify an atrial event which may then be used to identify an atrial timing fiducial. For example, the third heart sound (e.g., S3) occurs near the middle of passive filling. Passive filling may generate a very low frequency sound (in the range of 0 to 10 Hz) which may be detected by a DC capable pressure sensor. This may allow the S3 heart sound to be used to identify an atrial event and/or as an atrial timing fiducial over a range of heart rates. 
       FIG. 10  is a flow diagram showing an illustrative method  500  of generating a ventricular pulse using an LCP that is disposed with the right ventricle. In some cases, as indicated at block  502 , a first signal (e.g., an atrial artifact) indicating an atrial event of a patient&#39;s heart may be sensed with a sensing module of the LCP. A second different signal related to the atrial event of the patient&#39;s heart may also be detected, as indicated at block  504 . The second different signal may be sensed by the LCP, or may be received from another device (e.g. an SICD or another LCP) via a communication module of the LCP. 
     In some instances, the first signal and/or the second signal may be generated via one or more sensors within or on the housing the LCP. As described above, the sensing module of the LCP  400  may sense different events depending on whether attempting to identify active filling or passive filling (see  FIG. 7 ). Some illustrative sensing modalities for sensing active filling may include, but are not limited to impedance, strain, sound, rotation, or flow, any or all of which may be used to detect at least one of a P-wave, S2 heart sound, S3 heart sound, ventricular volume, ventricular wall dimension, ventricular blood movement, ventricular wall movement, tricuspid valve position, mitral valve position, and/or akinetic ventricular pressure. Some illustrative sensing modalities for sensing passive filling may include, but are not limited to pressure, impedance, strain, sound, rotation, acceleration, voltage, and flow, which may be used to detect at least one of a P-wave, A-wave, S1 heart sound, S4 heart sound, ventricular volume, ventricular wall dimension, cardiac tissue vibration, ventricular blood movement, ventricular wall movement, tricuspid valve position, and mitral valve position. 
     The circuitry within the LCP  400  may be configured to determine an atrial timing fiducial based at least in part on the first and/or second sensed signals, as indicated at block  506 . For example, the circuitry may be configured to determine when the A-wave occurs based on a sensed S4 heart sound and/or other atrial artifact. This is just one example. Those skilled in the art will recognize that any number of artifacts (or combinations thereof) can be used to determine an atrial timing fiducial. The circuitry may be configured to then generate and deliver a ventricular pacing pulse using the determined atrial timing fiducial, as indicated at block  508 . The control circuitry may delay delivering a pacing pulse to the ventricle until an appropriate AV delay expires after the determined atrial timing fiducial. Notably, a different AV delay may be used for different atrial timing fiducials (see  FIG. 14 ). 
     While the control timing of the pacing pulse may be triggered by an atrial timing fiducial that is based on arterial artifacts detected during a single heart beat, it is contemplated that the pacing pulse may be triggered by an atrial timing fiducial that is based on arterial and/or other artifacts detected during two or more previous heart beats. In some cases, the LCP may determine an average timing for a particular atrial artifact and/or atrial timing fiducial over multiple heart beats. 
     The circuitry of the LCP may further be configured to determine intrinsic intervals within the cardiac cycle. This capability may be provided within the control circuitry or provided as a separate interval determination module in the LCP. In some cases, the circuitry may be configured to identify intrinsic intervals including atrial to atrial event or artifact intervals, atrial to ventricle event or artifact intervals, ventricle to atrial event or artifact intervals, and/or ventricle to ventricle event or artifact intervals. This information may be useful in predicting when, for example, an atrial event (e.g. A-wave) is expected to occur. This may be useful in, for example, confirming an atrial event that is sensed by the LCP. This may also be useful in identifying a window of time around which an atrial event is expected, such that the LCP may increase amplification and/or add special filtering and/or signal averaging (e.g. see  FIG. 11 ) to help identify the atrial event during the window. 
     In some cases, the sensing module of the LCP may be configured to manipulate the signal prior to identifying an atrial event. For example, the sensing module may include one or more filters for filtering a signal. In some cases, the filter may include a first filter for passing a first frequency band, a second filter for passing a second frequency band, and a third filter for passing a third frequency band. The filter may include more than three frequency bands or fewer than three frequency bands, as desired. In some cases, the filter may be band-pass filter, a low pass filter, a high pass filter, and/or any other suitable filter. In some cases, a band-pass filter may be in the range of 1 to 5 Hz. In other cases, a bandpass filter may be in the range of 15 to 30 Hz. In yet another example, the filter may be a low-pass filter in the range of 0 to 10 Hz. These are just examples; other frequency ranges can be used, as desired. Also, filters may be employed that are not based on frequency, but rather some other signal feature such as amplitude, phase, etc. 
     In some cases it may be desirable to limit the time frame in which the LCP  400  is looking for an atrial artifact. For example, battery life may be increased when the circuitry is searching for an artifact only during a limited window or period of time that is less than an entire cardiac cycle. The method for determining a time window for searching for an atrial artifact may include first identifying an expected time frame for the atrial event (e.g., atrial contraction) and then defining a search window accordingly. Referring to  FIG. 11 , to begin, the control module may select one or more signals with a desirable characteristic for a first timing fiducial signal to use as a time reference. The signal may be one or more of a pressure signal, an acoustic single, an acceleration signal, an electrical signal, etc. It is contemplated that the fiducial signal may be a different signal from the signal used to identify an atrial artifact and hence an atrial event or atrial timing fiducial. In the example shown in  FIG. 12 , the selected signal may be an ECG  554  generated from electrical signals in the right ventricle. Within the ECG  554 , a specific feature, such as, but not limited to the R-wave may be selected as the fiducial reference feature  556 . The ECGs  554  signals for a plurality of cardiac cycles (e.g., at least two or more) may be averaged, with the fiducial reference features  556  in each ECG  554  aligned. This signal averaging technique may help reveal small signals by canceling out random noise. The signal averaging technique may also be used to identify various cardiac events, atrial event templates, appropriate A-V delays for a variety of different atrial timing fiducials (e.g. A-wave, P-wave, R-wave, and/or other atrial timing fiducial). 
     A window  560  where an atrial event is expected to occur can then be isolated. For example, an atrial event (e.g., atrial contraction) may be expected to occur within a time window  560  before the next R-wave. Using this time window  560 , the LCP may search for the atrial event. In some cases, the LCP may increase amplification and/or add special filtering and/or signal averaging to help identify the atrial event during the time window  560 . In some case, the window  560  can be used as a reference point for determining another window in which another signal should be recorded and searched to identify an atrial artifact from which an atrial event can be deduced. 
     In some cases, the timing window for identifying an atrial contraction may be based on artifacts occurring during passive filling of the ventricle. In some cases the down-stroke of the ventricular pressure (e.g., when the A-V valve opens) may be used to open a timing window for detecting an atrial artifact and/or atrial contraction. An upslope in ventricular pressure may trigger an open sensing window to detect the atrial kick.  FIG. 12  illustrates a portion of the pressure profile  600  of the right ventricle relative to the S3 and S4 heart sounds. The right ventricle may have an increase in pressure at the start of systole. The pressure may decrease as blood exits the ventricle. This sharp decrease in pressure may signal the control module to open a search window. For example, the search window may be opened in the general time frame indicated by box  602 . This may command the control module to begin searching for an atrial artifact that may be used to start a timing window. The timing window  604  may open at the S3 heart sound  606  and close at the R-wave  608 . The S4 heart sound and the atrial kick may occur within this timing window, as shown. It is contemplated that the control module may utilize automatic gain control to increase the sensitivity (e.g., reduce the threshold and/or increase the gain) over the period of the timing window to help increase the sensitivity when the expected event (e.g., atrial kick) is expected to occur. 
     In another example, the S2 heart sound may be used to identify the start of passive filling. It is contemplated that a pressure sensor in the LCP may be used to detect the pressure change associated with the atrial kick, or any of the atrial artifacts identified herein can be used either alone or in combination with the atrial kick as the atrial timing fiducial. The LCP  100 ,  400  can then pace the ventricle based off of the artifact, the atrial kick or a combination thereof. In another example, ventricular impedance may be used to identify volume changes in the ventricle, which may then be used to infer a pressure wave due to the atrial contraction. In another example, one or more atrial artifacts may be used to identify the end of passive filling for hemodynamic optimization. For example, passive filling may by typically completed approximately 500 milliseconds after the S2 heart sound. In yet another example, the timing window may be open between the S3 and S4 heart sounds. In some cases, the control module of the LCP may set a decreasing signal threshold to allow smaller signals to reach the input amplifier after the S3 heart sound in order to increase the signal. In some cases, the control module may be configured to run a continuous integration of the pressure signal as a surrogate for pressure, which may then be used to create a timing window. It is contemplated that changes in ventricular filling and/or pressures over time may be used to pick up respiration signals that may be used to support other features of the LCP  100 ,  400 . These are just some examples of how atrial artifacts can be detected by an LCP within the ventricle, which can then be used to identify an atrial timing fiducial for use in timing delivery of a pacing pulse to the ventricle. 
     It is contemplated that the control module of the LCP  100 ,  400  may be configured to search for an atrial artifact and identify a search window in more than one manner. In some cases, pacing can cover up, hide, or otherwise distort atrial artifacts and may make then difficult to identify. It may be desirable to allow the LCP to enter a listening mode in which the control circuitry does not issue pacing commands. The listening mode may be for a predefined window of time during a cardiac cycle that is less than the entire cardiac cycle. This may allow the LCP  400  to identify an atrial event without hiding or covering up the atrial artifact of interest (e.g. the A-wave). In some cases, such as when the patient is not pacing dependent, pacing can be paused for a cardiac cycle or two when no atrial activity is detected in order to determine if pacing is covering the atrial artifact(s) if interest. If the patient is pacing dependent, the pacing rate may be slowed (period extended) to allow for a larger period of time to search for the atrial artifacts without a pacing pulse present. Once the atrial artifact has been identified, the LCP may use the artifact to control the timing of the pacing pulse for a one or more cardiac cycles. In the event that an atrial artifact is not found, the LCP may return to its original pacing rate. In some cases, the LCP may be configured to pause or delay pacing and look for an atrial artifact and/or event on a predetermined time schedule. 
     It is further contemplated that in the event that the atrial artifact is not found the control module may be configured to deliver a pacing therapy at an altered pacing rate. In an example, the altered pacing rate may be less than the pacing rate delivered while atrial events are detected. In another example, the altered pacing rate may be greater than the pacing rate delivered while atrial events are detected. In a further example, the altered pacing rate may be static (e.g., remain constant) during the time there is a failure to detect atrial events. In yet another example, the altered pacing rate may be dynamic (e.g., change) during the time there is a failure to detect atrial events. 
     In another example, the control module may be configured to switch to a pacing only mode (in some cases a VOO mode). In this example, the control module may be configured to analyze the inputs received from the various sensor modules to determine if some sensors are providing a clearer signal than others. The control module may be configured to prioritize which sensor module is used to search for an atrial artifact and/or event before re-entering VDD mode. When in VOO mode, it may be desirable to pace off of the P-wave. However if this is not possible, it may be desirable to open a timing window based on other sensors including, but not limited to, pressure sensors and/or accelerometers to identify an atrial contraction. It is contemplated that the control module may be configured to switch between sensing modes and pacing modes as needed. 
     The control module may be configured to determine a quality threshold for a timing window, which may reflect the quality of the atrial artifact signal identified during the timing window. For example, the control module may be configured to analyze or grade a current A-wave timing window. If the current A-wave timing window does not meet certain quality metrics (e.g. percent of cardiac cycles in which an A-Wave is detected, the signal-to-noise ration of the detected A-wave signal, etc.), the control module may discard the window and use a previous window or calculate a new timing window. In some cases, the control module may prioritize one type of atrial artifact over another based on the quality of the detected signal. 
     As described above, the LCP  100 ,  400  may use different atrial and/or ventricle artifacts to determine when to search for an artifact and when to open the timing window. The LCP  100 ,  400  may include a sensing module that includes at least two of a pressure measurement module, an acoustic measurement module, an acceleration measurement module, and an electrogram measurement module. In some cases, the sensing module may include at least a pressure measurement module and at least one of an acoustic measurement module, an acceleration measurement module, and an electrogram measurement module. In some cases, the control module may use a ventricular event such as the R-wave to identify when to start a search window. In some cases, the control module may use different search windows to identify atrial artifacts from different measurement modules. The control module may identify a window of time during each of one or more cardiac cycles in which an atrial artifact and/or atrial event is expected to occur. The window of time may be less than an entire cardiac cycle. The control module may analyze information gathered by the sensing module (e.g., using at least one of the pressure measurement module, an acoustic measurement module, an acceleration measurement module, and an electrogram measurement module) to identify an atrial event (e.g., atrial kick). The control module may then deliver or command a pacing module to deliver a ventricular pacing pulse via the pacing electrodes of the LCP. The ventricular pacing pulse is delivered at a time that is based at least in part on the timing of the identified atrial event. For example, the pacing pulse may be delivered a predetermined length of time (e.g. A-V delay) after the identified atrial event. It is contemplated that the A-V delay that is used may depend on the particular atrial event that was identified. That is, different atrial events may cause different A-V delays to be applied. 
     The control module may be further configured to average the signals gathered from the sensing module in a similar manner to that described with respect to  FIG. 11 . For example, the control module may be configured to use signal averaging of the signals gather at the sensing module during each of a plurality of cardiac cycles to determine a signal average. The signal average may then be used to identify a window of time within a cardiac cycle. The identified window of time may then be used in subsequent cardiac cycles to search for and identify an atrial artifact and/or atrial event. 
     In some cases, the control module may be configured to move the window of time to search for an atrial artifact and/or atrial event. For example, if one of measurement modules of the sensor module is providing a better signal (e.g. better SNR), the control module may base the window around the detected artifact with a clearer signal. As the artifacts can occur at varying time points within the cardiac cycle, the window may be moved accordingly, sometimes cycle-to-cycle. The control module may be configured to select which measurement module to use dynamically or on a case to case basis. 
     In some cases, the control module may use different quality measurements to determine which measurement module to use. For example, the control module may select the measurement module with a better Signal-to-Noise Ratio (SNR). In another example, a p-wave detecting atrial activation by an electrogram measurement module may have a higher priority than a pressure signal detecting an atrial kick by a pressure measurement module. However, due to the inability of a ventricle only configuration to reliably sense a P-wave, the LCP may not rely solely on the P-wave to identify an atrial artifact and/or event. It may in fact, it may switch to detecting the A-wave when the P-wave is not available, and/or may use the A-wave to confirm the detection of a noisy P-wave. These are just examples. 
     In some cases, the control module may combine information gathered from more than one measurement module in identifying an atrial artifact and/or atrial event. For example, the control module may use both pressure data and electrocardiogram data in identifying an atrial artifact and/or event. In some cases, when data is used from two or more measurement modules, the data from each measurement module may be weighted differently (e.g., one may count more or be weighted more heavily than another). It is further contemplated that the control module may be configured to lengthen the window (e.g., make it longer) under certain conditions. For example, the window may not be long enough to identify an atrial artifact and/or event or a pacing pulse may be covering up the atrial event. In other cases, the window may be shortened (e.g., when noise is present, the noise may be reduced with a shortened window). 
     As described herein, the timing intervals for both searching and pacing may be based on pressure and/or heart sound fiducials (as well as other atrial artifacts described herein) as opposed to basing the intervals off solely of an electrocardiogram.  FIG. 13  is a graph  650  of illustrative cardiac signals including heart sounds, right ventricular pressure, and an electrocardiogram.  FIG. 13  also shows various intervals between various artifacts of these signals. It is contemplated that a number of different artifacts or characteristics during a cardiac cycle can be used to form a number of different timing intervals. For example, there can be intervals that extend between two electrocardiogram signals (E-E) such as between an R-wave amplitude  670  of a first cardiac cycle and an R-wave amplitude  672  of the next cardiac cycle, as indicated at arrow  652 . Another interval may be defined between two pressure signals (P-P), such as between an A-wave pressure  674  and a maximum systolic pressure  676  of the same cardiac cycle, as shown at arrow  654 , or between a maximum systolic pressure  676  of a first cardiac cycle and an A-wave pressure  678  in the subsequent cardiac cycle, as shown at arrow  656 . Another illustrative interval may be defined between two acoustic signals (A-A), such as between an S1 heart sound  680  and an S2 heart sound  682 , as shown at arrow  658 , between an S2 heart sound  682  and an S3 heart sound  684  as shown at arrow  660 , and/or between an S3 heart sound  684  and an S1 heart sound  686  of a subsequent cardiac cycle, as shown at arrow  662 . 
     As illustrated in  FIG. 13 , there can also be intervals defined between an electrocardiogram signal and a pressure signal (E-P), between a pressure signal and an electrocardiogram signal (P-E), between an electrocardiogram signal and an acoustic signal (E-A), between an acoustic signal and an electrocardiogram signal (A-E), between a pressure signal and an acoustic signal (P-A), and/or between an acoustic signal and a pressure signal (A-P). It is contemplated that any measurable parameter may serve as the beginning and/or end of an interval as desired, and the intervals are not limited to those explicitly described or shown in  FIG. 13 . 
       FIG. 14  is a graph  700  of illustrative cardiac signals including heart sounds, right ventricular pressure, and an electrocardiogram.  FIG. 14  also shows various intervals between various artifacts of these signals. As described herein, there can be a number of different intervals using various sensed parameters. Not only can there be various intervals from a sensed artifact to another sensed artifact, but also various intervals from a sensed artifact to a pacing pulse. 
     The E-E (R-wave to subsequent R-wave), A-A (S1 to subsequent S1), and P-P (max pressure to subsequent max pressure) intervals shown at  702  are three ventricular intervals. The E-E (P-wave to subsequent P-wave), A-A (S4 to subsequent S4), and P-P (atrial kick to subsequent atrial kick) intervals shown at  704  are three atrial intervals. These intrinsic same chamber intervals  706  have the same or roughly the same time interval between same sensed artifacts or events regardless of which parameter is used (e.g., R-wave to R-wave, S1 to S1, max pressure to max pressure). In contrast, intervals between chambers  708  vary substantially. As can be seen at  708  in  FIG. 14 , atrioventricular (A-V) intervals vary significantly depending on which atrial event is selected for the atrial timing fiducial. An E-E (P-wave to R-wave) interval, A-E (S4 to R-wave), and P-E (atrial kick to R-wave) intervals shown at  708  are three illustrative atrioventricular (AV) intervals each having a different duration. The duration of each of these AV intervals can be sensed during one or more intrinsic heart beats (e.g. no pacing). In some cases, the duration of each of these intervals can be sensed during a plurality of intrinsic heart beats (no pacing) and then averaged, resulting in an average AV interval for each of the different atrial timing fiducials as shown at  710 . 
     As described above, a P-wave may not be consistently detected in a device implanted in the ventricle. As such, it may be desirable to time the ventricular pacing pulse (V P ) using a pressure artifact (e.g., a-wave or atrial kick) as the atrial timing fiducial along with a corresponding AV interval. In another example, it may be desirable to time the ventricular pacing pulse (V P ) using an acoustic artifact (e.g., S4) as the atrial timing fiducial along with a corresponding AV interval. The corresponding AV interval used with an acoustic artifact (e.g., S4) may be different than the AV interval used with a pressure artifact, as seen at  710  in  FIG. 14 . In yet another example, it may be desirable to time the ventricular pacing pulse (V P ) using an electrical artifact (e.g., P-wave) as the atrial timing fiducial along with a corresponding AV interval, as seen at  712  in  FIG. 14 . These are just examples. The LCP may dynamically switch between these and other atrial timing fiducials, depending on a number of factors such as the quality of the signals that are currently sensed. In some cases, an atrial timing fiducial may be determined from two or more cardiac artifacts, sometimes with one weighted more than the others. 
     The sensing module of the LCP  100 ,  400  may include one or more of a pressure measurement module and an acoustic measurement module. However, other measurement modules may be used as desired, including but not limited to, measurement modules that include suitable sensors for determining the artifacts described with respect to  FIGS. 6 and 7 . For example, the sensing module may further include an electrogram measurement module. As described herein, the sensing module may be configured to gather information suitable for determining one or more atrial timing fiducials. The information may include, but is not limited to, an atrial artifact such as any of those discussed with reference to  FIGS. 6 and 7 . In some cases, information gathered from one of the measurement modules may be used to determine a blanking interval for another measurement module. 
     In some cases, a pressure measurement module may detect or determine at least one of a maximum pressure (atrial or ventricular), a minimum pressure (atrial or ventricular), a mean pressure (atrial or ventricular), a pressure time integral (atrial or ventricular), and/or a pressure time derivative (atrial or ventricular). An acoustic measurement module may detect or determine at least one of an S1 heart sound, an S2 heart sound, an S3 heart sound, and/or an S4 heart sound. An acceleration measurement module, if present, may detect or determine at least of an S1 heart sound, an S2 heart sound, an S3 heart sound, an S4 heart sound, myocardial (e.g., heart wall) movement, patient activity and/or patient posture. These and other artifacts may be used as the basis for an atrial timing fiducial. 
     In some cases, it may be desirable for the LCP  100 ,  400  to be configured to operate in a number of different pacing modes. Some illustrative pacing modes may include, but are not limited to VDD, VDDR, VVI, VVIR, VOO, and VOOR. As used herein, the pacing modes use the North American Society of Pacing and Electrophysiology (NASPE) and British Pacing and Electrophysiology Group (BPEG) pacemaker codes as outlined in Table 1 below: 
                     TABLE 1                  NASPE/BPEG Revised in 2002 NBG Pacemaker Code                                 Position I   Position II   Position III   Position IV   Position V               (Chamber Paced)   (Chamber Sensed)   (Response to Sensed Event)   (Programmability,   (Multisite Pacing)       O = none   O = none   O = none   Rate Modulation)   O = none       A = atrium   A = atrium   I = inhibited       A = atrium       V = ventricle   V = ventricle   T = triggered   O = none   V = ventricle       D = dual (A + V)   D = dual (A + V)   D = dual (T + I)   R = rate modulation   D = dual (A + V)               Miller R D. Miller’s Anesthesia, 6 th  ed. Philadelphia: Elsevier, Inc, 2005, pp 1417.            
A VDD device is a device pacing in the ventricle, sensing both the atrium and the ventricle, and using triggered and inhibited pacing.
 
     It is contemplated that a right ventricle LCP  100 ,  400  using remote tracking of atrial activity such as described herein may automatically revert from one pacing mode to another, depending on one or more sensed conditions. The reversionary behavior may be desirable for safe operation and/or for enhancing the effectiveness of the pacing therapy. The control module may be configured to search for and identify conditions that may indicate a reversion is desirable. Some conditions may include, but are not limited to: an atrial artifact (e.g., atrial timing fiducial) occurring too close to the R-wave (or other ventricular fiducial); a hemodynamic response that indicates that the present pacing therapy is worse than another pacing therapy or no pacing therapy, either actual or anticipated (due to one or more of posture, heart rate, respiratory rate, respiratory cycle, patient activity, physiological noise, environmental noise, etc.); continuous or intermittent loss of an atrial tracking artifact or fiducial; actual or anticipated continuous or intermittent loss of an atrial tracking artifact or fiducial due to search algorithms associated with reacquiring an atrial artifact or fiducial; a time period between adjacent atrial artifacts or fiducials being too short (e.g., due to over sensing caused by physiological or environmental noise or atrial tachyarrhythmia); and/or a ventricular interval being too short (e.g., due to over sensing caused by physiological or environmental noise or ventricular tachyarrhythmia). These are just some examples. Other events and conditions may be detected and cause reversionary behavior. 
     The LCP  100 ,  400  may experience or be configured to use different types of reversionary behavior based on the current conditions. In a first example, the control module may be configured to change pacing modes. In the event of a loss of an atrial timing fiducial, an atrial rate above a specified threshold, or atrial noise above a threshold, the LCP may be configured to automatically switch between VDD and VVI modes. In the event of ventricular noise being above a threshold, the LCP may be configured to automatically switch between VDD or VVI and VOO modes. In the event of a reduction in the hemodynamic signal, the LCP may be configured to automatically switch between VDD or VVI and OOO modes. 
     In an example LCP  100 ,  400  reverts to a VDI mode wherein the device continues to search and/or measure atrial artifacts but does not use any detected atrial artifacts to trigger ventricular pacing. If the LCP determines the atrial fiducial can be reliably determined the LCP reverts back to a mode that allows triggering of ventricular paces from the atrial fiducial (e.g. VDD mode). 
     In some cases, the control module may be configured to manipulate the tracking algorithm. For example, the control module may switch between continuous tracking and intermittent tracking with tracking estimate and search. In yet another example, the atrial timing fiducial signal may be reverted. In another example, the type of signal and/or portion of signal used to determine the atrial timing fiducial may be changed or switched. In yet another example, the atrial timing fiducial may be changed from a first weighted average of two or more signals to a second different weighted average of the same or different two or more signals. These are just examples. 
       FIG. 15  is a flow chart  800  of an illustrative method for determining whether the LCP  100 ,  400  should utilize reversion. The control module may continuously verify the current pacing mode is the best under the current conditions. If a reversion is needed, the control module may dynamically change the pacing mode (e.g., change on a beat-to-beat basis, if needed). The LCP  100 ,  400  may first deliver a pace, as shown at  802 . After delivering the pace, the control module may check to see if the atrial artifact and/or event (e.g., atrial timing fiducial) was detected, as shown at  804 . If the atrial timing fiducial was detected, the LCP  100 ,  400  may continue with its normal operational mode, which in some cases may be VDD tracking with the pacing occurring after the corresponding AV interval, as shown at  806 . An exception may occur when the normal VDD tracking inhibits a pacing pulse due to preventricular contraction, as shown at  808 . If the atrial timing fiducial was not detected or resolved, the LCP  100 ,  400  may enter a reversionary mode, as shown at  810 . In some cases, in the reversionary mode, the LCP  100 ,  400  may enter a VDD pseudo tracking mode in which the LCP  100 ,  400  paces using an estimated atrial timing fiducial time. Other reversionary modes may be used as appropriate. The control module may be configured to select a ventricular pacing therapy and/or mode based at least in part on one or more of the tracked atrial artifacts/events. In some cases, a first ventricular pacing therapy may have a first pacing rate and the reversionary (or second) ventricular pacing therapy may have a second pacing rate different from the first. For example, the reversionary ventricular pacing therapy may extend the pacing rate to aid in the search for the atrial timing fiducial. 
       FIG. 16  illustrates a comparison of pacing intervals on an electrocardiogram when the LCP  100 ,  400  is operating in a normal VDD mode  820  and pacing intervals on an electrocardiogram when the LCP  100 ,  400  is operating in a VDD pseudo tracking mode  830 . As can be seen in the normal VDD mode  820 , the control module is detecting an atrial timing fiducial  822 , and using the appropriate AV delay  824 , delivers the pacing pulse  826  at the appropriate time. The LCP  100 ,  400  will continue to operate in this manner unless conditions change that make VDD pacing unsafe or less desirable. 
       FIG. 16  illustrates an example in which the control module of the LCP  100 ,  400  has determined that the atrial artifact/event  832  is missing or unreliable. The control module may then sense a ventricular event  834  (such as but not limited to the R-wave) and essentially use the ventricular event  834  as the atrial pacing fiducial for the next cardiac cycle. The control module may determine an appropriate AV interval  836 , calculated using the ventricular intrinsic interval (e.g., R-wave to subsequent R-wave) minus a previously stored pace to R wave interval. In the example shown, starting at the sensed ventricular event  834 , the pacing pulse  838  may be delivered at a time equal to the R-wave to R-wave intrinsic interval minus a percentage of the historical AV interval. It is contemplated that the percentage of the historical AV interval may be in the range of about 30 to 70%. Alternatively, a fixed time period, such as, but not limited about 200 milliseconds may be used in place of the percentage of the historical AV interval or a previously stored pace to R wave interval. The control module may then continue to pace using the R-wave to R-wave intrinsic interval  840  as the timing interval using the pacing pulse  838  as the timing fiducial until a suitable atrial timing fiducial is identified. If the control module fails to re-acquire a suitable atrial timing fiducial, the control module may command the device to enter a search mode in an attempt to detect an intrinsic atrial and/or ventricular event. This is just one example of a reversionary scenario. 
     The control module may be configured to determine the accuracy of the atrial timing fiducial by analyzing the effectiveness of the pacing therapy. In one example, the control module may use the upstroke (e.g., dP/dt or peak pressure) on sequential cardiac cycles to estimate the accuracy of the A-wave detection. If the LCP  100 ,  400  is pacing at an incorrect time due to an inaccurate atrial timing fiducial, passive filling may be reduced thereby reducing the dP/dt. Similarly, the control module may use an integrator to find the area under the pressure waveform, which may represents the filling volume of the ventricle, or impedance between the electrodes of the LCP may be used as an indication of ventricle volume. Poor filling volume may indicate an inaccurate atrial timing fiducial. In yet another example, the control module may be configured to search for edges using, for example a high-pass pole to create a differentiator to help identify a sub-par atrial timing fiducial. If the atrial timing fiducial is determined to be inaccurate, the LCP may revert to asynchronous pacing (e.g. VOO mode). Alternatively, if the patient&#39;s intrinsic heart rate is high enough (e.g. 50 BPM), the LCP may revert to no pacing (e.g. OOO mode). 
     In addition to a first-order differentiator, higher-order differentiation could further provide better timing fiducials (e.g., more crisp) and other measures such as verification of signal quality. A third order time derivative of position is known as “jerk”, which is the change in acceleration with respect to time.  FIG. 17  illustrates a graph  900  of an illustrative relationship of higher order differentiation of a signal  902 . The signal  902  could be any suitable signal including an egram, a pressure signal, an acceleration signal, or any other suitable signal. The change in pressure with respect to time may be considered the first derivative  904 . The change in the first derivative  904  with respect to time may be considered the second derivative  906 . The change in second derivative with respect to time may be considered as an equivalent to jerk  908  (or the third derivative). When the signal is a pressure signal, the inflections produced by an A-wave may give third order blips which could be used for timing or verifying a quality of the signal. 
     Those skilled in the art will recognize that the present disclosure may be manifested in a variety of forms other than the specific examples described and contemplated herein. For instance, as described herein, various examples include one or more modules described as performing various functions. However, other examples may include additional modules that split the described functions up over more modules than that described herein. Additionally, other examples may consolidate the described functions into fewer modules. Accordingly, departure in form and detail may be made without departing from the scope and spirit of the present disclosure as described in the appended claims.