Patent Publication Number: US-10758737-B2

Title: Using sensor data from an intracardially implanted medical device to influence operation of an extracardially implantable cardioverter

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/397,905 filed on Sep. 21, 2016, 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 an intracardially implanted medical device such as 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, cardiac 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, cardioverters, 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. 
     SUMMARY 
     This disclosure generally relates to medical devices, and more particularly, to systems that use sensor data from an intracardially implanted medical device such as a leadless cardiac pacemaker to influence operation of an extracardially implantable cardioverter such as a subcutaneous implantable cardioverter defibrillator (SICD). In an example of the disclosure, a medical system for sensing and regulating cardiac activity of a patient includes a cardioverter that is configured to generate and deliver anti-arrhythmic therapy to cardiac tissue, and a leadless cardiac pacemaker (LCP) that is configured to sense cardiac activity and to communicate with the cardioverter. The cardioverter may be configured to detect a possible arrhythmia and, upon detecting the possible arrhythmia, may send a verification request to the LCP soliciting verification from the LCP that the possible arrhythmia is occurring. The LCP, upon receiving the verification request from the cardioverter, may be configured to use signals from one or more of a plurality of sensors of the LCP to attempt to confirm that the possible arrhythmia is occurring. 
     The LCP may be configured to send a confirmation response to the cardioverter if the LCP confirms that the possible arrhythmia is occurring. In some cases, the cardioverter may be configured to generate and deliver a therapy to cardiac tissue if the LCP confirms that the possible arrhythmia is occurring and to inhibit delivery of a therapy to cardiac tissue if the LCP did not confirm that the possible arrhythmia is occurring. 
     Alternatively or additionally to any of the embodiments above, at least some of the plurality of sensors of the LCP include a first sensor that when activated consumes a first level of power and a second sensor that when activated consumes a second level of power, wherein the second level of power is higher than the first level of power. Upon receiving the verification request from the cardioverter, the LCP may be configured to initially activate the first sensor to attempt to confirm that the possible arrhythmia is occurring. If the LCP confirms that the possible arrhythmia is occurring using the first sensor, the LCP may be configured to send the confirmation response to the cardioverter. If the LCP does not confirm that the possible arrhythmia is occurring using the first sensor, the LCP may be configured to activate the second sensor to attempt to confirm that the possible arrhythmia is occurring. If the LCP confirms that the possible arrhythmia is occurring using the second sensor, the LCP may be configured to send the confirmation response to the cardioverter. 
     Alternatively or additionally to any of the embodiments above, the LCP further includes a third sensor that when activated consumes a third level of power, wherein the third level of power is higher than the second level of power. If the LCP does not confirm that the possible arrhythmia is occurring using the second sensor, the LCP may be configured to activate the third sensor to attempt to confirm that the possible arrhythmia is occurring. If the LCP confirms that the possible arrhythmia is occurring using the third sensor, the LCP may be configured to send the confirmation response to the cardioverter. 
     Alternatively or additionally to any of the embodiments above, the LCP may include at least a first electrode and a second electrode, and the first sensor comprises detecting electrical cardiac activity via the first electrode and the second electrode. 
     Alternatively or additionally to any of the embodiments above, the second sensor may be configured to detect heart sounds. 
     Alternatively or additionally to any of the embodiments above, the second sensor may include an accelerometer disposed relative to the LCP. 
     Alternatively or additionally to any of the embodiments above, the second sensor may include a pressure sensor disposed relative to the LCP. 
     Alternatively or additionally to any of the embodiments above, the third sensor may include an optical sensor. 
     Alternatively or additionally to any of the embodiments above, the verification request from the cardioverter may include an indication of severity of the possible arrhythmia, and if the indication of severity exceeds a threshold severity level, the LCP may be configured to concurrently activate two or more of the plurality of sensors and to use the concurrently activated two or more of the plurality of sensors to attempt to confirm that the possible arrhythmia is occurring in an expedited manner. 
     Alternatively or additionally to any of the embodiments above, the LCP may be configured to concurrently activate two of the plurality of sensors upon receiving the verification request from the cardioverter and to examine both a signal from a first sensor of the plurality of sensors and a signal from a second sensor of the plurality of sensors to attempt to confirm that the possible arrhythmia is occurring. 
     In another example of the disclosure, a leadless cardiac pacemaker (LCP) that is configured for implantation relative to a patient&#39;s heart and to sense electrical cardiac activity and deliver pacing pulses when appropriate includes a housing, a first electrode that is secured relative to the housing and a second electrode that is secured relative to the housing and is spaced from the first electrode. A controller may be disposed within the housing and operably coupled to the first electrode and the second electrode such that the controller is capable of receiving, via the first electrode and the second electrode, electrical cardiac signals of the heart. In some cases, the first electrode and the second electrode form a first sensor that, when activated, consumes a first level of power. The LCP may include a second sensor that is disposed relative to the housing and operably coupled to the controller, the second sensor, when activated, consumes a second level of power that is higher than the first level of power. A communications module may be disposed relative to the housing and operably coupled to the controller, the communications module configured to receive a verification request from a cardioverter to confirm that a possible arrhythmia is occurring. Upon receipt of the verification request from the cardioverter via the communications module, the controller may be configured to initially sense cardiac activity using the first sensor to help confirm that the possible arrhythmia is occurring while the second sensor is in a lower power state. In some cases, if the possible arrhythmia is not confirmed using the first sensor, the controller may be configured to activate the second sensor from the lower power state to a higher power state, and then sense cardiac activity using the second sensor to help confirm that the possible arrhythmia is occurring. 
     Alternatively or additionally to any of the embodiments above, the second sensor may include an accelerometer or a pressure sensor. 
     Alternatively or additionally to any of the embodiments above, the LCP may further include a third sensor that is disposed relative to the housing and operably coupled to the controller, the third sensor, when activated, consumes a third level of power that is higher than the second level of power. If the possible arrhythmia is not confirmed using the second sensor, the controller may be configured to activate the third sensor from the lower power state to a higher power state, and then attempt to confirm that the possible arrhythmia is occurring using the third sensor. The controller sends the signal from the third sensor to the cardioverter so that the cardioverter may be able to determine whether the possible arrhythmia is occurring. 
     Alternatively or additionally to any of the embodiments above, the second sensor may include an accelerometer, and the third sensor may include a pressure sensor or an optical sensor. 
     Alternatively or additionally to any of the embodiments above, the verification request from the cardioverter may include an indication of severity of the possible arrhythmia, and if the indication of severity exceeds a threshold severity level, the controller may be configured to concurrently activate the first sensor and the second sensor in order to more quickly confirm or deny the possible arrhythmia. 
     Alternatively or additionally to any of the embodiments above, the cardioverter may be configured to examine a relationship between a signal from the first sensor and a signal from the second sensor to attempt to confirm that the possible arrhythmia is occurring. 
     Alternatively or additionally to any of the embodiments above, if the controller does not confirm that the possible arrhythmia is occurring using the first sensor, the controller may be configured to activate the first sensor and the second sensor and to send a signal to the cardioverter so that the cardioverter can examine a relationship between a signal from the first sensor and a signal from the second sensor to attempt to confirm that the possible arrhythmia is occurring. 
     In another example of the disclosure, a method of regulating a patient&#39;s heart includes using a medical system including a cardioverter and a leadless cardiac pacemaker (LCP). The cardioverter may be configured to monitor a cardiac EGM via electrodes disposed on an electrode support and deliver shock therapy via the electrodes and the LCP may configured to sense electrical cardiac activity via LCP electrodes disposed on the LCP and may include one or more additional sensors. The cardioverter may be used in a chronic monitoring mode in which the cardioverter monitors the cardiac EGM for indications of a possible arrhythmia. An acute mode may be activated if the cardioverter identifies a possible arrhythmia, and the LCP may be instructed to help confirm the possible arrhythmia using the LCP electrodes and/or at least one of the one or more additional sensors of the LCP. If the possible arrhythmia is confirmed (e.g. by the LCP or SICD), and if the possible arrhythmia is dangerous, shock therapy may be delivered to the heart via the electrodes of the cardioverter. If the possible arrhythmia is confirmed and is not dangerous, delivery of shock therapy to the heart via the electrodes of the cardioverter may be inhibited, and the acute mode continues in which the LCP electrodes and/or the at least one of the one or more additional sensors of the LCP are used to monitor cardiac activity. If the possible arrhythmia is not confirmed, delivery of shock therapy to the heart via the electrodes of the cardioverter may be inhibited and the acute mode may continue in which the LCP electrodes and/or the at least one of the one or more additional sensors of the LCP are used to monitor cardiac activity. 
     Alternatively or additionally to any of the embodiments above, the cardioverter may return to the chronic monitoring mode once the possible arrhythmia has terminated. 
     Alternatively or additionally to any of the embodiments above, the one or more additional sensors may include one or more of an accelerometer, a pressure sensor, and an optical sensor. 
     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 highly schematic diagram of an illustrative system in accordance with an example of the disclosure; 
         FIG. 2  is a graphical representation of an electrocardiogram (ECG) showing a temporal relationship between electrical signals of the heart and mechanical indications of contraction of the heart; 
         FIG. 3  is a graph showing example ECG signals, pressures, volumes and sounds within the heart over two example heart beats; 
         FIG. 4  is a schematic block diagram of an illustrative subcutaneous implantable cardioverter defibrillator (SICD) usable in the system of  FIG. 1 ; 
         FIG. 5  is a schematic block diagram of an illustrative leadless cardiac pacemaker (LCP) useable in the system of  FIG. 1 ; 
         FIG. 6  is a schematic block diagram of an illustrative leadless cardiac pacemaker (LCP) useable in the system of  FIG. 1 ; 
         FIG. 7  is a schematic block diagram of an illustrative leadless cardiac pacemaker (LCP) useable in the system of  FIG. 1 ; 
         FIG. 8  is a more detailed schematic block diagram of an illustrative LCP in accordance with an example of the disclosure; 
         FIG. 9  is a schematic block diagram of another illustrative medical device that may be used in conjunction with the LCP of  FIG. 8 ; 
         FIG. 10  is a schematic diagram of an exemplary medical system that includes multiple LCPs and/or other devices in communication with one another; 
         FIG. 11  is a schematic diagram of a system including an LCP and another medical device, in accordance with an example of the disclosure; 
         FIG. 12  is a side view of an illustrative implantable leadless cardiac device; and 
         FIG. 13  is a flow diagram of an illustrative method for regulating a patient&#39;s heart using the system of  FIG. 1 . 
     
    
    
     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. 
     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 forces 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. Implantable medical device are often used to treat such conditions by delivering one or more types of electrical stimulation therapy to the patient&#39;s heart. 
       FIG. 1  is a schematic diagram showing an illustrative system  10  that may be used to sense and/or pace a heart H. In some cases, the system  10  may also be configured to be able to shock the heart H. The heart H includes a right atrium RA and a right ventricle RV. The heart H also includes a left atrium LA and a left ventricle LV. In some cases, the system  10  may include a medical device that provides anti-arrhythmic therapy to the heart H. In some cases, the medical device is an SICD (subcutaneous implantable cardioverter defibrillator)  12 . While not shown in this Figure, in some cases the SICD  12  may include a lead that may be configured to be placed subcutaneously and outside of a patient&#39;s sternum. In other cases, the lead may extend around or through the sternum and may be fixed adjacent an inner surface of the sternum. In both cases, the lead is positioned extracardially (outside of the patient&#39;s heart). The SICD  12  may be configured to sense electrical activity generated by the heart H as well as provide electrical energy to the heart H in order to shock the heart H from an undesired heart rhythm to a desired heart rhythm. 
     In some cases, the system  10  may include an intracardially implanted medical device such as a cardiac monitor, a leadless cardiac pacemaker (LCP) or the like. In the example shown, the intracardially implanted medical device is an LCP  14  that is configured to sense and/or pace the heart H. While a single LCP  14  is illustrated, it will be appreciated that two or more LCPs  14  may be implanted in or on the heart H. The LCP  14  may be implanted into any chamber of the heart, such as the right atrium RA, the left atrium LA, the right ventricle RV and the left ventricle LV. When more than one LCP is provided, each LCP may be implanted in a different chamber. In some cases, multiple LCP&#39;s may be implanted within a single chamber of the heart H. 
     In some cases, the SICD  12  and the LCP  14  may be implanted at the same time. In some instances, depending on the cardiac deficiencies of a particular patient, the SICD  12  may be implanted first, and one or more LCPs  14  may be implanted at a later date if/when the patient develops indications for receiving cardiac resynchronization therapy and it becomes necessary to pace the heart H. In some cases, it is contemplated that one or more LCPs  14  may be implanted first, in order to sense and pace the heart H. When a need for possible defibrillation becomes evident, the SICD  12  may subsequently be implanted. Regardless of implantation order or sequence, it will be appreciated that the SICD  12  and the LCP  14  may communicate with each other using any desired communications modality, such as conducted communication, inductive communication, acoustic communication, RF communication and/or using any other suitable communication modality. 
     In situations in which the SICD  12  and the LCP  14  (or additional LCPs) are co-implanted, the SICD  12  may detect a possible arrhythmia. Rather than automatically delivering a defibrillation pulse, the SICD  12  may send a verification request to a co-implanted LCP  14 , requesting that the LCP  14 , from its vantage point, verify whether the LCP  14  is also detecting the possible arrhythmia. In some cases, the LCP  14 , upon receiving the verification request, may activate one or more of a plurality of sensors to determine whether the LCP is able to confirm or deny the possible arrhythmia seen by the SICD  12 . In some cases, activating a sensor may include powering up a sensor that was previously unpowered. In some cases, activating a sensor may include increasing a power level of the sensor from a first lower power level to a second higher power level that may for example provide increased sensitivity. 
     In some cases, the LCP  14  may initially activate a first sensor. If the first sensor provides verification of the arrhythmia, the LCP  14  may communicate the verification to the SICD  12 , which in response may deliver a shock to cardiac tissue. If the first sensor is not able to provide verification of the arrhythmia, or verify a lack of an arrhythmia, the LCP  14  may activate a second sensor that may, for example, be more sensitive than the first sensor at the expense of additional power consumption. If the second sensor is able to provide verification of the arrhythmia, or verify a lack of an arrhythmia, the LCP  14  may communicate the verification to the SICD  12 . If the second sensor is not able to provide verification of the arrhythmia, or verify the lack of an arrhythmia, the LCP  14  may activate a third sensor that may, for example, be more sensitive than the first sensor or the second sensor at the expense of additional power consumption. 
     In some cases, the verification request from the SICD  12  may include an indication of severity of the possible arrhythmia. If, for example, the possible arrhythmia is not severe, the LCP  14  may sequentially activate one sensor at a time, as described above, in order to verify or deny the possible arrhythmia without consuming more power than needed. In some cases, however, if the possible arrhythmia is deemed severe, such as if the indication of severity exceeds a threshold severity level, the LCP  14  may concurrently activate two or more sensors in order to more quickly provide either verification that the possible arrhythmia exists, or verification that the possible arrhythmia does not exist. If the LCP  14  does not verify the possible arrhythmia initially detected by the SICD  12 , the SICD  12  may delay or inhibit shock therapy. 
     In some cases, rather than sending a verification request upon initially sensing a possible arrhythmia, the SICD  12  may instead instruct the LCP  14  to activate a first sensor and then transmit a signal from the LCP  14  providing the SICD  12  with the signal from the first sensor. If the SICD  12  is not able to confirm the possible arrhythmia from the first sensor data, the SICD  12  may instruct the LCP  14  to activate a second sensor and then transmit a signal from the LCP  14  providing the SICD  12  with the signal from the second sensor. If the SICD  12  is not able to confirm the possible arrhythmia form the second sensor data, the SICD  12  may instruct the LCP  14  to activate a third sensor, or to activate several sensors simultaneously. The SICD  12  may generate and deliver, or may inhibit delivery of a shock to cardiac tissue based at least in part upon information received from the LCP  14 . 
     In some cases, the LCP  14  may not be able to confirm the possible arrhythmia, or the LCP  14  may not be able to communicate successfully with the SICD  12  even if the LCP  14  is able to confirm the possible arrhythmia. In some cases, for example, the SICD  12  may have a fail-safe mode in which the SICD  12  will automatically react to a possible arrhythmia if the LCP  14  is not able to confirm the possible arrhythmia and/or communicate its findings to the SICD  12 . In some cases, this may be a programmable setting. For some patients, a physician may program the SICD  12  to default to inhibiting therapy if the LCP  14  is unable to confirm. For other patients, a physician may program the SICD  12  to default to delivering therapy if the LCP  14  is unable to confirm the possible arrhythmia or communicate its findings. In some cases, the SICD  12  may be programmed, if an immediately dangerous or fatal arrhythmia is detected, to immediately deliver therapy without asking the LCP  14  for confirmation and/or without waiting for a reply from the LCP  14 . In some cases, how the SICD  12  responds to a possible arrhythmia, absent confirmation from the LCP  14 , may depend upon the severity of the possible arrhythmia. For example, some arrhythmias such as atrial fibrillation, superventricular tachycardia and low rate (under 150 beats per minutes) ventricular tachycardia may be considered as not immediately dangerous. Other arrhythmias, such as ventricular fibrillation and high rate (over 220 beats per minute) ventricular tachycardia may be considered as being immediately dangerous, for example. 
     With reference to  FIG. 2 , it will be appreciated that the heart H 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 SICD  12  and/or the LCP  14  of  FIG. 1 .  FIG. 2  includes a portion of an electrocardiogram (ECG)  16  along with a heart sounds trace  18 . As can be seen in the ECG  16 , a heartbeat includes a P-wave that indicates atrial depolarization. A QRS complex, including a Q-wave, an R-wave and an S-wave, represents ventricular depolarization. A T-wave indicates repolarization of the ventricles. It will be appreciated that the ECG  16  may be detected by implanted devices such as but not limited to the SICD  12  and/or the LCP  14  of  FIG. 1 . 
     A number of heart sounds may also be detectable while the heart H beats. It will be appreciated that the heart sounds may be considered as on 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 H detected by a pressure sensor of the LCP, acoustic signals caused by heart movements detected by an acoustic sensor (e.g. accelerometer, microphone, etc.) and/or any other suitable indication of a heart chamber beating. 
     An electrical signal typically instructs a portion of the heart H to contract, and then there is a corresponding mechanical response. In some cases, there may be a first heart sound that is denoted S 1  and that is produced by vibrations generated by closure of the mitral and tricuspid valves during a ventricle contraction, a second heart sound that is denoted S 2  and that is produced by closure of the aortic and pulmonary valves, a third heart sound that is denoted S 3  and that is an early diastolic sound caused by the rapid entry of blood from the right atrium RA into the right ventricle RV and from the left atrium LA into the left ventricle LV, and a fourth heart sound that is denoted S 4  and that is a late diastolic sound corresponding to late ventricular filling during an active atrial contraction. 
     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  16 , and the corresponding mechanical indication, indicated in the example shown by the heart sounds trace  18 . For example, the P-wave of the ECG  16  is an electrical signal triggering an atrial contraction. The S 4  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 S 4  heart sound. For example, if one of these signals may be detected, the relationship can be used as a timing mechanism to help 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 find and/or isolate the corresponding S 4  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 identity of an atrial contraction may be used to identify an atrial contraction timing fiducial (e.g. a timing marker of the atrial contraction). 
     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 H undergoes a cardiac cycle, the blood pressures and blood volumes within the heart H will vary over time.  FIG. 3  illustrates how these parameters match up with the electrical signals and corresponding mechanical indications. 
       FIG. 3  is a graph showing example pressures and volumes within a heart over time. More specifically,  FIG. 3  shows an illustrative example of the aortic pressure, left ventricular pressure, left atrial pressure, left ventricular volume, an electrocardiogram (ECG), and heart sounds of the heart H 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 ventricular 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 S 1  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 S 2  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. 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. 3 . In many cases, the cardiac pressure in the right ventricle is lower than the cardiac pressure in the left ventricle. 
       FIG. 4  is a schematic illustration of a subcutaneous implantable cardioverter defibrillator (SICD)  20  that may, for example, be considered as being a representative example of the SICD  12  shown in  FIG. 1 . In some cases, the SICD  20  includes a housing  22  and an electrode support  24  that is operably coupled to the housing  22 . In some cases, the electrode support  24  may be configured to place one or more electrodes in a position, such as subcutaneous or sub-sternal, that enables the one or more electrodes to detect cardiac electrical activity as well as to be able to deliver electrical shocks when appropriate to the heart. In the example shown, the housing  22  includes a controller  26 , a power supply  28  and a communications module  30 . As illustrated, the electrode support  24  includes a first electrode  32 , a second electrode  34  and a third electrode  36 . In some cases, the electrode support  24  may include fewer or more electrodes. In some cases, the electrode support  24  may include one or more other sensors such as an accelerometer or a gyro, for example. 
     It will be appreciated that the SICD  20  may include additional components which are not illustrated here for simplicity. The power supply  28  is operably coupled to the controller  26  and provides the controller  26  with power to operate the controller  26 , to send electrical power to the electrodes on or in the electrode support  24 , and to send signals to the communications module  30 , as appropriate. 
     In some cases, the controller  26  may be configured to sense a possible arrhythmia via the electrodes on or in the electrode support  24 , and may send a verification request to an LCP such as the LCP  14  ( FIG. 1 ) via the communications module  30 . The controller  26  may subsequently receive a signal from the LCP  14 , via the communications module  30 , that informs the controller  26  as to whether the possible arrhythmia has been confirmed (or not confirmed). In some cases, the controller  26  may subsequently receive a signal from the LCP  14 , via the communications module  30 , that confirms the existence of the possible arrhythmia and identifies the type of arrhythmia (e.g. Ventricular Tachycardia, Ventricular Fibrillation, Premature Ventricular Contractions, supraventricular Arrhythmias such as Supraventricular Tachycardia (SVT) or Paroxysmal Supraventricular Tachycardia (PSVT), atrial fibrillation). 
       FIG. 5  is a schematic illustration of a leadless cardiac pacemaker (LCP)  40  that may, for example, be considered as representing the LCP  14  shown in  FIG. 1 . In some cases, the LCP  40  includes a housing  42 . As illustrated, a first electrode  44  and a second electrode  47  are each disposed relative to the housing  42  and may, for example, be exposed to an environment exterior to the housing  42  when the LCP  40  is implanted in the heart. In some cases, the LCP  40  includes a controller  46 , a power supply  48  and a communications module  50 . The power supply  48  may be operably coupled to the controller  46  and provides the controller  46  with power to operate the controller  46 , to send communication signals such as but not limited to conducted communications through the first electrode  44  and the second electrode  47  via the communications module  50  as well as providing pacing pulses via the first electrode  44  and the second electrode  47 . While two electrodes  44 ,  47  are illustrated, it will be appreciated that in some cases the LCP  40  may include additional electrodes (not shown), and that different electrodes or vectors may be used for sensing and/or pacing, if desired. 
     In some cases, the communications module  50  may be configured to receive a verification request signal from the SICD  12  when the SICD  12  detects a possible arrhythmia. In some cases, the verification request includes a type of arrhythmia. The communications module  50  may be configured to subsequently send a signal to the SICD  12  that the possible arrhythmia has been confirmed. In some cases, the communications module  50  may send a signal to the SICD  12  that not only confirms the existence of the possible arrhythmia but also confirms the type of arrhythmia or notifies the SICD  12  of the type of arrhythmia. In some cases, the communications module  50  may be configured to send a signal to the SICD  12  that confirmed the absence of the possible arrhythmia. In some cases, the communications module  50  may be configured to send a signal to the SICD  12  that indicates neither the existence nor the absence of the possible arrhythmia could be confirmed. 
     The illustrative LCP  40  includes a plurality of sensors  52  that are operably coupled to the controller  46 . As illustrated, the plurality of sensors  52  includes a sensor  52   a , a sensor  52   b , a sensor  52   c  and a sensor  52   d . In some cases, the plurality of sensors  52  may include fewer sensors. In some instances, the plurality of sensors  52  may include additional sensors not shown. In some cases, it will be appreciated that the first electrode  44  and the second electrode  47  may, in combination, function as a sensor by sensing, for example, intrinsic and/or evoked cardiac electrical signals. It will be appreciated that one or more of the plurality of sensors  52  may, for example, include a sensor that is configured to detect heart sounds, pressure, cardiac wall movement, chamber volume, stroke volume, or other parameters. One or more of the plurality of sensors  52  may be an accelerometer, a pressure sensor, a gyro, and/or an optical sensor, for example. 
     In some cases, when the LCP  40  receives a verification request from the SICD  12  to verify the existence of a possible arrhythmia detected by the SICD  12 , the controller  46  may be configured to activate one or more of the plurality of sensors  52  from a lower power state to a higher power state, and to use the activated one or more of the plurality of sensors  52  to attempt to confirm that the possible arrhythmia is occurring. If the LCP  40  is able to confirm the possible arrhythmia using the activated one or more of the plurality of sensors  52 , the LCP  40  may be configured to send a confirmation response to the SICD  12  confirming the arrhythmia. It will be appreciated that in response to receiving the confirmation response, the SICD  12  may be configured to generate and deliver a shock to cardiac tissue if appropriate. In some cases, the SICD  12  may delay or otherwise inhibit delivery of a shock to cardiac tissue, particularly if the LCP  40  does not confirm the arrhythmia. 
     In some cases, the plurality of sensors  52  may include a first sensor such as sensor  52   a  that when activated consumes a first level of power and a second sensor such as sensor  52   b  that when activated consumes a second level of power that is higher than the first level of power. In some cases, the second sensor  52   b  may provide an increased level of accuracy or sensitivity relative to that provided by the first sensor  52   a , and/or may sense a different parameter that might be better suited to detect the particular arrhythmia. In some cases, upon receiving a verification request from the SICD  12 , the LCP  40  may initially activate the first sensor  52   a  in order to try to confirm or deny the possible arrhythmia. If the LCP  40  is able to do so using the first sensor  52   a , the LCP  40  may be configured to send a confirmation signal to the SICD  12 . However, if the LCP  40  is not able to confirm or deny the possible arrhythmia using the first sensor  52   a , the LCP  40  may be configured to activate the second sensor  52   b  in order to attempt to confirm or deny the possible arrhythmia. If the LCP  40  is able to confirm or deny the possible arrhythmia using the second sensor  52   b , the LCP  40  may send a confirmation response to the SICD  12 . 
     In some cases, the plurality of sensors  52  may include a third sensor such as sensor  52   c  that when activated, consumes a third level of power that is higher than that consumed by the second sensor  52   b  when activated. In some cases, the third sensor  52   c  may provide an increased accuracy or sensitivity, and/or may sense a different parameter that might be better suited to detect the particular arrhythmia, that justifies the increased power consumption. If the LCP  40  was not able to confirm or deny the possible arrhythmia using the first sensor  52   a  or the second sensor  52   b , the LCP  40  may be configured to activate the third sensor  52   c  in order to attempt to confirm or deny the possible arrhythmia. If the LCP  40  is able to confirm or deny the possible arrhythmia using the third sensor  52   c , the LCP  40  may be configured to send a confirmation response to the SICD  12 . 
     In some cases, the first sensor  52   a  represents the LCP  40  detecting electrical cardiac activity via the first electrode  44  and the second electrode  47 , or using one of the first electrode  44  and the second electrode  47  in combination with a third or fourth electrode (not illustrated). In some cases, the second sensor  52   b  may be configured to detect heart sounds. In some cases, the second sensor  52   b  may, for example, be an accelerometer that is disposed relative to the LCP  40 . In some cases, the second sensor  52   b  may be a pressure sensor that is disposed relative to the LCP  40 . In some cases, the third sensor  52   c  may be an optical sensor. These are just examples. By activating the sensors in sequence, often starting with the sensor with the lowest power consumption, power savings may be realized. 
     In some cases, the LCP  40  may concurrently activate two or more of the plurality of sensors  52 . In some cases, the LCP  40  may be configured to examine a relationship between a signal from the first of the two or more sensors and a signal from the second of the two or more sensors to attempt to confirm or deny the possible arrhythmia detected by the SICD  12 . For example, in some cases, a signal representing an S 2  heart sound may be compared with a signal representing a pressure waveform in order to confirm hemodynamic stability. If the patient is hemodynamically stable, the S 2  heart sound should be detected shortly after the ventricular pressure peaks, for example. If the timing is not as expected, this may provide verification of the possible arrhythmia seen by the SICD  12 . 
     In some cases, and as referenced above, the verification request that the LCP  40  receives from the SICD  12  may include an indication of severity of the possible arrhythmia. For example, if the possible arrhythmia is deemed not to be immediately dangerous to the health of the patient, the LCP  40  may sequentially activate the plurality of sensors  52  one at a time, as needed, in trying to confirm or deny the possible arrhythmia. In some cases, however, if the possible arrhythmia is deemed to be possibly immediately dangerous to the health of the patient, and thus the indication of severity exceeds a threshold severity level, the LCP  40  may be configured to concurrently activate two or more of the plurality of sensors  52  in order to more quickly confirm or deny the possible arrhythmia. In some cases, the threshold severity level may be programmed into the LCP  40  upon manufacture. In some instances, the threshold severity level may be customized for a particular patient, and in some cases may vary over time, by posture, by activity level, and/or any other suitable condition. 
     In some cases, there may be several LCPs  40  that are co-implanted (such as but not limited to the LCP  302  and the LCP  304  shown in  FIG. 10 ). In some cases, the two LCPs may cooperate to provide sensor functionality. For example, a first LCP may inject current into the heart using two or more of its electrodes. The second LCP may measure a resulting voltage across two or more of its electrodes. This may provide an impedance measurement of the tissue surrounding the first LCP and the second LCP. As another example, a first LCP may inject an ultrasonic pulse into the heart using an ultrasonic transmitting antenna. A second LCP may measure the ultrasonic energy using an ultrasonic receiving antenna, thereby providing a distance and/or density measurement of the tissue between the first LCP and the second LCP. 
       FIG. 6  is a schematic illustration of a leadless cardiac pacemaker (LCP)  60  that may, for example, be considered as representing the LCP  14  shown in  FIG. 1 . In some cases, the LCP  60  includes a housing  62 . As illustrated, a first electrode  64  and a second electrode  66  are each disposed relative to the housing  62  and may, for example, be exposed to an environment exterior to the housing  62  when the LCP  60  is implanted in the heart. In some cases, the LCP  60  includes a controller  68 , a power supply  70  and a communications module  72 . The power supply  70  may be operably coupled to the controller  68  and provides the controller  68  with power to operate the controller  68 , to send communication signals such as but not limited to conducted communications through the first electrode  64  and the second electrode  66  via the communications module  72  as well as providing pacing pulses via the first electrode  64  and the second electrode  66 . While two electrodes  64 ,  66  are illustrated, it will be appreciated that in some cases the LCP  60  may include additional electrodes (not shown). 
     In some cases, the communications module  72  may be configured to receive a verification request signal from the SICD  12  when the SICD  12  detects a possible arrhythmia. The communications module  72  may be configured to subsequently send a signal to the SICD  12  that the possible arrhythmia has been confirmed, or that an absence of an arrhythmia is confirmed. In some cases, the LCP  60  includes a first sensor  74  and a second sensor  76 . In some cases, the first sensor  74 , when activated, consumes a first level of power. In some cases, the second sensor  76 , when activated, consumes a second level of power that is higher than the first level of power. In some cases, and to justify the relatively higher power consumption, the second sensor  76  may provide an increased level of accuracy or sensitivity relative that that of the first sensor  74 , or is configured to detect a different parameter that might be better suited to detect the particular arrhythmia. 
     In some cases, upon receiving a verification request from the SICD  12  via the communications module  72 , the controller  68  may be configured to initially sense cardiac activity using the first sensor  74  to attempt to confirm that the possible arrhythmia is occurring while the second sensor  76  is in a lower power state. The lower power state may be in an off state that draws no power, or a sleep or other lower power state that draws some power. If the controller  68  is able to confirm that the possible arrhythmia is occurring using the first sensor  74 , the controller  68  may be configured to send a confirmation response to the SICD  12  via the communications module  72  confirming that the possible arrhythmia is occurring. 
     However, if the controller  68  is not able to confirm that the possible arrhythmia is occurring using the first sensor  74 , the controller  68  may be configured to activate the second sensor  76  from the lower power state to a higher power state, and then attempt to confirm that the possible arrhythmia is occurring using the second sensor  76  (e.g. using just the second sensor  76  or using the first sensor  74  and the second sensor  76 ). If the controller  68  is able to confirm that the possible arrhythmia is occurring using the second sensor  76 , the controller  68  may be configured to send the confirmation response to the SICD  12  via the communications module  72  confirming that the possible arrhythmia is occurring. If the controller  68  is not able to confirm the possible arrhythmia using only the first sensor  74 , the controller  68  may be configured to activate both the first sensor  74  and the second sensor  76  at the same time, as noted above, and then examine a relationship between a signal from the first sensor  74  and a signal from the second sensor  76  to attempt to confirm the possible arrhythmia. In some cases, the second sensor  76  may be an accelerometer or a pressure sensor. 
       FIG. 7  is a schematic illustration of a leadless cardiac pacemaker (LCP)  80  that may, for example, be considered as representing the LCP  14  shown in  FIG. 1 . In some cases, the LCP  80  includes a housing  82 . As illustrated, a first electrode  84  and a second electrode  86  are each disposed relative to the housing  82  and may, for example, be exposed to an environment exterior to the housing  82  when the LCP  80  is implanted in the heart. In some cases, the LCP  80  includes a controller  88 , a power supply  90  and a communications module  92 . The power supply  90  may be operably coupled to the controller  88  and provides the controller  88  with power to operate the controller  88 , to send communication signals such as but not limited to conducted communications through the first electrode  84  and the second electrode  86  via the communications module  92  as well as providing pacing pulses via the first electrode  84  and the second electrode  86 . While two electrodes  84 ,  86  are illustrated, it will be appreciated that in some cases the LCP  80  may include additional electrodes (not shown). 
     In some cases, the communications module  72  may be configured to receive a verification request signal from the SICD  12  when the SICD  12  detects a possible arrhythmia. The communications module  92  may be configured to subsequently send a signal to the SICD  12  that the possible arrhythmia has been confirmed, or that an absence of an arrhythmia is confirmed, or neither. In some cases, the first electrode  84  and the second electrode  86  may, in combination, form a first sensor that is able to sense cardiac electrical activity. In some cases, the LCP  80  includes a second sensor  94 . Optionally, the LCP  80  may include a third sensor  96 . As discussed with respect to the LCP  40  ( FIG. 5 ) and the LCP  60  ( FIG. 6 ), the controller  88  may be configured to sequentially or simultaneously, as desired, activate the first sensor, the second sensor  94  and the third sensor  96  to attempt to confirm or deny a possible arrhythmia. 
     To confirm the possible arrhythmia, the LCP  80  and/or SICD  12  may use a LCP electrogram (EGM) signal detected by electrodes  84 / 86  of the LCP  80  and/or an SICD EGM detected by electrodes of the SICD  12  to help confirm or deny a detected possible arrhythmia. For example, the LCP  80  and/or SICD  12  may use the LCP EGM signal to confirm the heart rate sensed by the SICD  12  by detecting T-waves that are sensed by the SICD  12  as R-waves. In another example, the LCP  80  and/or SICD  12  may use R-wave to R-wave variability between beats to confirm or deny if an unstable rhythm is present. The R-waves may be detected by the LCP  80 . The LCP  80  and/or SICD  12  may use the P-wave to R-wave interval to help differentiate between SVT from VT/VF. If the PR interval (e.g. P from the SICD EGM and R from the LCP EGM) is not stable, then the arrhythmia may be considered VF, and the SICD may proceed to delivery shock therapy. If the PR interval is stable, then the arrhythmia may be a possible sinus VT, in which case the LCP  80  may activate an accelerometer to detect if S 1 /S 2  is low and/or if the LV pressure is low. If either is low, the arrhythmia may be considered VF, and the SICD may proceed to delivery shock therapy. If neither is low, the SICD may delay or inhibit shock therapy. In another example, the LCP  80  and/or SICD  12  may use the association between the QRS complex and the S 1  heart sound, as detected by the LCP  80 , to confirm the heart rate. A measure of reliability of each sensor reading or combination of sensor readings may be used to determine a confidence level score in whether the possible arrhythmia is actually present. 
     In some cases, the LCP  80  and/or SICD  12  may use heart sounds detected by the LCP  80  (e.g. via second sensor  94 , such as an accelerometer) to help confirm or deny a possible arrhythmia. For example, the LCP  80  and/or SICD  12  may use the S 1 /S 2  amplitude to detect a measure of hemodynamic stability. In some cases, the LCP  80  and/or SICD  12  may use S 1 /S 2  timing to confirm heart rate. The LCP  80  and/or SICD  12  may use S 1 /S 2  variability to confirm or deny an unstable rhythm. The LCP  80  and/or SICD  12  may use R-wave to S 1 , R-wave to S 2 , and/or S 2  to R-wave variability to help differentiate between sinus VT from VF. 
     It is contemplated that the LCP  80  and/or SICD  12  may use pressure detected by the LCP  80  (e.g. via third sensor  96 , such as a pressure sensor) to help confirm or deny a possible arrhythmia. For example, the LCP  80  and/or SICD  12  may use variations of pressure in the heart to provide a measure of hemodynamic stability of the heart (e.g. standard deviation of max or min pressures, dp/dt and/or other pressure parameters over several beats, variation of max or min pressures, dp/dt or other pressure parameters over several beats, etc.). In some cases, the LCP  80  and/or SICD  12  may use the absolute value of the pressure to provide a measure of hemodynamic stability and/or perfusion status of the heart (e.g. End Systolic Pressure (ESP), End Diastolic Pressure (EDP), mean pressure, peak pressure). The LCP  80  and/or SICD  12  may use an association between the pressure wave and S 2  to confirm the heart rate and/or to confirm proper S 2  detection. 
     The LCP  80  and/or SICD  12  may use other parameters detected by the LCP  80  to help confirm or deny a possible arrhythmia. For example, the LCP may monitor an impedance between the LCP electrodes  84  and  86  to determine a measure of the volume of the chamber, and in some cases, a measure of stroke volume fluctuations. Alternatively, or in addition, the LCP may include one or more optical sensor to obtain a measure the volume of the chamber. These are just examples sensor measurements that may be used. 
       FIG. 8  depicts another illustrative leadless cardiac pacemaker (LCP) that may be implanted into a patient and may operate to deliver appropriate therapy to the heart, such as to deliver anti-tachycardia pacing (ATP) therapy, cardiac resynchronization therapy (CRT), bradycardia therapy, and/or the like. As can be seen in  FIG. 8 , the LCP  100  may be a compact device with all components housed within the or directly on a housing  120 . In some cases, the LCP  100  may be considered as being an example of one or more of the LCP  14  ( FIG. 1 ), the LCP  40  ( FIG. 5 ) and/or the LCP  60  ( FIG. 6 ). In the example shown in  FIG. 8 , the LCP  100  may include 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 an electrode arrangement  114 . The LCP  100  may include more or less modules, depending on the application. 
     The communication module  102  may be configured to communicate with devices such as sensors, other medical devices such as an SICD, and/or the like, that are located externally to the LCP  100 . Such devices may be located either external or internal to the patient&#39;s body. Irrespective of the location, external devices (i.e. external to the LCP  100  but not necessarily external to the patient&#39;s body) can communicate with the LCP  100  via communication module  102  to accomplish one or more desired functions. For example, the LCP  100  may communicate information, such as sensed electrical signals, data, instructions, messages, R-wave detection markers, etc., to an external medical device (e.g. SICD and/or programmer) through the communication module  102 . The external medical device may use the communicated signals, data, instructions, messages, R-wave detection markers, etc., to perform various functions, such as determining occurrences of arrhythmias, delivering electrical stimulation therapy, storing received data, and/or performing any other suitable function. The LCP  100  may additionally receive information such as signals, data, instructions and/or messages from the external 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, and/or performing any other suitable function. The communication module  102  may be configured to use one or more methods for communicating with external 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. 8 , the pulse generator module  104  may be electrically connected to the electrodes  114 . In some examples, the LCP  100  may additionally include electrodes  114 ′. In such examples, the pulse generator  104  may also be electrically connected to the electrodes  114 ′. The pulse generator module  104  may be configured to generate electrical stimulation signals. For example, the pulse generator module  104  may generate and deliver electrical stimulation signals by using energy stored in the 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) the pulse generator  104  delivers the electrical stimulation therapy. The pulse generator module  104  may generate and deliver 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 synchronization, 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 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 . For example, 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 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 in which the LCP  100  is implanted. For instance, if the LCP  100  is implanted within a ventricle of the heart (e.g. RV, LV), cardiac electrical signals sensed by the LCP  100  through the electrodes  114 / 114 ′ may represent ventricular cardiac electrical signals. In some cases, the LCP  100  may be configured to detect cardiac electrical signals from other chambers (e.g. far field), such as the P-wave from the atrium. 
     The mechanical sensing module  108  may include one or more sensors, such as an accelerometer, a pressure sensor, a heart sound sensor, a blood-oxygen sensor, a chemical sensor, a temperature sensor, a flow sensor and/or any other suitable sensors that are configured to measure one or more mechanical/chemical parameters of the patient. 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. 8  as separate sensing modules, in some cases, the electrical sensing module  206  and the mechanical sensing module  208  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 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 connected to the housing  120  through short connecting wires such that the electrodes  114 / 114 ′ are not directly secured relative to 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 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 the 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 . In some cases, one or more of the electrodes  114 / 114 ′ may be provided on a tail (not shown) that extends away from the housing  120 . 
     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, abnormalities in the operation of the heart H. Based on any determined abnormalities, the processing module  110  may control the pulse generator module  104  to generate and deliver electrical stimulation in accordance with one or more therapies to treat the determined abnormalities. 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 whether an abnormality is occurring, determine a type of abnormality, and/or to take particular action in response to the information. 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. 9  depicts an example of another medical device (MD)  200 , which may be used in conjunction with the LCP  100  ( FIG. 8 ) in order to detect and/or treat cardiac abnormalities. In some cases, the MD  200  may be considered as an example of the SICD  12  ( FIG. 1 ). 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 LCP  100 . Additionally, the battery  218  may be similar to the battery  112  of the LCP  100 . In some examples, however, the MD  200  may have a larger volume within the housing  220 . 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. 8 , 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 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 subcutaneously and outside of 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 delivery 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 accelerometers, acoustic sensors, blood pressure sensors, heart sound sensors, blood-oxygen 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 the 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. In some examples, the MD  200  may additionally be configured 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 other examples, 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 implanted lead. 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, but this is not required. In some instances, the lead(s) may have one or more electrodes that are placed subcutaneously and outside of the chest cavity. In other examples, the lead(s) may have one or more electrodes that are placed inside of the chest cavity, such as just interior of the sternum. 
     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. 
       FIG. 10  illustrates an example of a medical device system and a communication pathway through which multiple medical devices  302 ,  304 ,  306 , and/or  310  may communicate. In the example shown, the medical device system  300  may include LCPs  302  and  304 , external medical device  306 , and other sensors/devices  310 . The external device  306  may be any of the devices described previously with respect to the MD  200 . Other sensors/devices  310  may also be any of the devices described previously with respect to the MD  200 . In some instances, other sensors/devices  310  may include a sensor, such as an accelerometer, an acoustic sensor, a blood pressure sensor, or the like. In some cases, other sensors/devices  310  may include an external programmer device that may be used to program one or more devices of the system  300 . 
     Various devices of the system  300  may communicate via communication pathway  308 . For example, the LCPs  302  and/or  304  may sense intrinsic cardiac electrical signals and may communicate such signals to one or more other devices  302 / 304 ,  306 , and  310  of the system  300  via communication pathway  308 . In one example, one or more of the devices  302 / 304  may receive such signals and, based on the received signals, determine an occurrence of an arrhythmia. In some cases, the device or devices  302 / 304  may communicate such determinations to one or more other devices  306  and  310  of the system  300 . In some cases, one or more of the devices  302 / 304 ,  306 , and  310  of the system  300  may take action based on the communicated determination of an arrhythmia, such as by delivering a suitable electrical stimulation to the heart of the patient. It is contemplated that the communication pathway  308  may communicate using RF signals, inductive coupling, optical signals, acoustic signals, or any other signals suitable for communication. Additionally, in at least some examples, device communication pathway  308  may include multiple signal types. For instance, other sensors/device  310  may communicate with the external device  306  using a first signal type (e.g. RF communication) but communicate with the LCPs  302 / 304  using a second signal type (e.g. conducted communication). Further, in some examples, communication between devices may be limited. For instance, as described above, in some examples, the LCPs  302 / 304  may communicate with the external device  306  only through other sensors/devices  310 , where the LCPs  302 / 304  send signals to other sensors/devices  310 , and other sensors/devices  310  relay the received signals to the external device  306 . 
     In some cases, the communication pathway  308  may include conducted communication. Accordingly, devices of the system  300  may have components that allow for such conducted communication. For instance, the devices of system  300  may be configured to transmit conducted communication signals (e.g. current and/or voltage pulses) into the patient&#39;s body via one or more electrodes of a transmitting device, and may receive the conducted communication signals (e.g. pulses) via one or more electrodes of a receiving device. The patient&#39;s body may “conduct” the conducted communication signals (e.g. pulses) from the one or more electrodes of the transmitting device to the electrodes of the receiving device in the system  300 . In such examples, the delivered conducted communication signals (e.g. pulses) may differ from pacing or other therapy signals. For example, the devices of the system  300  may deliver electrical communication pulses at an amplitude/pulse width that is sub-threshold to the heart. Although, in some cases, the amplitude/pulse width of the delivered electrical communication pulses may be above the capture threshold of the heart, but may be delivered during a blanking period of the heart and/or may be incorporated in or modulated onto a pacing pulse, if desired. 
     Delivered electrical communication pulses may be modulated in any suitable manner to encode communicated information. In some cases, the communication pulses may be pulse width modulated or amplitude modulated. Alternatively, or in addition, the time between pulses may be modulated to encode desired information. In some cases, conducted communication pulses may be voltage pulses, current pulses, biphasic voltage pulses, biphasic current pulses, or any other suitable electrical pulse as desired. 
       FIG. 11  shows an illustrative medical device systems. In  FIG. 11 , an LCP  402  is shown fixed to the interior of the left ventricle of the heart  410 , and a pulse generator  406  is shown coupled to a lead  412  having one or more electrodes  408   a - 408   c . In some cases, the pulse generator  406  may be part of a subcutaneous implantable cardioverter-defibrillator (S-ICD), and the one or more electrodes  408   a - 408   c  may be positioned subcutaneously. In some cases, the one or more electrodes  408   a - 408   c  may be placed inside of the chest cavity but outside of the heart, such as just interior of the sternum. 
     In some cases, the LCP  402  may communicate with the subcutaneous implantable cardioverter-defibrillator (S-ICD). In some cases, the lead  412  may include an accelerometer  414  that may, for example, be configured to sense vibrations that may be indicative of heart sounds. 
     In some cases, the LCP  402  may be in the right ventricle, right atrium, left ventricle or left atrium of the heart, as desired. In some cases, more than one LCP  402  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. 
     When an LCP is placed in, for example, the left ventricle, and no LCP is placed in the left atrium, techniques of the present disclosure may be used to help determine an atrial contraction timing fiducial for the left atrium. This atrial contraction timing fiducial may then be used to determine a proper time to pace the left ventricle via the LCP, such as an AV delay after the atrial contraction timing fiducial. 
       FIG. 12  is a side view of an illustrative implantable leadless cardiac pacemaker (LCP)  610 . The LCP  610  may be similar in form and function to the LCP  100  described above. The LCP  610  may include any of the modules and/or structural features described above with respect to the LCP  100  described above. The LCP  610  may include a shell or housing  612  having a proximal end  614  and a distal end  616 . The illustrative LCP  610  includes a first electrode  620  secured relative to the housing  612  and positioned adjacent to the distal end  616  of the housing  612  and a second electrode  622  secured relative to the housing  612  and positioned adjacent to the proximal end  614  of the housing  612 . In some cases, the housing  612  may include a conductive material and may be insulated along a portion of its length. A section along the proximal end  614  may be free of insulation so as to define the second electrode  622 . The electrodes  620 ,  622  may be sensing and/or pacing electrodes to provide electro-therapy and/or sensing capabilities. The first electrode  620  may be capable of being positioned against or may otherwise contact the cardiac tissue of the heart while the second electrode  622  may be spaced away from the first electrode  620 . The first and/or second electrodes  620 ,  622  may be exposed to the environment outside the housing  612  (e.g. to blood and/or tissue). 
     In some cases, the LCP  610  may include a pulse generator (e.g., electrical circuitry) and a power source (e.g., a battery) within the housing  612  to provide electrical signals to the electrodes  620 ,  622  to control the pacing/sensing electrodes  620 ,  622 . While not explicitly shown, the LCP  610  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  612 . Electrical communication between the pulse generator and the electrodes  620 ,  622  may provide electrical stimulation to heart tissue and/or sense a physiological condition. 
     In the example shown, the LCP  610  includes a fixation mechanism  624  proximate the distal end  616  of the housing  612 . The fixation mechanism  624  is configured to attach the LCP  610  to a wall of the heart H, or otherwise anchor the LCP  610  to the anatomy of the patient. In some instances, the fixation mechanism  624  may include one or more, or a plurality of hooks or tines  626  anchored into the cardiac tissue of the heart H to attach the LCP  610  to a tissue wall. In other instances, the fixation mechanism  624  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  610  to the heart H. These are just examples. 
     The LCP  610  may further include a docking member  630  proximate the proximal end  614  of the housing  612 . The docking member  630  may be configured to facilitate delivery and/or retrieval of the LCP  610 . For example, the docking member  630  may extend from the proximal end  614  of the housing  612  along a longitudinal axis of the housing  612 . The docking member  630  may include a head portion  632  and a neck portion  634  extending between the housing  612  and the head portion  632 . The head portion  632  may be an enlarged portion relative to the neck portion  634 . For example, the head portion  632  may have a radial dimension from the longitudinal axis of the LCP  610  that is greater than a radial dimension of the neck portion  634  from the longitudinal axis of the LCP  610 . In some cases, the docking member  630  may further include a tether retention structure  636  extending from or recessed within the head portion  632 . The tether retention structure  636  may define an opening  638  configured to receive a tether or other anchoring mechanism therethrough. While the retention structure  636  is shown as having a generally “U-shaped” configuration, the retention structure  636  may take any shape that provides an enclosed perimeter surrounding the opening  638  such that a tether may be securably and releasably passed (e.g. looped) through the opening  638 . In some cases, the retention structure  636  may extend though the head portion  632 , along the neck portion  634 , and to or into the proximal end  614  of the housing  612 . The docking member  630  may be configured to facilitate delivery of the LCP  610  to the intracardiac site and/or retrieval of the LCP  610  from the intracardiac site. While this describes one example docking member  630 , it is contemplated that the docking member  630 , when provided, can have any suitable configuration. 
     It is contemplated that the LCP  610  may include one or more pressure sensors  640  coupled to or formed within the housing  612  such that the pressure sensor(s) is exposed to the environment outside the housing  612  to measure blood pressure within the heart. For example, if the LCP  610  is placed in the left ventricle, the pressure sensor(s)  640  may measure the pressure within the left ventricle. If the LCP  610  is placed in another portion of the heart (such as one of the atriums or the right ventricle), the pressures sensor(s) may measure the pressure within that portion of the heart. The pressure sensor(s)  640  may include a MEMS device, such as a MEMS device with a pressure diaphragm and piezoresistors on the diaphragm, a piezoelectric sensor, a capacitor-Micro-machined Ultrasonic Transducer (cMUT), a condenser, a micro-monometer, or any other suitable sensor adapted for measuring cardiac pressure. The pressures sensor(s)  640  may be part of a mechanical sensing module described herein. It is contemplated that the pressure measurements obtained from the pressures sensor(s)  640  may be used to generate a pressure curve over cardiac cycles. The pressure readings may be taken in combination with impedance measurements (e.g. the impedance between electrodes  620  and  622 ) to generate a pressure-impedance loop for one or more cardiac cycles as will be described in more detail below. The impedance may be a surrogate for chamber volume, and thus the pressure-impedance loop may be representative for a pressure-volume loop for the heart H. 
     In some embodiments, the LCP  610  may be configured to measure impedance between the electrodes  620 ,  622 . More generally, the impedance may be measured between other electrode pairs, such as the additional electrodes  114 ′ described above. In some cases, the impedance may be measure between two spaced LCP&#39;s, such as two LCP&#39;s implanted within the same chamber (e.g. LV) of the heart H, or two LCP&#39;s implanted in different chambers of the heart H (e.g. RV and LV). The processing module of the LCP  610  and/or external support devices may derive a measure of cardiac volume from intracardiac impedance measurements made between the electrodes  620 ,  622  (or other electrodes). Primarily due to the difference in the resistivity of blood and the resistivity of the cardiac tissue of the heart H, the impedance measurement may vary during a cardiac cycle as the volume of blood (and thus the volume of the chamber) surrounding the LCP changes. In some cases, the measure of cardiac volume may be a relative measure, rather than an actual measure. In some cases, the intracardiac impedance may be correlated to an actual measure of cardiac volume via a calibration process, sometimes performed during implantation of the LCP(s). During the calibration process, the actual cardiac volume may be determined using fluoroscopy or the like, and the measured impedance may be correlated to the actual cardiac volume. 
     In some cases, the LCP  610  may be provided with energy delivery circuitry operatively coupled to the first electrode  620  and the second electrode  622  for causing a current to flow between the first electrode  620  and the second electrode  622  in order to determine the impedance between the two electrodes  620 ,  622  (or other electrode pair). It is contemplated that the energy delivery circuitry may also be configured to deliver pacing pulses via the first and/or second electrodes  620 ,  622 . The LCP  610  may further include detection circuitry operatively coupled to the first electrode  620  and the second electrode  622  for detecting an electrical signal received between the first electrode  620  and the second electrode  622 . In some instances, the detection circuitry may be configured to detect cardiac signals received between the first electrode  620  and the second electrode  622 . 
     When the energy delivery circuitry delivers a current between the first electrode  620  and the second electrode  622 , the detection circuitry may measure a resulting voltage between the first electrode  620  and the second electrode  622  (or between a third and fourth electrode separate from the first electrode  620  and the second electrode  622 ) to determine the impedance. When the energy delivery circuitry delivers a voltage between the first electrode  620  and the second electrode  622 , the detection circuitry may measure a resulting current between the first electrode  620  and the second electrode  622  (or between a third and fourth electrode separate from the first electrode  620  and the second electrode  622 ) to determine the impedance. 
     In some instances, the impedance may be measured between electrodes on different devices and/or in different heart chambers. For example, impedance may be measured between a first electrode in the left ventricle and a second electrode in the right ventricle. In another example, impedance may be measured between a first electrode of a first LCP in the left ventricle and a second LCP in the left ventricle. In yet another example, impedance may be measured from an injected current. For example, a medical device (such as, but not limited to an SICD such as the SICD  12  of  FIG. 1 ), may inject a known current into the heart and the LCP implanted in the heart H may measure a voltage resulting from the injected current to determine the impedance. These are just some examples. 
       FIG. 13  is a flow diagram showing a method  700  for regulating a patient&#39;s heart using the medical system  10  ( FIG. 1 ) including the SICD  12  and the LCP  14 . As seen generally at block  702 , the SICD  12  may be used in a chronic monitoring mode in which the SICD  12  monitors a cardiac EGM for indications of a possible arrhythmia. An acute mode may be activated if the SICD  12  identifies a possible arrhythmia, and the LCP  14  may be instructed to help confirm the possible arrhythmia using the LCP electrodes and/or at least one of the one or more additional sensors of the LCP as noted at block  704 . If the possible arrhythmia is confirmed (e.g. by the LCP or SICD), and if the possible arrhythmia is dangerous, the SICD  12  may deliver shock therapy to the heart via the electrodes of the SICD  12 , as seen at block  706 . As indicated at block  708 , if the possible arrhythmia is confirmed and is not dangerous, inhibiting delivery of shock therapy to the heart via the electrodes of the SICD  12  and continuing in the acute mode in which the LCP electrodes and/or the at least one of the one or more additional sensors of the LCP are used to monitor cardiac activity. As noted at block  710 , if the possible arrhythmia is not confirmed, the SICD  12  may inhibit delivery of shock therapy to the heart and may continue in the acute mode in which the LCP electrodes and/or the at least one of the one or more additional sensors of the LCP  14  are used to monitor cardiac activity. In some cases, as seen at block  710 , if the possible arrhythmia is not confirmed, the SICD  12  may instead deliver shock therapy. In some cases, this is a programmable setting that a physician may select for a particular patient. For some patients, the SICD  12  may be programmed to inhibit shock therapy when the LCP  14  is either unable to confirm the possible arrhythmia or if communication with the LCP  14  fails. For other patients, the SICD  12  may be programmed to deliver a shock in these situations. In some cases, and as indicated at optional block  712 , the SICD  12  may return to the chronic monitoring mode once the possible arrhythmia has terminated. 
     It should be understood that this disclosure is, in many respects, only illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of steps without exceeding the scope of the disclosure. This may include, to the extent that it is appropriate, the use of any of the features of one example embodiment being used in other embodiments.