Patent Publication Number: US-10765871-B2

Title: Implantable medical device with pressure sensor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/413,766 filed on Oct. 27, 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 implantable medical devices with pressure sensors 
     BACKGROUND 
     Implantable medical devices are commonly used to perform a variety of functions, such as to monitor one or more conditions and/or delivery therapy to a patient. In some cases, an implantable medical device may deliver neurostimulation therapy to a patient. In some cases, an implantable medical device may simply monitor one or more conditions, such as pressure, acceleration, cardiac events, and may communicate the detected conditions or events to another device, such as another implantable medical device or an external programmer. 
     In some cases, an implantable medical device may be configured to deliver pacing and/or defibrillation therapy to a patient. Such implantable medical devices may 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. In some cases, heart conditions may lead to rapid, irregular, and/or inefficient heart contractions. To help alleviate some of these conditions, various devices (e.g., pacemakers, defibrillators, etc.) may be implanted into a patient&#39;s body. When so provided, such devices can monitor and provide therapy, such as electrical stimulation therapy, to the patient&#39;s heart to help the heart operate in a more normal, efficient and/or safe manner. In some cases, a patient may have multiple implanted devices that cooperate to monitor and/or provide therapy to the patient&#39;s heart. 
     SUMMARY 
     The present disclosure generally relates to implantable medical devices and more particularly to implantable medical devices with pressure sensors. 
     In a first example, a leadless cardiac pacemaker (LCP) may be configured to sense cardiac activity and to pace a patient&#39;s heart. The LCP may comprise a housing having a proximal end and a distal end, a first electrode secured relative to the housing and exposed to the environment outside of the housing, and a second electrode secured relative to the housing and exposed to the environment outside of the housing. The housing may have a diaphragm that is exposed to the environment outside of the housing. The diaphragm may be responsive to a pressure applied to the diaphragm by the environment outside of the housing. A pressure sensor may be within the housing may have a pressure sensor diaphragm that is responsive to a pressure applied to the pressure sensor diaphragm and provides a pressure sensor output signal that is representative of the pressure applied to the pressure sensor diaphragm. A fluid filled cavity may be in fluid communication with both the diaphragm of the housing and the pressure sensor diaphragm of the pressure sensor. The fluid filled cavity may be configured to communicate a measure related to the pressure applied by the environment to the diaphragm of the housing to the pressure sensor diaphragm of the pressure sensor. Circuitry in the housing may be in operative communication with the pressure sensor. The circuitry may be configured to determine a pressure exterior to the housing based on the pressure sensor output signal. 
     Alternatively or additionally to any of the examples above, in another example, the circuitry may be further operatively coupled to the first electrode and the second electrode, and may be further configured to use one or more cardiac signals sensed by the first electrode and the second electrode to determine when the patient&#39;s heart is in a first phase of a cardiac cycle and determine a pressure exterior to the housing based at least in part on the pressure sensor output signal taken during the first phase of a cardiac cycle. Alternatively or additionally to any of the examples above, in another example, the first phase of a cardiac cycle may be systole. 
     Alternatively or additionally to any of the examples above, in another example, the first phase of a cardiac cycle may be diastole. 
     Alternatively or additionally to any of the examples above, in another example, the circuitry may be configured to detect heart sounds of the patient&#39;s heart based at least in part on the pressure sensor output signal. 
     Alternatively or additionally to any of the examples above, in another example, the diaphragm of the housing may comprise a localized thinning of a housing wall of the housing. 
     Alternatively or additionally to any of the examples above, in another example, the diaphragm of the housing may comprise a region comprising a compliant material. 
     Alternatively or additionally to any of the examples above, in another example, the diaphragm of the housing may comprise one or more bellows. 
     Alternatively or additionally to any of the examples above, in another example, the LCP may further comprise a fixation member at the distal end of the housing for fixing the distal end of the housing to the patient&#39;s heart, and wherein the diaphragm of the housing may be adjacent the proximal end of the housing. 
     Alternatively or additionally to any of the examples above, in another example, the housing may include an elongated body with a distal end surface facing distally and a proximal end surface facing proximally, wherein the diaphragm of the housing may be situated on the proximal end surface of the housing. 
     Alternatively or additionally to any of the examples above, in another example, the housing may have a plurality of diaphragms that are exposed to the environment outside of the housing, each of the plurality of diaphragms are responsive to the pressure applied to the corresponding diaphragm by the environment outside of the housing. 
     Alternatively or additionally to any of the examples above, in another example, the fluid filled cavity may be filled with an incompressible fluid. 
     Alternatively or additionally to any of the examples above, in another example, the incompressible fluid may be a dielectric fluid. 
     Alternatively or additionally to any of the examples above, in another example, the LCP may further comprise an anti-thrombogenic coating disposed over the diaphragm of the housing. 
     Alternatively or additionally to any of the examples above, in another example, the diaphragm of the housing may have a first surface area and the pressure sensor diaphragm of the pressure sensor may have a second surface area, wherein a ratio of the first surface area to the second surface area is at least 5 to 1. 
     In another example, a leadless cardiac pacemaker (LCP) may be configured to sense cardiac activity and to pace a patient&#39;s heart. The LCP may comprise a housing having a proximal end and a distal end, a first electrode secured relative to the housing and exposed to the environment outside of the housing, and a second electrode secured relative to the housing and exposed to the environment outside of the housing. The housing may have a diaphragm that is exposed to the environment outside of the housing. The diaphragm may be responsive to a pressure applied to the diaphragm by the environment outside of the housing. One or more sensors may be coupled to the diaphragm of the housing for detecting a stress in the diaphragm of the housing, wherein the stress in the diaphragm is representative of the pressure applied to the diaphragm by the environment outside of the housing. Circuitry in the housing may be in operative communication with the one or more sensors for determining a pressure exterior to the housing based at least in part on the detected stress in the diaphragm of the housing. 
     Alternatively or additionally to any of the examples above, in another example, the diaphragm of the housing may comprise a localized thinning of a housing wall of the housing. 
     Alternatively or additionally to any of the examples above, in another example, the localized thinning of the housing wall may comprise a transition from a first thicker wall thickness to a second thinner wall thickness. 
     Alternatively or additionally to any of the examples above, in another example, the one or more sensors may comprise a piezo resistor, and wherein the stress in the diaphragm comprises one or more of compression and stretching. 
     In another example, an implantable medical device (IMD) may comprise a housing having a proximal end and a distal end, a first electrode secured relative to the housing and exposed to the environment outside of the housing, and a second electrode secured relative to the housing and exposed to the environment outside of the housing. The housing may have a diaphragm that is exposed to the environment outside of the housing. The diaphragm may be responsive to a pressure applied to the diaphragm by the environment outside of the housing. A pressure sensor may be positioned within the housing and may have a pressure sensor diaphragm that is responsive to a pressure applied to the pressure sensor diaphragm and provides a pressure sensor output signal that is representative of the pressure applied to the pressure sensor diaphragm. A fluid filled cavity may be in fluid communication with both the diaphragm of the housing and the pressure sensor diaphragm of the pressure sensor. The fluid filled cavity may be configured to communicate a measure related to the pressure applied by the environment to the diaphragm of the housing to the pressure sensor diaphragm of the pressure sensor. Circuitry in the housing may be in operative communication with the pressure sensor. The circuitry may be configured to determine a pressure exterior to the housing based on the pressure sensor output signal, and further configured to communicate with another device via the first and second electrodes. 
     The above summary is not intended to describe each embodiment or every implementation of the present disclosure. Advantages and attainments, together with a more complete understanding of the disclosure, will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure may be more completely understood in consideration of the following description of various illustrative embodiments in connection with the accompanying drawings, in which: 
         FIG. 1  is a schematic block diagram of an illustrative leadless cardiac pacemaker (LCP) according to one example of the present disclosure; 
         FIG. 2  is a schematic block diagram of another medical device (MD), which may be used in conjunction with an LCP  100  ( FIG. 1 ) in order to detect and/or treat cardiac arrhythmias and other heart conditions; 
         FIG. 3  is a schematic diagram of an exemplary medical system that includes multiple LCPs and/or other devices in communication with one another; 
         FIG. 4  is a schematic diagram of an exemplary medical system that includes an LCP and another medical device, in accordance with yet another example of the present disclosure; 
         FIG. 5  is a schematic diagram of an exemplary medical system that includes an LCP and another medical device, in accordance with yet another example of the present disclosure; 
         FIG. 6  is a side view of an illustrative implantable leadless cardiac pacing device; 
         FIG. 7A  is a plan view of an example leadless cardiac pacing device implanted within a heart during ventricular filling; 
         FIG. 7B  is a plan view of an example leadless cardiac pacing device implanted within a heart during ventricular contraction; 
         FIG. 8  is a graph showing example pressures and volumes within the heart over time; 
         FIG. 9  is a schematic cross-sectional view of a an illustrative leadless cardiac pacing device; 
         FIG. 10  is a schematic cross-sectional view of the proximal end of the illustrative leadless cardiac pacing device of  FIG. 9 ; 
         FIG. 11  is a schematic cross-sectional view of an illustrative pressure sensor for use with a leadless cardiac pacing device; 
         FIG. 12  is a schematic cross-sectional view of a proximal end portion of another illustrative leadless cardiac pacing device; 
         FIG. 13  is a schematic cross-sectional view of a proximal end portion of another illustrative leadless cardiac pacing device; 
         FIG. 14  is a schematic cross-sectional view of a proximal end portion of another illustrative leadless cardiac pacing device; 
         FIG. 15  is an end view of the proximal end of another illustrative leadless cardiac pacing device; 
         FIG. 16  is a schematic perspective view of another illustrative leadless cardiac pacing device; 
         FIG. 17A  is a schematic partial cross-sectional view of another illustrative leadless cardiac pacing device; 
         FIG. 17B  is a cross sectional view of the illustrative leadless cardiac pacing device of  FIG. 17A , taken along line  17 B- 17 B; and 
         FIG. 18  is a schematic perspective view of another illustrative leadless cardiac pacing device. 
     
    
    
     While the disclosure is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit aspects of the disclosure to the particular illustrative embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosure. 
     DESCRIPTION 
     The following description should be read with reference to the drawings in which similar elements in different drawings are numbered the same. The description and the drawings, which are not necessarily to scale, depict illustrative embodiments and are not intended to limit the scope of the disclosure. While the present disclosure is applicable to any suitable implantable medical device (IMD), the description below uses pacemakers and more particularly leadless cardiac pacemakers (LCP) as particular examples. 
     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. This contraction forces blood out of and into the heart, providing circulation of the blood throughout the rest of the body. However, many patients suffer from cardiac conditions that affect this contractility of their hearts. For example, some hearts may develop diseased tissues that no longer generate or conduct intrinsic electrical signals. In some examples, diseased cardiac tissues conduct electrical signals at differing rates, thereby causing an unsynchronized and inefficient contraction of the heart. In other examples, a heart may initiate 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. In some cases such an abnormality can develop into a fibrillation state, where the contraction of the patient&#39;s heart chambers are almost completely de-synchronized and the heart pumps very little to no blood. Implantable medical devices, which may be configured to determine occurrences of such cardiac abnormalities or arrhythmias and deliver one or more types of electrical stimulation therapy to patient&#39;s hearts, may help to terminate or alleviate these and other cardiac conditions. 
       FIG. 1  depicts an illustrative leadless cardiac pacemaker (LCP) that may be implanted into a patient and may operate to prevent, control, or terminate cardiac arrhythmias in patients by, for example, appropriately employing one or more therapies (e.g. anti-tachycardia pacing (ATP) therapy, cardiac resynchronization therapy (CRT), bradycardia therapy, defibrillation pulses, or the like). As can be seen in  FIG. 1 , the LCP  100  may be a compact device with all components housed within the LCP  100  or directly on the housing  120 . In the example shown in  FIG. 1 , 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 electrodes  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, 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, remote 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 the 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, etc., to an external medical device through the communication module  102 . The external medical device may use the communicated signals, data, instructions and/or messages to perform various functions, such as determining occurrences of arrhythmias, delivering electrical stimulation therapy, storing received data, analyzing received data, 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, analyzing received data, and/or performing any other suitable function. The communication module  102  may be configured to use one or more methods for communicating with remote devices. For example, the communication module  102  may communicate via radiofrequency (RF) signals, inductive coupling, optical signals, acoustic signals, conducted communication signals, and/or any other signals suitable for communication. 
     In the example shown in  FIG. 1 , the pulse generator module  104  may be electrically connected to the electrodes  114 . In some examples, the LCP  100  may include one or more additional electrodes  114 ′. In such examples, the pulse generator  104  may also be electrically connected to the additional electrodes  114 ′. The pulse generator module  104  may be configured to generate electrical stimulation signals. For example, the pulse generator module  104  may generate electrical stimulation signals by using energy stored in a battery  112  within the LCP  100  and deliver the generated electrical stimulation signals via the electrodes  114  and/or  114 ′. Alternatively, or additionally, the pulse generator  104  may include one or more capacitors, and the pulse generator  104  may charge the one or more capacitors by drawing energy from the battery  112 . The pulse generator  104  may then use the energy of the one or more capacitors to deliver the generated electrical stimulation signals via the electrodes  114  and/or  114 ′. In at least some examples, the pulse generator  104  of the LCP  100  may include switching circuitry to selectively connect one or more of the electrodes  114  and/or  114 ′ to the pulse generator  104  in order to select which of the electrodes  114 / 114 ′ (and/or other electrodes) the pulse generator  104  delivers the electrical stimulation therapy. The pulse generator module  104  may generate electrical stimulation signals with particular features or in particular sequences in order to provide one or multiple of a number of different stimulation therapies. For example, the pulse generator module  104  may be configured to generate electrical stimulation signals to provide electrical stimulation therapy to combat bradycardia, tachycardia, cardiac dyssynchrony, bradycardia arrhythmias, tachycardia arrhythmias, fibrillation arrhythmias, cardiac synchronization arrhythmias and/or to produce any other suitable electrical stimulation therapy. Some more common electrical stimulation therapies include bradycardia therapy, anti-tachycardia pacing (ATP) therapy, cardiac resynchronization therapy (CRT), and cardioversion/defibrillation therapy. 
     In some examples, the LCP  100  may not include a pulse generator  104  or may turn off the pulse generator  104 . When so provided, the LCP  100  may be a diagnostic only device. In such examples, the LCP  100  may not deliver electrical stimulation therapy to a patient. Rather, the LCP  100  may collect data about cardiac electrical activity and/or 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, cardiac electrical signals sensed by the LCP  100  through the electrodes  114 / 114 ′ may represent ventricular cardiac electrical signals. The mechanical sensing module  108  may include one or more sensors, such as an accelerometer, a blood pressure sensor, a heart sound sensor, a blood-oxygen sensor, a temperature sensor, a flow sensor and/or any other suitable sensors that are configured to measure one or more mechanical and/or 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. 1  as separate sensing modules, in some cases, the electrical sensing module  106  and the mechanical sensing module  108  may be combined into a single sensing module, as desired. 
     The electrodes  114 / 114 ′ can be secured relative to the housing  120  but exposed to the tissue and/or blood surrounding the LCP  100 . In some cases, the electrodes  114  may be generally disposed on 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 LCP  100  may have an insulative portion that electrically isolates the electrodes  114 / 114 ′ from adjacent electrodes, the housing  120 , and/or other parts of the LCP  100 . 
     The processing module  110  can be configured to control the operation of the LCP  100 . For example, the processing module  110  may be configured to receive electrical signals from the electrical sensing module  106  and/or the mechanical sensing module  108 . Based on the received signals, the processing module  110  may determine, for example, occurrences and, in some cases, types of arrhythmias. Based on any determined arrhythmias, the processing module  110  may control the pulse generator module  104  to generate electrical stimulation in accordance with one or more therapies to treat the determined arrhythmia(s). 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 arrhythmia is occurring, determine a type of arrhythmia, 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. 2  depicts an example of another medical device (MD)  200 , which may be used in conjunction with an LCP  100  ( FIG. 1 ) in order to detect and/or treat cardiac arrhythmias and other heart conditions. In the example shown, the MD  200  may include a communication module  202 , a pulse generator module  204 , an electrical sensing module  206 , a mechanical sensing module  208 , a processing module  210 , and a battery  218 . Each of these modules may be similar to the modules  102 ,  104 ,  106 ,  108 , and  110  of the LCP  100 . Additionally, the battery  218  may be similar to the battery  112  of the LCP  100 . In some examples, the MD  200  may have a larger volume within the housing  220  than LCP  100 . In such examples, the MD  200  may include a larger battery and/or a larger processing module  210  capable of handling more complex operations than the processing module  110  of the LCP  100 . 
     While it is contemplated that the MD  200  may be another leadless device such as shown in  FIG. 1 , in some instances the MD  200  may include leads such as leads  212 . The leads  212  may include electrical wires that conduct electrical signals between the electrodes  214  and one or more modules located within the housing  220 . In some cases, the leads  212  may be connected to and extend away from the housing  220  of the MD  200 . In some examples, the leads  212  are implanted on, within, or adjacent to a heart of a patient. The leads  212  may contain one or more electrodes  214  positioned at various locations on the leads  212 , and in some cases at various distances from the housing  220 . Some of the leads  212  may only include a single electrode  214 , while other leads  212  may include multiple electrodes  214 . Generally, the electrodes  214  are positioned on the leads  212  such that when the leads  212  are implanted within the patient, one or more of the electrodes  214  are positioned to perform a desired function. In some cases, the one or more of the electrodes  214  may be in contact with the patient&#39;s cardiac tissue. In some cases, the one or more of the electrodes  214  may be positioned substernally or subcutaneously but adjacent 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, blood pressure sensors, heart sound sensors, blood-oxygen sensors, acoustic sensors, ultrasonic sensors and/or other sensors which are configured to measure one or more mechanical/chemical parameters of the heart and/or patient. In some examples, one or more of the sensors may be located on the leads  212 , but this is not required. In some examples, one or more of the sensors may be located in the housing  220 . 
     While not required, in some examples, the MD  200  may be an implantable medical device. In such examples, the housing  220  of the MD  200  may be implanted in, for example, a transthoracic region of the patient. The housing  220  may generally include any of a number of known materials that are safe for implantation in a human body and may, when implanted, hermetically seal the various components of the MD  200  from fluids and tissues of the patient&#39;s body. 
     In some cases, the MD  200  may be an implantable cardiac pacemaker (ICP). In this example, the MD  200  may have one or more leads, for example leads  212 , which are implanted on or within the patient&#39;s heart. The one or more leads  212  may include one or more electrodes  214  that are in contact with cardiac tissue and/or blood of the patient&#39;s heart. The MD  200  may be configured to sense intrinsically generated cardiac electrical signals and determine, for example, one or more cardiac arrhythmias based on analysis of the sensed signals. The MD  200  may be configured to deliver CRT, ATP therapy, bradycardia therapy, and/or other therapy types via the leads  212  implanted within the heart or in concert with the LCP by commanding the LCP to pace. In some examples, the MD  200  may additionally be configured provide defibrillation therapy. 
     In some instances, the MD  200  may be an implantable cardioverter-defibrillator (ICD). In such examples, the MD  200  may include one or more leads implanted within a patient&#39;s heart. The MD  200  may also be configured to sense cardiac electrical signals, determine occurrences of tachyarrhythmias based on the sensed signals, and may be configured to deliver defibrillation therapy in response to determining an occurrence of a tachyarrhythmia. In some instances, the MD  200  may be a subcutaneous implantable cardioverter-defibrillator (S-ICD). In examples where the MD  200  is an S-ICD, one of the leads  212  may be a subcutaneously 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 cases, the S-ICD lead may extend subcutaneously from the S-ICD can, around the sternum and may terminate adjacent the interior surface of the sternum. 
     In some examples, the MD  200  may not be an implantable medical device. Rather, the MD  200  may be a device external to the patient&#39;s body, and may include skin-electrodes that are placed on a patient&#39;s body. In such examples, the MD  200  may be able to sense surface electrical signals (e.g. cardiac electrical signals that are generated by the heart or electrical signals generated by a device implanted within a patient&#39;s body and conducted through the body to the skin). In such examples, the MD  200  may be configured to deliver various types of electrical stimulation therapy, including, for example, defibrillation therapy. The MD  200  may be further configured to deliver electrical stimulation via the LCP by commanding the LCP to deliver the therapy. 
       FIG. 3  shows an example medical device system with 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 , an external medical device  306 , and other sensors/devices  310 . The external device  306  may be any of the devices described previously with respect to MD  200 . In some embodiments, the external device  306  may be provided with or be in communication with a display  312 . The display  312  may be a personal computer, tablet computer, smart phone, laptop computer, or other display as desired. In some instances, the display  312  may include input means for receiving an input from a user. For example, the display  312  may also include a keyboard, mouse, actuatable (e.g. pushable) buttons, or a touchscreen display. These are just examples. The other sensors/devices  310  may be any of the devices described previously with respect to the MD  200 . In some instances, the other sensors/devices  310  may include a sensor, such as an accelerometer or blood pressure sensor, or the like. In some cases, the 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 a 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 the 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. In another example, the LCPs  302  and/or  304  may sense indications of blood pressure (e.g. via one or more pressure sensors) and indications of volume (e.g. via an impedance between the electrodes of an LCP or between LCPs via an ultrasound transducer placed within the LCP, or via strain sensors placed on the heart in communication with the LCP). In one example, one or more of the devices  302 / 304  may receive such signals and, based on the received signals, determine a pressure-volume loop, and in some cases may communicate such information to one or more other devices  302 / 304 ,  306 , and  310  of the system  300  via the communication pathway  308 . 
     It is contemplated that the communication pathway  308  may communicate using RF signals, inductive coupling, conductive coupling optical signals, acoustic signals, or any other signals suitable for communication. Additionally, in at least some examples, the device communication pathway  308  may comprise multiple signal types. For instance, the 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, inductive 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 the other sensors/devices  310 , where the LCPs  302 / 304  send signals to the other sensors/devices  310 , and the 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 the 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 refractory 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. 
     In some cases, the communication pathway  308  may include inductive communication, and when so provided, the devices of the system  300  may be configured to transmit/receive inductive communication signals. 
       FIGS. 4 and 5  show illustrative medical device systems that may be configured to operate according to techniques disclosed herein. In  FIG. 4 , 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   b ,  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   b ,  408   c  may be positioned subcutaneously adjacent the heart. In some cases, the S-ICD lead may extend subcutaneously from the S-ICD can, around the sternum and one or more electrodes  408   a ,  408   b ,  408   c  may be positioned adjacent the interior surface of the sternum. In some cases, the LCP  402  may communicate with the subcutaneous implantable cardioverter-defibrillator (S-ICD). 
     In some cases, the LCP  402  may be in the right ventricle, right atrium 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. 
     In  FIG. 5 , an LCP  502  is shown fixed to the interior of the left ventricle of the heart  510 , and a pulse generator  506  is shown coupled to a lead  512  having one or more electrodes  504   a ,  504   b ,  504   c . In some cases, the pulse generator  506  may be part of an implantable cardiac pacemaker (ICP) and/or an implantable cardioverter-defibrillator (ICD), and the one or more electrodes  504   a ,  504   b ,  504   c  may be positioned in the heart  510 . In some cases, the LCP  502  may communicate with the implantable cardiac pacemaker (ICP) and/or an implantable cardioverter-defibrillator (ICD). 
     The medical device systems  400  and  500  may also include an external support device, such as external support devices  420  and  520 . The external support devices  420  and  520  can be used to perform functions such as device identification, device programming and/or transfer of real-time and/or stored data between devices using one or more of the communication techniques described herein. As one example, communication between the external support device  420  and the pulse generator  406  is performed via a wireless mode, and communication between the pulse generator  406  and the LCP  402  is performed via a conducted mode. In some examples, communication between the LCP  402  and the external support device  420  is accomplished by sending communication information through the pulse generator  406 . However, in other examples, communication between the LCP  402  and the external support device  420  may be via a communication module. In some embodiments, the external support devices  420 ,  520  may be provided with or be in communication with a display  422 ,  522 . The display  422 ,  522  may be a personal computer, tablet computer, smart phone, laptop computer, or other display as desired. In some instances, the display  422 ,  522  may include input means for receiving an input from a user. For example, the display  422 ,  522  may also include a keyboard, mouse, actuatable buttons, or be a touchscreen display. These are just examples. 
       FIGS. 4-5  illustrate two examples of medical device systems that may be configured to operate according to techniques disclosed herein. Other example medical device systems may include additional or different medical devices and/or configurations. For instance, other medical device systems that are suitable to operate according to techniques disclosed herein may include additional LCPs implanted within the heart. Another example medical device system may include a plurality of LCPs without other devices such as the pulse generator  406  or  506 , with at least one LCP capable of delivering defibrillation therapy. In yet other examples, the configuration or placement of the medical devices, leads, and/or electrodes may be different from those depicted in  FIGS. 4 and 5 . Accordingly, it should be recognized that numerous other medical device systems, different from those depicted in  FIGS. 4 and 5 , may be operated in accordance with techniques disclosed herein. As such, the examples shown in  FIGS. 4 and 5  should not be viewed as limiting in any way. 
       FIG. 6  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 herein. 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). 
     It is contemplated that the housing  612  may take a variety of different shapes. For example, in some cases, the housing  612  may have a generally cylindrical shape. In other cases, the housing  612  may have a half-dome shape. In yet other embodiments, the housing  612  may be a rectangular prism. It is contemplated that the housing may take any cross sectional shape desired, including but not limited to annular, polygonal, oblong, square, etc. 
     In some cases, the LCP  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. As shown in  FIG. 6 , 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 (not explicitly shown) extending from or recessed within the head portion  632 . The tether retention structure may define an opening configured to receive a tether or other anchoring mechanism therethrough. The retention structure may take any shape that provides an enclosed perimeter surrounding the opening such that a tether may be securably and releasably passed (e.g. looped) through the opening. In some cases, the retention structure 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 and/or otherwise operationally coupled with the environment outside the housing  612  to measure blood pressures within the heart. In some cases, the one or more pressure sensors  640  may be coupled to an exterior surface of the housing  612 . In other cases, the one or more pressures sensors  640  may be positioned within the housing  612  with a pressure acting on the housing and/or a port on the housing  612  to affect the pressure sensor  640 . 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. Some illustrative pressure sensor configurations will be described in more detail herein. 
     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 micromanometer, a surface acoustic wave (SAW) device, 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 sensor(s)  640  may measure/sense pressure in the chamber in which the LCP  610  is implanted. For example, an LCP  610  implanted in the left ventricle (LV) could sense LV pressure. The pressure sensor(s)  640  may be configured (either alone or in combination with other circuitry in the LCP  610 ) to derive change in pressure over time and used to adjust atrium to ventricle pacing delay to optimize cardiac resynchronization therapy (CRT). In some cases, the pressure sensor(s)  640  may be configured to detect a-waves and change the pacing timing of the LCP  610  for CRT optimization. It is further contemplated that sensing pressure could be used during the implant procedure to optimize the placement of the LCP  610  in the chamber (e.g., LV by sampling at different implant locations and using the best location. Frequent pressure monitoring may be beneficial for management of heart failure patients. Frequent pressure monitoring may also be useful for patients with chronic heart disease, hypertension, regurgitation, valve issues, atrial contraction detection, and to aid in addressing other problems. It is further contemplated that the pressure sensor(s)  640  may be used for monitoring respiration and associated diseases (e.g., chronic obstructive pulmonary disease (COPD), etc.). These are just examples. 
     In some cases, pressure readings may be taken in combination with a cardiac chamber volume measurement such an impedance measurement (e.g. the impedance between electrodes  620  and  622 ) to generate a pressure-impedance loop for one or more cardiac cycles. The impedance may be a surrogate for chamber volume, and thus the pressure-impedance loop may be representative of a pressure-volume loop for the heart H. 
       FIG. 7A  is a plan view of the example leadless cardiac pacing device  610  implanted within a left ventricle LV of the heart H during ventricular filling. The right ventricle RV, right atrium RA, left atrium LA, and aorta A are also illustrated.  FIG. 7B  is a plan view of the leadless cardiac pacing device  610  implanted within a left ventricle of the heart H during ventricular contraction. These figures illustrate how the volume of the left ventricle may change over a cardiac cycle. As can be seen in  FIGS. 7A and 7B , the volume of the left ventricle during ventricular filling is larger than the volume of the left ventricle of the heart during ventricular contraction. 
     In some cases, the processing module and/or other control circuitry may capture, at a time point within each of one or more cardiac cycles, a pressure within the heart (e.g. left ventricle), resulting in one or more pressure data points. These one or more data points may be used, in combination with other pressure data points taken at different times during the one or more cardiac cycles, to generate a pressure curve. In some cases, one or more parameters may be extracted or derived from the pressure curve. The pressure curve may be used to facilitate cardiac resynchronization therapy (CRT), patient health status monitoring, and/or the management of a non-CRT cardiac therapy. 
       FIG. 8  is a graph  800  showing example pressures and volumes within a heart over time. More specifically,  FIG. 8  depicts the aortic pressure, left ventricular pressure, left atrial pressure, left ventricular volume, an electrocardiogram (ECG or egram), and heart sounds of the heart H. 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 S1 heart sound. Before the aortic valve opens, an isovolumetric contraction phase occurs where the ventricle pressure rapidly increases but the ventricular volume does not significantly change. Once the ventricular pressure equals the aortic pressure, the aortic valve opens and the ejection phase begins where blood is ejected from the left ventricle into the aorta. The ejection phase continues until the ventricular pressure falls below the aortic pressure, at which point the aortic valve closes, generating the S2 heart sound. At this point, the isovolumetric relaxation phase begins and ventricular pressure falls rapidly until it is exceeded by the atrial pressure, at which point the mitral valve opens and the cycle repeats. Cardiac pressure curves for the pulmonary artery, the right atrium, and the right ventricle, and the cardiac volume curve for the right ventricle, similar to those illustrated in  FIG. 8  for the left part of the heart, may be likewise generated. Typically, the cardiac pressure in the right ventricle is lower than the cardiac pressure in the left ventricle. 
     In one example, the heart sound signals can be recorded using acoustic sensors, (for example, a microphone), which capture the acoustic waves resulted from heart sounds. In another example, the heart sound signals can be recorded using accelerometers or pressure sensors that capture the accelerations or pressure waves caused by heart sounds. The heart sound signals can be recorded within or outside the heart. These are just examples. 
       FIG. 9  is a cross-section of another illustrative implantable leadless cardiac pacemaker (LCP)  900 . The LCP  900  may be similar in form and function to the LCPs  100 ,  610  described above. The LCP  900  may include any of the modules and/or structural features described above with respect to the LCPs  100 ,  610 . The LCP  900  may include a shell or housing  902  having a proximal end  904  and a distal end  906 . In the example shown, the LCP  900  does not include a docking member. However, in some cases, a docking member may be provided, such as a cage extending proximally from adjacent the side walls of the housing  902 . The illustrative LCP  900  includes a first electrode  908  secured relative to the housing  902  and positioned adjacent to the distal end  906  of the housing  902 , and a second electrode (not explicitly shown) secured relative to the housing  902  and positioned adjacent to the proximal end  904  of the housing  902 . In some instances, the first electrode  908  may be positioned on a distal end surface facing distally. In some cases, the housing  902  may include a conductive material and may be insulated along a portion of its length. A section along the proximal end  904  may be free of insulation so as to define the second electrode. The electrodes  908  may be sensing and/or pacing electrodes to aid in providing electro-therapy and/or sensing capabilities. The first electrode  908  may be capable of being positioned against or may otherwise contact the cardiac tissue of the heart while the second electrode may be spaced away from the first electrode  908 . The first and/or second electrodes  908  may be exposed to the environment outside the housing  902  (e.g. to blood and/or tissue). 
     In some cases, the LCP  900  may include a pulse generator (e.g., electrical circuitry)  910  and a power source (e.g., a battery)  912  within the housing  902  to provide and/or receive electrical signals via the first and second electrodes. While not explicitly shown, the LCP  900  may also include a communications module, an electrical sensing module, a mechanical sensing module, and/or a processing module, and 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  902 . Electrical communication between the pulse generator and the electrodes may provide electrical stimulation to heart tissue and/or sense a physiological condition. 
     In the example shown, the LCP  900  further includes a fixation mechanism  914  proximate the distal end  906  of the housing  902 . The fixation mechanism  914  is configured to attach the LCP  900  to a wall of the heart H, or otherwise anchor the LCP  900  to the anatomy of the patient. As shown in  FIG. 9 , in some instances, the fixation mechanism  914  may include one or more, or a plurality of hooks or tines  916  anchored into the cardiac tissue of the heart H to attach the LCP  900  to a tissue wall. In other instances, the fixation mechanism  914  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  900  to the heart H. These are just examples. 
     Referring now to  FIG. 10 , which illustrates an enlarged cross-sectional view of the proximal end  904  of the LCP  900 . The housing  902  may include a proximal end surface  918  facing proximally (e.g., in a generally opposite direction from the distal end surface. In some instances, the proximal end surface  918  of the housing  902  may form a diaphragm  920 . In some cases, the diaphragm  920  may be formed from the housing material itself. When so provided, the wall thickness of the housing in the region of the diaphragm  920  may be thinned to increase the flexibility of the diaphragm  920 . In other cases, the diaphragm  920  may be formed from another material, such as but not limited to silicone, polyimides, etc. to form a deformable or movable diaphragm  920  that is responsive to a pressure applied to the diaphragm  920 . In any event, the diaphragm  920  may be fabricated to flex or deform as the pressure (external to the housing  902 ) in the heart (e.g., left ventricle) changes, as will be described in more detail herein. While the entire proximal end surface  918  may form the diaphragm  920 , it is contemplated that only a portion of the end surface  918  may form the diaphragm  920 . In some cases, the diaphragm  920  may be 1 millimeter in diameter or less. In other cases, the diaphragm  920  may be greater than 1 millimeter in diameter. In some cases, the diaphragm  920  may have a round shape. In other cases, the diaphragm  920  may have a square, rectangular or any other suitable shape. In the example shown, the diaphragm  920  may be configured to transfer an endocardial pressure external to the housing  902  to a pressure sensor  922  positioned within the housing  902 . 
     As will be described in more detail herein, the diaphragm  920  need not be placed on the proximal end surface  918  of the housing  902 . It is contemplated that the diaphragm  920  may be formed in any surface of the housing  902  (or docking member, if so provided) desired. In some cases, locating the diaphragm  920  on or adjacent to the proximal end  904  of the housing  902  may orientate the diaphragm towards the heart valves (when the LCP  900  is positioned in the apex of the heart) and in-line with maximum pressure changes, which may achieve higher signal levels. It may also locate the diaphragm  920  away from the heart tissue, may reduce the likelihood that diaphragm  920  will become fibrossed-over. In some cases, the diaphragm  920  may be coated with an anti-thrombogenic coating to help prevent tissue growth. 
     In  FIG. 10 , a pressure sensor  922  is positioned adjacent to, but not necessarily in direct contact with, the diaphragm  920 . When so provided, a fluid filled cavity  926  filled with a fluid  928  may be positioned between the diaphragm  920  of the housing and the pressure sensor  922 . In some cases, the pressure sensor  922  may include a diaphragm  934 , as best shown in  FIG. 11 , which is exposed to the fluid filled cavity  926 . The fluid filled cavity  926  may communicate a measure related to the pressure applied by the environment (e.g. endocardial pressures) to the diaphragm  920  of the housing  902  to the pressure sensor diaphragm  934  of the pressure sensor  922 . In some cases, the fluid filled cavity  926  may be confined to a portion of the volume of the housing, such as between the flexible diaphragm  920  of the housing and the diaphragm  934  of the pressure sensor  922 . An O-ring  923  or other seal may be used to confine the fluid  928 . In other instances, the fluid filled cavity  926  may encompass the entire volume of the housing  902  (e.g., the volume not filled by other components). 
     It is contemplated that the fluid filled cavity  926  may be filled with an incompressible fluid  928 . In some cases, the fluid  928  may also be dielectric or non-conductive. Some illustrative fluids  928  may include, but are not limited to, mineral oil, fluorocarbons perfluorohexane, perfluoro (2-butyl-tetrahydrofurane), perfluorotripentylamine, and/or Fluorinert™ (manufactured by the 3M Company, St. Paul, Minn.). In some cases, the fluid  928  may be highly soluble to gases that are likely to arise inside of the housing, particularly at body temperature (e.g. 37° C.). For example, the fluid  928  may be highly soluble to hydrogen, helium, nitrogen, argon, water, and/or other gases or liquids that might arise inside of the housing as a result of, for example, outgassing of internal components of the LCP  900 . As a pressure or external force  930  is applied to the outer surface of the diaphragm  920 . In response, the diaphragm  920  may flex inwards and the fluid  928  may transfer the force to the pressure sensor  922 , as shown at arrow  932 . The pressure sensor  922  may provide a pressure sensor signal that is representative of the pressure or external force  930 . 
     In some cases, the fluid  928  (and/or the diaphragm  920 ) may be selected to match the acoustic impedance of blood. This may facilitate the use of the pressure sensor  922  as an acoustic pressure sensor. In some cases, the pressure sensor  922  may be used to detect various sounds such as heart sounds, valve regurgitation, respiration, blood flow, blood turbulence and/or other suitable sounds. In some cases, sounds having a frequency of up to 200 Hz or more may be detected. 
     In some cases, the pressure sensor  922  may be a Micro-Electro-Mechanical System (MEMS) pressure sensor, such as shown in  FIG. 11 .  FIG. 11  illustrates a cross-sectional view of an illustrated MEMS pressure sensor. MEMS pressure sensors are often formed by anisotropically etching a recess into a back side of a silicon substrate, leaving a thin flexible diaphragm  934 . For an absolute pressure sensor, a sealed cavity  946  is created behind the diaphragm  934 . The sealed cavity  946  may be evacuated to a very low pressure, near zero. In operation, the front side of the diaphragm  934  is exposed to an input pressure, such as from the fluid  928 , and may flex or deform by an amount that is related to the difference between the input pressure and the vacuum pressure in the sealed cavity  946 . 
     The diaphragm  934  of the pressure sensor  922  may include one or more sense elements  936 , which may detect the flexing of the diaphragm  934 . In some cases, the sensor elements may include piezoresistors, the change resistance with increased stress in the diaphragm  934 . The piezoresistors may be connected to a circuit, such as a Wheatstone bridge circuit, that outputs a signal that is related to the amount of stress sensed in the diaphragm  934 , which is ultimately related to the amount of pressure applied to the outer surface of the diaphragm  920  of the housing. The stress may be one or more of compression or stretching of the diaphragm  920 . In some cases, the diaphragm  934  of the pressure sensor  922  and/or the diaphragm  920  of the housing may be made thinner and/or may include one or more support bosses to help increase the sensitivity and/or linearity of the flexure of the diaphragm. 
     In some cases, circuitry  938  may be fabricated in to the first substrate  940 , and may be connected to the sensor elements  936 . The circuitry  938  may be configured to provide some level of signal processing before providing an output signal to bond pads  948  of the pressure sensor  922 . The signal processing circuitry may filter, amplify, linearize, calibrate and/or otherwise process the raw sensor signal produced by the sensor elements (e.g. piezoresistors  936 ). While the sense elements  936  have been described as piezoresistors, it is contemplated that the sensor elements  936  may be configured to provide a capacitive output value. For example, the back side of the diaphragm  934  may support a first plate of a capacitor sensor, and the top side of the second substrate may support a second plate. As the diaphragm  934  flexes toward the second substrate, the distance between the first plate and the second plate changes. This changes the capacitance between the first plate and the second plate. This change in capacitance can be sensed by circuitry  938 . 
     The bond pads  948  may be electrically coupled to the circuitry  910  in the housing  902  of the LCP  900  through one or more electrical conductors  924  to relay one or more output signals of the pressure sensor  922  to the circuitry  910 . The circuitry  910  may be configured to determine a pressure exterior to the housing  902  based on the pressure sensor  922  output signal(s). 
     While the pressure sensor  922  has been described as a MEMS pressure sensor, it is contemplated that pressure sensor  922  may take any suitable form. In one alternative example, the pressure sensor may be formed in such a way that radio waves can be used to detect changes in pressure without sensor elements incorporated into the device. Such a pressure sensor may include a flexible base substrate, a bottom inductive coil positioned on the base substrate, a layer of pressure sensitive rubber pyramids positioned over the bottom inductive coil, a top inductive coil positioned on top of the rubber pyramids, and a top substrate positioned over the top inductive coil. As a pressure is exerted on the sensor, the inductive coils move closer together. Radio waves (from an applied source) reflected by the inductive coils have a lower resonance frequency when the coils are positioned closer together. Thus, the frequency of the radio waves can indicate the distance between the coils, which may then be correlated to the pressure exerted on the device. 
     As indicate above, the pressure sensor  922  may be configured to measure absolute pressure, rather than gauge pressure. A communication link may be used to determine atmospheric pressure such that the pressure readings provided to the physician are in terms of gauge pressure. Atmospheric pressure may be measured outside of the patient&#39;s body using an external device. The external device may be in communication with the LCP  900  through any of the wireless means described herein. An absolute pressure sensor  922  may provide higher accuracy with lower drift. In some cases, the absolute pressure readings may be transmitted to an external device and converted to gauge pressure at the external device where it can be viewed by a physician and/or transmitted to another external device viewable by the physician. 
     In some cases, the diaphragm  920  of the housing  902  may have a first surface area and the pressure sensor diaphragm  934  may have a second surface area. The ratio of the first surface area to the second surface area may be at least 5 to 1, greater than 10 to 1, greater than 20 to one, or more. In some instances, the pressure sensor  922  may be configured to obtain pressure measurements in the range of 0 to 240 mmHg (gauge) with an accuracy of 1 mmHg and a resolution of less than 1 mmHg. It is contemplated that in some instances, the pressure sensor  922  may be configured to obtain pressure measurements greater than 240 mmHg (for example, when the patient is under extreme exertion). The pressure sensor  922  may be configured to obtain pressure measurements at a sample rate of greater than 100 Hertz (Hz). This may allow for pressure measurements to be used to determine characteristics of the cardiac cycle including, but not limited to, dP/dT, dicrotic notch, etc. 
     In some embodiments, one or more sensor elements (e.g. piezoresistors) may be placed directly on the inner surface of the diaphragm  920  of the housing  902  and/or on the housing  902  itself. The sensor elements may then detect the stress in the diaphragm  920  of the housing  902  and/or the housing  902 . The sensor elements may be operatively coupled to circuitry (e.g. control electronics  910 ) through one or more electrical conductors  924 . This embodiment may eliminate the need for the fluid filled cavity  926 , the fluid  928 , the diaphragm  934  of the pressure sensor  922 , etc. 
     In the example of  FIG. 9 , the battery  912  is shown adjacent the pressure sensor  922 . However, many different configurations of the internal components of the LCP  900  are contemplated. In the example shown, the processing module (e.g., circuitry or control electronics)  910  may be positioned in a distal portion  906  of the housing  902  adjacent to the distal electrode  908 . The one or more electrical conductors  924  may be formed of a polyimide or similar interconnect having a cross-sectional dimension in the range of less than 250 microns. It is contemplated that the inside surface of the housing  902  may be electrically insulated and the electrical conductors  924  (e.g., trace) may be positioned on the inside surface of the housing  902  or along the outer surface of the battery  912 , as desired. Alternatively, wires or a ribbon cable may be used. These are just examples. 
     In some cases, the pressure sensor  922  may be configured to obtain pressure measurements at predetermined intervals over one or more cardiac cycles. In other instances, the pressure sensor may be configured to obtain a pressure measurement in response to a specific cardiac event or at a specific time in a cardiac cycle. For example, the circuitry  910  may be configured to use one or more cardiac signals sensed by the first electrode  908  and/or second electrode to determine when the patient&#39;s heart is in a first phase of a cardiac cycle. The circuitry  910  may be configured to determine a pressure exterior to the housing  902  based at least in part on the pressure sensor output signal from the pressure sensor  922  taken during the first phase of the cardiac cycle. In some cases, the first phase may be systole and in other cases the first phase may be diastole. The circuitry  910  may be configured to determine a pressure exterior to the housing  902  based at least in part on the pressure sensor output signal from the pressure sensor  922  taken during the first phase of the cardiac cycle. It is contemplated that the circuitry  910  may be further configured to detect heart sounds of the patient&#39;s heart based at least in part on the pressure sensor output signal. For example, the first heart sound may be a timing fiducial for a sudden increase in pressure while the second heart sound may be a timing fiducial for a sudden decrease in pressure. 
     In some cases, the circuitry  910  and/or pressure sensor  922  of the LCP  900  may be configured to obtain a plurality of pressure readings over one or more cardiac cycles. The pressure readings may be plotted (either by the circuitry  910  or an external device) to form a graph similar to the one shown in  FIG. 8 . Various parameters related to the function of the heart can be extrapolated from the graph including but not limited to peak to peak measurements, dP/dT, time averaged values, inotropic response of the ventricle, etc. In some instances, the pressure measurements may be compared to calibration values (e.g., measurements taken at the time of implantation of the LCP  900 ). 
     In some cases, a diaphragm  920  having a different material may not be provided. In other words, the diaphragm  920  may be formed of the same material and of the same thickness as the remaining portion of the housing  902 . For example, the housing  902  may flex or deform to transfer a pressure external to the housing  902  to the pressure sensor  922  located within the housing  612 . For example, the housing  902  may have a compliance such that the relative movement of the housing  902  in response to the external pressure may be coupled to the internal pressure sensor  922 . The internal pressure sensor  922  may be calibrated relative to external pressures prior to implantation of the LCP  902  in a patient. The calibration data may be stored in the memory and/or electrical circuitry of the LCP  900 . For example, once the LCP  900  is implanted, a measure related to the pressure applied by the environment (e.g. endocardial pressures) to the housing  902  may be communicated to a pressure sensing diaphragm  934  of the pressure sensor  922 . It is contemplated that there may be some pressure loss (e.g., in the range of 1-20%) between the pressure exerted on the housing  902  and the pressure reading obtained at the pressure sensor  922 . This pressure loss may be compensated for (e.g., nullified) by adjusting the pressure sensor signal from the pressure sensor  922  using the calibration data stored in the LCP  900 . It is contemplated that an incompressible fluid may couple the housing  902  and the pressure sensing diaphragm  934  of the pressure sensor  922  in a manner similar to that described herein. For example, the entire housing  902  or a portion of the housing  902  may be coupled to the pressure sensor  922  by the incompressible fluid. 
       FIG. 12  illustrates a proximal end portion  954  of another illustrative LCP  950  having a diaphragm  960  and a pressure sensor  962 . The LCP  950  may be similar in form and function to the LCPs  100 ,  610 ,  900  described above. The LCP  950  may include any of the modules and/or structural features described above with respect to the LCPs  100 ,  610 ,  900 . The diaphragm  960 , pressure sensor  962  and internal circuitry (not explicitly shown) may interact in a similar manner to the diaphragm  920 , pressure sensor  922  and circuitry  910  described above. 
     The LCP  950  may include a shell or housing  952  having a proximal end portion  954  and a distal end (not explicitly shown). The housing  952  may include a proximal end surface  956  facing proximally (e.g., in a generally opposite direction from the distal end surface. In some instances, the proximal end surface  956  of the housing  952  may include a region of localized thinning  958 . For example, the housing  952  may have a first wall thickness T 1  and the region of localized thinning  958  may have a second wall thickness T 2 . The second wall thickness T 2  may be less than the first wall thickness T 1 . In some embodiments, the region of localized thinning  958  may have a thickness T 2  in the range of 30 microns. This is just an example. The region of localized thinning  958  may have a thickness such that the region  958  may be deformable or movable to create a diaphragm  960  that is responsive to a pressure applied to the proximal end surface  956 . This may allow the diaphragm  960  to flex or deform as the pressure (external to the housing  952 ) in the heart (e.g., left ventricle) changes, as will be described in more detail herein. 
     In some cases, the region of localized thinning  958  may be created by removing material from the housing  952  from an interior of the housing  952 , which may reduce nucleation points for thrombus formation. The region of localized thinning  958  may transition from the first wall thickness T 1  to the second wall thickness T 2  in a tapered, sloped, or curved (e.g., exponential) manner. In other words, the region of localized thinning  158  may not have a uniform thickness across its width. A sloped transition between the first wall thickness T 1  and the second wall thickness T 2  may help reduce stress concentration and/or non-linearity in the proximal end surface  956 . However, in some cases, the region of localized thinning  958  may transition from the first wall thickness T 1  to the second wall thickness T 2  in an abrupt or step-wise manner. In other words, the region of localized thinning  158  may have a uniform thickness across its width (not explicitly shown). The region of localized thinning  958  may function as a diaphragm  960  formed by the housing  952 . In some cases, the diaphragm  960  may be as small as approximately 1 millimeter in diameter. The diaphragm  960  diameter and thickness may be configured so that the diaphragm  960  is able to suitable transfer a pressure external to the housing  952  (e.g. an endocardial pressure) to a pressure sensor  962  positioned within the housing  952 , via a cavity  968  filled with a fluid  970 . An O-ring  963  or other seal may be used to confine the fluid  970 . In other instances, the fluid filled cavity  970  may encompass the entire volume of the housing  952  (e.g., the volume not filled by other components) distal of the diaphragm  960 . 
       FIG. 13  illustrates a cross-sectional view of a proximal end portion  1004  of another illustrative LCP  1000  having a diaphragm  1006  and a force sensor  1010 . The LCP  1000  may be similar in form and function to the LCPs  100 ,  610 ,  900  described above. The LCP  1000  may include any of the modules and/or structural features described above with respect to the LCPs  100 ,  610 ,  900 . The diaphragm  1006 , a force sensor  1010  and internal circuitry (not explicitly shown) may interact in a similar manner to the diaphragm  920 , pressure sensor  922  and circuitry  910  described above. 
     The LCP  1000  may include a shell or housing  1002  having a proximal end portion  1004  and a distal end (not explicitly shown). The housing  1002  may include a proximal end surface  1006  facing proximally (e.g., in a generally opposite direction from the distal end surface. The proximal end surface  1006  may include a pair of generally opposing sidewalls  1014   a ,  1014   b  (collectively,  1014 ) extending distally therefrom. In this example, the sidewalls  1014  may include a crumple-zone  1008   a ,  1008   b  (collectively,  1008 ) formed therein. The crumple-zones  1008  may have an accordion or bellow-like structure including a plurality of peaks  1022   a ,  1022   b  (collectively,  1022 ) and valleys  1024   a ,  1024   b  (collectively,  1024 ) which allows the crumple-zones  1008  to compress in the distal direction  1016  or elongate in the proximal direction  1018 . This may allow the internal volume of the housing  1002  to change as an exteriorly applied pressure  1020  is applied to the housing  1002 . While the peaks  1022  and valleys  1024  are illustrated as having sharp or abrupt edges, the peaks  1022  and valleys  1024  may have gentle slopes or curves as desired. 
     A strut  1021  may extend from the proximal end surface  1006  to the force sensor  1010 . The force sensor  1010  may sense the force that is applied by the strut  1021 . The strut  1021  may transfer the force that is applied to the proximal end surface  1006  of the housing by an external pressure (e.g. endocardial pressure). The force applied to the proximal end surface  1006  of the housing by an external pressure (e.g. endocardial pressure) is amplified by the ratio of the surface area of the proximal end surface  1006  to the surface area of the strut that abuts the force sensor  1010 . 
     The force sensor  1010  may be operatively coupled to circuitry or control electronics (not explicitly shown) of the LCP  1000  through one or more electrical connections  1026 .  FIG. 13  illustrates the battery  1028  adjacent to the force sensor  1010 . However, many different configurations of the internal components are contemplated. The one or more electrical connections  1026  may be formed of a polyimide or similar interconnect having a cross-sectional dimension in the range of less than 250 microns. It is contemplated that the inside surface of the housing  1002  may be electrically insulated and the electrical connection  1026  (e.g., trace) positioned on the inside surface of the housing  1002  or along the outer surface of the battery  1028 , as desired. Alternatively, wires or a ribbon cable may be used. These are just examples. 
       FIG. 14  illustrates a cross-sectional view of a proximal end portion  1054  of another illustrative LCP  1050  having a diaphragm  1058  and a pressure sensor  1060 . The LCP  1050  may be similar in form and function to the LCPs  100 ,  610 ,  900  described above. The LCP  1050  may include any of the modules and/or structural features described above with respect to the LCPs  100 ,  610 ,  900 . The diaphragm  1058 , pressure sensor  1060  and internal circuitry (not explicitly shown) may interact in a similar manner to the diaphragm  920 , pressure sensor  922  and circuitry  910  described above. 
     The LCP  1050  may include a shell or housing  1052  having a proximal end portion  1054  and a distal end (not explicitly shown). The housing  1052  may include a docking member  1056  extending proximally from the proximal end portion  1054 . The docking member  1056  may be configured to facilitate delivery and/or retrieval of the LCP  1050 . For example, the docking member  1056  may extend from the proximal end portion  1054  of the housing  1052  along a longitudinal axis of the housing  1052 . The docking member  1056  may include a head portion  1062  and a neck portion  1064  extending between the housing  1052  and the head portion  1062 . The head portion  1062  may be an enlarged portion relative to the neck portion  1064 . An access port  1068  may extend through the head portion  1062  and the neck portion  1064  to fluidly couple the diaphragm  1058  with the blood in the heart. The diaphragm  1058  may be constructed using any of the materials and/or configurations described herein. Alternatively, the diaphragm  1058  may be positioned at the proximal opening  1070  of the access port  1068 . 
     A pressure sensor  1060  may be positioned adjacent to, but not necessarily in direct contact with the diaphragm  1058 . The pressure sensor  1060  may be operatively coupled to circuitry or control electronics (not explicitly shown) of the LCP  1050  through one or more electrical connections  1072 .  FIG. 14  illustrates the battery  1078  adjacent to the pressure sensor  1060 . However, many different configurations of the internal components of the LCP  1050  are contemplated. The one or more electrical connections  1072  may be formed of a polyimide or similar interconnect having a cross-sectional dimension in the range of less than 250 microns. It is contemplated that the inside surface of the housing  1052  may be electrically insulated and the electrical connection  1072  (e.g., trace) positioned on the inside surface of the housing  1052  or along the outer surface of the battery  1078 , as desired. Alternatively, wires or a ribbon cable may be used. These are just examples. 
     The pressure sensor  1060  may be positioned in or adjacent to a cavity  1074  filled with a fluid  1076 . The fluid filled cavity  1074  is in fluid communication with the diaphragm  1058  and the pressure sensor  1060 , such that the fluid filled cavity  1074  may communicate a measure related to the pressure applied by the environment to the diaphragm  1058  to a pressure sensor diaphragm of the pressure sensor  1060 . In some cases, the fluid filled cavity  1074  may be confined to a portion of the volume of the housing  1052 , such as between the flexible diaphragm  1058  and a sensor diaphragm (not shown) of the pressure sensor  1060 . An O-ring  1073  or other seal may be used to confine the fluid  1076 . In other instances, the fluid filled cavity  1074  may encompass the entire volume of the housing  1052  (e.g., the volume not filled by other components) distal of the diaphragm  1058 . In the example shown in  FIG. 14 , the diaphragm  1058  is located distal of the docking member  1056 . An access port  1068  is provided through the docking member  1056  so allow endocardial pressure  1080  to engage the diaphragm  1058 . 
     In some cases, a plurality of access ports  1108   a - 1108   d  may be provided to the diaphragm  1058 , such as shown in  FIG. 15 .  FIG. 15  illustrates a proximal end view of another illustrative LCP  1100  having a diaphragm and an internally positioned pressure sensor (not explicitly shown). The LCP  1100  may be similar in form and function to the LCP  1050  described above. The LCP  1100  may include any of the modules and/or structural features described above with respect to the LCPs  100 ,  610 ,  900 ,  1050 . A diaphragm, pressure sensor and internal circuitry (not explicitly shown in  FIG. 11 ) may interact in a similar manner to the diaphragm  1058 , pressure sensor  1060  and circuitry of  FIG. 14 . 
     The LCP  1100  may include a shell or housing having a proximal end region  1104  and a distal end (not explicitly shown). The housing  1102  may include a docking member  1106  extending proximally from the proximal end region  1104 . The docking member  1106  may be configured to facilitate delivery and/or retrieval of the LCP  1100 . For example, the docking member  1106  may extend from the proximal end region  1104  of the housing  1102  along a longitudinal axis of the housing  1102 . One or more access ports  1108   a ,  1108   b ,  1108   c ,  1108   d  (collectively,  1108 ) may be formed through the docking member  1106  and/or through the proximal end region  1104  of the housing  1102 . It is contemplated that the access ports  1108  may allow endocardial pressure  1080  to engage the diaphragm inside of the housing, similar to that shown and described with respect to  FIG. 14 . 
       FIG. 16  is a side view of another illustrative implantable leadless cardiac pacemaker (LCP)  1150 . The LCP  1150  may be similar in form and function to the LCPs  100 ,  610 ,  900  described above. The LCP  1150  may include any of the modules and/or structural features described above with respect to the LCPs  100 ,  610 ,  900 . The LCP  1150  may include a shell or housing  1152  having a proximal end  1154  and a distal end  1156 . In the example shown, the LCP  1150  does not include a docking member. However, in some cases, a docking member may be provided at the proximal end  1154  of the LCP  1150 . The illustrative LCP  1150  includes a first electrode  1158  secured relative to the housing  1152  and positioned adjacent to the distal end  1156  of the housing  1152 , and a second electrode  1160  secured relative to the housing  1152  and positioned adjacent to the proximal end  1154  of the housing  1152 . In some instances, the first electrode  1158  may be positioned on a distal end surface facing distally. In some cases, the housing  1152  may include a conductive material and may be insulated along a portion of its length. A section along the proximal end  1154  may be free of insulation so as to define the second electrode. The electrodes  1158 ,  1160  may be sensing and/or pacing electrodes to provide electro-therapy and/or sensing capabilities. The first electrode  1158  may be capable of being positioned against or may otherwise contact the cardiac tissue of the heart while the second electrode may be spaced away from the first electrode  1158 . The first and/or second electrodes  1158 ,  1160  may be exposed to the environment outside the housing  1152  (e.g. to blood and/or tissue). 
     In some cases, the LCP  1150  may include a pulse generator (e.g., electrical circuitry) and a power source (e.g., a battery) within the housing  1152  to provide electrical signals to the electrodes  1158 ,  1160  to control the pacing/sensing electrodes  1158 ,  1160 . While not explicitly shown, the LCP  1150  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  1152 . 
     In the example shown, the LCP  1150  includes a fixation mechanism  1162  proximate the distal end  1156  of the housing  1152 . The fixation mechanism  1162  is configured to attach the LCP  1150  to a wall of the heart, or otherwise anchor the LCP  1150  to the anatomy of the patient. As shown in  FIG. 16 , in some instances, the fixation mechanism  1162  may include one or more, or a plurality of hooks or tines  1164  anchored into the cardiac tissue of the heart to attach the LCP  1150  to a tissue wall. In other instances, the fixation mechanism  1162  may include one or more, or a plurality of passive tines, configured to entangle with trabeculae within the chamber of the heart and/or a helical fixation anchor configured to be screwed into a tissue wall to anchor the LCP  1150  to the heart. These are just examples. 
     The housing  1152  may include a plurality of pressure sensitive regions or diaphragms  1166   a ,  1166   b  (collectively,  1166 ). While the example shown in  FIG. 16  illustrates two diaphragms  1166 , the LCP  1150  may include any number of diaphragms  1166  desired such as, but not limited to, one, two, three, four, or more. It is further contemplated that the diaphragms  1166  may be uniformly or eccentrically positioned about the circumference of the housing  1152 . In some cases, the diaphragms  1166  may be spaced equally about the entire circumference of the housing  1152 , while in other cases, the diaphragms  1166  may be positioned about a portion of the circumference of the housing  1152 . In some embodiments, the diaphragms  1166  may be formed from a flexible or compliant material, such as, but not limited to silicone, polyimides, etc. to form a deformable or movable diaphragm  1166  that is responsive to a pressure applied to the diaphragm  1166 . In other cases, the diaphragms  1166  may be formed from a thinned wall of the housing itself. In either case, this may allow the diaphragms  1166  to flex or deform as the pressure (external to the housing  1152 ) in the heart (e.g., left ventricle) changes, as will be described in more detail herein. Alternatively, the diaphragms  1166  may be a combination of flexible materials and/or regions of localized thinning, and/or any other suitable configuration. While the diaphragms  1166  are illustrated as positioned adjacent to the proximal end  1154 , it is contemplated that the diaphragms  1166  may be positioned anywhere along the length of the LCP  1150  as desired. 
     In some cases, locating the diaphragms  1166  on or adjacent to the proximal end  1154  of the housing  1152  may orientate the diaphragm towards the heart valves (when the LCP  1150  is positioned in the apex of the heart), which may achieve higher levels of sensitivity. Locating the diaphragm  1166  further away from the heart tissue may also reduce the likelihood of the diaphragm becoming fibrossed-over. However, the diaphragm  1166  may be coated with an anti-thrombogenic coating to help reduce such tissue growth. 
     One or more pressure sensors may be positioned adjacent to, but not necessarily in direct contact with, the diaphragms  1166 . In some embodiments, piezoresistors may be placed directly on the inner surface of the diaphragms  1166 . The pressure sensor may be operatively coupled to the circuitry or control electronics of the LCP  1050  through one or more electrical connections. The one or more electrical connections may be formed of a polyimide or similar interconnect having a cross-sectional dimension in the range of less than 250 microns. It is contemplated that the inside surface of the housing  1152  may be electrically insulated and the electrical connection (e.g., trace) positioned on the inside surface of the housing  1152  or along the outer surface of the battery, as desired. Alternatively, wires or a ribbon cable may be used. These are just examples. 
     The one or more pressure sensors may be positioned in or adjacent to one or more cavities filled with a fluid. The one or more fluid filled cavities may be in fluid communication with the diaphragms  1166  and the one or more pressure sensors such that the one or more fluid filled cavities may communicate a measure related to the pressure applied by the environment to the diaphragms  1166  of the housing  1152  to a pressure sensor diaphragm of a corresponding pressure sensor. In some instances, a fluid filled cavity may encompass the entire volume of the housing  1152  (e.g., the volume not filled with other components). In other embodiments, the one or more fluid filled cavities may be a portion of the volume of the housing between the flexible diaphragms  1166  and the pressure sensor. In some cases, each of the diaphragms  1166  has a corresponding pressure sensor, such that a separate pressure signal can be derived for each of the diaphragms  1166 . When so provided, pressures at different locations of the housing may be detected. In some cases, these different pressures can be used to detect a variety of different conditions including, for example, blood flow around the housing, pressure waves as they pass over the housing, relative direction of a detected heart sound using the relative locations of the diaphragms  1166  and the delay between detection times. These are just examples. 
       FIG. 17A  is a partial cross-sectional view of another illustrative implantable leadless cardiac pacemaker (LCP)  1200 . The LCP  1200  may be similar in form and function to the LCPs  100 ,  610 ,  900  described above. The LCP  1200  may include any of the modules and/or structural features described above with respect to the LCPs  100 ,  610 ,  900 . The LCP  1200  may include a shell or housing  1202  having a proximal end region  1204  and a distal end  1206 . The housing  1202  may include a docking member  1210  extending proximally from the proximal end region  1204 . The docking member  1210  may be configured to facilitate delivery and/or retrieval of the LCP  1200 . For example, the docking member  1210  may extend from the proximal end region  1204  of the housing  1202  along a longitudinal axis of the housing  1202 . 
     The illustrative LCP  1200  includes a first electrode  1208  secured relative to the housing  1202  and positioned adjacent to the distal end  1206  of the housing  1202 , and a second electrode (not explicitly shown) secured relative to the housing  1202  and positioned adjacent to the proximal end region  1204  of the housing  1202 . In some instances, the first electrode  1208  may be positioned on a distal end surface facing distally. In some cases, the housing  1202  may include a conductive material and may be insulated along a portion of its length. A section along the proximal end region  1204  may be free of insulation so as to define the second electrode. The electrodes  1208  may be sensing and/or pacing electrodes to provide electro-therapy and/or sensing capabilities. The first electrode  1208  may be capable of being positioned against or may otherwise contact the cardiac tissue of the heart while the second electrode may be spaced away from the first electrode  1208 . The first and/or second electrodes  1208  may be exposed to the environment outside the housing  1202  (e.g. to blood and/or tissue). 
     In some cases, the LCP  1200  may include a pulse generator  1218  (e.g., electrical circuitry) and a power source  1220  (e.g., a battery) within the housing  1202  to provide electrical signals to the electrodes  1208  to control the pacing/sensing electrodes  1208 . The LCP  1200  may also include a communications module  1222 , an electrical sensing module  1224 , a mechanical sensing module  1226 , and/or a processing module  1228 , 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  1202 . The modules  1218 ,  1222 ,  1224 ,  1226 ,  1228  may be positioned on a flexible board (e.g., a flexible polyimide)  1232 , which allows for electrical communication between the various modules. In some cases, the flexible board  1232  may be folded over to allow for different size components and/or more components to be present. In order to accommodate the flexible board  1232 , the battery  1220  may have a generally “D” shaped cross-section. This is better illustrated in  FIG. 17B  which illustrates a cross-section of the LCP  1200  taken at line B-B of  FIG. 17A . 
     In the example shown, the LCP  1200  includes a fixation mechanism  1212  proximate the distal end  1206  of the housing  1202 . The fixation mechanism  1212  is configured to attach the LCP  1200  to a wall of the heart H, or otherwise anchor the LCP  1200  to the anatomy of the patient. As shown in  FIG. 16 , in some instances, the fixation mechanism  1212  may include one or more, or a plurality of hooks or tines  1214  anchored into the cardiac tissue of the heart to attach the LCP  1200  to a tissue wall. In other instances, the fixation mechanism  1212  may include one or more, or a plurality of passive tines, configured to entangle with trabeculae within the chamber of the heart and/or a helical fixation anchor configured to be screwed into a tissue wall to anchor the LCP  1200  to the heart. These are just examples. 
     The housing  1202  may include one or more pressure sensitive regions or diaphragms (not explicitly shown). The diaphragm may be positioned in the proximal end surface  1234  of the housing  1202 , in the docking member  1210 , and/or in the side wall of the housing  1202  as desired. In some cases, all or a majority of the housing may function as a diaphragm. When so provided, the housing may have a wall thickness that will flex under endocardial pressures. In some cases, the entire housing may be filled with an incompressible and non-conductive fluid. 
     A pressure sensor  1230  may be positioned adjacent to, but not necessarily in direct contact with, the diaphragm(s). In some embodiments, a piezoresistors may be placed directly on the inner surface of the diaphragm(s). The pressure sensor  1230  may be operatively coupled to the circuitry or control electronics through one or more electrical connections (e.g., flexible board  1232 ). The pressure sensor  1230  may be positioned in or adjacent to a cavity  1236  filled with a fluid  1238 . The fluid filled cavity  1236  is in fluid communication with the diaphragm(s) and the pressure sensor  1230  such that the fluid filled cavity  1236  may communicate a measure related to the pressure applied by the environment to the diaphragm(s) of the housing  1202  to a pressure sensor diaphragm of the pressure sensor  1230 . As noted above, and in some instances, the fluid filled cavity  1236  may encompass the entire volume of the housing  1202  (e.g., the volume not filled with other components). In other embodiments, the fluid filled cavity  1236  may be a portion of the volume of the housing, and may extend between the flexible diaphragm and the pressure sensor  1230 . The fluid filled cavity  1236  may be filled with an incompressible fluid  1238 . In some cases, the fluid filled cavity  1236  may be filled with a non-conductive fluid  1238 . In some cases, the fluid  1238  may be highly soluble to gases that are likely to arise inside of the housing, particularly at body temperature (e.g. 37° C.). For example, the fluid  1238  may be highly soluble to hydrogen, helium, nitrogen, argon, water, and/or other gases or liquids that might arise inside of the housing as a result of, for example, outgassing of internal components of the LCP  1200 . 
     In some cases, the fluid  1238  (and/or the diaphragm material) may be selected to match the acoustic impedance of blood. This may facilitate the use of the pressure sensor  1230  as an acoustic pressure sensor. In some cases, the pressure sensor  1230  may be used to detect various sounds such as heart sounds, valve regurgitation, respiration, blood flow, blood turbulence and/or other suitable sounds. In some cases, sounds having a frequency of up to 200 Hz or more may be detected. 
     A pressure exterior to the housing  1202  may be communicated to the pressure sensor diaphragm of the pressure sensor  1230  through the fluid  1238 . For example, as the diaphragm of the housing  1202  deflects inward, the incompressible fluid  1238  may transfer or communicate the pressure exterior to the housing  1202  to the pressure sensor diaphragm of the pressure sensor  1230 . The pressure sensor  1230  may use the deflection of the pressure sensor diaphragm to determine an output signal related to the pressure exterior to the housing  1202 . The pressure sensor output signal may be communicated to the circuitry  1218 ,  1222 ,  1224 ,  1226 , and/or  1228  in the housing  1202  through one or more electrical connections. The circuitry is configured to determine a pressure exterior to the housing  1202  based on the pressure sensor output signal. 
       FIG. 18  is a partial cross-sectional a side view of another illustrative implantable leadless cardiac pacemaker (LCP)  1250 . The LCP  1250  may be similar in form and function to the LCPs  100 ,  610 ,  900  described above. The LCP  1250  may include any of the modules and/or structural features described above with respect to the LCPs  100 ,  610 ,  900 . The LCP  1250  may include a shell or housing  1252  having a proximal end region  1254  and a distal end  1256 . The housing  1252  may include a docking member  1260  extending proximally from the proximal end region  1254 . The docking member  1260  may be configured to facilitate delivery and/or retrieval of the LCP  1250 . For example, the docking member  1260  may extend from the proximal end region  1254  of the housing  1252  along a longitudinal axis of the housing  1252 . 
     The illustrative LCP  1250  includes a first electrode  1258  secured relative to the housing  1252  and positioned adjacent to the distal end  1256  of the housing  1252  and a second electrode (not explicitly shown) secured relative to the housing  1252  and positioned adjacent to the proximal end region  1254  of the housing  1252 . In some instances, the first electrode  1258  may be positioned on a distal end surface facing distally. In some cases, the housing  1252  may include a conductive material and may be insulated along a portion of its length. A section along the proximal end region  1254  may be free of insulation so as to define the second electrode. The electrodes  1258  may be sensing and/or pacing electrodes to provide electro-therapy and/or sensing capabilities. The first electrode  1258  may be capable of being positioned against or may otherwise contact the cardiac tissue of the heart while the second electrode may be spaced away from the first electrode  1258 . The first and/or second electrodes  1258  may be exposed to the environment outside the housing  1252  (e.g. to blood and/or tissue). 
     In the example shown, the LCP  1250  includes a fixation mechanism  1262  proximate the distal end  1256  of the housing  1252 . The fixation mechanism  1262  is configured to attach the LCP  1250  to a wall of the heart H, or otherwise anchor the LCP  1250  to the anatomy of the patient. As shown in  FIG. 18 , in some instances, the fixation mechanism  1262  may include one or more, or a plurality of hooks or tines  1264  anchored into the cardiac tissue of the heart to attach the LCP  1250  to a tissue wall. In other instances, the fixation mechanism  1262  may include one or more, or a plurality of passive tines, configured to entangle with trabeculae within the chamber of the heart and/or a helical fixation anchor configured to be screwed into a tissue wall to anchor the LCP  1250  to the heart. These are just examples. 
     In some cases, the LCP  1250  may include a pulse generator  1268  (e.g., electrical circuitry) and a power source  1270  (e.g., a battery) within the housing  1252  to provide electrical signals to the electrodes  1258  to control the pacing/sensing electrodes  1258 . The LCP  1250  may also include other modules  1272  including but not limited to 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  1252 . The modules  1268 ,  1272 , may be positioned on a flexible board (e.g., a flexible polyimide)  1274  which allows for electrical communication between the various modules. In some cases, the flexible board  1274  may be folded over to allow for different size components and/or more components to be present. In order to minimize the feed thru and wiring requirements, the modules  1268 ,  1272  may be positioned adjacent to the distal end  1256  of the LCP  1250 . 
     The housing  1252  may include one or more pressure sensitive regions or diaphragms (not explicitly shown). The diaphragm may be positioned in the proximal end surface of the housing  1252 , in the docking member  1260 , and/or in the side wall of the housing  1252  as desired. The diaphragm may be formed of any of the various materials and/or configurations described herein. 
     A pressure sensor  1276  may be positioned adjacent to, but not necessarily in direct contact with, the diaphragms. The pressure sensor  1276  may be operatively coupled to the circuitry or control electronics through one or more electrical connections (e.g., flexible board  1274 ). In some cases, the pressure sensor  1276  may be positioned in the distal end  1256  of the LCP  1250  in or adjacent to a cavity  1278  filled with a fluid  1280 . The fluid filled cavity  1278  is in fluid communication with the diaphragm and the pressure sensor  1276  such that the fluid filled cavity  1278  may communicate a measure related to the pressure applied by the environment to the diaphragm of the housing  1252  to a pressure sensor diaphragm of the pressure sensor  1276 . In some embodiments, the battery  1270  may include a port or lumen  1282  extending therethrough. This may allow the pressure sensor  1276  to be positioned in the distal end  1256  of the LCP  1250  while allowing the diaphragm(s) to be positioned adjacent to the proximal end region  1254 . A fluid passage  1282  provides a fluid pathway to allow the diaphragm of the housing  1252  to be in fluid communication with the pressure sensor diaphragm. The port  1282  may extend through the center of the battery  1270  as shown. Rather than providing the port  1282 , it is contemplated that the fluid filled cavity  1278  may encompass the entire volume of the housing  1252  (e.g., the volume not filled with other components). 
     A pressure exterior to the housing  1252  may be communicated to the pressure sensor diaphragm through the fluid  1280 . For example, as the diaphragm of the housing  1252  deflects inward, an incompressible fluid  1280  may transfer or communicate the pressure exterior to the housing  1252  to the pressure sensor diaphragm, sometimes via fluid in the port  1282 . The pressure sensor  1276  may use the deflection of the pressure sensor diaphragm to determine an output signal related to the pressure exterior to the housing  1252 . The pressure sensor output signal may be communicated to the circuitry  1268 ,  1272 , in the housing  1252  through one or more electrical connections. The circuitry is configured to determine a pressure exterior to the housing  1252  based on the pressure sensor output signal. 
     Those skilled in the art will recognize that the present disclosure may be manifested in a variety of forms other than the specific examples described and contemplated herein. For instance, as described herein, various examples include one or more modules described as performing various functions. However, other examples may include additional modules that split the described functions up over more modules than that described herein. Additionally, other examples may consolidate the described functions into fewer modules. Accordingly, departure in form and detail may be made without departing from the scope and spirit of the present disclosure as described in the appended claims.