Patent Publication Number: US-11020595-B2

Title: Systems and methods for treating cardiac arrhythmias

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of co-pending U.S. patent application Ser. No. 15/587,033, filed May 4, 2017, which is a continuation of co-pending U.S. patent application Ser. No. 15/012,443 filed Feb. 1, 2016, which claims the benefit of U.S. Provisional Patent Application Ser. No. 62/113,173 filed on Feb. 6, 2015, both of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally relates to systems, devices, and methods for treating cardiac arrhythmias, and more particularly, to systems, devices, and methods for detecting cardiac arrhythmias and coordinating therapy between multiple devices. 
     BACKGROUND 
     Pacing instruments can be used to treat patients suffering from various heart conditions that result in a reduced ability of the heart to deliver sufficient amounts of blood to a patient&#39;s body. These 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.) have been implanted in a patient&#39;s body. Such devices may monitor and provide electrical stimulation to the 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. 
     SUMMARY 
     The present disclosure generally relates to systems, devices, and methods for treating cardiac arrhythmias, and more particularly, to systems, devices, and methods for detecting cardiac arrhythmias and coordinating treatment of anti-tachycardia pacing (ATP) therapy and defibrillation shock therapy between a leadless cardiac pacemaker and another medical device. 
     In one embodiment, a subcutaneous implantable cardioverter defibrillator (SICD) for delivering a defibrillation shock to a heart of a patient comprises two or more electrodes, a charge storage device for storing a charge that can be delivered to shock the heart via two or more of the electrodes, and a controller operatively coupled to two or more of the electrodes and the charge storage device. The controller may be configured to monitor cardiac activity of the heart of the patient via cardiac signals received via two or more of the electrodes, detect an occurrence of a cardiac arrhythmia based on the cardiac activity, and determine a type of the detected cardiac arrhythmia from two or more types of cardiac arrhythmias. If the determined type of cardiac arrhythmia is one of a first set of cardiac arrhythmia types, the controller may send an instruction via two or more of the electrodes for reception by a Leadless Cardiac Pacemaker (LCP) to initiate the application of ATP therapy by the LCP. If the determined type of cardiac arrhythmia is not one of the first set cardiac arrhythmia types, the controller may not send the instruction. 
     Additionally, or alternatively, in the previous embodiment, the instruction may also include one or more ATP parameters that define one or more characteristics of the ATP therapy. 
     Additionally, or alternatively, in any of the previous embodiments, the one or more characteristics of the ATP therapy may comprise a method of ATP therapy. 
     Additionally, or alternatively, in any of the previous embodiments, the one or more characteristics of the ATP therapy may comprise a number of ATP bursts applied during the ATP therapy. 
     Additionally, or alternatively, in any of the previous embodiments, the controller may further be configured to initiate charging of the charge storage device if the determined type of the cardiac arrhythmia is one of a second set cardiac arrhythmia types, and wait to initiate charging of the charge storage device if the determined type of the cardiac arrhythmia is one of the first set of cardiac arrhythmia types. 
     Additionally, or alternatively, in any of the previous embodiments, if an instruction was sent to initiate the application of ATP therapy by the LCP, the controller may be further configured to determine if the application of the ATP therapy by the LCP was successful in terminating the cardiac arrhythmia, and if so, not initiate charging of the charge storage device if the determined type of the cardiac arrhythmia is one of the first set of cardiac arrhythmia types. 
     Additionally, or alternatively, in any of the previous embodiments, if an instruction was sent to initiate the application of ATP therapy by the LCP, the controller may be further configured to determine if the application of the ATP therapy by the LCP was successful in terminating the cardiac arrhythmia, and if so, terminate the charging of the charge storage device if the determined type of the cardiac arrhythmia is one of the second cardiac arrhythmia types. 
     Additionally, or alternatively, in any of the previous embodiments, the first set of type of cardiac arrhythmia types may include Monomorphic Ventricular Tachycardia (MVT). 
     Additionally, or alternatively, in any of the previous embodiments, the second set of cardiac arrhythmia types may include one or more of Polymorphic Ventricular Tachycardia (PVT), Supra Ventricular Tachycardia (SVT), and Ventricular Fibrillation (VF). 
     Additionally, or alternatively, in any of the previous embodiments, the second set of cardiac arrhythmia types may include Polymorphic Ventricular Tachycardia (PVT). 
     Additionally, or alternatively, in any of the previous embodiments, the second set of cardiac arrhythmia types may include Supra Ventricular Tachycardia (SVT). 
     Additionally, or alternatively, in any of the previous embodiments, the second set of cardiac arrhythmia types may include Monomorphic Ventricular Tachycardia (MVT). 
     Additionally, or alternatively, in any of the previous embodiments, the first set of cardiac arrhythmia types and the second set of cardiac arrhythmia types may share one or more common cardiac arrhythmia types, but this is not required. 
     Additionally, or alternatively, in any of the previous embodiments, the cardiac arrhythmia types in the first set of cardiac arrhythmia types may be user selectable. 
     Additionally, or alternatively, in any of the previous embodiments, the SICD may further comprise an energy storage module, and the cardiac arrhythmia types in the first set of cardiac arrhythmia types may depend at least partially on a charge level of the storage module. 
     Additionally, or alternatively, in any of the previous embodiments, to determine a type of the detected cardiac arrhythmia from two or more types of cardiac arrhythmias, the controller may be configured to compare the cardiac signals to one or more templates of cardiac signals. 
     In another embodiment, a subcutaneous implantable cardioverter defibrillator (SICD) for delivering a defibrillation shock to a heart of a patient may comprise two or more electrodes, a charge storage device for storing a charge that can be delivered to shock the heart via two or more of the electrodes, and a controller operatively coupled to two or more of the electrodes and the charge storage device. The controller may be configured to monitor cardiac activity of the heart of the patient via cardiac signals received via two or more of the electrodes, detect an occurrence of a cardiac arrhythmia based on the cardiac activity; and determine a type of the detected cardiac arrhythmia from two or more types of cardiac arrhythmias. If the determined type of cardiac arrhythmia is one of a first set of cardiac arrhythmia types, the controller may send an instruction via two or more of the electrodes for reception by a Leadless Cardiac Pacemaker (LCP) to initiate the application of ATP therapy by the LCP. If the determined type of cardiac arrhythmia is not one of the first set of cardiac arrhythmia types, the controller may not send the instruction. 
     Additionally, or alternatively, in the previous embodiment, the instruction may also include one or more ATP parameters that define one or more characteristics of the ATP therapy. 
     Additionally, or alternatively, in any of the previous embodiments, the one or more characteristics of the ATP therapy may comprise a method of ATP therapy. 
     Additionally, or alternatively, in any of the previous embodiments, the one or more characteristics of the ATP therapy may comprise a number of ATP bursts applied during the ATP therapy. 
     Additionally, or alternatively, in any of the previous embodiments, the controller may be further configured to initiate charging of the charge storage device if the determined type of the cardiac arrhythmia is one of a second set of cardiac arrhythmia types, and wait to initiate charging of the charge storage device if the determined type of the cardiac arrhythmia is not one of the second set of cardiac arrhythmia types. 
     Additionally, or alternatively, in any of the previous embodiments, if an instruction was sent to initiate the application of ATP therapy by the LCP, the controller may be further configured to determine if the application of the ATP therapy by the LCP was successful in terminating the cardiac arrhythmia, and if so, not initiate charging of the charge storage device if the determined type of the cardiac arrhythmia is one of the first set of cardiac arrhythmia types. 
     Additionally, or alternatively, in any of the previous embodiments, if an instruction was sent to initiate the application of ATP therapy by the LCP, the controller may be further configured to determine if the application of the ATP therapy by the LCP was successful in terminating the cardiac arrhythmia, and if so, terminate the charging of the charge storage device if the determined type of the cardiac arrhythmia is one of the second set of cardiac arrhythmia types. 
     Additionally, or alternatively, in any of the previous embodiments, the first set of cardiac arrhythmia types may include Monomorphic Ventricular Tachycardia (MVT). 
     Additionally, or alternatively, in any of the previous embodiments, the second set of cardiac arrhythmia types may include one or more of Polymorphic Ventricular Tachycardia (PVT), Supra Ventricular Tachycardia (SVT), and Ventricular Fibrillation (VF). 
     Additionally, or alternatively, in any of the previous embodiments, the second set of cardiac arrhythmia types may include Polymorphic Ventricular Tachycardia (PVT). 
     Additionally, or alternatively, in any of the previous embodiments, the second set of cardiac arrhythmia types may include Supra Ventricular Tachycardia (SVT). 
     Additionally, or alternatively, in any of the previous embodiments, the second set of cardiac arrhythmia types may include Monomorphic Ventricular Tachycardia (MVT). 
     Additionally, or alternatively, in any of the previous embodiments, the first set of cardiac arrhythmia types and the second set of cardiac arrhythmia types may share one or more common cardiac arrhythmia types, but this is not required. 
     In yet another embodiment, an implantable cardioverter defibrillator (ICD) for delivering a defibrillation shock to a heart of a patient may comprise a charge storage device for storing a charge that can be delivered to shock the heart and a controller operatively coupled to the charge storage device. The controller may be configured to monitor cardiac activity of the heart of the patient, detect an occurrence of a cardiac arrhythmia based on the cardiac activity, and determine a type of the detected cardiac arrhythmia from two or more types of cardiac arrhythmias. If the determined type of cardiac arrhythmia is one of a first set of cardiac arrhythmia types, the controller may be configured to send an instruction for reception by a Leadless Cardiac Pacemaker (LCP) to initiate the application of ATP therapy by the LCP. If the determined type of cardiac arrhythmia is not one of the first set of cardiac arrhythmia types, the controller may not send the instruction. 
     Additionally, or alternatively, in the previous embodiment, the instruction may include one or more ATP parameters that define one or more characteristics of the ATP therapy. 
     Additionally, or alternatively, in any of the previous embodiments, the one or more characteristics of the ATP therapy may comprise a method of ATP therapy. 
     Additionally, or alternatively, in any of the previous embodiments, the one or more characteristics of the ATP therapy may comprise a number of ATP bursts applied during the ATP therapy. 
     Additionally, or alternatively, in any of the previous embodiments, the controller may be further configured to initiate charging of the charge storage device if the determined type of the cardiac arrhythmia is one of a second set of cardiac arrhythmia types, and wait to initiate charging of the charge storage device if the determined type of the cardiac arrhythmia is not one of the second set of cardiac arrhythmia types. 
     Additionally, or alternatively, in any of the previous embodiments, if an instruction was sent to initiate the application of ATP therapy by the LCP, the controller may be further configured to determine if the application of the ATP therapy by the LCP was successful in terminating the cardiac arrhythmia, and if so, not initiate charging of the charge storage device if the determined type of the cardiac arrhythmia is the first set of cardiac arrhythmia types. 
     Additionally, or alternatively, in any of the previous embodiments, if an instruction was sent to initiate the application of ATP therapy by the LCP, the controller may be further configured to determine if the application of the ATP therapy by the LCP was successful in terminating the cardiac arrhythmia, and if so, terminate the charging of the charge storage device if the determined type of the cardiac arrhythmia is one of the second set of cardiac arrhythmia types. 
     In still another embodiment, a method implemented by an implantable cardioverter defibrillator (ICD) may comprise determining an occurrence of a cardiac arrhythmia and determining a type of the detected cardiac arrhythmia from two or more types of cardiac arrhythmias. The method may further comprise, if the determined type of cardiac arrhythmia is one of a first set of cardiac arrhythmia types, sending an instruction for reception by a Leadless Cardiac Pacemaker (LCP) to initiate the application of ATP therapy by the LCP. Additionally, the method may also comprise, if the determined type of cardiac arrhythmia is not one of the first set of cardiac arrhythmia types, not sending the instruction to the LCP. 
     Additionally, or alternatively, in the previous embodiment, the method may further comprise charging the charge storage device if the determined type of the cardiac arrhythmia is one of a second set of cardiac arrhythmia types, and waiting to charge the charge storage device if the determined type of the cardiac arrhythmia is not one of the second set of cardiac arrhythmia types. 
     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 embodiment of the present disclosure; 
         FIG. 2  is a schematic block diagram of another illustrative medical device that may be used in conjunction with the LCP of  FIG. 1 ; 
         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 a system including an LCP and another medical device, in accordance with another embodiment of the present disclosure; 
         FIG. 5  is a schematic diagram of a system including a leadless cardiac pacemaker (LCP) and another medical device, in accordance with yet another embodiment of the present disclosure; 
         FIG. 6  is a flow diagram of an illustrative method that may be implemented by a medical device or medical device system, such as the illustrative medical devices and medical device systems described with respect to  FIGS. 1-5 ; 
         FIG. 7  is a flow diagram of an illustrative method that may be implemented by a medical device or medical device system, such as the illustrative medical devices and medical device systems described with respect to  FIGS. 1-5 ; and 
         FIG. 8  is a flow diagram of an illustrative method that may be implemented by a medical device or medical device system, such as the illustrative medical devices and medical device systems described with respect to  FIGS. 1-5 . 
     
    
    
     While the disclosure is amenable to various modifications and alternative forms, specifics thereof have been shown by way of embodiment 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. 
     This disclosure describes systems, devices, and methods for detecting and treating cardiac arrhythmias, and more particularly, to systems, devices, and methods implementing different treatment protocols for different types of arrhythmias. One option for treating tachyarrhythmias, one type of cardiac arrhythmia, includes using anti-tachycardia therapy (ATP) techniques. Some embodiments of these techniques include delivering pacing pulses to the heart of the patient at a faster rate than the tachycardia in an effort to get the heart to track the ATP pulses, thereby terminating the physiologically induced tachycardia. If ATP therapy does not work, then other measures, such as delivering a defibrillation pulse to the heart may be employed to attempt to terminate the tachycardia. Tachycardias may be classified into a number of different types of tachycardias—including monomorphic ventricular tachycardia (MVT), polymorphic ventricular tachycardia (PVT), supraventricular tachycardia (SVT) and Ventricular Fibrillation (VF). ATP therapy may be more likely to be effective at terminating some of these tachycardias than other of these tachycardias. For example, ATP may be most effective at terminating MVT (perhaps 80-90% termination). Efficacy may decrease for PVT (e.g. e.g. 20-40% termination), and SVT and VF (10-20% termination). Moreover, applying ATP therapy to only certain well-suited tachyarrhythmias may help reduce acceleration of other less-suited tachyarrhythmias into the defibrillation/cardioversion zone. Accordingly, it may be beneficial to employ different therapy protocols for different types of tachycardias, as will be more fully detailed below. 
       FIG. 1  is a conceptual drawing of an exemplary leadless cardiac pacemaker (LCP) that may be implanted into a patient and may operate to sense physiological signals and parameters and deliver one or more types of electrical stimulation therapy to tissues of the patient. Example electrical stimulation therapy includes bradycardia pacing, rate responsive pacing therapy, cardiac resynchronization therapy (CRT), anti-tachycardia pacing (ATP) therapy and/or the like. As can be seen in  FIG. 1 , LCP  100  may be a compact device with all components housed within LCP  100  or directly on housing  120 . LCP  100  may include communication module  102 , pulse generator module  104 , electrical sensing module  106 , mechanical sensing module  108 , processing module  110 , energy storage module  112 , and electrodes  114 . 
     As depicted in  FIG. 1 , LCP  100  may include electrodes  114 , which can be secured relative to housing  120  and electrically exposed to tissue and/or blood surrounding LCP  100 . Electrodes  114  may generally conduct electrical signals to and from LCP  100  and the surrounding tissue and/or blood. Such electrical signals can include communication pulses, electrical stimulation pulses, and intrinsic cardiac electrical signals, to name a few. Intrinsic cardiac electrical signals may include electrical signals generated by the heart and may be represented by an electrocardiogram (ECG). 
     Electrodes  114  may include 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, electrodes  114  may be generally disposed on either end of LCP  100  and may be in electrical communication with one or more of modules  102 ,  104 ,  106 ,  108 , and  110 . In embodiments where electrodes  114  are secured directly to housing  120 , an insulative material may electrically isolate the electrodes  114  from adjacent electrodes, housing  120 , and/or other parts of LCP  100 . In some instances, some or all of electrodes  114  may be spaced from housing  120  and connected to housing  120  and/or other components of LCP  100  through connecting wires. In such instances, the electrodes  114  may be placed on a tail (not shown) that extends out away from the housing  120 . As shown in  FIG. 1 , in some embodiments, LCP  100  may include electrodes  114 ′. Electrodes  114 ′ may be in addition to electrodes  114 , or may replace one or more of electrodes  114 . Electrodes  114 ′ may be similar to electrodes  114  except that electrodes  114 ′ are disposed on the sides of LCP  100 . In some cases, electrodes  114 ′ may increase the number of electrodes by which LCP  100  may deliver communication pulses and/or electrical stimulation pulses, and/or may sense intrinsic cardiac electrical signals, communication pulses, and/or electrical stimulation pulses. 
     Electrodes  114  and/or  114 ′ may assume any of a variety of sizes and/or shapes, and may be spaced at any of a variety of spacings. For example, electrodes  114  may have an outer diameter of two to twenty millimeters (mm). In other embodiments, electrodes  114  and/or  114 ′ may have a diameter of two, three, five, seven millimeters (mm), or any other suitable diameter, dimension and/or shape. Example lengths for electrodes  114  and/or  114 ′ may include, for example, one, three, five, ten millimeters (mm), or any other suitable length. As used herein, the length is a dimension of electrodes  114  and/or  114 ′ that extends away from the outer surface of the housing  120 . In some instances, at least some of electrodes  114  and/or  114 ′ may be spaced from one another by a distance of twenty, thirty, forty, fifty millimeters (mm), or any other suitable spacing. The electrodes  114  and/or  114 ′ of a single device may have different sizes with respect to each other, and the spacing and/or lengths of the electrodes on the device may or may not be uniform. 
     In the embodiment shown, communication module  102  may be electrically coupled to electrodes  114  and/or  114 ′ and may be configured to deliver communication pulses to tissues of the patient for communicating with other devices such as sensors, programmers, other medical devices, and/or the like. Communication pulses, as used herein, may be any modulated signal that conveys information to another device, either by itself or in conjunction with one or more other modulated signals. In some embodiments, communication pulses may be limited to sub-threshold signals that do not result in capture of the heart yet still convey information. The communication pulses may be delivered to another device that is located either external or internal to the patient&#39;s body. Communication module  102  may additionally be configured to sense for communication pulses delivered by other devices, which may be located external or internal to the patient&#39;s body. 
     Communication module  102  may communicate to help accomplish one or more desired functions. Some example functions include delivering sensed data, using communicated data for determining occurrences of events such as arrhythmias, coordinating delivery of electrical stimulation therapy, and/or other functions. In some cases, LCP  100  may use communication pulses to communicate raw information, processed information, messages and/or commands, and/or other data. Raw information may include information such as sensed electrical signals (e.g. a sensed ECG), signals gathered from coupled sensors, and the like. In some embodiments, the processed information may include signals that have been filtered using one or more signal processing techniques. Processed information may also include parameters and/or events that are determined by the LCP  100  and/or another device, such as a determined heart rate, timing of determined heartbeats, timing of other determined events, determinations of threshold crossings, expirations of monitored time periods, activity level parameters, blood-oxygen parameters, blood pressure parameters, heart sound parameters, and the like. Messages and/or commands may include instructions or the like directing another device to take action, notifications of imminent actions of the sending device, requests for reading from the receiving device, requests for writing data to the receiving device, information messages, and/or other messages commands. 
     In at least some embodiments, communication module  102  (or LCP  100 ) may further include switching circuitry to selectively connect one or more of electrodes  114  and/or  114 ′ to communication module  102  in order to select which electrodes  114  and/or  114 ′ that communication module  102  delivers communication pulses. It is contemplated that communication module  102  may communicating with other devices via conducted signals, radio frequency (RF) signals, optical signals, acoustic signals, inductive coupling, and/or any other suitable communication methodology. 
     In the embodiment shown, a pulse generator module  104  may be electrically connected to one or more of electrodes  114  and/or  114 ′. Pulse generator module  104  may be configured to generate electrical stimulation pulses and deliver the electrical stimulation pulses to tissues of a patient via one or more of the electrodes  114  and/or  114 ′ in order to effectuate one or more electrical stimulation therapies. Electrical stimulation pulses as used herein are meant to encompass any electrical signals that may be delivered to tissue of a patient for purposes of treatment of any type of disease or abnormality. For example, when used to treat heart disease, the pulse generator module  104  may generate electrical stimulation pacing pulses for capturing the heart of the patient, i.e. causing the heart to contract in response to the delivered electrical stimulation pulse. In another embodiment, the electrical stimulation pulses may be defibrillation/cardioversion pulses for shocking the heart out of fibrillation. In yet another embodiment, the electrical stimulation pulses may be anti-tachycardia pacing (ATP) pulses. These are just some examples. When used to treat other ailments, the pulse generator module  104  may generate electrical stimulation pulses suitable for neurostimulation therapy or the like. Pulse generator module  104  may include one or more capacitor elements and/or other charge storage devices to aid in generating and delivering appropriate electrical stimulation pulses. In the embodiment shown, pulse generator module  104  may use energy stored in energy storage module  112  to generate the electrical stimulation pulses. 
     Pulse generator module  104  may include the capability to modify the electrical stimulation pulses, such as by adjusting the pulse width and/or amplitude of the electrical stimulation pulses. When pacing the heart, this may help tailor the electrical stimulation pulses to capture the heart a particular patient, sometimes with reduced battery usage. For neurostimulation therapy, adjusting the pulse width and/or amplitude may help tailor the therapy for a particular application and/or help make the therapy more effective for a particular patient. 
     In some embodiments, LCP  100  may include an electrical sensing module  106  and mechanical sensing module  108 . Electrical sensing module  106  may be configured to sense intrinsic cardiac electrical signals conducted from electrodes  114  and/or  114 ′ to electrical sensing module  106 . For example, electrical sensing module  106  may be electrically connected to one or more electrodes  114  and/or  114 ′ and electrical sensing module  106  may be configured to receive cardiac electrical signals conducted through electrodes  114  and/or  114 ′. In some embodiments, the cardiac electrical signals may represent local information from the chamber in which LCP  100  is implanted. For instance, if LCP  100  is implanted within a ventricle of the heart, cardiac electrical signals sensed by LCP  100  through electrodes  114  and/or  114 ′ may represent ventricular cardiac electrical signals. Mechanical sensing module  108  may include, or be electrically connected to, various sensors, such as accelerometers, blood pressure sensors, heart sound sensors, blood-oxygen sensors, and/or other sensors which measure one or more physiological parameters of the heart and/or patient. Mechanical sensing module  108 , when present, may gather signals from the sensors indicative of the various physiological parameters. Both electrical sensing module  106  and mechanical sensing module  108  may be connected to processing module  110  and may provide signals representative of the sensed cardiac electrical signals and/or physiological signals to processing module  110 . Although described with respect to  FIG. 1  as separate sensing modules, in some embodiments, electrical sensing module  106  and mechanical sensing module  108  may be combined into a single module. 
     Processing module  110  may be configured to control the operation of LCP  100 . For example, processing module  110  may be configured to receive cardiac electrical signals from electrical sensing module  106  and/or physiological signals from mechanical sensing module  108 . Based on the received signals, processing module  110  may determine, for example, occurrences and types of arrhythmias. Processing module  110  may further receive information from communication module  102 . In some embodiments, processing module  110  may additionally use such received information to determine occurrences and types of arrhythmias. However, in other embodiments, LCP  100  may use the received information instead of the signals received from electrical sensing module  106  and/or mechanical sensing module  108 —for instance if the received information is more accurate than the signals received from electrical sensing module  106  and/or mechanical sensing module  108  or if electrical sensing module  106  and/or mechanical sensing module  108  have been disabled or omitted from LCP  100 . 
     Based on a determined arrhythmia, processing module  110  may control pulse generator module  104  to generate electrical stimulation pulses in accordance with one or more electrical stimulation therapies to treat the determined arrhythmia. For example, processing module  110  may control pulse generator module  104  to generate pacing pulses with varying parameters and in different sequences to effectuate one or more electrical stimulation therapies. For example, in controlling pulse generator module  104  to deliver bradycardia pacing therapy, processing module  110  may control pulse generator module  104  to deliver pacing pulses designed to capture the heart of the patient at a regular interval to help prevent the heart of a patient from falling below a predetermined threshold. In some cases, the rate of pacing may be increased with an increased activity level of the patient (e.g. rate adaptive pacing). For ATP therapy, processing module  110  may control pulse generator module  104  to deliver pacing pulses at a rate faster than an intrinsic heart rate of a patient in attempt to force the heart to beat in response to the delivered pacing pulses rather than in response to intrinsic cardiac electrical signals. Once the heart is following the pacing pulses, processing module  110  may control pulse generator module  104  to reduce the rate of delivered pacing pulses down to a safer level. In CRT, processing module  110  may control pulse generator module  104  to deliver pacing pulses in coordination with another device to cause the heart to contract more efficiently. In cases where pulse generator module  104  is capable of generating defibrillation and/or cardioversion pulses for defibrillation/cardioversion therapy, processing module  110  may control pulse generator module  104  to generate such defibrillation and/or cardioversion pulses. In some cases, processing module  110  may control pulse generator module  104  to generate electrical stimulation pulses to provide electrical stimulation therapies different than those examples described above. 
     Aside from controlling pulse generator module  104  to generate different types of electrical stimulation pulses and in different sequences, in some embodiments, processing module  110  may also control pulse generator module  104  to generate the various electrical stimulation pulses with varying pulse parameters. For example, each electrical stimulation pulse may have a pulse width and a pulse amplitude. Processing module  110  may control pulse generator module  104  to generate the various electrical stimulation pulses with specific pulse widths and pulse amplitudes. For example, processing module  110  may cause pulse generator module  104  to adjust the pulse width and/or the pulse amplitude of electrical stimulation pulses if the electrical stimulation pulses are not effectively capturing the heart. Such control of the specific parameters of the various electrical stimulation pulses may help LCP  100  provide more effective delivery of electrical stimulation therapy. 
     In some embodiments, processing module  110  may further control communication module  102  to send information to other devices. For example, processing module  110  may control communication module  102  to generate one or more communication pulses for communicating with other devices of a system of devices. For instance, processing module  110  may control communication module  102  to generate communication pulses in particular sequences, where the specific sequences convey different information. Communication module  102  may also receive communication signals for potential action by processing module  110 . 
     In further embodiments, processing module  110  may control switching circuitry by which communication module  102  and pulse generator module  104  deliver communication pulses and/or electrical stimulation pulses to tissue of the patient. As described above, both communication module  102  and pulse generator module  104  may include circuitry for connecting one or more electrodes  114  and/ 114 ′ to communication module  102  and/or pulse generator module  104  so those modules may deliver the communication pulses and electrical stimulation pulses to tissue of the patient. The specific combination of one or more electrodes by which communication module  102  and/or pulse generator module  104  deliver communication pulses and electrical stimulation pulses may influence the reception of communication pulses and/or the effectiveness of electrical stimulation pulses. Although it was described that each of communication module  102  and pulse generator module  104  may include switching circuitry, in some embodiments, LCP  100  may have a single switching module connected to the communication module  102 , the pulse generator module  104 , and electrodes  114  and/or  114 ′. In such embodiments, processing module  110  may control the switching module to connect modules  102 / 104  and electrodes  114 / 114 ′ as appropriate. 
     In some embodiments, processing module  110  may include a pre-programmed chip, such as a very-large-scale integration (VLSI) chip 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 LCP  100 . By using a pre-programmed chip, processing module  110  may use less power than other programmable circuits while able to maintain basic functionality, thereby potentially increasing the battery life of LCP  100 . In other instances, processing module  110  may include a programmable microprocessor or the like. Such a programmable microprocessor may allow a user to adjust the control logic of LCP  100  after manufacture, thereby allowing for greater flexibility of LCP  100  than when using a pre-programmed chip. 
     Processing module  110 , in additional embodiments, may include a memory circuit and processing module  110  may store information on and read information from the memory circuit. In other embodiments, LCP  100  may include a separate memory circuit (not shown) that is in communication with processing module  110 , such that processing module  110  may read and write information to and from the separate memory circuit. The memory circuit, whether part of processing module  110  or separate from processing module  110 , may be volatile memory, non-volatile memory, or a combination of volatile memory and non-volatile memory. 
     Energy storage module  112  may provide a power source to LCP  100  for its operations. In some embodiments, energy storage module  112  may be a non-rechargeable lithium-based battery. In other embodiments, the non-rechargeable battery may be made from other suitable materials. In some embodiments, energy storage module  112  may include a rechargeable battery. In still other embodiments, energy storage module  112  may include other types of energy storage devices such as super capacitors. 
     To implant LCP  100  inside a patient&#39;s body, an operator (e.g., a physician, clinician, etc.), may fix LCP  100  to the cardiac tissue of the patient&#39;s heart. To facilitate fixation, LCP  100  may include one or more anchors  116 . Anchor  116  may include any number of fixation or anchoring mechanisms. For example, anchor  116  may include one or more pins, staples, threads, screws, helix, tines, and/or the like. In some embodiments, although not shown, anchor  116  may include threads on its external surface that may run along at least a partial length of anchor  116 . The threads may provide friction between the cardiac tissue and the anchor to help fix anchor  116  within the cardiac tissue. In other embodiments, 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 embodiment of another medical device (MD)  200 , which may operate to sense physiological signals and/or parameters and deliver one or more types of electrical stimulation therapy to tissues of the patient. In the embodiment shown, 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 an energy storage module  218 . Each of modules  202 ,  204 ,  206 ,  208 , and  210  may be similar to modules  102 ,  104 ,  106 ,  108 , and  110  of LCP  100 . Additionally, energy storage module  218  may be similar to energy storage module  112  of LCP  100 . In some embodiments, however, MD  200  may have a larger volume within housing  220 . In such embodiments, MD  200  may include a larger energy storage module  218  and/or a larger processing module  210  capable of handling more complex operations than processing module  110  of LCP  100 . 
     While MD  200  may be another leadless device such as shown in  FIG. 1 , in some instances MD  200  may include leads, such as leads  212 . Leads  212  may include electrical wires that conduct electrical signals between electrodes  214  and one or more modules located within housing  220 . In some cases, leads  212  may be connected to and extend away from housing  220  of MD  200 . In some embodiments, leads  212  are implanted on, within, or adjacent to a heart of a patient. Leads  212  may contain one or more electrodes  214  positioned at various locations on leads  212  and various distances from housing  220 . Some leads  212  may only include a single electrode  214 , while other leads  212  may include multiple electrodes  214 . Generally, electrodes  214  are positioned on leads  212  such that when 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 other cases, the one or more of the electrodes  214  may be positioned subcutaneously but adjacent the patient&#39;s heart. The electrodes  214  may conduct intrinsically generated electrical cardiac signals to leads  212 . Leads  212  may, in turn, conduct the received electrical cardiac signals to one or more of the modules  202 ,  204 ,  206 , and  208  of MD  200 . In some cases, MD  200  may generate electrical stimulation signals, and leads  212  may conduct the generated electrical stimulation signals to electrodes  214 . Electrodes  214  may then conduct the electrical stimulation signals to the cardiac tissue of the patient (either directly or indirectly). MD  200  may also include one or more electrodes  214  not disposed on a lead  212 . For example, one or more electrodes  214  may be connected directly to housing  220 . 
     Leads  212 , in some embodiments, may additionally contain one or more sensors, such as accelerometers, blood pressure sensors, heart sound sensors, blood-oxygen sensors, and/or other sensors which are configured to measure one or more physiological parameters of the heart and/or patient. In such embodiments, mechanical sensing module  208  may be in electrical communication with leads  212  and may receive signals generated from such sensors. 
     While not required, in some embodiments MD  200  may be an implantable medical device. In such embodiments, housing  220  of MD  200  may be implanted in, for example, a transthoracic region of the patient. 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 MD  200  from fluids and tissues of the patient&#39;s body. In such embodiments, leads  212  may be implanted at one or more various locations within the patient, such as within the heart of the patient, adjacent to the heart of the patient, adjacent to the spine of the patient, or any other desired location. 
     In some embodiments, MD  200  may be an implantable cardiac pacemaker (ICP). In these embodiments, 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. 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. MD  200  may be configured to deliver CRT, ATP therapy, bradycardia therapy, and/or other therapy types via leads  212  implanted within the heart. In some embodiments, MD  200  may additionally be configured to provide defibrillation/cardioversion therapy. 
     In some instances, MD  200  may be an implantable cardioverter-defibrillator (ICD). In such embodiments, MD  200  may include one or more leads implanted within a patient&#39;s heart. MD  200  may also be configured to sense electrical cardiac signals, determine occurrences of tachyarrhythmias based on the sensed electrical cardiac signals, and deliver defibrillation and/or cardioversion therapy in response to determining an occurrence of a tachyarrhythmia (for example by delivering defibrillation and/or cardioversion pulses to the heart of the patient). In other embodiments, MD  200  may be a subcutaneous implantable cardioverter-defibrillator (SICD). In embodiments where MD  200  is an SICD, one of leads  212  may be a subcutaneously implanted lead. In at least some embodiments where MD  200  is an SICD, MD  200  may include only a single lead which is implanted subcutaneously but outside of the chest cavity, however this is not required. 
     In some embodiments, MD  200  may not be an implantable medical device. Rather, MD  200  may be a device external to the patient&#39;s body, and electrodes  214  may be skin-electrodes that are placed on a patient&#39;s body. In such embodiments, MD  200  may be able to sense surface electrical signals (e.g. electrical cardiac 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 embodiments, MD  200  may be configured to deliver various types of electrical stimulation therapy, including, for example, defibrillation therapy. 
       FIG. 3  illustrates an embodiment of a medical device system and a communication pathway through which multiple medical devices  302 ,  304 ,  306 , and/or  310  of the medical device system may communicate. In the embodiment shown, medical device system  300  may include LCPs  302  and  304 , external medical device  306 , and other sensors/devices  310 . External device  306  may be a device disposed external to a patient&#39;s body, as described previously with respect to MD  200 . Other sensors/devices  310  may be any of the devices described previously with respect to MD  200 , such as ICPs, ICDs, and SICDs. Other sensors/devices  310  may also include various diagnostic sensors that gather information about the patient, such as accelerometers, blood pressure sensors, or the like. In some cases, other sensors/devices  310  may include an external programmer device that may be used to program one or more devices of system  300 . 
     Various devices of system  300  may communicate via communication pathway  308 . For example, 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 system  300  via communication pathway  308 . In one embodiment, one or more of devices  302 / 304  may receive such signals and, based on the received signals, determine an occurrence of an arrhythmia. In some cases, device or devices  302 / 304  may communicate such determinations to one or more other devices  306  and  310  of system  300 . In some cases, one or more of devices  302 / 304 ,  306 , and  310  of 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. One or more of devices  302 / 304 ,  306 , and  310  of system  300  may additionally communicate command or response messages via communication pathway. The command messages may cause a receiving device to take a particular action whereas response messages may include requested information or a confirmation that a receiving device did, in fact, receive a communicated message or data. 
     It is contemplated that the various devices of system  300  may communicate via pathway  308  using conducted signals, RF signals, inductive coupling, optical signals, acoustic signals, or any other signals suitable for communication. In some instances, the various devices of system  300  may communicate via pathway  308  using different signal types. For instance, other sensors/device  310  may communicate with external device  306  using a first signal type (e.g. RF communication) but communicate with LCPs  302 / 304  using a second signal type (e.g. conducted communication). Further, in some embodiments, communication between devices may be limited. For instance, as described above, in some embodiments, LCPs  302 / 304  may communicate with external device  306  only through other sensors/devices  310 , where LCPs  302 / 304  send signals to other sensors/devices  310 , and other sensors/devices  310  relay the received signals to external device  306 . This is just one example. 
     In some cases, the various devices of system  300  may communicate via pathway  308  using conducted communication signals. Accordingly, devices of system  300  may have components that allow for such conducted communication. For instance, the devices of system  300  may be configured to transmit conducted communication signals (e.g. current and/or voltage pulses) into the patient&#39;s body via one or more electrodes of a transmitting device, and may receive the conducted communication signals (e.g. pulses) via one or more electrodes of a receiving device. The patient&#39;s body may “conduct” the conducted communication signals (e.g. pulses) from the one or more electrodes of the transmitting device to the electrodes of the receiving device in the system  300 . In such embodiments, the delivered conducted communication signals (e.g. pulses) may differ from pacing pulses, defibrillation and/or cardioversion pulses, or other electrical stimulation therapy signals. For example, the devices of system  300  may deliver electrical communication pulses at an amplitude/pulse width that is sub-threshold (e.g. does not capture the heart, phrenic nerve, and/or other tissue). Although, in some cases, the amplitude/pulse width of the delivered electrical communication pulses may be above a capture threshold, but may be delivered during an irrelevant time period. For example, the amplitude/pulse width of the delivered electrical communication pulses may be above a 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, as 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 and/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. 
       FIGS. 4 and 5  show illustrative medical device systems that may be configured to operate according to techniques disclosed herein. For example, the systems may include multiple devices that are implanted within a patient and are configured to sense physiological signals, determine occurrences of cardiac arrhythmias, and deliver electrical stimulation to treat detected cardiac arrhythmias. In  FIG. 4 , an LCP  402  is shown fixed to the interior of the right ventricle of the heart  410 , and a pulse generator  406  is shown coupled to a lead  412  having one or more electrodes  408   a - 408   c . In some cases, the pulse generator  406  may be part of a subcutaneous implantable cardioverter-defibrillator (SICD), and the one or more electrodes  408   a - 408   c  may be positioned subcutaneously adjacent the heart. LCP  402  may communicate with the SICD, such as via communication pathway  308 . The locations of LCP  402 , pulse generator  406 , lead  412 , and electrodes  408   a - c  depicted in  FIG. 4  are just exemplary. In other embodiments of system  400 , LCP  402  may be positioned in the left ventricle, right atrium, or left atrium of the heart, as desired. In still other embodiments, LCP  402  may be implanted externally adjacent to heart  410  or even remote from heart  410 . 
     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   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   c  may be positioned in the heart  510 . In some cases, LCP  502  may communicate with the implantable cardiac pacemaker (ICP) and/or an implantable cardioverter-defibrillator (ICD), such as via communication pathway  308 . As with  FIG. 4 , the locations of LCP  502 , pulse generator  506 , lead  512 , and electrodes  504   a - c  depicted in  FIG. 5  are just exemplary. In other embodiments of system  500 , LCP  502  may be positioned in the right ventricle, right atrium, or left atrium of the heart, as desired. In still other embodiments, LCP  502  may be implanted externally adjacent to heart  510  or even remote from heart  510 . Additionally, in some embodiments lead  512  and/or electrodes  504   a - c  may be disposed in different chambers of heart  510 , or pulse generator may include additional leads and/or electrodes that are disposed within or adjacent to heart  510 . 
     The medical device systems  400  and  500  may also include an external support device, such as external support devices  420  and  520 . 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 embodiment, communication between external support device  420  and the pulse generator  406  is performed via a wireless mode, and communication between the pulse generator  406  and LCP  402  is performed via a conducted mode. In some embodiments, communication between the LCP  402  and external support device  420  is accomplished by sending communication information through the pulse generator  406 . However, in other embodiments, communication between the LCP  402  and external support device  420  may be via a communication module. 
       FIGS. 4-5  only illustrate a few embodiments 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 with or without other devices such as pulse generator  406  or  506 , with at least one LCP capable of delivering defibrillation therapy. Still another embodiment may include one or more LCPs implanted along with a transvenous pacemaker and with or without an implanted SICD. In yet other embodiments, 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 illustrated in  FIGS. 4 and 5 , may be operated in accordance with techniques disclosed herein. The embodiments systems shown in  FIGS. 4 and 5  should not be viewed as limiting. 
     Using the system of  FIG. 4  as one example, LCP  402  and the ICD (which can be a non-subcutaneously implanted device, or a subcutaneously implanted device—an SICD), which can include pulse generator  406 , may determine occurrences of cardiac arrhythmias and discriminate between different types of cardiac arrhythmias. In some embodiments, the types of cardiac arrhythmias include tachyarrhythmias, and the ICD may further identify occurrences of tachyarrhythmias as specific types of tachyarrhythmias. As used herein, the term tachyarrhythmia may include ventricular fibrillation (VF). Based on the determined type of tachyarrhythmia, LCP  402  and/or the ICD may implement differing treatment protocols. By tailoring the specific treatment protocol to the different types of tachyarrhythmias, the system of LCP  402  and the ICD may more effectively conserve battery life and/or reduce the amount of unnecessary defibrillation and/or cardioversion pulses delivered to the patient—which can be a painful and scary experience for the patient. 
     In some embodiments, the ICD may operate to determine occurrences of tachyarrhythmias and types of tachyarrhythmias in accordance with the illustrative flow chart shown in  FIG. 6 . To determine an occurrence of a tachyarrhythmia, the ICD may first compare a detected heart rate to a first heart rate threshold, as at  601 . For example, the ICD may use received cardiac electrical signals, in some embodiments in conjunction with other received physiological signals, to determine a heart rate. If the ICD determines that the heart rate is greater than or equal to the first heart rate threshold, the ICD may compare the morphology of the cardiac electrical signal of a current heart beat with the morphology of a template of a normal heart beat, as at  603 . For example, the ICD may isolate a region of cardiac electrical activity surrounding a QRS complex of a current heart beat and perform a correlation analysis between the morphology of the isolated QRS complex (or part thereof) and the template containing a QRS complex (or part thereof) of a normal heart beat. If the correlation between the current beat and the template beat is greater than or equal to a first correlation threshold, the ICD may determine that there is no tachyarrhythmia present, as at  605 . In these cases, the heart rate may be elevated due to exercise or stress, or some other factor, rather than due to an abnormal physiological process. 
     If, however, the correlation between the current heart beat and the template beat is not greater than or equal to the first correlation threshold, the ICD may then compare the morphology of the current heart beat with the morphology of one or more previous heart beats, as at  607 . For example, the ICD may isolate the QRS complexes of the current heart beat and the previous heart beat and perform a correlation analysis between the morphologies of the two beats. If the correlation between the two beats is less than a second correlation threshold, the ICD may determine that the tachyarrhythmia is a polymorphic ventricular tachyarrhythmia (PVT), as at  609 . However, if the correlation between the two beats is greater than or equal to the second correlation threshold, the ICD may further compare the width of the QRS complex of the current beat with the width of the QRS complex of the template normal beat, as at  611 . For example, the ICD may compare the difference in QRS widths between the current beat and the template beat to a QRS width threshold. If the difference in QRS widths is greater than or equal to the QRS width threshold (indicating that the width of the QRS complex of the current beat is wider than the width of the QRS complex of the template beat by a threshold amount), the ICD may determine that the tachyarrhythmia is a monomorphic ventricular tachyarrhythmia (MVT), as at  613 . However, if the ICD determines that the difference in QRS widths is less than the QRS width threshold (indicating that the width of the QRS complex of the current beat is narrower than the width of the QRS complex of the template beat), the ICD may determine that the tachyarrhythmia is a supraventricular tachyarrhythmia (SVT), as at  615 . 
     It should be noted that the flow chart of  FIG. 6  is only one embodiment in which the ICD may determine occurrences and/or discriminate types of tachyarrhythmias. For instance, in some additional embodiments, the ICD may further compare the determine heart rate to a second heart rate threshold, where the second heart rate threshold is greater than the first heart rate threshold. If the heart rate is greater than or equal to the second heart rate threshold, the ICD may determine that the heart is in ventricular fibrillation (VF). Additionally, or alternatively, in some embodiments, the ICD may additionally use the width of the current QRS complex and the width of the QRS complex of the template beat in performing the correlation analysis between the current beat and the template beat in step  607 . Of course, in still other embodiments, the ICD may use different methods for determining occurrences of tachyarrhythmias and discriminating between various types of tachyarrhythmias, which may include fewer or greater numbers of steps than those listed in  FIG. 6 . 
     Where the ICD discriminates between different types of tachyarrhythmias, the ICD may coordinate with LCP  402  to implement differing treatment protocols. For example, the ICD may have stored in memory a first set of tachyarrhythmia types. If the determined tachyarrhythmia is one of the types of tachyarrhythmias in the first set of tachyarrhythmia types, the ICD may communicate an instruction to LCP  402  to initiate application of ATP therapy. In some cases, the ICD may also wait to initiate charging of its charge storage device for delivery of defibrillation and/or cardioversion therapy. The ICD may then monitor the cardiac electrical signals to determine if the ATP therapy delivered by LCP  402  terminates the tachyarrhythmia. If the ICD determines that the tachyarrhythmia was not terminated by the ATP, the ICD may initiate charging of its charge storage device and deliver defibrillation and/or cardioversion therapy once the charge is complete. In determining whether the delivery of ATP therapy has terminated the tachyarrhythmia, the ICD may determine the current heart rate and compare it to a threshold or compare it to the heart rate at the time the tachyarrhythmia was detected. If the heart rate is less than the comparison heart rate, the ICD may determine that the tachyarrhythmia was terminated. In other embodiments, the ICD may perform the process detailed in  FIG. 6  to determine if the ATP therapy terminated the tachyarrhythmia. 
     In embodiments where the types of tachyarrhythmias in the first set of tachyarrhythmia types are types that are likely to be susceptible to ATP therapy, the ICD may conserve energy by waiting to charge its charge storage device until after confirmation that the ATP therapy failed to terminate the tachyarrhythmia. In some embodiments, the types of tachyarrhythmia in the first set of tachyarrhythmia types may include MVT. However, in other embodiments, PVT may also be included in the first set of tachyarrhythmia types. In still other embodiments, SVT and/or VF may also be included in the first set of tachyarrhythmia types. These are just examples. 
     In embodiments where the type of tachyarrhythmia is not one of the tachyarrhythmias in the first set of tachyarrhythmia types, the ICD may not send the instruction to LCP  402  to initiate ATP therapy. Instead, the ICD may initiate charging of the charge storage device and deliver defibrillation and/or cardioversion therapy once the charging is complete. In some embodiments, VF may be excluded from the first set of tachyarrhythmia types. In some embodiments, PVT and/or SVT may be excluded from the first set of tachyarrhythmia types. 
     In some instances, the ICD may alter the treatment protocol based on the determined type of tachyarrhythmia. For instance, if the ICD determines that the tachyarrhythmia is one of the tachyarrhythmia types in the first set of tachyarrhythmia types, the ICD may send the instruction to LCP  402  to initiate ATP therapy but also begin charging its charge storage device for delivery of defibrillation and/or cardioversion therapy. This may best be applied when the determined type of tachyarrhythmia has some non-trivial chance that ATP therapy would terminate the tachyarrhythmia, but is still un-likely to be successful. The ICD may monitor the cardiac electrical activity while charging the charge storage device and while LCP  402  is delivering ATP therapy. If delivery of ATP therapy does actually terminate the tachyarrhythmia, the ICD may terminate the charging of its charge storage device, and then subsequently slowly leak off the accumulated charge without performing defibrillation and/or cardioversion therapy. In such embodiments, the ICD may save some energy by not unnecessarily fully charging its charge storage device, and may also not harm or scare the patient by delivering an unnecessary defibrillation and/or cardioversion pulse. If delivery of ATP therapy fails to terminate the tachyarrhythmia, the ICD may continue to fully charge the charge storage device and perform defibrillation and/or cardioversion therapy. In these embodiments, the ICD has saved time between the detection of the tachyarrhythmia and the delivery of the defibrillation and/or cardioversion therapy by initiating charging of its charge storage device earlier than if the determined tachyarrhythmia was one of the first set of tachyarrhythmia types. In some embodiments, the charge storage device may begin charging, without first waiting for ATP therapy, when the tachyarrhythmia is a PVT or SVT type tachyarrhythmia. In other embodiments, the charge storage device may begin charging, without first waiting for ATP therapy, when the tachyarrhythmia is a MVT and/or VF. 
     In additional or alternative embodiments, the instruction sent to LCP  402  to initiate ATP therapy may include one or more ATP parameters that define one or more characteristics of the ATP therapy. For example, the instruction may specify a number of ATP therapy attempts that are to be attempted by LCP  402  before terminating the ATP therapy protocol. For instance, if the tachyarrhythmia type is a first type of tachyarrhythmia, the ICD may communicate an instruction that LCP  402  should attempt a defined ATP therapy two, or three, or five, or any other suitable number of times. Each attempt may include applying a plurality of spaced ATP pulses to the heart. If the tachyarrhythmia type is a second type of tachyarrhythmia, different and distinguishable from the first type of tachyarrhythmia, the ICD may communicate an instruction that LCP  402  should only provide one ATP therapy attempt. For example, for MVT, ATP therapy may be applied in multiple bursts (sometimes as programmed by a clinician) and more time may be allowed to terminate the tachyarrhythmia given the higher chance of success for ATP therapy when applied to MVT, whereas for other types of tachyarrhythmia (e.g. PVT, SVT and/or VF), it may be more appropriate to apply a single burst (single ATP attempt) to help reduce the time to defibrillation and/or cardioversion therapy given the lower chance of success for ATP therapy. 
     In some cases, the charge storage device of the ICD may be charged in parallel with the ATP therapy delivered by the LCP  402 . In these cases, the instruction sent to LCP  402  may command LCP  402  to perform a number of ATP therapy attempts that tend to fill up the time it takes the ICD to charge its charge storage device. In other instances, each instruction the ICD sends to LCP  402  commanding LCP  402  to perform ATP therapy may instruct LCP  402  to perform a single ATP attempt. Then, if the ICD determines that the ATP attempt did not terminate the detected tachyarrhythmia, the ICD may communicate an additional instruction to LCP  402  to perform another ATP attempt. In such instances, the result may be that the ICD may communicate a different number of instructions for delivery of ATP therapy, depending on whether the ATP therapy was successful or not. 
     The instructions sent by the ICD to the LCP  402  commanding LCP  402  to perform ATP may define one or more characteristics of the ATP therapy. Example parameters may include a number of ATP bursts—e.g. ATP pulses—to be delivered by LCP  402  during each ATP therapy attempt. This parameter may be in addition to the number of ATP therapy attempts or an alternative. In some embodiments, the number of ATP pulses may be specified differently for each ATP therapy attempt, where LCP  402  is instructed to perform multiple ATP therapy attempts. Also, the number of ATP pulses may be specified differently depending on the determined type of tachyarrhythmia. Additionally, in embodiments where the instruction to LCP  402  includes a number of ATP therapy attempts, the instructions may, in some embodiments, further include specific rates of ATP pulse delivery during each ATP therapy attempt. As with the number of ATP pulses in each attempt, the rates of ATP pulse delivery may differ between ATP therapy attempts and/or depending on the detected tachyarrhythmia type. Additionally, or alternatively, the instructions may specify a length of a break period between ATP therapy attempts. Other example parameters may include a pulse amplitude and/or a pulse width of the ATP pulses to be delivered during each ATP therapy attempt. As with the other parameters, the instructions may specify different amplitudes and/or pulse widths for each ATP therapy attempt and/or based on the determined type of tachyarrhythmia. 
     The instructions sent by the ICD to the LCP  402  commanding LCP  402  to perform ATP may additionally, or alternatively, include an instruction to perform ATP according one of a number of methods. According to a burst method, LCP  402  may deliver consecutive electrical stimulation pulses with a constant time interval between each electrical stimulation pulse. Additionally, when delivering ATP according to the burst method, LCP  402  may deliver each sequence of electrical stimulation pulses with a constant time interval between each of the sequences of electrical stimulation pulses. In another method, the ramp method, LCP  402  may deliver electrical stimulation pulses within a sequence of electrical stimulation pulses, or ATP therapy attempt, with a decreasing time interval between each pair of successive electrical stimulation pulses. In yet another method, the scan method, LCP  402  may deliver sequences of electrical stimulation pulses, or ATP therapy attempts, with a time interval between electrical stimulation pulses within each sequence of electrical stimulation pulses that decreases for each successive sequence of electrical stimulation pulses. In still another method, the ramp/scan method, LCP  402  may deliver ATP therapy according to the features of both the ramp method and the scan method. 
     In embodiments where the ICD includes an instruction to perform ATP according to specific method, the specific method may depend at least partially on the type of cardiac arrhythmia. For instance, if the arrhythmia is one of the first set of arrhythmia types, the ICD may communicate a message to LCP  402  to perform ATP therapy according to the burst method. However, if the arrhythmia type is one of the second set of arrhythmia types, the ICD may communicate a message to LCP  402  to perform ATP therapy according to the ramp method. Of course, in other embodiments, the type of method associated with each arrhythmia type may differ and may be any of the burst, ramp, scan, or ramp/scan methods. 
     In some embodiments, the types of tachyarrhythmias in the first and second sets of tachyarrhythmia types may be user programmable. For instance, as described with respect to  FIG. 4 , the ICD, including pulse generator  406 , may be able to communicate with external support device  420 , which in some embodiments may act as a programmer. A clinician may interact with the programmer to specify which tachyarrhythmias types are to be included in each of the first and second sets of tachyarrhythmia types, and which tachyarrhythmia types are not in either set. In some cases, the first set of cardiac arrhythmia types and the second set of cardiac arrhythmia types may be the same cardiac arrhythmia types. In other cases, the first set of cardiac arrhythmia types and the second set of cardiac arrhythmia types may be mutually exclusive. In other cases, the first set of cardiac arrhythmia types and the second set of cardiac arrhythmia types may share one or more common cardiac arrhythmia types. 
     In some embodiments, the types of tachyarrhythmias in the first and second sets of tachyarrhythmia types may depend at least partially on the level of charge in the energy storage module that powers the ICD. For example, the ICD may determine a percentage of remaining energy capacity of energy storage module  218 . The ICD may begin with, for example, MVT and PVT in the first set of tachyarrhythmia types, and PVT in the second set of tachyarrhythmia types. That is, if either MVT or PVT is detected, the ICD may send an instruction to the LCP  402  to initiate the application of ATP therapy by the LCP  402 . If PVT is detected, the ICD may initiate charging of the charge storage device, and if MVT is detected, the ICD may wait on initiating charging of the charge storage device until ATP is given a chance to terminate the tachyarrhythmia. Once the ICD determines that the percentage of capacity of remaining energy storage module  218  has dropped below, for example, fifty percent, the ICD may recommend and/or automatically remove PVT from the first set of tachyarrhythmia types. Then, if the ICD detects PVT, the ICD may not send an instruction to the LCP  402  to initiate the application of ATP therapy by the LCP  402 . Removing PVT from the first set of tachyarrhythmia types may help increase the remaining life of the battery of the ICD by not performing ATP therapy in cases wherein ATP therapy is less likely to be successful (e.g. PVT verses MVT). This is just one example. In other embodiments, different types of tachyarrhythmias may be moved, added or deleted from each of the sets. In some cases, one or more thresholds may be used for adjusting which tachyarrhythmia types are in each set of tachyarrhythmia types. 
       FIG. 7  is a flow diagram of an illustrative method  700  that may be implemented by a medical device, such as that shown in  FIG. 2 , which in some embodiments may be an ICD. Although the method of  FIG. 7  will be described with respect to MD  200 , the illustrative method of  FIG. 7  may be performed by any suitable medical device or medical device system. 
     In some embodiments, MD  200  may determine an occurrence of a cardiac arrhythmia, as shown at  701 . In some cases, MD  200  may determine an occurrence of a cardiac arrhythmia, and specifically a tachyarrhythmia, according to the flow diagram of  FIG. 6 . However, in other embodiments, MD  200  may use additional or alternative techniques to determine an occurrence of a cardiac arrhythmia. After determining an occurrence of a cardiac arrhythmia, MD  200  may determine a type of the detected cardiac arrhythmia from two or more types of cardiac arrhythmias, as shown at  703 . For example, MD  200  may discriminate between two or more different cardiac arrhythmias types. In embodiments where MD  200  discriminates between different types of tachyarrhythmias, MD  200  may operate according to a method detailed in  FIG. 6 . However, in other embodiments, MD  200  may operate differently than described with respect to  FIG. 6  to discriminate between tachyarrhythmia types. 
     After determining a type of cardiac arrhythmia, MD  200  may, if the determined type of cardiac arrhythmia is one of a first set of cardiac arrhythmia types, send an instruction for reception by a Leadless Cardiac Pacemaker (LCP) to initiate the application of ATP therapy by the LCP, as shown at  705 . In some embodiments, the instruction may include one or more parameters that define a characteristic of the ATP therapy. Some example parameters include a number of ATP therapy attempts, a number of ATP bursts in each ATP therapy attempt, a length of a break between ATP therapy attempts, an amplitude and/or pulse width of each ATP burst, among other parameters. If the determined type of cardiac arrhythmia is not one of the first set of cardiac arrhythmia types, MD  200  may not send the instruction to the LCP, as shown at  707 . 
     In some additional or alternative embodiments, MD  200  may further begin charging a charge storage device (for instance, a charge storage device that may be a part of pulse generator module  204 ) if the determined type of the cardiac arrhythmia is one of a second set of cardiac arrhythmia types in addition to sending the instruction. However, if the determined type of tachyarrhythmia is not one of the second set of cardiac arrhythmia types, MD  200  may wait to charge the charge storage device until after confirming that the delivered ATP therapy failed to terminate the tachyarrhythmia. 
       FIG. 8  is a flow diagram of an illustrative method  800  that may be implemented by a medical device, such as that shown in  FIG. 2 , which in some embodiments may be an ICD. Although the method of  FIG. 8  will be described with respect to MD  200 , the illustrative method of  FIG. 8  may be performed by any suitable medical device or medical device system. 
     In some embodiments, MD  200  may determine an occurrence of a cardiac arrhythmia, as shown at  801 . In some cases, MD  200  may determine an occurrence of a cardiac arrhythmia, and specifically a tachyarrhythmia, according to the flow diagram of  FIG. 6 . However, in other embodiments, MD  200  may use additional or alternative techniques to determine an occurrence of a cardiac arrhythmia. After determining an occurrence of a cardiac arrhythmia, MD  200  send an instruction for reception by a Leadless Cardiac Pacemaker (LCP) to initiate application of ATP therapy, as at  803 . 
     After sending the instruction, MD  200  may determine a type of the detected cardiac arrhythmia from two or more types of cardiac arrhythmias, as shown at  805 . For example, MD  200  may discriminate between two or more different cardiac arrhythmias types. In embodiments where MD  200  discriminates between different types of tachyarrhythmias, MD  200  may operate according to a method detailed in  FIG. 6 . However, in other embodiments, MD  200  may operate differently than described with respect to  FIG. 6  to discriminate between tachyarrhythmia types. 
     After determining a type of cardiac arrhythmia, MD  200  may, if the determined type of cardiac arrhythmia is one of a first set of cardiac arrhythmia types, wait to initiate charging of its charge storage device, as shown at  807 . For instance, MD  200  may monitor received electrical cardiac signals during and after delivery of ATP therapy by the LCP. In these embodiments, MD  200  may attempt to save battery energy by waiting until confirming that the delivery of ATP therapy did not terminate the tachycardia. Once MD  200  has confirmed that delivery of ATP therapy has failed to terminate the tachycardia, MD  200  may initiate charging of its charge storage device for delivery of defibrillation and/or cardioversion therapy. In instances where delivery of ATP therapy did terminate the tachycardia, MD  200  has saved energy by not charging its charge storage device. 
     However, if the determine type of cardiac arrhythmia is not one of the first set of cardiac arrhythmia types, MD  200  may initiate charging of its charge storage device, as shown at  809 . In some embodiments, while charging its charge storage device, MD  200  may monitor received cardiac electrical signals. MD  200  may determine, based at least in part on the received signals, whether delivery of ATP therapy by the LCP has terminated the tachycardia. If MD  200  determines that the delivery of ATP therapy has terminated the tachycardia, MD  200  may cease charging its charge storage device. MD  200  may then slowly leak off the accumulated charge. However, if MD  200  determines that the delivery of ATP therapy has not terminated the tachycardia, MD  200  may deliver defibrillation and/or cardioversion therapy once charging of its charge storage device is complete. 
     The above description of determining occurrences of tachyarrhythmias and discriminating between the various tachyarrhythmia types used a system including an ICD/SICD and an LCP as an example only. In other embodiments, other devices may be used as part of the system implementing the disclosed techniques. In still other embodiments, a system implementing the disclosed techniques may include additional devices. In such embodiments, determining occurrences of tachyarrhythmias, discriminating between the different types of tachyarrhythmias, communicating instructions, delivering ATP therapy, and/or delivering defibrillation/cardioversion therapy may be coordinated between the devices, as desired. 
     Those skilled in the art will recognize that the present disclosure may be manifested in a variety of forms other than the specific embodiments described and contemplated herein. For instance, as described herein, various embodiments include one or more modules described as performing various functions. However, other embodiments may include additional modules that split the described functions up over more modules than that described herein. Additionally, other embodiments 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.