ADJUSTMENT BASED ON SENSED HEART RATE

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Patent Application Serial No. 62/480,784 filed on April 3, 2017, the disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

[0002] The disclosure relates generally to implantable medical devices, and more particularly to implantable cardiac pacemakers that have a post shock pacing capability.

BACKGROUND

[0003] Implantable medical devices (IMDs) are commonly used to perform a variety of functions, such as monitor one or more conditions and/or delivery therapy to a patient. In some cases, IMDs 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's body. Some heart conditions may lead to low heart rates (e.g. bradycardia), while others may lead to rapid, irregular, and/or inefficient heart contractions (tachycardia). To help alleviate these and other conditions, various devices (e.g., pacemakers, defibrillators, etc.) can be implanted into a patient's body. When so provided, such devices can monitor and provide therapy, such as electrical stimulation therapy, to the patient's heart to help the heart operate in a more normal, efficient and/or safe manner. In some cases, an IMD may be configured to deliver pacing and/or defibrillation therapy to a patient's heart. In other cases, a patient may have multiple implanted devices that cooperate to deliver pacing and/or defibrillation therapy to the patient's heart.

[0004] In some instances, an IMD may perform demand pacing to help ensure that the heart rate of a patient does not fall below a lower heart rate threshold. When performing demand pacing, the IMD may pace the heart at the lower heart rate threshold when the intrinsic heart rate falls below the lower heart rate threshold. In some instances, the heart may be susceptible to cardiac fibrillation, which may be characterized by rapid, irregular, and/or inefficient heart contractions. When this happens, an Implantable Cardioverter Defibrillator (ICD) can be used to deliver a shock to the heart of the patient to defibrillate the heart. The heart typically stops beating for a moment in response to a delivered shock event, but then resumes in a normal rhythm. Often post-shock pacing pulses are delivered after the shock event to help bring the heart back into the normal rhythm. In some cases, the post-shock pacing pulses are delivered at a higher amplitude than the pacing pulses that are used during demand pacing. In some instances, an ICD may deliver both demand pacing and defibrillation shock therapy. In other instances, an IMD may deliver demand pacing while a separate ICD may deliver defibrillation shock therapy.

[0005] What would be desirable is an IMD that can deliver demand pacing, and can also anticipate a coming shock event from a remote ICD based on a detected heart rate condition, and then on its own increasing the energy level for subsequently delivered pacing pulses over a temporarily period of time. Such an IMD may, for example, deliver post shock pacing pulses with increased energy levels without requiring communication between the IMD and the remote ICD.

SUMMARY

[00Θ6] The disclosure relates generally to implantable medical devices, and more particularly to implantable cardiac pacemakers that have a post shock pacing capability. While a Leadless Cardiac Pacemaker (LCP) is used as an example implantable cardiac pacemaker, it should be recognized that the disclosure may be applied to any suitable implantable medical device as desired.

[0007] In an example of the disclosure, a cardiac pacemaker that is free from an Implantable Cardioverter Defibrillator (ICD) may include one or more sensors for sensing one or more physiological parameters of a patient, and two or more pacing electrodes for delivering pacing pulses to the heart of the patient. Electronics operatively coupled to the one or more sensors and the two or more pacing electrodes may be configured to determine a heart rate of the patient based at least in part on the one or more phy siological parameters sensed by the one or more sensors and may pace the heart of the patient via the two or more pacing electrodes in a manner that attempts to keep the heart rate of the patient from falling below a demand heart rate threshold. If the heart rate is below an upper heart rate threshold, the pacing pulses may be delivered at a capture pacing energy level. If the heart rate rises above the upper heart rate threshold, the pacing pulses may be temporarily delivered at an enhanced energy level above the capture pacing energy level for a period of time, and after the period of time, the pacing pulses may again be delivered at the capture energy level. During the period of time, it is contemplated that the pacing pulses delivered at the enhance energy level may comprise demand-pacing pacing pulses, post-shock pacing pulses, and/or anti-tachyarrhythmia-pacing (ATP) pulses, depending on what is deemed appropriate therapy at any given time.

[0008] Alternatively or additionally to any of the embodiments above, the one or more sensors may comprise two or more sensing electrodes, and at least one of the physiological parameters may comprise a cardiac electrical signal.

[0009] Alternatively or additionally to any of the embodiments above, at least one of the two or more sensing electrodes may be one of the pacing electrodes.

[0010] Alternatively or additionally to any of the embodiments above, the one or more sensors may comprise an accelerometer, and at least one of the physiological parameters may comprise one or more of a heart motion and a heart sound.

[0011] Alternatively or additionally to any of the embodiments above, the heart rate determined by the electronics may be an average heart rate of "n" previous heart beats, wherein "n" may be an integer greater than one.

[0012] Alternatively or additionally to any of the embodiments above, the pacing pulses may have a first amplitude and first pulse width at the capture pacing energy level, and a second amplitude and second pulse width at the enhanced energy level, wherein the second amplitude may be greater than the first amplitude and the second pulse width may be the same as the first pulse width.

[0013] Alternatively or additionally to any of the embodiments above, the pacing pulses may- have a first amplitude and first pulse width at the capture pacing energy level, and a second amplitude and second pulse width at the enhanced energy level, wherein the second amplitude may ^¬ be the same as the first amplitude and the second pulse width may be greater than the first pulse width.

[0014] Alternatively or additionally to any of the embodiments above, the pacing pulses may have a first amplitude and first pulse width at the capture pacing energy level, and a second amplitude and second pulse width at the enhanced energy level, wherein the second amplitude may be greater than the first amplitude and the second pulse width may be greater than the first pulse width.

[0015] Alternatively or additionally to any of the embodiments above, the period of time may ^¬ be a predetermined period of time. [0016] Alternatively or additionally to any of the embodiments above, the predetermined period of time may be programmable.

[0017] Alternatively or additionally to any of the embodiments above, the period of time may be greater than 3 minutes.

[0018] Alternatively or additionally to any of the embodiments above, the period of time may ^¬ be less than 1 hour.

[0019] Alternatively or additionally to any of the embodiments above, further comprising a communication module, wherein the electronics can receive commands from a remote device via the communication module, and wherein in response to receive an ATP command, the electronics may be configured to deliver a burst of ATP pacing pulses at the enhanced energy level.

[0020] Alternatively or additionally to any of the embodiments above, the cardiac pacemaker may be a leadless cardiac pacemaker (LCP) that may be configured to be implanted within a chamber of the heart of the patient.

[0021] In another example of the disclosure, a leadless cardiac pacemaker (LCP) may comprised a housing, and a plurality of electrodes for sensing electrical signals emanating from outside of the housing. An energy storage module may be disposed within the housing. The LCP may further include a pulse generator for delivering pacing pulses via two or more of the plurality of electrodes, wherein the pulse generator may be capable of changing an energy level of the pacing pulses. A control module disposed within the housing may be operative! y coupled to the pulse generator and at least two of the plurality of electrodes. The control module may be configured to receive one or more cardiac signals via two or more of the plurality of electrodes, determine a heart rate based at least in part on the received one or more cardiac signals, instruct the pulse generator to pace the heart with pacing pulses at a capture pacing energy level in a manner that attempts to keep the heart rate from falling below a demand heart rate threshold, determine if the heart rate rises above an upper heart rate threshold, and in response to determining that the heart rate has risen above the upper heart rate threshold, instruct the pulse generator to increase the energy level of the pacing pulses to an enhanced energy level for a period of time, and after the period of time, instruct the pulse generator to decrease the energy level of the pacing pulses back to the capture pacing energy level. During the period of time, it is contemplated that the pacing pulses delivered at the enhance energy level may comprise demand-pacing pacing pulses, post- shock pacing pulses, and/or anti-tachyarrhythmia-pacing (ATP) pulses, depending on what is deemed appropriate therapy at any given time.

[0022] Alternatively or additionally to any of the embodiments above, the pulse generator may change an amplitude of the pacing pulses to increase the energy level of the pacing pulses to the enhanced energy level.

[0023] Alternatively or additionally to any of the embodiments above, the pulse generator may change a pulse width of the pacing pulses to increase the energy level of the pacing pulses to the enhanced energy level.

[0024] Alternatively or additionally to any of the embodiments above, the pulse generator may change an amplitude and a pulse width of the pacing pulses to increase the energy level of the pacing pulses to the enhanced energy level.

[0025] In another example of the disclosure, a method for pacing a heart of a patient may comprise determining a heart rate of the patient, and pacing the heart of the patient in a manner that attempts to keep the heart rate of the patient from falling below a demand heart rate threshold. If the heart rate is below an upper heart rate threshold, pacing pulses may be delivered at a capture pacing energy level. If the heart rate rises above the upper heart rate threshold, pacing pulses may temporarily be delivered at an enhanced energy level above the capture pacing energy level for a period of time, and after the period of time, the pacing pulses may again be delivered at the capture pacing energy level. During the period of time, it is contemplated that the pacing pulses delivered at the enhance energy level may comprise demand-pacing pacing pulses, post-shock pacing pulses, and/or anti-tachyarrhythraia-pacing (ATP) pulses, depending on what is deemed appropriate therapy at any given time.

[0026] Alternatively or additionally to any of the embodiments above, the heart rate may be determined by an average heart rate of "n" previous heart beats, wherein "n" may be an integer greater than one.

[0027] The above summary of some illustrative embodiments is not intended to describe each disclosed embodiment or every implementation of the present disclosure. The Figures and Description which follow more particularly exemplify these and other illustrative embodiments.

BRIEF DESCRIPTION OF THE FIGURES

[0028] The disclosure may be more completely understood in consideration of the following description in connection with the accompanying drawings, in which: [0029] Figure 1 is a schematic block diagram of an illustrative LCP, in accordance with an example of the disclosure;

[0030] Figure 2 is a side view of an illustrative implantable LCP:

[0031] Figure 3 is a schematic diagram of an LCP implanted in a chamber of a patient's heart, in accordance with an example of the disclosure;

[0032] Figure 4 is a schematic diagram of a co-implanted transvenous implantable cardioverter-defibrillator (T-ICD) and LCP, in accordance with an example of the disclosure;

[0033] Figure 5 is a schematic diagram of a co-implanted subcutaneous or substernum implantable cardioverter-defibrillator (S-ICD) and LCP, in accordance with an example of the disclosure; and

[0034] Figures 6A-6G are timing diagrams showing illustrative operations of an LCP under various operating conditions.

[0035] 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 the disclosure to the particular 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

[0036] For the following defined terms, these definitions shall be applied, unless a different definition is given in the claims or elsewhere in this specification.

[0037] All numeric values are herein assumed to be modified by the term "about," whether or not explicitly indicated. The term "about" generally refers to a range of numbers that one of skill in the art would consider equivalent to the recited value (i.e., having the same function or result). In many instances, the terms "about" may include numbers that are rounded to the nearest significant figure.

[0038] The reci tation of numerical ranges by endpoints includes all numbers within that range (e.g. 1 to 5 includes 1, 1.5, 2, 2,75, 3, 3.80, 4, and 5).

[0039] As used in this specification and the appended claims, the singular forms "a", "an", and "the" include plural referents unless the content clearly dictates otherwise. As used in this specification and the appended claims, the term "or" is generally employed in its sense including "and/or" unless the content clearly dictates otherwise.

[0040] It is noted that references in the specification to "an embodiment", "some embodiments", "other embodiments", etc., indicate that the embodiment described may include one or more particular features, structures, and/or characteristics. However, such recitations do not necessarily mean that all embodiments include the particular features, structures, and/or characteristics. Additionally, when particular features, structures, and/or characteristics are described in connection with one embodiment, it should be understood that such features, structures, and/or characteristics may also be used connection with other embodiments whether or not explicitly described unless clearly stated to the contrary.

[0041] The following description should be read with reference to the drawings in which similar structures in different drawings are numbered the same. The drawings, which are not necessarily to scale, depict illustrative embodiments and are not intended to limit the scope of the disclosure.

[0042] Figure 1 depicts an illustrative cardiac pacemaker (e.g., a leadless cardiac pacemaker (LCP) 00) that may be implanted into a patient and may operate to deliver appropriate therapy to the heart, such as to deliver demand pacing therapy (e.g. for bradycardia), anti-tachycardia pacing (ATP) therapy, post-shock pacing therapy, cardiac resynchronization therapy (CRT) and/or the like. While a Leadless Cardiac Pacemaker (LCP) is used as an example implantable cardiac pacemaker, it should be recognized that the disclosure may be applied to any suitable implantable medical device as desired.

[0043] As can be seen in Figure 1 , the LCP 100 may be a compact device with a control module or electronics including all of its components housed within or directly on a housing 120. In some cases, the LCP 100 may be considered as being an example of an implantable medical device (IMD). In the example shown in Figure 1, the control module or electronics of 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 1 10, a battery 112, and an electrode arrangement 1 14. The control module or electronics of the LCP 100 may include more or less modules, depending on the application.

[0044] The electrical sensing module 106 may be configured to sense one or more physiological parameters of a patient. In some examples, the physiological parameters may include the cardiac electrical activity of the heart. For example, the electrical sensing module 106 may be connected to sensors 118 and the electncal sensing module 106 may be configured to sense the physiological parameters of the patient via the sensors 118. In some examples, the electrical sensing module 106 may be connected to electrodes 114/114', and the electrical sensing module 106 may be configured to sense one or more of the physiological parameters of the patient, including cardiac electrical signals, via the electrodes 114/114'. In this case, the electrodes 114/114' are the sensors.

[0045] According to various embodiments, the physiological parameters may be indicative of the state of the patient and/or the state of the heart of the patient. For example, in some cases, the physiological parameters may include temperature (e.g., blood temperature, body tissue temperature, etc.), respiration activity, cardiac electrical signals, etc. In addition, in some examples, the cardiac electrical signals may represent local information from the chamber in which the LCP 100 is implanted. For instance, if the LCP 100 is implanted within a ventricle of the heart (e.g. V, LV), cardiac electrical signals sensed by the LCP 100 through the electrodes 114/114' and/or sensors 1 18 may represent ventricular cardiac electrical signals. In some cases, the LCP 100 may be configured to detect cardiac electrical signals from other chambers (e.g. far field), such as the P-wave from the atrium.

[0046] In some examples, the mechanical sensing module 108, when provided, may be configured to sense one or more physiological parameters of the patient. For example, in certain embodiments, the mechanical sensing module 108 may include one or more sensors, such as an accelerometer, a pressure sensor, a heart sound sensor, a blood-oxygen sensor, a chemical sensor, a temperature sensor, a flow sensor and/or any other suitable sensor that is configured to detect one or more mechanical/chemical physiological parameters of the patient (e.g., heart motion, heart sound, etc.). The mechanical sensing module 108 may receive and measure the physiological parameters. Both the electrical sensing module 106 and the mechanical sensing module 108 may ^¬ be connected to a processing module 1 10, which may provide signals representati ve of the sensed parameters. Although described with respect to Figure 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.

[0047] The electrodes 114/114' can be secured relative to the housing 120 and may be exposed to the tissue and/or blood surrounding the LCP 100. In some cases, depending on the sensor type, the sensors 118 may be internal to the housing or exposed to the tissue and/or blood surrounding the LCP 00. In some cases, the electrodes 114 may be generally disposed on either end of the LCP 100. In some examples, the electrodes 114/1 14' and sensors 1 18 may be in electrical communication with one or more of the modules 102, 104, 106, 108, and 110. The electrodes 114/114' and/or sensors 118 may be supported by the housing 120. In some examples, the electrodes 114/114' and/or sensors 118 may be connected to the housing 120 through short connecting wires such that the electrodes 114/114' and/or sensors 118 are not directly secured relative to the housing 120 but rather located on a tail that is connected the housing. 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 physiological parameters, deliver electrical stimulation, and/or communicate with an external medical device. The electrodes 4/1 14' and/or sensors 118 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' and/or sensors 118 connected to the LCP 100 may have an insulative portion that electrically isolates the electrodes 114/114' and/or sensors 118 from adjacent electrodes/sensors, the housing 120, and/or other parts of the LCP 100.

[0048] The processing module 110 may include electronics that is configured to control the operation of the LCP 100. For example, the processing module 110 may be configured to receive electrical signals from the electrical sensing module 106 and/or the mechanical sensing module 108. Based on the received signals, the processing module 110 may determine, for example, a heart rate of the patient, abnormalities in the operation of the heart, etc. Based on the determined conditions, the processing module 110 may control the pulse generator module 104 to generate and deliver pacing pulses in accordance with one or more therapies to treat the determined conditions. The processing module 110 may further receive information from the communication module 102, In some examples, the processing module 1 10 may use such received information to help determine the current conditions of the patient, determine whether an abnormality is occurring given the current condition, and/or to take a 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.

[0049] 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. In some cases, the pre-programmed chip may implement a state machine that performs the desired functions. 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 1 10 may read and write information to and from the separate memory.

[0050] The battery 1 12 may provide power to the LCP 100 for its operations. 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, such as a fuel cell or the like, as desired.

[0051] In the example shown in Figure 1, the pulse generator module 104 may be electrically connected to the electrodes 1 14/114'. In some cases, the sensors 118 may also have electrical stimulation functionality and may be electrically connected to the pulse generator module 104 when desired. Said another way, one or more of the electrodes 114/1 4' may function as a sensor 118 electrode, such as for sensing cardiac electrical signals. In some cases, the LCP 100 may have a controllable switch that connects one or more of the electrodes 1 14/114' to the pulse generator module 104 when the pulse generator module 104 delivers a pacing pulse, and may connect one or more of the electrodes 114/1 14' to the electrical sensing module 106 when the pulse generator module 104 is not delivering a pacing pulse.

[0052] The pulse generator module 104 may be configured to generate electrical stimulation signals. For example, the pulse generator module 104 may generate and deliver electrical pacing pulses by using energy stored in the battery 112 within the LCP 100 and deliver the generated pacing pulses via the electrodes 1 14, 114' and/or sensors 118. 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 pacing pulses via the electrodes 114, 114', and/or sensors 118. In at least some examples, the pulse generator 104 of the LCP 100 may include switching circuitiy to selectively connect one or more of the electrodes 114, 114' and/or sensors 118 to the pulse generator 104 in order to select which of the electrodes 114/114' and/or sensors 118 (and/or other electrodes) the pulse generator 104 uses to deliver the electrical stimulation therapy. The pulse generator module 104 may be configured to deliver pacing pulses at two or more different energy levels. This may be accomplished by controlling the amplitude, pulse width, pulse shape and/or any other suitable characteristic of the pacing pulses.

[0053] According to various embodiments, the sensors 118 may be configured to sense one or more physiological parameters of a patient and send a signal to the electrical sensing module 106 and/or the mechanical sensing module 108. For example, the physiological parameters may include a cardiac electrical signal and the sensors 118 may send a response signal to the electrical sensing module 106. In some examples, one or more of the sensors 118 may be an accelerometer and the physiological parameters may alternatively or additionally include heart motion and/or heart sounds and the sensors 1 18 may send a corresponding signal to the mechanical sensing module 108. Based on the sensed signals, the sensing modules 106 and/or 108 may determine or measure one or more physiological parameters, such as heart rate, respiration rate, activity level of the patient and/or any other suitable physiological parameters. The one or more physiological parameters may then be passed to the processing module 110.

[0054] In some cases, the intrinsic heart rate of the patient may reach and/or fall below a demand heart rate threshold and into a "Normal Demand Zone". In this case, the processing module 1 10 may perform demand pacing by instructing the pulse generator module 104 to deliver pacing pulses at a set energy level using the electrodes 114/1 14' in a manner that attempts to keep the heart rate of the patient from falling below the demand heart rate threshold. The demand heart rate threshold may be a fixed heart rate such as a lower rate limit, or may be a dynamic heart rate that is dependent on the activity level of the patient. In order to help conserve battery power, the pacing pulses may be delivered at a capture pacing energy level, which is above the capture threshold of the heart but less than the maximum allowed pacing energy level. [0055J In some cases, the intrinsic heart rate may rise to and/or above a normal heart rate upper threshold and into an "ATP Zone". In this case, the intrinsic heart rate observed may be a fast but regular rhythm, such as that observed during ventricular tachycardia. Similar to the demand heart rate threshold, the normal heart rate upper threshold may be a fixed rate or a dynamic heart rate that is dependent on the activity level of the patient. In response to the intrinsic heart rate reaching and/or exceeding the normal heart rate upper threshold, the processing module 110 may be configured to automatically perform anti-tachyarrhythmia-pacing (ATP) therapy by instructing the pulse generator module 104 to deliver ATP pulses at the capture pacing energy level (or an enhanced level if desired). Note, in this case, the LCP 100 may autonomously initiate ATP therapy based on the detected heart rate without having to first receive a command from another medical device notifying the LCP to deliver ATP pulses.

[0056] If the intrinsic heart rate rises to and/or above an upper heart rate threshold and into a "Post Shock Zone", the processing module 110 may instruct the pulse generator module 104 to temporarily set the energy level of pacing pulses, if delivered, to an enhanced energy level above the capture pacing energy level for a period of time. After the period of time expires, the energy level of pacing pulses may be returned to the capture energy level. During the period of time, it is contemplated that the pacing pulses, if delivered, may be demand-pacing pacing pulses, post-shock pacing pulses, and/or anti-tachyarrhythmia-pacing (ATP) pulses, depending on what is deemed appropriate therapy by the processing module 1 10 at any given time.

[0057] The upper heart rate threshold may be a threshold that may be fixed or programmable. The upper heart rate threshold may be set at a rate that is above a safe heart rate of the patient, such that if the patient's heart rate rises above the upper heart rate threshold, the patient may be experiencing tachycardia and even cardiac fibrillation. Anticipating that a shock may be delivered to the heart via another medical device (e.g. an Implantable Cardioverter Defibrillator), the processing module 110 may instruct the pulse generator module 104 to temporarily set the energy level of pacing pulses, if delivered, to an enhanced energy level above the capture pacing energy- level for a period of time. The period of time may be 30 seconds, 1 minute, 3 minutes, 5 minutes, 10 minutes, 20 minutes, 30 minutes, 1 hour, 1 day, or any other suitable time period. While this may consume extra power during this period of time by delivering some pacing pulses at the enhanced energy level, the pulses will be more appropriate for post-shock pacing should a shock be delivered to the heart by another medical device. Note, this allows the LCP 100 to autonomously set the pacing pulses to an enhanced energy level for post shock-pacing without having to first detect a high energy shock pulse or receive a communication from another medical device notifying the LCP that a shock will be delivered. Whether a shock pulse is actually delivered or not, the processing module 110 may instruct the pulse generator module 104 to temporarily set the energy level of pacing pulses, if delivered, to an enhanced energy level until the end of the time period, and then return the energy level back to the capture energy level. In some cases, the period of time may be reset each time the measured heart rate is above the upper heart rate threshold. When so provided, the pulse generator module 104 keep the energy level at the enhanced energy level until the heart rate remains below the upper heart rate threshold for at least the period of time.

[0058] In some case, the processing module 1 10 may detect when the sensed heart rate falls at a rate that is above a threshold rate and/or falls below a floor heart rate. When the heart rate falls at a rate that is outside the bounds of normal physiology, or falls below a heart rate that is below what is necessary to sustain life, it may be assumed that the heart has been shocked by an ICD or the like. In response, the processing module may instruct the pulse generator module 104 to deliver pacing pulses (e.g. post shock pacing pulses) at the enhanced energy level until the end of the time period, and then return the energy level back to the capture energy level. This may be an alternative trigger for temporarily delivering pacing pulses at the enhanced energy level for a period of time.

[0059] In certain embodiments, the LCP 100 may include the communication module 102. In some cases, the communication module 102 may be configured to communicate with devices such as remote sensors, other medical devices such as an SICD, and/or the like, that are located externally to the LCP 100. Such devices may be located either external or internal to the patient's body. Irrespective of the location, external devices (i.e. external to the LCP 100 but not necessarily- external to the patient's body) can communicate with the LCP 100 via communication module 102 to accomplish one or more desired functions. For example, the LCP 100 may communicate information, such as sensed electrical signals, data, instructions, messages, R-wave detection markers, etc., to an external medical device (e.g. SICD and/or programmer) through the communication module 102. The external medical device may use the communicated signals, data, instructions, messages, R-wave detection markers, etc., to perform various functions, such as determining occurrences of arrhythmias, delivering electrical stimulation therapy, storing received data, and/or performing any other suitable function. The LCP 100 may additionally receive information such as signals, data, instructions and/or messages from the external medical device through the communication module 102, and the LCP 100 may use the received signals, data, instructions and/or messages to perform various functions, such as determining occurrences of arrhythmias, delivering electrical stimulation therapy, storing received data, and/or performing any other suitable function. The communication module 102 may be configured to use one or more methods for communicating with external devices. For example, the communication module 102 may communicate via radiofrequency (RF) signals, inductive coupling, optical signals, acoustic signals, conducted communication signals, and/or any other signals suitable for communication.

[006Θ] To implant the LCP 100 inside a patient's body, an operator (e.g., a physician, clinician, etc.), may fix the LCP 100 to the cardiac tissue of the patient'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 1 16 within the cardiac tissue. In other examples, the anchor 1 16 may include other structures such as barbs, spikes, or the like to facilitate engagement with the surrounding cardiac tissue.

[0061] Figure 2 is a side view of an illustrative implantable leadless cardiac pacemaker (LCP) 210. The LCP 210 may be similar in form and function to the LCP 100 described above. The LCP 210 may include the control module having any of the modules and/ or structural features described above with respect to the LCP 100 described above. The LCP 210 may include a shell or housing 212 having a proximal end 214 and a distal end 216. The illustrative LCP 210 includes a first electrode 220 secured relative to the housing 212 and positioned adjacent to the distal end 216 of the housing 212 and a second electrode 222 secured relative to the housing 212 and positioned adjacent to the proximal end 214 of the housing 212. The electrodes 220, 222 may be sensing and/or pacing electrodes to provide electro-therapy and/or sensing capabilities. The first electrode 220 may be capable of being positioned against or may otherwise contact the cardiac tissue of the heart while the second electrode 222 may be spaced away from the first electrode 220. The first and/or second electrodes 220, 222 may be exposed to the environment outside the housing 212 (e.g. to blood and/or tissue).

[0062] In some cases, the LCP 210 may include a pulse generator (e.g., electrical circuitry) and an energy storage module (e.g., a battery, super capacitor and/or other power source) within the housing 212 to provide electrical signals to the electrodes 220, 222 to control the pacing/sensing electrodes 220, 222. While not explicitly shown, the LCP 210 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 212. Electrical connections between the pulse generator and the electrodes 220, 222 may allow electrical stimulation to heart tissue and/or sense a physiological parameter.

[0063] In the example shown, the LCP 210 includes a fixation mechanism 224 proximate the distal end 216 of the housing 212. The fixation mechanism 224 is configured to attach the LCP 210 to a wall of the heart, or oth erwise anchor the LCP 210 to the anatomy of the patient. In some instances, the fixation mechanism 224 may include one or more, or a plurality of hooks or tines 226 anchored into the cardiac tissue of the heart to attach the LCP 210 to a tissue wall. In other instances, the fixation mechanism 224 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 210 to the heart. These are just examples.

[0064] The LCP 210 may further include a docking member 230 proximate the proximal end 214 of the housing 212, The docking member 230 may be configured to facilitate delivery and/or retrieval of the LCP 210. For example, the docking member 230 may extend from the proximal end 214 of the housing 212 along a longitudinal axis of the housing 212. The docking member 230 may include a head portion 232 and a neck portion 234 extending between the housing 212 and the head portion 232. The head portion 232 may be an enlarged portion relative to the neck portion 234. For example, the head portion 232 may have a radial dimension from the longitudinal axis of the LCP 210 that is greater than a radial dimension of the neck portion 234 from the longitudinal axis of the LCP 210. In some cases, the docking member 230 may further include a tether retention structure 236 extending from or recessed within the head portion 232. The tether retention structure 236 may define an opening 238 configured to receive a tether or other anchoring mechanism therethrough. While the retention structure 236 is shown as having a generally "U- shaped" configuration, the retention structure 236 may take any shape that provides an enclosed perimeter surrounding the opening 238 such that a tether may be securably and releasably passed (e.g. looped) through the opening 238. In some cases, the retention structure 236 may extend though the head portion 232, along the neck portion 234, and to or into the proximal end 214 of the housing 212. The docking member 230 may be configured to facilitate delivery of the LCP 210 to the intracardiac site and/or retrieval of the LCP 210 from the intracardiac site. While this describes one example docking member 230, it is contemplated that the docking member 230, when provided, can have any suitable configuration.

[0065] It is contemplated that the LCP 210 may include one or more pressure sensors 240 coupled to or formed within the housing 212 such that the pressure sensor(s) is exposed to the environment outside the housing 212 to measure blood pressure within the heart. For example, if the LCP 210 is placed in the left ventricle, the pressure sensor(s) 240 may measure the pressure within the left ventricle. If the LCP 210 is placed in another portion of the heart (such as one of the atriums or the right ventricle), the pressures sensor(s) may measure the pressure within that portion of the heart. The pressure sensor(s) 240 may include a MEMS device, such as a MEMS device with a pressure diaphragm and piezoresistors on the diaphragm, a piezoelectric sensor, a capacitor-Mi cro-machined Ultrasonic Transducer (cMUT), a condenser, a micro-monometer, or any other suitable sensor adapted for measuring cardiac pressure. The pressures sensor(s) 240 ma ^¬ be part of a mechanical sensing module described herein. It is contemplated that the pressure measurements obtained from the pressures sensor(s) 240 may be used to generate a pressure curve over cardiac cycles. The pressure readings may be taken in combination with impedance measurements (e.g. the impedance between electrodes 220 and 222) to generate a pressure- impedance loop for one or more cardiac cycles as will be described in more detail below. The impedance may be a surrogate for chamber volume, and thus the pressure-impedance loop may be representative for a pressure-volume loop for the heart.

| 0066] In some embodiments, the LCP 210 may be configured to measure impedance between the electrodes 220, 222. More generally, the impedance may be measured between other electrode pairs, such as the additional electrodes 114' described above. In some cases, the impedance may ^¬ be measured between two spaced LCP's, such as two LCP's implanted within the same chamber (e.g. LV) of the heart, or two LCP's implanted in different chambers of the heart (e.g. RV and LV). The processing module of the LCP 210 and/ or external support devices may derive a measure of cardiac volume from intracardiac impedance measurements made between the electrodes 220, 222 (or other electrodes). Primarily due to the difference in the resistivity of blood and the resistivity of the cardiac tissue of the heart, the impedance measurement may vary during a cardiac cycle as the volume of blood (and thus the volume of the chamber) surrounding the LCP changes. In some cases, the measure of cardiac volume may be a relative measure, rather than an actual measure. In some cases, the intracardiac impedance may be correlated to an actual measure of cardiac volume via a calibration process, sometimes performed during implantation of the LCP(s). During the calibration process, the actual cardiac volume may be determined using fluoroscopy or the like, and the measured impedance may be correlated to the actual cardiac volume.

[0067] In some cases, the LCP 210 may be provided with energy delivery circuitry operatively coupled to the first electrode 220 and the second electrode 222 for causing a current to flow between the first electrode 220 and the second electrode 222 in order to determine the impedance between the two electrodes 220, 222 (or other electrode pair). It is contemplated that the energy delivery circuitry may also be configured to deliver pacing pulses via the first and/or second electrodes 220, 222. The LCP 210 may further include detection circuitry operatively coupled to the first electrode 220 and the second electrode 222 for detecting an electrical signal received between the first electrode 220 and the second electrode 222, In some instances, the detection circuitry may be configured to detect cardiac signals received between the first electrode 220 and the second electrode 222.

[0068] When the energy delivery circuitry delivers a current between the first electrode 220 and the second electrode 222, the detection circuitry may measure a resulting voltage between the first electrode 220 and the second electrode 222 (or between a third and fourth electrode separate from the first electrode 220 and the second electrode 222, not shown) to determine the impedance. When the energy delivery circuitry delivers a voltage between the first electrode 220 and the second electrode 222, the detection circuitry may measure a resulting current between the first electrode 220 and the second electrode 222 (or between a third and fourth electrode separate from the first electrode 220 and the second electrode 222) to determine the impedance.

[0069] From these and other measurements, heart rate, respiration, stroke volume, contractility, and other physiological parameters can be derived.

[0070] Figure 3 shows an illustrative LCP 300 implanted in a heart 306. In Figure 3, the LCP 300 is shown fixed to the interior of the left ventricle (LV) of the heart 306. In some cases, the LCP 300 may be i the right ventricle, right atrium, left ventricle or left atrium of the heart, as desired. In some cases, more than one LCP 300 may be implanted. For example, one LCP 300 may be implanted in the right ventricle and another may be implanted in the right atrium. In another example, one LCP 300 may be implanted in the right ventricle and another may be implanted in the left ventricle. In yet another example, one LCP 300 may be implanted in each of the chambers of the heart.

[0071] According to various embodiments, the LCP 300 may include a housing 302 having electrodes 304 for sensing electrical signals emanating from outside of the housing 302. The electrodes 304 may be configured to provide sensed cardiac signals to a control module disposed with the housing 302. The control module may then determine a heart rate of the heart 306 based on the cardiac signals and instruct a pulse generator to deliver pacing pulses to the heart 306 via the electrodes 304.

[0072] According to various embodiments, the control module may cause the LCP 300 to deliver demand pacing. In demand pacing, the LCP 300 may monitor the heart rate and send an electrical pacing pulse or electrical pacing pulses to the heart 306 if the intrinsic heart rate is too slow and/or if beats are being missed. Said another way, the LCP 300 may pace the heart at a lower heart rate threshold when the intrinsic heart rate falls below the lower heart rate threshold, and missed intrinsic beats may be paced. In one example, in demand pacing, when the control module receives cardiac signals from the electrodes 304, the control module may analyze the cardiac signals and determine a measure of heart rate. In some cases, the determined measure of the heart rate may be an average heart rate of more than one or a set of previously recorded heart beats. In certain embodiments, the control module may then compare the measure of the heart rate to the lower heart rate threshold (e.g. a fixed heart rate threshold or a demand heart rate threshold). In some cases, the control module may be programmed to keep the measure of the heart rate from falling below the lower heart rate threshold. As a result, if the intrinsic heart rate falls below the lower heart rate threshold, such as 60 bpm, the control module instructs the pulse generator to deliver electrical pacing pulses at the lower heart rate threshold and at a first energy level using the electrodes 304. The lower heart rate threshold may be any suitable heart rate, such as 70 bpm, 60 bpm, 50 bpm, 45 bpm, 40 bpm, etc. It is contemplated that the lower heart rate threshold may be a fixed heart rate such as a lower rate limit, or may be a dynamic heart rate that is dependent on the activity level of the patient. 10073] In various embodiments, the desired energy level of the pacing pulses may dictate the amplitude and/or the pulse width of the electrical pacing pulses that are delivered to the heart. In certain embodiments, the first energy level may deliver pacing pulses each having an electrical pulse width of 1 ms and an amplitude of 5.0 V. In some embodiments, the first energy level may deliver pacing pulses each having an electrical pulse width of 0.5 ms and an amplitude of 4.0 V. In further embodiments, the first energy level may deliver pacing pulses each having an electrical pulse width of 0.25 ms and an amplitude of 3.0 V amplitude. These are just examples and other amplitudes and pulse widths may be designated for the first energy level at which the demand pacemaker delivers electrical pulses. In some instances, the first energy level may be set based on the results of a capture threshold test. For example, the first energy level may be set at the capture threshold plus a capture threshold margin. In some instances, changing the energy level may only change the amplitude and keep the pulse width the same, or change the pulse width and keep the amplitude the same, or change both the amplitude and pulse width.

[0074] In some cases, while the pulse generator delivers pacing pulses at the first energy level, the control module may continue to use the electrodes 304 to sense the cardiac signals and determine and monitor the heart rate. In some cases, the control module may continue to instruct the pulse generator to deliver the electrical pacing pulses until the intrinsic rate is above the lower heart rate threshold.

[0075] In various embodiments, the control module may cause the LCP 300 to deliver ATP therapy pulses. In ATP therapy, the LCP 300 may monitor the heart rate and send an electrical pacing pulse or electrical pacing pulses to the heart 306 if the intrinsic heart rate is above a normal heart rate. In one example, in ATP therapy, when the control module receives cardiac signals from the electrodes 304, the control module may analyze the cardiac signals and determine a measure of heart rate. In some cases, the determined measure of the heart rate may be an average heart rate of more than one or a set of previously recorded heart beats. In certain embodiments, the control module may then compare the measure of the heart rate to the higher heart rate threshold (e.g. a fixed heart rate threshold or a normal heart rate threshold). In some cases, the control module may be programmed to attempt to keep the measure of the heart rate from rising above the higher heart rate threshold by delivering ATP therapy if appropriate. As a result, if the intrinsic heart rate rises above the higher heart rate threshold, such as 140 bpm, the control module may instruct the pulse generator to deliver ATP therapy pulses at the first energy level using the electrodes 304. The higher heart rate threshold may be any suitable heart rate, such as 155 bpm, 50 bpm, 145 bpm, 135 bpm, etc. It is contemplated that the higher heart rate threshold may be a fixed heart rate such as a higher rate limit, or may be a dynamic heart rate that is dependent on the activity level of the patient.

[0076] In some cases, while the pulse generator delivers ATP pulses at the first energy level, the control module may continue to use the electrodes 304 to sense the cardiac signals and determine and monitor the heart rate. In some cases, the control module may continue to instruct the pulse generator to deliver the ATP pulses until the intrinsic rate falls below the higher heart rate threshold or until a predetermined period has passed.

[0077] While monitoring the measure of the heart rate, the control module may detect when the heart rate rises above an upper heart rate threshold. If the heart rate is below the upper heart rate threshold, the pacing pulses may be delivered at the first energy level as discussed above. However, if the heart rate rises above the upper heart rate threshold, the control module may cause pacing pulses to be temporarily delivered at an enhanced energy level above the first energy level for a period of time, and after the period of time, the pacing pulses may again be delivered at the first energy level. During the period of time, it is contemplated that the pacing pulses delivered at the enhance energy level may comprise demand-pacing pacing pulses, post-shock pacing pulses, and/or anti-tachyarrhythraia-pacing (ATP) pulses, depending on what is deemed appropriate therapy at any given time.

[0078] The upper heart rate threshold may be a threshold that may be fixed or programmable. The upper heart rate threshold may be set at a rate that is above a safe heart rate of the patient, such that if the patient's heart rate rises above the upper heart rate threshold, the patient may be experienced tachycardia and even cardiac fibrillation. Anticipating that a shock may be delivered to the heart via another medical device (e.g. an Implantable Cardioverter Defibrillator), the control module of the LCP 300 may instruct a pulse generator module of the LCP 300 to temporarily deliver pacing pulses at the enhanced energy level above the first energy level for a period of time. The period of time may be 30 seconds, 1 minute, 3 minutes, 5 minutes, 10 minutes, 20 minutes, 30 minutes, 1 hour, 1 day, or any other suitable time period. While this may consume extra power during this period of time by delivering pacing pulses at the enhanced energy level rather than the lower first energy level, the pacing pulses will be delivering pacing pulses that are more appropriate for post-shock pacing pulses should a shock be delivered to the heart by another medical device. Note, this allows the LCP 300 to autonomously set the pacing pulses to an enhanced energy level for post shock-pacing without having to have circuitry to detect a high energy shock pulse or receive a communication from another medical device notifying the LCP 300 that a shock will be delivered. Whether a shock pulse is actually delivered or not, the control module may instruct the pulse generator module to temporarily deliver pacing pulses at the enhanced energy level until the end of the time period, and then return to delivering pacing pulses at the first energy level. In some cases, the period of time may be reset each time the measured heart rate is above the upper heart rate threshold. When so provided, the control module of the LCP 300 may deliver pacing pulses at the enhanced energy level until the heart rate remains below the upper heart rate threshold for the period of time. During the period of time, it is contemplated that the pacing pulses delivered at the enhance energy level may comprise demand-pacing pacing pulses, post-shock pacing pulses, and/or anti-tachyarrhythmia-pacing (ATP) pulses, depending on what the control module deems appropriate therapy at any given time.

[0079] In some case, the control module may detect when the sensed heart rate falls at a rate that is above a threshold and/or falls below a floor heart rate. When the heart rate falls at a rate that is outside the bounds of normal physiology, or falls below a heart rate that is below what is necessary to sustain life, the control module may assume that the heart has just been shocked by an ICD. In response, the control module may instruct the pulse generator module of the LCP 300 to temporarily deliver pacing pulses at the enhanced energy level until the end of a time period, and then return to delivering pacing pulses at the first energy level. This may be an alternative trigger for temporarily delivering pacing pulses at the enhanced energy level for a period of time. jOOSOJ In certain embodiments, the enhanced energy level may have an electrical pulse width of 1.5 ms and a 5.0 V amplitude. In some embodiments, the enhanced energy level may have an electrical pulse width of 1.5 ms and a 7.0 V amplitude. In some embodiments, the enhance energy- level may have an electrical pulse width of 2 ms and an 8.0 V amplitude. In further embodiments, the enhanced energy level may have an electrical pulse width of 2.5 ms and an 8.5 V amplitude. These are just examples and other amplitudes and pulse widths may be designated for the enhanced energy level. In some cases, the enhanced energy level may have an amplitude that is a maximum voltage, and the pulse width is the same or larger than that used for the first energy level. In some cases, when changing between the first energy level and the enhanced energy level, the control module may only change the pulse amplitude of the electrical pacing pulses and leave the pulse widths the same, only change the pulse widths and leave the pulse amplitudes the same, or both change pulse amplitude and pulse width.

[0081] Figure 4 is a schematic diagram of a co-implanted transvenous implantable cardioverter-defibrillator (T-ICD) 400 and LCP 402, in accordance with an example of the disclosure. In Figure 4, the ICD 400 may include a pulse generator 403 coupled to a lead 406 having one or more electrodes 404. In some cases, the electrodes 404 may be positioned in the heart 410. The location of the pulse generator 403, the lead 406, and electrodes 404 are just exemplary. In some cases, the pulse generator 403, the lead 406 and/or electrodes 404 may be disposed in different chambers of the heart 410, or the pulse generator 403 may include additional leads and/or electrodes that are disposed within or adjacent to heart 410. According to various embodiments, the ICD 400 may be configured to deliver a shock to the heart 410.

[0082] The LCP 402 may operate similar to the LCPs 100, 210 and 300 discussed above. The LCP 402 may be configured to deliver demand pacing. In demand pacing, the LCP 402 may monitor the heart rate and send an electrical pacing pulse or electrical pacing pulses to the heart 410 if the intrinsic heart rate is too slow and/or if beats are being missed. Said another way, the LCP 402 may pace the heart at a lower heart rate threshold when the intrinsic heart rate falls below the lower heart rate threshold, and missed intrinsic beats may be paced. In one example, in demand pacing, when the LCP 402 receives cardiac signals from its electrodes, the LCP 402 may analyze the cardiac signals and determine a measure of heart rate. In some cases, the determined measure of the heart rate may be an average heart rate of more than one or a set of previously recorded heart beats. In certain embodiments, the LCP 402 may then compare the measure of the heart rate to the lower heart rate threshold (e.g. a fixed heart rate threshold or a demand heart rate threshold). In some cases, the LCP 402 may be programmed to keep the measure of the heart rate from falling below the lower heart rate threshold. As a result, if the intrinsic heart rate falls below the lower heart rate threshold, such as 60 bpm, the LCP 402 delivers electrical pacing pulses at the lower heart rate threshold and at a first energy level.

[0083] While monitoring the measure of the heart rate, the control module may detect when the heart rate rises above a higher heart rate threshold. If the heart rate rises above the higher heart rate threshold, the LCP 402 may be configured to deliver ATP therapy. In ATP therapy, the LCP 402 may monitor the heart rate and send an electrical pacing pulse or electrical pacing pulses to the heart 410 if the intrinsic heart rate is too high. In one example, in ATP therapy, when the LCP 402 receives cardiac signals from its electrodes, the LCP 402 may analyze the cardiac signals and determine a measure of heart rate. In some cases, the determined measure of the heart rate may be an average heart rate of more than one or a set of previously recorded heart beats. In certain embodiments, the LCP 402 may then compare the measure of the heart rate to the higher heart rate threshold (e.g. a fixed heart rate threshold or a demand heart rate threshold). In some cases, the LCP 402 may attempt to keep the measure of the heart rate from rising above the higher heart rate threshold by applying ATP therapy if appropriate. As a result, if the intrinsic heart rate rises above the higher heart rate threshold, such as 140 bpm, the LCP 402 may delivers ATP therapy at the first energy level.

[0084] While monitoring the measure of the heart rate, the control module may detect when the heart rate rises above an upper heart rate threshold. If the heart rate is below the upper heart rate threshold, the pacing pulses may be delivered at the first energy level as discussed above. However, if the heart rate rises above the upper heart rate threshold, the LCP 402 may cause pacing pulses to be temporarily delivered at an enhanced energy level above the first energy level for a period of time, and after the period of time, the pacing pulses may again be delivered at the first energy level. During the period of time, it is contemplated that the pacing pulses delivered at the enhanced energy level may comprise demand-pacing pacing pulses, post-shock pacing pulses, and/or anti-tachyarrhythraia-pacing (ATP) pulses, depending on what is deemed appropriate therapy at any given time.

[0085] The upper heart rate threshold may be a threshold that may be fixed or programmable. The upper heart rate threshold may be set at a rate that is above a safe heart rate of the patient, such that if the patient's heart rate rises above the upper heart rate threshold, the patient may be experiencing tachycardia and even cardiac fibrillation. Anticipating that a shock may be delivered to the heart via the ICD 400, the LCP 402 may temporarily deliver pacing pulses at the enhanced energy level above the first energy level for a period of time. While this may consume extra power during this period of time by delivering pacing pulses at the enhanced energy level rather than the lower first energy level, the pacing pulses will be delivering pacing pulses that are more appropriate for post-shock pacing pulses should a shock be delivered to the heart 410 by the ICD 400. Note, this allows the LCP 402 to autonomously set the pacing pulses to an enhanced energy level for post shock-pacing without having to have circuitry to detect a high energy shock pulse or receive a communication from ICD 400 notifying the LCP 402 that a shock will be delivered. Whether a shock pulse is actually delivered or not by the ICD 400 during the time period, the LCP 402 may temporarily deli ver pacing pulses at the enhanced energy level until the end of the time period, and then return to delivering pacing pulses at the first energy level. In some cases, the period of time may be reset each time the measured heart rate is above the upper heart rate threshold. When so provided, the LCP 402 may deliver pacing pulses at the enhanced energy level until the heart rate remains below the upper heart rate threshold for the period of time. During the period of time, it is contemplated that the pacing pulses delivered at the enhance energy level may comprise demand-pacing pacing pulses, post-shock pacing pulses, and/or anti-tachyarrhythmia- pacing (ATP) pulses, depending on what the control module deems appropriate therapy at any given time.

[0086] In some case, the LCP 402 may detect when the sensed heart rate falls at a rate that is above a threshold and/or falls below a floor heart rate. When the heart rate falls at a rate that is outside the bounds of normal physiology, or falls below a heart rate that is below what is necessary to sustain life, the LCP 402 may assume that the heart has just been shocked by the ICD 400. In response, the LCP 402 may temporarily deliver pacing pulses at the enhanced energy level until the end of a time period, and then return to delivering pacing pulses at the first energy level. This may be an alternative trigger for temporarily delivering pacing pulses at the enhanced energy level for a period of time.

[0087] In some cases, the ICD 400 may monitor the heart 410 and determine if the heart 410 is experiencing cardiac fibrillation or other condition that necessitates deliver}' of a high energy- shock therapy. This may include the detection of rapid, irregular, and/or inefficient heart contractions. In some cases, before delivering the shock therapy, the ICD 400 may communicate an ATP command to the LCP 402 to deliver anti-tachyarrhythmia-pacmg (ATP) pulses to the heart 410. In some cases, anti-tachyarrhythmia-pacmg (ATP) pulses may cause the heart 410 to return to a normal rhythm without delivering a high energy shock. The LCP 402 may receive the ATP command and deli ver the requested ATP pulses. In some cases, the energy le vel of the ATP pulses may be at an enhanced energy level. In some cases, the LCP 402 may have already detected a high heart rate and already adjusted the energy level of the delivered pulses (for a period of time) to the enhanced energy level. In other cases, the LCP 402 may adjust the energy level of the delivered pulses in response to receiving the ATP command from the ICD 400.

[0088] Figure 5 is a schematic diagram of a co-implanted subcutaneous or substernum implantable cardioverter-defibrillator (S-ICD) 500 and LCP 502, in accordance with an example of the disclosure. In Figure 5, the LCP 502 is shown fixed to the interior of the left ventricle of the heart 510. A subcutaneous or substernum implantable cardioverter-defibrillator (S-ICD) 500 is shown implanted near the heart. The illustrative subcutaneous or substernum implantable cardioverter-defibrillator (S-ICD) 500 includes a pulse generator 506 that may be implanted subcutaneous, and a lead 512 with one or more electrodes 508a-508c that extends subcutaneous or substernum (e.g. just interior of the sternum) adjacent but outside of the heart 510. The pulse generator 506 is configured to deliver a shock to the heart via one or more of the electrodes 508a- 508c. The LCP 502 may operate in a similar manner to that described above with respect to Figure 4, but with the subcutaneous or substernum implantable cardioverter-defibrillator (S-ICD) 500 configured to deliver the shock therapy rather than the transvenous ICD 400.

[0089] Figures 6A-6F are timing diagrams showing illustrative operations of an LCP under various operating conditions. As shown in Figures 6A-6F, traces for an intrinsic heart rate 602, a demand heart rate threshold 604, an ATP threshold 624, an upper limit threshold 606, a life sustaining threshold 608, an active energy level 610, and a pacing therapy 612. According to various embodiments, an LCP may monitor the intrinsic heart rate 602, and may deliver electrical pacing pulses to the heart when the intrinsic heart rate 602 is too low, when there is a sudden drop in the heart rate, and/or when intrinsic heart beats are being skipped or missed.

[0090] Turning specifically to Figure 6A, the intrinsic heart rate 602 is initially above the demand heart rate threshold 604 and below the upper limit threshold 606. In some cases, the intrinsic heart rate 602 may be an average heart rate of more than one or a set of previously recorded heart beats. The demand heart rate threshold 604 that may vary according to fluctuations in hemodynamic demand, as detected by changes in patient activity, respiration, blood temperature, body tissue temperature, etc. Although not shown in Figures 6A-6f, it is contemplated that the demand heart rate threshold 604 may be a fixed heart rate threshold (e.g., 70 bpm, 60 bpm, 50 bpm, 40 bpm, etc.).

[0091] The upper limit threshold 606 may be a threshold that may be variable (e.g. depend on fluctuations in hemodynamic demand), fixed and/or programmable. In Figure 6A, the upper limit threshold 606 is a fixed heart rate threshold. In various embodiments, the upper limit threshold 606 may be set at a rate that is above a safe heart rate for the patient, such that if the patient's heart rate rises above the upper limit threshold 606, the patient may be experiencing tachycardia and even cardiac fibrillation. Initially, the intrinsic heart rate 602 is below the upper limit threshold 606, and because the intrinsic heart rate 602 is not falling at a rate that is above a maximum decrease threshold, the active energy level 610 at which the LCP would delivers pacing pulses may be set at a first energy level.

|0092] As shown in Figure 6A, at point A, the intrinsic heart rate 602 has fallen below the demand heart rate threshold 604. In some cases, the LCP may pace the heart when the intrinsic heart rate falls below the demand heart rate threshold 604. As shown in Figure 6A, pacing therapy 612 is delivered at the first energy level until point B, where the intrinsic heart rate 602 rises above the demand heart rate threshold 604. This is an example of demand pacing, which helps ensure that the heart rate of a patient does not fall below a lower heart rate threshold such as the demand heart rate threshold 604. When performing demand pacing, the LCP may pace the heart at the lower heart rate threshold (e.g. demand heart rate threshold 604) when the intrinsic heart rate falls below the lower heart rate threshold (e.g. demand heart rate threshold 604).

[0093] After point B, the active energy level 610 is still set at the first energy level as the intrinsic heart rate 602 continues to rise. At point C, the intrinsic heart rate 602 has reached the upper limit threshold 606, at which point the LCP increases the active energy level 610 to an enhanced energy level. When demand pacing, and as shown in Figure 6A, the LCP may not deliver pacing therapy 612 to the heart at the enhanced energy level while the intrinsic heart rate 602 remains above the demand heart rate threshold 604. In certain embodiments, the active energy level 610 may be increased to the enhanced energy level for a period time. In Figure 6A, the period of time has been set for 10 minutes. During the 10 minute time period, it is contemplated that pacing therapy 612 will be delivered with pacmg pulses having the enhanced energy level. The pacing therapy 612 may be demand-pacing pacing pulses, post-shock pacing pulses, anti- tachyarrhythmia-pacmg (ATP) pulses and/or any other suitable pacing therapy depending on what is deemed appropriate pacmg therapy at any given time. As shown in Figure 6A, the intrinsic heart rate 602 falls back under the upper limit threshold 606 before the 10 minute time period has expired. The LCP then decreases the active energy level 6 0 back to the first energy level at point D, when the 10 minute time period has expired.

[0094] Turning now to Figure 6B, the intrinsic heart rate 602 is initially below the upper limit threshold 606 and the intrinsic heart rate 602 is not failing at a rate that is above a maximum decrease threshold. As a result, the LCP may set the active energy level 610 at which the LCP would deliver pacing pulses to the first energy level. The intrinsic heart rate 602 then rises and reaches the upper limit threshold 606 at point A. In response, the LCP may set the active energy level 610 to the enhanced energy level for a time period such as 10 minutes. At point B, the intrinsic heart rate 602 falls back under the upper limit threshold 606 before the 10 minutes has expired, but the active energy level 610 may remain at the enhanced energy level. At point C, the intrinsic heart rate 602 once again rises above the upper limit threshold 606. In some cases, as depicted in Figure 6B, if the intrinsic heart rate 602 rises above the upper limit threshold 606 before the 10 minute time period has expired from the previous time (point A) that the intrinsic heart rate 602 went above the upper limit threshold 606, the time period may be reset back to zero such that the active energy level 610 may remain at the enhanced energy level until 10 minutes after point C. At point D, the intrinsic heart rate 602 falls back under the upper limit threshold 606 before the reset 10 minute timer has expired, and the active energy level 610 may remain at the enhanced energy level. At point E, the intrinsic heart rate 602 again rises above the upper limit threshold 606 for a third time, and the 10 minute time period is once again reset. At point F, the intrinsic heart rate 602 falls back under the upper limit threshold 606 before the 10 minutes has expired. The active energy level 610 remains at the enhanced energy level until point G, when the 10 minute time period finally expires, in this example, the active energy level 610 remains at the enhanced energy level for 10 minutes following the last time the intrinsic heart rate 602 rises in a positive direction above the upper limit threshold 606.

[0095] At point H, the intrinsic heart rate 602 again rises to the upper limit threshold 606 and the active energy level 610 may be again increased from the first energy level to the enhanced energy level for a 10 minute time period, which might be extended as described above. At point I, the intrinsic heart rate 602 has started falling at a rate that is above the maximum decrease threshold. This may indicate that an ICD may have delivered a shock to the patient's heart. As a result, the LCP may be configured to deliver post shock pacing therapy 614 at the enhanced energy level. As shown in Figure 6B, the post shock pacing therapy 614 may be delivered until point J, where the intrinsic heart rate 602 rises above the demand heart rate threshold 604. In some cases, the post shock pacmg therapy 614 may be deli vered for a predetermined period of time or until the intrinsic heart rate 602 has stabilized. Even though the post shock pacing therapy 614 is no longer being delivered after point J, in this example, the active energy level 610 may not decrease back to the first energy level until point K, when the 10 mmute window has expired. |Ό096] Turning now to Figure 6C, the initial intrinsic heart rate 602 is below the upper limit threshold 606 and the intrinsic heart rate 602 is not falling at a rate that is above the maximum decrease threshold. As a result, the active energy level 610 may be set at a first energy level at which the LCP delivers pacing pulses. At point A, the intrinsic heart rate 602 has started falling at a rate that is above the maximum decrease threshold. In response, in this example, the active energy level 610 may instantaneously be increased to the enhanced energy level and a post shock pacing therapy 614 may be delivered using pulses at the enhanced energy level. As shown in Figure 6C, the post shock pacing therapy 614 may be delivered until point B, where the intrinsic heart rate 602 has stabilized (or after a predetermined period of time). In this example, because the intrinsic heart rate 602 did not rise above the upper limit threshold 606 before it started falling at a rate above the maximum decrease threshold, the active energy level 610 may be decreased back to the first energy level once the intrinsic heart rate 602 has stabilized (or after a predetermined period of time). However, it is contemplated that the active energy level 610 may remain at the enhanced energy level for a period of time.

At point C, the intrinsic heart rate 602 once again begins falling at a rate that is above the maximum decrease threshold and continues to fall below the demand heart rate threshold 604 and the life sustaining threshold 608. In this case, the active energy level 610 may once again be instantaneously increased to the enhanced energy level and post shock pacing therapy 614 ma ^¬ be delivered until point D, where the intrinsic heart rate 602 rises above the demand heart rate threshold 604. In some cases, the post shock pacing therapy 614 may be delivered for a predetermined period of time or until the intrinsic heart rate 602 has stabilized. Once again, in this example, because the intrinsic heart rate 602 did not rise above the upper limit threshold 606 before it started failing at a rate above the maximum decrease threshold, the active energy level 610 may be decreased back to the first energy level once the intrinsic heart rate 602 has stabilized (or after a predetermined period of time). However, it is contemplated that the active energy level 610 may remain at the enhanced energy level for a period of time.

[0097] At point E, the intrinsic heart rate 602 has fallen below the demand heart rate threshold 604. In this example, demand pacing therapy 612 is delivered at the first energy level until point F, where the intrinsic heart rate 602 rises above the demand heart rate threshold 604. After point F, the active energy level 610 is still set at the first energy level as the intrinsic heart rate 602 continues to rise. Turning now to Figure 6D, the initial intrinsic heart rate 602 is below the upper limit threshold 606 and the intrinsic heart rate 602 is not falling at a rate that is above the maximum decrease threshold. As a result, the active energy level 610 may be set at a first energy level at which the LCP delivers pacing pulses. As shown in Figure 6D, the intrinsic heart rate 602 rises to the upper limit threshold 606 at point A. In response, and in this example, the active energy level 610 may be increased to the enhanced energy level for a 10 minute time period. At point B, the 10 minute time period has expired and the intrinsic heart rate 602 remains above the upper limit threshold 606. In this example, the active energy level 610 may decrease back to the first energy level even though the intrinsic heart rate 602 has not yet fallen below the upper limit threshold 606. At point C, the intrinsic heart rate 602 falls below the upper limit threshold 606. At point D, the intrinsic heart rate 602 once again rises to the upper limit threshold 606 and the active energy level 610 may once again be increased to the enhanced energy level for the 10 minute time period. At point E, the intrinsic heart rate 602 starts falling at a rate that is above the maximum decrease threshold. This may indicate that an ICD may have delivered a shock to the patient's heart. As a result, the post shock pacing therapy 614 may be delivered at the enhanced energy level. As shown in Figure 6D, the post shock pacing therapy 614 may be delivered until point F, where the intrinsic heart rate 602 rises above the demand heart rate threshold 604. In some cases, the post shock pacing therapy 614 may be delivered for a predetermined period of time or until the intrinsic heart rate 602 has stabilized. The active energy level 61 0 may not decrease back to the first energy level until point G, when the 10 minute time period has expired.

[0098] Turning now to Figure 6E, the initial intrinsic heart rate 602 is below the upper limit threshold 606 and the intrinsic heart rate 602 is not falling at a rate that is above the maximum decrease threshold. As a result, the active energy level 610 may be set at a first energy level at which the LCP delivers pacing pulses. As shown in Figure 6E, the intrinsic heart rate 602 rises to the upper limit threshold 606 at point A. In response, the active energy level 610 may be increased to the enhanced energy level for a 10 minute time period as shown. At point B, the 10 minute time period has expired and the intrinsic heart rate 602 remains above the upper limit threshold 606. In this example, the active energy level 610 remains at the enhanced energy level even though the 10 time period has expired. At point C, the intrinsic heart rate fails below the upper limit threshold 606 and the active energy level 610 is now decreased back to the first energy level. At point D, the intrinsic heart rate 602 once again rises to the upper limit threshold 606 and the active energy level 6 0 may once again be increased to the enhanced energy level for another 10 minute time period. At point E, the intrinsic heart rate 602 starts falling at a rate that is above the maximum decrease threshold. This may indicate that an ICD may have delivered a shock to the patient's heart. As a result, post shock pacing therapy 614 may be delivered at the enhanced energy level. As shown in Figure 6E, the pacing therapy 612 may be delivered until point F, where the intrinsic heart rate 602 rises above the demand heart rate threshold 604. In some cases, the post shock pacing therapy 614 may be delivered for a predetermined period of time or until the intrinsic heart rate 602 has stabilized. The active energy level 610 may not decrease back to the first energy level until point G, when the 10 minute time period has expired.

[0099] Turning now to Figure 6F, the intrinsic heart rate 602 is initially below the upper limit threshold 606 and the intrinsic heart rate 602 is not falling at a rate that is above a maximum decrease threshold. As a result, the LCP may set the active energy level 610 at which the LCP would deliver pacing pulses to the first energy level. The intrinsic heart rate then falls below the demand heart rate threshold 604 at point A. As shown in Figure 6F, demand pacing therapy 612 is delivered at the first energy level until point B, where the intrinsic heart rate 602 rises above the demand heart rate threshold 604. This is an example of demand pacing, which helps ensure that the heart rate of a patient does not fall below a lower heart rate threshold such as the demand heart rate threshold 604. When performing demand pacing, the LCP may pace the heart at the lower heart rate threshold (e.g. demand heart rate threshold 604) when the intrinsic heart rate falls below the lower heart rate threshold (e.g. demand heart rate threshold 604).

[0100] After point B, the active energy level 610 is still set at the first energy level as the intrinsic heart rate 602 continues to rise. At point C, the intrinsic heart rate 602 has reached the ATP threshold 624. As shown in Figure 6F, ATP therapy 616 is delivered at the first energy level until point D, where the intrinsic heart rate 602 falls below the ATP threshold 624. This is an example of ATP therapy that was effective at terminating a tachyarrhythmia, which helps ensure that the heart rate of the patient does not rise abo ve a higher heart rate threshold such as the ATP threshold 624. When performing ATP therapy, the LCP may pace the heart with a burst of pulses.

[0101] Turning now to Figure 6G, the intrinsic heart rate 602 is initially below the upper limit threshold 606 and the intrinsic heart rate 602 is not falling at a rate that is above a maximum decrease threshold. As a result, the LCP may set the active energy level 610 at which the LCP would deliver pacing pulses to the first energy level. The intrinsic heart rate then falls below the demand heart rate threshold 604 at point A. As shown in Figure 6F, demand pacing therapy 612 is delivered at the first energy level until point B, where the intrinsic heart rate 602 rises above the demand heart rate threshold 604. After point B, the active energy level 610 is still set at the first energy level as the intrinsic heart rate 602 continues to rise. At point C, the intrinsic heart rate 602 has reached the ATP threshold 624 and ATP therapy 616 is delivered. In some cases, the ATP therapy 616 may not be capable of terminating the tachyarrhythmia and bringing the intrinsic heart rate 602 below the ATP threshold 624. In some cases, as shown in Figure 6G, the intrinsic heart rate 602 may continue to rise and at point D, reach the upper limit threshold 606. At point D, the LCP may stop delivering the ATP therapy 616 at the first energy level and the active energy level 610 may be increased to the enhanced energy level for a 10 minute time period, which might be extended as described above. At point E, the intrinsic heart rate 602 starts falling at a rate that is above the maximum decrease threshold. This may indicate that an ICD has delivered a shock to the patient's heart. As a result, the LCP may be configured to deliver post shock pacing therapy 614 at the enhanced energy level. As shown in Figure 6G, the post shock pacing therapy 6 4 may be delivered until point F, where the intrinsic heart rate 602 rises above the demand heart rate threshold 604. In response, and in some cases, post shock pacing therapy 614 may be delivered for a predetermined period of time or until the intrinsic heart rate 602 has s tabilized. Even though the post shock pacing therapy 614 is no longer being delivered after point F, in this example, the active energy level 610 may not decrease back to the first energy level until point G, when the 10 minute window has expired.

[0102] It should be understood that this disclosure is, in many respects, only illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of steps without exceeding the scope of the disclosure. This may include, to the extent that it is appropriate, the use of any of the features of one example embodiment being used in other embodiments.