IMPLANTABLE MEDICAL DEVICE

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Application Serial No. 62/239,025 filed on October 8, 2015, the disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure generally relates to systems, devices, and methods for adjusting a pacing rate of an implantable medical device such as in a rate responsive manner, and more particularly to systems, devices, and methods for limiting the duration of elevated pacing rates in an implantable medical device.

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'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.) can be implanted in a patient'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, such devices may have a rate response capability that changes the pacing rate based on the current metabolic demand of the patient. In some cases, the devices may be part of an implantable medical device system.

SUMMARY

The present disclosure generally relates to systems, devices, and methods for adjusting a pacing rate of an implantable medical device such as in a rate responsive manner, and more particularly to systems, devices, and methods for limiting the duration of elevated pacing rates in an implantable medical device.

In a first illustrative embodiment, a Leadless Cardiac Pacemaker (LCP) may comprise an outer housing, a plurality of exposed electrodes connected relative to the outer housing, and a controller disposed within the outer housing and operatively connected to the plurality of exposed electrodes. The controller may be configured to: deliver pacing pulses via the plurality of exposed electrodes at a base pacing rate; detect a measure of metabolic demand, wherein the measure of metabolic demand varies over time; deliver pacing pulses at an elevated pacing rate relative to the base pacing rate after determining an elevated metabolic demand with respect to a base measure of metabolic demand, wherein the elevated pacing rate varies with time and is dependent on the detected measure of metabolic demand; change a heart stress tracking value during times when pacing pulses are delivered at the elevated pacing rate, wherein the heart stress tracking value is changed faster during times when higher elevated pacing rates are delivered relative to times when lower elevated pacing rates are delivered; and determine if the heart stress tracking value crosses a predetermined heart stress threshold, and if so, reduce the elevated pacing rate back toward the base pacing rate.

Additionally, or alternatively, in any of the above embodiments with respect to the first illustrative embodiment, the controller may be configured to increase the heart stress tracking value monotonically over time.

Additionally, or alternatively, in any of the above embodiments with respect to the first illustrative embodiment, the controller may be configured to decrease the heart stress tracking value monotonically over time.

Additionally, or alternatively, in any of the above embodiments with respect to the first illustrative embodiment, if the measure of metabolic demand is no longer elevated with respect to the base measure of metabolic demand, the controller may be configured to deliver pacing pulses at the base pacing rate.

Additionally, or alternatively, in any of the above embodiments with respect to the first illustrative embodiment, the controller may be configured to reset the heart stress tracking value if the pacing pulses are no longer being delivered at the elevated pacing rate for at least a predetermined length of time.

Additionally, or alternatively, in any of the above embodiments with respect to the first illustrative embodiment, the controller may be further configured to pause changing the heart stress tracking value if the elevated pacing rate falls below a threshold pacing rate that is higher than the base pacing rate.

Additionally, or alternatively, in any of the above embodiments with respect to the first illustrative embodiment, the LCP may further comprise an accelerometer disposed within the outer housing, and wherein the controller may be configured to detect the measure of metabolic demand by monitoring an output of the accelerometer.

Additionally, or alternatively, in any of the above embodiments with respect to the first illustrative embodiment, the controller may be configured to detect the measure of metabolic demand by monitoring an impedance signal via the plurality of exposed electrodes.

Additionally, or alternatively, in any of the above embodiments with respect to the first illustrative embodiment, the impedance signal provides a measure of respiration rate.

Additionally, or alternatively, in any of the above embodiments with respect to the first illustrative embodiment, the controller may be configured to determine if the heart stress tracking value crosses the predetermined heart stress threshold, and if so, reduce the elevated pacing rate to the base pacing rate.

Additionally, or alternatively, in any of the above embodiments with respect to the first illustrative embodiment, the base pacing rate may be a lower rate limit.

Additionally, or alternatively, in any of the above embodiments with respect to the first illustrative embodiment, a counter may be used to change the heart stress tracking value during times when pacing pulses are delivered at the elevated pacing rate.

Additionally, or alternatively, in any of the above embodiments with respect to the first illustrative embodiment, a counter may be incremented at a faster rate during times when higher elevated pacing rates are delivered relative to times when lower elevated pacing rates are delivered.

in a second illustrative embodiment, a method for adjusting a pacing rate of a medical device may comprise: generating, by the medical device, pacing pulses at a pacing rate that vanes over time based on a determined metabolic demand; adjusting, by the medical device, a heart stress tracking value while generating pacing pulses at or above a first pacing rate, wherein a rate of adjustment of the heart stress tracking value is dependent on a current pacing rate; determining, by the medical device, when the heart stress tracking value crosses a predetermined heart stress threshold; and after determining that the heart stress tracking value crossed the predetermined heart stress threshold, generating, by the medical device, pacing pulses at a second pacing rate, wherein the second pacing rate is lower than the first pacing rate.

Additionally, or alternatively, in any of the above embodiments with respect to the second illustrative embodiment, the method may further comprise resetting the heart stress tracking value, by the medical device, after generating pacing pulses at pacing rates below a pacing rate threshold that is below the first pacing rate for a predetermined amount of time.

In a third illustrative embodiment, an implantable medical device for generating pacing pulses to a heart of a patient may comprise a plurality of electrodes for delivering pacing pulses to the heart of the patient, a pulse generator operatively coupled to the plurality of electrodes, and a controller operatively coupled to the pulse generator. The pulse generator may be configured to generate pacing pulses at a current pacing rate and deliver the pacing pulses to the plurality of electrodes. The controller may comprise a pacing rate circuit configured to determine the current pacing rate for the pulse generator and a watchdog circuit. The watchdog circuit may be configured to: monitor the current pacing rate over time, and if the current pacing rate rises above a predetermined pacing rate threshold at a start time and remains above the predetermined pacing rate threshold for a predetermined threshold amount of time, force the current pacing rate of the pulse generator to be at a base pacing rate, and if the current pacing rate falls below the predetermined pacing rate threshold before the predetermined threshold amount of time expires, and remains below the predetermined pacing rate threshold for at least a threshold amount of time, reset the start time to a next time the current pacing rate rises above the predetermined pacing rate threshold.

Additionally, or alternatively, in any of the above embodiments with respect to the third illustrative embodiment, after forcing the current pacing rate of the pulse generator to be at the base pacing rate, the watchdog circuit may be configured to maintain the current pacing rate at the base pacing rate for at least a predetermined rest period of time.

Additionally, or alternatively, in any of the above embodiments with respect to the second illustrative embodiment, the predetermined threshold amount of time may be dependent on the current pacing rate while the current pacing rate remains above the predetermined pacing rate threshold.

Additionally, or alternatively, in any of the above embodiments with respect to the second illustrative embodiment, the pacing rate circuit may determine the current pacing rate in a rate responsive manner.

Additionally, or alternatively, in any of the above embodiments with respect to the second illustrative embodiment, the implantable medical device may be a leadless cardiac pacemaker configured to be implanted on or within a heart. The above summary is not intended to describe each embodiment or every implementation of the present disclosure. Advantages and attainments, together with a more complete understanding of the disclosure, will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may be more completely understood in consideration of the following description of various illustrative embodiments in connection with the accompanying drawings, in which:

FIG. 1 is a schematic block diagram of an illustrative leadless cardiac pacemaker (LCP) according to one example 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 block diagram of an example of a controller circuit for implementing a watchdog function, according to one aspect of the present disclosure;

FIG. 6 depicts a graph of an illustrative metabolic demand of a patient and a graph of a corresponding pacing rate;

FIG. 7 is block diagram of an illustrative circuit for incrementing a counter at different rates;

FIG. 8 depicts a graph of a current pacing rate versus time and a corresponding graph of how a heart stress tracking value may modified based on the current pacing rate, in accordance with aspects of the present disclosure;

FIG. 9 depicts a graph of a current pacing rate versus time and a corresponding graph of a heart stress tracking value and how the current pacing rate may be modified based on the heart stress tracking value, in accordance with aspects of the present disclosure; FIG. 10 depicts another graph of a current pacing rate versus time and a corresponding graph of a heart stress tracking value and how the current pacing rate may he modified based on the heart stress tracking value, in accordance with aspects of the present disclosure;

FIG. 11 depicts another graph of a current pacing rate versus time and a corresponding graph of how a heart stress tracking value may be modified based on the current pacing rate, in accordance with aspects of the present disclosure;

FIG. 12 depicts a block diagram of another example circuit for implementing a watchdog function, according to one aspect of the present disclosure; and

FIG. 13 depicts a block diagram of yet another example circuit for implementing a watchdog function, according to one aspect of the present disclosure.

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 i s 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.

FIG. 1 is a conceptual schematic block diagram of an exemplar}' leadless cardiac pacemaker (LCP) that may be implanted on the heart or within a chamber of the heart and may operate to sense physiological signals and parameters and deliver one or more types of electrical stimulation therapy to the heart of the patient. Example electrical stimulation therapy may include 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. In some instances, 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 1 14 may generally conduct electrical signals to and from LCP 100 and the surrounding tissue and/or blood. Such electrical signals can include communication signals, 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 1 14 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, 04, 106, 108, and 1 10. In embodiments where electrodes 1 14 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 1 14 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 1 00 may include electrodes 114' . Electrodes 114' may be in addition to electrodes 1 14, or may replace one or more of electrodes 1 14. Electrodes 114' may be similar to electrodes 114 except that electrodes 1 14' 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 signals and/or electrical stimulation pulses, and/or may sense intrinsic cardiac electrical signals, communication signals, 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 spacmgs. 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 1 14 and/or 1 14' 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 1 14' that extends away from the outer surface of the housing 120. In some instances, at least some of electrodes 1 14 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 signals, 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 signals may be limited to subthreshold signals that do not result in capture of the heart yet still convey information. The communication signals may be delivered to another device that is located either external or internal to the patient's body. In some instances, the communication may take the form of distinct communication pulses separated by various amounts of time. In some of these cases, the timing between successive pulses may convey information. Communication module 102 may additionally be configured to sense for communication signals delivered by other devices, which may be located external or internal to the patient'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 signals 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, accelerometer signals, 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 be communicating with other devices via conducted signals, radio frequency (RF) signals, optical signals, acoustic signals, inductive coupling, and/or any other suitable communication methodology. Where communication module 102 generates electrical communication signals, communication module 102 may include one or more capacitor elements and/or other charge storage devices to aid in generating and delivering communication signals. In the embodiment shown, communication module 02 may use energy stored in energy storage module 1 12 to generate the communication signals. In at least some examples, communication module 102 may include a switching circuit that is connected to energy storage module 112 and, with the switching circuitry, may connect energy storage module 1 12 to one or more of electrodes 1 14/114' to generate the communication signals.

As shown in FIG. 1, a pulse generator module 104 may be electrically connected to one or more of electrodes 114 and/or 1 14'. 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 1 14 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 some of these cases, LCP 100 may vary the rate at which pulse generator 104 generates the electrical stimulation pulses, for example in rate responsive pacing. In other embodiments, the electricai stimulation pulses may include defibrillation/cardioversion pulses for shocking the heart out of fibrillation or into a normal heart rhythm. In yet other embodiments, the electrical stimulation pulses may include anti-tachycardia pacing (ATP) o pulses. It should be understood that 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 at least some embodiments, pulse generator module 104 may use energy stored in energy storage module 112 to generate the electrical stimulation pulses. In some particular embodiments, pulse generator module 104 may include a switching circuit that is connected to energy storage module 112 and may connect energy storage module 112 to one or more of electrodes 114/114' to generate electrical stimulation pulses.

LCP 100 may further include an electrical sensing module 106 and mechanical sensing module 108. Electrical sensing module 06 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 1 14 and/or 114' and electrical sensing module 106 may be configured to receive cardiac electrical signals conducted through electrodes 1 14 and/or 114' via a sensor amplifier or the like. 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, including multi-axis accelerometers such as two- or three-axis accelerometers, gyroscopes, including multi-axis gyroscopes such as two- or three-axis gyroscopes, blood pressure sensors, heart sound sensors, piezoelectric 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 1 10 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. In at least some examples, LCP 100 may only include one of electrical sensing module 106 and mechanical sensing module 108. In some cases, any combination of the processing module 1 10, electrical sensing module 106, mechanical sensing module 108, communication module 102, pulse generator module 104 and/ or energy storage module may be considered a controller of the LCP 100.

Processing module 1 10 may be configured to direct the operation of LCP 100 and may, in some embodiments, be termed a controller. 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 and other determinations such as whether LCP 100 has become dislodged. Processing module 110 may further receive information from communication module 102. In some embodiments, processing module 1 10 may additionally use such received information to determine occurrences and types of arrhythmias and/or and other determinations such as whether LCP 100 has become dislodged. In still some additional 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 deemed to be 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.

After determining an occurrence of an 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. As one example, in controlling pulse generator module 104 to deliver bradycardia pacing therapy, processing module 1 10 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 responsive pacing). For instance, processing module 1 10 may monitor one or more physiological parameters of the patient which may indicate a need for an increased heart rate (e.g. due to increased metabolic demand). Processing module 110 may then increase the rate at which pulse generator 104 generates electrical stimulation pulses.

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 04 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 1 10 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 1 10 may control pulse generator module 104 to generate the various electrical stimulation pulses with specific pulse widths and pulse amplitudes. For example, processing module 0 may cause pulse generator module 04 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 signals for communicating with other devices of a system of devices. For instance, processing module 110 may control communication module 102 to generate communication signals in particular pulse 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 signals 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 signals 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 signals and electrical stimulation pulses may influence the reception of communication signals 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 1 10 may control the switching module to connect modules 102/104 and electrodes 114/1 14' 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. In still other embodiments, processing module 1 10 may not be a single component. For example, processing module 1 10 may include multiple components positioned at disparate locations within LCP 100 in order to perform the various described functions. For example, certain functions may be performed in one component of processing module 110, while other functions are performed in a separate component of processing module 110. Processing module 1 10, in additional embodiments, may include a memory circuit and processing module 1 10 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, nonvolatile 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 capacitors or super capacitors.

To implant LCP 100 inside a patient's body, an operator (e.g., a physician, clinician, etc.), may fix LCP 100 to the cardiac tissue of the patient's heart. To facilitate fixation, LCP 100 may include one or more anchors 1 16. The one or more anchors 116 are shown schematically in FIG. 1. The one or more anchors 1 16 may include any number of fixation or anchoring mechanisms. For example, one or more anchors 116 may include one or more pins, staples, threads, screws, helix, tines, and/or the like. In some embodiments, although not shown, one or more anchors 116 may include threads on its external surface that may run along at least a partial length of an anchor member. The threads may provide friction between the cardiac tissue and the anchor to help fix the anchor member withm the cardiac tissue. In some cases, the one or more anchors 116 may include an anchor member that has a cork-screw shape that can be screwed into the cardiac tissue. In other embodiments, anchor 1 16 may include other structures such as barbs, spikes, or the like to facilitate engagement with the surrounding cardiac tissue.

In some examples, LCP 100 may be configured to be implanted on a patient's heart or within a chamber of the patient's heart. For instance, LCP 100 may be implanted within any of a left atrium, right atrium, left ventricle, or right ventricle of a patient's heart. By being implanted within a specific chamber, LCP 100 may be able to sense cardiac electrical signals originating or emanating from the specific chamber that other devices may not be able to sense with such resolution. Where LCP 100 is configured to be implanted on a patient's heart, LCP 100 may be configured to be implanted on or adjacent to one of the chambers of the heart, or on or adjacent to a path along which intrinsically generated cardiac electrical signals generally follow. In these examples, LCP 100 may also have an enhanced ability to sense localized intrinsic cardiac electrical signals and deliver localized electrical stimulation therapy.

FIG. 2 depicts an embodiment of another device, medical device (MD) 200, which may operate to sense physiological signals and 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 02, 104, 106, 108, and 110 of LCP 100. Additionally, energy storage module 218 may be similar to energy storage module 1 2 of LCP 100. However, in some embodiments, 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 lead! ess device such as shown in FIG. 1 , in some instances MD 200 may mclude leads, such as leads 212. Leads 212 may include electrical wires that conduct electrical signals between electrodes 214 and one or more modules located withm 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, withm, 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's cardiac tissue. In other cases, the one or more of the electrodes 214 may be positioned subcutaneously but adjacent the patient'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'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'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'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 (I CD). In such embodiments, MD 200 may include one or more leads implanted within a patient'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's body, and electrodes 214 may be skin-electrodes that are placed on a patient'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's body and conducted through the body to the skin). MD 200 may further be configured to deliver various types of electrical stimulation therapy, including, for example, defibrillation therapy via skin-electrodes 214.

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's body, as described previously with respect to MD 200. In at least some examples, external device 306 may represent an external support device such as a device programmer, as will be described in more detail below. Other sensors/devices 310 may be any of the devices described previously with respect to MD 200, such as ICPs, ICDs, and S CDs. Other sensors/devices 310 may also include various diagnostic sensors that gather information about the patient, such as acceierometers, 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 308. 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 RF signals, inductive coupling, optical signals, acoustic signals, or any other signals suitable for communication. Additionally, in at least some embodiments, the various devices of system 300 may communicate via pathway 308 using multiple 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.

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. a voltage and/or current waveform punctuated with current and/or voltage pulses, referred herein as electrical communication pulses) into the patient's body via one or more electrodes of a transmitting device, and may receive the conducted communication signals via one or more electrodes of a receiving device. The patient's body may "conduct" the conducted communication signals 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 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. That is, the communication pulses have an amplitude/pulse width designed to not capture the heart. In some cases, the amplitude/pulse width of the delivered electrical communication pulses may be above the capture threshold of the heart, but may be delivered during a refractory period of the heart and/or may be incorporated in or modulated onto a pacing pulse, if desired.

Additionally, unlike normal eiectncal stimulation therapy pulses, the electrical communication pulses may be delivered in specific sequences which convey information to receiving devices. For instance, 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, a predefined sequence of communication pulses may represent a corresponding symbol (e.g. a logic "I" symbol, a logic "0" symbol, an ATP therapy trigger symbol, etc.). In some cases, conducted communication pulses may be voltage pulses, current pulses, biphasic voltage pulses, biphasic current pulses, or any other suitable electrical pulse as desired.

FIG. 4 depicts an illustrative medical device system 400 that may be configured to operate together. For example, system 400 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 some embodiments, the devices of system 400 may be configured to determine occurrences of dislodgment of one or more devices of system 400. 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 408a-408c. In some cases, pulse generator 406 may be part of a subcutaneous implantable cardioverter-defibrillator (SICD), and the one or more electrodes 408a-408c 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 408a-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. Medical device system 400 may also include external support device 420. External support device 420 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, or other functions involving communication with one or more devices of system 400. As one example, communication between external support device 420 and pulse generator 406 can be performed via a wireless mode, and communication between pulse generator 406 and LCP 402 can be performed via a conducted communication mode. In some embodiments, communication between LCP 402 and external support device 420 is accomplished by sending communication information through pulse generator 406. However, in other embodiments, communication between the LCP 402 and external support device 420 may be via a communication module.

FIG. 4 only illustrates one example embodiment of a medical device system 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, with at least one LCP capable of delivering defibrillation therapy. Still another example 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 FIG. 4. Accordingly, it should be recognized that numerous other medical device systems, different from system 400 depicted in FIG. 4, may be operated in accordance with techniques disclosed herein. As such, the embodiment shown in FIG. 4 should not be viewed as limiting in any way.

As mentioned LCP 100 and/or MD 200 may be configured to deliver electrical stimulation therapy to a heart of a patient. For instance, LCP 100 and/or MD 200 may be configured to generate pacing pulses and deliver those pacing pulses to the heart of the patient in order to pace the heart. Additionally, LCP 100 and/or MD 200 may be configured to adjust the rate of pacing pulse generation and delivery in a rate responsive manner in order to increase or decrease the cardiac output based on determined factors relating to metabolic demand. In some cases, it may be beneficial to limit the amount of time LCP 100 and/or MD 200 generate and deliver pacing pulses above a certain pacing rate in order to reduce a stress level of the heart and give the heart a chance to rest.

FIG. 5 depicts an example controller circuit 500 of a medical device, such as LCP 100 or MD 200. The illustrative controller circuit 500 includes rate determination module (RDM) 502 connected to watchdog module 504. Watchdog module 504 is further connected to pulse generator module 506.

In some embodiments, controller circuit 500 may be part of a medical device that is configured to deliver pacing pulses to a heart of a patient in order to control a heart rate. The medical device may be further configured to deliver the pacing pulses in a rate responsive manner, increasing or decreasing the rate of delivery of pacing pulses based on one or more sensed signals and/or determined metrics. For instance, RDM 502 may represent a circuit that outputs a current pacing rate. RDM 502 may receive one or more signals sensed from one or more sensors and/or determine one or more metrics based on the received signals. Some example signals that RDM 502 may receive and/or use in determining a current pacing rate may include transthoracic impedance, temperature, and aecelerometer signals. Some example parameters that RDM 502 may determine based on such signal(s) include a respiration rate (using transthoracic impedance signal), a blood temperature (using the blood temperature signal), and an activity level and/or posture of the patient (using the aecelerometer signal). Of course, in other embodiments, a different circuit (e.g. SICD) can receive these or other signals and/or determine such parameters and then communicate the signals and/or parameters to RDM 502.

Using the received signals and/or determined parameters, RDM 502 may determine a current pacing rate. The received signals and/or determined parameters may represent measures of metabolic demand of the patient. For example, an elevated respiration rate, an elevated blood temperature, and/or an elevated activity level, all in comparison to individual baseline levels, may indicate that the patient needs increased cardiac output, which can be achieved by an increased pacing rate. In additional, or alternative embodiments, certain chemical markers or levels, body position, and/or pacing rate of another implanted device may all further be measures of metabolic demand. LCP 100 may sense any number or all of these measures, or may receive indications of these measures from another device in order for RDM 502 to determine a pacing rate. However, in still other embodiments, LCP 100 may not be the device that determines a pacing rate. For instance, another device may sense one or more measures related to metabolic demand, or receive such measures from still another device, and determine a pacing rate based on the sensed or received measures. In these embodiments, this other device may then communicate the pacing rate to LCP 100, which uses the received pacing rate in conjunction with the presently disclosed watchdog techniques to deliver pacing pulses to the heart.

In some embodiments, RDM 502 may determine a pacing rate based on only one of these measures of metabolic demand. Alternatively, RDM 502 may determine a pacing rate based individually on each of two or more of these measures of metabolic demand, and determine the current pacing rate as the highest pacing rate determined based on the individual measures. Alternatively, RDM 502 may blend each of the individual measures into an overall measure of metabolic demand and determined the current pacing rate based on the overall measure.

RDM 502 may additionally enforce a lower rate limit (LRL), representing a minimum pacing rate. Regardless of the measures of metabolic demand, the LRL may be the lowest rate at which the medical device is configured to deliver pacing pulses in order to ensure a safe base level of cardiac output for the patient.

Accordingly, RDM 502 may vary the current pacing rate over time based on one or more measures of metabolic demand, such as depicted in the example shown in FIG. 6. FIG. 6 depicts a graph 510 of metabolic demand 514 versus time along with a graph 512 of a current pacing rate 516 as generated by RDM 502, in some embodiments, the medical device may use thresholding to determine the current pacing rate based on the sensed or determined metabolic demand 514. For instance, the medical device may have three thresholds, thresholds 522, 524, and 526 which may represent different levels of metabolic demand, and which may drive changes in the current pacing rate 516. As can be seen in FIG. 6, threshold 528 of current pacing rate 516 may- represent an LRL, and the medical device may not output a current pacing rate lower than the LRL. Accordingly, where metabolic demand 514 is below demand threshold 522, the medical device may maintain the current pacing rate at the LRL. Once metabolic demand 514 crosses demand threshold 522, the medical device may determine that there is a need for increased cardiac output and may thus increase the current pacing rate 516, as can be seen between times 511 and 513. Additionally, as metabolic demand 514 crosses demand thresholds 524 and 526, the medical device may again increase the current pacing rate 516, as seen between times 513, 515 and between times 515, 517. Further, as metabolic demand drops below demand thresholds 526, 524, and 522, the medical device may decrease the current pacing rate 516, as seen between times 517, 519, between times 519, 521, and after time 521.

It should be understood that FIG. 6 shows only one example by which the medical device may adjust the pacing rate over time. For instance, the medical device may use a continuous function based on metabolic demand 514 to determine the current pacing rate 516, rather than the thresholding approach shown in FIG. 6. Additionally, although FIG. 6 depicts the current pacing rate 516 transitioning over time between pacing rates, in other embodiments the current pacing rate 516 may be abruptly changed between different pacing rates if desired.

As RDM 502 determines the current pacing rate, RDM 502 may communicate the current pacing rate to watchdog module 504 (see FIG. 5). In general, watchdog module 504 may be configured to monitor the received current pacing rate and determine when the current pacing rate is elevated above a baseline pacing rate like the LRL. Watchdog module 504 may be further configured to prevent the current pacing rate from staying at an elevated rate for a prolonged period of time, as this may cause undue stress on the heart.

In order to determine a relative stress level of the heart and to determine when the current pacing rate has been at an elevated rate for too long a period of time, watchdog module 504 may be configured to determine a heart stress tracking value (HSTV). In at least some embodiments, which will be discussed in more detail below, the HSTV may be dependent at least in part on the current pacing rate.

Watchdog module 504 may be implemented with a base pacing rate. In some embodiments, the base pacing rate may be the LRL. However, in other embodiments, the base pacing rate may be higher than the base pacing rate. As some examples, the LRL may be set at around 60 pulses per minute (could be patient dependent). In embodiments where the base pacing rate is higher than the LRL, the base pacing rate may be set at about 65 pulses per minute or 70 pulses per minute. However, these are just some examples. This may allow some variation in the pacing rate without trigging the watchdog feature.

Watchdog module 504 may determine when the current pacing rate is equal to or above the base pacing rate, indicating that the current pacing rate is at an elevated pacing rate. Once watchdog module 504 determines that the current pacing rate is at an elevated pacing rate, watchdog module 504 may begin to modify the HSTV. While the current pacing rate stays above the base pacing rate, watchdog module 504 may continue to modify the HSTV and determine if and when the HSTV crosses an HSTV threshold. When the HSTV crosses the HSTV threshold, watchdog module 504 may be configured to change the current pacing rate to a rest pacing rate, and maintain the current pacing at the rest pacing rate for a predetermined period of time. Examples of these operations will be described in more detail below.

One example HSTV that watchdog module 504 may implement is a simple counter that may count up or count down in a monotonic fashion. In these embodiments, once watchdog module 504 determines that the current pacing rate is at or above the base pacing rate, watchdog module 504 may begin counting up or counting down, depending on the implementation. For instance, watchdog module 504 may include a clock, or use a clock signal from another portion of the medical device. Once every clock cycle, watchdog module 504 may increment or decrement a counter. When the counter reaches a predetermined threshold, watchdog module 504 may implement the rest pacing rate. When the watchdog module 504 is incrementing a counter from zero, the predetermined threshold may be some positive value. Where watchdog module 504 is decrementing a counter, the initial value may be a positive value and the predetermined threshold may be zero. In some cases, it is contemplated that the positive value may be programmable in either implantation.

In some instances, the rate at which watchdog module 504 increments or decrements the counter may be dependent at least in part on the pacing rate. For example, FIG. 7 depicts an illustrative circuit 530 that may be included in watchdog module 504. Circuit 530 includes a clock module 532, a counter 534, and a frequency modifier module 536. Clock module 532 may function similarly to clocks in other electrical devices by operating at a particular frequency to drive operation of the device or circuit. Counter 534, when the current pacing rate is equal to or above the base pacing rate, may increment or decrement the counter (e.g. an HSTV value) each clock cycle as determined by the operating frequency of clock module 532.

Frequency modifier module 536 may be configured to modify the frequency of clock module 532 based on the current pacing rate. FIG. 8 illustrates one example implementation of how frequency modifier module 536 may operate. FIG. 8 depicts a current pacing rate 540 versus time along with a corresponding HSTV 542. In the example of FIG. 8, the base pacing rate may be equal to LRL 544. For instance, once current pacing rate 540 increases above LRL 544, such as at time 541, HSTV 542 begins to increase. As can be seen in FIG. 8, while current pacing rate 540 is at or below a first value and above LRL 544, as in region 546 between time 541 and 543, HSTV 542 increases at a first rate. Once current pacing rate 540 increases above the first value, such as at time 543, and while current pacing rate 540 is within in region 548 between times 543 and 545, HSTV 542 increases at a second rate that is greater than the first rate. Additionally, as seen in FIG. 8, when current pacing rate 540 falls back below the first value, such as at time 545, HSTV 542 may return to increasing at the first rate. HSTV 542 may maintain tins first rate while current pacing rate 540 is between LRL 544 and the first value, such as between times 545 and 547. After time 547, where current pacing rate 540 fails back to LRL 544, HSTV 542 may simply maintain its value.

The increase in the rate of increase of HSTV 542 while current pacing rate 540 is above the first value may be effectuated by frequency modifier module 536. For instance, frequency modifier module 536 may modify the frequency of operation of clock module 532, such as by increasing the frequency. Accordingly, this increased frequency of operation of clock module 532 also increases the rate at which counter 534 increments a counter value (e.g. an HSTV value) - thereby increasing HSTV 542 at a faster rate. In some embodiments, frequency modifier module 536 may double the frequency. Additionally, other embodiments may have additional threshold levels where frequency modifier module 536 modifies the frequency. For example, some embodiments may implement another threshold level above the first value of FIG. 8 where frequency modifier module 536 again increases the frequency of operation of clock module 532. In some of these embodiments, frequency modifier module 536 may again double the frequency of operation. However, it should be understood these are only examples. In general, frequency modifier module 536 may be configured to modify the frequency of operation of clock module 532 in any manner using any amount of thresholds or on a continuous basis.

Another implementation of an HSTV may be a heartbeat or pacing pulse counter. For instance, once current pacing rate 540 increases above the base pacing rate, and while current pace rate 540 stays above the base pacing rate, watchdog module 504 may begin counting each heart beat or delivered pacing pulse. In some instances, watchdog module 504 may receive detected heart beats from another portion of the medical device that is directly connected to electrodes of the medical device and that is able to sense cardiac electrical signals. Alternatively, watchdog module 504 may receive an indication when a pacing pulse is generated. In these embodiments, watchdog module 504 may count the heartbeats or the pacing pulses in a counter and compare the counter to a heartbeat or pacmg pulse threshold (e.g. HSTV threshold). Once the counter reaches the threshold, watchdog module 504 may modify the current pacing rate to be the rest pacing rate. In these embodiments, as the pacing rate increases, the counter naturally begins accumulating faster as more heartbeats occur, or pacing pulses are generated, per minute.

In still other embodiments, watchdog module 504 may determine the integral of pacing rate 540 above the base pacing rate as the HSTV. The integral of the pacing rate 540 may then be compared to an integral threshold value, and if it exceeds the integral threshold value (e.g. HSTV threshold), the watchdog module 504 may modify the current pacing rate to be the rest pacing rate.

As mentioned, once watchdog module 504 has determined that the HSTV has reached the HSTV threshold, watchdog module 504 may implement a rest pacing period. FIG. 9 illustrates current pacing rate 560 along with HSTV 562 where watchdog module 504 implements a rest pacing period. Similar to FIG. 8, when current pacing rate 560 increase above base pacing rate 564, HSTV 562 may begin to increase at a first rate, for instance at time 561. Also, if current pacing rate 560 increases above a first level, HSTV 562 may increase at a second rate that is greater than the first rate, such as at time 563. While current pacing rate 560 is above base pacing rate 564, watchdog module 504 may be comparing HSTV 562 to an HSTV threshold 566.

While HSTV 562 is below HSTV threshold 566, watchdog module 504 may pass through the current pacing rate 560 to pulse generator module 506. Pulse generator module 506 may be configured to generate pacing pulses at the current pacing rate and to deliver those generated pacing pulses to the heart of the patient.

If and/or when watchdog module 504 determines that HSTV 562 reaches HSTV threshold 566, such as at time 565, watchdog module 504 may implement a rest pacing period. Instead of simply passing through the current pacing rate 560 to the pulse generator module 506, watchdog module 504 may modify the current pacing rate 560 before passing the current pacing rate 560 through to the pulse generator module 506. For instance, watchdog module 504 may adjust the current pacmg rate 560 to be equal to a rest pacing rate. In some embodiments, the rest pacing rate may be equal to the base pacing rate 564. Base pacing rate 564 may be equal to an LRL of the medical device, but this is not necessary in all embodiments. In other embodiments, instead of adjusting the current pacing rate 560 to be equal to the base pacing rate 564, watchdog module 504 may adjust the current pacing rate 560 to be equal to a pacing rate 567, which is higher than base pacing rate 564 but still allows the heart to have a rest period.

Watchdog module 504 may maintain this adjustment to the current pacing rate 560 for a predetermined amount of time. In some embodiments, the predetermined amount of time may be between about fifteen minutes and about four hours and fifteen minutes, although this is just one example. In more specific embodiments, the predetermined amount of time may be about fifteen minutes, about thirty minutes, about forty-five minutes, about one hour, about one hour and fifteen minutes, about one hour and thirty minutes, about one hour and forty-five minutes, about two hours minutes, about two hours and fifteen minutes, about two hours and thirty minutes, about two hours and forty-five minutes, about three hours, about three hours and fifteen minutes, about three hours and thirty minutes, about three hours and forty-five minutes, about four hours, about four hours and fifteen minutes, or any other suitable amount of time. In some cases, the predetermined amount of time may be programmable, such as by a physician.

In some cases, watchdog module 504 may pause and/or reset the HSTV under certain conditions. FIG. 10 depicts one example of how watchdog module 504 may operate after the current pacing rate falls back to the base pacing rate before the HSTV has crossed the HSTV threshold. FIG. 10 depicts current pacing rate 570 versus time along with the corresponding HSTV 572. Once current pacing rate 570 rises above the base pacing rate 574, as at time 571 , watchdog module 504 may begin to increment HSTV 572 at a first rate as shown. However, before HSTV 572 reaches HSTV threshold 578, current pacing rate 570 falls back to base pacing rate 574 at time 573. While current pacing rate 570 stays at or below base pacing rate 574, watchdog module 504 may not further increment HSTV 572. Instead, watchdog module 504 may begin tracking a period of time. If the period of time expires before the current pacing rate 570 again increases above base pacing rate 574, watchdog module 504 may clear or reset HSTV 572. In the example of FIG. 10, before the period of time expires, current pacing rate 570 increases above base pacing rate 574 again at time 575. Watchdog module 504 then resumes incrementing HSTV 572 as shown. Eventually, HSTV 572 reaches HSTV threshold 578 and watchdog module 504 begins a rest pacing period at time 577.

Although current pacing rate 570 is shown falling back to base pacing rate 574 before watchdog module 504 pauses incrementing HSTV 572, in other embodiments, watchdog module 504 may operate differently. For instance, watchdog module 504 may pause incrementing HSTV 572 before current pacing rate 570 falls all the way back to base pacing rate 574, such as when current pacing rate 570 falls to pacing rate 579. Further variations are contemplated by this disclosure of when and how watchdog module 504 pauses incrementing HSTV 572.

FIG. 11 depicts an example where watchdog module 504 resets the HSTV. FIG. 11 depicts current pacing rate 580 versus time along with the corresponding HSTV 582. As with the example of FIG. 10, when current pacing rate 580 rises above base pacing rate 584, watchdog module 504 may begin incrementing HSTV 582, as at time 581. However, once current pacing rate 580 falls back to base pacing rate 584 before HSTV 582 reaches HSTV threshold 588, as at time 583, watchdog module 504 pauses incrementing HSTV 582. At this point, watchdog module 504 may begin tracking a period of time. Once the period of time expires, watchdog module 504 may reset HSTV 582, such as at time 585. FIG. 1 1 goes on to show that, after resetting HSTV 582 at time 585, once current pacing rate 580 again rises above base pacing rate 584 at time 587, watchdog module 504 may again begin to increment HSTV 582. Further, when current pacing rate 580 falls back to base pacing rate 584 at time 589, watchdog module 504 once again pauses incrementing HSTV 582.

In some instances, the period of time that watchdog module 504 tracks before resetting the HSTV may be dependent on the value of the HSTV in relation to the HSTV threshold. For instance, the closer the HSTV is to the HSTV threshold, the longer the period of time may be that watchdog module 504 tracks before resetting the HSTV value.

In some cases, instead of pausing incrementing the HSTV when the current pacing rate falls below a threshold, watchdog module 504 may begin decrementing the HSTV. As one example, in FIG. 10, instead of pausing incrementing HSTV 572 at time 573, watchdog module 504 may begin decrementing HSTV 572 such that the graph of HSTV 572 would have a negative slope between times 573 and 575. It should be understood that a similar approach may be applied to other embodiments of the HSTV, for instance where the HSTV is decremented down from a staring value to zero as opposed to incremented up to a threshold value. In such a case, the watchdog module 504 may begin incrementing HSTV 572 when the current pacing rate falls below a threshold.

FIGS. 12 and 13 depicts other illustrative ways in which the pacing circuit may be implemented. For instance, FIG. 12 depicts controller circuit 600 that includes a rate determination module 602, a watchdog module 604, and a pulse generator module 606. As opposed to controller circuit 500 of FIG. 5, controller circuit 600 includes watchdog module 604 connected in parallel with rate determination module 602 and pulse generator module 606. In these embodiments, instead of acting as a pass through for the current pacing rate that rate determination module 602 generates, watchdog module 604 may monitor the current pacing rate that rate determination module 602 communicates to pulse generator module 606 and may send separate control signals to the pulse generator module 606 to modify the pacing rate of pulse generator module 606, in accordance with the above disclosed techniques. Alternatively, watchdog module 604 may monitor the current pacing rate that rate determination module 602 communicates to pulse generator module 606 and may send separate control signals to the rate determination module 602 to modify the pacing rate of pulse generator module 606, in accordance with the above disclosed techniques.

In the example of FIG. 13, the rate determination module 621 , may be comprised of sub rate determination modules, such as rate determination module A 622, rate determination module B 624, and rate determination module C 626. Each of rate determination module A 622, rate determination module B 624, and rate determination module C 626 may represent a different rate drive that produces a pacing rate based on different information (e.g. using different sensor or sensor combinations to estimate the current metabolic demand). In these embodiments, watchdog module 628 may monitor the pacing rate produced by each of rate determination module A 622, rate determination module B 624, and rate determination module C 626 and may pass through the highest determined pacing rate to rate determination module 630. Watchdog module 628 may further act in accordance with the disclosed techniques to limit the amount of time pulse generator 630 spends generating pacing pulses at an elevated pacing rate.

In some cases, some or all of the different thresholds and time periods may be programmable. For instance, as described with respect to FIG. 4 a medical device implementing the disclosed techniques, such as LCP 100 or MD 200, may communicate with a device such as external support device 420. When so provided, a user (e.g. physician) may interact with external support device 420 to communicate with LCP 100 or medical device 200, such as to change one or more programmable features of LCP 100 or MD 200. Some example parameters that may be programmable include the lower rate limit (LRL), the base pacing rate, the rest pacing rate, the metabolic demand thresholds, the length of the rest pacing period, the length of the time period before resetting the HSTV, the rates at winch the HSTV is incremented or decremented, or any of the other thresholds or time periods disclosed herein.

In some cases, the techniques of the present disclosure may be implemented by a system of devices. For instance, a patient may be implanted with multiple devices that may be in communication with each other, such as in the example of FIG. 4. In such cases, one of the devices, for instance pulse generator 406, may determine the current pacing rate and may communicated the current pacing rate to another other device of the system, such as LCP 402, and LCP 402 may deliver pacing pulses to the heart of the patient at the received current pacing rate. In these embodiments, either device, e.g. pulse generator 406 or LCP 402, may implement the watchdog module described herein. Where pulse generator 406 implements the watchdog module, the watchdog module may modify the current pacing rate, as described herein, before pulse generator 406 communicates the current pacing rate to LCP 402. Where LCP 402 implements the watchdog module, after receiving the current pacing rate from pulse generator 406, the watchdog module of LCP 402 may modify the current pacing rate or otherwise cause an adjustment of the rate of deliver}' of pacing pulses by LCP 402 in accordance with the disclosed techniques.

Additionally, as mentioned previously, in some embodiments another device may determine the pacing rate and communicate the pacing rate to LCP 100, which implements the presently disclosed watchdog techniques. However, in still other embodiments, the other device that determines the pacing rate may further implement the presently disclosed watchdog techniques. For instance, before communicating the pacing rate to LCP 100, the other device may run the determine pacing rate through the disclosed watchdog processes to produce an processed pacing rate. This processed pacing rate, which may have been modified by the watchdog processes, is then communicated to LCP 100.

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. Although various features may have been described with respect to less than all embodiments, this disclosure contemplates that those features may be included or combined with any embodiment. Further, although the embodiments described herein may have omitted some combinations of the various described features, this disclosure contemplates embodiments that include any combination of each described feature. 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.