Patent Description:
A wide variety of implantable medical devices (IMDs) for delivering therapeutic electrical stimulation pulses to treat a medical condition have been used clinically or proposed for clinical use in patients. Examples include IMDs that deliver electrical stimulation therapy to the heart, muscle, nerve, brain, stomach or other tissue. Such IMDs employ electrodes for the delivery of therapeutic electrical signals to such organs or tissues. An IMD may include electrodes for sensing intrinsic physiological electrical signals within the patient, which may be propagated by such organs or tissue, and/or other sensors for sensing physiological signals of a patient. The electrodes used for delivering electrical stimulation pulses may be carried by a medical electrical lead extending from the IMD or carried on the housing of an IMD that encloses the electronic circuitry of the IMD.

For example, a leadless intracardiac pacemaker having housing-based electrodes may be implanted within a heart chamber for sensing cardiac electrical signals and delivering cardiac pacing pulses to detect and treat cardiac arrhythmias. A leadless intracardiac pacemaker may be miniaturized in order to facilitate implantation wholly within a single heart chamber. Other IMDs, such as neurostimulators for treating pain, incontinence, paralysis, tremor, or other neurological disorders or symptoms, may be miniaturized to reduce the complexity, invasiveness, or complications of an implantation procedure, reduce patient discomfort, or enable implantation in a limited anatomical space.

IMDs capable of delivering electrical stimulation therapy may be programmable devices capable of wireless communication with an external programming device, e.g., using radio-frequency (RF) telemetry. IMDs may transmit data about the therapy delivered, signals sensed from the patient or device diagnostic data to an external programming or monitoring device via the RF telemetry. In this way a clinician can adjust the therapy as needed or obtain additional information about the patient's condition or the IMD itself. <CIT> relates to mobile applications and methods for conveying performance information of a cardiac pacemaker. <CIT> relates to descriptive transtelephonic pacing intervals for use by an implantable pacemaker. <CIT> relates to a leadless pacemaker with conduction communication.

Further embodiments are defined in dependent claims <NUM>-<NUM>.

The techniques of this disclosure generally relate to communicating a numerical value of a monitored variable through modulation of a rate of delivered electrical stimulation pulses by an IMD. An IMD operating according to the techniques disclosed herein determines a numerical value of a monitored variable and coverts the numerical value to a sequence of modulated stimulation rate intervals that represent the numerical value. A rate monitor detects the modulated rate of activation of excitable tissue caused by the modulated stimulation rate intervals and demodulates the detected rate to determine the numerical value of the monitored variable without requiring direct detection of the stimulation pulses by the rate monitor or radio frequency or other wirelessly transmitted signals to communicate the numerical value. In some examples, the IMD is a pacemaker delivering cardiac pacing pulses at modulated pacing rate intervals and the rate monitor is a heart rate monitor.

In one example, the disclosure provides a medical device system including an IMD having a control circuit configured to determine a numerical value of a variable that is monitored by the IMD according to a monitoring protocol and convert the numerical value to a data sequence of modulated stimulation rate intervals. The IMD includes a pulse generator configured to deliver electrical stimulation pulses according to the data sequence of the modulated stimulation rate intervals to cause a modulated rate of activation of an excitable tissue of a patient corresponding to the modulated stimulation rate intervals. The modulated rate of activation is detectable by a rate monitor for demodulation of the modulated rate of activation to the numerical value of the monitored variable value. The modulated stimulation rate sequence includes a header comprising at least one modulated stimulation rate interval and the data sequence following the header. The control circuit is also configured to set the at least one modulated stimulation rate interval of the header to indicate information about the data sequence.

In another example, the disclosure provides a method, not part of the claimed invention but useful for the general understanding of the invention, including determining by an IMD a numerical value of a variable that is monitored by the IMD according to a monitoring protocol and converting the numerical value to a data sequence comprising a plurality of modulated stimulation rate intervals. The method further includes delivering electrical stimulation pulses by the IMD according to the data sequence of the modulated stimulation rate intervals to cause a modulated rate of activation by an excitable tissue of a patient corresponding to the modulated stimulation rate intervals. The modulated rate of activation is detectable by a rate monitor for demodulation of the modulated rate of activation to the numerical value of the monitored variable.

In yet another example, the disclosure provides a non-transitory, computer-readable storage medium, not part of the claimed invention but useful for the general understanding of the invention, comprising a set of instructions which, when executed by a control circuit of an IMD, cause the device to determine a numerical value of a variable that is monitored by the IMD according to a monitoring protocol and convert the numerical value to a data sequence of modulated stimulation rate intervals. The instructions further cause the IMD to deliver electrical stimulation pulses according to the data sequence of the modulated stimulation rate intervals to cause a modulated rate of activation of an excitable tissue a patient corresponding to the modulated stimulation rate intervals. The modulated rate of activation is detectable by a rate monitor for demodulation of the modulated rate of activation to the numerical value of the monitored variable.

In general, this disclosure describes an IMD system configured to monitor device- and/or patient-related variables to determine a numerical value of a monitored variable and communicate the numerical value of the monitored variable by modulating rate intervals of electrical stimulation pulses delivered by the IMD. Each electrical stimulation pulse delivered at the modulated rate intervals is intended to cause capture or activation of an excitable tissue being stimulated. As used herein, the term "excitable tissue" refers to body tissue comprising cells that can generate an action potential at its membrane in response to depolarization and may transmit an impulse along the membrane. The term "activation" refers to an electrical depolarization or subsequent mechanical contraction caused by the electrical depolarization due to delivery of an electrical stimulation pulse by the IMD. Examples of excitable tissue include skeletal muscle, smooth muscle, cardiac muscle, nerves, spinal cord and brain. A rate monitor detects the rate of depolarization or contraction of the excitable tissue caused by the modulated rate of stimulation pulses by detecting a physiological signal comprising waveforms occurring at the rate of depolarization or contraction of the excitable tissue. The rate monitor thereby detects modulated rates of activations of the excitable tissue from the physiological signal and demodulates the detected rates to determine the numerical value of the monitored variable.

In some examples of the techniques disclosed herein, the IMD is a pacemaker, cardioverter/defibrillator or other IMD capable of generating and delivering the electrical stimulation pulses as cardiac pacing pulses intended to capture the myocardium or nerves of the heart. The cardiac pacing pulses are delivered at modulated cardiac pacing rate intervals to cause depolarization and subsequent mechanical contraction of the heart at the modulated rate intervals. A heart rate monitor can detect the resulting modulated heart rate and demodulated detected heart rates to determine the numerical value of a patient-related or device-related variable monitored by the IMD.

This data communication technique including modulating the rate of electrical stimulation pulses delivered by the IMD does not require the IMD to communicate the numerical value using a radio frequency (RF) transceiver or other dedicated circuitry used for transmitting wireless communication signals. Further, the rate monitor is not required to detect the electrical stimulation pulses delivered by the IMD. The rate monitor detects a rate of activation, e.g., the electrical depolarization or the resulting mechanical contraction, of the stimulated tissue without detecting the actual stimulation pulses or any specific feature of the stimulation pulses themselves. For instance, the rate monitor may detect a heart rate from a physiological sensor signal for demodulating a modulated rate of cardiac pacing pulses without detecting the cardiac pacing pulses themselves.

As used herein, a "monitored variable" is a patient- or device-related variable that is monitored or measured by the IMD and is not set or directly controlled by the IMD, the rate monitor or any other device as a control parameter for controlling a function of the IMD or rate monitor. For example, monitored variables may include remaining battery voltage of the IMD, cardiac pacing capture threshold, electrical impedance of a cardiac pacing vector, or the resulting frequency of cardiac pacing out of a total number of sensed cardiac and paced cardiac events over a specified time period as examples. A monitored variable is not directly set or controlled by the IMD in contrast to a control parameter that is programmed or automatically set by the IMD to control IMD functions.

A "control parameter" as used herein is a parameter that is programmed into or set or adjusted by the IMD to control IMD sensing and therapy delivery functions. A sensing control parameter, for example, is used to control the sensing of physiological signals, e.g., sensing of cardiac electrical signals such as R-waves or P-waves. Examples of sensing control parameters may include, with no limitation intended, a sensing threshold, sensitivity, blanking period, and refractory period. While these control parameters are used to control how the IMD senses events from a physiological signal, the number of sensed events, e.g., the number of sensed R-waves may be a monitored or measured variable that is not directly set by the IMD. Examples of therapy delivery or electrical stimulation control parameters may include, with no limitation intended, pacing lower rate, pacing pulse amplitude, and pacing pulse width. These parameters are directly programmed into, set or adjusted by the IMD. The capture threshold used to set the pacing pulse amplitude, however, is an example of a monitored variable that has a numerical value and is not set or controlled by the IMD. The pacing rate modulation techniques disclosed herein are used for communicating a numerical value of a monitored variable.

A "physiological signal" as used herein is a signal that is produced by sensor, e.g., an electrical, chemical, mechanical or optical sensor of the IMD or rate monitor, in response to a physiological change in a body tissue. A physiological signal as used herein does not refer to a signal such as an electrical stimulation pulse that is produced by a device and detected by another device. One example of a physiological signal is a cardiac electrical signal that is produced by a cardiac electrical sensing circuit from signals received by sensing electrodes and includes depolarization signals attendant to the electrical depolarization of myocardial tissue. Other examples of physiological signals, with no limitation intended, include pulsatile signals that can be sensed by mechanical or optical sensors and include pulsatile signals produced by cyclical changes in blood volume corresponding to the cardiac cycle or cyclical changes in motion of the heart chambers or blood vessel walls. In the communication techniques disclosed herein, the rate monitor senses a physiological signal and determines activation rates corresponding to modulated rate intervals of electrical stimulation pulses delivered by the IMD. In the examples presented herein, the rate monitor does not directly detect the electrical stimulation pulses delivered by the IMD or demodulate the characteristics or features of the actual stimulation pulses delivered by the IMD. The rate monitor detects the rate of activations, e.g., electrical depolarizations or mechanical contractions, represented by pulsatile or cyclical waveforms of a physiological signal sensed by a sensor of the rate monitor.

<FIG> is a conceptual diagram illustrating an IMD system <NUM> that may be used to deliver therapeutic electrical stimulation pulses and modulate the rate of the stimulation pulses for communicating data to another device. In the illustrative example of <FIG>, system <NUM> includes an IMD shown as an intracardiac pacemaker <NUM> and an external, wearable heart rate monitor (HRM) <NUM>. Pacemaker <NUM> is configured to deliver therapeutic pacing pulses to heart <NUM> to treat or prevent arrhythmias. Pacemaker <NUM> is configured to modulate pacing rate intervals to deliver pacing pulses at different rates in accordance with an encoding scheme. As described herein the encoding scheme may be based on a binary, ternary, quaternary or other base number system or other encoding scheme that includes predetermined pacing rate intervals used to cause the heart to beat at modulated heart rates. The HRM <NUM> detects the modulated heart rates and demodulates the heart rates to a numerical value of a variable monitored by the pacemaker <NUM>.

As used herein, "therapeutic" pacing pulses includes pacing pulses intended to capture the heart <NUM> to cause a pacing-evoked myocardial depolarization to cause the heart to contract or "beat" at an intended rate. As examples, pacemaker <NUM> may deliver bradycardia pacing therapy by delivering therapeutic pacing pulses at a programmed lower pacing rate in the absence of intrinsic R-waves being sensed at or above the programmed lower pacing rate. Rate responsive pacing therapy may include therapeutic pacing pulses delivered at a temporary pacing rate above the programmed lower pacing rate to increase the heart rate during physical activity. As disclosed herein, each pacing pulse delivered at a modulated pacing rate interval may be considered a "therapeutic" pacing pulse in that it is delivered outside the physiological refractory period of the myocardial tissue at a pacing pulse energy intended to capture the heart and cause a heartbeat above a programmed lower pacing rate thereby preventing asystole or bradycardia.

The pacing pulses delivered at modulated pacing rate intervals (also referred to herein as "modulated pacing intervals") may be delivered without suspending bradycardia pacing in a pacing dependent patient, for instance, in that pacing pulses delivered at the modulated pulse intervals capture the heart at a rate interval within an acceptable range of a programmed lower pacing rate or other targeted therapeutic pacing rate or a sensed intrinsic heart rate. For example, if the programmed lower rate for bradycardia pacing is <NUM> to <NUM> pulses per minute, corresponding to a pulse rate interval of <NUM> to <NUM> seconds, modulated pacing rate intervals for communicating a numerical value of a monitored variable may be in the range of about <NUM> to about <NUM> corresponding to overdrive pacing rates of about <NUM> to about <NUM> pulses per minute, as examples with no limitation intended. Since the number of modulated pacing rate intervals required to communicate one or more numerical values of one or more monitored variables can generally be delivered within a few minutes or even one minute or less, the modulated pacing rates can be delivered without compromising the intended therapeutic benefit of a pacing therapy such as bradycardia pacing. For example, modulated pacing rates may account for about <NUM>% or even <NUM>% or less of the total number of pacing pulses delivered by pacemaker <NUM> over <NUM> hours.

In the example of <FIG>, pacemaker <NUM> is shown in the right ventricle (RV) of the patient's heart <NUM> for sensing cardiac electrical signals and delivering cardiac pacing pulses. Pacemaker <NUM> may be a transcatheter intracardiac pacemaker adapted for implantation wholly within a heart chamber, e.g., wholly within the RV <NUM>, wholly within the left ventricle (LV) <NUM>, or wholly with the right atrium (RA) <NUM>. Pacemaker <NUM> may be reduced in size compared to subcutaneously implanted pacemakers and may be generally cylindrical in shape to enable transvenous implantation in a heart chamber via a delivery catheter.

The techniques disclosed herein are not limited to the pacemaker location shown in the example of <FIG> and other positions within heart <NUM> are possible. For example, a pacemaker <NUM> performing the techniques disclosed herein may be positioned at other locations in or on the heart <NUM> for delivering electrical stimulation pulses that capture and pace the heart at the rate of the electrical stimulation pulses. Pacemaker <NUM> may deliver the cardiac pacing pulses via electrodes on the outer housing of the pacemaker, referred to herein as "housing based electrodes. " Pacemaker <NUM> may be configured to sense a cardiac electrical signal from the RV <NUM> using the housing based electrodes for detecting ventricular R-waves for producing a ventricular electrogram (EGM) signal. The cardiac electrical signals may be sensed using the housing based electrodes that are also used to deliver pacing pulses to the heart <NUM>.

HRM <NUM> may be a wearable device that is capable of detecting the patient's heart rate and demodulate the heart rate to extract information communicated from pacemaker <NUM>. HRM <NUM> may be a dedicated heart rate tracking device provided to the patient receiving pacemaker <NUM> for detecting and demodulating modulated heart rates for obtaining numerical values of variables monitored by pacemaker <NUM>. In other examples, HRM <NUM> may be a commercially available smart watch, activity tracker, or other wearable accessory capable of detecting the patient's heart rate and storing and executing an application for demodulating the detected heart rate to extract data encoded in the modulated heart rate. In still other examples, HRM <NUM> may be an implantable device, e.g., a device that is implantable for sensing a subcutaneous ECG signal such as the Reveal LINQ® Insertable Cardiac Monitoring System available from Medtronic, Inc. , Minneapolis MN. As described below, HRM <NUM> may detect a modulated heart rate and demodulate the heart rate to determine a numerical value of one or more patient-related and/or device-related variables monitored by pacemaker <NUM>. HRM <NUM> may provide a variety of responses to a numerical value of a monitored variable such as generating a patient notification that data is available for viewing, generating an alert that a numerical value is outside a normal range, generating a display of the numerical value which may include historic values of the monitored variable, generating a notification of the status of the monitored variable based on the numerical value, and/or transmitting the numerical value to another device.

HRM <NUM> may be capable of bidirectional wireless communication with an external device <NUM> for transmitting data demodulated from the detected heart rate to external device <NUM>. External device <NUM> may be a home medical monitor, a smart phone or other hand held device, a personal computer or other device that may be used by the ambulatory patient or at a fixed location in the patient's home or other location for wirelessly retrieving data from HRM <NUM>. External device <NUM> is shown with a wireless communication link <NUM> established with HRM <NUM>. Communication link <NUM> may be established using a radio frequency (RF) link such as BLUETOOTH®, Wi-Fi, Medical Implant Communication Service (MICS) or other communication bandwidth. In some examples, HRM <NUM> and external device <NUM> are BLUETOOTH® enabled devices to allow data extracted from the detected heart rates by HRM <NUM> to be transmitted to external device <NUM> or cloud-based storage any time a BLUETOOTH® connection is available. External device <NUM> may be a MYCARELINK™ Patient Monitor available from Medtronic, Inc. Minneapolis MN, USA, in one example.

HRM <NUM> and/or external device <NUM> may display numerical values or a status indicator of the numerical value demodulated from the detected heart rate to the patient or other user to view, enabling the patient or other user to take any corrective action as needed. Notifications or alerts may be initiated by the HRM <NUM> for display to the patient directly or via the external device <NUM>. Accessibility to patient-related or device-related variables that are monitored or measured by pacemaker <NUM> is provided without requiring pacemaker <NUM> to perform wireless radio frequency transmission. The operation of pacemaker <NUM> for communicating data to a patient or clinician via an external device is improved by making the communication process power efficient (powering of an RF transmitter is not required). Numerical values of monitored parameters can be made available from pacemaker <NUM> multiple times throughout an hour, day, week, etc., when HRM <NUM> is detecting the patient's heart rate. Patient alerts or notifications can be initiated by HRM without time constraint or waiting for an interrogation command from another device. Data may be obtained from pacemaker <NUM> with a higher compliance rate when data is communicated without reliance on patient or user interaction with system <NUM>, e.g., by manually initiating a communication session.

It is contemplated that HRM and/or external device <NUM> may be in wired or wireless connection to a communications network via a telemetry circuit that includes a transceiver and antenna or via a hardwired communication line for transferring data to a centralized database or computer to allow remote management of the patient. Remote patient management systems including a remote patient database may be configured to receive data and information determined by HRM <NUM> to enable a clinician to review data communicated through heart rate modulation by pacemaker <NUM>.

While a pacemaker <NUM> and wearable HRM <NUM> are shown in system <NUM>, it is contemplated that other types of implantable electrical stimulation devices may deliver therapeutic individual pulses or individual pulse trains, e.g., high frequency pulse trains for muscle stimulation or pain control, separated by rate intervals. The implantable electrical stimulation device may be configured to determine a monitored variable data value relating to a patient or device condition and convert the numerical data value to a sequence of modulated rate intervals (between individual stimulation pulses or between high frequency trains of pulses). A rate monitor, which may be a wearable or implantable monitor, may be configured to detect a signal including the resulting evoked responses or activations of the stimulated muscle or nerve tissue occurring at the modulated rate. The rate detecting device may determine the rate of evoked responses or activations, e.g., from a neural signal, electromyogram signal or mechanical sensor signal, and demodulate the rate to determine a numerical value of a device-related or patient-related variable. The HRM <NUM> monitor may be configured only for monitoring in some examples but may include therapy delivery capabilities in other examples. For instance, the HRM configured to demodulate a detected heart rate may be a pacemaker, implantable cardioverter defibrillator, neurostimulator, or other medical device capable of both detecting a rate of activations produced by the modulated stimulation rate of the IMD and delivering a therapy, such as an electrical stimulation therapy, drug delivery or other therapy.

<FIG> is a conceptual diagram of the intracardiac pacemaker <NUM> shown in <FIG>. Pacemaker <NUM> includes leadless electrodes <NUM> and <NUM> spaced apart on the housing <NUM> of pacemaker <NUM> for sensing cardiac electrical signals and delivering pacing pulses. Electrode <NUM> is shown as a tip electrode extending from a distal end <NUM> of pacemaker <NUM>, and electrode <NUM> is shown as a ring electrode along a mid-portion of the lateral wall of housing <NUM>, for example adjacent proximal end <NUM>. Electrode <NUM> may circumscribe a portion of the lateral sidewall of housing <NUM> that extends from distal end <NUM> to proximal end <NUM>. Distal end <NUM> is referred to as "distal" in that it is expected to be the leading end as pacemaker <NUM> is advanced through a delivery tool, such as a catheter, and placed against a targeted pacing site.

Electrodes <NUM> and <NUM> form an anode and cathode pair for bipolar cardiac pacing and cardiac electrical signal sensing. In alternative embodiments, pacemaker <NUM> may include two or more ring electrodes, two tip electrodes, and/or other types of electrodes exposed along pacemaker housing <NUM> for delivering electrical stimulation to heart <NUM> and sensing cardiac electrical signals. Electrodes <NUM> and <NUM> may be positioned at locations along pacemaker <NUM> other than the locations shown. Electrodes <NUM> and <NUM> may be, without limitation, titanium, platinum, iridium or alloys thereof and may include a low polarizing coating, such as titanium nitride, iridium oxide, ruthenium oxide, platinum black among others.

Housing <NUM> is formed from a biocompatible material, such as a stainless steel or titanium alloy. In some examples, the housing <NUM> may include an insulating coating. Examples of insulating coatings include parylene, urethane, PEEK, or polyimide among others. The entirety of the housing <NUM> may be insulated, but only electrodes <NUM> and <NUM> uninsulated. Electrode <NUM> may serve as a cathode electrode and be coupled to internal circuitry, e.g., a pacing pulse generator and cardiac electrical signal sensing circuitry, enclosed by housing <NUM> via an electrical feedthrough crossing housing <NUM>. Electrode <NUM> may be formed as a conductive portion of housing <NUM> defining a ring electrode that is electrically isolated from the other portions of the housing <NUM> as generally shown in <FIG>. In other examples, the entire periphery of the housing <NUM> may function as an electrode that is electrically isolated from tip electrode <NUM>, instead of providing a localized ring electrode such as anode electrode <NUM>. Electrode <NUM> defined by an electrically conductive portion of housing <NUM> serves as a return anode during pacing and sensing.

The housing <NUM> includes a control electronics subassembly <NUM>, which houses the electronics for sensing cardiac signals, producing pacing pulses and controlling therapy delivery and other functions of pacemaker <NUM> as described below in conjunction with <FIG>. Housing <NUM> further includes a battery subassembly <NUM>, which provides power to the control electronics subassembly <NUM>.

Pacemaker <NUM> may include a set of fixation tines <NUM> to secure pacemaker <NUM> to patient tissue, e.g., by actively engaging with the ventricular endocardium and/or interacting with the ventricular trabeculae. Fixation tines <NUM> are configured to anchor pacemaker <NUM> to position electrode <NUM> in operative proximity to a targeted tissue for delivering therapeutic electrical stimulation pulses. Numerous types of active and/or passive fixation members may be employed for anchoring or stabilizing pacemaker <NUM> in an implant position. Pacemaker <NUM> may include a set of fixation tines as disclosed in commonly-assigned <CIT>).

Pacemaker <NUM> may optionally include a delivery tool interface <NUM>. Delivery tool interface <NUM> may be located at the proximal end <NUM> of pacemaker <NUM> and is configured to connect to a delivery device, such as a catheter, used to position pacemaker <NUM> at an implant location during an implantation procedure, for example within the LV.

<FIG> is a schematic diagram of an example configuration of pacemaker <NUM> shown in <FIG>. Pacemaker <NUM> includes a pulse generator <NUM>, a cardiac electrical signal sensing circuit <NUM> (also referred to herein as "sensing circuit <NUM>"), a control circuit <NUM>, memory <NUM>, telemetry circuit <NUM> and a power source <NUM>. The various circuits represented in <FIG> may be combined on one or more integrated circuit boards which include a specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, state machine or other suitable components that provide the described functionality.

Cardiac electrical signal sensing circuit <NUM> is configured to receive a cardiac electrical signal via electrodes <NUM> and <NUM> by a pre-filter and amplifier circuit <NUM>. Pre-filter and amplifier circuit may include a high pass filter to remove DC offset, e.g., a <NUM> to <NUM> high pass filter, or a wideband filter having a passband of <NUM> to <NUM> to remove DC offset and high frequency noise. Pre-filter and amplifier circuit <NUM> may further include an amplifier to amplify the "raw" cardiac electrical signal passed to analog-to-digital convertor (ADC) <NUM>. ADC <NUM> may pass a multi-bit, digital electrogram (EGM) signal to control circuit <NUM> for use in detecting cardiac events and determining a patient's heart rhythm. The digital signal from ADC <NUM> may be passed to rectifier and amplifier circuit <NUM>, which may include a rectifier, bandpass filter, and amplifier for passing the filtered and rectified cardiac electrical signal to cardiac event detector <NUM>.

Cardiac event detector <NUM> may include a sense amplifier or other detection circuitry that compares the incoming rectified, cardiac electrical signal to an R-wave detection threshold amplitude, which may be an auto-adjusting threshold. When the incoming signal crosses the R-wave detection threshold, the cardiac event detector <NUM> produces an R-wave sensed event signal that is passed to control circuit <NUM>. R-wave sensed event signals passed from cardiac event detector <NUM> to control circuit <NUM> may be used for scheduling ventricular pacing pulses by pace timing circuit <NUM>, determining ventricular rate intervals or RR intervals (between two consecutively received R-wave sensed event signals. Control circuit <NUM> may determine an intrinsic heart rate from the RR intervals.

Control circuit <NUM> includes pace timing circuit <NUM> and processor <NUM>. Control circuit <NUM> may receive R-wave sensed event signals and/or digital cardiac electrical signals from sensing circuit <NUM> for use in detecting and confirming cardiac events and controlling ventricular pacing. For example, R-wave sensed event signals may be passed to pace timing circuit <NUM> for inhibiting scheduled ventricular pacing pulses. Pace timing circuit <NUM> (or processor <NUM>) may receive R-wave sensed event signals from cardiac event detector <NUM> for use in controlling the timing of pacing pulses delivered by pulse generator <NUM> and determining an intrinsic heart rate when pacemaker <NUM> is not delivering pacing pulses.

Control circuit <NUM> may retrieve programmable pacing control parameters, such as pacing pulse amplitude and pacing pulse width, which are passed to pulse generator <NUM> for controlling pacing pulse delivery from memory <NUM>. In addition to providing control signals to pace timing circuit <NUM> and pulse generator <NUM> for controlling pacing pulse delivery, control circuit <NUM> may provide sensing control signals to sensing circuit <NUM> (e.g., R-wave sensing threshold, sensitivity, and/or various blanking and refractory intervals applied to the cardiac electrical signal).

Pulse generator <NUM> generates electrical pacing pulses that are delivered to the RV of the patient's heart via cathode electrode <NUM> and return anode electrode <NUM>. Pulse generator <NUM> may include charging circuit <NUM>, switching circuit <NUM> and an output circuit <NUM>. Charging circuit <NUM> may include a holding capacitor that may be charged to a pacing pulse amplitude by a multiple of the battery voltage signal of power source <NUM> under the control of a voltage regulator. The pacing pulse amplitude may be set based on a control signal from control circuit <NUM>. Switching circuit <NUM> may control when the holding capacitor of charging circuit <NUM> is coupled to the output circuit <NUM> for delivering the pacing pulse. For example, switching circuit <NUM> may include a switch that is activated by a timing signal received from pace timing circuit <NUM> upon expiration of a pacing interval and kept closed for a programmed pacing pulse width to enable discharging of the holding capacitor of charging circuit <NUM>. The holding capacitor, previously charged to the pacing pulse voltage amplitude, is discharged across electrodes <NUM> and <NUM> through the output capacitor of output circuit <NUM> for the programmed pacing pulse duration. Examples of pacing circuitry generally disclosed in <CIT>) and in commonly assigned <CIT>), may be implemented in pacemaker <NUM> for charging a pacing capacitor to a predetermined pacing pulse amplitude under the control of control circuit <NUM> for generating and delivering a pacing pulse.

Pacemaker <NUM> may include a sensor <NUM> for producing a physiological signal used by control circuit <NUM> for monitoring a patient-related variable. For example, sensor <NUM> may include a motion sensor such as an accelerometer for detecting motion of the heart and/or motion caused by patient physical activity. Additionally or alternatively, sensor <NUM> may include a pressure sensor, optical sensor, acoustical sensor, temperature sensor, pH sensor, or any combination thereof. Control circuit <NUM> may derive a numerical value of a patient condition from a signal received from sensor <NUM> according to a monitoring protocol.

Control circuit <NUM> is configured to monitor one or more patient-related and/or device related variables. Among the variables that may be monitored by control circuit <NUM> are battery voltage of power source <NUM>, pacing electrode impedance, pacing capture threshold determined by performing a pacing capture threshold test, percentage of pacing pulses delivered out of all paced and sensed events, number of premature ventricular contractions (PVC) or runs of PVCs detected, number of atrial arrhythmia episodes or atrial arrhythmia burden, percentage of time pacing at a rate response rate, percentage of time pacing at a programmed lower rate, percentage of time at a resting physical activity level, blood oxygen saturation, or blood pressure as examples. Control circuit <NUM> may monitor a variable by detecting or determining a numerical value of the variable according to a predetermined schedule or protocol stored in memory <NUM>. The numerical value of a monitored device-related variable may be determined by circuitry included in pacemaker <NUM>, e.g., for measuring voltage of a battery included in power source <NUM>. The numerical value of a monitored patient-related variable may be determined using a signal from sensor <NUM> or cardiac electrical signal sensing circuit <NUM>, e.g., for determining blood oxygen saturation, blood pressure, cardiac impedance, pacing capture threshold or other patient related variables. It is recognized that numerous types of variables may be monitored by pacemaker <NUM> on a weekly, daily, hourly, or more or less frequent basis or on a triggered basis. The methods for communicating a numerical value of a monitored variable by pacing rate interval modulation disclosed herein are not limited any specific type of monitored variables.

Control circuit <NUM> is configured to convert the numerical value of a monitored variable to a data sequence of modulated pacing rate intervals. The modulated pacing rate intervals may correspond to a binary sequence of zeros and ones in some examples. For instance, a binary representation of the monitored variable numerical value may be converted to a modulated pacing rate interval data sequence by converting all zeros in the binary representation of the numerical variable value to a first pacing interval and converting all ones in the sequence to a second, different pacing interval. For example, for each zero in the sequence, a pacing pulse interval may be set to <NUM>. For each one in the sequence, a pacing pulse interval may be set to <NUM>. When a ternary encoding scheme is used, three pulse intervals, e.g., <NUM>, <NUM> and <NUM> may be used. Four pulse intervals, e.g., <NUM>, <NUM>, <NUM> and <NUM> intervals, may be used in converting a numeric value of a monitored variable to a sequence of pacing intervals representing a quaternary value and so on. When a higher base number system is used to convert a variable numerical value to modulated pacing intervals, fewer modulated pacing intervals may be required to encode the numerical value in a sequence of modulated pacing intervals. For example, a quaternary encoding scheme may require half the number of modulated pacing intervals to encode a numerical value of a monitored variable than a binary encoding scheme.

In other examples, control circuit <NUM> converts the numerical value of a monitored variable to modulated pacing rate intervals by setting the modulated pacing rate interval to a fixed pacing interval for a number of intervals equal to a digit of the numerical value. For example, if a digit of the numerical value is a <NUM>, the pacing rate interval is set to a fixed rate interval for <NUM> pacing cycles to communicate the numerical value of a "<NUM>. " Multiple sets of pacing rate intervals may be delivered for communicating a multi-digit numerical value where each set has a number of pacing rate intervals equal to the value of one digit of the multi-digit numerical value.

Control circuit <NUM> establishes a sequence of modulated pacing rate intervals representing the numerical value of a monitored variable by controlling pace timing circuit <NUM> to set timers or control signals according to the modulated pacing rate intervals. Pulse generator <NUM> responds to signals received from pace timing circuit <NUM> by generating and delivering therapeutic pacing pulses, e.g., having a pacing pulse amplitude and pulse width that captures the myocardial tissue to cause a pacing evoked depolarization, according to the modulated pacing rate interval sequence. In various examples, the modulated pacing rate interval sequence may include a header including one or more pacing pulses delivered at interval(s) that signal that subsequent pulses are being delivered at modulated pulse intervals for communicating one or more monitored data values. The modulated pacing rate interval sequence may be repeated one or more times to increase the likelihood of successful detection and demodulation of the modulated heart rate by the HRM <NUM>. Other examples of sequences of modulated pacing rate intervals and associated techniques are described below.

Memory <NUM> may include computer-readable instructions that, when executed by processor <NUM> of control circuit <NUM>, cause control circuit <NUM> to perform various functions attributed throughout this disclosure to pacemaker <NUM>. The computer-readable instructions may be encoded within memory <NUM>. Memory <NUM> may include any non-transitory, computer-readable storage media including any volatile, non-volatile, magnetic, optical, or electrical media, such as a random access memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM (EEPROM), flash memory, or other digital media. Memory <NUM> may store timing intervals and other data used by control circuit <NUM> to control the delivery of pacing pulses by pulse generator <NUM> according to the techniques disclosed herein.

Power source <NUM> may correspond to battery subassembly <NUM> shown in <FIG> and provides power to each of the other circuits and components of pacemaker <NUM> as required. Power source <NUM> may include one or more energy storage devices, such as one or more rechargeable or non-rechargeable batteries. The connections between power source <NUM> and other pacemaker circuits and components are not shown in <FIG> for the sake of clarity but are to be understood from the general block diagram of <FIG>. For example power source <NUM> may provide power to charging circuit <NUM> for charging a holding capacitor to a pacing voltage amplitude, current to switch <NUM> and other circuitry included in pulse generator <NUM> as needed to generate and deliver pacing pulses. Power source <NUM> also provides power to telemetry circuit <NUM>, sensing circuit <NUM> and sensor <NUM> as needed as well as memory <NUM>.

Pacemaker <NUM> may optionally have a telemetry circuit <NUM> including a transceiver <NUM> and antenna <NUM> for transferring and receiving data, e.g., via a radio frequency (RF) communication link with an external programmer or home monitor. Telemetry circuit <NUM> may be used to transmit larger amounts of data and for receiving programming commands for setting programmable pacing and cardiac sensing parameters, for example. Telemetry circuit <NUM> may be capable of bi-directional communication with external device <NUM> (<FIG>), for example, when external device <NUM> is a programmer or home monitor used to transmit programming commands to pacemaker <NUM> and retrieve data from pacemaker <NUM>. Cardiac electrical signals, marker channel data depicting the timing of cardiac event sensing and pacing, currently programmed parameters or other data may be transmitted by telemetry circuit <NUM> to external device <NUM>. Programmable control parameters and programming commands for controlling cardiac electrical signal sensing and cardiac pacing may be received by telemetry circuit <NUM> and stored in memory <NUM> for access by control circuit <NUM>.

The functions attributed to pacemaker <NUM> herein may be embodied as one or more processors, controllers, hardware, firmware, software, or any combination thereof. Depiction of different features as specific circuitry is intended to highlight different functional aspects and does not necessarily imply that such functions must be realized by separate hardware, firmware or software components or by any particular circuit architecture. Rather, functionality associated with one or more circuits described herein may be performed by separate hardware, firmware or software components, or integrated within common hardware, firmware or software components. For example, pacing rate modulation operations performed by pacemaker <NUM> may be implemented in control circuit <NUM> executing instructions stored in memory <NUM> for controlling pulse generator <NUM>.

The operation of circuitry included in pacemaker <NUM> as disclosed herein should not be construed as reflective of a specific form of hardware, firmware and software necessary to practice the techniques described. It is believed that the particular form of software, hardware and/or firmware will be determined primarily by the particular system architecture employed in the pacemaker <NUM> and by the particular sensing and therapy delivery circuitry employed by the pacemaker <NUM>. Providing software, hardware, and/or firmware to accomplish the described functionality in the context of any modern IMD, given the disclosure herein, is within the abilities of one of skill in the art. The functionality and components described in conjunction with <FIG> for performing cardiac pacing rate modulation for communicating a value of a monitored variable may generally be included in any IMD or wearable medical device configured to deliver electrical stimulation therapy to cause depolarization or activation of excitable tissue at a modulated rate.

<FIG> is a conceptual diagram of the HRM <NUM> of <FIG> according to one example. HRM <NUM> may take a variety of forms but is generally an implantable or wearable device equipped with a sensor for producing a pulsatile signal having a frequency correlated to heart rate and a processor for detecting heart rate from the signal and demodulating the detected heart rate to a numerical value of a variable monitored by pacemaker <NUM>. HRM <NUM> is shown as a smart watch in the example of <FIG> but may be configured as a wearable band, strap or patch, such as a chest strap, arm or wrist band, head band or other wearable device such as earbuds. HRM <NUM> may include a heart rate sensor <NUM>, processor <NUM>, memory <NUM>, communication circuit <NUM>, alert circuit <NUM>, power source <NUM>, display <NUM> and user interface <NUM>. In other examples, HRM <NUM> may be a subcutaneously implantable device that does not include a display <NUM> and user interface <NUM> but is capable of monitoring heart rate and transmitting data values demodulated from the heart rate via communication circuit <NUM> via BLUETOOTH® or other RF wireless communication. As indicated previously, an example of an implantable HRM that may be included in the disclosed system is the Reveal LINQ® Insertable Cardiac Monitor available from Medtronic, Inc. , Minneapolis MN. In still other examples, the HRM may be an implantable or wearable medical device having therapy delivery capabilities in addition to the capability of detecting a rate of depolarizations or contractions produced by electrical stimulation pulses generated and delivered by another medical device. Such devices may include pacemakers, implantable cardioverter defibrillators, neurostimulators, drug delivery pumps, or the like. While a therapy delivery circuit or module is not shown in <FIG>, it is to be understood that the functions attributed to HRM <NUM> may be included in a medical device having additional components than those shown in <FIG>, which may include circuitry and components for generating or delivering a therapy.

Power source <NUM> provides power to processor <NUM>, heart rate sensor <NUM> and the other circuits of HRM <NUM> as needed. Power source <NUM> may include one or more rechargeable or non-rechargeable batteries. Heart rate sensor <NUM> may include electrodes for receiving electrical signals produced by the patient's heart and circuitry for sensing R-waves attendant to ventricular depolarizations for detecting heart beats. In other examples, heart rate sensor <NUM> includes an optical sensor and circuitry for determining heart rate using photoplethysmography (PPG), a bioimpedance sensor for detecting heart rate from cyclical tissue impedance changes, or another sensor capable of producing a pulsatile signal correlated to the patient's heart rate.

Processor <NUM> determines the patient's heart rate from a signal received from heart rate sensor <NUM>. Processor <NUM> may determine the heart rate by detecting and counting individual pulses of the pulsatile signal over a predetermined or variable time interval, timing the intervals between consecutive individual pulses of the pulsatile signal, performing fast Fourier transform (FFT) over a predetermined time interval for determining the frequency of the pulsatile signal, or other technique for detecting the patient's heart rate from the heart rate sensor signal. Processor <NUM> may execute instructions stored in memory <NUM> for determining heart rate and for demodulating heart rate intervals for determining a numerical value of a monitored, variable data value that is communicated from pacemaker <NUM> via heart rate modulation. HRM <NUM> may be configured to monitor the user's heart rate continuously, at specified times of day, at regularly scheduled time intervals, or according to a predetermined monitoring protocol, which may be customizable for a given patient. An application may be stored in memory <NUM> for execution by processor <NUM> each time the heart rate is determined to detect one or more heart rate intervals that are known to be included in the encoding scheme for establishing a sequence of modulated pacing rate intervals.

Processor <NUM> may determine one or more numerical values corresponding to a respective number of monitored variables by demodulating heart rate intervals and may display a notification to the user on display <NUM> to notify the user that new data is available for viewing. In some examples, processor <NUM> may transmit the determined numerical values to another device, such as external device <NUM> or another implantable or wearable medical device, via communication circuit <NUM>. Communication circuit <NUM> includes a wireless transceiver for transmitting and receiving radio frequency signals. For example, communication circuit <NUM> may be a BLUETOOTH® enabled device for transmitting and receiving data from external device <NUM>. In this way, numerical values of device-related and/or patient-related variables may be communicated to HRM <NUM> by pacemaker <NUM> using only modulated pacing rate intervals without requiring wireless radio frequency communication transmission by pacemaker <NUM> and the associated power consumption and without disruption of cardiac pacing.

In some examples, processor <NUM> may compare a determined numerical data value to a threshold value or range and generate an alert or notification by alert circuit <NUM> to notify the user of the corresponding status of the monitored variable, e.g., as being within a normal or outside a normal range. The user may be alerted when a monitored variable value is outside a normal range and may be advised to contact his/her physician or seek medical attention. Alert circuit <NUM> may generate a vibration or audible tone to alert the user, and/or a visual display may be generated by display <NUM> notifying the user that the numerical data value determined from the modulated heart rate intervals is out of a normal range. In other examples, the alert generated based on the numerical data value may be an indication that the pacemaker <NUM> (or other IMD performing the electrical stimulation rate modulation) is changing a therapy delivery mode, stopping therapy delivery, performing a temporary operation or other status indicator. The alert may be transmitted to another medical device, which may respond to the alert by notifying a clinician or altering its own therapy delivery and/or monitoring functions in the medical device system <NUM>.

HRM <NUM> may include a user interface <NUM> which may include a touch screen, buttons, voice response, or other interface or combinations thereof that enable the user to set user-selectable settings that control notifications and alerts or other functions of HRM <NUM>. User interface <NUM> and display <NUM> may be configured to enable a user to select monitored variables and display current and/or historic numerical values of a monitored variable in a tabular or graphical format. For example, a user may select to be notified by HRM <NUM> each time new data is available, once a day or other selected frequency. In other examples, a user may select to be notified by HRM <NUM> only when a numeric value is outside a normal range. At other times, a user may select to disable patient notifications relating data values derived from the modulated heart rate and only enable transmission of numerical values of monitored variables to another medical device, e.g., to external device <NUM>, to another implantable or wearable therapy delivery device, to a clinician's computer, phone or other device, or to a central patient management database, etc..

<FIG> is a flow chart <NUM> of a method performed by pacemaker <NUM> according to one example. Initially, pacemaker <NUM> may be operating in a normal pacing mode at block <NUM>, delivering pacing therapy according to programmed therapy control parameters without modulation of the pacing rate intervals. At block <NUM>, control circuit <NUM> determines a numerical value of a monitored variable to be communicated via heart rate modulation. The numerical value may be determined according to a monitoring protocol, e.g., once hourly, daily, weekly or at longer, shorter or variable time intervals. The numerical values that may be determined at block <NUM> may be related to device diagnostics and/or patient related variables as described above. Various examples of numerical values that may be communicated by pacemaker <NUM> via heart rate modulation include, with no limitation intended, pacemaker battery voltage; percentage of paced events out of all paced and sensed events over a given time period; impedance measurements, and pacing capture threshold. The numerical values that may be communicated via heart rate modulation may generally include any numerical value that can be converted to a single or multi-bit word in a binary, ternary, quaternary, or other base number system or represented according to a number of pacing intervals equal to a digit of the numerical value in a base-<NUM> number system. In some cases, a data value may be converted to multiple bytes if the data value cannot be communicated as a single multi-bit word. In other examples, the data values may be converted to multiple sets of pacing intervals each representing a numerical value of a digit between a defined range, e.g., between <NUM>-<NUM>.

At block <NUM>, control circuit <NUM> converts each numerical value to a sequence of modulated pacing intervals. In some examples, the numerical value is determined in a binary number system with one predetermined pacing interval representing a binary digit of "<NUM>" and another predetermined pacing interval representing a binary digit of "<NUM>. " The second pacing rate interval representing a binary digit of "<NUM>" is different than the first pacing rate interval representing a binary digit of "<NUM>" and is distinguishable from the first pacing rate interval by the heart rate detection technique and resolution of HRM <NUM>. The pacing rate intervals established by the IMD system, e.g., system <NUM> in <FIG>, for representing binary "<NUM>" and "<NUM>" may be selected to be pacing rate intervals that are different than the pacing intervals corresponding to available programmable pacing rates or at least different than the pacing interval(s) corresponding to commonly programmed pacing rates. For instance, <NUM> pulses per minute is a commonly programmed lower pacing rate and corresponds to a pacing interval of <NUM>. The pacing intervals established for use in rate modulation for data communication using a binary encoding scheme may avoid the <NUM> interval. In one example, a pacing interval of <NUM> (corresponding to a pacing rate of about <NUM> pulses per minute) may be designated as a binary "<NUM>" and a pacing interval of <NUM> (corresponding to a pacing rate of <NUM> pulses per minute) may be designated as a binary "<NUM>. " The selected pacing intervals representing a binary "<NUM>" and binary "<NUM>" may be reserved intervals that are not programmable in pacemaker <NUM> as an intended therapy pacing rates in some examples.

In other examples, a ternary encoding scheme may be used in which the numerical value of the monitored variable is converted from a first base number system, e.g., a binary or base <NUM> numerical value, to the ternary number system then converted to a sequence of pacing rate intervals representing the numerical value in the ternary number system with each one of three distinct pacing rate intervals representing a respective "<NUM>", "<NUM>" or "<NUM>" for each digit in the ternary value of the monitored variable. In some examples, therefore, control circuit <NUM> may be configured to convert a numerical value of the monitored variable determined in one number system (e.g., binary or base <NUM>) to a different base number system (e.g., ternary or quaternary) then convert each digit or bit of the converted numerical value to a corresponding predetermined pacing rate interval.

The selected pacing intervals corresponding to each possible value of a digit of the numerical value may be selected and spaced apart from each other such that beat-to-beat changes between the two pacing intervals is clearly recognized by the HRM as being communication intervals as opposed to physiological changes in heart rate or normal changes in pacing rate according to a therapeutic pacing protocol. For instance normal changes in pacing rate are expected to increase gradually, decrease gradually or remain stable as opposed to a series of heart beats that includes only two specified rates (in the case of a binary value) with rate changes occurring only between those two specified rates.

In one illustrative example, the numerical value to be communicated by pacemaker <NUM> is the pacemaker battery voltage. The normal range of battery voltage for power source <NUM> may be <NUM> V to <NUM> V in one example, with <NUM> possible voltage values to the hundredths place. Each possible voltage value may be determined as a unique, six bit binary value as listed in TABLE I below.

After determining the battery voltage value as a six bit binary value, the binary value is converted to a pacing rate interval sequence as shown in TABLE I where each interval represents either a digital "<NUM>" or a digital "<NUM>. " As such, at block <NUM>, after determining the numerical value of a monitored variable in a selected base number system, the numerical value is converted to a sequence of modulated pacing intervals, where each interval in the sequence corresponds to one possible value of a digit in the selected base number system. Control circuit <NUM> may establish a modulated pacing rate sequence at block <NUM> that includes the pacing rate intervals representing the numerical value and may include other information. As described below, in addition to the modulated pacing intervals representing each numerical value of one or more monitored variables to be communicated, the modulated pacing rate sequence may include modulated pacing rate intervals that represent a header, a footer, and error detection code. In other examples, the modulated pacing rate intervals may include a sequence of modulated pacing rate intervals indicating the operational status of the pacemaker <NUM> such as a therapy mode status, a patient status, or other categorical data that may be represented by a predetermined sequence of modulated pacing rate intervals. Control circuit <NUM> waits for a time to transmit the modulated pacing sequence at block <NUM>. In some examples, the modulated pacing sequence may be delivered without delay, as long as the intrinsic heart rate or currently delivered therapy rate is less than the lowest pacing rate included in the modulated pacing sequence. The modulated pacing sequence overdrive paces the heart to control the heart rate detected by HRM <NUM>.

In some instances, control circuit <NUM> may compare the intrinsic heart rate, determined based on sensed event signals received from sensing circuit <NUM> to the lowest heart rate associated with the modulated pacing sequence. If the intrinsic heart rate is faster than the lowest heart rate associated with the modulated pacing sequence, control circuit <NUM> waits at block <NUM> until the intrinsic heart rate falls to a rate less than the lowest modulated pacing rate included in the modulated pacing sequence. For example, RR intervals (RRIs) may be determined from the R-wave sensed event signals received from sensing circuit <NUM>. An RRI is the time interval between two consecutively received R-waves sensed by sensing circuit <NUM>. Control circuit <NUM> may wait for a threshold number of consecutive RRIs that are greater than the longest modulated pacing rate interval by a predetermined safety interval or percentage to be detected before initiating the modulated pacing rate sequence at block <NUM>.

In other examples, control circuit <NUM> may wait for a predetermined time of day for initiating the modulated pacing rate sequence. The HRM <NUM> may be programmed or set to detect the patient's heart rate at a specified time of day. Pacemaker <NUM> and HRM <NUM> may be synchronized for data communication by programming pacemaker <NUM> to deliver the modulated pacing rate sequence at a specified time of day or specified time intervals that the HRM <NUM> is scheduled to detect heart rate. The pacemaker <NUM> may be programmed to start the modulated pacing rate sequence at a short time delay to ensure that the HRM <NUM> is operating to detect heart rate at the time the modulated pacing rate sequence is delivered.

In some cases, the scheduled time of day may be at night or when the patient is expected to be asleep. In this way, the intrinsic heart rate is expected to be less than the lowest modulated pacing rate and the patient may be less likely to perceive heart rate fluctuations or any symptoms due to the overdrive pacing required to deliver the modulated pacing rate sequence. Control circuit <NUM> may detect a time of day and/or detect a low patient activity level, corresponding to inactivity or rest, when pacemaker <NUM> includes an accelerometer or other type of patient physical activity sensor. The modulated pacing rate sequence may be delivered at any time that the patient is expected to be at rest.

In some examples, control circuit <NUM> may be configured to determine a sensor indicated pacing rate from the patient physical activity sensor (e.g., sensor <NUM> in <FIG>) to provide rate responsive pacing that automatically adjusts the pacing rate according to patient physical activity level. Control circuit <NUM> may verify that the sensor indicated pacing rate is at the programmed lower rate (corresponding to rest) or at least less than lowest pacing rate included in the modulated pacing sequence at block <NUM>. If the sensor indicated pacing rate is greater than the lowest pacing rate included in the modulated pacing rate sequence, control circuit <NUM> waits for the sensor indicated pacing rate to decrease before delivering the modulated pacing rate sequence at block <NUM>.

In some cases, pacemaker <NUM> may be configured to detect tachyarrhythmia, e.g., tachycardia or fibrillation, and may deliver an anti-tachyarrhythmia pacing therapy (ATP). If a tachyarrhythmia is being detected and/or ATP or other therapy is being delivered to terminate a tachyarrhythmia, the control circuit <NUM> waits at block <NUM> for the heart rhythm to return to a normal sinus rhythm or a paced rhythm that is at a rate less than the lowest pacing rate included in the modulated pacing rate sequence. Accordingly, control circuit <NUM> may be configured to apply one or more requirements at block <NUM>, such as time of day, patient activity level, sensor indicated pacing rate, intrinsic heart rate, cardiac rhythm, or any combination thereof, for determining the appropriate time to generate the modulated pacing sequence to transmit the monitored variable value(s) via a modulated paced heart rate.

When it is time to deliver the modulated pacing rate sequence, control circuit <NUM> controls pulse generator <NUM> to schedule and deliver cardiac pacing pulses according to the modulated sequence of pacing rate intervals at block <NUM>. Upon completion of the modulated pacing rate sequence, delivered for communicating one or more numerical values of one or more monitored variables, pulse generator <NUM> returns to non-modulated pacing at block <NUM>. The first non-modulated pacing interval after delivering a sequence of modulated pacing rate intervals may be equal to the last non-modulated pacing interval prior to the modulated pacing sequence. In other examples, the non-modulated pacing interval returned to at block <NUM> may be adjusted since the last non-modulated pacing interval prior to the modulated pacing sequence. For example, pacemaker <NUM> may have updated a sensor indicated pacing rate to provide rate responsive pacing and deliver the first pacing pulse after the modulated pacing sequence according to a non-modulated interval set to provide rate responsive pacing. In general, abrupt changes in heart rate may be avoided by delivering the first non-modulated pacing interval at a rate smoothing interval that is within a threshold difference from the ending modulated pacing interval of the sequence. It is recognized that in some instances returning to non-modulated pacing at block <NUM> may result in an episode of cardiac sensing with no pacing delivery required based on the programmed pacing therapy control parameters.

At block <NUM>, control circuit <NUM> may determine that the modulated pacing rate sequence should be repeated. In some examples, the modulated pacing rate sequence is repeated multiple times, which may be spread out over twenty-four hours for example, to increase the likelihood of HRM <NUM> detecting and correctly demodulating the modulated heart rate. In other examples, the modulated pacing rate sequence may be repeated if an intrinsic sensed event occurs during the delivery of the first delivered sequence. The intrinsic heart rate may spontaneously increase, a premature contraction may occur, or non-cardiac noise may be oversensed by sensing circuit <NUM> (and be oversensed by HRM <NUM> depending on the heart rate sensor employed by HRM <NUM>). When one or more sensed event signals are received from sensing circuit <NUM> by control circuit <NUM>, control circuit <NUM> may determine that the modulated pacing rate sequence should be repeated.

When control circuit <NUM> determines that the modulated pacing rate sequence should be repeated, control circuit <NUM> returns to block <NUM> to wait for an appropriate time to repeat the modulated pacing rate sequence, which may be repeated one or more times. When no further repetitions of the modulated pacing rate sequence are required, control circuit <NUM> returns to block <NUM> to continue monitoring one or more variables and determine the next numerical value(s) to be communicated via pacing rate modulation.

It is contemplated that some device related and/or patient related variables that are monitored by pacemaker <NUM> may only be communicated to HRM <NUM> when the numerical data value is above or below a given threshold or outside a given range. For example, the battery voltage of power source <NUM> may be monitored by control circuit <NUM>. An elective replacement battery voltage may be established. The elective replacement battery voltage may be a voltage level that is associated with a limited remaining functional life of the power source <NUM>, e.g., three months, based on therapy delivery demand. The numerical value of the battery voltage may be communicated through pacing rate modulation only when the numerical value is within a predetermined range, equal to or less than the elective replacement battery voltage. In this example, the battery voltage may be determined periodically, e.g., once a day, but an indication of the numerical battery voltage value may not be included in a modulated pacing rate sequence unless it has reached a threshold relative to the elective replacement battery voltage or fallen below the elective replacement battery voltage. In other examples, a modulated pacing rate sequence may be established that indicates that the battery voltage is equal to or less than the elective replacement battery voltage and is effectively a data flag or warning that is detected by HRM <NUM>.

As such, pacemaker <NUM> may be configured to monitor multiple device-related and/or patient-related variables, and control circuit <NUM> may be configured to determine which data values are to be communicated to HRM <NUM>. Some data values, e.g., that fall within a normal or expected range, may not be communicated every time they are determined. A numerical data value for a particular monitored variable may be communicated via paced heart rate modulation only if outside a normal range or represents a threshold change from a preceding data value in some examples. In other examples, a numerical data value may be communicated relatively less frequently, e.g., once a week, when within a normal range and with increasing frequency, e.g., once a day, when the numerical data value is approaching or outside the normal range. Data values determined at block <NUM> that do not require communication to HRM <NUM>, may be stored in pacemaker memory <NUM> for later communication, comparison to future measurements for detecting trends, or for radio frequency transmission during an interrogation session with external device <NUM>, as examples.

<FIG> is a timing diagram <NUM> illustrating ventricular pacing pulses <NUM> delivered by pacemaker <NUM> at modulated pacing pulse intervals <NUM> and <NUM> to communicate a numerical data value to a HRM. In the example shown, pacing pulses <NUM> are delivered according to a sequence of modulated pacing pulse intervals including relatively shorter pacing pulse intervals <NUM> corresponding to a digital "<NUM>" and relatively longer pacing pulse intervals <NUM> corresponding to a digital "<NUM>" in a binary encoding scheme. The HRM in this example is configured to detect the series of heart beats occurring at the modulated rate intervals and demodulate the rate intervals to generate a binary value of <NUM>. The HRM may convert the binary value to a corresponding numerical base <NUM> value. In the example of battery voltage of Table I, the digital signal of <NUM> may be converted to a battery voltage value of <NUM> V. The HRM may generate a display of the numerical value and/or transmit the numerical value to another device, such as external device <NUM> shown in <FIG>. In other examples, the numerical value may be compared to an alert threshold by HRM <NUM>. As long as the numerical value is in a normal range, HRM <NUM> may not take any action. If the data value is in an alert range, however, HRM <NUM> may generate a notification by producing a visual and/or audible alert. In examples where the HRM <NUM> includes therapy delivery capabilities, e.g., a pacemaker, ICD, neurostimulator, the HRM may respond to the numerical value by starting, stopping, or adjusting a therapy to account for any change in therapy being delivered by pacemaker <NUM>. For example, if the battery voltage indicates an end of service voltage of the pacemaker power supply, the HRM detecting the modulated rate intervals may initiate a pacing or other therapy delivery function to replace a therapy no longer being delivered by pacemaker <NUM>.

The pacing pulses <NUM> delivered at the modulated pacing rate intervals <NUM> and <NUM> are otherwise unchanged from pacing pulses that are delivered according to a pacing therapy. For example, pacing pulse voltage amplitude, pulse width, polarity, pulse shape and other characteristics of each ventricular pacing pulse <NUM> may remain constant between normal pacing therapy delivery and pacing rate modulation to communicate numerical data values. In some examples, the only pacing control parameter that is changed during data communication is the pacing pulse intervals, and therefore the pacing rate, according to a modulation protocol. Accordingly, each pacing pulse <NUM> shown in <FIG>, including the first, leftmost pacing pulse that is the last pacing pulse delivered prior to modulated intervals <NUM> and <NUM>, may be delivered according to identical pacing pulse amplitude, pulse width, polarity and pulse shape or waveform control parameters.

<FIG> is a timing diagram illustrating a modulated pacing rate sequence <NUM> for communicating data according to another example. In pacing rate sequence <NUM>, a header <NUM> including one or more pacing pulses delivered at one or more predefined pacing interval(s) <NUM> may be delivered ahead of the modulated pacing cycles representing a numerical data value. The header <NUM> is a predefined sequence of one or more pacing rate intervals that is to be recognized by HRM <NUM> as an indication that subsequent heart rate intervals contain encoded data. The header <NUM> may be two or more cycles at a single predefined pacing interval, two or more cycles at two or more predefined pacing intervals in a specified pattern or a single overdrive pacing cycle at a predefined pacing interval. In the example shown, three ventricular pacing pulses are delivered at a fixed header pacing interval <NUM>, which may be the same or different than the pacing intervals <NUM> and <NUM> designating a digital "<NUM>" and digital "<NUM>," respectively. While the header <NUM> is shown having a fixed pacing interval <NUM>, two alternating intervals or other pattern may be employed as a header sequence that is recognizable by HRM <NUM>. Starting modulated pacing rate sequence <NUM> with a header <NUM> including one or more pacing pulses delivered according to a predefined header sequence may increase the likelihood of HRM <NUM> successfully detecting the subsequent modulated heart rate.

Modulated pacing rate sequence <NUM> includes a data sequence <NUM> of six modulated pacing intervals representing digital zeros and ones as described in conjunction with <FIG>. The data sequence <NUM> may include one or more pacing cycles representing one or more numerical values of one or more monitored variables encoded as pacing intervals according to the rate modulation protocol. The modulated pacing rate sequence <NUM> may be terminated by a footer <NUM> which may be the same or different than header <NUM>. Footer <NUM> follows the data sequence <NUM> to indicate that the modulated pacing rate for data communication is completed. Footer <NUM> may include one or more pacing cycles at one or more predefined pacing intervals <NUM> to produce a heart rate over one or more cycles that is recognizable by HRM <NUM> as the footer <NUM> and end of heart rate modulation for data communication.

In the illustrative modulated pacing sequence <NUM> shown in <FIG>, the header <NUM>, data sequence <NUM>, and footer <NUM> are shown as consecutively following one another within modulated pacing sequence <NUM>. In other examples, one or more pacing cycles at one or more "normal" pacing pulse interval(s) may be delivered between header <NUM> and data sequence <NUM> and/or between data sequence <NUM> and footer <NUM>. A "normal" pacing pulse interval is a pacing pulse interval applied according to the pacing therapy protocol as opposed to pacing pulse intervals that are modulated according to a rate modulation protocol for communication. The normal pacing pulse interval may be a programmed lower rate interval as an example. For instance, if the programmed lower rate is <NUM> pulses per minute, corresponding to <NUM>, the header and footer pulse intervals <NUM> and <NUM> may be delivered at <NUM> intervals. The modulated pacing data sequence intervals <NUM> and <NUM> may be delivered at <NUM> and <NUM> intervals. The header <NUM> and footer <NUM> may be separated from data sequence <NUM> by one or more "normal" intervals at <NUM> at the lower rate of <NUM> pulses per minute.

When a header precedes and/or a footer follows the data sequence of modulated pacing rate intervals, the interval and/or number of intervals included in the header or footer may include information that is demodulated by the HRM <NUM>. For example, header <NUM> may be delivered according to a unique pattern or number of header pacing intervals <NUM> that identifies the monitored variable that is being communicated by data sequence <NUM>. In one example, three header intervals <NUM> may indicate that the data sequence <NUM> that follows is a numerical value of the battery voltage. In another example, five header intervals <NUM> may indicate that data sequence <NUM> that follows is a numerical value of the pacing capture threshold. In other examples, two or more different pacing intervals may be included in header <NUM> according to a predefined sequence or pattern that is identified by HRM <NUM> as indicating an associated monitored variable.

Footer <NUM> may be common to all data sequences, independent of the specific monitored variable being communicated, and merely indicates that the data sequence is complete. In other examples, footer <NUM> may replicate header <NUM> and indicate the monitored variable being communicated as well as completion of the data sequence. In other examples, footer <NUM> is optional. Data sequence <NUM> may be followed by another header distinct from header <NUM> in number of header pacing intervals, duration of pacing intervals, and/or pattern of two or more different pacing intervals. This next header is delivered according to predefined pacing rate interval modulation to indicate what the next monitored variable is for which a numerical value is to be communicated by a subsequent data sequence.

In still other examples, a single header <NUM> may be a modulated sequence of pacing intervals that indicates the number and/or type of monitored variables to be communicated by a corresponding number of subsequent data sequences. For example, the number of pacing intervals <NUM> included in header <NUM> may indicate a corresponding number of monitored variables to be communicated in a like number of subsequent consecutive data sequences that may or may not be separated by a footer <NUM> or by one or more separation beats delivered according to a predefined pacing interval or as "normal" pacing intervals. For instance, the three pacing intervals <NUM> may indicate three monitored variable data values are to be communicated. Three subsequent data sequences may follow, which may or may not be separated by footer <NUM> or other separation beats. The three subsequent data sequences may be delivered according to a predefined order, e.g., battery voltage, electrode impedance, and pacing capture threshold, and according to a pacing rate modulation protocol.

In some examples, the data sequence <NUM> may include pacing intervals set according to a cyclic redundancy check (CRC), Hamming or other error detection code. Accordingly data sequence <NUM> may include additional pacing cycles modulated to the digital "<NUM>" or " <NUM>" pacing intervals, consecutive with the modulated intervals that represent the numerical data value, to provide error detection bits in the stream of pacing intervals that are detected and demodulated by HRM <NUM>. Including error detection code with the data sequence <NUM>, header <NUM>, and/or footer <NUM> may increase the likelihood of the HRM <NUM> correctly detecting the modulated heart rate and reduce the likelihood of random pacing or intrinsic heart rate fluctuations from appearing as a modulated heart rate corresponding to real data.

<FIG> is a timing diagram of a modulated pacing rate data sequence <NUM> according to another example. In this example, data sequence <NUM> includes one or more sets of modulated pacing intervals, with each set representing a numerical value of one digit of the numerical data value being communicated. The number of pacing cycles delivered at the modulated rate interval in each set equals the numerical value of the given digit. For example, to transmit a numerical value of <NUM>, a set of <NUM> pacing cycles at a predefined modulated pacing rate interval is delivered. To transmit a multi-digit value, two or more sets of pacing intervals, one set for each digit, may be delivered sequentially in order from lowest to highest place value or from highest to lowest place value. Each set may be separated from a subsequent set, to separate the "digits" by one or more separation beats, which may be a "normal" pacing interval or a predefined separation beat pacing interval, particularly when each set is delivered at the same pacing interval as shown in <FIG>. When different pacing intervals are used for each set indicating a digit value, separation beats may not be required.

In the illustrative example shown in <FIG>, a numerical value of <NUM> is being communicated by data sequence <NUM>. The numerical value <NUM> may represent <NUM> Volts, indicating the battery voltage of power source <NUM>. A first set <NUM> of two modulated pacing intervals <NUM> are delivered to indicate the digital value of <NUM> in the highest place value. The pacing interval <NUM> is a predefined interval, e.g., <NUM>, <NUM> or <NUM> as examples, which may be a reserved pacing interval not used by a pacing therapy protocol.

The first set <NUM> may be separated from the second set <NUM> by a separation interval <NUM>. Separation interval <NUM> may be a "normal" pacing cycle according to the pacing therapy, e.g., a programmed lower rate interval as described above. Separation interval <NUM> may alternatively be a predefined modulated interval that is different that the modulated pacing interval <NUM> used within a given set <NUM>, <NUM> or <NUM> of modulated pacing intervals. A predefined number of one or more separation intervals <NUM> may separate the sets <NUM>, <NUM> and <NUM> in data sequence <NUM>.

The second set <NUM> of modulated pacing intervals <NUM> includes six intervals representing the numerical value of <NUM> for the second place value. The third set <NUM>, separated from the second set <NUM> by one or more separation intervals <NUM>, includes two modulated pacing intervals <NUM> representing a value of <NUM> in the third place value. Going from greatest to least place value, HRM <NUM> demodulates a numerical value of <NUM> from the data sequence <NUM>.

The data sequence <NUM> may be preceded by a header and/or terminated by a footer as generally described above in conjunction with <FIG> to indicate the onset and or termination of the data sequence <NUM>. In this example, the header and/or footer may include modulated information indicating, e.g., the number of data sets, i.e., the number of place values, represented by the data sequence <NUM> and/or the order from highest to lowest or lowest to highest place values.

Depending on the possible range of a numerical value of a given monitored variable, the value of a digit in a given place value may be inferred by HRM <NUM> without being represented by a set of pacing intervals within data sequence <NUM>. For example, since a normal range of battery voltage is from <NUM> to <NUM>, a set of modulated intervals representing the numerical tenths place value and a set representing the numerical hundredths place value may be included in data sequence <NUM>. A set of modulated intervals representing the numerical ones place value may be omitted. Since any tenths place value of <NUM> or higher must be preceded by a value of <NUM> in the ones place, and any tenths place value of <NUM> or less must be preceded by a value of <NUM> in the ones place to span the normal range of battery voltages from <NUM> to <NUM>, the HRM <NUM> may infer the ones place value based on the tenths place value. Thus, in some cases, all battery voltages may be communicated by a data sequence including two sets of modulated pacing intervals, with each set including the number of the modulated pacing intervals equal to the numerical digit of the respective tenths or hundredths place value. Furthermore, when the sets are transmitted from highest to lowest place value for instance, a single set of pacing intervals at the modulated pacing rate interval <NUM> may be interpreted by the HRM <NUM> as representing the tenths digit with the hundredths digit being a numerical value of zero, in this example of battery voltage. In other multi-digit data values, a specified number of separation intervals <NUM> may indicate a numerical value of zero for a place value that is in between two other non-zero place values. When the numerical value is zero for two consecutive place values, for example a battery voltage of <NUM>, and the ones place value is omitted to be inferred by HRM <NUM>, the numerical values of two consecutive zeros in the tenths and hundredths places may be represented by predefined series of separation intervals <NUM> delivered at a modulated pacing interval different than the modulated pacing interval <NUM> used within a set of pacing intervals such as sets <NUM>, <NUM> and <NUM>.

While two examples of pacing rate modulation for producing a data sequence that is detected by HRM <NUM> as modulated heart rates that can be demodulated into numerical data values have been described in conjunction with <FIG>, <FIG>, it is recognized that numerous protocols are conceivable for modulating the pacing rate to represent a numerical value of a monitored variable. The particular modulation protocol may be defined based on the heart rate detection method employed by HRM <NUM>. For instance, in the examples of <FIG>, <FIG>, it is assumed that HRM <NUM> is capable of detecting the heart rate beat by beat to distinguish the heart rate interval from one cardiac cycle to the next and therefore detect each modulated cardiac cycle length and distinguish it from a different modulated cardiac cycle length and cycle lengths (or heart rates) corresponding to "normal" pacing intervals of the programmed pacing therapy and even intrinsic heart beats.

In other examples, HRM <NUM> may detect heart rate by counting a number of PPG pulses, bioimpedance pulses, R-waves or other heart rate sensor signal feature indicative of a heartbeat over a given time interval to convert the number of heartbeats counted over the given time interval to a heart rate. Conversely, HRM <NUM> may be configured determine the time interval over which a predetermined number of sensor signal pulses or R-waves occurs and convert that time interval to a heart rate. In still other examples, HRM <NUM> may use FFT or other techniques for deriving a frequency of the heart rate sensor signal and converting the frequency to a heart rate. In these examples where the heart rate is detected over a fixed or variable time interval by counting heart beats or performing Fourier transform or other techniques for deriving a heart rate from the sensor signal, the pacing rate modulation communication protocol may involve delivering a fixed, modulated pacing rate over a time interval that is sufficiently long to be detected as the modulated heart rate by the HRM <NUM> as opposed to a single beat by beat heart rate detection as described above in conjunction with <FIG>.

<FIG> is a timing diagram <NUM> of pacing rate modulation according to another example. Rather than delivering a modulated pacing rate interval for a single pacing cycle to be detected as a modulated heart rate by the HRM <NUM>, a modulated pacing rate interval may be delivered for a plurality of cycles in order to enable the HRM to detect the corresponding heart rate. In this case, pacing pulses are delivered at a modulated pacing rate interval for a predetermined number of pacing cycles or a predetermined time interval to enable HRM <NUM> to detect the heart rate corresponding to the modulated pacing rate by counting pulses over a fixed or variable time interval as described above or performing FFT or other frequency analysis. For example, during a first data time segment <NUM>, a fixed pacing rate at a first modulated pacing rate interval <NUM> may be delivered and detected by the HRM <NUM> as digital "<NUM>" (or "<NUM>"). The fixed heart rate detected over data time segment <NUM> may be demodulated to a numerical value of a monitored variable or one of its digits. A second modulated pacing rate may be delivered over a second data time segment <NUM> to be detected as a different heart rate by HRM <NUM>. The pacing rate interval <NUM> of data time segment <NUM> may represent a digital "<NUM>" (or "<NUM>") in a binary encoding scheme or be assigned to a numerical value in another number system, such as a <NUM>, <NUM>, <NUM> or <NUM> in a base <NUM> system.

The two modulated data time segments <NUM> and <NUM> are shown separated by separation time segment <NUM> that may include pacing according to the programmed pacing therapy, e.g., according to the programmed lower rate interval. In other examples, separation time segment <NUM> may include pacing at a uniquely defined overdrive pacing rate interval <NUM> different than the modulated pacing rate intervals <NUM> and <NUM> of data time segments <NUM> and <NUM>. The second data time segment <NUM> is followed by another separation time segment <NUM> and the pacing modulation sequence <NUM> may continue with additional modulated pacing rate time intervals delivered over predetermined time segments as needed to complete communication of one or more numerical data values of a corresponding number of monitored variables. In some instances, the separation time segment <NUM> may include intrinsic heart beats, e.g., that occur at a heart rate that is faster than the programmed lower rate but may be slower than the modulated pacing rate intervals <NUM> and <NUM>.

The overall total time and number of modulated pacing cycles required to communicate a numerical value of a single monitored variable using the technique of <FIG> may be longer than the examples of <FIG>, <FIG>, however the longer time segments at a fixed modulated interval may be required depending on the heart rate detection method employed by HRM <NUM>. Time segments <NUM>, <NUM> and <NUM> may be set to be long enough to cover an entire sampling interval used by HRM <NUM> for detecting one heart rate. For example, if HRM <NUM> samples the heart rate sensor signal every <NUM> seconds to determine a heart rate value, each data time segment <NUM> and <NUM> may be <NUM> seconds long or more to promote reliable detection of the corresponding heart rate by HRM <NUM>, sampling once every <NUM> seconds. Control circuit <NUM> may extend a data time segment <NUM> or <NUM> in response to an increase in intrinsic heart rate during the time segment. For example, if one or more R-wave sensed event signals are received by control circuit <NUM> from sensing circuit <NUM> during time segment <NUM>, time segment <NUM> may be increased to three times the sampling interval, e.g., <NUM> seconds, to enable HRM <NUM> to discard a rate from one sampling interval that changed unexpectedly. Any of the pacing rate modulation examples presented in conjunction with <FIG>, or combinations or variations thereof, may be performed in the process of <FIG> according to a modulated pacing rate communication protocol.

<FIG> is a flow chart <NUM> of a method that may be performed by HRM <NUM> for detecting heart rate and demodulating the detected heart rate to determine a numerical data value according to one example. At block <NUM>, HRM <NUM> determines the heart rate according to a heart rate monitoring protocol using the heart rate detection technique implemented in HRM <NUM>. For example, HRM <NUM> may be configured to detect the heart at scheduled times of day, at predetermined time intervals, or on an ongoing basis. In order to detect a modulated heart rate due to a modulated pacing rate sequence delivered by pacemaker <NUM>, HRM <NUM> may determine the time interval between consecutive pulses or beats, i.e. the cardiac cycle length, detected by the heart rate sensor implemented in HRM <NUM>. For example, RRIs may be determined by HRM <NUM> when HRM <NUM> includes electrodes for sensing an ECG signal or cardiac cycle lengths may be determined between pulse peaks when HRM <NUM> includes an optical sensor for PPT. In other examples, HRM <NUM> may detect the heart rate over a sampling interval using FFT or other techniques.

At block <NUM>, HRM <NUM> determines if a heart rate is detected that corresponds to a modulated heart rate used for data communication. Processor <NUM> of HRM <NUM> may compare individual heart rate intervals or a detected heart rate to the modulated pacing rates established for data communication. Modulated pacing rates corresponding to the pacing rate intervals used by pacemaker <NUM> may be stored in memory <NUM> of HRM <NUM> for comparison to detected heart rates for recognizing a modulated heart rate. For example, a heart rate corresponding to the modulated pacing rate interval established as a digital "<NUM>" or a digital "<NUM>" or a known header interval may be detected over one or more cardiac cycles or a heart rate sampling interval. If a heart rate that corresponds to a modulated pacing rate interval is not detected at block <NUM>, the HRM <NUM> does not look for additional modulated heart rates and returns to block <NUM> to monitor the heart rate according to the heart rate monitoring protocol.

In some examples, when a heart rate corresponding to a modulated pacing rate is not detected, HRM <NUM> may compare the detected heart rate to a threshold rate at block <NUM>. If the detected heart rate is less than a low threshold rate or greater than a high threshold rate at block <NUM>, HRM <NUM> may remain enabled for heart rate detection by returning to block <NUM> to continue to detect the heart rate beat-by-beat or sampling interval by sampling interval to wait for a modulated heart rate. In some cases, a very low heart rate or very high heart rate, e.g., a heart rate less than <NUM> beats per minute or greater than <NUM> beat per minute, or other established thresholds appropriate for the patient, may indicate a concerning condition. Pacemaker <NUM> may be configured to communicate one or more patient related and/or device related monitored variables which may be useful in detecting or diagnosing a serious patient condition associated with the low or high heart rate that may warrant medical attention. In anticipation of important patient-related or device-related monitored variables being communicated, HRM <NUM> may remain enabled to detect a modulated heart rate when an abnormal heart rate is detected.

If a heart rate corresponding to a modulated pacing rate interval is detected at block <NUM>, processor <NUM> determines the heart rate for subsequent consecutive cardiac cycles, beat-by-beat, or subsequent heart rate sampling intervals depending on the heart rate detection technique that is implemented. Consecutive heart rates may be determined until a threshold number of detected heart rates do not correspond to a modulated pacing rate interval. For example the heart rate may be detected one or more times (on one or more cardiac cycles or sampling intervals) corresponding to the pacing lower rate or otherwise not matching the modulated pacing rate intervals defined by the modulation scheme. When a threshold number of non-modulated heart rate detections are made, the HRM <NUM> may or may not continue determining the heart rate on a continuous basis, e.g., on consecutive cardiac cycles or consecutive sampling intervals, in accordance with the normal heart rate monitoring protocol performed by HRM. In some examples, HRM <NUM> detects a modulated heart rate at block <NUM> by detecting a header including one or more modulated rate intervals as described in conjunction with <FIG>. Consecutive heart rates, beat-by-beat or over consecutive sampling intervals, may be determined until a threshold number of non-modulated heart rates are detected, until a predefined number of expected modulated heart rates are detected based on information included in the header, or until a footer is detected.

As the modulated heart rates are detected at block <NUM>, or after a complete modulated heart rate sequence is detected, the processor <NUM> demodulates the sequence of heart rates by converting each rate to a corresponding value according to the encoding scheme. For example, each single cardiac cycle length (e.g., corresponding to the data sequence in <FIG>) may be converted to a binary "<NUM>" or "<NUM>" to determine a binary word that is converted to a numerical (base <NUM>) value of the monitored variable. In other examples, HRM detects a set of cardiac cycle lengths (e.g., corresponding to the data sequence in <FIG>) and converts the number of cardiac cycles to the value of one digit of the numerical value of the monitored variable. In still other examples, HRM <NUM> converts a heart rate detected over one or more sampling intervals to a corresponding binary value (or other base number system). Consecutive cardiac cycles, sets of cardiac cycles, or sampling intervals are converted to corresponding digit values.

At block <NUM>, HRM processor <NUM> may be configured to confirm the received data which may include error detection coding as described previously. Header and/or footer detection may be used to verify that a demodulated data sequence is valid. For example, if the header indicates a specific monitored variable data value is being communicated, but the subsequently detected and demodulated heart rates do not correspond to a valid numerical value of the monitored variable (e.g., outside a specified range of possible values), the HRM processor <NUM> may determine the that the numerical value is invalid. In other examples, HRM <NUM> may be configured to detect an outlier of a monitored variable. For example, if a demodulated numerical value of the monitored variable is within a range of possible values but is greater than a threshold change from a preceding value, HRM <NUM> may flag the data value as a possible outlier. If the next numerical value of the monitored variable received after the flagged value is within a threshold change of the first preceding value, the intervening flagged data value may be deemed an outlier and considered a spurious or invalid data value.

The HRM <NUM> may provide a variety of responses to the demodulated data values. In some examples, a numerical data value is compared to alert criteria at block <NUM>. The numerical data value may be compared to an alert range indicating that medical follow-up is recommended. If the alert criteria are satisfied, as determined at block <NUM>, the HRM <NUM> may generate a display of the data and/or generate an audible, visual or other sensory alert such as a vibration at block <NUM> to notify the patient to follow up with a clinician.

In some examples, the HRM <NUM> may be configured to adjust a therapy that is delivered by the IMD system, e.g., system <NUM> shown in <FIG>. HRM <NUM> may be configured to adjust a therapy either directly or via communication with another medical device included in the IMD system. For instance, when the HRM is implemented as an ICD, the ICD may enable a pacing function to replace a pacing function provided by pacemaker <NUM> when the numerical value of a monitored battery voltage meets alert criteria or other therapy adjustment criteria. When the battery voltage reaches a threshold level, e.g., an elective replacement voltage or end of service voltage, the HRM may determine that pacing therapy delivered by pacemaker <NUM> may be stopped or adjusted to a power conserving pacing mode. The HRM may adjust its own pacing mode or other therapy based on the demodulated numerical value of the monitored variable. In other examples, the HRM may transmit a therapy adjustment command to another medical device, e.g., via external device <NUM>, to cause a therapy delivery adjustment at block <NUM>.

In another example, when the numerical value of a monitored parameter is demodulated to a value that indicates that pacemaker <NUM> may be performing temporary operations, the HRM may respond to the demodulated value by adjusting the therapy delivery at block <NUM>. For example, a transmitted value of a pacing capture threshold may be an indication that a pacing catpure threshold test is going to be performed by pacemaker <NUM>. The HRM may adjust a pacing therapy, cardiac rhythm detection or other functionality of the IMD system to avoid interference with the capture test or confounding rhythm detection results. As such, the HRM may adjust its own operation or generate a command or notification that an adjustment by another medical device included in the IMD system is needed in response to the demodulated numerical value of the monitored variable.

In still other examples, the need for a system operation adjustment may be based on any categorical data communicated according to predefined sequences of modulated pacing rate intervals. For example, the current pacing mode of pacemaker <NUM> may be demodulated by HRM from the modulated heart rates and based on the current pacing mode, the pacing mode of the HRM or another medical device may be adjusted at block <NUM>. For example, when the HRM is an extracardiovascular ICD, the ICD may enable anti-tachycardia therapy, back-up pacing therapy, or other pacing therapy previously delivered by pacemaker <NUM> when the pacing mode of pacemaker <NUM> has changed, e.g., due to a low battery voltage condition.

Each time data is received, whether or not alert criteria are met and/or system operation adjustment is needed, the HRM <NUM> may generate a notification that new data is available at block <NUM>. The user may view the data using an application programmed in the HRM <NUM>. The application may allow the user to open a window of a display, e.g., a graph or table, of the new data values, which may include historical data and/or indicators of normal ranges of the displayed numerical data values.

At block <NUM>, the HRM <NUM> may transmit the data values to a patient monitor or external device, such as external device <NUM>, or to a centralized patient monitoring database, e.g., via a BLUETOOTH ® connection. The transmitted data values may exclude data values identified as out of range or spurious values identified at block <NUM>. In some examples, HRM <NUM> may transmit detected modulated heart rates or heart rate intervals without demodulating the heart rates or heart rate intervals prior to transmission. Demodulation may be performed by a receiving device, e.g., external device <NUM> or another pacemaker, implantable cardioverter defibrillator or other medical device, or a centralized database. In this way, BLUETOOTH® capabilities of HRM <NUM> may be utilized for transmitting data communicated from pacemaker <NUM> without requiring pacemaker <NUM> to have or utilize BLUETOOTH® or other RF transmission capabilities, saving space and/or power consumption of pacemaker <NUM>.

<FIG> is a flow chart <NUM> of a method performed by system <NUM> of <FIG> according to another example. In some examples, pacemaker <NUM> may determine that an interrogation session is needed (block <NUM>) and modulate the pacing rate according to an interrogation command code at block <NUM>. For instance, pacemaker <NUM> may determine that an interrogation session is needed according to a regular schedule stored in memory <NUM>, e.g., once per day, once per week etc. The interrogation session is scheduled to retrieve data from pacemaker <NUM> using wireless RF transmission by an external device, e.g., either HRM <NUM> or external device <NUM>. Control circuit <NUM> may determine that an interrogation session is needed at a scheduled time at block <NUM> and modulate the pacing rate according to an interrogation command code at block <NUM>. In other examples, control circuit <NUM> may determine that an interrogation session is needed at block <NUM> because a scheduled interrogation session is past due or an expected interrogation session has not yet occurred. Control circuit <NUM> may not modulate the pacing rate at block <NUM> to communicate an interrogation command code to HRM <NUM> unless an expected interrogation session has not occurred or is past due by at least a threshold time interval, e.g., <NUM> hours, <NUM> hours or other threshold time interval.

In other examples, pacemaker <NUM> may determine that an interrogation session is needed at block <NUM> in response to detecting a patient-related or device-related event. For instance, pacemaker <NUM> may detect an arrhythmia episode, a change in pacing impedance or capture threshold, an out of range monitored parameter or other event that may warrant medical attention. In response to an event that warrants data transmission via wireless RF transmission as opposed to only pacing rate modulation, as described in the various examples given above, control circuit <NUM> may modulate the pacing rate at block <NUM> according to an interrogation command code to initiate the interrogation process. The modulated pacing rate for communicating an interrogation command code at block <NUM> may be repeated multiple times until an RF interrogation command transmitted by another device is received by telemetry circuit <NUM>.

At block <NUM>, HRM <NUM> detects the patient's heart rate. HRM <NUM> determines if the detected heart rate corresponds to a modulated heart rate. A modulated heart rate is detected at block <NUM> based on detection of a heart rate or heart rate intervals corresponding to designated, reserved rates or intervals used for communication or detection of a header or specified pattern of heart rates or rate intervals. When a modulated heart rate is detected at block <NUM>, HRM <NUM> determines the modulated rates and compares the modulated rate or pattern to an interrogation command code at block <NUM>. A predefined reserved heart rate, e.g., <NUM> beats per minute, for a predefined number of heart beats or time intervals may correspond to the interrogation command code. The interrogation command code may be defined according to a specified number of heart beats or specified number of heart rate time intervals including one or more specified heart rates occurring in a specified pattern.

When the interrogation command code is not detected at block <NUM>, HRM <NUM> may demodulate the detected modulated heart rate to determine numerical values of monitored parameters as described above in conjunction with any of <FIG>. When the interrogation command code is detected at block <NUM>, HRM <NUM> generates an interrogation alert at block <NUM>. The interrogation alert may by a visual display or audible sound produced by HRM <NUM> to alert the patient than an interrogation session is needed. The patient may be instructed to respond to an alert by taking any steps necessary to enable an interrogation session to occur. For example, the patient may be required to open an application on HRM <NUM> or external device <NUM> and/or hold HRM <NUM> or external device <NUM> within a specified proximity to pacemaker <NUM>, e.g., within one foot.

In other examples, the interrogation alert produced by HRM <NUM> may be a signal transmitted from HRM <NUM> to another external device, e.g., external device <NUM> shown in <FIG>. The external device <NUM> may respond to receipt of the interrogation alert by automatically starting an interrogation session at block <NUM> by transmitting a wake-up command or other signal to pacemaker <NUM> to establish a bidirectional communication link with pacemaker <NUM>. Additionally or alternatively, external device <NUM> may respond to receipt of the interrogation alert transmitted from HRM <NUM> by generating a patient alert, e.g., a notification on a display of external device <NUM>, text message, audible alarm or other patient notification method. In this case, the patient may be instructed to respond to the alert from external device <NUM> by taking necessary steps to enable the interrogation session to occur.

At block <NUM>, external device <NUM> starts the interrogation session by establishing a bidirectional communication link and transmitting an interrogation command to pacemaker <NUM>. In response to receiving the interrogation command at block <NUM>, pacemaker <NUM> transmits data via wireless RF transmission by telemetry circuit <NUM>. Data that may be transmitted during an interrogation session may include data that is not available for communication via pacing rate modulation alone. Such data may generally include larger amounts of data such as recorded cardiac electrical signal episodes and marker channel data, detected rhythms, therapy delivery and sensing control parameters, sensing and pacing data, or other data stored by pacemaker <NUM> in memory <NUM> or cardiac electrical signals transmitted in real time.

The operation of system <NUM> is improved when pacemaker <NUM> is configured to determine that an interrogation session is needed and communicate an interrogation command code via pacing rate modulation for detection by HRM <NUM> because power is conserved by avoiding frequent wakeups of a telemetry circuit of pacemaker <NUM> for listening for a ping or communication request from another device. The power required for pacemaker <NUM> to initiate a communication session by modulating the pacing rate for communicating an interrogation command is minimal since it is the same or similar power used to deliver the pacing therapy. The overall system <NUM> is further improved because compliance of the patient is promoted in retrieving data from pacemaker <NUM> for review by a clinician. Increased patient compliance enables the clinician to confirm appropriate pacemaker operation and identify any changes needed to sensing and/or therapy operations of pacemaker <NUM> in order to best meet the patient's needs. The reduced size of an intracardiac pacemaker, such as pacemaker <NUM> shown in <FIG>, may limit the power available for wireless data transmission, such as via BLUETOOTH®, and the depth of implant of an intracardiac pacemaker may limit the range of RF signals being transmitted. The ability of pacemaker <NUM> to transmit larger amounts of data to an external device is improved by communicating the interrogation command via pacing rate modulation first to subsequently promote successful and efficient RF data transmission to an external device that is placed within the transmission range of pacemaker <NUM>.

It should be understood that, depending on the example, certain acts or events of any of the methods described herein can be performed in a different sequence, Moreover, in certain examples, acts or events may be performed concurrently, e.g., through multi-threaded processing, interrupt processing, or multiple processors, rather than sequentially. In addition, while certain aspects of this disclosure are described as being performed by a single circuit or unit for purposes of clarity, it should be understood that the techniques of this disclosure may be performed by a combination of units or circuits associated with, for example, a medical device.

Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media (e.g., RAM, ROM, EEPROM, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer).

Claim 1:
A medical device system (<NUM>), comprising:
an implantable medical device (<NUM>) comprising:
a control circuit (<NUM>) configured to:
determine a numerical value of a monitored variable that is monitored by the implantable medical device (<NUM>) based on a monitoring protocol; and
convert the numerical value of the monitored variable to a data sequence comprising a plurality of modulated stimulation rate intervals; and
a pulse generator (<NUM>) configured to deliver a plurality of electrical stimulation pulses according to the plurality of modulated stimulation rate intervals of the data sequence to cause a modulated rate of activation of an excitable tissue of a patient that corresponds to the modulated stimulation rate intervals, the modulated rate of activation being detectable by a rate monitor (<NUM>) for demodulation of the modulated rate of activation to the numerical value of the monitored variable,
characterized in that
the pulse generator (<NUM>) is configured to deliver a modulated stimulation rate sequence (<NUM>) including:
a header (<NUM>) comprising at least one modulated stimulation rate interval; and
the data sequence (<NUM>) following the header (<NUM>),
wherein the control circuit (<NUM>) is configured to set the at least one modulated stimulation rate interval of the header (<NUM>) to indicate information about the data sequence (<NUM>).