Patent Publication Number: US-2011077707-A1

Title: Dual-use sensor for rate responsive pacing and heart sound monitoring

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/778,527, filed on Jul. 16, 2007, which is a continuation of U.S. patent application Ser. No. 10/703,175, filed on Nov. 6, 2003, now issued as U.S. Pat. No. 7,248,923, the specifications of which are incorporated herein by reference. 
     This application is related to commonly assigned U.S. patent application Ser. No. 10/307,896, entitled “PHONOCARDIOGRAPHIC IMAGE-BASED ATRIOVENTRICULAR DELAY OPTIMIZATION,” filed on Dec. 2, 2002, now issued as U.S. Pat. No. 7,123,962, and U.S. patent application Ser. No. 10/334,694, entitled “METHOD AND APPARATUS FOR MONITORING OF DIASTOLIC HEMODYNAMICS,” filed on Dec. 30, 2002, which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This document relates generally to cardiac rhythm management systems and particularly, but not by way of limitation, to such a system sensing heart sounds and delivering rate responsive pacing. 
     BACKGROUND 
     A heart is the center of a person&#39;s circulatory system. It includes an electromechanical system performing two major pumping functions. The left portions of the heart draw oxygenated blood from the lungs and pump it to the organs of the body to provide the organs with their metabolic needs for oxygen. The right portions of the heart draw deoxygenated blood from the organs and pump it into the lungs where the blood gets oxygenated. The body&#39;s metabolic need for oxygen increases with the body&#39;s physical activity level. The pumping functions are accomplished by contractions of the myocardium (heart muscles). An increase in the body&#39;s metabolic need for oxygen is satisfied primarily by a higher frequency of the contractions, i.e., a higher heart rate. In a normal heart, the sinoatrial node, the heart&#39;s natural pacemaker, generates electrical impulses, called action potentials, that propagate through an electrical conduction system to various regions of the heart to excite myocardial tissues in these regions. Coordinated delays in the propagations of the action potentials in a normal electrical conduction system cause the various regions of the heart to contract in synchrony such that the pumping functions are performed efficiently. 
     The functions of the sinoatrial node and the electrical conduction system are indicated by electrocardiography (ECG) with at least two electrodes placed in or about the heart to sense the action potentials. When the heart contracts irregularly or otherwise abnormally, one or more ECG signals indicate that contractions at various cardiac regions are chaotic and unsynchronized. Such conditions are known as cardiac arrhythmias. Cardiac arrhythmias result in a reduced pumping efficiency of the heart, and hence, diminished blood circulation. 
     Pacing therapy treats cardiac arrhythmias by using an implantable pacemaker to deliver electrical pulses that substitute for the action potentials to excite the myocardium, thereby restoring the functions of the sinoatrial note and/or the natural electrical conduction system. To ensure that the body receives sufficient oxygen to satisfy its metabolic needs, a pacing mode referred to as rate responsive pacing, or rate adaptive pacing, uses an indication of the body&#39;s physical activity level to dynamically adjust the pacing rate, which determines the frequency of the contractions. 
     Various mechanical functions of the heart, as well as electro-mechanical association between the electrical conduction system and the myocardium, are indicated by heart sounds. For example, amplitudes of the third heart sound (S3) and fourth heart sound (S4) are related to filing pressures of the left ventricle during diastole. Fundamental frequencies of S3 and S4 are related to ventricular stiffness and dimension. Chronic changes in S3 amplitude is correlated to left ventricular chamber stiffness and degree of restrictive filling. Change in the interval between atrial contraction and S4 is correlated to the changes in left ventricular end of diastolic pressure. Such parameters, being correlated to the heart&#39;s mechanical properties and electromechanical association, quantitatively indicate abnormal cardiac conditions such as heart failure, including degrees of severity, and need of appropriate therapies. 
     For these and other reasons, there is a need for an implantable pacemaker that senses the body&#39;s physical activity level and the heart sounds. Implantability requires that any circuit or functional module of the implantable pacemaker to be designed for the minimum size and energy consumption. 
     SUMMARY 
     An implantable medical device includes a dual-use sensor such as a single accelerometer that senses an acceleration signal. A sensor processing circuit processes the acceleration signal to produce an activity level signal and a heart sound signal. The implantable medical device provides for rate responsive pacing in which at least one pacing parameter, such as the pacing interval, is dynamically adjusted based on the physical activity level. The implantable medical device also uses the heart sounds for pacing control purposes or transmits a heart sound signal to an external system for pacing control and/or diagnostic purposes. 
     In one embodiment, a cardiac rhythm management system includes a sensing circuit, a pacing circuit, a dual-use sensor, a sensor processing circuit, and a controller. The sensing circuit senses at least one electrogram. The pacing circuit delivers pacing pulses. The dual-use sensor senses a signal indicative of activities and heart sounds. The sensor processing circuit produces an activity level signal and a heart sound signal from the sensed signal. The controller includes a rate responsive pacing algorithm execution module dynamically adjusting at least a pacing interval based on at least the activity level signal. 
     In one embodiment, a cardiac rhythm management system includes an accelerometer, a processing circuit, and a controller. The accelerometer senses an acceleration signal indicative of physical activities and heart sounds. The processing circuit has an input to receive the acceleration signal, an amplifier, and a band-pass filter. The amplifier has a programmable gain. The band-pass filter has one or more cutoff frequencies programmable for producing an activity level signal during first time periods and producing a heart sound signal during second time periods. The controller includes a processing circuit programming module adapted to program the gain and the cutoff frequencies. 
     In one embodiment, a cardiac rhythm management system includes an accelerometer to sense an acceleration signal, a first processing circuit, and a second processing circuit. The first processing circuit includes a first input to receive the acceleration signal, a first output indicative of a physical activity level, and a first gain-and-filter circuit to provide for a first gain and a first set of cutoff frequencies. The second processing circuit includes a second input to receive the acceleration signal, a second output indicative of heart sounds, and a second gain-and-filter circuit to provide for a second gain and a second set of cutoff frequencies. 
     In one embodiment, a signal indicative of activities and heart sounds is sensed using a single implantable sensor. The sensed signal is processed to produce an activity level signal and a heart sound signal. A rate responsive pacing algorithm dynamically adjusts at least one pacing parameter based on the activity level signal. At least one type of heart sounds is detected from the heart sound signal. 
     In one embodiment, an acceleration signal indicative of an activity level and heart sounds is sensed. An amplifier is programmed with a first gain suitable for sensing the activity level for a first time period. A band-pass filter is programmed with a first set of cutoff frequencies suitable for sensing the activity level for the first time period. The sensed acceleration signal is amplified and filtered to produce an activity level signal. The amplifier is programmed with a second gain suitable for sensing the heart sounds for a second time period. The band-pass filter is programmed with a second set of cutoff frequencies suitable for sensing the heart sounds for the second time period. The sensed acceleration signal is amplified and filtered to produce a heart sound signal. 
     In one embodiment, an acceleration signal is sensed. An activity level signal and a heart sound signal is produced concurrently from the acceleration signal by amplifying and filtering. The acceleration signal is amplified with a first gain and filtered with a first set of cutoff frequencies suitable for producing the activity level signal, and is amplified with a second gain and filtered with a second set of cutoff frequencies suitable for producing the heart sound signal. 
     This Summary is an overview of some of the teachings of the present application and not intended to be an exclusive or exhaustive treatment of the present subject matter. Further details about the present subject matter are found in the detailed description and appended claims. Other aspects of the invention will be apparent to persons skilled in the art upon reading and understanding the following detailed description and viewing the drawings that form a part thereof, each of which are not to be taken in a limiting sense. The scope of the present invention is defined by the appended claims and their equivalents. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, which are not necessarily drawn to scale, like numerals describe similar components throughout the several views. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document. 
         FIG. 1  is a block diagram illustrating an embodiment of a cardiac rhythm management system, including an implantable medical device, and portions of an environment in which it is used. 
         FIG. 2A  is a block diagram illustrating an embodiment of a circuit of the implantable medical device. 
         FIG. 2B  is a block diagram illustrating an embodiment of a controller being a part of the circuit of the implantable medical device. 
         FIG. 3  is a block diagram illustrating an embodiment of a circuit including a dual-use sensor and a sensor processing circuit for sensing a physical activity level and a heart sound signal. 
         FIG. 4  is a block diagram illustrating another embodiment of the circuit including the dual-use sensor and the sensor processing circuit for sensing the physical activity level and the heart sound signal. 
         FIG. 5A  is a block diagram illustrating an embodiment of the circuit of  FIG. 3  in which the sensor processing circuit includes an additional preconditioning circuit. 
         FIG. 5B  is a block diagram illustrating an embodiment of the circuit of  FIG. 4  in which the sensor processing circuit includes an additional preconditioning circuit. 
         FIG. 5C  is a block diagram illustrating an embodiment of a circuit of the additional preconditioning circuit. 
         FIG. 6  is a flow chart illustrating an embodiment of a method for sensing the physical activity level and the heart sounds by using the circuit of  FIGS. 3 and 5 . 
         FIG. 7  is a flow chart illustrating an embodiment of a method for sensing the physical activity level and the heart sounds by using the circuit of  FIGS. 4 and 5 . 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are discussed in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that the embodiments may be combined, or that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the spirit and scope of the present invention. The following detailed description provides examples, and the scope of the present invention is defined by the appended claims and their equivalents. 
     It should be noted that references to “an”, “one”, or “various” embodiments in this disclosure are not necessarily to the same embodiment, and such references contemplate more than one embodiment. 
     This document discusses, among other things, a cardiac rhythm management system including a dual-use sensor, such as a single accelerometer, for rate responsive pacing and heart sound sensing. The cardiac rhythm management system includes, for example, an implantable medical device including such as a pacemaker, a pacemaker/defibrillator, a pacemaker/drug delivery device, or a cardiac resynchronization therapy (CRT) device. The implantable medical device provides for rate responsive pacing and heart sound sensing. 
     Rate response pacing, also referred to as rate adaptive pacing, uses an indication of a patient&#39;s gross physical activity level to adjust a pacing rate, such that the cardiac output as a result of pacing meets or approaches the patient&#39;s metabolic need. One example of rate responsive pacing using acceleration to adjust the pacing rate is discussed in U.S. Pat. No. 5,179,947, entitled “ACCELERATION-SENSITIVE CARDIAC PACEMAKER AND METHOD OF OPERATION,” assigned to Cardiac Pacemakers, Inc., which is hereby incorporated by reference in its entirety. 
     Known and studied heart sounds include the “first heart sound,” or S1, the “second heart sound,” or S2, the “third heart sound,” or S3, the “fourth heart sound,” or S4, and their various sub-components. S1 is known to be indicative of, among other things, mitral valve closure, tricuspid valve closure, and aortic valve opening. S2 is known to be indicative of, among other things, aortic valve closure and pulmonary valve closure. S3 is known to be a ventricular diastolic filling sound often indicative of certain pathological conditions including heart failure. S4 is known to be a ventricular diastolic filling sound resulted from atrial contraction and is usually indicative of pathological conditions. The term “heart sound” hereinafter refers to any heart sound (e.g., S1) and any components thereof (e.g., M1 component of S1, indicative of Mitral valve closure and Mitral regurgitation). Heart sounds are used, for example, to calculate pacing parameters for improving the patient&#39;s hemodynamic performance and diagnosing a pathological condition such as heart failure. Examples of such uses are discussed in U.S. patent application Ser. No. 10/307,896, now issued as U.S. Pat. No. 7,123,962, entitled “PHONOCARDIOGRAPHIC IMAGE-BASED ATRIOVENTRICULAR DELAY OPTIMIZATION,” and U.S. patent application Ser. No. 10/334,694, entitled “METHOD AND APPARATUS FOR MONITORING OF DIASTOLIC HEMODYNAMICS,” both assigned to Cardiac Pacemakers, Inc., the specifications of which are incorporated herein by reference in their entirety. 
     An accelerometer can be used to sense both the physical activity level for rate responsive pacing and the heart sounds because the two signals feature substantially distinguishable spectrums. The acceleration measured in the direction normal to a person&#39;s chest wall is indicative of both the physical activity level and the heart sounds. The sensor specifications required for sensing the physical activity level and the sensor specifications required for sensing the heart sounds, such as bandwidth, sensitivity, noise floor, robustness, size, and power consumption are sufficiently close such that they can be satisfied by a single accelerometer having adequate size and power consumption for used in an implantable medical device. A sensor processing circuit processes the signal sensed by such an accelerometer to produce an activity level signal indicative of the physical activity level for rate responsive pacing and a heart sound signal from which heart sounds of each type can be detected. 
     Throughout this document, a “heart sound signal” includes audible and inaudible mechanical vibrations of the heart that can be sensed with a sensor such as an accelerometer. A “heart sound” refers to a recognized event in the heart sound signal. Unless noted otherwise, S1, S2, S3, and S4 refer to the first, second, third, and fourth heart sounds, respectively, as a heart sound type, or as one or more occurrences of the corresponding type heart sounds, depending on the context. An “electrogram” includes an electrocardiogram (ECG) sensed with at least an intracardiac electrode placed in the heart or an epicardial electrode placed on the heart. A “user” includes a physician or other caregiver who examines and/or treats a patient using one or more of the methods and apparatuses discussed in the present document. 
       FIG. 1  is a block diagram illustrating an embodiment of a cardiac rhythm management system  100 , and portions of an environment in which it is used. System  100  includes an implantable medical device  110 , a lead system  108 , an external system  170 , and a wireless telemetry link  160 . 
     After implantation, implantable medical device  110  operates within a body  102  to sense activities of a heart  105  and deliver one or more therapies to heart  105 . Implantable medical device  110  includes a pacemaker capable of rate responsive pacing. In one embodiment, implantable medical device  110  is an implantable pacemaker. In another embodiment, implantable medical device  110  includes a pacemaker module and one or more other therapeutic modules, such as a cardioversion-defibrillation module and a drug delivery module. In one specific embodiment, the pacemaker includes a cardiac resynchronization therapy module capable of delivering multi-site biventricular pacing. Implantable medical device  110  senses an activity level for rate responsive pacing purposes and heart sounds for various diagnostic and/or therapy control purposes. A dual-use sensor such as an accelerometer is used for sensing both the activity level and the heart sounds. In one embodiment, the dual-use sensor is within implantable medical device  110 . 
     Lead system  108  provides one or more electrical connections between implantable medical device  110  and heart  105 . It includes one or more pacing leads each having one or more electrodes for electrogram sensing and pacing pulse delivery. In one embodiment, the dual-use sensor is incorporated into a lead of lead system  108  and connected to implantable medical device through the lead. 
     External system  170  communicates with implantable medical device  110 . It allows a user and/or a patient to communicate and/or to control the operation of implantable medical device  110 . In one embodiment, external system  170  includes an external programmer. In another embodiment, external system  170  includes an advanced patient management system, such as discussed in U.S. patent application Ser. No. 10/323,604, entitled “ADVANCED PATIENT MANAGEMENT FOR DEFINING, IDENTIFYING AND USING PREDETERMINED HEALTH-RELATED EVENTS,” assigned to Cardiac Pacemakers, Inc., the specification of which is incorporated herein by reference in its entirety. The advanced patient management system allows the user to access implantable medical device  110  from a remote location. 
     Telemetry link  160  provides for data transmissions between implantable medical device  110  and external system  170 . In one embodiment, telemetry link  160  is an inductive telemetry link. In an alternative embodiment, telemetry link  160  is a far-field radio-frequency telemetry link. Telemetry link  160  provides for data transmission from implantable medical device  110  to external system  170 . This may include, for example, transmitting real-time physiological data acquired by implantable medical device  110 , extracting physiological data acquired by and stored in implantable medical device  110 , extracting therapy history data stored in implantable medical device  110 , and extracting data indicating an operational status of implantable medical device  110  (e.g., battery status and lead impedance). Telemetry link  160  also provides for data transmission from external system  170  to implantable medical device  110 . This may include, for example, programming implantable medical device  110  to acquire physiological data, programming implantable medical device  110  to perform at least one self-diagnostic test (such as for a battery status and lead impedance status), and programming implantable medical device  110  to deliver at least one therapy. Examples of signals represented by the physiological data include, but are not limited to, electrograms, heart sounds or signals indicative of heart sounds, activity level signals, and respiratory signals. In one embodiment, the physiological data also include parameters measured from one or more of these signals. In one embodiment, external system  170  or a user determines and/or adjusts a therapy based on these signals and/or physiological data. 
       FIG. 2A  is a block diagram illustrating an embodiment of a circuit of implantable medical device  110 . Implantable medical device  110  includes a sensing circuit  212 , a pacing circuit  214 , a dual-use sensor  216 , a sensor processing circuit  218 , a heart sound detector  220 , an implant controller  222 , a memory circuit  224 , and an implant telemetry module  226 . In one embodiment, these circuit elements, and possibly additional circuit elements of implantable medical device  110 , are encapsulated in a hermetically sealed implantable housing. In another embodiment, some of these circuit elements, such as dual-use sensor  216  or implant telemetry module  224 , are located outside of the hermetically sealed implantable housing. 
     Sensing circuit  212  and pacing circuit  214  are both electrically coupled to heart  105  via lead system  108 . Sensing circuit  212  includes an amplifier circuit suitable for sensing one or more electrograms from heart  108  through lead system  108 . Pacing circuit  214  includes a pulse generator generating electrical pacing pulses that are delivered to heart  105  through lead system  108 . 
     Dual-use sensor  216  senses a signal indicative of two activities, events, or quantities having distinguishable frequency characteristics. The term “dual-use” refers to the fact that the two activities, events, or quantities are extracted separately from the signal and used for different purposes serving the functions of system  100 . In one embodiment, dual-use sensor  216  includes a single accelerometer that senses an acceleration signal indicative of a patient&#39;s gross physical activity level and heart sounds of the patient. In one embodiment, dual-use sensor  216  is an integrated circuit accelerometer. A specific example of such an integrated circuit accelerometer is a piezoelectric accelerometer made by Endevco Corporation (Model 12 Picochip Accelerometer). Other examples include piezoresistive and capacitive accelerometers. In one embodiment, dual-use sensor  216  is encapsulated in the hermetically sealed implantable housing. This embodiment provides the accelerometer with an environment ensuring a stable operation. In an alternative embodiment, dual-use sensor  216  is incorporated into a lead of lead system  108 . This embodiment allows the accelerometer to be located in or near heart  105 , thus being more sensitive to the heart&#39;s mechanical activities such as vibrations (heart sounds). 
     Sensor processing circuit  218  processes the acceleration signal to produce an activity level signal indicative of the patient&#39;s gross physical activity level and a heart sound signal indicative of the patient&#39;s heart sounds. Embodiments of sensor processing circuit  218  are discussed below, with reference to  FIGS. 3-5 . 
     In one embodiment, heart sound detector  220  detects heart sounds from the heart sound signal produced by sensor processing circuit  218 . In one embodiment, implantable controller  222  receives the detected heart sounds use it, in addition to the activity level signal, for rate responsive pacing purposes. In a further or alternative embodiment, implantable controller  222  uses the detected heart sounds for adjusting atrioventricular and/or interventricular pacing delays, such as in a cardiac resynchronization therapy for heart failure. The detected heart sounds, and/or parameters measured from the detected heart sounds, are also transmitted to external system  170  through telemetry link  160  for further analysis by the system or the user. In an alternative embodiment, the heart sound signal is transmitted to external system  170  though telemetry link  160 . External system  170  detects and analyzes the heart sound signal for diagnostic and/or pacing control purposes. Heart sound detector  220  is configured and/or programmed by external system  170  to detect one or more of the S1, S2, S3, and S4 type heart sounds. 
     Implant controller  222  controls the operation of the entire implantable medical device  110 . An embodiment of implantable device  222  is discussed below, with reference to  FIG. 2B . In one embodiment, implant controller  222  is implemented using a microprocessor. Memory circuit  224  provides a storage medium for a device control code, parameters for the operation of implantable medical device  110 , and the data acquired by implantable medical device  110 . In one embodiment, memory circuit  224  includes a buffer for storing the signal sensed by dual-use sensor  216 , the activity level signal produced by sensor processing circuit  218 , and/or the heart sound signal produced by sensor processing circuit  218 . In another embodiment, the buffer also stores the one or more electrograms sensed by sensing circuit  212 . In a further embodiment, implant controller  222  includes an event detector to detect cardiac events and a maker generator to generate event markers representing the cardiac events. Examples of such cardiac events include sensed events (intrinsic depolarizations) and paced events (paced contractions or pulse deliveries) associated with one or more cardiac sites. Each event mark is indicative of the type and the timing of one cardiac event. In this embodiment, the buffer further stores the event markers. In one embodiment, the activity level signal and/or the heart sound signal are synchronized with the event markers such that the event markers serve as a timing reference relating the activity level and/or heart sound to the cardiac events. In one embodiment, implant controller  222  includes an analog-to-digital converter to digitize one or more of the signal sensed by dual-use sensor  216 , the activity level signal, the heart sound signal, and the electrograms for storage and/or further processing. The analog-to-digital converter has a programmable sampling rate. Implantable controller  222  includes a digitization control module to control this sampling rate. In one embodiment, the sampling rate is programmable through external system  170 . 
     Implant telemetry module  226  includes an antenna and a transceiver to support two-way communications with external system  170  via telemetry link  160 . In one embodiment, one or more of the electrograms, the event markers, the signal sensed by dual-use sensor  216 , the activity level signal, and the heart sound signal are transmitted to external system  170  in real time. In another embodiment, one or more of the electrograms, the event markers, the signal sensed by dual-use sensor  216 , the activity level signal, and the heart sound signal are stored in the buffer of memory circuit  224  and retrieved from the buffer when needed. In one embodiment, the retrieval occurs at predetermined times as controlled by implant controller  222 . In another embodiment, the retrieval occurs in response to a command from external system  170 . 
       FIG. 2B  is a block diagram illustrating an embodiment of implant controller  222 . Implant controller  222  executes the device control code stored in memory circuit  224 . It includes, among other control modules, a rate responsive pacing algorithm execution module  230  and a sensor processing circuit programming module  232 . 
     Rate responsive pacing algorithm execution module  230  controls the timing of the pacing pulse delivery from pacing circuit  214 , based on predefined pacing logic and timing rules and one or more of the activity level signal, the sensed electrograms, timing of previous pacing pulse deliveries, the detected heart sounds, and possibly other physiological signals indicative of electrical events, mechanical activities, and/or hemodynamic performance of heart  105 . It includes a pacing interval calculator to calculate a pacing interval based on at least the activity level signal and predetermined maximum and minimum pacing intervals. When the pacing interval calculator produces a new value for the pacing interval, rate responsive pacing algorithm execution module  230  updates the pacing interval with the new value. In one embodiment, rate responsive pacing algorithm execution module  230  performs the calculation and the update dynamically, for each and every heart beat. 
     In one embodiment, sensor processing circuit programming module  232  controls the timing, gain, and/or frequency responses of sensor processing circuit  218  to produce the activity level signal and the heart sound signal. The programming of sensor processing circuit  218  is discussed below with reference to  FIGS. 3 and 4 . 
       FIG. 3  is a block diagram illustrating an embodiment of a circuit including an accelerometer  316  for sensing the acceleration signal and a sensor processing circuit  318  for producing the activity level signal and the heart sound signal from the acceleration signal. Accelerometer  316  is one embodiment of dual-use sensor  216  or a portion thereof. Sensor processing circuit  318  is one embodiment of sensor processing circuit  218  or a portion thereof. 
     Sensor processing circuit  318  includes an amplifier  340 , a band-pass filter  342 , and a demultiplexer (DEMUX)  343 . It produces the activity level signal and the heart sound signal from the acceleration signal sense by accelerometer  316  on a time-sharing basis. During predetermined first time periods, sensor processing circuit  318  produces the activity level signal. During predetermined second time periods, sensor processing circuit  318  produces the heart sound signal. The first and second time periods do not overlap. Sensor processing circuit programming module  232  controls the first time periods for producing the activity level signal and the second time periods for producing the heart sound signal by programming the gain of amplifier  340 , the cutoff frequencies of band-pass filter  342 , and the connections within demultiplexer  343 . Thus, sensor processing circuit  318  has an input to receive the acceleration signal, an output representative of the activity level signal during the first periods, and another output representative of the heart sound signal during the second periods. In one embodiment, the gain and/or the cutoff frequencies are predetermined and stored in memory circuit  224 . In one specific embodiment, the gain and/or the cutoff frequencies are empirically determined based on data collected from the patient treated with system  100 , and programmed into implantable medical device  110  by using external system  170 . In one embodiment, the gain and/or the cutoff frequencies are adjustable by the user, when necessary, after the implantation of implantable medical device  110 . The adjustments may become necessary when, for example, the range of the amplitude of the sensed acceleration signal has significantly changed, or when a different type of the heart sound is sought. Demultiplexer  343  receives the output of band-pass filter  342  and provides two outputs separately representing the activity level signal and the heart sound signal. 
     For producing the activity level signal, the gain of amplifier  340  is a first gain programmable in the range of 100 to 500. The cutoff frequencies of band-pass filter  342  are a first set of cutoff frequencies including a first low cutoff frequency programmable in a range of 0.5 Hz to 2 Hz and a first high cutoff frequency programmable in a range of 5 Hz to 15 Hz. In one specific embodiment, sensor processing circuit programming module  232  programs the first gain to 125, the first low cutoff frequency to 1 Hz, and the first high cutoff frequency to 10 Hz during the predetermined first time periods. For producing the heart sound signal, the gain of amplifier  340  is a second gain programmable in the range of 500 to 2000. The cutoff frequencies of band-pass filter  342  are a second set of cutoff frequencies including a second low cutoff frequency programmable in a range of 5 Hz to 10 Hz and a second high cutoff frequency programmable in a range of 50 Hz to 200 Hz. In one specific embodiment, sensor processing circuit programming module  232  programs the second gain to 1000, the second low cutoff frequency to 10 Hz, and the second high cutoff frequency to 100 Hz during the predetermined second time periods. 
     Sensor processing circuit  318  requires only one set of an amplifier and a filter to produce both the activity level signal and the heart sound signal. It is suitable for applications in which the activity level and the heart sounds need not be sensed concurrently. For an implantable pacemaker that already requires an accelerometer for the purpose of rate responsive pacing, sensor processing circuit  318  provides for heart sound sensing with minimal additional requirement for circuit size and energy consumption. 
       FIG. 4  is a block diagram illustrating another embodiment of the circuit including accelerometer  316  for sensing the acceleration signal and a sensor processing circuit  418  for concurrently producing the activity level signal and the heart sound signal from the acceleration signal. Accelerometer  316  is one embodiment of dual-use sensor  216  or a portion thereof. Sensor processing circuit  418  is one embodiment of sensor processing circuit  218  or a portion thereof. 
     Sensor processing circuit  418  includes a first processing circuit  450 A for producing the activity level signal and a second processing circuit  450 B for producing the heart sound signal. First processing circuit  450 A includes a first amplifier  440 A having a first gain and a first band-pass filter  442 A having a first set of cutoff frequencies. Second processing circuit  450 B includes a second amplifier  440 B having a second gain and a second band-pass filter  442 B having a second set of cutoff frequencies. First processing circuit  450 A and second processing circuit  450 B operate in parallel to allow concurrent sensing of the physical activity level and the heart sounds. Thus, sensor processing circuit  418  has an input to receive the acceleration signal, an output representative of the activity level signal, and another output representative of the heart sound signal. In one embodiment, the gains and/or the cutoffs frequencies are predetermined and stored in memory circuit  224 . While there is no need to program the gains and the cutoff frequencies for the time-sharing purpose, in one embodiment, the gains and/or the cutoffs frequencies are programmable to ensure proper sensing under each patient&#39;s particular circumstances. In one embodiment, the gain and/or the cutoff frequencies are empirically determined based on data collected from the patient, and programmed into implantable medical device  110  by using external system  170 . In one embodiment, the gain and/or the cutoff frequencies are adjustable by the user, when necessary, after the implantation of implantable medical device  110 . 
     The gain of amplifier  440 A (the first gain) is programmable in the range of 100 to 500. The cutoff frequencies of band-pass filter  442 A (the first set of cutoff frequencies) include a first low cutoff frequency programmable in a range of 0.5 Hz to 2 Hz and a first high cutoff frequency programmable in a range of 5 Hz to 15 Hz. In one specific embodiment, sensor processing circuit programming module  232  programs the first gain to 125, the first low cutoff frequency to 1 Hz, and the first high cutoff frequency to 10 Hz during the predetermined first time periods. The gain of amplifier  440 B (the second gain) is programmable in the range of 500 to 2000. The cutoff frequencies of band-pass filter  442 B (the second set of cutoff frequencies) include a second low cutoff frequency programmable in a range of 5 Hz to 10 Hz and a second high cutoff frequency programmable in a range of 50 Hz to 200 Hz. In one specific embodiment, sensor processing circuit programming module  232  programs the second gain to 1000, the second low cutoff frequency to 10 Hz, and the second high cutoff frequency to 100 Hz during the predetermined second time periods. 
     Sensor processing circuit  418  allows concurrent sensing of the activity level and the heart sounds. With sensor processing circuit  418 , dual-use sensor  216  is usable for sensing the activity level and the heart sounds simultaneously when needed. 
       FIG. 5A  is a block diagram illustrating an embodiment of the circuit of  FIG. 3  with an additional preconditioning circuit  552 . In this embodiment, sensor processing circuit  218  includes preconditioning circuit  552  with its input connected to accelerometer  316  and its output connected to sensor processing circuit  318 . 
       FIG. 5B  is a block diagram illustrating an embodiment of the circuit of  FIG. 4  with an additional preconditioning circuit  552 . In this embodiment, sensor processing circuit  218  includes preconditioning circuit  552  with its input connected to accelerometer  316  and its output connected to sensor processing circuit  418 . 
       FIG. 5C  is a block diagram illustrating an embodiment of a circuit of preconditioning circuit  552 . Preconditioning circuit  552  provides for initial conditioning or processing of the acceleration signal before being processed for producing the activity level signal and the heart sound signal. 
     In one embodiment, preconditioning circuit  552  includes a preconditioning amplifier  554  having a preconditioning gain and a preconditioning band-pass filter  556  having a set of preconditioning cutoff frequencies. In one embodiment, the preconditioning gain is programmable in the range of 100 to 500. The overall gains for producing the activity signal and the heart sound signal are products of the preconditioning gain (gain of preconditioning amplifier  554 ) multiplied by the gains of sensor processing circuit  318  or  418  as discussed above. The gains to be programmed to sensor processing circuit  318  or  418  are calculated by dividing the gains discussed above by the programmed preconditioning gain. That is, the gain of amplifier  340  includes a first gain in the range of 100 to 500 divided by the preconditioning gain for producing the activity signal, and a second gain in the range of 500 to 2000 divided by the preconditioning gain for producing the heart sound signal. The gain of amplifier  440 A (the first gain) is in the range of 100 to 500 divided by the preconditioning gain. The gain of amplifier  440 B (the second gain) is in the range of 500 to 2000 divided by the preconditioning gain. In one embodiment, the set of preconditioning cutoff frequencies includes a low preconditioning cutoff frequency programmable in the range of 0.5 to 2 Hz, and a high preconditioning cutoff frequency programmable in the range of 50 to 200 Hz. In one specific embodiment, with sensor processing circuit  318 , the preconditioning gain is 125, the low preconditioning cutoff frequency is 1 Hz, the high preconditioning cutoff frequency programmable is 100 Hz, the first gain for amplifier  340  is 1, and the second gain for amplifier  340  is 8. In an alternative specific embodiment, with sensor processing circuit  418 , the preconditioning gain is 125, the low preconditioning cutoff frequency is 1 Hz, the high preconditioning cutoff frequency programmable is 100 Hz, the gain for amplifier  440 A is 1, and the gain for amplifier  440 B is 8. 
     In one embodiment, preconditioning circuit  552  further includes an analog-to-digital converter (ADC)  558  to digitize the acceleration signal. This allows sensor processing circuit  318  or sensor processing circuit  418  to be implemented with digital technology. That is, the activity level signal and the heart sound signal are digital signals produced from the digitized acceleration signal using digital signal processing. In one embodiment, ADC  558  has a programmable sampling rate, and implantable controller  222  includes a digitization control module to control this sampling rate. In one further embodiment, the sampling rate is programmable through external system  170 . In general, sensor processing circuit  218  can be implemented with hardware, software, and a combination of both. 
       FIG. 6  is a flow chart illustrating an embodiment of a method for sensing the physical activity level and the heart sounds with the circuit illustrated in  FIGS. 3 and 5 . The method illustrates by way of example, but not by way of limitation, a use of the circuit that includes accelerometer  316 , preconditioning circuit  552 , and sensor processing circuit  318 . 
     Accelerometer  316  senses an acceleration signal at  600 . Preconditioning circuit  552  preconditions the sensed acceleration signal at  605 . In one embodiment, preconditioning circuit  552  amplifies and filters the sensed acceleration signal. In a further embodiment, preconditioning circuit  552  digitizes the sensed acceleration signal. The digitization allows subsequent processing to be performed using digital signal processing technology. 
     Sensor processing circuit  318  is programmed with the first gain and the first set of cutoff frequencies for first time periods at  610 . During the first time periods, sensor processing circuit  318  amplifies and filters the acceleration signal to produce the activity level signal at  620 . Sensor processing circuit  318  is programmed with the second gain and the second set of cutoff frequencies for second time periods at  630 . During the second time periods, sensor processing circuit  318  amplifies and filters the acceleration signal to produce the heart sound signal at  640 . In one embodiment, the first and second time periods are programmed into memory circuit  224  for use by sensor processing circuit programming module  232 , which programs the gain and the cutoff frequencies of sensor processing circuit  318 . The first and second time periods do not overlap. 
     Rate responsive pacing algorithm execution module  230  adjusts a pacing parameter such as the pacing interval based on at least the activity level signal at  650 . In one embodiment, rate responsive pacing algorithm execution module  230  also adjusts the pacing interval and/or one or more other pacing parameters based on other signals such as the electrograms and the heart sound signal. 
     Heart sounds are detected from the heart sound at  660 . In one embodiment, heart sound detector  220 , which is a part of implantable medical device  110 , detects the heart sounds from the heart sound signal. In one embodiment, the detected hearts sounds are used by implant controller  222  for pacing control purposes. In another embodiment, the heart sound signal and/or information extracted from the detected heart sounds are transmitted to external system  170 . In an alternative embodiment, the heart sound signal is transmitted to external system  170 , which includes a heart sound detector to detect the heart sounds. The heart sound detection includes detection of predetermined types of heart sounds including one or more of S1, S2, S3, and S4. The heart sounds are analyzed at  670 . The analysis includes measurement of, for example, one or more of amplitude of any type heart sound, relative amplitude between any two types of heart sounds, duration of each type heart sound, interval between any type or types of heart sounds, interval between any type heart sound and any type electrical event of the heart, fundamental frequency of each type heart sound, and harmonic frequency of each type heart sound. In one embodiment, the results of the analysis, such as parameters generated from the above measurements, are used to determine a pacing parameter based on the heart sounds at  680 . One example of determining a pacing parameter based on the heart sounds is discussed in U.S. patent application Ser. No. 10/307,896. In one embodiment, one or more parameters generated from the above measurements are trended at  690 . One example of trending heart sound related parameters is discussed in U.S. patent application Ser. No. 10/334,694. 
       FIG. 7  is a flow chart illustrating an embodiment of a method for sensing the physical activity level and the heart sounds with the circuit illustrated in  FIGS. 4 and 5 . The method illustrates by way of example, but not by way of limitation, a use of the circuit that includes accelerometer  316 , preconditioning circuit  552 , and sensor processing circuit  418 . 
     Accelerometer  316  senses an acceleration signal at  700 . Preconditioning circuit  552  preconditions the sensed acceleration signal at  705 . In one embodiment, preconditioning circuit  552  amplifies and filters the sensed acceleration signal. In a further embodiment, preconditioning circuit  552  digitizes the sensed acceleration signal. The digitization allows subsequent processing to be performed using digital signal processing technology. 
     First processing circuit  450 A of sensor processing circuit  418  produces the activity level signal from the acceleration signal at  710 . This includes amplifying the acceleration signal with the first gain at  712  and filtering the acceleration signal with the first set of cutoff frequencies at  714 . Rate responsive pacing algorithm execution module  230  adjusts a pacing parameter such as the pacing interval based on the activity level signal at  720 . In one embodiment, rate responsive pacing algorithm execution module  230  also adjusts the pacing interval and/or one or more other pacing parameters based on other signals such as the electrograms and the heart sound signal. 
     Second processing circuit  450 B of sensor processing circuit  418  produces the heart sound signal from the acceleration signal at  730 . This includes amplifying the acceleration signal with the second gain at  732  and filtering the acceleration signal with the second set of cutoff frequencies at  734 . Heart sounds are detected from the heart sound signal at  740  and analyzed at  750 . In one embodiment, the results of the analysis are used to determine a pacing parameter based on the heart sounds at  760 . In one embodiment, the results of the analysis are used for trending one or more parameters measured from the heart sounds at  770 . In one embodiment, step  660  is identical or similar to step  740 , step  670  is identical or similar to step  750 , step  680  is identical or similar to step  760 , and step  690  is identical or similar to step  770 . 
     Because first processing circuit  450 A and second processing circuit  450 B are separate circuits producing distinguished signals from a common signal, step  710  (and its subsequent step  720 ) and step  730  (and its subsequent steps  740 ,  750 ,  760 , and  770 ) can be performed concurrently. 
     It is to be understood that the above detailed description is intended to be illustrative, and not restrictive. For example, the sensor processing circuit can be expanded to produce additional signals from the acceleration signal, such as a respiration-indicative signal, if the additional signals each have a distinguishable spectrum. Other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.