Abstract:
An implantable device and method for monitoring S1 heart sounds with a remotely located accelerometer. The device includes a transducer that converts heart sounds into an electrical signal. A control circuit is coupled to the transducer. The control circuit is configured to receive the electrical signal, identify an S1 heart sound, and to convert the S1 heart sound into electrical information. The control circuit also generates morphological data from the electrical information. The morphological data relates to a hemodynamic metric, such as left ventricular contractility. A housing may enclose the control circuit. The housing defines a volume coextensive with an outer surface of the housing. The transducer is in or on the volume defined by the housing.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 13/605,397, filed Sep. 6, 2012, now issued as U.S. Pat. No. 8,535,235, which is a continuation of application Ser. No. 13/178,945, filed Jul. 8, 2011, now issued as U.S. Pat. No. 8,277,389, which is a continuation of U.S. application Ser. No. 12/703,533, filed Feb. 10, 2010, now issued as U.S. Pat. No. 8,007,442, which is a continuation of U.S. application Ser. No. 11/142,851, filed Jun. 1, 2005, now issued as U.S. Pat. No. 7,670,298, each of which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     This document relates to cardiac rhythm management devices generally, and more particularly to cardiac rhythm management devices that employ a sensing device to detect a heart sound and to extract morphological data therefrom, in order to relate the heart sound to a hemodynamic metric. 
     BACKGROUND 
     Cardiac pacemakers generally provide functions including sensing electrical signals generated by the heart, controlling stimulation of excitable tissues in the heart, sensing the response of the heart to such stimulation, and responding to inadequate or inappropriate stimulus or response (e.g., dysrhythmia) to deliver therapeutic stimuli to the heart. Some pacemakers employ cardiac resynchronization therapy. Some existing cardiac pacemakers also function to communicate with an external programmer device to support a variety of monitoring, diagnostic and configuration functions. 
     Certain cardiac pacemakers, defibrillators with pacing and/or cardiac resynchronization therapy (CRT) capabilities, and CRT devices (collectively referred to herein by the term “pacemaker”) include an internal accelerometer for measuring the level of activity of the patient (e.g., movement caused by walking around, or by muscle twitches). Such pacemakers process (e.g., filter) the accelerometer signal to reduce noise interfering with the measurement of the patient&#39;s motion-related activity, such as the sounds generated by the heart itself, and then use the processed signals as inputs to one or more algorithms for generating the signals used to control the stimulation of the heart. For example, if the accelerometer indicates that a patient is walking briskly, the pacemaker may stimulate the heart to beat at a faster rate (often subject to an upper rate limit) than when the patient is at rest. 
     Pacemakers are typically electrically coupled to a patient&#39;s heart by a lead system. The lead system may include one or multiple leads that may provide electrical contact with one or multiple chamber of a patient&#39;s heart. Some leads may contain an accelerometer at their distal end. When implanted, the accelerometer is located within a patient&#39;s heart, and may detect sounds emitted by the heart. Such a scheme may be used, for example, to detect an S1 heart sound (an S1 heart sound is the first sound made by the heart during a cardiac cycle). It is known that an S1 heart sound contains data content related to left ventricular contractility, a characteristic of the heart that reveals the capacity of the myocardium to shorten, and therefore to circulate blood through the body. A pacemaker system such as the one described may measure S1 heart sounds as a means to gather information about the contractility of the patient&#39;s heart. 
     The above-described scheme exhibits certain shortcomings, however. Such a scheme may lead to the use of two accelerometers—an internal accelerometer for use in adjusting the pacing rate during instances of physical exertion by the patient, and an external accelerometer situated in the heart for the purpose of monitoring heart sounds. Disposing an accelerometer on the tip of a lead is costly, and could be avoided if an internal accelerometer could be used to detect heart sounds with a sufficient signal-to-noise ratio to permit extraction of data content related to cardiac performance (such as left ventricular contractility). 
     SUMMARY 
     Against this backdrop the present invention was developed. According to one embodiment, an implantable device includes a transducer that converts heart sounds into an electrical signal. A control circuit is coupled to the transducer. The control circuit is configured to receive the electrical signal, identify an S1 heart sound, and convert the S1 heart sound into morphological data that relates to a rate of change of pressure within a ventricle of a heart. A housing encloses the control circuit. The transducer is located in a region in or on the housing. 
     According to another embodiment, a method includes using a transducer located outside of a heart to detect an S1 heart sound. The S1 heart sound is converted into an electrical signal using the transducer. Morphological data is extracted from the electrical signal. The morphological data relates to a rate of change of pressure within a ventricle of the heart. 
     According to yet another embodiment, a system includes an implantable device and an external system. The implantable device includes a transducer located in or on the implantable device. The transducer is configured to convert heart sounds into an electrical signal. A first control circuit is coupled to the transducer, and is configured to receive the electrical signal. The implantable device also includes a first interface circuit for communicating with the external system. The external system includes a second interface circuit for communicating with the implantable device. A second control circuit is coupled to the second interface circuit. The first and second control circuits cooperate to identify an S1 heart sound, and to generate morphological data from the S1 heart sound. The morphological data relates to a rate of change of pressure in a ventricle of a heart. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts data supporting the notion that the S1 heart sound contains certain data content related to maximum rate of change of left ventricular pressure. 
         FIG. 2  depicts a method of analyzing accelerometer data to determine a morphological characteristic related to a hemodynamic metric, according to some embodiments of the present invention. 
         FIG. 3A  depicts a chart presenting rate of change in left ventricular pressure (y-axis) versus time (x-axis). 
         FIG. 3B  depicts a chart presenting conditioned accelerometer data (y-axis) versus time (x-axis). 
         FIG. 4  depicts a signal flow scheme that may be used to implement the method of  FIG. 2 , according to some embodiments of the present invention. 
         FIG. 5  depicts an exemplary system for performance of the methods and schemes disclosed herein. 
         FIG. 6  depicts a method of adjusting parameters that influence a process, according to some embodiments of the present invention. 
         FIG. 7  depicts a method of identifying occurrence of a cardiac event, according to some embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     During the course of a cardiac cycle, blood flows from the peripheral venous system to the right atrium. From the right atrium, blood passes through the tricuspid valve to the right ventricle. Blood exits the right ventricle, through the pulmonic valve, into the pulmonary artery, and is directed through the lungs, so that the blood may be reoxygenated. Oxygenated blood from the lungs is drawn from the pulmonary vein to the left atrium. From the left atrium, blood passes though the mitral valve to the left ventricle. Finally, the blood flows from the left ventricle, through the aortic valve, to the peripheral arterial system in order to transfer oxygenated blood to the organs of the body. 
     As the blood circulates and the various valves open and close (as just described), certain heart sounds are produced. The heart sounds occur in a fixed sequence and are respectively referred to as S1, S2, S3 and S4. 
     The S1 heart sound is caused by acceleration and deceleration of blood, and closure of the mitral and tricuspid valves. The S1 heart sound generated during a given cardiac cycle exhibits morphological characteristics that relate to the maximum rate of change of pressure in the left ventricle during the given cardiac cycle. The maximum rate of change of pressure in the left ventricle is related to, and may be used as a proxy measurement for, left ventricular contractility. Left ventricular contractility is important, because it indicates the capacity of the left ventricle to contract, and therefore to circulate blood through the peripheral arterial system. 
       FIG. 1  depicts data illustrating that the S1 heart sound contains certain data content related to maximum rate of change of left ventricular pressure.  FIG. 1  presents a chart having an x-axis and a y-axis. Maximum rate of change of left ventricular pressure for a given cardiac cycle is measured along the x-axis in units of millimeters of mercury per second (mmHg/s). Median peak-to-peak amplitude exhibited by S1 heart sounds over the past N cardiac cycles is measured along the y-axis in units of mG, where N is on the order of 10, for example, between 5 and 25. (One scheme by which “median peak-to-peak amplitude” is determined is discussed below). 
     To obtain the data presented in  FIG. 1 , animal testing was performed. During the test, the animal was at rest, and its left ventricular filling pressure was monitored and determined to be constant. Over a span of time, a drug known to modify myocardial contractility was administered. At intervals, the animal&#39;s maximum rate of left ventricular pressure change was measured, and was mated with the median peak-to-peak amplitude exhibited by S1 heart sounds over the past N=approximately 10 (5-25) cardiac cycles, as measured by an accelerometer located at a point remote from the animal&#39;s heart. (The accelerometer was located within a cardiac rhythm management device implanted in the animal). Further, the accelerometer data was signal conditioned (discussed below) prior to measurement of the peak-to-peak amplitude. Thus, a given data point on the chart of  FIG. 1  is determined by the maximum rate of left ventricular pressure change during a given cycle, and the median peak-to-peak amplitude exhibited by S1 heart sounds over the past N cardiac cycles. 
     As can be seen from  FIG. 1 , the median peak-to-peak amplitude exhibited over a span of cardiac cycles increases (approximately linearly) with the maximum rate of left ventricular pressure change. Therefore, by measuring the median peak-to-peak amplitude exhibited over a span of N cardiac cycles, the maximum rate of left ventricular pressure change may be determined. 
     Accordingly,  FIG. 2  illustrates a method useful in arriving at data indicative of left ventricular contractility. The example of  FIG. 2  begins with the reception of raw accelerometer data, as shown in operation  200 . Thereafter, the raw accelerometer data is conditioned (discussed below), for example, to remove noise, respiratory components, and baseline wander (operation  202 ). The resulting data stream substantially represents the sounds emitted by the heart. 
     This conditioned signal is then processed so as to isolate the S1 complex, as shown in operation  204 . The result of such a process is depicted in  FIG. 3B .  FIG. 3B  presents conditioned accelerometer data (y-axis) versus time (x-axis).  FIG. 3A  presents rate of change in left ventricular pressure (y-axis) versus time (x-axis). 
       FIG. 3B  contains a region identified by a dashed box. The dashed box identifies the S1 complex. The process of isolating the S1 complex refers to identifying a point in time t m  at which the S1 complex begins, and a point in time t n  at which the S1 complex ends. 
     Returning to  FIG. 2 , after isolation of the S1 complex at operation  204 , peak values are extracted at operation  206 . For example, the exemplary isolated S1 complex depicted in  FIG. 3B  contains a global maxima labeled “Max,” and a global minima labeled “Min.” The minima and maxima are “global” over the span of time between t m  and t n . The combined result of operations  200 - 206  is that, for each cardiac cycle, the amplitude values at each global maxima and minima exhibited by an S1 heart sound are extracted and stored in a manner to preserve their relationship to the cardiac cycle from which they were extracted. 
     Next, in operations  208  and  210 , the median minima and the median maxima exhibited over the last N cardiac cycles are found. For example, assuming that the peak values had been extracted from the J th  cardiac cycle in a given instance of execution of operation  206 , then operations  208  and  210  yield the median minima and the median maxima exhibited over cardiac cycles J−N+1 through J. 
     Finally, in operation  212 , the median minima determined in operation  208  is subtracted from the median maxima determined in operation  210 . The result of operation  212  is an example of a “median peak-to-peak-amplitude,” as referred to above with reference to  FIG. 1 . 
       FIG. 4  depicts a signal flow scheme that may be used to implement the method of  FIG. 2 . As can be seen from  FIG. 4 , raw accelerometer data is supplied to both a bandpass filter  400  and a baseline estimator  402 . The bandpass filter  400  is characterized by upper and lower cutoff frequencies that are set to pass frequency content included in heart sounds. In one example, the lower and upper cutoff frequencies are approximately 10 Hz and 50 Hz, respectively. The cutoff frequencies are also set to reject frequency content due to movement of the patient (e.g., walking or muscular twitching), and to pass frequency content of the heart sounds. 
     Typically, the upper and lower cutoff frequencies of a bandpass filter (such as bandpass filter  400 ) are determined by two sets of poles. A first set of poles determines the lower cutoff frequency (e.g., placement of poles at 10 Hz generates a lower cutoff frequency at approximately 10 Hz). Similarly, a second set of poles determines the upper cutoff frequency (again, placement of poles at 50 Hz generates an upper cutoff frequency at approximately 50 Hz). In order to yield a narrow passband region, the sets of poles determining the upper and lower cutoff frequencies may be oriented in proximity to one another (i.e., the poles may be “squeezed” together). Unfortunately, such an approach tends to exhibit a drawback: the filter may ring when driven by signals with sharp transitions. Since S1 heart sounds tend to exhibit sharp transitions, the bandpass filter  400  may ring if its passband is narrowed by way of “squeezing” its poles close together. 
     To address the problem of ringing, the baseline estimator  402  is introduced. The baseline estimator  402  yields an estimate of a low-frequency baseline upon which the heart sounds in the accelerometer data are riding. For example, baseline estimator  402  may be an exponentially-weighted historical averaging unit. By subtracting the baseline estimate yielded by the estimator  402  from the output of the bandpass filter  400  (using the subtracting unit  404 ), unwanted low-frequency content is removed from the signal passed by the filter  400 . This means that the lower cutoff frequency of the filter  400  may be relaxed (i.e., set at a relatively lower frequency), and that the combined functioning of the baseline estimator  402  and subtraction unit  404  will remove low frequency content. By relaxing the lower cutoff frequency of the filter, the frequency space between the sets of poles may also be broadened, diminishing the likelihood of ringing in the filter  400 . 
     The combined functioning of passband filter  400 , baseline estimator  402 , and subtraction unit  404  operate to achieve the effect described with reference to operation  202  in  FIG. 2 . Thus, the signal yielded by subtraction unit  404  primarily includes content related to heart sounds. 
     The signal from the subtraction unit  404  is passed to an S1 isolator unit  406 , which functions to achieve the result described with reference to operation  204  in  FIG. 2 . An exemplary method for automatically processing accelerometer signals to isolate S1, S2, and S3 heart sounds is disclosed in U.S. Pat. No. 5,792,195, issued to Carlson et al. on Aug. 11, 1998, which is incorporated by reference herein in its entirety. 
     The output of the S1 isolator unit  406  is a set of time-sequenced data representing an S1 heart sound. Such data is passed to a minima/maxima extractor  408  to find the global minima and maxima of the S1 complex, as described with reference to operation  206  in  FIG. 2 . 
     Finally, the output of the minima/maxima extractor  408  is passed to a median calculator  410  to find the median minima and median maxima exhibited by the last N S1 heart sounds, as described with reference to operations  208  and  210  in  FIG. 2 . 
     Returning briefly to  FIG. 1 , a formula presented therein describes a linear regression of the data contained in the graph. (In the case of the data presented in  FIG. 1 , the formula is y=0.00009x−0.0625). The “reliability” of the linear regression is indicated by R 2 , which is a measure of the variance explained by the regression model (in the case of the data presented in the graph of  FIG. 1 , R 2 =0.9866). In one example, the reliability of the data is improved (and therefore the reliability of the linear regression model, as understood by R 2  is improved) by performing the data processing schemes of  FIGS. 2-4  upon heart sounds obtained during periods of exhalation; accelerometer data obtained from heart sounds occurring during periods of inhalation is ignored. To distinguish periods of exhalation and inhalation, transthoracic impedance may be examined (transthoracic impedance may decrease during exhalation, for example). 
       FIG. 5  depicts an exemplary system useful for detecting S1 heart sounds, and extracting therefrom one or more morphological characteristics related to left ventricular contractility. 
     In  FIG. 5 , an exemplary system  500  for detecting and processing heart sounds includes an implantable system  502  and an external system  504 . The implantable system  502  and external system  504  are configured to communicate via a communications link  506 . 
     The implantable system  502  includes an implantable device  508  operatively coupled to a patient&#39;s heart by a lead system  512 . The components of the implantable device  508  include an atrial sense amplifier  514 , a ventricular sense amplifier  516 , an atrial stimulating circuit  518 , a ventricular stimulating circuit  520 , a controller  522 , a memory  524 , an accelerometer  526 , an analog pre-processing circuit  528 , an analog-to-digital (A/D) converter  530 , and an input/output (I/O) interface  532 . The components of implantable device  508  are housed within an implantable housing (indicated by the broken lined box in  FIG. 5 ), which may be implanted within the patient&#39;s chest cavity (e.g., in the pectoral region) or elsewhere. 
     The atrial sense amplifier  514 , ventricular sense amplifier  516 , atrial stimulating circuit  518  and ventricular stimulating circuit  520  are operatively coupled to lead system  512  via a pair of conductors  534 . The lead system  512  may include an atrial sensing electrode and an atrial stimulating electrode adapted to be disposed in the right atrial chamber of heart and a ventricular sensing electrode and a ventricular stimulating electrode adapted to be disposed in the right ventricular chamber of the heart. 
     Sensed atrial and ventricular electrical signals generated by the sensing electrodes are applied to the atrial and ventricular sense amplifiers  514  and  516 , respectively. Similarly, atrial and ventricular stimulating signals generated by the atrial and ventricular stimulating circuits  518  and  520  are applied to the atrial and ventricular stimulating electrodes, respectively. The atrial sense amplifier  514 , ventricular sense amplifier  516 , atrial stimulating circuit  518 , and ventricular stimulating circuit  520 , are each also operatively coupled to the controller  522 . 
     In other embodiments, other sensing electrode configurations are used for internally sensing one or more electrical signals of heart. In one example, only one sensing electrode may be used. Alternatively, one or more electrodes placed within the body but outside of the heart are used for sensing cardiac electrical signals. In yet another example, a sensing electrode is placed on the implantable housing. In each of these examples, the sensing electrodes are operatively coupled to the controller  522 . 
     In the embodiment shown in  FIG. 5 , the sensing electrodes and the stimulating electrodes are disposed in the right side of heart. In other embodiments, one or more sensing electrode(s) and one or more stimulating electrode(s) are disposed in the left side of the heart (in lieu of being disposed in the right side of the heart, or in addition to sensing electrode(s) and stimulating electrode(s) disposed in the right side of the heart). The addition of left heart sensing may advantageously allow for the resolution of ambiguities due to disassociation of right and left heart conduction. 
     The controller  522  includes a microcontroller or microprocessor which is configured to execute a program stored in a read-only memory (ROM) portion of a memory unit  524 , and to read and write data to and from a random access memory (RAM) portion of the memory unit  524 . By executing the program stored in memory  524 , the controller  522  is configured to process the atrial and ventricular electrical signals from the atrial and ventricular sense amplifiers  514  and  516 , and to provide control signals to the atrial and ventricular stimulating circuits  518  and  520 . In response, the stimulating circuits  518  and  520  provide stimulating pulses to heart via atrial and ventricular stimulating electrodes at appropriate times. In other embodiments, the controller  522  may include other types of control logic elements or circuitry. 
     The implantable device  508  may be referred to as a dual-chamber pacemaker since pacemaking functions are provided to both atrial and ventricular chambers of heart. In another embodiment, the implantable system includes a single-chamber pacemaker that senses electrical signals and provides stimulating pulses to a single chamber of heart. In yet another embodiment, the implantable system does not provide any stimulation of heart tissues, but includes one or more sensing electrodes for sensing one or more electrical signals of heart, and for providing corresponding sensed signals to controller  522 . In still another embodiment, the implantable system does not provide any sensing electrodes for sensing any cardiac electrical signals, but is configured to sense and transmit signals representing heart sounds using a sensor such as the accelerometer  526 , as described below. 
     In the remainder of this description, the implantable device  508  is described as a dual-chamber pacemaker for the sake of illustration. It is to be understood, however, that implantable system  502  need not provide the stimulation functions described herein, and may provide other functions which are not described herein. 
     In some embodiments, a minute ventilation output channel and a minute ventilation input channel may be interposed between the controller  522  and the ventricular lead. The minute ventilation output channel generates a high-frequency, low-voltage signal that is transmitted from the ventricular lead (in either unipolar or bipolar mode). The input channel receives and conditions the signal. The content of the conditioned signal reveals respiration information. 
     An accelerometer  526  may be configured to provide sensed signals to the analog pre-processing circuit  528 , which generates an analog output signal which is digitized by A/D converter  530 . The digitized accelerometer signal is received by the controller  522 . In the embodiment of  FIG. 5 , the accelerometer  526  is located within the housing of implantable device  508 . In another embodiment, the accelerometer  526  is located on the housing of the implantable device. The accelerometer  526  may include, for example, a piezo-electric crystal accelerometer sensor of the type used by pacemakers to sense the level of activity of the patient, or may include other types of accelerometers. To detect heart sounds, other types of sound-detecting sensors or microphones may also be used, such as a pressure sensor or a vibration sensor configured to respond to sounds made by the heart. 
     In another embodiment, the system  500  includes two or more sound-detecting sensors. In such an embodiment, the plurality of sensed heart sound signals from the plurality of sensors may be individually transmitted to external system  504  for display as individual traces, may be combined (e.g., averaged) by external system  504  before being displayed as a single trace, or may be combined by controller  522  before being transmitted to external system  504  as a single heart sound signal. These sensors may include different types of sensors, sensors that are located in different locations, or sensors that generate sensed signals which receive different forms of signal processing. 
     In one embodiment, the accelerometer  526  is configured to generate sensed signals representative of two distinct physical parameters: (1) the level of activity of the patient; and (2) the heart sounds generated by heart. Accordingly, the analog pre-processing circuit  528  is configured to pre-process the sensed signals from the accelerometer  526  in a manner which conforms to the signal characteristics of both of these physical parameters. For example, if the frequencies of interest for measuring the patient&#39;s level of activity are below 10 Hz, while the frequencies of interest for detecting heart sounds are between 0.05 Hz and 50 Hz, then analog pre-processing circuit  528  may include a low-pass filter having a cutoff frequency of 50 Hz. The controller  522  may then perform additional filtering in software, as described above with reference to  FIGS. 2-4 , for example. Along with filtering, analog pre-processing circuit  528  may perform other processing functions including automatic gain control (AGC) functions. 
     The analog pre-processing circuit  528  may perform the filtering and baseline wander removal functions described with reference to operation  202  ( FIG. 2 ) and may include modules  400 ,  402 , and  404  (shown in  FIG. 4 ). Alternatively, the analog pre-processing circuit  528  may simply provide automatic gain control functionality. 
     In some embodiments, the controller  522  performs one or more of steps  204 - 212  ( FIG. 2 ), and may include one or more of modules  406 - 410  ( FIG. 4 ). In the context of an embodiment in which the analog pre-processing circuit  528  performs only automatic gain control, the controller may perform operations  200  and  202  ( FIG. 2 ), and may include modules  400 - 404  ( FIG. 4 ). As discussed below, any operations  200 - 212  or modules  400 - 410  to be performed digitally by a controller may be performed cooperatively by the controller  522  within the implantable device  508  and another controller. For example, the controller  522  in the implantable device  508  may perform operation  204 , and communicate the result to an external controller (contained in a programmer, for example) that performs operation  206 - 212 . Alternatively, an external controller may perform all of the operations  200  described in  FIG. 2 . 
     In another embodiment, the implantable device  508  has two pre-processing channels for receiving sensed signals from accelerometer  526 . In still another embodiment, implantable device  508  includes two accelerometers, with one accelerometer configured to generate sensed signals representative of the level of activity of the patient and the other accelerometer configured to generate sensed signals representative of heart sounds. In these latter two embodiments, any hardware and/or software processing performed on the sensed signals can conform to the specific characteristics of the respective sensed signals. For example, the analog pre-processing circuit used for the level-of-activity sensed signals can provide a low-pass filter with a cutoff frequency of 10 Hz, while the analog preprocessing circuit for the heart-sound sensed signals can provide a band-pass filter with cutoff frequencies of 0.05 and 50 Hz. In the latter case, each accelerometer can be selected, located and/or oriented to maximize the detection of the respective physical parameter. In yet another embodiment, if the implantable device does not need to sense the level of activity of the patient, the accelerometer  526  may measure only the sounds made by heart. 
     The controller  522  is capable of bi-directional communications with an external system  504  via an I/O interface  532 . In one embodiment, the I/O interface  532  communicates using RF signals, which may be understood to include inductive coupling. In other embodiments, the I/O interface  532  communicates using optical signals, or a combination of RF and optical signals (e.g., RF signals for receiving data from the external system  504  and optical signals for transmitting data to external system  504 , or vice-versa). The controller  522  uses the I/O interface  532  for bi-directional communications with the external system  504  to support conventional monitoring, diagnostic and configuration pacemaker functions. The controller  522  may also use the I/O interface  532  to telemeter data representative of the heart sounds sensed by accelerometer  526  to the external system  504 . In various embodiments, the controller  522  further uses the I/O interface  532  to telemeter data representative of cardiac electrical signals (i.e., electrogram or EGM signals), which may include data representative of atrial electrical signals, sensed by the atrial sensing electrode, and/or data representative of ventricular electrical signals, sensed by the ventricular sensing electrode. Thus, implantable system  502  is capable of sensing heart sounds, atrial electrical signals and ventricular electrical signals, and of telemetering data representative of the heart sounds and/or cardiac electrical signals to external system  504 . In other embodiments, the controller  522  telemeters data representative of cardiac electrical signals which were sensed by other configurations of internal cardiac sensing electrodes. 
     The external system  504  may include an external device  542 . The external device  542  may include an external controller  546 , an I/O interface  548 , user input device(s)  550 , and user output device(s)  552 . Using the I/O interface  548 , the external controller  546  is configured for bi-directional communications with the implantable device  508 , for receiving input signals from input device(s)  550 , and for applying control signals to output device(s)  552 . The input device(s)  550  include at least one input device which allows a user (e.g., a physician, nurse, medical technician, etc.) to generate input signals to control the operation of external device  542 , such as at least one user-actuatable switch, knob, keyboard, pointing device (e.g., mouse), touch-screen, voice-recognition circuit, etc. The output device(s)  552  include at least one display device (e.g., CRT, flat-panel display, etc.), audio device (e.g., speaker, headphone), or other output device which generates user-perceivable outputs (e.g., visual displays, sounds, etc.) in response to control signals. The external controller  546  may be configured to receive the data representative of heart sounds, atrial electrical signals and/or ventricular electrical signals from implantable system  502 , and to generate control signals that, when applied to output device(s)  552 , cause the output device(s) to generate outputs that are representative of the heart sounds, the atrial electrical signals and/or the ventricular electrical signals. 
     The external controller  546  may cooperate with the internal controller  522  to perform any or all of the steps in  FIG. 2 . For example, the implantable device  508  may telemeter conditioned accelerometer data (yielded from operation  202  in  FIG. 2 ) to the external controller  546  via the communication link  506 . The external controller  546  may perform operations  204 - 212  upon the telemetered data. 
     In one embodiment, the system  500  further includes a remote system  554  operatively coupled to communicate with the external system  504  via transmission media  556 . The remote system  554  includes one or more user input device(s)  558 , and one or more user output device(s)  560 , which allow a remote user to interact with remote system  554 . The transmission media  556  includes, for example, a telephone line, electrical or optical cable, RF interface, satellite link, local area network (LAN), wide area network (WAN) such as the Internet, etc. The remote system  554  cooperates with external system  504  to allow a user located at a remote location to perform any of the diagnostic or monitoring functions that may be performed by a user located at external system  504 . For example, data representative of heart sounds and/or cardiac electrical signals are communicated by the external system  504  to the remote system  554  via the transmission media  556  to provide a visual display and/or an audio output on the output device(s)  560 , thereby allowing a physician at the remote location to aid in the diagnosis of a patient. The system  554  is “remote” in the sense that a user of remote system  554  is not physically capable of actuating input device(s)  550  and/or of directly perceiving outputs generated by output device(s)  552 . For example, the system  554  may be located in another room, another floor, another building, another city or other geographic entity, across a body of water, at another altitude, etc., from the external system  504 . 
     Although not depicted in  FIG. 5 , the I/O interface  532  may establish a communication link with a communication device in physical proximity to the patient. For example, the I/O interface may establish a data link with a personal digital assistant, and may upload or download any of the data mentioned previously or hereafter. The personal digital assistant may, in turn, establish a link with an access point, so that the link may be effectively extended over a network, such as the Internet. 
     To this point, the disclosure has discussed schemes for detecting a heart sound, generating morphological data from the heart sound, and relating the morphological data to a hemodynamic metric ( FIGS. 1-4 ). The disclosure has also discussed an exemplary device and system for performing such acts ( FIG. 5 ). The remainder of the disclosure relates to methods which may make use of the methods discussed with reference to  FIGS. 1-4 , and may make use of the exemplary device or system disclosed with reference to  FIG. 5 . 
       FIG. 6  depicts a method of adjusting parameters that influence a process. Any of the acts depicted in  FIG. 6  may be performed by any controller in the exemplary system of  FIG. 5 . The method begins with performing a process known to affect a characteristic of the heart that influences sound, as shown in operation  600 . For example, operation  600  may include titration of a drug known to influence a heart sound. An inotrope, for instance, enhances the inotropic state of the heart, resulting in greater myocardial contractility, which influences the S1 heart sound (as shown in  FIG. 1 ). Alternatively, operation  600  may include administration of therapy to the heart. For instance, an implantable device (such as the one shown in  FIG. 5 ) may administer cardiac resynchronization therapy to the heart. Over time, cardiac resynchronization therapy may make the heart stronger, resulting in greater myocardial contractility. Again, contractility is known to influence the S1 heart sound. 
     Next, in operation  602 , the heart sounds are detected using an accelerometer that is located at a point that is remote from the heart (e.g., within an implantable device, such as the one depicted in  FIG. 5 ). The monitoring operation  602  may include, for example, performing the acts discussed with reference to  FIGS. 2-4 . Thus, the median peak-to-peak amplitude exhibited by S1 heart sounds may be monitored (after being conditioned, as discussed with reference to  FIGS. 2-4 ) to obtain information regarding the contractility of the left ventricle. 
     After monitoring the heart sounds in operation  602 , it is determined whether the cardiac characteristic known to relate to the heart sound is in the desired state. For example, in the context of monitoring S1 heart sounds during administration of cardiac resynchronization therapy, the goal may be to increase or maximize contractility of the heart by administration of such therapy. Accordingly, operation  604  may involve a determination of whether the contractility of the heart has been maximized or sufficiently increased. If it is determined that the given cardiac characteristic (e.g., contractility) is, indeed, in the desired state, then the process may come to an end, as shown in operation  606 . On the other hand, if the desired state has not been reached then control may be passed to operation  608 . 
     Operation  608 , is optional. In operation  608 , one or more parameters influencing execution of the process of operation  600  are changed. For example, if operation  600  involved administration of cardiac resynchronization therapy, then one or more resynchronization parameters may be changed in operation  608 . For example, an atrioventricular pacing delay parameter, biventricular delay parameter, electrode site selection parameter, etc. may be altered. Execution of operation  608  may be performed automatically by an implanted cardiac rhythm management device. Alternatively, it may be performed automatically by an external processor (e.g., accelerometer data is telemetered to a programmer or other external device; the accelerometer data is processed according to  FIG. 2 ; the programmer automatically determines how to adjust one or more parameters on the basis of such processing, and new parameters are telemetered to the implantable device). Operation  608  may be performed manually as well (e.g., accelerometer data is telemetered to a programmer; the accelerometer data is processed according to  FIG. 2 ; a health care professional analyzes the accelerometer data to determine how to adjust one or more parameters, and new parameters are telemetered to the implantable device). After execution of operation  608 , control returns to operation  600 , and the controlled process continues. 
     Operation  608  may be omitted entirely. For example, in the context of titration, rather than increasing or decreasing the titration rate in operation  608 , the titration rate may remain constant, meaning that control returns to operation  600  and titration simply continues. 
     Thus, in sum, the loop defined by operations  600 ,  602 ,  604 , and  608 , functions to monitor and control a process, until a cardiac characteristic is observed (by virtue of detection of heart sounds with a remote accelerometer) to be in a desired state. 
       FIG. 7  depicts a method of identifying an occurrence of a cardiac event. Any of the acts depicted in  FIG. 7  may be performed by any controller in the exemplary system of  FIG. 5 . The method begins with detecting heart sounds using an accelerometer that is located at a point that is remote from the heart (e.g., within an implantable device, such as the one depicted in  FIG. 5 ), as shown in operation  700 . The monitoring operation of operation  700  may include, for example, performing the acts discussed with reference to  FIGS. 2-4 . Thus, the median peak-to-peak amplitude exhibited by S1 heart sounds may be monitored (after being conditioned, as discussed with reference to  FIGS. 2-4 ) to obtain information regarding the contractility of the left ventricle. 
     Thereafter, control may be passed to operation  702 . Operation  702  is optional, and may be omitted altogether. In operation  702 , the information obtained by monitoring of heart sounds in operation  700  is combined with other information (e.g., exertion level as indicated by rate of respiration and/or motion-related acceleration, etc.). The heart sound information may be combined with any of the information normally measured by a cardiac rhythm management device, for example. 
     Next, in operation  704 , the information obtained from operations  700  and  702  (if performed) is analyzed to determine whether a cardiac event of interest is occurring. For example, the information may be analyzed to determine whether ischemia or acute heart failure decompensation is being exhibited by a patient. If no such event is detected, control is returned to operation  700 , and the monitoring continues. On the other hand, if such an event is detected, control is passed to operation  706 . 
     Operation  706  is optional. In operation  706 , occurrence of the detected event is communicated. The communication may occur between an implanted cardiac management device (performing operations  700 - 706 ) and a programmer, a personal digital assistant, an access point to a network, such as a wireless or wired network, or to a wireless communication device that may forward the message to an access point for communication to a remote computing system, for example. (The programmer or personal digital assistant may relay such a message to a remote computing). Alternatively, the communication may occur between the detection routine executing on the internal controller and a history logging routine executed by the same internal controller. Thus, the occurrence of the detected event is logged, so that a health care professional may become aware of the event, for example, the next time he or she reads the data contained in the log. After execution of operation  706 , control is returned to operation  700 , and monitoring continues. 
     The method of  FIG. 7  may be performed continually or at intervals. For example, a patient may be monitored for a given cardiac condition once per day (e.g., at night) or several times each day, meaning that the method of  FIG. 7  would be executed once per day or several times per day. 
     Embodiments of the invention may be implemented in one or a combination of hardware, firmware, and software. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by at least one processor to perform the operations described herein. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer). For example, a machine-readable medium may include read-only memory (ROM), random-access memory (RAM), magnetic disc storage media, optical storage media, flash-memory devices, electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. 
     The Abstract is provided to comply with 37 C.F.R. Section 1.72(b) requiring an abstract that will allow the reader to ascertain the nature and gist of the technical disclosure. It is submitted with the understanding that it will not be used to limit or interpret the scope or meaning of the claims. 
     In the foregoing detailed description, various features are occasionally grouped together in a single embodiment for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments of the subject matter require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus, the following claims are hereby incorporated into the detailed description, with each claim standing on its own as a separate preferred embodiment.