Patent Publication Number: US-6714811-B1

Title: Method and apparatus for monitoring heart rate

Description:
This application claims the benefit of Provisional application Ser. No. 60/123,002, filed Mar. 5, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to implantable medical devices and more particularly to implantable medical devices intended for use in monitoring a patient&#39;s heart rhythm. 
     Implantable pacemakers and cardioverters monitor the heart&#39;s rhythm in order to detect arrhythmias and deliver appropriate therapies to terminate detected arrhythmias. In conjunction with this function, the ability of the device is to store information with regard to monitored heart rhythms has dramatically increased over the past two years. Examples of implantable pacemakers and defibrillators which have the capability of storing information related to monitor heart rhythms include U.S. Pat. No. 4,223,678 issued to Langer et al., U.S. Pat. No. 5,722,999 issued to Snell, U.S. Pat. No. 5,513,645 issued to Jacobsen et al. and U.S. Pat. No. 5,312,446 issued to Holschbach et al. In addition, there have recently been developed implantable monitoring devices that do not deliver any anti-arrhythmia therapies to the heart but simply store information regarding a patient&#39;s heart rhythms for later uplink to an external device. Such devices are disclosed in U.S. Pat. No. 5,331,966 issued to Bennett et al., U.S. Pat. No. 5,135,004 issued to Adams and U.S. Pat. No. 5,497,780 issued to Zehender. 
     In conjunction with implantable devices as described above, information stored relating to a patient&#39;s heart rhythm may include information relating to heart rate trends over time, as disclosed in U.S. Pat. No. 5,088,488 issued to Markowitz et al., U.S. Pat. No. 5,330,513 issued to Nichols et al. and U.S. Pat. No. 5,603,331 issued to Heemels et al. as well as information relating to heart rate variability over time, as disclosed in U.S. Pat. No. 5,749,900 issued to Schroeppel et al., U.S. Pat. No.5,466,245 issued to Spinelli et al., U.S. Pat. No. 5,411,131 issued to Yomtov et al. and U.S. Pat. No. 5,437,285 issued to Verrier et al. Typically, measurements of heart rate trend in such devices are accomplished by continually measuring heart rate over a defined time period, and calculating average heart rates for successive shorter time periods within the defined time period for later telemetry to an external device. Gradual increases in average heart rate over extended time periods are known to be an indicator of decompensation, a phenomenon that takes place during the progression of clinical heart failure. 
     SUMMARY OF THE INVENTION 
     The present invention is directed toward an implantable device having enhanced capabilities for monitoring a patient&#39;s heart rate trends over extended periods of time. The information collected by the implantable device is stored and telemetered to an associated external device such as a device programmer for display and analysis. Heart rates are measured by measuring the time intervals between sensed depolarizations of a chamber of the patient&#39;s heart and preceding sensed depolarizations or delivered pacing pulses. Intervals may be measured in the ventricle and/or atrium of the patient&#39;s heart. The measured intervals are referred to hereafter as “heart intervals”. The measured heart intervals during defined time periods are used to calculate average heart rates or average heart intervals associated with the time periods. Preferably the average heart rate takes the form of a mean heart rate, but in some embodiments, the median heart rate over the time periods may be employed or the most common heart rate or interval based on a in a histogram of measured heart intervals or other equivalent value may be substituted. For purposes of the present application, the term “average heart rate” should be understood to include mean, median or other equivalent values indicative of the general heart rate or heart interval. 
     Rather than simply measuring average heart rate values over successive time periods, the implantable device instead measures successive average values of heart rates measured during discontinuous time periods, preferably chosen to occur during times of particular interest, for example during defined time periods during the night and/or day. Preferably the measurements are taken and stored over a period of weeks or months. In a first embodiment, measurements are during the night during a period of time in which the patient is likely to be sleeping. In this context, measurement of the trend of night heart rates taken, for example over the period of time between 12:00 a.m. and 4:00 a.m . is believed to be particularly valuable. Night heart rate is predominantly controlled by the parasympathetic nervous system, and progression of heart failure is usually associated with abnormal excitation of the sympathetic nervous system, leading to increases in night heart rate. 
     In addition, long-term trends of daytime heart rates may also be collected, for example over periods of time between 8:00 a.m. and 8:00 p.m. Daytime heart rate is primarily controlled by the sympathetic nervous system and thus differences in day and night heart rates can be used as a measure of autonomic dysfunction and have been shown to be different in heart failure patients when compared to age matched individuals with normal hearts. In the context of an implantable pacemaker, comparisons of trends of day and night heart rates to the lower or base pacing rate of the pacemaker may also provide useful physiological information. This comparison may be especially valuable in pacemakers which store information regarding trends of physiologic sensor outputs or regarding trends of pacing rates based upon physiologic sensor outputs as in U.S. patent application Ser. No. 09/078,221, filed May 13, 1998 by Stone et al, incorporated herein by reference in its entirety. 
     In a preferred embodiment of the invention, the implantable device includes a sensor indicative of exercise level either measured directly using a physiologic sensor such as an accelerometer or piezo-electric sensor or measured indirectly by means of a sensor of metabolic demand such as a pressure sensor, oxygen saturation sensor, stroke volume sensor or respiration sensor. In this embodiment of the invention, measurements of heart rhythms are made only in response to the sensor&#39;s determination that the patient is at rest, in order to produce a long-term trends of resting heart rates during the defined time intervals. Even over relatively long time frames, a patient&#39;s level of activity may vary substantially, and changes in average heart rates can be masked by such variations in exercise level. By limiting the measurements of heart rates to times during which the patient is known to be at rest, a more accurate indication of the true long-term progression of heart rates can be obtained. In such embodiments the implantable device may collect heart rate information continuously during longer time periods, typically extending at least over several hours. During the longer time periods the device may define a series of shorter time periods, typically extending over several minutes, and will employ heart rate information collected during a preceding one of the shorter time periods only if the sensor indicates the patient was at rest during the shorter time period. 
     In some preferred embodiments, particularly those intended for use in patients known to suffer from tachyarrhythmias, the implantable device is also configured to reject intervals between depolarizations associated with tachyarrhythmias. In such embodiments the implantable device may define a minimum cumulative duration of non-rejected heart intervals as a prerequisite to calculation of an average rate value for a defined time period. 
     In devices employing physiologic sensors, the device may correspondingly also store values indicative of the general levels of sensor output during daytime and nighttime periods may also be collected. In such embodiments, average sensor output values, including the various types of averages discussed above in conjunction with calculation of average heart rates may be employed. Alternatively, a sum or total of all generated sensor outputs during relevant time periods may be employed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates an implantable pacemaker/cardioverter/ defibrillator of a type useful in practicing the present invention, in conjunction with a human heart. 
     FIG. 2 illustrates an implantable pacemaker of a type useful in practicing the present invention, in conjunction with a human heart. 
     FIG. 3 illustrates an implantable monitor of a type useful in practicing the present invention. 
     FIG. 4 is a perspective view of a programmer of a type useful in practicing the present invention. 
     FIG. 5 is a functional schematic diagram of an implantable pacemaker/cardioverter/defibrillator of a type useful in practicing the present invention. 
     FIG. 6 is a functional schematic diagram of an implantable pacemaker of a type useful in practicing the present invention. 
     FIG. 7 is a functional schematic diagram of an implantable monitor of a type useful in practicing the present invention. 
     FIG. 8 is a functional schematic diagram of a programmer of a type useful in practicing the present invention. 
     FIG. 9 is a functional flow chart illustrating a first method of monitoring heart rate trends, which may be employed in conjunction with the present invention. 
     FIG. 10 is a functional flow chart illustrating a second method of monitoring heart rate trends, which may be employed in conjunction with the present invention. 
     FIG. 11 is a functional flow chart illustrating a method of monitoring sensor output trends, which may be employed in conjunction with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a defibrillator and lead set of a type in which the present invention may usefully be practiced. The ventricular lead includes an elongated insulative lead body  16 , carrying three mutually insulated conductors. Located adjacent the distal end of the lead are a ring electrode  24 , an extendable helix electrode  26 , mounted retractably within an insulative electrode head  28 , and an elongated coil electrode  20 . Each of the electrodes is coupled to one of the conductors within the lead body  16 . Electrodes  24  and  26  are employed for cardiac pacing and for sensing ventricular depolarizations. At the proximal end of the lead is a bifurcated connector  14  that carries three electrical connectors, each coupled to one of the coiled conductors. 
     The atrial/SVC lead includes an elongated insulative lead body  15 , also carrying three mutually insulated conductors. Located adjacent the J-shaped distal end of the lead are a ring electrode  21  and an extendible helix electrode  17 , mounted retractably within an insulative electrode head  19 . Each of the electrodes is coupled to one of the conductors within the lead body  15 . Electrodes  17  and  21  are employed for atrial pacing and for sensing atrial depolarizations. An elongated coil electrode  23  is provided, proximal to electrode  21  and coupled to the third conductor within the lead body  15 . At the proximal end of the lead is a bifurcated connector  13  that carries three electrical connectors, each coupled to one of the coiled conductors. 
     The coronary sinus lead includes an elongated insulative lead body  6 , carrying one conductor, coupled to an elongated coiled defibrillation electrode  8 . Electrode  8 , illustrated in broken outline, is located within the coronary sinus and great vein of the heart. At the proximal end of the lead is a connector plug  4 , which carries an electrical connector, coupled to the coiled conductor. 
     The pacemaker/cardioverter/defibrillator  10  includes a hermetic enclosure  11  containing the electronic circuitry used for generating cardiac pacing pulses for delivering cardioversion and defibrillation shocks and for monitoring the patient&#39;s heart rhythm. Pacemaker/cardioverter/defibrillator  10  is shown with the lead connector assemblies  4 ,  13  and  14  inserted into the connector block  12 , which serves as a receptacle and electrical connector for receiving the connectors,  4 ,  13  and  14  and interconnecting the leads to the circuitry within enclosure  11 . An activity sensor  30  is illustrated schematically by broken outline, and may be an accelerometer or a piezoelectric transducer. Sensor  30  may be used for verifying that the patient is at rest, in conjunction with measurement of long-term heart rate trends according to the present invention as well as for regulation of pacing rate based upon demand for cardiac output. 
     Optionally, insulation of the outward facing portion of the housing  11  of the pacemaker/cardioverter/defibrillator  10  may be provided or the outward facing portion may instead be left uninsulated, or some other division between insulated and uninsulated portions may be employed. The uninsulated portion of the housing  11  optionally serves as a subcutaneous defibrillation electrode, used to defibrillate either the atria or ventricles. Other lead configurations and electrode locations may of course be substituted for the lead set illustrated. For example, atrial defibrillation and sensing electrodes might be added to either the coronary sinus lead or the right ventricular lead instead of being located on a separate atrial lead, allowing for a two-lead system. 
     FIG. 2 illustrates a cardiac pacemaker of a type appropriate for use in practicing the present invention in conjunction with its associated lead system, illustrated in relation to a patient&#39;s heart. The pacemaker  120  includes a hermetic enclosure  124  containing the electronic circuitry used for generating cardiac pacing pulses and for monitoring the patient&#39;s heart rhythm. An activity sensor  126  is illustrated schematically by broken outline, and may be an accelerometer or a piezoelectric transducer as discussed above in conjunction with FIG.  1 . Mounted to the enclosure  124  is a header  122  which serves as a receptacle and electrical connector for receiving the connectors  132  and  134  of pacing leads  128  and  130  and interconnecting the leads to the circuitry within enclosure  124 . Lead  128  is a ventricular lead provided with electrodes  140  and  142  for monitoring right ventricular heart signals. Also illustrated on lead  128  is a physiologic sensor  144  which may optionally be included in addition to or as an alternative to the activity sensor  126 , and which may take the form of an oxygen sensor, pressure sensor, temperature sensor, other sensor of any of the various types employed for monitoring demand for cardiac output or for measuring heart hemodynamics. Sensor  144  may be used in conjunction with or as an alternative to the activity sensor  126  for verifying that the patient is at rest, in conjunction with measurement of long-term heart rate trends according to the present invention. Atrial lead  130  carries electrodes  136  and  138  and is employed for sensing and pacing the patient&#39;s atrium. 
     FIG. 3 illustrates a subcutaneously implantable monitor of a type appropriate for use in practicing the present invention. The monitor shares the external configuration of the Medtronic Reveal ® implantable monitor, and is provided with a hermetically sealed enclosure  104  containing the electronic circuitry used for generating cardiac pacing pulses and for monitoring the patient&#39;s heart rhythm and which carries a molded plastic header  108 . The enclosure  104  and the header  108  each carry an electrode  102  and  106 , respectively for monitoring heart rhythm. Also  30  mounted in the header  108  is an antenna  110  for use in communicating between the device and an external programmer. Illustrated in broken outline at  112  is an internal activity sensor, of the type typically employed in the context of rate responsive cardiac pacemakers, taking the form either of an accelerometer or a piezo-electric transducer. Heart signals are detected between the electrodes  102  and  106  and measurements of physical activity are detected by sensor  112  for use in storing and calculating heart rate trends and heart rate variability measurements according to the present invention. 
     FIG. 4 is a plan view of an external programmer of a sort appropriate for use in conjunction with the practice of the present invention in conjunction with any of the devices of FIGS. 1-3. The programmer  420  is a microprocessor controlled device which is provided with a programming head  422  for communicating with an implanted device, a set of surface electrogram electrodes  459  for monitoring a patient&#39;s electrogram, a display  455  which is preferably a touch sensitive display, control buttons or keys  465 , and a stylist  456  for use in conjunction with the touch sensitive screen  455 . By means of the control keys  465  and the touch sensitive screen  455  and stylus  456 , the physician may format commands for transmission to the implantable device. By means of the screen  455 , the physician may observe information telemetered from the implantable device. The programmer is further provided with a printer  463  which allows for hard copy records of displays of signals received from the implanted device such as electrograms, stored parameters, programmed parameters and information as to heart rate trends according to the present invention. While not visible in this view, the device may also be provided with a floppy disk or CD ROM drive and/or a port for insertion of expansion cards such as P-ROM cartridges, to allow for software upgrades and modifications to the programmer  420 . 
     In the context of the present invention, programmer  420  may serve simply as a display device, displaying information with regard to heart rate variability and heart rate trends as calculated by the implanted device or instead may receive uplinked raw data related to heart intervals and may calculate the heart rate trends and heart rate variability values according to the present invention. It is believed that it is preferable for the implanted device to perform the bulk of the computations necessary to practice the invention, and in particular that it is preferable for the implanted device to at least calculate average rate values, to reduce the storage requirements within the implanted device. However, allocation of functions between the implanted device and the programmer may differ from the preferred embodiments and still result in a workable system. 
     FIG. 5 is a functional schematic diagram of an implantable pacemaker/cardioverter/defibrillator of the type illustrated in FIG. 3, in which the present invention may usefully be practiced. This diagram should be taken as exemplary of one type of anti-tachyarrhythmia device in which the invention may be embodied, and not as limiting, as it is believed that the invention may usefully be practiced in a wide variety of device implementations, including devices providing therapies for treating atrial arrhythmias instead of or in addition to ventricular arrhythmias, cardioverters and defibrillators which do not provide anti-tachycardia pacing therapies, anti-tachycardia pacers which do not provide cardioversion or defibrillation, and devices which deliver different forms of anti-arrhythmia therapies such nerve stimulation or drug administration. 
     The device is provided with a lead system including electrodes, which may be as illustrated in FIG.  1 . Alternate lead systems may of course be substituted. If the electrode configuration of FIG. 1 is employed, the correspondence to the illustrated electrodes is as follows. Electrode  311  corresponds to electrode  11 , and is the uninsulated portion of the housing of the implantable pacemaker/cardioverter/defibrillator. Electrode  320  corresponds to electrode  20  and is a defibrillation electrode located in the right ventricle. Electrode  310  corresponds to electrode  8  and is a defibrillation electrode located in the coronary sinus. Electrode  318  corresponds to electrode  28  and is a defibrillation electrode located in the superior vena cava. Electrodes  324  and  326  correspond to electrodes  24  and  26 , and are used for sensing and pacing in the ventricle. Electrodes  317  and  321  correspond to electrodes  19  and  21  and are used for pacing and sensing in the atrium. 
     Electrodes  310 ,  311 ,  318  and  320  are coupled to high voltage output circuit  234 . Electrodes  324  and  326  are coupled to the R-wave amplifier  200 , which preferably takes the form of an automatic gain controlled amplifier providing an adjustable sensing threshold as a function of the measured R-wave amplitude. A signal is generated on R-out line  202  whenever the signal sensed between electrodes  324  and  326  exceeds the present sensing threshold. 
     Electrodes  317  and  321  are coupled to the P-wave amplifier  204 , which preferably also takes the form of an automatic gain controlled amplifier providing an adjustable sensing threshold as a function of the measured R-wave amplitude. A signal is generated on P-out line  206  whenever the signal sensed between electrodes  317  and  321  exceeds the present sensing threshold. The general operation of the R-wave and P-wave amplifiers  200  and  204  may correspond to that disclosed in U.S. Pat. No. 5,117,824, by Keimel, et al., issued Jun. 2, 1992, for an Apparatus for Monitoring Electrical Physiologic Signals, incorporated herein by reference in its entirety. However, any of the numerous prior art sense amplifiers employed in implantable cardiac pacemakers, defibrillators and monitors may also usefully be employed in conjunction with the present invention. 
     Switch matrix  208  is used to select which of the available electrodes are coupled to wide band amplifier  210  for use in digital signal analysis. Selection of electrodes is controlled by the microprocessor  224  via data/address bus  218 , which selections may be varied as desired. Signals from the electrodes selected for coupling to bandpass amplifier  210  are provided to multiplexer  220 , and thereafter converted to multi-bit digital signals by A/D converter  222 , for storage in random access memory  226  under control of direct memory access circuit  228 . Microprocessor  224  may employ digital signal analysis techniques to characterize the digitized signals stored in random access memory  226  to recognize and classify the patient&#39;s heart rhythm employing any of the numerous signal-processing methodologies known to the art. 
     Telemetry circuit  330  receives downlink telemetry from and sends uplink telemetry to the patient activator by means of antenna  332 . Data to be uplinked to the activator and control signals for the telemetry circuit are provided by microprocessor  224  via address/data bus  218 . Received telemetry is provided to microprocessor  224  via multiplexer  220 . The atrial and ventricular sense amp circuits  200 ,  204  produce atrial and ventricular EGM signals, which also may be digitized, and uplink telemetered to an associated programmer on receipt of a suitable interrogation command. The device may also be capable of generating so-called marker codes indicative of different cardiac events that it detects. A pacemaker with marker-channel capability is described, for example, in U.S. Pat. No.4,374,382 to Markowitz, which patent is hereby incorporated by reference herein in its entirety. The particular telemetry system employed is not critical to practicing the invention, and any of the numerous types of telemetry systems known for use in implantable devices may be used. In particular, the telemetry systems as disclosed in U.S. Pat. No. 5,292,343 issued to Blanchette et al., U.S. Pat. No. 5,314,450, issued to Thompson, U.S. Pat. No.5,354,319, issued to Wybomy et al. U.S. Pat. No. 5,383,909, issued to Keimel, U.S. Pat. No. 5,168,871, issued to Grevious, U.S. Pat. No. 5,107,833 issued to Barsness or U.S. Pat. No. 5,324,315, issued to Grevious, all incorporated herein by reference in their entireties, are suitable for use in conjunction with the present invention. However, the telemetry systems disclosed in the various other patents cited herein which are directed to programmable implanted devices, or similar systems may also be substituted. The telemetry circuit  330  is of course also employed for communication to and from an external programmer, as is conventional in implantable anti-arrhythmia devices. 
     The device of FIG. 5 may additionally is provided with an activity sensor  344 , mounted to the interior surface of the device housing or to the hybrid circuit within the device housing. The sensor  344  and sensor present in circuitry  342  may be employed in the conventional fashion described in U.S. Pat. No. 4,428,378 issued to Anderson et al, incorporated herein by reference in its entirety, to regulate the underlying pacing rate of the device in rate responsive pacing modes and also serves as in an indicator of the patient&#39;s activity level for use in conjunction with the measurement of heart rate at rest or during sleep, as discussed above and as discussed in more detail below in conjunction with FIGS. 10 and 12. In addition, the sensor  344  may be employed to track the functional status of the patient as in the above-cited application by Stone et al. In such case, the device may also store trend information with regard to the number of and/or durations of periods in which the patient&#39;s physical activity meets or exceeds a defined level. Comparisons of the stored trend of day and/or night heart rate with trend information related to sensor output may be especially valuable. 
     The remainder of the circuitry is dedicated to the provision of cardiac pacing, cardioversion and defibrillation therapies, and, for purposes of the present invention may correspond to circuitry known in the prior art. An exemplary apparatus is disclosed for accomplishing pacing, cardioversion and defibrillation functions as follows. The pacer timing/control circuitry  212  includes programmable digital counters which control the basic time intervals associated with DDD, VVI, DVI, VDD, AAI, DDI, DDDR, VVIR, DVIR, VDDR, AAIR, DDIR and other modes of single and dual chamber pacing well known to the art. Circuitry  212  also controls escape intervals associated with anti-tachyarrhythmia pacing in both the atrium and the ventricle, employing, any anti-tachyarrhythmia pacing therapies known to the art. 
     Intervals defined by pacing circuitry  212  include atrial and ventricular pacing escape intervals, the refractory periods during which sensed P-waves and R-waves are ineffective to restart timing of the escape intervals and the pulse widths of the pacing pulses. The durations of these intervals are determined by microprocessor  224 , in response to stored data in memory  226  and are communicated to the pacing circuitry  212  via address/data bus  218 . Pacer circuitry  212  also determines the amplitude of the cardiac pacing pulses under control of microprocessor  224 . 
     During pacing, the escape interval counters within pacer timing/control circuitry  212  are reset upon sensing of R-waves and P-waves as indicated by signals on lines  202  and  206 , and in accordance with the selected mode of pacing on time-out trigger generation of pacing pulses by pacer output circuits  214  and  216 , which are coupled to electrodes  317 ,  321 ,  324  and  326 . The escape interval counters are also reset on generation of pacing pulses, and thereby control the basic timing of cardiac pacing functions, including anti-tachyarrhythmia pacing. 
     The durations of the intervals defined by the escape interval timers are determined by microprocessor  224 , via data/address bus  218 . The value of the count present in the escape interval counters when reset by sensed R-waves and P-waves may be used to measure the durations of R-R intervals, P-P intervals, PR intervals and R-P intervals, which measurements are stored in memory  226  and are used in conjunction with the present invention to measure heart rate variability and heart rate trends and in conjunction with tachyarrhythmia detection functions. 
     Microprocessor  224  operates as an interrupt driven device, and is responsive to interrupts from pacer timing/control circuitry  212  corresponding to the occurrences of sensed P-waves and R-waves and corresponding to the generation of cardiac pacing pulses. These interrupts are provided via data/address bus  218 . Any necessary mathematical calculations to be performed by microprocessor  224  and any updating of the values or intervals controlled by pacer timing/control circuitry  212  take place following such interrupts. Microprocessor  224  includes associated ROM in which the stored program controlling its operation as described below resides. A portion of the memory  226  (FIG. 2) may be configured as a plurality of recirculating buffers, capable of holding series of measured intervals, which may be analyzed in response to the occurrence of a pace or sense interrupt to determine whether the patient&#39;s heart is presently exhibiting atrial or ventricular tachyarrhythmia. 
     The arrhythmia detection method of the present invention may include any of the numerous available prior art tachyarrhythmia detection algorithms. One preferred embodiment may employ all or a subset of the rule-based detection methods described in U.S. Pat. No. 5,545,186 issued to Olson et al. or in U.S. Pat. No. 5,755,736 issued to Gillberg et al., both incorporated herein by reference in their entireties. However, any of the various other arrhythmia detection methodologies known to the art might also Insert Walt&#39;s case 
     In the event that an atrial or ventricular tachyarrhythmia is detected, and an anti-tachyarrhythmia pacing regimen is desired, timing intervals for controlling generation of anti-tachyarrhythmia pacing therapies are loaded from microprocessor  224  into the pacer timing and control circuitry  212 , to control the operation of the escape interval counters therein and to define refractory periods during which detection of R-waves and P-waves is ineffective to restart the escape interval counters. 
     In the event that generation of a cardioversion or defibrillation pulse is required, microprocessor  224  employs the escape interval counter to control timing of such cardioversion and defibrillation pulses, as well as associated refractory periods. In response to the detection of atrial or ventricular fibrillation or tachyarrhythmia requiring a cardioversion pulse, microprocessor  224  activates cardioversion/defibrillation control circuitry  230 , which initiates charging of the high voltage capacitors  246 ,  248  via charging circuit  236 , under control of high voltage charging control line  240 . The voltage on the high voltage capacitors is monitored via VCAP line  244 , which is passed through multiplexer  220  and in response to reaching a predetermined value set by microprocessor  224 , results in generation of a logic signal on Cap Full (CF) line  254 , terminating charging. Thereafter, timing of the delivery of the defibrillation or cardioversion pulse is controlled by pacer timing/control circuitry  212 . Following delivery of the fibrillation or tachycardia therapy the microprocessor then returns the device to cardiac pacing and awaits the next successive interrupt due to pacing or the occurrence of a sensed atrial or ventricular depolarization. In the illustrated device, delivery of the cardioversion or defibrillation pulses is accomplished by output circuit  234 , under control of control circuitry  230  via control bus  238 . Output circuit  234  determines whether a monophasic or biphasic pulse is delivered, whether the housing  311  serves as cathode or anode and which electrodes are involved in delivery of the pulse. 
     FIG. 6 is a functional schematic diagram of the pacemaker  120  illustrated in FIG.  2 . The pacemaker of FIGS. 2 and 6 is essentially a set of subcomponents of the implantable pacemaker/cardioverter/defibrillator illustrated in FIGS. 1 and 5. Like the device of FIG. 5, the pacemaker is a microprocessor-controlled device with microprocessor  189  operating under control of programming stored in Read Only Memory (ROM)  191 . In the device as illustrated, electrodes  136  and  138 , intended for location in the atrium of the patient&#39;s heart are coupled to an atrial amplifier  181  which may correspond to atrial amplifier  204  in FIG.  5 . Similarly, ventricular electrodes  140  and  142  are coupled to ventricular amplifier  182 , which may correspond to ventricular amplifier  200  in FIG.  5 . The outputs of atrial and ventricular amplifiers  181  and  182  are input into timing and control circuitry  183  which conforms generally to the pacer timing and control circuitry  212  of FIG. 5, and which measures intervals between detected depolarizations and controls intervals between delivered pacing pulses as well as generating interrupts via data/address  192  to awake microprocessor  189  in response to delivery of a pacing pulse or sensing of a cardiac depolarization. Intervals between depolarizations measured by timing/control circuitry  183  are stored in Random Access Memory (RAM)  190  until processed by microprocessor  189  to derive average heart rate values. Atrial and ventricular pacing pulses delivered according to one or more of the standard pacing modes described in conjunction with FIG. 5 are produced by atrial and ventricular pulse generator circuits  184  and  185  which may correspond to pulse generator circuits  215  ad  216  in FIG.  5 . 
     The sensor illustrated in FIG. 6 may correspond to either an activity sensor  126  as described in conjunction with FIG. 2 above, a respiration sensor, for example as disclosed in U.S. Pat. No. 5,562,711 issued to Yerich et al or a hemodynamic sensor  140 , as described in conjunction with FIG.  2 . If the sensor is an activity sensor, then sensor-processing circuitry  186  may correspond to sensor processing circuitry  342  discussed in conjunction with FIG.  5 . However, if the sensor is a respiration or hemodynamic sensor, the sensor processing circuitry would correspond to the sort of processing circuitry typically associated with respiration or hemodynamic sensors. For purposes of the present invention, the hemodynamic sensor may be, for example, an oxygen saturation sensor in conjunction with associated processing circuitry as described in U.S. Pat. No. 5,903,701 issued to Moore, a pressure or temperature sensor and associated sensor processing circuitry as described in U.S. Pat. No. 5,564,434 issued to Halperin et al. or an impedance sensor and associated sensor processing circuitry as described in U.S. Pat. No. 5,824,029issued to Weijand et al., all incorporated herein by reference in their entireties, or may correspond to other types of physiologic sensors, as may be appropriate. As discussed in more detail below, in the context of the present invention, the sensor  126 ,  140  is employed to determine when the patient is in a resting state, for purposes of controlling the gathering and storage of information related to long term heart rate trends. Telemetry circuitry  187  in conjunction with antenna  188  serves to transmit information to and receive information from an external programmer precisely as described above in conjunction with the device of FIG. 5, including information related to stored median interval values and heart rate variability measurements in RAM  190 , as calculated by microprocessor  189 . 
     FIG. 7 illustrates the functional organization of the subcutaneously implantable heart monitor  100  illustrated in FIG.  3 . This device consists essentially of a set of subcomponents of the more complex embodiment of the invention disclosed in FIG. 5, and includes a sense amplifier  152  coupled to electrodes  102  and  106 , illustrated in FIG.  1 . Sense amplifier  152  may correspond to sense amplifier  204  or  200  in FIG.  5 . Like the device of FIG. 5, the implantable monitor may be a microprocessor control device operating under control microprocessor  156  with its functionality controlled primarily by software stored in the read only memory associated therein. In this context, amplifier  152  detects the occurrence of heart depolarizations, with timing/control circuitry  154  serving to measure the durations between the detected heart depolarizations and to generate interrupts awakening microprocessor  156  so that it may store, analyze and process the detected intervals. Random Access Memory (RAM)  158  serves to store measured and calculated parameters including the calculated average heart rate values for later telemetry to an external device. Like the device in FIG. 5, timing and control circuitry communicates with the microprocessor and the remaining circuitry by means of the address/data bus  168 . Telemetry system  162  may correspond to telemetry system  330  in FIG. 5 and, via antenna  110  transmits and receives information from the external programmer, including transmitting information with regard to the calculated median rate values and heart variability values stored in RAM  158 . Sensor  112  may correspond to sensor  344  in FIG.  5  and it may be a physical activity sensor as discussed above. The output of sensor  112  is passed through sensor processing circuitry  166  which may correspond to sensor processing circuitry  342  in FIG.  5 . 
     FIG. 8 is a functional schematic of a programmer as illustrated in FIG. 4 appropriate for use in conjunction with the invention. Programmer  420  is a personal computer type, microprocessor-based device incorporating a central processing unit  450 , which may be, for example, an Intel  80386  or  80486  or Pentium microprocessor or the like. A system bus  451  interconnects CPU  450  with a hard disk drive  452  storing operational programs and data and with a graphics circuit  453  and an interface controller module  454 . A floppy disk drive  466  or a CD ROM drive is also coupled to bus  451  and is accessible via a disk insertion slot within the housing of the programmer  420 . Programmer  420  further comprises an interface module  457 , which includes digital circuit  458 , non-isolated analog circuit  459 , and isolated analog circuit  460 . Digital circuit  448  enables interface module  457  to communicate with interface controller module  454 . 
     In order for the physician or other caregiver or user to communicate with the programmer  420 , control buttons  465  or optionally a keyboard coupled to CPU  50  are provided. However the primary communication mode is through graphics display screen  455  of the well-known “touch sensitive” type controlled by graphics circuit  453 . A user of programmer  420  may interact therewith through the use of a stylus  456 , also coupled to graphics circuit  453 , which is used to point to various locations on screen  455 , which display menu choices for selection by the user or an alphanumeric keyboard for entering text or numbers and other symbols. 
     Graphics display  455  also displays a variety of screens of telemetered out data or real time data including measurements of heart rate variability and heart rate trends according to the present invention. Programmer  420  is also provided with a strip chart printer  463  or the like coupled to interface controller module  454  so that a hard copy of a patient&#39;s ECG, EGM, marker channel or of graphics displayed on the display  455  can be generated. 
     As will be appreciated by those of ordinary skill in the art, it is often desirable to provide a means for programmer  20  to adapt its mode of operation depending upon the type or generation of implanted medical device to be programmed. Accordingly, it may be desirable to have an expansion cartridge containing EPROM&#39;s or the like for storing software programs to control programmer  420  to operate in a particular manner corresponding to a given type or generation of implantable medical device. In addition, in accordance with the present invention, it is desirable to provide the capability through the expansion cartridge or through the floppy disk drive  66  or CD ROM drive. 
     The non-isolated analog circuit  459  of interface module  457  is coupled to a programming head  422 , which is used to establish the uplink and downlink telemetry links between the pacemaker  410  and programmer  420  as described above. Uplink telemetered EGM signals are received in programming head  422  and provided to non-isolated analog circuit  459 . Non-isolated analog circuit  459 , in turn, converts the digitized EGM signals to analog EGM signals and presents these signals on output lines A EGM OUT and V EGM OUT. These output lines may then be applied to a strip-chart recorder  463  to provide a hard-copy printout of the A EGM or V EGM for viewing by the physician. Similarly, the markers received by programming head  422  are presented on the MARKER CHANNEL output line from non-isolated analog circuit  459 . 
     Isolated analog circuit  460  in interface module  547  is provided to receive external ECG and electrophysiologic (EP) stimulation pulse signals. In particular, analog circuit  460  receives ECG signals from patient skin electrodes  459  and processes these signals before providing them to the remainder of the programmer system in a manner well known in the art. Circuit  460  further operates to receive the EP stimulation pulses from an external EP stimulator for the purposes of non-invasive EP studies, as is also known in the art. 
     In order to ensure proper positioning of programming head  422  over the antenna of the associated implanted device, feedback is provided to the physician that the programming head  422  is in satisfactory communication with and is receiving sufficiently strong RF signals. This feedback may be provided, for example, by means of a head position indicator, e.g. a light-emitting diode (LED) or the like that is lighted to indicate a stable telemetry channel. 
     FIG. 9 illustrates a functional flow chart describing a first method of calculation of average heart rates taken during desired time ranges over the course of a day. For example, calculation of a daily heart rate average and a night heart rate average, to be employed in constructing day heart rate trends and night heart rate trends, for display on the associated external programmer. In this context, the flow chart of FIG. 9 starts from the assumption that the implanted device will collect the measured heart intervals and calculate and store the average heart interval values for day heart rate and/or night heart rate, with the calculated average day heart rate and night simply displayed on the external device associated with the implanted device. In this context, it should also be understood that all calculations and processing of the measured heart intervals is performed by the microprocessor within the implanted device. However, as noted above, alternate divisions of tasks between the implanted and external devices are still believed to be within the scope of the invention. 
     At  600 , the device is initialized and thereafter sets SUMNN=0 at  602 . SUMNN is a running sum of the total duration of measured heart intervals retained for use in calculation of average heart rate according to the present invention. The device also sets the value of NN=0 in  602 . NN is the running total of measured heart intervals employed in calculation of average day or night heart rates according to the present invention. The device then waits until the time of day falls within the desired time window extending from a start time “A” to an end time “B”. In the context of monitoring of average daily heart rate, the defined time range may extend between 8:00 a.m. and 8:00 p.m., for example. In the context of a device which measures average nightly heart rate, the defined range may extend between 12:00 a.m. and 4:00 a.m., for example. It should be also understood that the same device may make and store measurements of both average day heart rate and average night heart rate. 
     If the device determines that present time T is within the defined desired time range for heart range monitoring, in response to a sensed or paced depolarization at  606 , the device at  608  stores the measured heart interval separating the sensed or paced depolarization  606  from the preceding paced or sensed depolarization, as measured in milliseconds. At  610 , the device determines whether the measured heart interval is acceptable to be retained for use in measuring average heart rate or should be rejected. The desirability of rejecting measured heart intervals will depend upon the condition of the patient and the type of device implanted. For example, in the case of a patient who is subject to atrial or ventricular tachycardia, wherein the device employing the present invention is an implantable pacemaker/cardioverter/defibrillator, it may be desirable to discard all measured heart intervals associated with detection and treatment of tachyarrhythmias. For example the device may reject all intervals which meet tachyarrhythmia detection criteria due to their relatively short duration, all intervals obtained during charging of the output capacitors of such a device prior to delivery of a cardioversion or defibrillation shock and all intervals sensed during delivery of anti-tachyarrhythrnia therapies such as anti-tachycardia pacing, cardioversion and defibrillation. In contrast, if the invention is embodied in a simple VVI-type pacemaker, and the patient is not subject to tachyarrhythmias, there may be no need to discard any heart intervals ending on a sensed depolarization. In addition or as an alternative, in which the invention is embodied to include a dual chamber pacemaker capable of switching between various pacing modes in response to detected atrial tachyarrhythmias, it may be desirable to discard heart intervals measured during operation of the mode switch between pacing modes. 
     If the measured heart interval is not rejected, the value of the interval is added to SUMNN at  612 , and the value of NN is incremented by one at  614 . The device continues to increment the values of SUMNN and NN according to this mechanism until the present time T equals or exceeds the defined expiration time B for heart rate monitoring. At  616 , the device compares the total duration of measured and saved intervals to a desired total duration “X” which may reflect a predetermined proportion of the duration of the monitoring interval. For example, the value of SUMNN may have to exceed 20% of the defined monitoring period. In the event that the value of SUMNN is inadequate, the device stores an indication that no heart rate has been calculated for the monitoring period presently in effect at  620 , and the device resets the values of SUMNN and NN to zero at  602 , awaiting the next defined monitoring interval. If the value of SUMNN is adequate, the average heart rate HR is calculated by means of the equation HR=60,000/(SUMNN/NN) at  622 , and the value of HR, representing the average heart rate over the monitoring period is stored at  624  for later telemetry to the associated external device and for display by the associated external device. The method of operation illustrated in FIG. 9 may be employed to collect and calculate average daily rates, average night heart rates, or both, for display on the associated external device. 
     FIG. 10 illustrates an alternative embodiment of the present invention in which an associated activity sensor or other metabolic sensor is employed in order to assure that during the defined heart rate monitoring periods, only heart intervals indicative of the patient at rest are employed in calculating average heart rates. It should be noted that the method of operation illustrated in FIG. 10 also permits the calculation of average resting heart rates over 24 hour periods, by simply designating the desired monitoring period it is to be successive 24 hour periods, as opposed to discreet periods within each 24 hour period. 
     After initialization at  700 , the device sets SUMNN and NN to zero at  702 , as discussed above in conjunction with FIG. 9, and awaits the beginning of the defined monitoring period at  704 . At  706 , the device initiates the relatively shorter time period T 1 , over which the patient&#39;s physical activity or other metabolic indicator of demand for cardiac output is to be monitored. The values of INTCOUNT, indicative of the number of intervals counted during this shorter time interval T 1  and INTSUM, reflective of the total duration of intervals stored during interval T 1  are reset to zero at  706 . The value of T 1  is preferably fairly short, for example, in the range of a few minutes, for example, about two to five minutes. Thereafter, until expiration of the shorter period T 1  at  712 , each time a paced or sensed depolarization is occurs at  708 , the heart interval separating the depolarization from the preceding depolarization is stored at  710 , and the device determines whether the stored interval should be rejected at  726 , in a fashion analogous to that described in conjunction with FIG. 9 above. If the interval is saved, the value of INTCOUNT is incremented by one at  728  and the value of INTSUM is incremented by the duration of the stored heart interval at  730 . This process continues until expiration of time period T 1  at  712 . Following expiration of T 1  at  712 , the device checks the output of the sensor over the preceding time period T 1  and compares the output to a defined threshold to determine whether the patient is at rest. For example, if the sensor output takes the form of successive numerical values (e.g. counts) generated over T 1 , the sum, mean, or median of the numerical values generated during T 1  may be calculated and analyzed, for example by comparison to a threshold value, to determine whether the patient was at rest during T 1 . If the sensor&#39;s output based on directly measured activity or other measured metabolic demand indicator indicates the patient was not at rest, the intervals collected during the preceding shorter T 1  period are discarded, and the next T 1  period is initiated at  706 . If the activity sensor or other indicator of metabolic demand indicates that the patient was at rest during the preceding shorter time period T 1 , the value of NN is incremented by the value of INTCOUNT at  732  and the value of SUMNN is incremented by INTSUM at  734 . This process continues until the device determines at  736  that the present time T is equal to or after the expiration point B of the defined monitoring period. 
     On expiration of the defined monitoring period, the device checks at  724  to determine whether the value of SUMNN exceeds a desired total duration, precisely as described above, in conjunction with FIG.  9 . If the total duration of stored heart intervals is less than the desired total, the device stores an indication that no measurement of average heart rate was stored for the monitoring period at  720 . However, if the total duration of measured heart intervals is sufficiently great, the value of the average heart rate is calculated at  718  in the same fashion as discussed in conjunction with FIG. 10 above, and the stored value of the average heart rate for the monitoring interval is stored at  716  for later telemetry to an associated external device for display thereon. 
     FIG. 11 illustrates a functional flow chart describing an alternative embodiment of the present invention in which sensor outputs are monitored over daytime or nighttime periods, in a manner analogous to the collection of heart rate information as discussed in conjunction with FIGS. 9 and 10 above. The term “average” in the context of FIG. 11 is the same as discussed above in conjunction with monitoring of heart rates. The sensor may be an activity sensor as described above or any of the various known physiologic sensors available for implant in the human body, including but not limited to sensors of metabolic demand for oxygenated blood, including oxygen saturation sensors, blood pressure sensors, blood temperature sensors, Ph sensors, respiration sensors and the like, as discussed above. 
     Calculation of a daily sensor output and a night sensor output value may be used, for example, in constructing day sensor output trends and night sensor output trends for display on the associated external programmer. In this context, the flow chart of FIG. 11 starts from the assumption that the implanted device will collect the measured sensor output values and calculate and store average or total values for day sensor output and/or night sensor output, with the calculated average or total value displayed on the external device associated with the implanted device. In this context, it should also be understood that all calculations and processing of the measured sensor output values are performed by the microprocessor within the implanted device. However, as noted above, alternate divisions of tasks between the implanted and external devices are still believed to be within the scope of the invention. 
     At  800 , the device is initialized and thereafter sets SUMSENS=0 at  602 . SUMSENS is a running sum of the total of measured sensor outputs retained for use in calculation of average or total sensor output according to the present invention. The device then waits until the time of day falls within the desired time window extending from a start time “A” to an end time “B”. In the context of monitoring of daily sensor output, the defined time range may extend between 8:00 a.m. and 8:00 p.m., for example. In the context of a device that measures nightly sensor output, the defined range may extend between 12:00 a.m. and 4:00 a.m., for example. It should be also understood that the same device may make and store measurements of both day and night sensor outputs. 
     If the device determines that present time T is within the defined desired time range for heart range monitoring, in response to a new output sensor value at  806 , the device at  808  stores the measured sensor output as a numerical value. The value of the sensor output (SO) is added to SUMSENS at  812 . The device continues to increment the values of SUMSENS according to this mechanism until the present time T equals or exceeds the defined expiration time B for sensor output monitoring at  816 . On expiration of the defined time for sensor output monitoring, the device either stores SUMSENS at  820  or optionally calculates and stores an average sensor output value at  822  and  824 , for example calculated based on SUMSENS and the duration of the defined time for sensor output monitoring or based on SUMSENS and the total number of sensor outputs included in SUMSENS, in a fashion analogous to that employed to calculate heart rate averages according to the method illustrated in FIG.  9 .