Patent Publication Number: US-7917197-B1

Title: Methods and devices for determining exercise diagnostic parameters

Description:
PRIORITY CLAIM 
     The present application is a continuation of, and claims priority to, U.S. patent application Ser. No. 10/828,883, entitled “METHODS AND DEVICES FOR DETERMINING EXERCISE DIAGNOSTIC PARAMETERS,” filed Apr. 20, 2004, now U.S. Pat. No. 7,031,766. 
     CROSS REFERENCE TO RELATED APPLICATIONS 
     The present invention relates to the following commonly assigned application, which is incorporated herein by reference: U.S. patent application No. Ser. 11/351,401, entitled “Methods and Devices for Determining Exercise Diagnostic Parameters,” filed Feb. 10, 2006, now U.S. Pat. No. 7,668,590. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to implantable cardiac devices and, more particularly, to an implantable cardiac device with the capability of measuring exercise diagnostic parameters. 
     2. Background Art 
     An implantable cardiac device is a medical device that is implanted in a patient to monitor electrical activity of the heart and to deliver appropriate electrical and/or drug therapy, as required. Implantable cardiac devices include, for example, pacemakers, cardioverters and defibrillators. The term “implantable cardioverter defibrillator” or simply “ICD” is used herein to refer to any implantable cardiac device. An ICD employs a battery to power its internal circuitry and to generate electrical therapy. The electrical therapy can include, for example, pacing pulses, cardioverting pulses and/or defibrillator pulses. 
     Heart failure is a growing medical challenge. In clinical practice today, most patients are managed effectively through pharmacological therapy such as beta-blockers, ACE inhibitors, and diuretics. If a patient&#39;s condition worsens, treatment may become more aggressive to include biventricular pacing and other implantable cardiac device therapy. Along with providing the primary objectives in the treatment of heart failure of improving symptoms, increasing the quality of life, and slowing disease progression, devices need to provide heart failure physicians with diagnostic parameters to monitor the patient&#39;s progress. 
     Currently, medical history and physical examination are the most important tools that a physician uses to determine and mark the progress of a heart failure patient. This involves much of the physician&#39;s time with the patient, as this may lead to the primary management program for the patient. 
     Included in most management programs is an exercise routine. It has been written extensively that adherence to exercise is a priority in improving or in maintaining good heath. Exercise diagnostics may help clinicians assess the compliance of the management programs prescribed to their patients, and possibly assist the patient in meeting those goals. 
     During exercise, the heart rate is a parameter or indicator of the amount of work that was required to provide blood and oxygen to the body. The maximum heart rate for a level of exercise corresponds to the conditioning of the heart. Other parameters, such as heart rate intensity, percent oxygen consumption (% VO 2 ) reserve, metabolic equivalents (METS), and workload also provide data that is indicative of heart conditioning. 
     Heart rate recovery after exercise is evaluated as a clinical marker of good vagal activity and cardiac health. As the heart rate increases due to a reduction in vagal tone, the heart rate also decreases with a reactivation of vagal activity. A delayed response to the decreasing heart rate may be a good prognostic marker of overall mortality (Cole, C. et al., NEJM 341:18, 1351-1357 (1999)) and cardiac health. Cole suggests that a reduction of only 12 beats per minute after one minute from peak exercise has been shown to be an abnormal value. 
     It would be advantageous to be able to obtain accurate exercise diagnostics over time from the patient without the cost and time of a physical examination, such as, for example, a treadmill test. 
     BRIEF SUMMARY OF THE INVENTION 
     The inventor has discovered that a device, such as an implantable medical device, can be used to determine exercise diagnostics in a patient, minimizing time and expense in monitoring a patient&#39;s progress. 
     The present invention includes a device, such as an implantable cardiac device, and method for determining a maximum observed heart rate of a patient during exercise. The method includes monitoring a changing heart rate of the patient and producing heart rate measurements, monitoring activity level of the patient, and identifying a heart rate as the maximum observed heart rate. The maximum observed heart rate is identified when the activity level exceeds an activity threshold, a heart rate measurement is greater than a stored heart rate measurement, and a difference between the heart rate measurement and the stored heart rate measurement does not exceed a predetermined threshold. 
     The present invention also includes a device, such as an implantable cardiac device, and method for determining workload of a patient during exercise. The method includes monitoring a changing heart rate of the patient and producing heart rate measurements, monitoring activity level of the patient, and determining workload of the patient using at least one heart rate measurement when the activity level exceeds an activity threshold. 
     The present invention also includes a device, such as an implantable cardiac device, and method for determining heart rate recovery of a patient. The method includes monitoring a changing heart rate of the patient and producing heart rate measurements, identifying a first heart rate, identifying a second heart rate, and using the first heart rate and the second heart rate to determine a measure of heart rate recovery. The first heart rate is identified when at least one heart rate measurement exceeds a first heart rate measurement threshold and/or an activity level of the patient exceeds a first activity threshold. The second heart rate is identified when at least one heart rate measurement falls below a second heart rate measurement and/or an activity level of the patient falls below a second activity threshold. 
     The embodiments of the present invention related to the device for determining a maximum observed heart rate of a patient during exercise, determining workload of a patient during exercise, and determining heart recovery of a patient include means for performing the above-described methods. 
     Further features and advantages of the present invention as well as the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
       The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the relevant art(s) to make and use the invention. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, in most drawings, the leftmost digit of a reference number identifies the drawing in which the reference number first appears. 
         FIG. 1A  is a simplified diagram illustrating an exemplary ICD in electrical communication with at least three leads implanted into a patient&#39;s heart for delivering multi-chamber stimulation and shock therapy. 
         FIG. 1B  is a functional block diagram of an exemplary ICD, which can provide cardioversion, defibrillation and pacing stimulation in four chambers of a heart. 
         FIG. 2  is a flow chart illustrating an embodiment of a method for determining an observed maximum heart rate of a patient during exercise in accordance with the present invention. 
         FIG. 3A ,  FIG. 3B ,  FIG. 3C , and  FIG. 3D  illustrate representations of sample exercise diagnostic results in accordance with the present invention. 
         FIG. 4  is a flow chart illustrating an embodiment of a method for determining an exercise diagnostic such as work of a patient during exercise in accordance with the present invention. 
         FIG. 5  is a flow chart illustrating an embodiment of a method for determining heart rate recovery of a patient after exercise in accordance with the present invention. 
         FIG. 6  is a flow chart illustrating another embodiment of a method for determining an observed maximum heart rate of a patient during exercise in accordance with the present invention, as described in a first example. 
         FIG. 7  is a flow chart illustrating another embodiment of a method for determining heart rate recovery of a patient in accordance with the present invention, as described in a second example. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description of the present invention refers to the accompanying drawings that illustrate exemplary embodiments consistent with this invention. Other embodiments are possible, and modifications may be made to the embodiments within the spirit and scope of the present invention. Therefore, the following detailed description is not meant to limit the invention. Rather, the scope of the invention is defined by the appended claims. 
     It will be apparent to one of skill in the art that the present invention, as described below, may be implemented in many different embodiments of hardware, software, firmware, and/or the entities illustrated in the figures. Any actual software and/or hardware described herein is not meant to limit the scope of the present invention. Thus, the structure, operation and behavior of the present invention will be described with the understanding that many modifications and variations of the embodiments are possible, given the level of detail presented herein. 
     Before describing the invention in detail, it is helpful to describe an example environment in which the invention may be implemented. The present invention is particularly useful in the environment of an implantable cardiac device. Implantable cardiac devices include, for example, pacemakers, cardioverter-defibrillators, and hemodynamic monitors. The term “implantable cardioverter defibrillator” or simply “ICD” is used herein to refer to any implantable cardiac device or implantable cardioverter-defibrillator.  FIGS. 1A and 1B  illustrate such an environment. 
     As shown in  FIG. 1A , there is an exemplary ICD  10  in electrical communication with a patient&#39;s heart  12  by way of three leads,  20 ,  24  and  30 , suitable for delivering multi-chamber stimulation and pacing therapy. To sense atrial cardiac signals and to provide right atrial chamber stimulation therapy, ICD  10  is coupled to implantable right atrial lead  20  having at least an atrial tip electrode  22 , which typically is implanted in the patient&#39;s right atrial appendage. 
     To sense left atrial and ventricular cardiac signals and to provide left-chamber pacing therapy, ICD  10  is coupled to “coronary sinus” lead  24  designed for placement in the “coronary sinus region” via the coronary sinus for positioning a distal electrode adjacent to the left ventricle and/or additional electrode(s) adjacent to the left atrium. As used herein, the phrase “coronary sinus region” refers to the vasculature of the left ventricle, including any portion of the coronary sinus, great cardiac vein, left marginal vein, left posterior ventricular vein, middle cardiac vein, and/or small cardiac vein or any other cardiac vein accessible by the coronary sinus. 
     Accordingly, exemplary coronary sinus lead  24  is designed to receive atrial and ventricular cardiac signals and to deliver left ventricular pacing therapy using at least a left ventricular tip electrode  26 , left atrial pacing therapy using at least a left atrial ring electrode  27 , and shocking therapy using at least a left atrial coil electrode  28 . 
     ICD  10  is also shown in electrical communication with the patient&#39;s heart  12  by way of an implantable right ventricular lead  30  having, in this embodiment, a right ventricular tip electrode  32 , a right ventricular ring electrode  34 , a right ventricular (RV) coil electrode  36 , and an SVC coil electrode  38 . Typically, right ventricular lead  30  is transvenously inserted into heart  12  so as to place the right ventricular tip electrode  32  in the right ventricular apex so that RV coil electrode  36  will be positioned in the right ventricle and SVC coil electrode  38  will be positioned in the superior vena cava. Accordingly, right ventricular lead  30  is capable of receiving cardiac signals and delivering stimulation in the form of pacing and shock therapy to the right ventricle. 
       FIG. 1B  shows a simplified block diagram of ICD  10 , which is capable of treating both fast and slow arrhythmias with stimulation therapy, including cardioversion, defibrillation, and pacing stimulation. While a particular multi-chamber device is shown, it is shown for illustration purposes only, and one of skill in the art could readily duplicate, eliminate or disable the appropriate circuitry in any desired combination to provide a device capable of treating the appropriate chamber(s) with the desired cardioversion, defibrillation and pacing stimulation. 
     A housing  40  of ICD  10 , shown schematically in  FIG. 1B , is often referred to as the “can,” “case” or “case electrode” and may be programmably selected to act as the return electrode for all “unipolar” modes. Housing  40  may further be used as a return electrode alone or in combination with one or more of coil electrodes,  28 ,  36 , and  38  for shocking purposes. Housing  40  further includes a connector (not shown) having a plurality of terminals,  42 ,  44 ,  46 ,  48 ,  52 ,  54 ,  56 , and  58  (shown schematically and, for convenience, the names of the electrodes to which they are connected are shown next to the terminals). As such, to achieve right atrial sensing and pacing, the connector includes at least a right atrial tip terminal (AR TIP)  42  adapted for connection to atrial tip electrode  22 . 
     To achieve left chamber sensing, pacing and shocking, the connector includes at least a left ventricular tip terminal (VL TIP)  44 , a left atrial ring terminal (AL RING)  46 , and a left atrial shocking terminal (AL COIL)  48 , which are adapted for connection to left ventricular ring electrode  26 , left atrial tip electrode  27 , and left atrial coil electrode  28 , respectively. 
     To support right chamber sensing, pacing, and shocking the connector also includes a right ventricular tip terminal (VR TIP)  52 , a right ventricular ring terminal (VR RING)  54 , a right ventricular shocking terminal (RV COIL)  56 , and an SVC shocking terminal (SVC COIL)  58 , which are configured for connection to right ventricular tip electrode  32 , right ventricular ring electrode  34 , RV coil electrode  36 , and SVC coil electrode  38 , respectively. 
     At the core of ICD  10  is a programmable microcontroller  60  which controls the various modes of stimulation therapy. As is well known in the art, microcontroller  60  typically includes a microprocessor, or equivalent control circuitry, designed specifically for controlling the delivery of stimulation therapy and can further include RAM or ROM memory, logic and timing circuitry, state machine circuitry, and I/O circuitry. Typically, microcontroller  60  includes the ability to process or monitor input signals (data) as controlled by a program code stored in a designated block of memory. The details of the design of microcontroller  60  are not critical to the present invention. Rather, any suitable microcontroller  60  can be used to carry out the functions described herein. The use of microprocessor-based control circuits for performing timing and data analysis functions are well known in the art. In specific embodiments of the present invention, microcontroller  60  performs some or all of the steps associated with the exercise diagnostics in accordance with the present invention. 
     Representative types of control circuitry that may be used with the invention include the microprocessor-based control system of U.S. Pat. No. 4,940,052 (Mann et al.) and the state-machines of U.S. Pat. Nos. 4,712,555 (Thornander et al.) and 4,944,298 (Sholder). For a more detailed description of the various timing intervals used within the ICDs and their inter-relationship, see U.S. Pat. No. 4,788,980 (Mann et al.). The &#39;052, &#39;555, &#39;298 and &#39;980 patents are incorporated herein by reference. 
     As shown in  FIG. 1B , an atrial pulse generator  70  and a ventricular pulse generator  72  generate pacing stimulation pulses for delivery by right atrial lead  20 , right ventricular lead  30 , and/or coronary sinus lead  24  via an electrode configuration switch  74 . It is understood that in order to provide stimulation therapy in each of the four chambers of the heart, atrial and ventricular pulse generators  70 ,  72 , may include dedicated, independent pulse generators, multiplexed pulse generators, or shared pulse generators. Pulse generators  70  and  72  are controlled by microcontroller  60  via appropriate control signals  76  and  78 , respectively, to trigger or inhibit the stimulation pulses. 
     Microcontroller  60  further includes timing control circuitry  79  which is used to control pacing parameters (e.g., the timing of stimulation pulses) as well as to keep track of the timing of refractory periods, PVARP intervals, noise detection windows, evoked response windows, alert intervals, marker channel timing, etc., which are well known in the art. Examples of pacing parameters include, but are not limited to, atrio-ventricular (AV) delay, interventricular (RV-LV) delay, atrial interconduction (A-A) delay, ventricular interconduction (V-V) delay, and pacing rate. 
     Switch  74  includes a plurality of switches for connecting the desired electrodes to the appropriate I/O circuits, thereby providing complete electrode programmability. Accordingly, switch  74 , in response to a control signal  80  from microcontroller  60 , determines the polarity of the stimulation pulses (e.g., unipolar, bipolar, combipolar, etc.) by selectively closing the appropriate combination of switches (not shown) as is known in the art. 
     Atrial sensing circuits  82  and ventricular sensing circuits  84  may also be selectively coupled to right atrial lead  20 , coronary sinus lead  24 , and right ventricular lead  30 , through switch  74  for detecting the presence of cardiac activity in each of the four chambers of the heart. Accordingly, the atrial (ATR. SENSE) and ventricular (VTR. SENSE) sensing circuits  82  and  84  may include dedicated sense amplifiers, multiplexed amplifiers, or shared amplifiers. Switch  74  determines the “sensing polarity” of the cardiac signal by selectively closing the appropriate switches, as is also known in the art. In this way, the clinician may program the sensing polarity independent of the stimulation polarity. 
     Each sensing circuit,  82  and  84 , preferably employs one or more low power, precision amplifiers with programmable gain and/or automatic gain control, bandpass filtering, and a threshold detection circuit, as known in the art, to selectively sense the cardiac signal of interest. The automatic sensitivity control enables ICD  10  to deal effectively with the difficult problem of sensing the low amplitude signal characteristics of atrial or ventricular fibrillation. Such sensing circuits,  82  and  84 , can be used to determine cardiac performance values used in the present invention. 
     The outputs of atrial and ventricular sensing circuits  82  and  84  are connected to microcontroller  60  which, in turn, are able to trigger or inhibit atrial and ventricular pulse generators,  70  and  72 , respectively, in a demand fashion in response to the absence or presence of cardiac activity, in the appropriate chambers of the heart. Sensing circuits  82  and  84 , in turn, receive control signals over signal lines  86  and  88  from microcontroller  60  for purposes of measuring cardiac performance at appropriate times, and for controlling the gain, threshold, polarization charge removal circuitry (not shown), and timing of any blocking circuitry (not shown) coupled to the inputs of sensing circuits  82  and  84 . 
     For arrhythmia detection, ICD  10  utilizes the atrial and ventricular sensing circuits  82  and  84  to sense cardiac signals to determine whether a rhythm is physiologic or pathologic. The timing intervals between sensed events (e.g., P-waves, R-waves, and depolarization signals associated with fibrillation which are sometimes referred to as “F-waves” or “Fib-waves”) are then classified by microcontroller  60  by comparing them to a predefined rate zone limit (i.e., bradycardia, normal, low rate VT, high rate VT, and fibrillation rate zones) and various other characteristics (e.g., sudden onset, stability, physiologic sensors, and morphology, etc.) in order to determine the type of remedial therapy that is needed (e.g., bradycardia pacing, anti-tachycardia pacing, cardioversion shocks or defibrillation shocks, collectively referred to as “tiered therapy”). 
     Microcontroller  60  utilizes arrhythmia detection circuitry  75  and morphology detection circuitry  76  to recognize and classify arrhythmia so that appropriate therapy can be delivered. 
     Cardiac signals are also applied to the inputs of an analog-to-digital (A/D) data acquisition system  90 . Data acquisition system  90  is configured to acquire intracardiac electrogram signals, convert the raw analog data into a digital signal, and store the digital signals for later processing and/or telemetric transmission to an external device  102 . Data acquisition system  90  is coupled to right atrial lead  20 , coronary sinus lead  24 , and right ventricular lead  30  through switch  74  to sample cardiac signals across any pair of desired electrodes. 
     Advantageously, data acquisition system  90  can be coupled to microcontroller  60 , or other detection circuitry, for detecting an evoked response from heart  12  in response to an applied stimulus, thereby aiding in the detection of “capture.” Capture occurs when an electrical stimulus applied to the heart is of sufficient energy to depolarize the cardiac tissue, thereby causing the heart muscle to contract. Microcontroller  60  detects a depolarization signal during a window following a stimulation pulse, the presence of which indicates that capture has occurred. Microcontroller  60  enables capture detection by triggering ventricular pulse generator  72  to generate a stimulation pulse, starting a capture detection window using timing control circuitry  79  within microcontroller  60 , and enabling data acquisition system  90  via control signal  92  to sample the cardiac signal that falls in the capture detection window and, based on the amplitude, determines if capture has occurred. 
     The implementation of capture detection circuitry and algorithms are well known. See for example, U.S. Pat. No. 4,729,376 (DeCote, Jr.); U.S. Pat. No. 4,708,142 (DeCote, Jr.); U.S. Pat. No. 4,686,988 (Sholder); U.S. Pat. No. 4,969,467 (Callaghan et al.); and U.S. Pat. No. 5,350,410 (Kleks et al.), which patents are hereby incorporated herein by reference. The type of capture detection system used is not critical to the present invention. Microcontroller  60  also contains maximum observed heart rate (HR max ) detector  62 , workload detector  64 , and/or a heart rate recovery detector  66 . The operation of the HR max  detector, workload detector, and heart rate recovery detector are discussed below in connection with the methods of the present invention. 
     Microcontroller  60  is further coupled to a memory  94  by a suitable data/address bus  96 , wherein the programmable operating parameters used by microcontroller  60  are stored and modified, as required, in order to customize the operation of ICD  10  to suit the needs of a particular patient. Such operating parameters define, for example, pacing pulse amplitude, pulse duration, electrode polarity, rate, sensitivity, automatic features, arrhythmia detection criteria, and the amplitude, waveshape and vector of each shocking pulse to be delivered to the patient&#39;s heart  12  within each respective tier of therapy. 
     Advantageously, the operating parameters of ICD  10  may be non-invasively programmed into memory  94  through a telemetry circuit  100  in telemetric communication with external device  102 , such as a programmer, transtelephonic transceiver, or a diagnostic system analyzer. Telemetry circuit  100  is activated by microcontroller  60  by a control signal  106 . Telemetry circuit  100  advantageously allows intracardiac electrograms and status information relating to the operation of ICD  10  (as contained in microcontroller  60  or memory  94 ) to be sent to external device  102  through an established communication link  104 . 
     For examples of such devices, see U.S. Pat. No. 4,809,697, entitled “Interactive Programming and Diagnostic System for use with Implantable Pacemaker” (Causey, III et al.); U.S. Pat. No. 4,944,299, entitled “High Speed Digital Telemetry System for Implantable Device” (Silvian); and U.S. Pat. No. 6,275,734, entitled “Efficient Generation of Sensing Signals in an Implantable Medical Device such as a Pacemaker or ICD” (McClure et al), which patents are hereby incorporated herein by reference. 
     In one embodiment, ICD  10  further includes a physiologic sensor  108  that can be used to detect changes in cardiac performance or changes in the physiological condition of the heart. Accordingly, microcontroller  60  can respond by adjusting the various pacing parameters (such as rate, AV Delay, RV-LV Delay, V-V Delay, etc.) in accordance with the embodiments of the present invention. Microcontroller  60  controls adjustments of pacing parameters by, for example, controlling the stimulation pulses generated by the atrial and ventricular pulse generators  70  and  72 . While shown as being included within ICD  10 , it is to be understood that physiologic sensor  108  may also be external to ICD  10 , yet still be implanted within or carried by the patient. More specifically, sensor  108  can be located inside ICD  10 , on the surface of ICD  10 , in a header of ICD  10 , or on a lead (which can be placed inside or outside the bloodstream). As discussed below, sensor  108  can also be used to measure activity level. 
     ICD  10  additionally includes a battery  110  which provides operating power to all of the circuits shown in  FIG. 1B . For ICD  10 , which employs shocking therapy, battery  110  must be capable of operating at low current drains for long periods of time, and then be capable of providing high-current pulses (for capacitor charging) when the patient requires a shock pulse. Battery  110  must also have a predictable discharge characteristic so that elective replacement time can be detected. Accordingly, ICD  10  preferably employs lithium/silver vanadium oxide batteries, as is true for most (if not all) current devices. 
     ICD  10  further includes a magnet detection circuitry (not shown), coupled to microcontroller  60 . It is the purpose of the magnet detection circuitry to detect when a magnet is placed over ICD  10 , which magnet may be used by a clinician to perform various test functions of ICD  10  and/or to signal microcontroller  60  that the external programmer  102  is in place to receive or transmit data to microcontroller  60  through telemetry circuit  100 . 
     As further shown in  FIG. 1B , ICD  10  is shown as having an impedance measuring circuit  112  which is enabled by microcontroller  60  via a control signal  114 . The known uses for an impedance measuring circuit  120  include, but are not limited to, lead impedance surveillance during the acute and chronic phases for proper lead positioning or dislodgement; detecting operable electrodes and automatically switching to an operable pair if dislodgement occurs; measuring respiration or minute ventilation; measuring thoracic impedance for determining shock thresholds; detecting when the device has been implanted; measuring stroke volume; and detecting the opening of heart valves, etc. The impedance measuring circuit  112  is advantageously coupled to switch  74  so that any desired electrode may be used. The impedance measuring circuit  112  is not critical to the present invention and is shown only for completeness. 
     In the case where ICD  10  is intended to operate as a cardioverter, pacer or defibrillator, it must detect the occurrence of an arrhythmia and automatically apply an appropriate electrical therapy to the heart aimed at terminating the detected arrhythmia. To this end, microcontroller  60  further controls a shocking circuit  116  by way of a control signal  118 . The shocking circuit  116  generates shocking pulses of low (up to 0.5 Joules), moderate (0.5-10 Joules), or high energy (11 to 40 Joules), as controlled by microcontroller  60 . Such shocking pulses are applied to the patient&#39;s heart  12  through at least two shocking electrodes (e.g., selected from left atrial coil electrode  28 , RV coil electrode  36 , and SVC coil electrode  38 ). As noted above, housing  40  may act as an active electrode in combination with RV electrode  36 , or as part of a split electrical vector using SVC coil electrode  38  or left atrial coil electrode  28  (i.e., using the RV electrode as a common electrode). 
     Cardioversion shocks are generally considered to be of low to moderate energy level (so as to minimize pain felt by the patient), and/or synchronized with an R-wave and/or pertaining to the treatment of tachycardia. Defibrillation shocks are generally of moderate to high energy level (i.e., corresponding to thresholds in the range of 5-40 Joules), delivered asynchronously (since R-waves may be too disorganized to be recognized), and pertaining exclusively to the treatment of fibrillation. Accordingly, microcontroller  60  is capable of controlling the synchronous or asynchronous delivery of the shocking pulses. 
     With the description of an example environment, such as an ICD, in mind, features of the present invention are described in more detail below. 
     A method  200  of determining a maximum observed heart rate (HR max ) of a patient during exercise in accordance with the present invention is illustrated in  FIG. 2 . According to an embodiment of the invention, the method  200  begins at step  202 , in which the heart rate and activity level of the patient are monitored. The heart rate and activity level of the patient may be continuously monitored during the method  200 . 
     The patient&#39;s heart rate may be determined by any suitable method. Many variations on how to determine heart rate are known to those of ordinary skill in the art, and any of these of reasonable accuracy may be used for the purposes of the invention. In embodiments of the invention, heart rate can be determined by measurement of an R-R interval cycle length (or P-P), which is the inverse of heart rate. As used herein, the heart rate (in beats per second) can be seen as the inverse to cycle length, determined by 60,000 divided by the cycle length (in milliseconds). 
     Heart rate measurements can be produced based upon the monitored heart rate. Such heart rate measurements include but are not limited to heart rate and heart rate intensity. 
     The activity level of the patient may also be determined by any suitable method. For example, the activity level may be determined by an accelerometer, piezoelectric crystal, minute ventilation, photoplethysmography, or a derivative thereof, such as the sensor indicated rate. In one embodiment, activity level is determined using physiologic sensor  108 . In this embodiment, sensor  108  is an accelerometer, a piezoelectric crystal, an impedance sensor, or a photoplethysmography sensor. 
     In step  204 , the measured activity level is compared with a predetermined activity threshold to determine whether the activity level exceeds the threshold. The predetermined activity threshold can be a value that corresponds to a certain level of exercise. It should be appreciated that the activity threshold value can be tailored for a specific patient&#39;s condition. Illustratively, an activity threshold value which correlates with walking or some other low level of exercise may be, for example, 50 milligravities as measured by an accelerometer. 
     It should be understood that in the context of the present invention, when comparing a measurement to a threshold, the terms “exceeds” or “is greater than” encompass instances when the measurement is equal to the threshold value. Similarly, it should be understood that the terms “falls below” or “is less than” a threshold value encompass instances when the measurement is equal to the threshold value. A person skilled in the relevant art will recognize that selection of a threshold value, and how to treat the condition of equality between the threshold and the measurement, are design choices. 
     The activity level can be compared with an activity threshold at various time intervals or periodically to determine whether the activity level exceeds the predetermined threshold. The particular selected time interval for monitoring is not critical. In one embodiment of the invention, the activity level is monitored and compared with the activity threshold at time intervals of 30 seconds (i.e., every 30 seconds). 
     If the patient activity level exceeds the predetermined activity threshold, then the method proceeds to step  206 . Illustratively, if 50 milligravities activity is a threshold that correlates well with walking or some low level of exercise, and the implantable medical device is programmed at this threshold, then if the measured activity level exceeds 50 milligravities, the method proceeds to step  206 . 
     Steps  206  and  208  can be performed when the patient activity level exceeds the predetermined activity threshold for a predetermined period of time. This predetermined period of time can be an amount that one skilled in the art would understand to be sufficient for the heart to react to the exercise by the patient (which can be indicated by, e.g., the activity level exceeding the predetermined activity threshold). Illustratively, the predetermined period of time may be 10 seconds to five minutes, preferably about two to three minutes, more preferably about two minutes. 
     In step  206 , a heart rate measurement is compared with a stored heart rate measurement. The stored heart rate measurement can be, for example, a heart rate measurement previously obtained during exercise, including a previously determined HR max  during exercise. Prior to first occurrence of the method, the stored heart rate measurement can be set to a predetermined default value. If the heart rate measurement exceeds the previously stored heart rate measurement, then the method proceeds to step  208 . Otherwise, step  204  is repeated. That is, the method continues to monitor heart rate and activity level and produce heart rate measurements. 
     In step  208 , the difference between the heart rate measurement and the stored heart rate measurement is compared to a predetermined threshold. The predetermined threshold difference may be selected to correspond to a value above which may be indicative of noise, PACs, PVCs, and/or arrhythmias. If the difference between the heart rate measurement and the stored heart rate measurement exceeds the threshold difference, the measured heart rate is not considered to be a HR max . The threshold may even be step-size units, so as to show a gradual (physiologic) increase. 
     In accordance with one embodiment of the invention, step  208  is not performed. However, this embodiment is less preferred, as the resulting HR max  could be inaccurate due to noise and/or premature heartbeats. 
     It should be understood that the order of comparison steps  206  and  208  is not limited to that depicted in the figure and may be performed in reverse order or conducted simultaneously. 
     If, in step  206 , the heart rate measurement is greater than the stored heart rate measurement and, in step  208 , the difference between the heart rate measurement and the stored heart rate measurement does not exceed a predetermined threshold, then the heart rate associated with the heart rate measurement may be identified as a maximum observed heart rate (HR max ). In other words, a heart rate can be identified as a HR max  when the comparison steps  204 ,  206 , and  208  are met. 
     The maximum observed heart rate may be recorded as a stored value, and the method  200  repeated, using the HR max  as a new stored heart rate measurement. The HR max  determination may be continued until activity level and/or heart rate is indicative of a slow-down of exercise. 
     Based on the HR max  obtained, further values may be obtained that are indicative of heart conditioning. These values include heart rate intensity, percent oxygen consumption (% VO 2 ) reserve, metabolic equivalents (METS), percentage METS, workload, and absolute oxygen uptake. 
     For example, heart rate intensity (also known as percent heart rate reserve, heart rate capacity, target heart rate, or % HRR) may be calculated by dividing HR max  by the predicted age compensated maximum heart rate as follows: 
     
       
         
           
             
               % 
               ⁢ 
               HRR 
             
             = 
             
               
                 
                   
                     HR 
                     max 
                   
                   - 
                   
                     Resting 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     HR 
                   
                 
                 
                   
                     Age 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     compensated 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     maximum 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     HR 
                   
                   - 
                   
                     Resting 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     HR 
                   
                 
               
               * 
               100 
             
           
         
       
     
     In the equation indicated above, resting heart rate of the patient may be obtained by any suitable method including, for example, a heart rate measurement taken when the activity level of the patient is sufficiently low to be considered inactive. The age compensated maximum heart rate can be calculated by the formula: (220−age). 
     With % HRR, it is also possible to calculate % VO 2  reserve. Swain et al. have shown a close correlation between % HRR and % VO 2  reserve (“Heart rate reserve is equivalent to % VO 2  reserve, not % VO 2max   ,” Med Sci. Sports Exercise  29:410-414 (1997)). 
     % VO 2  reserve is an intensity scale or index that describes the percentage of oxygen intake used during exercise. The value between % VO 2  reserve and 100% is the amount of oxygen intake reserves available. This value may be obtained by the following equation (Swain et al., Target HR for the development of CV fitness,  Medicine  &amp;  Science in Sports  &amp;  Exercise  26(1):112-116):
 
% VO 2 reserve=(%  HRR− 37)/0.64
 
where % HRR is calculated as described above.
 
     Workload is measure of intensity times duration, and may be seen by the following equation:
 
Workload=Intensity*Duration
 
where Intensity is VO 2observed , but may also be seen as an index such as heart rate intensity (% HRR) or % VO 2  reserve as discussed above, and Duration is the time during exercise when activity is above a predetermined threshold. It is possible to use in the calculation of work only % HRR values above a predetermined threshold (e.g., &gt;40%), reflective of at least moderate exercise. An additional method involves multiplying the mean % HRR above the predetermined threshold by the total duration.
 
     A primary expression of intensity throughout the clinical community is metabolic equivalents (METS). METS is a measure of Intensity or functional capacity. One (1) MET is equivalent to the amount of energy used at rest (oxygen uptake of 3.5 ml/(kg*min)), or the resting VO 2 .
 
1 MET= 3.5 mL/(kg*min)=VO 2resting  
 
     METS are linked to heart rate intensity. See, Strath et al., “Evaluation of Heart Rate as a Method for Accessing Moderate Intensity Physical Activity,”  Med.  &amp;  Sci. in Sports  &amp;  Exerc.,  465-470 (2000). 
     One method for determining METS has been described by Wilkoff, B. L., et al. (“A Mathematical Model of the Cardiac Chronotropic Response to Exercise,”  J. Electrophysiol.  3:176-180 (1989)), in which a mathematical model was developed describing the relationship of percentage metabolic equivalents (% METS) to heart rate intensity using the CAEP and Bruce exercise protocols. They found that the relationship was linear, with a slope of approximately 1 (1.06), by the equation:
 
%  METS= 1.06*(%  HRR )−4.87
 
     Observed METS during exercise can be obtained through the following equation: 
               %   ⁢   METS     =           (       METS   observed     -     METS   rest       )       (       METS   max     -     METS   rest       )       *   100   ⁢   %   ⁢           ⁢   with   ⁢           ⁢     METS   rest       =   1           
The value for METS max  to be used in the above equation may be obtained as follows:
 
Predicted  METS   max =16.6−0.16(age)
 
     This predicted METS max  value is an approximation, as it was obtained by a nomogram of sedentary men who participated in the USAir Force School of Aerospace Medicine Protocol, and who did not have a history of CHF. See Morris et al., “Nomogram Based on Metabolic Equivalents and Age for Assessing Aerobic Exercise Capacity in Men,”  J. Am. Coll. Cardiol.  22:175-182 (1993). However, if this approximation is used as a best fit method for maximal METS expected for each patient, METS observed  can thus be calculated as:
 
 METS   observed =(%  METS/ 100)*((16.6−0.16*(age))−1)+1
 
     METS can also be determined by the following method by alternatively solving for % VO 2  reserve. % VO 2  reserve can be calculated by the following equation: 
               %   ⁢     VO   2     ⁢   reserve     =           (       VO     2   ⁢   observed       -     VO     2   ⁢   rest         )       (       VO     2   ⁢           ⁢   max       -     VO     2   ⁢           ⁢   rest         )       *   100   ⁢   %   ⁢           ⁢   with   ⁢           ⁢     VO     2   ⁢           ⁢   rest         =   1           
where VO 2max  can be obtained from the non-exercise prediction equation of Jackson et al., “Prediction of functional aerobic capacity without exercise testing,”  Med. Sci. Sports  &amp;  Exerc J.  22:863-870 (1990) by:
 
VO 2max =50.513+1.589*(activity scale[0 . . . 7])−0.289*(age)−0.552*(% fat)+5.863*( F= 0 ,M= 1).
 
     Or, for those times when % fat may be difficult to obtain, the following equation by Jackson et al. allows for use of Body Mass Index (BMI):
 
VO 2max =56.363+1.921*(activity scale[0 . . . 7])−0.381*(age)−0.754*(BMI)+10.987*( F= 0 ,M= 1)
 
     In the above two equations for VO 2max , activity scale can be related to % HRR as a level of activity, % fat or BMI is either calculated as an average over the population or a value to be uploaded to the ICD, and F and M designate female and male, respectively. 
     When VO 2max  is plugged back into the % VO 2  equation, VO 2observed  can be obtained (units of mL/[kg*min]). METS observed  can be obtained by dividing VO 2observed  by 3.5. 
     Another way to determine VO 2max  is by the Astrand single-stage submaximal method, with the following equation:
 
VO 2max =VO 2observed *[(Age compensated max. HR− K )/(HR observed   −K )]
 
where K=63 for men and 73 for women. (Astrand, P. O., and Rodah, K.,  Textbook of Work Physiology,  3 rd  Ed. New York: McGraw-Hill, 1986, p. 318-325 and 340-358.)
 
     Once METS observed  has been calculated, it is possible to get the following values: 
     Relative Oxygen consumption (ml/(kg*min)): METS/3.5 
     Absolute Oxygen Uptake (L/min): VO 2max *Weight 
     Calories (kcal): 1 L O 2 =5 kcal: (VO 2max *duration)/5 
     Joules: 1 Kcal=4186 J 
     If the value for METS has a large standard deviation over the above equations, it can be further worked into a descriptive intensity scale (light, moderate, vigorous) as defined by Ainsworth B E et al., Compendium of physical activities: an update of activity codes and MET intensities,  Med Sci. Sports Exerc.;  9:S498-S516 (2000)) where these can be defined by:
 
 METS   60% max cardiorespiratory capacity =[10.6*(60−0.55*(age)]/3.5 for men, and
 
 METS   60% max cardiorespiratory capacity =[0.6*(48−0.37*(age)]/3.5 for women
 
with 60% max cardiorespiratory capacity (MCC) considered vigorous. Therefore, light intensity would be, for example, between 20-40%, and moderate activity would be, for example, between 40-60%.
 
       FIGS. 3A to 3D  illustrate how exercise data, such as exercise intensities may be displayed. The data illustrated in these Figures are prophetic. In  FIG. 3A , measured heart rate intensity (% HRR) data is displayed as a function of time in the graph. The table above the chart illustrates the corresponding time to HR max  and the total duration of HR max . In  FIG. 3B , measured heart rate intensity and total duration of HR max  are illustrated on one graph. In  FIG. 3C , measured exercise intensity in the units of METS is illustrated, with corresponding amounts of vigorous, moderate, and low intensities, and the duration of each amount. In  FIG. 3D , measured workload is illustrated, with corresponding duration of the workload. Each data point illustrated on the table and graphs of  FIGS. 3A-3D  represent an average over one week. 
     As is illustrated from  FIGS. 3A-3D , the invention also encompasses determining the time period associated with exercise intensities. For example, the time to and duration of HR max  and workload can be determined. 
     The above-described method  200  for determining the maximum observed heart rate of a patient during exercise may be implemented by hardware, software, or firmware of a pacing system, such as the ICD described earlier with reference to  FIGS. 1A and 1B , with particular reference to HR max  detector  62 . 
     In another embodiment of the invention, a method for determining exercise diagnostics, such as workload, heart rate intensity, percent oxygen consumption (% VO 2 ) reserve, metabolic equivalents (METS), percentage METS, and absolute oxygen uptake may be obtained without obtaining HR max . This method includes monitoring a changing heart rate of a patient and producing heart measurements, monitoring activity level, and determining an exercise diagnostic, such as workload of the patient using at least one heart rate measurement when the activity level exceeds an activity threshold. 
     A method  400  of determining workload of a patient during exercise in accordance with the present invention is illustrated in  FIG. 4 . According to an embodiment of the invention, the method  400  begins at step  402 , in which the heart rate and activity level of the patient is monitored. The heart rate and activity level of the patient may be continuously monitored during the method  400 . 
     As discussed above in conjunction with the method for determining HR max , the patient&#39;s heart rate and activity level may be determined by any suitable method, and heart rate measurements can be generated based upon the monitored heart rate. In embodiments of the invention, the heart rate measurements include heart rate intensity. 
     In step  404 , the measured activity level is compared with a predetermined activity threshold to determine whether the activity level exceeds the threshold. As discussed above, the predetermined activity threshold can be a value that corresponds to a certain level of exercise and can be tailored for a specific patient&#39;s condition. 
     The activity level can be compared with an activity threshold at various time intervals to determine whether the activity level exceeds the predetermined threshold for a predetermined period of time. The time interval or frequency of comparing the activity level with the activity threshold is not critical to the invention. In an embodiment of the invention, the activity level is monitored and compared with the activity threshold at time intervals of 30 seconds. 
     If it is determined in step  404  that the patient activity level exceeds a predetermined activity threshold, then step  406  is performed. As discussed above in conjunction with determining HR max , step  406  can be performed when the patient activity level exceeds the predetermined activity threshold for at least a predetermined period of time. This predetermined period of time may correlate to the amount of time for the heart to react to the exercise by the patient. Illustratively, the predetermined period of time may be 10 seconds to five minutes, preferably about two to three minutes, more preferably about two minutes. 
     In step  406 , workload of the patient is determined using at least one heart rate measurement. Preferably, a heart rate measurement that is used to determine work of the patient during the exercise is heart rate intensity. 
     Specifically, workload of a patient during exercise can be determined by the summation of intensities over time over the full time of exercise (i.e., for the entire period that the activity level exceeds the predetermined threshold), where intensities are calculated from the previous equations discussed to obtain VO 2observed . Alternately, as discussed above, workload may be described as a unitless index by multiplying intensities such as % HRR or % VO 2  reserve and time. Illustratively, after the activity level exceeds an activity threshold, work values can be calculated (Intensity*Duration) for each data-point until the cessation of exercise (i.e., when the activity level no longer exceeds the predetermined threshold)). The determination of work of the patient during the exercise can also be represented by the following formula:
 
ΣIntensity(x)*(Time(x)−Time(x−1))
 
where x=0:n.
 
     Based on the workload value obtained above, other exercise diagnostics, such as heart rate intensity, percent oxygen consumption (% VO 2 ) reserve, metabolic equivalents (METS), percentage METS, and absolute oxygen uptake may be obtained. For example, heart rate intensity may be found by dividing the work by the total time of exercise. 
     The above-described method  400  for determining workload of a patient during exercise may be implemented by hardware, software, or firmware of a pacing system, such as the ICD described earlier with reference to  FIGS. 1A and 1B , with particular reference to work detector  64 . 
     The present invention is also directed to a method and device for determining heart rate recovery of a patient. Heart rate recovery involves analyzing how the heart recovers from a maximum rate during exercise. The heart rate recovery value may not change in a matter of days, but possibly in a matter of weeks. Obtaining the heart rate recovery value only during episodes of peak exercise, as opposed to any low-level exercise, may provide a more accurate reflection of cardiac health through heart rate recovery. 
     A method  500  of determining a measure of heart rate recovery in accordance with the present invention is illustrated in  FIG. 5 . According to an embodiment of the invention, the method  500  begins at step  502 , in which the heart rate and activity level of the patient are monitored. The heart rate and activity level of the patient may be continuously monitored during the method  500 . The heart rate and activity level can be monitored by any suitable method, including those discussed above. 
     Heart rate measurements can be produced based upon the monitored heart rate. As discussed above, such heart rate measurements include but are not limited to heart rate and heart rate intensity. 
     In step  504 , a heart rate measurement is compared with a first heart rate measurement threshold, and an activity level is compared with a first activity threshold. The first heart rate measurement threshold and first activity threshold may be indicative of exercise, preferably vigorous or peak exercise. 
     In step  504 , if a heart rate measurement exceeds a first heart rate measurement threshold, and/or an activity level exceeds a first activity threshold, then the method proceeds to step  506 . In step  506 , the heart rate is identified as a first heart rate. That is, the heart rate taken at the time (1) a heart rate measurement exceeded a first heart rate measurement threshold and/or (2) the activity level exceeded the first activity threshold is used as a first heart rate value for further computations. 
     In one embodiment of the invention, in step  504 , the first heart rate is identified when at least one heart rate measurement exceeds the first heart rate measurement threshold. In another embodiment, the first heart rate is identified when at least one heart rate measurement exceeds the first heart rate measurement threshold for a predetermined period of time. In other embodiments, the first heart rate can be identified when an average value of heart measurements (taken over a predetermined time period, such as, for example, one minute) exceeds the first heart rate measurement threshold. 
     In another embodiment of the invention, the first heart rate can be identified when the activity level exceeds the first activity threshold. In yet another embodiment, the first heart rate can be identified when the activity level exceeds the first activity threshold for a predetermined period of time. In still yet another embodiment, the first heart rate can be identified when, for the predetermined period of time, an average activity level exceeds the first activity threshold. 
     Preferably, the first heart rate is identified when both the activity level exceeds a first activity threshold and a heart rate measurement exceeds a first heart rate measurement threshold. 
     Even more preferably, the first heart rate is identified when the mean activity level value exceeds a first activity threshold for a predetermined period of time, and a mean heart rate measurement value, such as heart rate intensity, exceeds a first heart rate measurement threshold for a predetermined period of time. 
     In accordance with embodiments of the invention, the first heart rate is identified only during peak exercise, only after a stringent set of conditions have been met. These conditions can include certain levels of heart rate intensity, activity level and duration of time. This first heart rate may be referred to as a peak exercise heart rate. 
     Illustratively, a peak exercise heart rate can be identified when the mean activity level exceeds a first activity threshold and the heart rate intensity exceeds a heart rate intensity threshold, such as, e.g., 80%, for a period of time of at least about five minutes. 
     As illustrated by step  508 , heart rate and activity level continue to be monitored. It should be understood that the identified first heart rate can be overwritten by a subsequent heart rate (including a slower heart rate), provided that the first heart rate criteria described above are still met. 
     Heart rate and activity level also continue to be monitored, as illustrated by step  508 , for determining the next parameter used to determine heart rate recovery, a second heart rate. The second heart rate is the heart rate after a slow-down in exercise, and is compared with the first heart rate to determine a measure of heart rate recovery. In accordance with embodiments of the present invention, heart rate measurements (such as, for example heart rate) continued to be produced. 
     In step  510 , a heart rate measurement is compared with a second heart rate measurement threshold, and an activity level is compared with a second activity threshold. The second heart rate measurement threshold and second activity threshold can be indicative of a slowing down or cessation of exercise. 
     If a heart rate measurement falls below a second heart rate measurement threshold, and/or an activity level falls below a second activity threshold, then in step  512  the monitored heart rate is identified as a second heart rate. 
     In one embodiment of the invention, the second heart rate is identified when the activity level falls below the second activity threshold for a predetermined period of time. Preferably, the second heart rate is identified when a mean activity level falls below the second activity threshold for a predetermined period of time. The comparison can also be done based on an average activity level over a predetermined period of time. 
     In another embodiment of the invention, the second heart rate is identified when a heart rate measurement falls below a second heart rate measurement threshold for a predetermined period of time. For example, if a heart rate measurement (e.g. heart rate) falls below a predetermined threshold and/or the mean activity level falls below a predetermined activity threshold, then the heart rate and activity levels can be recorded for a predetermined period of time, such as, for example one, two, or three minutes. 
     After the predetermined period of time, if a heart rate measurement is less than the heart rate measurement prior to the predetermined period of time, and the activity level is less than a third activity threshold (which can be the same as or lower than the second activity threshold), then a second heart rate is identified. Preferably, the slowest heart rate measured during the predetermined period of time is identified as the second heart rate. 
     In step  514 , once a first heart rate and a second heart rate are identified, the first and second heart rates are used to determine a measure of heart rate recovery. For example, the second heart rate is subtracted from the first heart rate to obtain a heart rate difference. The difference is a heart rate recovery value. 
     It should be understood that additional second heart rate values can be identified after the first heart rate and compared to the first heart rate to determine a measure of heart rate recovery. Accordingly, the term “second heart rate” is intended to encompass one or more heart rates that meet the above-described criteria for identification of the second heart rate. In other words, the second heart rate may be several heart rates over consecutive periods of time (e.g. minutes). 
     Illustratively, heart rates measured at discrete times after the identified first heart rate and that meet the second heart rate identification criteria can be compared with the first heart rate to determine a measure of heart rate recovery. For example, the difference between the first heart rate and each of the second heart rates can provide a measure of heart rate recovery. Also, a listing of the first heart rate and heart rates meeting the second heart rate criteria as they decrease over time can also be a measure of heart rate recovery. 
     The invention also encompasses identifying the first heart rate at the time the criteria for identifying the second heart rate is met. For example, if a heart rate measurement exceeds a first heart rate measurement threshold or an activity level exceeds a first activity threshold, and subsequently a heart rate measurement falls below a second heart rate measurement or the activity level falls below a second activity threshold, a first heart rate can be identified at or near the inflection point between meeting the first and second heart rate identification criteria. 
     The second heart rate then can be identified as one or more heart rates measured subsequent to the identified first heart rate. For example, provided that the measured heart rates meet the second heart rate identification criteria, a second heart rate can be identified one minute, two minutes, and/or three minutes following the first heart rate. The difference between the first heart rate and the second heart rate at one, two, and/or three minutes post-first heart rate identification provides values that determine a measure of heart rate recovery. 
     To illustrate, a patient exercises (e.g. runs) for five minutes, and then stops running and sits down for three minutes. Provided that the patient met the first and second heart rate identification criteria described above, the first heart rate would be identified at the five minute mark, and the second heart rates would be identified at the six, seven, and eight minute mark. The first heart rate would be compared with each of the second heart rates at the six, seven, and eight minute mark to determine a measure of heart rate recovery. 
     Preferably, in the method  500  for determining a measure of heart rate recovery, heart rate measurements are filtered to remove noise and premature heart beats such as arrhythmias, PACs, and PVCs. 
     The above-described method  500  for determining the measure of heart rate recovery of a patient may be implemented in software, or firmware of a pacing system, such as the ICD described earlier with reference to  FIGS. 1A and 1B , with particular reference to HR Recovery Detector  66 . 
     EXAMPLES 
     Example 1 
     Determination of Maximum Observed Heart Rate 
     A method for determining a maximum observed heart rate (HR max ) of a patient during exercise is illustrated in  FIG. 6 . 
     To calculate HR max  for exercise conditioning, it is preferred that the patient has maintained a certain level of activity for a certain period of time. Thus, in the illustrative method, the maximum observed heart rate is not calculated unless the activity level is above a threshold activity for a certain period of time. 
     In method  600 , the current cycle length (inverse of heart rate) and activity level are obtained as illustrated in step  602 . It should be understood that the cycle length and activity level can be continuously or periodically monitored. 
     In step  604 , the activity level measured is compared with an activity threshold. If the activity is less than an activity threshold, then the method returns to step  602 . In this manner, steps  602  and  604  result in a continuous (or, optionally, periodic) monitoring of activity level. 
     If the measured activity level is greater than the activity threshold, then in step  606  the elapsed time (i.e., the period during which the activity level is greater than the activity threshold) is compared with a time threshold. The time threshold can be, for example, 2-3 minutes. Once this comparison indicates that sufficient time has elapsed, then step  608  is performed. Thus, before step  608  is performed, there has been a sufficient activity level for a sufficient period of time to indicate actual exercise by the patient. 
     In step  608 , the current cycle length is compared with the previous cycle lengths, preferably a previous average cycle length. In step  610 , if the difference of cycle length is too large (e.g., greater than or equal to about 100 milliseconds), this may indicate noise, PACs, PVCs or arrhythmias, and will not be identified as the HR max . If this occurs, as illustrated in step  610 , the method returns to step  602  to obtain a new, current cycle length and activity. 
     In step  610 , if the difference of cycle length is less than a threshold (indicating that the current cycle length is not due to noise or a premature heart beat), then in step  612 , the current cycle length is compared with the previous cycle length. If the current recorded cycle length is not less than the previously recorded value, then the current cycle length is not identified as the HR max , and step  602  is repeated. However, if the current cycle length is less than the previous recorded value, then in step  614  the current cycle length is identified and stored as the new HR max . 
     Example 2 
     Method of Determining Heart Rate Recovery 
     A method for determining heart rate recovery of a patient is illustrated in  FIG. 7 . 
     In accordance with the illustrated method  700 , the first heart rate, a peak exercise heart rate is obtained by analyzing cycle lengths only when the qualifications for HR max  have been met. 
     In step  702 , the current cycle length is obtained. In step  704 , if the cycle length is near HR max , the value of the heart rate intensity is determined. If this value is greater than a predetermined threshold such as, e.g., 65%, and if the length of time is greater than a predetermined threshold, such as, e.g., 5 minutes, then in step  706  the cycle length is recorded as the first, peak exercise heart rate. Otherwise, the heart rate intensity for each cycle length continues to be determined. 
     The cycle length recorded in step  706  is continuously or periodically recorded, and may be overwritten by slower rates. However, if a noticeable slowdown occurs, in step  708  a new buffer collects the recorded cycle length. In step  710 , if the current cycle length is greater than a predetermined threshold, such as, e.g., 20 milliseconds, or the mean activity level is less than a predetermined threshold, each indicative of a drop in activity, then in step  712  cycle length values are continuously recorded for three minutes. 
     In step  714 , if after three minutes, the cycle length is greater than the cycle length measured three minutes previously, and the activity level is less than a threshold value, then in step  716  the largest cycle length for each of the three minutes is recorded as the second set of heart rate recovery values. If these criteria in step  714  are not met, then in step  718  a second heart rate is not recorded, as the exercise is coming too slowly to a stop. 
     Once the cessation of exercise has been determined, in step  716  both the first, peak exercise heart rate and the three heart rate recovery value cycle lengths are converted to beats per minute and subtracted from each other. The values obtained are the times of heart rate recovery. 
     It will be appreciated by those skilled in the art that the above methods  200 ,  400 ,  500 ,  600 , and  700  can be used within the hardware, software, and/or firmware of a pacing system, such as the ICD described earlier with reference to  FIGS. 1A and 1B , for example. 
     Example embodiments of the methods, systems, and components of the present invention have been described herein. As noted elsewhere, these example embodiments have been described for illustrative purposes only, and are not limiting. Other embodiments are possible within the scope of the invention. Such embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. 
     For example, although the inventive methods are described with reference to an ICD, the methods are not limited to such a use. Illustratively, the inventive methods could be carried out with one or more external devices affixed to a patient&#39;s body to monitor heart rate and activity level, produce heart rate measurements and to determine exercise diagnostics such as, for example HR max , work, and heart rate recovery. Illustratively, the patient may have affixed to their body a holter recording device to measure heart rate and an accelerometer to determine activity level. 
     Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.