HEART RATE VARIABILITY AS AN INDICATOR
OF EXERCISE CAPACITY
CROSS REFERENCE TO RELATED
 This application is a continuation-in-part of application Ser. No. 09/093,118, filed Jun. 8, 1998.
BACKGROUND OF THE INVENTION  I. Field of the Invention
 This invention relates generally to a method and apparatus for assessing patient well-being, and more particularly to a method and apparatus for indirectly determining a patient's peak oxygen uptake (V02MAX) by measuring his/her heart rate variability (HRV).
 II. Discussion of the Prior Art
 It is known in the art that HRV, i.e., the beat-to-beat variance in sinus cycle length over a period of time, is a predictor of mortability and morbidity. Patients exhibiting low HRV show a significantly increased risk of sudden cardiac death. See "Heart Rate Variability" by Zsolt Ori, et al., Cardiology Clinics, Vol. 10, No. 3, August 1992, pp. 499-537 and "Depressed Heart Rate Variability As An Independent Predictor of Death in Chronic Congestive Heart Failure Secondary to Ischemic or Idiopathic Dilated Cardiomyopathy" by Piotr Ponikowski, et al., The American Journal of Cardiology, Vol. 79, Jun. 15, 1997, pp. 16451650. See also "Correlations Among Time and Frequency Domain Measures of Heart Period Variability Two Weeks After Acute Myocardial Infarction" by J. Thomas Bigger et al., The American Journal of Cardiology, Vol. 69, Apr. 1, 1992. In addition, the Spinelli et al. U.S. Pat. No. 5,466,245 provides a very detailed method for automatically determining AV delay based on evaluating a frequency domain measure of heart rate variability (HRV).
 In the Heemels et al. U.S. Pat. No. 5,603,331, assigned to applicant's assignee, a method and apparatus is described for efficiently processing, logging and disseminating essential features relating to HRV accumulated from a continuous, long-term monitoring of cardiac activity. As pointed out in that patent, the method sufficiently conserves data memory, program memory and power consumption that it may be incorporated within an implantable pacemaker or defibrillator to log a 24-hour period of cardiac activity accumulated for subsequent telemetry to an external monitor. The patent further describes a method of processing and displaying HRV data in a manner that is readily understandable by clinicians which provides an improved graphical contrast between normal and abnormal HRV patterns.
 Further studies which we have recently conducted on several patients has revealed a high correlation between HRV and a patient's exercise capacity as measured by V02MAX. Based upon this realization, a method has been established for indirectly assessing a patient's maximum oxygen uptake by computing the standard deviation of five minute mean RR intervals, i.e., the SDANN Index. In an alternative approach, a two-dimensional histogram array is plotted with RR intervals along one axis and the absolute value of the time difference between successive RR intervals plotted along the second axis. By measuring the area on the plot occupied by the two-dimensional histogram, V02MAX can be estimated.
SUMMARY OF THE INVENTION
 In accordance with a first aspect of the invention, an estimation of a patient's maximum oxygen uptake can be derived by the following method:
 First, a patient's ECG waveform is sensed and recorded over a 24-hour period. In a pacemaker implementation, the whole 24-hour recording is not possible. Instead, the R-R interval is calculated in real time (on the fly) . The recording is then analyzed and a determination is made as to the average length of normal RR intervals in 288 five-minute intervals. When the standard deviation of the 288 averages is computed, it yields an index exhibiting a high correlation with the patient's peak oxygen consumption.
 In an alternative method, rather than computing the SDANN Index, the length of RR intervals in the recorded ECG waveform during a plurality of time segments of a predetermined length are measured and the absolute value of the time difference between successive RR intervals is determined. By plotting this data as a two-dimensional histogram and then measuring the area on the plot occupied by the two-dimensional histogram, the patient's peak oxygen uptake can be inferred. That is to say, studies have shown that there is a high correlation between the area or "footprint" of the histogram and the patient's peak oxygen uptake.
 Irrespective of which of the above methods is employed, the information arrived at can be utilized in assessing the efficacy of a given mode of drug therapy or electrical cardiac stimulation on a patient's exercise capacity. By plotting the HRV Index computed over predetermined time intervals in accordance with the first method or the footprint area determined in accordance with the alternative method, trends in a patient's peak oxygen uptake resulting from a predetermined therapy regimen can be determined and used in adjusting the regimen. This is achieved without the need for conducting a breath-by-breath analysis of ventilatory flow and subjecting the patient to a treadmill test or the like.
DESCRIPTION OF THE DRAWINGS  FIG. 1 is a graphical representation of a patient data collection schedule for ECG waveforms and V02max measurements;
 FIG. 2 is a plot of V02max and the computed SDANN index plotted on the same time axis and showing the correlation of each as the pacing protocol is changed;
 FIG. 3 is a pseudo three-dimensional histogram plot of RR interval vs. the absolute value of the difference in RR interval between successive heart beats;
 FIG. 4 is a two-dimensional histogram plot taken prior to pacer implant where frequency of occurrence of predetermined HRV values are represented by a gray scale;
 FIG. 5 is a plot like that of FIG. 4 but representing HRV data taken following four weeks of pacing therapy;
 FIG. 6 is a plot showing the correlation between histogram of footprint size and V02max for the data collection schedule of FIG. 1;
 FIG. 7 is a flow chart of the algorithm for determining an optimum pacing mode based upon the relationship of HRV to peak oxygen uptake;