HEART-RATE-VARIABILITY AS A PROGNOSTIC TOOL IN CARDIOLOGY - A CONTRIBUTION TO THE PROBLEM FROM A THEORETICAL POINT-OF-VIEW

Background Recent clinical studies have proposed standard deviation of heart rate as a diagnostic tool for the outcome of cardiac infarction . Mathematical analysis of heart rate variability shows that heart rat e is influenced by different frequency components derived from differe nt parts of the autonomous nervous system. In the experimental part of this study, we investigated the possibility of calculating a variable describing the parasympathetic branch of the autonomous nervous syste m exclusively. Methods and Results In 60 healthy volunteers, heart rat e was measured to 1 millisecond during two different conditions: 5 min utes of rest, and 5 minutes of intermittent handgrip dynamometry; the latter is known to increase sympathetic arousal selectively. Heart rat e was found to be lower at rest (65.9+/-9.7 beats per minute) than dur ing dynamometry (72.8+/-10.4 beats per minute, P<.001). Respiratory si nus arrhythmia (RSA) calculated from the mean absolute differences bet ween successive heart beats showed no significant change (3.01+/-1.62 beats per minute at rest versus 2.97+/-1.30 beats per minute during dy namometry). In contrast, standard deviation increased from 5.19+/-1.98 to 9.22+/-3.56 beats per minute (P<.001). Conclusions It can be concl uded from these data as well as from other plots presented in this art icle that RSA is a measure of the parasympathetic vagal tone, whereas standard deviation is increased by both sympathetic and parasympatheti c arousal. Clinical evidence and data from physiological experiments a re presented to show that a selective measure of vagal tone like RSA m ay offer advantages over standard deviation as a prognostic tool in ca rdiology.