BackgroundRecent 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 rate is influenced by different frequency components derived from different 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 system exclusively.Methods and ResultsIn 60 healthy volunteers, heart rate was measured to 1 millisecond during two different conditions: 5 minutes of rest, and 5 minutes of intermittent handgrip dynamometry; the latter is known to increase sympathetic arousal selectively. Heart rate was found to be lower at rest (65.9±9.7 beats per minute) than during dynamometry (72.8±10.4 beats per minute,P<.001). Respiratory sinus arrhythmia (RSA) calculated from the mean absolute differences between 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 dynamometry). In contrast, standard deviation increased from 5.19±1.98 to 9.22±3.56 beats per minute (P<.001).ConclusionsIt can be concluded from these data as well as from other plots presented in this article that RSA is a measure of the parasympathetic vagal tone, whereas standard deviation is increased by both sympathetic and parasympathetic arousal. Clinical evidence and data from physiological experiments are presented to show that a selective measure of vagal tone like RSA may offer advantages over standard deviation as a prognostic tool in cardiology.