A measure of parasympathetic influences on the heart as measured by the variability of inter-beat intervals in heart rate (R-to-R wave intervals on an electrocardiogram). This variability, a function of respiratory sinus arrhythmia (RSA), fluctuates at the same frequency as respiratory activity. Characteristically, the inter-beat interval will begin with inhalation, and shorten with exhalation. It has been investigated as a possible alternative measure of stress in a PDD setting. See Miller (1994).