Comparison of the physiologic and prognostic implications of the heart rate versus the RR interval

BACKGROUND Heart rate (HR) and RR interval are inversely related. OBJECTIVE The purpose of this study was to determine which parameter better describes the autonomic changes that occur after exercise and which provides stronger prognostic significance. METHODS Healthy volunteers (n = 33) underwent sequential bicycle exercise tests with selective autonomic blockade during exercise to define HR and RR interval changes in recovery due to parasympathetic effect, sympathetic effect, and sympathetic-para-sympathetic interaction. The prognostic significance of resting HR and RR interval was assessed in a cohort study (n = 33,781). The prognostic significance of exercise HR and RR interval and 1-minute HR and RR interval recovery was assessed in patients referred for exercise testing (n = 2387). RESULTS Parasympathetic effect on HR and RR interval both increased in recovery (P <.001), while the sympathetic effect on HR declined (P <.001) and the sympathetic effect on the RR interval paradoxically increased. Significant sympathetic-parasympathetic interaction was noted with the HR analysis but not with the RR interval. Resting HR and RR interval had similar prognostic implications by age and gender. While resting and exercise HR and RR interval had similar prognostic implications, 1-minute HR recovery was a multivariate predictor of mortality (HR 0.81; 95% CI 0.69-0.95), while 1-minute RR interval recovery was not. CONCLUSION Based on these findings, HR (and its changes) is not necessarily interchangeable with the RR interval (and its changes) in either physiologic or prognostic studies. It is important to consider underlying physiologic constraints and identify wisely which parameter (or even other transformation of these parameters) is most suitable for a given analysis.