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

BackgroundHeart rate (HR) and RR interval are inversely related.ObjectiveThe purpose of this study was to determine which parameter better describes the autonomic changes that occur after exercise and which provides stronger prognostic significance.MethodsHealthy 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-parasympathetic 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).ResultsParasympathetic 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.ConclusionBased 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.