HEART-RATE AND HEART-RATE-VARIABILITY IN NORMAL YOUNG-ADULTS

Introduction: The relationships between heart rate (HR) and HR variabi lity (HRV) are not simple. Because both depend on the autonomic nervou s system (ANS), they are not independent variables. Technically, the q uantification of HRV is influenced by the duration of the cardiac cycl es. The complexity of these relationships does not justify ignoring HR when studying HRV, as frequently occurs. Methods and Results: Using s pectral and nonspectral methods, the HR and various normalized and non -normalized indices of HRV were studied in 24-hour recordings of a hom ogeneous cohort of seventeen 20-year-old healthy males. The HR-HRV rel ationships were appraised by analyzing the same data in two different ways. The 24 mean hourly values provide consistent information on the circadian behavior of the indices, while tbe average 24-hour individua l data show a wide spectrum of normality. Combined approaches allow as sessment of the direct impact of RR interval on HRV evaluation. The co rrelations between HR and normalized indices of HRV are weaker in 24-h our individual data than in pooled hourly data of the same individuals . These correlations are close to 1 in the latter case, which does not mean that measuring HRV is simply another method of evaluating HR, bu t that normal physiology supposes a harmonious behavior of the various indices. When considered individually without normalization, the spec ific indices of vagal modulation (high-frequency band of the spectrum, shortterm HR oscillations of the nonspectral analysis) consistently i ncrease at night and diminish during the day. However, the low-frequen cy power, which supposedly reflects sympathetic influences, also incre ases at night, whereas more logically the longer HR oscillations would predominate during the day. Moreover, the selective analysis of HR os cillations during HR acceleration or decrease indicates that their beh avior differs accordingly. Conclusion: We recommend that closer attent ion be paid to the complex relationships between HR and HRV. The stron g correlations found in healthy subjects may reflect either the physio logical harmony of ANS functions or simple redundancy. Their tendency to deteriorate in diseased hearts suggests that redundancy is not the cause and that abnormalities of ANS functions are not demonstrated by HRV analysis alone.