SPECTRAL-ANALYSIS OF HEART-RATE-VARIABILITY DURING HEAT EXPOSURE AND REPEATED EXERCISE

This study examined indices of parasympathetic (PNS) and sympathetic ( SNS) nerve activity during exposure to heat and/or two successive bout s of exercise. Seven healthy males [age = 27.1 (3.6) years; mean (SD), maximum oxygen consumption (V0(2max))=48.1 (7.6) ml . kg(-1). min(-1) ] were assigned to each of four experimental conditions according to a randomized-block design. While in a thermoneutral (23 degrees C) or h eated (40 degrees C, 30% relative humidity) climatic chamber subjects performed exercise on a cycle ergometer (two 30-min bouts at approxima te to 50% VO2max, separated by a 45-min recovery period, (CEx and HEx, respectively) or remained seated (CS and HS, respectively) for 2 h. T he RR intervals of the subjects' ECGs were analyzed for selected near- steady-state time periods [termed Phase I (25-40 min) and Phase II (10 0-115 min)] according to the method of Yamamoto and Hughson (J Appl Ph ysiol 71:1143-1150, 1991). Total (P-T), low-frequency (P-LF = ?0-0.15 Hz) and high-frequency (P-HF = 0.15-0.5 Hz) power spectra were calcula ted using coarse-graining spectral analysis. Heat exposure alone did n ot alter autonomic balance or levels of circulating catecholamines sig nificantly. Exercise in both environmental conditions induced a signif icant decrease in an index of PNS tone (P-HF:P-T) [PNS indicator for C S = 0.084 (0.04) vs CEx = 0.023 (0.015) and HS = 0.065 (0.027) vs HEx = 0.015 (0.009)], with an increase in catecholamine concentrations. Al though the index of SNS activity (P-LF:P-HF) tended to rise with exerc ise in both environmental conditions, increments reached levels of sig nificance only during exercise in the heat [SNS indicator for CS = 8.2 2 (5.58) vs CEx = 34.06(21.73) and HS = 8.94 (5.49) vs HEx = 54.29 (49 .80)]. The relative magnitudes of SNS and PNS indicators did not diffe r significantly between the first and second bouts of exercise. These results indicate the substantial contribution of vagal withdrawal and catecholamine secretion to the increase in heart rate that occurs duri ng repeated moderate exercise at room temperature and the additional c ontribution from SNS activity during such exercise in the heat.