SYMPATHETIC AND PARASYMPATHETIC INDICATORS OF HEART-RATE CONTROL AT ALTITUDE STUDIED BY SPECTRAL-ANALYSIS

The adaptive responses of the cardiovascular system to altitude appear to be dominated by increased sympathetic neural activity. We investig ated the combined roles of the sympathetic and parasympathetic nervous systems (SNS and PNS, respectively) in the early (days 4-5) and subse quent (clays 11-12) phases of acclimatization on Pike's Peak, CO (4,30 0 m), by spectral analysis of heart rate variability. Male subjects we re randomly assigned to groups receiving oral propranolol (240 mg/day; n = 6) or a matched placebo (n = 3). On ascent to altitude, the high- frequency, fractal, and total spectral powers were reduced in the plac ebo group during days 4-5 and 11-12. At altitude during days 4-5, all three placebo group subjects increased SNS and decreased PNS activitie s compared with at sea level, and during days 11-12 SNS decreased and PNS increased compared with days 4-5. Relative to the placebo group, p ropranolol caused lengthening of the R-R interval; increases in high-f requency power, total spectral power, and the PNS indicator; and a dec rease in the SNS indicator. Total spectral power tended to decrease at altitude, but there were no effects of altitude on PNS and SNS indica tors in the propranolol group. The data from the placebo and propranol ol groups suggest that both the PNS and SNS are involved in the elevat ed heart rate during the early phase of altitude acclimatization. Chan ges in heart rate variability during clays 11-12 at altitude must be c onsidered in light of the possible reductions in sympathetic receptor number noted in previous studies.