Body Temperature,Autonomic Responses,and Acute Mountain Sickness

Abstract

A few studies have reported increased body temperature (T_o) associated with acute mountain sickness (AMS), but these usually include exercise, varying environmental conditions over days, and pulmonary edema. We wished to determine whether T_o would increase with AMS during early exposure to simulated altitude at rest. Ninety-four exposures of 51 men and women to reduced P_B (423 mmHg = 16,000 ft = 4850 m) were carried out for 8 to 12 h. AMS was evaluated by LL and AMS-C scores near end of exposure, and T_o was measured by oral digital thermometer before altitude and after 1 (A1), 6 (A6), and last (A12) h at simulated altitude. Other measurements included ventilation, O₂ consumption and autonomic indicators of plasma catecholamines, HR, and HR variability. Average T_o increased by 0.5°F from A1 to A12 in all subjects (p < 0.001). Comparison between 16 subjects with lowest AMS scores (mean LL = 1.0, range = 0 to 2.5) and 16 other subjects with highest AMS scores (mean LL = 7.4, range = 5 to 11) demonstrated a transient decline in T_o from A1 to A6 in AMS, in contrast to a rise in non-AMS (p = 0.001). Catecholamines, HR, and HR variability (increased low F/high F ratio) indicated significant elevation of sympathetic activity in AMS, where T_o fell, but no change in metabolic rate. The apparently greater heat loss during early AMS suggests increased hypoxic vasodilation in spite of enhanced sympathetic drive. Greater hypoxic vasodilation and elevated HR in AMS in the absence of other changes suggest that augmentation of β-adrenergic tone may be involved in early AMS pathophysiology.