EUCAPNIC HYPOXIA LOWERS HUMAN COLD THERMOREGULATORY RESPONSE THRESHOLDS AND ACCELERATES CORE COOLING

Hypoxia alters the basic thermoregulatory responses of animals and hum ans. In cold-exposed animals, hypoxia increases core temperature (T-co ) cooling rate and suppresses shivering thermogenesis. In humans, the experimental effects of hypoxia on thermoregulation are equivocal. Als o, the effect of hypoxia has not been separated from that of hypocapni a consequent to hypoxic hyperventilation. To determine the islolated e ffects of hypoxia on warm and cold thermoregulatory responses and core cooling during mild cold stress, we examined the T-co thresholds for sweating, vasoconstriction, and shivering as well as the core cooling rates of eight subjects immersed in 28 degrees C water under eucapnic conditions. On 2 separate days, subjects exercised on an underwater cy cle ergometer to elevate T-co above the sweating threshold. They then rested and cooled until they shivered vigorously. Subjects inspired hu midified room air during the control trial. For the eucapnic hypoxia t rial, they inspired 12% O-2-balance N-2 with CO2 added to maintain euc apnia. Eucapnic hypoxia lowered the T-co thresholds for vasoconstricti on and shivering by 0.14 and 0.19 degrees C, respectively, and increas ed core cooling rate by 33% (1.83 vs. 1.38 degrees C/h). These results demonstrate that eucapnic hypoxia enhances the core cooling rate in h umans during mild cold stress. This may be attributed in part to a del ay in the onset of vasoconstriction and shivering as well as increased respiratory heat loss during hypoxic hyperventilation.