Eucapnic hypoxia lowers human cold thermoregulatory response
thresholds and accelerates core cooling.
Johnston, Chad E., Matthew D. White, Mingpu Wu, Gerald K. Bristow, and
Gordon G. Giesbrecht.
Laboratory for Exercise and Environmental Medicine, the Health
Leisure and Human Performance Research Institute, University of
Manitoba, Winnipeg, Canada, R3T 2N2
APStracts 2:0408A, 1995.
Hypoxia alters the basic thermoregulatory responses of animals and
humans. In cold-exposed animals, hypoxia increases core temperature
(Tco) cooling rate and suppresses shivering thermogenesis. In humans
the experimental effects of hypoxia on thermoregulation are
equivocal. As well, the effect of hypoxia has not been separated from
that of hypocapnia consequent to hypoxic hyperventilation. To
determine the islolated effects of hypoxia on warm and cold
thermoregulatory responses and core cooling during mild cold stress,
we examined the Tco thresholds for sweating, vasoconstriction and
shivering as well as core cooling rates of eight subjects immersed in
28 C water under eucapnic conditions. On two separate days, subjects
exercised on an underwater cycle ergometer to elevate Tco above the
sweating threshold. They then rested and cooled until they shivered
vigorously. Subjects inspired humidified room air during the control
trial. For the eucapnic hypoxia trial, they inspired 12% O2/balance
N2, with CO2 added to maintain eucapnia. Eucapnic hypoxia lowered the
Tco thresholds for vasoconstriction and shivering by 0.14 C and 0.19
C respectively, and increased core cooling rate by 33% (1.83 vs 1.38
C.hr-1). These results demonstrate that eucapnic hypoxia enhances the
core cooling rate in humans during mild cold stress. This may be
attributed in part to a delay in the onset of vasoconstriction and
shivering as well as increased respiratory heat loss during hypoxic
hyperventilation.
Received 4 November 1994; accepted in final form 13 September
1995.
APS Manuscript Number A1141-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 31 October 95