Journal of Applied Physiology, Vol 47, Issue 1 197-200, Copyright © 1979 by American Physiological Society
Heat and acute dehydration effects on acceleration response in man
S. A. Nunneley and R. F. Stribley
Though heat and dehydration each impair acceleration tolerance,
interactions among these stresses have not previously been studied. Seven
men were dehydrated in heat by 0, 1, and 3% of body weight before a series
of +Gz, gradual-onset centrifuge runs with the capsule first 38 degrees C,
then 20 degrees C. Heat alone raised heart rate by 6.5 beats/min
independent of other stresses. Dehydration and acceleration appeared to act
synergistically in raising HR. Heat lowered relaxed G tolerance by 0.3 G;
dehydration tended to lower G tolerance and increased the variability of
response to heat. A high-tolerance subgroup (n = 4) could normally sustain
+7 Gz for 60 s with anti-G suit and straining, but 3% dehydration reduced
mean time to 35 s. Dehydration was associated with a decrease in the loss
of plasma volume at 7 G. Heat-induced tolerance loss appears similar for
both gradual- and rapid-onset centrifuge profiles. In contrast, dehydration
effects are greater in rapid-onset runs, evidence that normal anti-G
protective mechanisms can partly counteract the effect of fluid deficit.
The results are relevant for crew members of high-performance aircraft,
where unexpected diminution of their normally high G tolerance can have
disastrous consequences.
