Abstract

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 hightolerance 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. Dehydrationwas associated with a decrease in the loss of plasma volume at 7 G. Heatinduced tolerance loss appears similar for both gradual- and rapid-onset centrifuge profiles. In contrast, dehydration effects are greater in rapidonset 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. Verf.-Referat