title: Heat and acute dehydration effects on acceleration response in man.
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 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.
mesh_terms: D000054:Acceleration / Q000009:adverse effects*; D000208:Acute Disease; D000328:Adult; D000337:Aerospace Medicine; D003681:Dehydration / Q000150:complications* / Q000209:etiology; D006339:Heart Rate; D006358:Hot Temperature / Q000009:adverse effects*; D006801:Humans; D008297:Male; D010953:Plasma Volume