Influences of hypobaric hypoxia on skin blood flow and sweating responses during exercise in neutral and hot environments

Exposure to hot environments augments cutaneous vasodilation and sweating during exercise compared with these responses in cooler environments. The effects of hypobaric hypoxia on these responses are less clear, as are the effects of heat and simulated altitude combined. We evaluated the individual and potential additive effects of environmental heat and hypobaric hypoxia on skin blood flow and sweating responses during exercise. Thirteen volunteers (11 M, 2 F; age 25.3 +/- 6.1 yr; height 177 +/- 9 cm; weight 81.2 +/- 16.8 kg) completed 30 min of steady-state (SS) exercise on a cycle ergometer at 50% (V) over dotO(2peak) during four separate conditions: 1) sea level thermoneutral (SLTN; 250 m, 20 degrees C, 30-50% RH), 2) sea level hot (SLH; 250 m, 35 degrees C, 30% RH), 3) simulated altitude thermoneutral (ATN; 3,000 m, 20 degrees C, 30-50% RH), and 4) simulated altitude hot (AH; 3,000 m, 35 degrees C, 30% RH). Skin blood flow and local sweating rate (LSR) were recorded on the ventral forearm. During exercise, SS cutaneous vascular conductance in AH (63 +/- 31% peak) and SLH (52 +/- 19% peak) were significantly higher than both SLTN (20 +/- 9% peak, P < 0.001) and ATN (25 +/- 12% peak, P < 0.05) but were not different from each other (P > 0.05). SS LSR was similarly increased in the hot environments but unaffected by simulated altitude. We propose that multiple antagonistic mechanisms during exposure to 3,000-m simulated altitude result in no net effect on skin blood flow or sweating responses during exercise in thermoneutral or hot environments.