Cutaneous vascular responses to heat simulated at a high altitude of 5,600 m

Six healthy young men were studied in a high-altitude chamber during a 60-min heat exposure at a simulated altitude of 5,600 m or 0.5 atmosphere absolute (ATA). The heat load was provided by increasing the chamber temperature to 38 degrees C at the rate of 1 degree C/min after a 60-min equilibrium period at thermoneutrality (28 degrees C). Our question was whether or not hypoxia causes differential changes in regional cutaneous circulation during heat exposure. Skin blood flow in the forearm (FBF) and the finger (FiBF), temperatures of the esophagus (Tes) and of the skin, and cardiac output (CO) were measured during the heat exposure at 0.5 ATA and at the sea level (1 ATA). During the equilibrium period, hypoxia increased the mean skin temperature and mean heat transfer coefficient, as well as FBF and forearm vascular conductance. The increased blood flow in the cutaneous circulation during the hypoxic exposure may reflect cutaneous vasodilation and vasoconstriction in other regions of the body, since there was no alteration in CO and total peripheral resistance. During heat exposure, Tes rose faster at high altitude than at sea level. However, at the end of the 60-min heat exposure, all thermal as well as circulatory parameters showed no difference between the two altitudes, except for the FiBF. An attenuated vasodilation in the fingers during heat exposure at high altitude suggests differential vascular controls and possible impairment of thermoregulation when additional stress, such as heat, is imposed. The data suggest that cutaneous blood flow during heat exposure is not uniform throughout the entire skin in a hypoxic environment.