HUMAN TEMPERATURE REGULATION DURING EUCAPNIC HYPOXIA

At altitude decreased inspired P02 results in arterial hypoxemia. It is well known that hypoxemia alters the basic thermoregulatory responses to cold challenge. In animals, hypoxemia delays the onset of shivering (I), decreases the maximal shivering response (2-4) and inhibits nonshivering thermogenesis (3). Hypoxemia is also associated with increased core temperature (Teo) cooling in conscious cats (2). In humans, hypoxemia elicits peripheral vasodilation (with consequent increases in skin temperature and surface heat loss) and inhibits heat production (5, 6). The effects of hypoxemia on total heat balance are not so clear. Core temperature has been demonstrated to either remain constant (6) or decrease (5, 7) during hypoxic cold exposure. There are several limitations to previous work on the effects of hypoxia on human thermoregulation. First, the hypoxic stimulus has not been isolated from that of arterial hypocapnia subsequent to hypoxic hyperventilation. Second, little is known of the thermoregulatory effects of hypoxia during heat challenge. Therefore this study was performed to determine the effects of eucapnic hypoxia on Tco thresholds for warm and cold thermoregulatory responses as well as the rate of core cooling during mild cold stress. METHOD In eight subjects, we used the protocol of Mekjavic et al. (8) to determine the Tco thresholds for sweating