Circulatory responses to work at simulated altitudes under different ambient temperatures.

4,eOO m in a hypobaric chamber under ambient temperatures of 16°C , 20"C, 24°C and 28°C . Oxygen uptake (Vo2), cardiac output (Q ; C02 rebreathing method), arteriovenous 02 difference (AVOD), heart rate (HR), and stroke volume (SV) were rneasured during two successive submaximal exercises after 2 hours of exposure. Vo2 and SV at high altitudes were similar to those at sea level. Q and HR at a given "o2 significantly increased at high altitudes, and AVOD decreased at high altitudes. No effect of ambient temperature on Q and AVOD was found, but HR showed a combined effect of temperature and altitude : at sea level and at an altitude of 2,OOO m, HR tended to increase when temperature increased ; but at an altitude of 4,OOO m, HR at low temperature was equal to the Iever at high temperature. These results indicated the possibility of vasodilation in the peripheral region even at low ambient temperature at high