Orthostatic responses in heat tolerant and intolerant subjects compared by three different methods.

: The orthostatic responses of 10 heat tolerant and five intolerant subjects were evaluated by three different test methods--standing, head-up tilt (HUT), and head-up tilt after prolonged exercise (HUTPE)--in a hot environment of 40 degrees C, 40% RH. No difference was found within the normal group between responses in the standing and HUT test. In the HUTPE test, orthostatic responses were inadequate and 5 of 10 subjects fainted. The heat intolerant subjects reacted to all three tests similarly to the normal group, though their heart rate was higher and 3 out of 5 fainted at an earlier phase in the HUTPE test.