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.