The human heat tolerance time (TT) is generally determined by his/her thermoregulatory capacity. Altered thermoregulation cases have been reported following simulated microgravity (bed rest). It is necessary to determine a TT for the rescue operations in case of a thermal emergency occurred in space flight. But the TT has not been investigated experimentally after the exposure to a bed rest.
Six healthy non-heat-acclimated males (18–20 years old) underwent a 7-days duration at 6° head-down tilt (HDT), at 22°C, and 50% relative humidity. The subjects, dressed in shorts, have been exposed to simulated microgravity at 60° C and a water vapor pressure of 21.5 mmHg on two separated occasions: one for the ground control of heat tolerance prior to a 7-day period of bed rest and the other in determining TT at the end of the 7-day HDT. The TT was defined as the time of occurrence of a rectal temperature at 38.6° C and/or a heart rate at 160 beat/min or pre-syncope symptoms.
The TTs have been significantly reduced from 69.4 ± 3.3 min on ground control to 56.1 ± 2.3 min after HDT by a reduction of 19% (P=0.0006). Comparing with pre-DHT heat trials, the total body water loss was significantly declined after HDT (2.3 ± 0.3 vs. 1.7 ± 0.2% of total body weight, P<0.05), while the sweating rate was unaffected. The mean skin and rectal temperatures, and the body heat storage in post-HDT heat exposure were significantly greater than their pre-HDT controls. The symptoms at termination have included one case of hyperventilation in pre-HDT heat trial and 3 cases of gray sight, headache and nausea, and hyperventilation during post-HDT heat exposure.
The HDT has induced hypohydration and cardiovascular deconditioning, impaired human thermoregulation, and reduced heat tolerance in this hot and dry environment.