Thermoregulation during a six-hour exposure to warm, humid hyperbaric conditions

Purpose: In a disabled submarine scenario, a pressurized rescue module (PRM) may be deployed to rescue survivors. If the PRM were to become disabled, conditions could become hot and humid exposing the occupants to heat stress. We tested the hypothesis that the rise in core temperature and fluid loss from sweating would increase with rising dry bulb temperature.

Methods: Twelve males (age 22 ± 3 years; height 179 ± 7 cm; mass 77.4 ± 8.3 kg) completed this study. On three occasions, subjects were exposed to high humidity and either 28-, 32-, or 35˚C for six hours in a dry hyperbaric chamber pressurized to 6.1 msw. Changes in core temperature (Tc) and body mass were recorded and linear regression lines fit to estimate the predicted rise in Tc and loss of fluid from sweating.

Results: Heart rate was higher in the 35°C condition compared to the 28°C and 32°C conditions. Tc was higher in the 32°C condition compared to 28°C and higher in 35°C compared to the 28˚°C and 32°C conditions. Projected fluid loss in all of the tested conditions could exceed 6% of body mass after 24 hours of exposure endangering the health of sailors in a DISSUB or disabled PRM. A fluid intake of 1.0 to 3.5 L would be required to limit dehydration to 2% or 4% of initial mass depending upon condition.

Conclusions: Prolonged exposure to 35°C conditions under pressure results in uncompensable heat stress. 32°C and 35°C exposures were compensable under these conditions but further research is required to elucidate the effect of increased ambient pressure on thermoregulation.