Predicting the Magnitudes of Hyperthermia and Hypohydration during Prolonged Exposures to Warm and Very Humid Environments
Authors: Chapman Christopher L , Stooks Jocelyn J , Johnson Blair D , Hostler David , Schlader Zachary J 
Published: June 2021

In the event of a disabled submarine a pressurized rescue module (PRM) may be deployed. Safe deployment of the PRM depends on understanding the challenges should failures occur. If a PRM were to become disabled, temperature and humidity are predicted to rise quickly, and submariners may need to endure environments approaching 35°C, 95% relative humidity (RH) until rescued. Models predicting increases in core temperature (i.e., hyperthermia) and the magnitude of hypohydration incurred over 24 h in 95% RH environments yield physiologically impossible outcomes at ambient temperatures ≥32°C. Thus, it is not possible to accurately predict the magnitudes of hyperthermia and hypohydration during prolonged exposures to warm and very humid environments. By extension, the fluid prescription required to prevent unsafe levels of hypohydration, defined as a loss of body weight >4%, in a disabled PRM is also unknown.PurposeTest the hypothesis that the predicted magnitudes of hyperthermia and hypohydration during a 24 h exposure to a 95% RH environment are dependent on ambient temperature.Methods10 healthy males (23 ± 3 y) sat in a 95 ± 2% RH normobaric environment for 8 h on three occasions separated by ≥7 days. Trials differed by temperature (32 ± 0°C, 33 ± 0°C, 35 ± 0°C). The order of the trials was randomly assigned. Subjects were not allowed to eat or drink throughout. Core temperature (telemetry pill) and percent changes in body weight (%ΔBW), an index of changes in total body water, were measured every hour. Sweat rate was calculated from %ΔBW. Linear regression models were fit to core temperature (over the final 3 h of exposure), and %ΔBW and sweat rate (over the entire 8 h exposure) over time for each subject. The resulting equations were used to predict the magnitudes of hyperthermia, %ΔBW, and sweat losses for up to 24 h. The volume of fluid required to prevent >4% loss of body weight was calculated accordingly. Data are presented as mean ± SD.ResultsAt the end of the 8 h exposure, core temperature was higher in 35°C (38.0 ± 0.3°C, P<0.01) compared to 32°C (37.6 ± 0.2°C) and 33°C (37.5 ± 0.2°C). At this time, %ΔBW was greater in 35°C (−2.0 ± 0.5%, P<0.01) compared to 32°C (−1.5 ± 0.5%) and 33°C (−1.7 ± 0.4%). At 24 h, predicted core temperature was higher in 35°C (39.0 ± 1.1°C, P<0.01) compared to 32°C (37.7 ± 1.0°C) and 33°C (37.8 ± 0.9°C), and the predicted %ΔBW was greater in 35°C (−6.1 ± 1.2%, P<0.01) compared to 32°C (−4.5 ± 1.2%) and 33°C (−5.3 ± 1.3%). Predicted sweat loss at 24 h was greater in 35°C (−5.2 ± 1.4 L, P<0.01) compared to 32°C (−3.8 ± 1.6 L) and 33°C (−4.4 ± 1.1 L). The predicted fluid prescription required to prevent >4% loss of body weight was greater in 35°C (1.8 ± 0.9 L) compared to 32°C (0.7 ± 0.8 L, P<0.02), and 33°C (1.0 ± 0.8 L, P=0.06).ConclusionIn a 95% RH environment, the magnitudes of hyperthermia and hypohydration predicted to occur during a 24 h exposure are dependent on ambient temperature. Moreover, the volume of fluid required to prevent >4% loss in body weight during a 24 h exposure in ambient temperatures ≤35°C is ~1.8 L.Support or Funding InformationNaval Sea Systems Command Award N00024‐18‐C‐4316.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.