Validation of a human thermoregulatory model during prolonged immersion in warm water

This study validates the Six Cylinder Thermoregulatory Model (SCTM) during prolonged warm water immersion, which underpins the Probability of Survival Decision Aid (PSDA) currently in use by the United States Coast Guard (USCG). PSDA predicts survival time for hypothermia and dehydration. USCG has been using PSDA for search and rescue operation since 2010. In 2019, USCG organized a workshop to review PSDA performance and concluded that PSDA is an essential tool for operation, although it occasionally overestimates survival times in warm waters above 16 °C. Forty-six human subjects were immersed from the neck down in 18, 22, and 26 °C water for 45 min up to 10 h. Rectal temperature (T_core), 10-site mean skin temperature (T_sk), and water loss were measured. At the end of immersion, T_core ranged from 35.2 to 38.0 °C, and T_sk ranged from 19.7 to 27.4 °C. The SCTM-predicted T_core, T_sk and water loss were compared to the measured values. Root mean squared deviation (RMSD) was used to test for acceptable predictions. T_core RMSDs were 0.2, 0.14, and 0.3 °C in 18, 22, and 26 °C water respectively. T_sk RMSDs were 1.44, 0.76, and 1.1 °C in 18, 22, and 26 °C water respectively. SCTM underpredicted water loss by 84%. Overall, SCTM predicted T_core and T_sk with acceptable accuracy in 18 and 22 °C water for up to 10 h, but overpredicted in 26 °C water. Future studies and algorithm development are required to improve water loss prediction as well as T_core and T_sk prediction in 26 °C water.