A model of human thermoregulation during water immersion

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 (Tcore), 10-site mean skin temperature (Tsk), and water loss were measured. At the end of immersion, Tcore ranged from 35.2 to 38.0 °C, and Tsk ranged from 19.7 to 27.4 °C. The SCTM-predicted Tcore, Tsk and water loss were compared to the measured values. Root mean squared deviation (RMSD) was used to test for acceptable predictions. Tcore RMSDs were 0.2, 0.14, and 0.3 °C in 18, 22, and 26 °C water respectively. Tsk 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 Tcore and Tsk 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 Tcore and Tsk prediction in 26 °C water.

Prediction of the thermoregulatory response for clothed immersion in cold water

A multi-compartmental thermoregulatory model was applied to data of ten resting clothed males immersed for 3 h in water at 10 and 15 degrees C. Clothing consisted of a dry suit and either a light or heavy undergarment, representing a total insulation of 0.15 (0.95) or 0.20 m2 degrees CW-1 (1.28 clo), respectively. Data were grouped according to low (less than 14%) and high (14 to 24%) body fat individuals. Mean decreases in rectal temperature ranged from 0.79 to 1.38 degrees C, mean decreases in the mean weighted skin temperature ranged from 6.3 to 10.2 degrees C, and mean increases in the metabolic rate ranged from 33.9 to 80.8 W. The model consists of eight segments, each representing a specific region of the body. Each segment is comprised of compartments representing the core, muscle, fat, skin, and clothing. Each compartment is assigned thermophysical values of heat conduction and heat capacitance, and with the exception of clothing, physiological values of blood flow and metabolic heat production. During cold exposure, responses are directed towards increased heat production in the form of shivering and heat conservation in the form of vasoconstriction and convective heat exchange at the vascular level. Agreement between the model predictions and the experimental observations was obtained by adjusting the parameters governing these responses.(ABSTRACT TRUNCATED AT 250 WORDS)

The simulation of human core temperature

Two fit subjects performed two cycle-ergometer exercise bouts, one of 200 W for 48 min with a following 12-min recovery phase and one of 250 W for 30 min with a subsequent 15-min monitored recovery period. Experiments were performed in ambient conditions of 18 +/- 1 degrees C, 52% relative humidity +/- 5% and 0.5 m.s-1 + 0.5 air velocity (mean +/- range). Experimental values obtained from rectal and tympanic temperature sites were compared with simulation figures produced from a FORTRAN model of human thermoregulation. Data obtained provide qualified experimental support for the concept of a simple feedback model to simulate human core temperature during exercise in ambient temperatures below 20 degrees C.