Factors affecting an increase in core body temperature and heat tolerance during hot water immersion

The aim of the present study was to clarify the factors affecting an increase in core body temperature during 40°C water immersion to the subclavian level. Fifteen healthy males were immersed in water for 60 min. Rectal temperature (Trec) and skin temperature (Tsk) at four skin sites were determined. Minute ventilation (VE) was measured, and metabolic rate was determined by indirect calorimetry. Skin blood flow and sweat rate at the forehead were assessed using laser-Doppler flowmetry (%LDFhead) and dew hygrometry (SRhead), respectively. Hot feeling was assessed with a visual analog scale. When Trec reached 39°C or participants reported an extremely hot feeling, the experiment was ceased. Eleven participants were unable to complete the protocol (ten participants due to Trec > 39°C; and one due to excessive hot feeling). Trec increased with immersion period. Mean Tsk was unchanged from 20 min. VE and metabolic rate increased with immersion period. %LDFhead and SRhead increased after immersion and remained unchanged from 15 and 30 min, respectively. Change in Trec from the baseline at 15, 30, and 45 min was correlated to cumulative change in metabolic rate from the baseline at 0-15, 0-30, and 0-45 min. No correlations were observed between change in Trec and cumulative changes in VE, %LDFhead, and SRhead from baseline, hot feeling, body weight and body composition. Water immersion at 40°C induced a large difference in the increase of Trec, in which metabolic responses to heat stress may be involved. The relationship between heat tolerance and change in Trec is different among individuals