Influence Of Passive Heating On Physiological Responses To Different Body Hydration Levels In Healthy Men

Abstract

At times humans are required to withstand high environmental heat stress in a passive manner. Depending on access to fluids, the physiological responses to this passive heat stress (PHS) may differ. PURPOSE To determine the effects of PHS on physiological responses of healthy men at three levels of body hydration. A secondary purpose was to compare ingestible temperature sensor (ITS) and rectal thermistor methods for measurement of core body temperature during PHS. METHODS Ten healthy men participated (mean ± SD, age: 28 ± 3 yrs; wgt: 80.7 ± 12.4 kg; % body fat: 17.2 ± 4.8 %; body surface area: 1.96 ± 0.13 m2). Each subject underwent 80 min of PHS (35 °C, 70 % RH, WBGT=30) with lower legs submersed in re-circulated water (∼43 °C) on 3 occasions. A dehydration trial (DT; no fluid intake) was always performed first followed by trials of forced (FT; 120% of dehydration in DT) and voluntary drinking (VT; ad libitum) of tap water. FT and VT were counterbalanced. During each session, heart rate (HR) and skin temperatures were recorded. Core body temperature was determined with a rectal thermistor and ITS. RESULTS Body hydration in each session was significantly (p<0.05) different than each other session and averaged −1.5 ± 0.5 % (DT), 0.7 ± 0.6 % (FT), and −0.6 ± 0.7 % (VT). HR increased by 40 ± 26 beats/min (DT), 21 ± 11 beats/min (FT), and 29 ± 12 beats/min (VT), with the smaller increases in FT and VT significantly (p<0.05) different than DT. The increase in forearm (∼4.5 °C) and anterior thigh (∼5.0 °C) temperature did not differ between trials. However, the increase in forehead temperature (3.0 ± 0.9 °C) during VT was greater than during FT (2.5 ± 0.8 °C). In each trial, rectal temperature (RT) increased from ∼37.1 °C at rest to 37.94 ± 0.47 °C (DT), 37.46 ± 0.25 °C (FT) and 37.88 ± 0.39 °C (VT) by the end of heat exposure. Although small, the absolute change in RT was significantly (p<0.05) attenuated during FT. RT correlated significantly with ITS (r=0.67, p<0.05). However, there was poor agreement between the two measures with ITS overestimating RT by 0.14 °C (±1SD=0.36°C). There were no differences between sessions in sweating rate, which averaged 0.84 ± 0.30 L/hour. CONCLUSION The results indicate that passive heating is associated with significant physiological alterations that can be minimized with either voluntary or forced fluid intake. Thus, humans exposed to high heat stress under passive conditions (e.g., fighter pilots, race car drivers) will benefit from access to fluids. Furthermore, ITS tracks changes in body temperature under conditions of PHS, but may not be an accurate surrogate for rectal temperature.