Reduced sweating threshold during exercise-induced hyperthermia

Abstract

Two methods have been proposed previously for evaluating thermoregulatory responses to isolated core temperature perturbations. One involves clamping skin temperature at 28°C by water immersion and elevating core temperature by exercise. In the other, core temperature is reduced by central venous infusion of cold fluid while skin temperature is kept constant near 36°C. The sweating-to-shivering temperature range determined using the first protocol is ≈0.6±0.2°C, but is 1.4±0.6°C using the second. The authors tested the hypothesis that the disparity results from an exercise-induced reduction in the sweating threshold. The sweating threshold was determined three times each in six men, with skin temperature kept constant (36.5–37.0°C) throughout. The first threshold (control) was in response to gradual core hyperthermia that resulted from a skin temperature sufficiently high to prevent dissipation of metabolic heat. The second threshold (exercise) was in response to exercise-induced hyperthermia, and followed a period of core cooling (≈ 1°C) induced by central venous infusion of cold lactated Ringer's solution. The purpose of the central venous cooling was to reduce core temperature to a level well below the sweating threshold. The third threshold (recovery) again resulted from a skin temperature sufficiently high to prevent dissipation of metabolic heat. The control sweating threshold was 0.7±0.2°C greater than during exercise (P<0.01), but virtually identical to that during recovery. Since the control and recovery thresholds were similar, hyperhydration (necessary in this protocol) appears not to have influenced sweating thresholds. Our results thus indicate that exercise per se reduces the sweating threshold sufficiently to explain reported differences in the sweating-to-shivering range.