The Effect Of Hypohydration On Hyperthermic Hyperpnea And Cutaneous Vasodilation During Exercise

Abstract

In exercising humans, hypohydration reportedly attenuates thermoregulatory responses such as cutaneous vasodilation and sweating elicited by increases in core temperature. Since respiratory heat loss induced by increases in ventilation also may contribute to heat dissipation in humans, it is plausible that hyperthermic hyperpnea is also attenuated by hypohydration during exercise, though this possibility has never been directly tested. PURPOSE: To test whether hypohydration attenuates hyperthermic hyperpnea in humans exercising in the heat. METHODS: On separate occasions, 13 male subjects performed a fluid replacement (FR) trial and a no fluid replacement (NFR) trial in random order. Each trial entailed two bouts of cycle exercise (Ex1 and Ex2, each 30-60 min) at 50% VO2peak at 35°C separated by a 70-80 min rest period, during which they drank sodium water in the FR trial but not the NFR trial. We plotted ventilatory parameters (minute ventilation (VE), ventilatory equivalent for VO2 and VCO2, tidal volume and respiratory frequency and forearm vascular conductance (FVC) as functions of Tes. RESULTS: In the FR trial, intermediate drinking nearly restored body fluid to the euhydrated condition, so that the body fluid status prior to Ex2 were different (p<0.05) in the two trials (the difference in plasma osmolality prior to Ex 2 was 9.4 mosm l−1, plasma volume was 7.6% and body weight was 2.5%). The slopes and intercepts of the linear relationships between the ventilatory parameters and Tes did not significantly differ between Ex1 and Ex2, or between the FR and NFR trials (e.g., the slopes of the linear relationships between VE and Tes were within 8.8 - 9.9 l min−1 °C−1). On the other hand, during Ex2 in the NFR trial, the Tes threshold for the onset of increased FVC (36.94±0.28 °C−1) was higher, and the slope of the relationship between FVC and Tes (9.3±4.4 Units (ml 100 ml−1 min−1 100mmHg−1 °C−1)) were lower than during Ex1 in the NFR trial (36.63±0.29 °C−1, 11.1±4.8 Units) and during Ex2 in the FR trial (36.72±0.30 °C−1, 11.6±5.5 Units) (p<0.05). CONCLUSIONS: These findings suggest that hypohydration does not affect hyperthermic hyperpnea during exercise, though it markedly attenuates the cutaneous vasodilatory response.