Comparison of hyperthermic hyperpnea elicited during rest and submaximal, moderate-intensity exercise

Abstract

We tested the hypothesis that, in humans, hyperthermic hyperpnea elicited in resting subjects differs from that elicited during submaximal, moderate-intensity exercise. In the rest trial, hot-water legs-only immersion and a water-perfused suit were used to increase esophageal temperature (Tes) in 19 healthy male subjects; in the exercise trial, Tes was increased by prolonged submaximal cycling [50% peak O2 uptake (V̇o2)] in the heat (35°C). Minute ventilation (V̇e), ventilatory equivalent for V̇o2 (V̇e/V̇o2) and CO2 output (V̇e/V̇co2), tidal volume (Vt), and respiratory frequency (f) were plotted as functions of Tes. In the exercise trial, V̇e increased linearly with increases (from 37.0 to 38.7°C) in Tes in all subjects; in the rest trial, 14 of the 19 subjects showed a Tes threshold for hyperpnea (37.8 ± 0.5°C). Above the threshold for hyperpnea, the slope of the regression line relating V̇e and Tes was significantly greater for the rest than the exercise trial. Moreover, the slopes of the regression lines relating V̇e/V̇o2, V̇e/V̇co2, and Tes were significantly greater for the rest than the exercise trial. The increase in V̇e reflected increases in Vt and f in the rest trial, but only f in the exercise trial, after an initial increase in ventilation due to Vt. Finally, the slope of the regression line relating Tes and Vt or f was significantly greater for the rest than the exercise trial. These findings indicate that hyperthermic hyperpnea does indeed differ, depending on whether one is at rest or exercising at submaximal, moderate intensity.