Effect of initial core temperature on hyperthermic hyperventilation during prolonged submaximal exercise in the heat

Abstract

We investigated whether a core temperature threshold for hyperthermic hyperventilation is seen during prolonged submaximal exercise in the heat when core temperature before the exercise is reduced and whether the evoked hyperventilatory response is affected by altering the initial core temperature. Ten male subjects performed three exercise trials at 50% of peak oxygen uptake in the heat (37°C and 50% relative humidity) after altering their initial esophageal temperature (Tes). Initial Tes was manipulated by immersion for 25 min in water at 18°C (Precooling), 35°C (Control), or 40°C (Preheating). Tes after the water immersion was significantly higher in the Preheating trial (37.5 ± 0.3°C) and lower in the Precooling trial (36.1 ± 0.3°C) than in the Control trial (36.9 ± 0.3°C). In the Precooling trial, minute ventilation (V̇e) showed little change until Tes reached 37.1 ± 0.4°C. Above this core temperature threshold, V̇e increased linearly in proportion to increasing Tes. In the Control trial, V̇e increased as Tes increased from 37.0°C to 38.6°C after the onset of exercise. In the Preheating trial, V̇e increased from the initially elevated levels of Tes (from 37.6 to 38.6°C) and V̇e. The sensitivity of V̇e to increasing Tes above the threshold for hyperventilation (the slope of the Tes-V̇e relation) did not significantly vary across trials (Precooling trial = 10.6 ± 5.9, Control trial = 8.7 ± 5.1, and Preheating trial = 9.2 ± 6.9 L·min−1·°C−1). These results suggest that during prolonged submaximal exercise at a constant workload in humans, there is a clear core temperature threshold for hyperthermic hyperventilation and that the evoked hyperventilatory response is unaffected by altering initial core temperature.