Comparison of hyperthermic hyperventilation during passive heating and prolonged light and moderate exercise in the heat

Elevation of core temperature leads to increases in ventilation in both resting subjects and those engaged in prolonged exercise. We compared the characteristics of the hyperthermic hyperventilation elicited during passive heating at rest and during prolonged moderate and light exercise. Twelve healthy men performed three trials: a rest trial in which subjects were passively heated using hot-water immersion (41°C) and a water-perfused suit and two exercise trials in which subjects exercised at 25% (light) or 50% (moderate) of peak oxygen uptake in the heat (37°C and 50% relative humidity) after first using water immersion (18°C) to reduce resting esophageal temperature (T(es)). This protocol enabled detection of a T(es) threshold for hyperventilation during the exercise. When minute ventilation (Ve) was expressed as a function of T(es), 9 of the 12 subjects showed T(es) thresholds for hyperventilation in all trials. The T(es) thresholds for increases in Ve during light and moderate exercise (37.1 ± 0.4 and 36.9 ± 0.4°C) were both significantly lower than during rest (38.3 ± 0.6°C), but the T(es) thresholds did not differ between the two exercise intensities. The sensitivity of Ve to increasing T(es) (slope of the T(es)-Ve relation) above the threshold was significantly lower during moderate exercise (8.7 ± 3.5 l · min(-1) · °C(-1)) than during rest (32.5 ± 24.2 l · min(-1) · °C(-1)), but the sensitivity did not differ between light (10.4 ± 13.0 l · min(-1) · °C(-1)) and moderate exercise. These results suggest the core temperature threshold for hyperthermic hyperventilation and the hyperventilatory response to increasing core temperature in passively heated subjects differs from that in exercising subjects, irrespective of whether the exercise is moderate or light.