Comparison of hyperthermic hyperventilation during passive heating and 1 prolonged light and moderate exercise in the heat

31 Elevation of core temperature leads to increases in ventilation in both resting subjects and 32 those engaged in prolonged exercise. We compared the characteristics of the hyperthermic 33 hyperventilation elicited during passive heating at rest and during prolonged moderate and light 34 exercise. Twelve healthy men performed three trials: a rest trial in which subjects were passively 35 heated using hot water immersion (41°C) and a water-perfused suit, and two exercise trials in 36 which subjects exercised at 25% (light) or 50% (moderate) of peak oxygen uptake in the heat 37 (37°C and 50% relative humidity) after first using water immersion (18°C) to reduce resting 38 esophageal temperature (Tes). This protocol enabled detection of a Tes threshold for 39 hyperventilation during the exercise. When minute ventilation ( ) was expressed as a function 40 of Tes, 9 of the 12 subjects showed Tes thresholds for hyperventilation in all trials. The Tes 41 thresholds for increases in during light and moderate exercise (37.1 ± 0.4 and 42 36.9 ± 0.4°C) were both significantly lower than during rest (38.3 ± 0.6°C), but the Tes 43 thresholds did not differ between the two exercise intensities. The sensitivity of to 44 increasing Tes (slope of the Tesrelation) above the threshold was significantly lower during 45 moderate exercise (8.7 ± 3.5 l·min·°C) than during rest (32.5 ± 24.2 l·min·°C), but the 46 sensitivity did not differ between light (10.4 ± 13.0 l·min·°C) and moderate exercise. These 47 results suggest the core temperature threshold for hyperthermic hyperventilation and the 48 hyperventilatory response to increasing core temperature in passively heated subjects differ 49 from that in exercising subjects, irrespective of whether the exercise is moderate or light. 50 (250 words) 51 52