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

Abstract

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.

Ventilatory response to increasing body temperature: Characteristics and effect on central fatigue

“…However, there are several ways in which the increases in ventilation seen in resting subjects differ from those seen during exercise. At rest, ventilation does not increase until esophageal temperature (Tes) exceeds 38.0-38.5 °C [4][5][6] . Above this Tes threshold, minute ventilation (V ・ E), tidal volume and respiratory frequency all increase linearly in proportion to the increase in Tes 5,6) .…”

“…At rest, ventilation does not increase until esophageal temperature (Tes) exceeds 38.0-38.5 °C [4][5][6] . Above this Tes threshold, minute ventilation (V ・ E), tidal volume and respiratory frequency all increase linearly in proportion to the increase in Tes 5,6) . On the other hand, White and Cabanac 7) reported that during incremental exercise, there is a core temperature threshold for hyperventilation around a tympanic temperature of 37.0-37.7 °C or a Tes of 37.5-37.9 °C, while Beaudin et al 8) and Sancheti and White 9) reported the Tes threshold for hyperventilation during incremental exercise to be around 37.1-38.2 °C.…”

“…On the other hand, White and Cabanac 7) reported that during incremental exercise, there is a core temperature threshold for hyperventilation around a tympanic temperature of 37.0-37.7 °C or a Tes of 37.5-37.9 °C, while Beaudin et al 8) and Sancheti and White 9) reported the Tes threshold for hyperventilation during incremental exercise to be around 37.1-38.2 °C. In addition, after pre-cooling subjects in cold water (18°C) to decrease body temperature enough to cause a 1°C drop in Tes, Tsuji et al 6,10) found that there is a Tes threshold for hyperventilation around 37°C during prolonged steadystate light and moderate intensity exercise (25% and 50% of peak oxygen uptake [V ・ O2peak], respectively). Above this threshold, V ・ E and respiratory frequency increased in proportion to the increase in Tes 3,5,6,10) (Fig.…”

“…In addition, after pre-cooling subjects in cold water (18°C) to decrease body temperature enough to cause a 1°C drop in Tes, Tsuji et al 6,10) found that there is a Tes threshold for hyperventilation around 37°C during prolonged steadystate light and moderate intensity exercise (25% and 50% of peak oxygen uptake [V ・ O2peak], respectively). Above this threshold, V ・ E and respiratory frequency increased in proportion to the increase in Tes 3,5,6,10) (Fig. 2), though no increases in tidal volume were seen.…”

“…It thus appears that the range within which the core temperature threshold for hyperventilation during exercise can fall is somewhat wide. Nonetheless, in all cases, the core temperature threshold Correspondence: khayashi@u-shizuoka-ken.ac.jp We also evaluated the ventilatory sensitivity to increasing body temperature (slope of the regression line between V ・ E and Tes) and found that the sensitivity is higher at rest than during exercise, and that the sensitivity is not influenced by exercise intensity (25% vs. 50% of V ・ O2peak) 5,6) . Table 1 summarizes the findings for threshold and sensitivity of hyperthermia-induced hyperventilation.…”