Core temperature thresholds for hyperpnea during passive hyperthermia in humans

Abstract

Humans have higher ventilation when they are hyperthermic but it is not known whether core temperature thresholds for ventilation exist, nor has a physiological rationale been presented for this response. To examine this question, ventilation was studied in relation to core temperatures in humans rendered hyperthermic in a warm bath. Seven subjects [mean (SE), 23.3 (1.4) years] wearing only shorts and a thick felt hat with ear flaps were immersed to the neck in a bath at 41 (0.5)°C for 25 min. Tympanic (T _ty), esophageal (T _es), thigh skin and forehead skin temperatures, heart rate, inspired minute ventilation (V _I at body temperature and pressure, saturated), ventilation frequency and oxygen consumption (VO₂ at standard temperature and pressure, dry) were recorded at 30-s intervals. At immersion V _I briefly increased to 18.6 (3.0)l·min⁻¹ returned to about the pre-immersion value,, and significantly increased to 19.3 (3.0) l·min⁻¹ by the end of immersion. VO₂ increased significantly from the pre-immersion value of 0.27 l·min⁻¹ to 0.67 l·min⁻¹ by the first 0.5 min of immersion, but then returned to its pre-immersion value. T _ty increased to 38.7 (0.2)°C and T _es increased to 39.0 (0.2)°C by the end of immersion. Core temperature thresholds for increases in V _I were evident at 38.1°C when expressed against T _ty and at 38.5°C when expressed against T _es. The results indicated that during body warming core temperature thresholds for V _I are reached and subsequently a hyperpnea was evident, despite VO₂ remaining at a resting value. This hyperpnea is seen as a thermoregulatory response likely to participate in selective brain cooling.