Core temperature thresholds for hyperpnea during passive hyperthermia in humans.

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) degrees C for 25 min. Tympanic (Tty), esophageal (Tes), thigh skin and forehead skin temperatures, heart rate, inspired minute ventilation (V1 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 V1 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. Tty increased to 38.7 (0.2) degrees C and Tes increased to 39.0 (0.2) degrees C by the end of immersion. Core temperature thresholds for increases in V1 were evident at 38.1 degrees C when expressed against Tty and at 38.5 degrees C when expressed against Tes. The results indicated that during body warming core temperature thresholds for V1 are reached and subsequently a hyperpnea was evident, despite VO2 remaining at a resting value.(ABSTRACT TRUNCATED AT 250 WORDS)