Effects of hyperthermia on hypoxic ventilatory response in normal man

Increased body temperature stimulates hyperventilation in man but little is known about its effects on ventilatory responsiveness to hypoxia. Hence this study examined the effects of hyperthermia on hypoxic ventilatory response (HVR), hypercapnic ventilatory response (HCVR), and oxygen consumption (VO2). Six fasting subjects had these variables measured under basal conditions and at two levels of hyperthermia. Hypoxic ventilatory response was measured as the shape paramater A of the VE/PAO2 curves. Since hyperthermia produces hyperventilation and, therefore, hypocapnia, HVR was measured at the hyperthermic (hypocapnic alveolar CO2 tension (PACO2) and at the basal (normothermic) PACO2. Hypoxic ventilatory response (A) increased when measured at basal PACO2 levels, from 113 +/- 8.8 (S.E.M.) to 189 +/- 21.8 at 0.7 degrees C. and 240 +/- 34.0 at + 1.40 degrees C. (P less than 0.005). HVR measured during hyperthermic hypocapnia also increased at each temperature level but did not reach statistical significance (P = 0.1). Hypercapnic ventilatory response, as measured by the slope S of VE/PACO2 lines, increased significantly at each temperature elevation (P less than 0.025). We conclude that raising body temperature causes a significant augmentation of ventilatory responses to hypoxia (during normothermic PACO2 conditions) and to hypercapnia.