Ventilatory response to 2-h sustained hypoxia in humans

Abstract

We used two protocols to determine if hypoxic ventilatory decline (HVD) involves changes in slope and/or intercept of the isocapnic HVR (hypoxic ventilatory response, expressed as the increase in V ̇ i per percentage decrease in Sa O 2 ). Isocapnia was defined as 1.5 mmHg above hyperoxic P et CO 2 . HVD was recorded in protocol I during two sequential 25 min exposures to isocapnic hypoxia (85 and 75% Sa O 2 , n=7) and in protocol II during 14 min of isocapnic hypoxia (90% Sa O 2 , F i O 2 =0.13, n=15), extended to 2 h of hypoxia with CO 2-uncontrolled in eight subjects. HVR was measured by the step reduction to sequentially lower levels of Sa O 2 in protocol I and by 3 min steps to 80% Sa O 2 at 8, 14 and 120 min in protocol II. The intercept of the HVR ( V ̇ i predicted at Sa O 2 =100%) decreased after 14 and 25 min in both protocols ( P<0.05). Changes in slope were observed only in protocol I at Sa O 2 =75%, suggesting that the slope of the HVR is more sensitive to depth than duration of hypoxic exposure. After 2 h of hypoxia the HVR intercept returned toward control value ( P<0.05) with still no significant changes in the HVR slope. We conclude that HVD in humans involves a decrease in hyperoxic ventilatory drive that can occur without significant change in slope of the HVR. The partial reversal of the HVD after 2 h of hypoxia may reflect some components of ventilatory acclimatization to hypoxia.