Control of breathing and cardio-respiratory response to normobaric and hypobaric hypoxia

We examined the control of breathing, cardio-respiratory effects and the prevalence of acute mountain sickness (AMS) in humans exposed to hypobaric hypoxia (HH), normobaric hypoxia (NH), and under two control conditions (hypobaric normoxia and normobaric normoxia). Subjects (n = 11) were familiarised with all tests prior to their first exposures. The order of conditions was randomized, each exposure lasted for 6 hours, and consecutive exposures were separated by a one-week washout period. Prior to and following exposures, subjects underwent hyperoxic and hypoxic Duffin rebreathing tests, measuring CO₂ threshold and sensitivity, and a hypoxic ventilatory response test (HVR), measuring sensitivity to O₂. Inside the environmental chamber, minute ventilation (VE), tidal volume (VT), frequency of breathing (fB), blood oxygenation (SPO₂), heart rate (HR) and blood pressure (BP) were measured at 5min, 30min and hourly until exit. Symptoms of AMS were evaluated hourly using the Lake Louise score (LLS). Both the hyperoxic and hypoxic CO₂ thresholds were lowered after HH and NH during the Duffin rebreathing test. Hypoxic sensitivity in the Duffin rebreathing test was only increased after HH exposure. No changes occurred in the HVR after any of the four exposures. Ventilatory parameters, SPO₂ and HR were higher in the hypoxic exposures as opposed to the normoxic exposures. No major differences were observed for VE or any other cardio-respiratory variables between NH than HH. The LLS was greater in AMS-susceptible than in AMS-resistant subjects, but LLS was similar in HH and NH. We conclude that 6 hours of hypoxic exposure is sufficient to lower the peripheral and central CO₂ threshold, but it is too short in duration to induce differences in cardio-respiratory variables between HH and NH or to create differences in AMS severity.Education, Faculty ofKinesiology, School ofGraduat