Sleep and respiration under acute hypobaric hypoxia

The purpose of this study was to elucidate the effects of acute hypobaric hypoxia on nocturnal sleep architecture and respiratory responses in a hypobaric simulator. Five healthy young males (19-23 years old) were recruited to sleep for 8 h at sea level and at simulated altitudes of 1,500, 3,000, and 4,000 m in the simulator (61.2 m3, 20 degrees C, and 60% RH). Each experimental run was separated by at least 3 d. Standard polysomnograph, respiration, and arterial oxygen saturation (SaO2) during sleep were observed. 1) SaO2 decreased significantly with increasing altitude. At 4,000 m, SaO2 showed its lowest value during 1 to 3 h after sleep onset. 2) Sleep architecture below 3,000 m showed almost the same pattern. However, reduction in REM sleep and increased wakefulness were observed at 4,000 m, though such sleep disturbance was not observed in the first one-third of the night spent in bed. 3) Periodic breathing (PB) with apnea and/or hypopnea developed in all subjects above 3,000 m. PB tended to appear in light sleep, though sleep was not always disturbed by PB. It might be concluded that there was no sleep disturbance up to 3,000 m altitude. Nocturnal sleep at 4,000 m, however, was disturbed after a few hours from sleep onset by severe hypoxemia induced by multiplicative effects of hypoxia and hypoventilation during deep sleep. At high altitude, PB seems to not induce arousals consistently, which was different from sleep apnea syndrome at sea level.