Effects of sleep state on ventilatory acclimatization to hypoxia in humans

We assessed the influence of sleep state on ventilatory acclimatization to hypoxia. Ventilation, arterial O2 saturation (SaO2), and arterial acid-base status were monitored in healthy adult males during wakefulness, nonrapid-eye-movement (NREM) sleep, and rapid-eye-movement (REM) sleep in normoxia [barometric pressure (PB) = 740 Torr] and over 4 continuous days of hypobaric hypoxia (PB = 455 Torr). The relative hypoventilation observed during sleep compared with wakefulness in normoxia was also observed during all stages of hypoxic acclimatization. The characteristic time-dependent changes associated with acclimatization to chronic hypoxia were similar during wakefulness and all sleep states: 1) arterial CO2 partial pressure (PaCO2) decreased 27-31% by night 4 with approximately half of this fall occurring acutely (0.3-3 h hypoxia); 2) minute ventilation increased progressively with duration of hypoxic exposure including increased levels of hyperventilation throughout the initial night of sleep in hypoxia; 3) SaO2 was lowest acutely and gradually increased coincident with the progressive hyperventilation; and 4) pHa increased acutely and remained unchanged despite additional hyperventilation due to a compensatory reduction in [HCO3-]a. In addition, in the acclimatized subject hyperventilation persisted following acute restoration of normoxia, and this continued hyperventilation was similar in magnitude during both wakefulness and NREM sleep. These results indicate that suprapontine influences on ventilatory control associated with the state of wakefulness are not required in the process of ventilatory acclimatization to chronic hypoxia.