Ventilatory and waking responses to hypoxia in sleeping dogs.

We examined waking and ventilatory responses to acute hypoxia in four dogs during natural sleep. Progressive hypoxia was induced by a rebreathing technique in which alveolar CO2 pressure (PACO2) was held at the eucapnic level. Arterial O2 saturation (SaO2) was measured with an ear oximeter, and sleep stage was determined by electroencephalographic and behavioral criteria. Arousal from eucapnic hypoxia occurred at a SaO2 of 87.5 +/- 2.6% (mean +/- SE) during slow-wave sleep (SWS), and at a SaO2 of 70.5 +/- 3.4% during rapid-eye-movement (REM) sleep (P less than 0.005). The irregular pattern of breathing typical of REM sleep persisted during hypoxia. However linear regression analysis of breath-by-breath instantaneous minute volume of ventilation (VI) against SaO2 revealed regression coefficients in REM sleep that were similar to those found in SWS and wakefulness. This finding contrasts with earlier observations of a decreased response of VI to CO2 during REM sleep. The results indicate that although waking responses to hypoxia are delayed in REM sleep, ventilatory responses remain intact and therefore may be of importance in maintaining adequate ventilation during this stage of sleep.