Determinants of the ventilatory responses to hypoxia during sleep.

Disagreement exists on the effect of sleep on hypoxic ventilatory responses. We hypothesized that these differences were due to variabilities in methodology of inducing hypoxia, specifically, as they pertained to the PCO2 level during the studies. We therefore measured ventilatory responses to hypoxia with (eucapnic) and without (hypocapnic) added CO2 during wakefulness and sleep in 7 goats. Eucapnic responses to hypoxia were significantly decreased during both slow wave (SWS) and REM sleep. This decrease was not apparent when hypocapnia was allowed to occur. In 4 goats also provided with electromagnetic flow probes for brain blood flow (BBF) measurements, hypocapnia significantly attenuated the increase in BBF induced by hypoxia during both the awake and SWS stages. Concomitantly measured cerebral venous blood also showed lower oxygen tension during hypocapnia. We postulate that under hypocapnic conditions, the depressant effects of brain hypoxia may contribute to the obscuring of differences in hypoxic responses during wakefulness and sleep.