DOES SLEEP ENHANCE THE EFFECT OF SUBANESTHETIC ISOFLURANE ON HYPOXIC VENTILATION

After surgery, patients may receive little audiovisual stimulation and may sleep. Lack of audiovisual stimulation enhances the suppression o f the hypoxic ventilatory response (HVR) by 0.1 minimum alveolar anest hetic concentration (MAC) isoflurane. Sleep also reduces the HVR and m ay thus increase the risk of hypoxia in patients at this time. We ther efore measured the ventilatory response in volunteers to a sustained s tep hypoxic stimulus (mean arterial oxygen saturation [SaO(2)] 80% [SE M 0.3] for 20 min) in the presence of 0.1 MAC isoflurane, with subject s in the awake and asleep states. The behavioral states were studied i n random order in nine male subjects. The combination of isoflurane an d sleep significantly reduced (P < 0.05) normoxic ventilation (6.71 [0 .39] vs 8.24 [0.29] L/min) and increased end-tidal PCO2 (PETCO(2)) (43 .1 [0.5] vs 40.4 [0.8] mm Hg) compared with the awake state. However, ventilation was similar in the asleep and awake states during early (1 5.10 [1.35] vs 15.50 [1.61] L/min) and late (10.45 [0.97] vs 11.03 [0. 39] L/min) hypoxia in the presence of isoflurane. Thus sleep did not r educe ventilation during hypoxia in the presence of isoflurane sedatio n. The increase in PETCO(2) during sleep may have offset suppression o f the HVR.