CEREBRAL AWAKENING CONCENTRATION OF SEVOFLURANE AND ISOFLURANE PREDICTED DURING SLOW AND FAST ALVEOLAR WASHOUT

We studied 49 patients of ASA physical status I to determine cerebral anesthetic concentration on awakening calculated with end-tidal anesth etic concentration, when the end-tidal concentration decreased spontan eously. We also attempted to explain the difference in the average of the bracketing alveolar anesthetic concentration that allows and preve nts the response to verbal command during recovery from anesthesia (MA C-Awake) between slow and fast alveolar washout by comparing the cereb ral anesthetic concentrations with MAC-Awake determined by fast and sl ow washout. Slow washout was obtained by decreasing anesthetic concent rations in predetermined steps of 15 min, assuming equilibration betwe en brain and alveolar partial pressures. Fast alveolar washout was obt ained by discontinuation of the inhaled anesthetic, which had been mai ntained at 0.5 minimum alveolar anesthetic concentration (MAC) for at least 15 min. MAC-Awake values for sevoflurane and isoflurane obtained by slow washout were 0.34 +/- 0.05 and 0.31 +/- 0.05 (mean +/- SD), r espectively, when MAC-Awake was expressed as a ratio to age-adjusted M AC. MAC-Awake values obtained by fast washout (0.22 +/- 0.07 MAC for s evoflurane, 0.22 +/- 0.05 MAC for isoflurane) were significantly small er than those obtained by slow washout. Anesthetic concentrations in t he brain at first eye opening calculated with end-tidal concentrations during fast alveolar washout (0.34 +/- 0.08 MAC for sevoflurane, 0.30 +/- 0.08 MAC for isoflurane) were nearly equal to MAC-Awake obtained by slow alveolar washout. The difference in MAC-Awake between fast and slow alveolar washout could be explained by arterial-to-cerebral and end-tidal-to-arterial anesthetic differences.