Cerebral Awakening Concentration of Sevoflurane and Isoflurane Predicted During Slow and Fast Alveolar Washout

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We studied 49 patients of ASA physical status I to determine cerebral anesthetic concentration on awakening calculated with end-tidal anesthetic concentration, when the end-tidal concentration decreased spontaneously. We also attempted to explain the difference in the average of the bracketing alveolar anesthetic concentration that allows and prevents the response to verbal command during recovery from anesthesia (MAC-Awake) between slow and fast alveolar washout by comparing the cerebral anesthetic concentrations with MAC-Awake determined by fast and slow washout. Slow washout was obtained by decreasing anesthetic concentrations in predetermined steps of 15 min, assuming equilibration between brain and alveolar partial pressures. Fast alveolar washout was obtained by discontinuation of the inhaled anesthetic, which had been maintained 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), respectively, when MAC-Awake was expressed as a ratio to age-adjusted MAC. MAC-Awake values obtained by fast washout (0.22 ± 0.07 MAC for sevoflurane, 0.22 ± 0.05 MAC for isoflurane) were significantly smaller than those obtained by slow washout. Anesthetic concentrations in the 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.

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