Minimum alveolar concentration-awake of Xenon alone and in combination with isoflurane or sevoflurane.

Background: The minimum alveolar concentration (MAC)-awake is a traditional index of hypnotic potency of an inhalational anesthetic. The MAC-awake of xenon, an inert gas with anesthetic properties (MAC = 71%), has not been determined. It is also unknown how xenon interacts with isoflurane or sevoflurane on the MAC-awake. Methods: In the first part of the study, 90 female patients received xenon, nitrous oxide (N 2 O), isoflurane, or sevoflurane supplemented with epidural anesthesia (n = 36 for xenon and n = 18 per group for other anesthetics). In the second part, 72 additional patients received either xenon or N 2 O combined with the 0.5 times MAC-awake concentration of isoflurane or sevoflurane (0.2% and 0.3%, respectively, based on the results of the first part; n = 18 per group). During emergence, the concentration of an assigned anesthetic (xenon or N 2 O only in the second part) was decreased in 0.1 MAC decrements every 15 min from 0.8 MAC or from 70% in the case of N 2 O until the patient followed the command to either open her eyes or to squeeze and release the investigator's hand. The concentration midway between the value permitting the first response to command and that just preventing it was defined as the MAC-awake. Results: The MAC-awake were as follows: xenon, 32.6 ± 6.1% (mean ± SD) or 0.46 ± 0.09 MAC; N 2 O, 63.3 ± 7.1% (0.61 ± 0.07 MAC); isoflurane, 0.40 ± 0.07% (0.35 ± 0.06 MAC); and sevoflurane, 0.59 ± 0.10% (0.35 ± 0.06 MAC). Addition of the 0.5 MAC-awake concentrations of isoflurane and sevoflurane reduced the MAC-awake of xenon to 0.50 ± 0.15 and 0.51 ± 0.16 times its MAC-awake as a sole agent, but that of N 2 O to the values significantly greater than 0.5 times its MAC-awake as a sole agent (0.68 ± 0.12 and 0.66 ± 0.14 times MAC-awake; P < 0.01, analysis of variance and Dunnett's test). Conclusions: The MAC-awake of xenon is 33% or 0.46 times its MAC. In terms of the MAC-fraction, this is smaller than that for N 2 O but greater than those for isoflurane and sevoflurane. Unlike N 2 O, xenon interacts additively with isoflurane and sevoflurane on MAC-awake.