Minimum Alveolar Concentrations in Man on Awakening from Methoxyflurane, Halothane, Ether and Fluroxene Anesthesia

Alveolar anesthetic concentrations at the first response to command and those concentrations just preventing the response were determined in man during recovery from methoxyflurane, halothane, ether and fluroxene anesthesia. The authors assumed equilibration of cerebral anesthetic concentration with alveolar concentration after alveolar concentration had been kept constant for at least 15 minutes. The anesthetic concentration midway between the value permitting the response and that just preventing the response was defined as “MAC awake.” MAC awake values were 0.081 ± 0.021 (SD) per cent methoxyflurane, 0.41 ± 0.05 per cent halothane, 1.41 ± 0.22 per cent ether, and 2.20 ± 0.49 per cent fluroxene. MAC awake-to-MAC ratios were fairly close for the four agents, being 0.52, 0.52, 0.67, and 0.60 for methoxyflurane, halothane, ether, and fluroxene, respectively. When the alveolar concentrations were allowed to fall spontaneously, falsely low MAC awake values were obtained for halothane and fluroxene, while MAC awake for methoxyflurane was unchanged from that found at constant alveolar concentration.