Influence of age on awakening concentrations of sevoflurane and isoflurane

We determined whether age, duration of anesthesia, gender, or type of surgery significantly influenced end-tidal concentrations on awakening from anesthesia with sevoflurane and isoflurane in 39 healthy ASA physical status I patients. Postoperatively, the end-tidal anesthetic concentration was maintained at a constant level at least for 15 min. If patients failed to open their eyes on request, the end-tidal concentration was decreased and again maintained at a constant level for 15 min. The anesthetic concentration midway between the value permitting the response and that just preventing the response was recorded. The end-tidal concentrations on awakening from anesthesia were 0.62 +/- 0.02% (mean +/- SE) for sevoflurane and 0.41 +/- 0.02% for isoflurane. Awakening concentration of sevoflurane and isoflurane correlated significantly with age (P < 0.001), but not with duration of anesthesia, gender, or type of surgery. The authors conclude that awakening concentration decreases at the similar rate of decrease in minimum alveolar concentration (MAC) with increasing age; and therefore, the ratios to MAC are fairly constant, being 0.34 for both sevoflurane and isoflurane.