Reduction of the minimum alveolar concentration of isoflurane by dexmedetomidine

Background: alpha 2-Adrenergic agonists have been shown to reduce anesthetic requirements of other anesthetics, and they may even act as complete anesthetics by themselves at high doses in animal models. The present study was designed to define the interaction of intravenous infusion of dexmedetomidine, an alpha 2-adrenergic agonist, and isoflurane in patients having surgery by using the minimum alveolar concentration (MAC) of isoflurane as the measure of anesthetic potency. Methods: Forty-nine women scheduled for abdominal hysterectomy mere randomly allocated to receive either a placebo infusion (n = 16) or a two-stage infusion of dexmedetomidine with target plasma concentration of 0.3 ng/ml (n = 17) or 0.6 ng/ml (n = 16). The study drug infusion was commenced 15 min before induction of anesthesia with thiopental and alfentanil and was continued until skin incision. The end-tidal concentration of isoflurane for each patient was predetermined according to the ''up-down'' method of Dixon, and it was maintained for at least 15 min before the patient's response to skin incision was assessed. Results: The MAC of isoflurane was 0.85% end-tidal in the control group, 0.55% end-tidal with the low dose of dexmedetomidine, and 0.45% end-tidal with the high dose of dexmedetomidine. Conclusions: The MAC of isoflurane in the control group was lower than that reported previously in similar patients having surgery, probably due to anesthesia induction with thiopental and alfentanil. Nevertheless, with the high dose of dexmedetomidine, the MAC of isoflurane was still 47% less than that without dexmedetomidine.