The effect of fentanyl on sevoflurane requirements for somatic and sympathetic responses to surgical incision

Background: Fentanyl produces a reduction in the minimum alveolar concentra tion (MAC) of isoflurane and desflurane needed to blockade adrenergic respo nse (BAR) to surgical incision in 50% of patients (MAC-BAR). MAC-BAR of sev oflurane and the reduction in MAC-BAR of sevoflurane by fentanyl have not b een described previously. The purpose of this study was to determine the MA C and MAC-BAR reduction of sevoflurane by fentanyl with and without nitrous oxide (N2O). Methods: Two hundred twenty-six patients were randomly assigned to one of t wo groups: a sevoflurane group and a sevoflurane/N2O group. Patients in eac h group were randomly assigned to one of five different fentanyl concentrat ion subgroups. Patients were anesthetized with sevoflurane and fentanyl in the sevoflurane group and with sevoflurane, fentanyl, and N2O (66 vol%) in the sevoflurane/N2O group. Somatic and sympathetic responses to surgical in cision were observed for MAC and MAC-BAR assessment at predetermined concen trations of sevoflurane. Results: Fentanyl produced an initial steep reduction in the MAC and MAC-BA R of sevoflurane, with 3 ng/ml resulting in a 61% reduction in MAC and an 8 3% reduction in MAC-BAR. A ceiling effect was observed for MAC and MAC-BAR with 6 ng/ml fentanyl providing only an additional 13% and 9% reduction in MAC and MAC-BAR, respectively. In the presence of 66 vol% N2O, MAC and MAC- BAR of sevoflurane were reduced with increasing concentrations of fentanyl A ceding effect was not observed for reduction in MAC and MAC-BAR in the pr esence of N2O. Conclusions: MAC and MAC-BAR decreased similarly with increasing concentrat ions of fentanyl in plasma, showing an initial steep reduction followed by a ceiling effect. In the presence of N2O MAC and MAC-BAR decreased similarl y but did not exhibit a ceiling effect.