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

Background: Fentanyl produces a reduction in the minimum alveolar concentration (MAC) of isoflurane and desflurane needed to blockade adrenergic response (BAR) to surgical incision in 50% of patients (MAC-BAR). MAC-BAR of sevoflurane and the reduction in MAC-BAR of sevoflurane by fentanyl have not been described previously. The purpose of this study was to determine the MAC and MAC-BAR reduction of sevoflurane by fentanyl with and without nitrous oxide (N(2)O). Methods: Two hundred twenty-six patients were randomly assigned to one of two groups: a sevoflurane group and a sevoflurane/N(2)O group. Patients in each group were randomly assigned to one of five different fentanyl concentration subgroups. Patients were anesthetized with sevoflurane and fentanyl in the sevoflurane group and with sevoflurane, fentanyl, and N(2)O (66 vol%) in the sevoflurane/N(2)O group. Somatic and sympathetic responses to surgical incision were observed for MAC and MAC-BAR assessment at predetermined concentrations of sevoflurane. Results: Fentanyl produced an initial steep reduction in the MAC and MAC-BAR of sevoflurane, with 3 ng/ml resulting in a 61% reduction in MAC and an 83% 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% N(2)O, 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 presence of N(2)O. Conclusions: MAC and MAC-BAR decreased similarly with increasing concentrations of fentanyl in plasma, showing an initial steep reduction followed by a ceiling effect. In the presence of N(2)O MAC and MAC-BAR decreased similarly but did not exhibit a ceiling effect.