The effect of remifentanil on the minimum alveolar concentration of isoflurane in children.

Study Objective: To evaluate the effect of remifentanil on the isoflurane end-tidal concentration required to eliminate movement reaction upon surgical incision in children.

Design: Prospective, double blinded, serial study.

Setting: Operating room of a university-affiliated hospital.

Patients: Patients of ASA status 1 or 2, aged 4 to 7 years, scheduled for either inguinal hernia repair or orchidopexy surgery with general anesthesia.

Interventions And Measurements: After endotracheal intubation, 108 children serially received 1 of 6 dose (nil, 0.05, 0.10, 0.15, 0.20, or 0.25 μg kg(-1) min(-1)) of remifentanil. End-tidal isoflurane concentration was adjusted according to a Dixon's up-and-down approach. Twenty-five minutes after starting the remifentanil infusion, the surgical incision was performed. The response of patients was classified as either "response" or "no response." Response was defined as a purposeful response in response to skin incision.

Main Results: The MAC of isoflurane were 1.50 ± 0.16%, 1.33 ± 0.27%, 0.93 ± 0.13%, 0.73 ± 0.27%, 0.63 ± 0.19%, and 0.60 ± 0.15% for remifentanil infusion rates of nil, 0.05, 0.10, 0.15, 0.20, and 0.25 μg kg(-1) min(-1), respectively.

Conclusion: The MAC of isoflurane decreased with increasing infusion rate of remifentanil, showing an initial step reduction followed by a ceiling effect.