Summary

Background and objective: The aim of this prospective, randomized, double-blind study was to determine the effects of adding nitrous oxide on sevoflurane requirement for blunting sympathetic responses after surgical incision combined with two different target-controlled concentrations of remifentanil (1 and 3 ng mL−1) in female.

Methods: 102 female patients, aged 20–50 yr, ASA I, undergoing general anaesthesia for elective abdominal surgery were enrolled and randomly allocated to receive sevoflurane anaesthesia alone (Group A, n = 53), or with the addition of 60% nitrous oxide (Group N, n = 49). Patients of both groups were further assigned to receive a target-controlled remifentanil infusion with an effect-site concentration of either 1 ng mL−1 (Group N1, n = 27; Group A1, n = 30), or 3 ng mL−1 (Group N3, n = 22; Group A3, n = 23). Sympathetic responses to surgical incision were determined after a 20-min period of stable end-tidal sevoflurane and target-controlled remifentanil concentrations. Predetermined end-tidal sevoflurane concentrations and minimum alveolar concentration (MAC) for each group were determined using an up-and-down sequential allocation technique.

Results: The MAC of sevoflurane was 3.96% (95% confidence interval, CI95: 3.69–4.23%) in Group A1 and 1.2% (CI95: 0.9–1.3%) in Group N1 (P < 0.01), while in Groups A3 and N3 the MAC of sevoflurane was 0.36% (CI95: 0.24–0.47%) and 0.18% (CI95: 0.1–0.3%), respectively (P < 0.05).

Conclusion: Adding 60% nitrous oxide reduces the MAC of sevoflurane by 70% when using a remifentanil concentration of 1 ng mL−1 and 50% when using a remifentanil concentration of 3 ng mL−1.