Effect of nitrous oxide on desflurane MACBAR at two target-controlled concentrations of remifentanil.

Background. The aim of this prospective, randomized, double-blind study was to determine the effect of nitrous oxide on the desflurane requirement for blunting sympathetic response following surgical incision (MACBAR) when desflurane was combined with two different target-controlled concentrations of remifentanil (1 and 3 ng/mL). Methods. A total of 103 patients, aged 20-50 years, ASA physical status I, scheduled to undergo general anesthesia for elective abdominal surgery, were randomly allocated to receive anesthesia with desflurane alone (Group A, n = 53), or with the addition of 60% nitrous oxide (Group N, n = 50). Patients of both groups were further assigned to receive a target-controlled plasma concentration of 1 ng/mL (Group Al, n = 27; Group N1, n = 26) or 3 ng/mL remifentanil (Group A3, n = 26, Group N3, n = 24). Sympathetic responses to surgical incision were determined after a 20 min period of constant end-tidal desflurane and target-controlled remifentanil concentrations. Predetermined end-tidal desflurane concentrations and the MACBAR for each group were determined using an up-and-down sequential allocation technique. Results. A total of 98 patients completed the study. The MACBAR of desflurane was 5.2% (95% confidence interval [CI95: 4.9-5.5%]) in Group Al and 2.7% (CI95: 2.6-2.8%) in Group N1 (P<0.001), while in Groups A3 and N3 the MACBAR of desflurane was 2.2% (CI95:2-2.4%) and 2% (CI95:1.9-2.2%), respectively (P<0.01). When considering a minimum anesthetic concentration (MAC) value with a contribution of 60% nitrous oxide (0.55 MAC) in this population, the combined MACBAR values (expressed as multiples of the MAC) were 1.9 MAC for group Al, 1 MAC for group N1, 0.8 MAC for group A3, and 0.7 MAC for group N3. Conclusion. The addition of 60% nitrous oxide reduces the MACBAR of desflurane by 52% when using a remifentanil concentration of 1 ng/mL, and reduces the MACBAR by 10% when using a remifentanil concentration of 3 ng/mL.