Fig. 6. Optimal combinations of remifentanil and sevoflurane to maintain adequate anesthesia and promote rapid emergence. The combinations that produced the quickest time to regain responsiveness (Observer's Assessment of Alertness/Sedation score ≥ 4) at various durations (in hours) are shown. For example: In a 1-h procedure, target concentrations of 1.05 vol% sevoflurane and 4.3 ng/ml remifentanil result in the fastest return of responsiveness. The simulations show that optimal combination changes as a function of duration of procedure. Although a target concentration pair with higher remifentanil concentrations provides a faster recovery in longer cases, remifentanil–sevoflurane mixtures in which sevoflurane is less than 0.75 vol% show no significant advantage.