Figure 11-27 Mean alfentanil plasma concentration-effect curves for the intraoperative period in each surgical group of patients, intubation, skin incision, and skin closure (with nitrous oxide), and the relationship between the alfentanil plasma concentration (without nitrous oxide) and the probability of needing naloxone to restore adequate spontaneous ventilation. The Cp₅₀ and slope of these curves were determined by averaging the estimates of individual patients. Bars indicate standard deviation of Cp₅₀ . Cp₅₀ , the concentration needed to obtund responses to stimuli adequately in 50% of patients. (From Ausems ME, Hug CC Jr, Stanski DR, Burm AG: Plasma concentrations of alfentanil required to supplement nitrous oxide anesthesia for general surgery. Anesthesiology 65:362–367, 1986.)