Figure 11-11 Pharmacokinetic and pharmacodynamic variability of the opioid. The upper panel demonstrates alfentanil plasma concentration in 45 patients administered with 50 µg/kg alfentanil. The lower panel shows dose-response relationships of alfentanil from 34 patients receiving alfentanil and 66% nitrous oxide during intraabdominal surgery. The black circles represent the Cp₅₀ from each patient. Cp₅₀ , the concentration needed to obtund responses to stimuli adequately in 50% of patients. (From Maitre PO, Vozeh S, Heykants J, et al: Population pharmacokinetics of alfentanil: The average dose-plasma concentration relationship and interindividual variability in patients. Anesthesiology 66:3–12, 1987; and 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.)