Figure 12-8 Interaction between alfentanil and propofol on three different end points: response to intubation (red line), maintenance of anesthesia (black line), and concentrations associated with emergence from anesthesia (gray line). The curve shows the concentrations associated with a 50% probability of the respective end point. (Adapted from Vuyk J, Lim T, Engbers FH, et al: The pharmacodynamic interaction of propofol and alfentanil during lower abdominal surgery in women. Anesthesiology 83:8–22, 1995.)