Figure 12-20 Simulation of the interaction of propofol and fentanyl in preventing a somatic response at skin incision and time to recovery. On the x axis is the fentanyl concentration, and on the y axis, the propofol concentration. The curve in the lower plane shows the propofol-fentanyl interaction required to provide adequate anesthesia. When the infusion is turned off, the concentrations of each drug decrease, as shown on the z axis. The curve drawn on the recovery surface shows the time to emergence from anesthesia for combinations of fentanyl and propofol after an anesthetic of 10 minutes' (A), 60 minutes' (B), 300 minutes' (C), or 600 minutes' (D) duration. Note that the optimal combination for the most rapid recovery is a propofol concentration of 3.0 to 3.5 µg/mL of propofol combined with 1.5 ng/mL of fentanyl. As the concentration of propofol or fentanyl increases, the time for recovery increases. In addition, the longer the duration of drug infusion, the longer that recovery takes, especially if the optimal combination is not used. Adapted from Vuyk et al. ^[87]