Figure 10-4 Measured arterial propofol and fentanyl concentrations at which patients did and did not respond to a verbal command 10 minutes after the initiation of infusion of these drugs. Solid lines denote the modeled concentrations of propofol according to the decade of age when combined with the measured fentanyl concentrations at which 50% of patients did not respond to verbal command (Cp₅₀ s). (Redrawn from Smith C, McEwan A, Jhaveri R: Reduction of propofol Cp50 by fentanyl. Anesthesiology 81:820–828, 1994.)