Figure 12-31 Simulated plasma fentanyl concentrations over time for a brief anesthetic showing the rapid rise and fall and impression of precise titration that can be created by a target-controlled drug delivery device (A). B, Effect-site concentrations for the same anesthetic course demonstrating that precise control of the plasma concentration does not necessarily translate into precise control at the site of drug effect. (Adapted from Shafer SL, Gregg K: Algorithms to rapidly achieve and maintain stable drug concentrations at the site of drug effect with a computer controlled infusion pump. J Pharmacokinet Biopharm 20:147–169, 1992.)