Figure 12-4 Steps involved in a pharmacokinetic model-driven infusion. Typically, pharmacokinetic models are derived from experiments in which plasma drug concentrations are measured at intervals after bolus administration of the drug. Nonlinear regression is used to fit a monoexponential, biexponential, or triexponential curve to the resulting concentration-versus-time data. There is an algebraic relationship between the exponential decay curves and a one-, two-, or three-compartment pharmacokinetic model. The "BET" infusion scheme is developed and consists of a bolus, a continuous infusion to replace drug eliminated from the body, and an exponentially declining infusion to replace drug transferred out of plasma to other body compartments. A BET infusion results in maintenance of a constant specified plasma drug concentration. Practical implementation of the BET scheme with real infusion pumps and infusion rates that change only at discrete intervals results in a plasma drug concentration profile that approximates the profile resulting from a BET infusion.