Figure 12-2 Pharmacokinetic simulation of (upper curve) a single large bolus of fentanyl (50 µg/kg) and (lower curve) a smaller fentanyl bolus (8 µg/kg) followed by intermittent boluses (1.5 µg/kg) every 15 minutes. The single large bolus results in plasma drug concentrations far in excess of those required, whereas the intermittent bolus scheme results in plasma concentrations that periodically fall below the therapeutic range. Ideally, the plasma drug concentration should be continuously within the therapeutic range, which can best be accomplished by a continuous infusion.