Pharmacokinetics of fentanyl in neonatal humans and lambs: effects of age.

To determine whether the clearance of fentanyl in neonates varies with age, the authors determined the pharmacokinetics of fentanyl in 14 humans ages 1-71 days and 15 lambs ages 3-37 days. In humans, fentanyl, 54.1 +/- 2.3 (mean +/- SD) micrograms/kg, was administered as a 2-min iv infusion; in lambs, fentanyl, 50 micrograms/kg, was administered as an iv bolus. Ventilation was controlled to maintain end-tidal or arterial PCO2 normal, and potent inhaled anesthetics were not administered; in humans, additional anesthesia was provided with iv morphine. Arterial or venous samples were obtained for 12 h, and plasma concentrations of fentanyl were determined by radioimmunoassay. Plasma concentration versus time data were fitted to two- and three-compartment pharmacokinetic models, and clearance, volume of distribution at steady-state (Vdss), and elimination half-life were determined. Clearance increased with age in both humans and lambs. Two humans who had intraabdominal surgery had no clearance of fentanyl: plasma concentrations of fentanyl remained constant for approximately 10 h after an initial distribution phase. In lambs, but not in humans, Vdss increased with age; elimination half-life did not change with age in either lambs or humans. The authors conclude that at least two factors--postnatal age and the type of surgery--affect fentanyl clearance during the neonatal period. The effect of other factors, such as inhaled anesthetics, remains to be determined.