Pharmacokinetics of fentanyl during constant rate i.v. infusion for the relief of pain after surgery.

Forty-five patients in four groups undergoing orthopaedic, upper abdominal, prolonged or cardiac surgery received a constant rate i.v. infusion of fentanyl 100 micrograms h-1, for 24 h starting 2 h before surgery. A single bolus dose was given i.v. at the induction of anaesthesia. Plasma fentanyl concentrations, measured by radioimmunoassay were between 1 and 3 ng ml-1 until the infusions were discontinued. Clearance of fentanyl was decreased in the cardiac surgery group only. The elimination half-life was 7.3-9.7 h. This simple regimen produced effective analgesia.