PHARMACOKINETICS OF ALFENTANIL ADMINISTERED AT A VARIABLE-RATE DURING3 TYPES OF SURGERY

To determine if the pharmacokinetics of alfentanil varied with the typ e of surgery (site of operation and intensity of surgical stimulation) or with the mode of administration, three groups of patients were giv en a variable-rate infusion of alfentanil (supplemented with bolus inj ections) as an adjunct to nitrous oxide-oxygen anaesthesia for breast surgery, lower abdominal surgery, or upper abdominal surgery. A two-co mpartment pharmacokinetic model adequately characterized the plasma co ncentration profiles of alfentanil for 31 of 34 patients. Mean distrib ution half-life (6 min), elimination half-life (87 min), initial volum e of distribution (10 litres), steady state volume of distribution (31 litres), total body clearance (308 ml min-1), and distribution cleara nce (656 ml min-1) did not vary with the type of surgery. The pharmaco kinetics could not be determined for three patients undergoing upper a bdominal surgery, probably because of nonstationarity. Comparison of o ur data with those obtained after a single bolus injection or a consta nt-rate infusion indicates that the pharmacokinetics of alfentanil do not vary with the mode of administration.