Pharmacokinetics of alfentanil administered at a variable rate during three types of surgery

To determine if the pharmacokinetics of alfentanil varied with the type of surgery (site of operation and intensity of surgical stimulation) or with the mode of administration, three groups of patients were given a variable-rate infusion of alfentanil (supplemented with bolus injections) as an adjunct to nitrous oxide-oxygen anaesthesia for breast surgery, lower abdominal surgery, or upper abdominal surgery. A two-compartment pharmacokinetic model adequately characterized the plasma concentration profiles of alfentanil for 31 of 34 patients. Mean distribution half-life (6 min), elimination half-life (87 min), initial volume of distribution (10 litres), steady state volume of distribution (31 litres), total body clearance (308 ml min-1), and distribution clearance (656 ml min-1) did not vary with the type of surgery. The pharmacokinetics could not be determined for three patients undergoing upper abdominal surgery, probably because of nonstationarity. Comparison of our data with those obtained after a single bolus injection or a constant-rate infusion indicates that the pharmacokinetics of alfentanil do not vary with the mode of administration.