Pharmacokinetics of computer-controlled alfentanil administration in children undergoing cardiac surgery

Cardiopulmonary bypass (CPB) induces changes in the pharmacokinetics of drugs. The purpose of this study was to model the pharmacokinetics of alfentanil in children undergoing cardiac surgery to provide accurate dosage titration intraoperatively as well as in the postoperative period.

Fourteen children (aged 3 months to 8 yr) undergoing cardiac surgery with CPB were administered alfentanil via a computer-controlled infusion pump. During surgery, the computer-controlled infusion pump was set to target plasma alfentanil concentrations of 500-2500 micrograms/ml. After surgery, the computer-controlled infusion pump was set to target plasma concentrations of 200-500 micrograms/ml. Parameters for children previously published by Goresky et al. were programmed into the device. Arterial blood samples were taken throughout the infusion. Plasma samples were assayed by radioimmunoassay. Alfentanil pharmacokinetics were estimated using a pooled-data approach with a simple weight-proportional, three-compartment mamillary model with parameters expressed in volumes and clearances as well as a CPB-adjusted, three-compartment model in which the parameters were allowed to change before, during, and after CPB. The accuracy of the three models was compared using cross-validation.

Plasma alfentanil concentrations during computer-controlled infusion pump administration exceeded target concentrations for the first 10 min of drug administration, and from 300 min to the end of the study. The median absolute performance error was 33%. Pharmacokinetic modeling estimated a set of parameters for a simple three-compartment model with a median absolute weighted residual of 18.4%. A CPB-adjusted model nominally decreased the median absolute weighted residual to 17.0%. The performance of these models as measured by cross-validation performance was 18.9% median absolute performance error for the simple model and 18.4% median absolute performance error for the CPB-adjusted model. Parameters for the simple three-compartment model are: V1 = 19.2 ml.kg-1; V2 = 99 ml.kg-1; V3 = 2344 ml.kg-1; Cl1 = 2.5 ml.kg-1.min-1; Cl2 = 38 ml.kg-1.min-1; and Cl3 = 15 ml.kg-1.min-1. In the CPB-adjusted model V1, V2, and Cl2 changed with the onset of CPB. After CPB, V1 and Cl2 returned to the initial values, while V2 was described by a third value.

The population pharmacokinetics of alfentanil in children undergoing cardiac surgery were well described by both a simple weight-proportional, three-compartment model and a weight-proportional, CPB-adjusted three-compartment model. Cross-validation estimated an expected median inaccuracy of approximately 18-20% with the estimated models in identical experimental circumstances. The flexible CPB-adjusted pharmacokinetic model could be used for modeling any drug with linear pharmacokinetics given in the context of CPB.