Extracorporeal circuit sequestration of fentanyl and alfentanil.

Fentanyl or alfentanil, in doses approximating to those used in clinical practice, was added to the priming fluid of an extracorporeal circuit before the institution of cardiopulmonary bypass (CPB). The concentrations of both drugs in the priming fluid were measured over a 20-min period. The concentration of fentanyl decreased at neutral or high pH values, suggesting drug adsorption to the circuit. The concentration of alfentanil was unaffected. The administration of fentanyl to the priming fluid may produce lower anaesthetic concentrations than anticipated.