Binding of fentanyl and alfentanil to the extracorporeal circuit.

Adsorption of fentanyl and alfentanil to the cardiopulmonary bypass (CPB) equipment was studied in vitro by adding one of the analgesics to the priming solution consisting of either saline or a mixture of saline and blood. Opiate concentrations in the solutions were measured during a 60-min circulation period of a closed CPB system. When the saline prime was used, 29% of the predicted fentanyl level of 30 ng ml-1 was found at the end of the experiment, while the recovery of alfentanil was 80% of the calculated level of 1500 ng ml-1. When blood was added to the prime, experiments with fentanyl produced similar results to those with pure saline prime, but recovery exceeding the calculated concentration was obtained with alfentanil. The difference between the alfentanil levels in the two primes may reflect the poor distribution of this analgesic into red blood cells. In another set of experiments, the CPB circuit was primed with fentanyl or alfentanil and circulated for 10 min before connection of the apparatus to patients undergoing cardiac surgery under high-dose opiate anaesthesia. This priming prevented the steep reduction in plasma opiate concentration regularly observed during the institution of CPB. It is concluded that in a clinically relevant dose range a smaller fraction of alfentanil is sequestered by the CPB apparatus than fentanyl.