Alfentanil infusions: relationship between pharmacokinetics and pharmacodynamics in man

Fourteen fit young patients undergoing body surface surgery received an infusion of alfentanil at either 50 or 100 micrograms kg-1 hr-1 to supplement nitrous oxide anaesthesia. The alfentanil infusion was continued for two hours post-operatively at the lower rate of 20 micrograms kg-1 hr-1. Resting ventilation, carbon dioxide responsiveness and pain scores were measured post-operatively. Values for clearance and elimination half life were similar to data following single doses of alfentanil but showed considerable interindividual variation. However, there was a greater systemic clearance (P = 0.02) when determined using the post-infusion decay data compared with that calculated during anaesthesia (527 ml min-1 compared with 434 ml min-1). This is in accord with observations for other intravenous drugs.