Plasma concentrations of alfentanil required to supplement nitrous oxide anaesthesia for lower abdominal surgery

In 15 patients about to undergo lower abdominal surgery, anaesthesia was induced with alfentanil 150 micrograms kg-1 i.v. and 66% nitrous oxide in oxygen. Thereafter, nitrous oxide anaesthesia was supplemented with a continuous infusion of alfentanil at a rate which varied between 25 and 150 micrograms kg-1, as indicated by the patient's responses to the stimulation of surgery. Small bolus doses of alfentanil 7 micrograms kg-1 were administered to suppress precisely defined somatic, haemodynamic and other responses to stimulation. With one exception, all responses were rapidly controlled by increments of alfentanil. By measuring the arterial plasma concentrations of alfentanil at times with and without a response, we were able to estimate the alfentanil concentrations required (with nitrous oxide) to suppress responses to different stimuli during surgery. The results demonstrate the feasibility of continuous infusion of alfentanil in anaesthesia.