Comparison of a computer-assisted infusion versus intermittent bolus administration of alfentanil as a supplement to nitrous oxide for lower abdominal surgery.

The anesthesiologist attempts to balance the dose or concentration of an anesthetic against the intensity of noxious stimulation so as to: 1) maintain a satisfactory anesthetic state, 2) minimize side effects and toxicity of the anesthetic, and 3) allow for a rapid recovery from anesthesia. The development of infusion pumps controlled by computers programmed according to pharmacokinetic principles should facilitate the achievement of these objectives for intravenous drugs. To test this hypothesis, the authors compared anesthetic conditions achieved with a computer-controlled infusion to those produced by the traditional method of intermittent intravenous injections. In both cases, the intravenous opiate, alfentanil, was used to supplement nitrous oxide anesthesia, and the dose/dose-rate of alfentanil after the induction dose was guided by the use of precisely defined clinical signs of inadequate anesthesia. One group of ten patients received 10 mg of alfentanil and 66% N2O to induce anesthesia, and was subsequently given 1 or 2 mg iv doses of alfentanil whenever the depth of anesthesia was inadequate. A second group of ten patients had a target alfentanil concentration of 475 ng/ml of plasma established by the computer-controlled infusion, which subsequently raised or lowered the concentration by 50 or 100 ng/ml according to the presence or absence of clinical signs of inadequate anesthesia. Regular measurements of alfentanil concentrations in plasma showed that the computer-assisted infusion produced relatively stable concentrations that closely paralleled those predicted (prediction error of -64 +/- 40 ng/ml [+/- SD] in the range of 150-600 ng/ml). The traditional method of intermittent injections resulted in continuous, rapid fluctuations in alfentanil concentrations. Both methods were successful in controlling the patients' responses to noxious stimuli, but the infusion group had: 1) a lower incidence of responsiveness, 2) greater hemodynamic stability, 3) no patients requiring naloxone for satisfactory ventilation postoperatively, and 4) an incidence of side effects that tended to be lower. The previously reported alfentanil concentration versus anesthetic effect relationships were confirmed.