Plasma concentration of fentanyl, with 70% nitrous oxide, to prevent movement at skin incision

The Cp50 (minimal steady state plasma concentration of an intravenous analgesic/anesthetic required to prevent a somatic response in 50% of patients following skin incision) and the Cp50-BAR (minimal plasma concentration of an analgesic/anesthetic required to prevent either a somatic, hemodynamic, or autonomic response in 50% of patients following skin incision) have been recently proposed as a measure, like minimum alveolar concentration (MAC; and MAC-BAR), to establish the relative potency of intravenous analgesics. This study was conducted to establish the Cp50 for fentanyl.

Unpremedicated patients were administered fentanyl (in the presence of 70% N2O) via computer-assisted continuous infusion, a pharmacokinetic model-driven infusion device. After induction of anesthesia with fentanyl, the randomized target fentanyl concentration was entered into computer-assisted continuous infusion. This target fentanyl concentration was maintained until skin incision. Before induction, prior to skin incision, and immediately after skin incision, arterial blood samples were obtained for measurement of fentanyl and norepinephrine concentrations. At skin incision, patients were observed for a somatic, hemodynamic, or autonomic response. Only patients in whom the pre- and postincision fentanyl concentrations were within +/- 30% were included in the calculation of the Cp50. The Cp50 was calculated using logistic regression.

The Cp50 for fentanyl was 3.26 ng/ml, and the Cp50-BAR was 4.17 ng/ml.

Comparing these results with the previously published Cp50 of alfentanil, the potency of fentanyl relative to alfentanil is 1:58. Establishing the Cp50, once effect site equilibration has occurred, will allow pharmacodynamic comparisons between the opioids at equipotent concentrations.