FENTANYL OR ALFENTANIL DECREASES THE MINIMUM ALVEOLAR ANESTHETIC CONCENTRATION OF ISOFLURANE IN SURGICAL PATIENTS

The minimum alveolar anesthetic concentration (MAC) is an accepted pot ency measure for inhaled anesthetics. There is no generally accepted i ntraoperative measure of opioid potency, partly because of the difficu lty in obtaining steady state biophase concentrations. We have studied the relative potency of fentanyl and alfentanil by using computer-ass isted continuous infusions (CACI), in terms of reduction of isoflurane MAC. Data are presented from 79 patients in whom anesthesia was induc ed with thiopental and maintained with a CACI of fentanyl (set to achi eve a plasma concentration of 0, 1, 3, 6, or 8 ng/mL) or alfentanil (0 , 70, 210, 400, or 1000 ng/mL) with a predetermined end-tidal concentr ation of isoflurane. A determination of movement in response to skin i ncision was made. Without any opioid, the MAC of isoflurane was 1.25%. The maximum likelihood solution to a logistic regression model showed that fentanyl 0.5 ng/mL (95% confidence intervals [CI], 0-4.6 ng/mL) or alfentanil 28.8 ng/mL (95% CI, 0-70.9 ng/mL) resulted in a 50% isof lurane MAC reduction. In the logistic regression model, age or weight were not significant factors in the model. These results suggest that the comparative intraoperative potency ratio for alfentanil and fentan yl is 58:1, and that this methodology allows for direct intraoperative comparisons of opioid potency.