Cl. Westmoreland et al., FENTANYL OR ALFENTANIL DECREASES THE MINIMUM ALVEOLAR ANESTHETIC CONCENTRATION OF ISOFLURANE IN SURGICAL PATIENTS, Anesthesia and analgesia, 78(1), 1994, pp. 23-28
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.