The Anesthetic Potency of Fentanyl in Terms of Its Reduction of Enflurane MAC

Infusion rates for fentanyl were calculated to produce stable plasma concentrations at which the ability of fentanyl to reduce enflurane MAC could be studied utilizing the tail clamp method and measurement of end-tidal enflurane. Following the determination of control enflurane MAC in each animal, an infusion of fentanyl was begun. Group 1 received continuous successive infusion rates of 0.05, 0.1, and 0.2 micrograms . kg-1 . min-1 with respective loading doses (given over 20 min) of 15, 15, and 30 micrograms/kg; Group 2 received infusions of 0.2, 0.8, and 3.2 micrograms . kg-1 . min-1 with loading doses of 30, 90, and 270 micrograms/kg, respectively. Group 3 was studied in the same manner except that fentanyl was omitted from the infusion solution. Enflurane MAC was determined at each infusion level and blood samples were analyzed for the concentration of fentanyl. Fentanyl concentrations in plasma were proportional to the infusion rate. Enflurane MAC was decreased significantly in proportion to fentanyl plasma concentrations up to 30 ng/ml where a reduction of MAC by 65% was evident. A threefold higher concentration produced a minimal further reduction. In Group 3 dogs, no change in enflurane MAC was seen. It was concluded that predictable, stable levels of fentanyl in plasma can be achieved, that there is a close relationship between the concentration of fentanyl in plasma and its enflurane sparing effect, and that there is a ceiling to this concentration-response relationship.