The enflurane sparing effect of sufentanil in dogs

There is a ceiling to the reduction of enflurane MAC by fentanyl in the dog. Sufentanil (SUF), a more potent narcotic, may be more efficacious in reducing enflurane MAC. To test this hypothesis, 25 mongrel dogs were studied in three groups. Group 1 (n=8) received SUF in progressively increasing infusion rates from 0.005 μg.kg^-1.min^-1 to a maximum of 1.215 μg.kg^-1.min^-1. MAC was determined at stable SUF concentrations in plasma [SUF] during each infusion rate. Group 2 (n=10) received SUF at a dose rat (0.007 μg.kg^-1.min^-1) designed to produce approximately 35% MAC reduction, and MAC determinations were made at regular intervals over a mean infusion time of 7.6 ± 0.43 h (mean ± SEM). Group 3 (n=7) received 1.215 μg.kg^-1.min^-1 and were studied as in group 2 over an infusion time of 6.7 ± 0.42 h. In group 1, the highest infusion rate (1.215 μg.kg^-1.min^-1) produced [SUF]=48 ng/ml and reduced MAC by 71 ± 6%. This was not statistically different from the reduction which occurred at [SUF]=0.92 ng/ml (57 ± 7%; infusion rate 0.015 μg.kg^-1.min^-1; P=0.21). In group 2, the degree of MAC reduction achieved by stable [SUF] (0.54 ±0.08 ng/ml) declined over time (MAC reduction at start = 34 ± 2% versus 18 ± 4.0% at the end of infusion; P=0.001), suggesting the development of tolerance. In group 3 (1.215 μg.kg^-1.min^-1), there was no statistically significant difference demonstrated between the degree of MAC reduction at the beginning versus the end of the infusion. The maximum reduction achieved by an infusion of 1.215 μg.kg^-1.min^-1 (group 3) for nearly 7 h did not differ from that achieved with a series of progressively increasing infusion rates up to 1.215 μg.kg^-1.min^-1 (group 1-70.5% reduction at [SUF]=48 versus group 3-78% at 41 ng/ml). The authors conclude that there was a ceiling of approximately 70% reduction of enflurane MAC by SUF in the dog. This reduction was independent of the dose-rate at which it was achieved. At plasma concentrations below those producing a maximum reduction of MAC, acute tolerance was evident.