The enflurane-sparing effect of alfentanil in dogs

Some investigators believe that the dog is less sensitive than are humans to the anesthetic/analgesic actions of opioids. The alfentanil plasma concentration [ALF] vs anesthetic effect relationship has been determined for humans undergoing surgery. This study was designed to determine the [ALF] vs anesthetic relationship for alfentanil in the enflurane-anesthetized dog and thereby to provide data by which the [ALF] vs anesthetic effect relationships in the dog and in humans could be compared. Mongrel dogs (n = 10) were anesthetized with enflurane, and enflurane MAC (EMAC) was determined in each dog. After this, each dog received at least three incremental infusions of alfentanil using infusion rates of 0.625, 1.6, 8, 32, or 80 micrograms.kg-1.min-1. EMAC and [ALF] were determined during each infusion rate. There was a linear increase in [ALF] produced by incremental infusions of alfentanil (r = 0.999). Administration of alfentanil produced a dose-dependent reduction of EMAC up to a maximum of 72.5 +/- 3.7% (mean +/- SEM) at 32 micrograms.kg-1.min-1 ([ALF] = 960 +/- 86 ng/ml); a ceiling effect was evident. The degree of EMAC reduction (69%) produced by an infusion rate of 8 micrograms.kg-1.min-1 ([ALF] = 223 +/- 13 ng/ml) was not statistically different from the EMAC reductions produced by infusion rates of 32 (73% reduction at [ALF] = 960 +/- 86 ng/ml) or 80 micrograms.kg-1.min-1 (70% reduction at [ALF] = 2613 +/- 247 ng/ml) (P greater than 0.05). The relative potency of alfentanil was one-seventh to one-tenth that of fentanyl studied under identical conditions.(ABSTRACT TRUNCATED AT 250 WORDS)