The Enflurane-Sparing Effect of Alfentanil in Dogs

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Abstract
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 μg·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 μg·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 μg·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 80μg·kg−1·min−1 (70% reduction at [ALF] = 2613 ± 247 ng/ml) (P > 0.05). The relative potency of alfentanil was one-seventh to one-tenth that of fentanyl studied under identical conditions. The [ALF] producing maximal EMAC reduction in the enflurane anesthetized dog in response to a tail-clamp stimulus (223 ng/ml) was similar to the [ALF] required to provide adequate anesthesia of patients for skin incision in the presence of nitrous oxide (mean plasma concentration required to prevent response to skin incision, 279 ± 20 ng/ml; 95% confidence interval, 238–320 ng/ml). It is concluded that alfentanil, in common with other μ-receptor agonists, is capable of reducing enflurane MAC by only 70% in dogs. The anesthetic efficacy of alfentanil in humans and in the dog appears to be similar.