The Enflurane Sparing Effect of Morphine, Butorphanol, and Nalbuphine

Abstract

The potencies of morphine and of the narcotic analgesic agonist-antagonists, butorphanol and nalbuphine, in terms of their ability to decrease enflurane MAC [minimum anesthetic requirement] were studied. Following the determination of control MAC for enflurane in each dog, an i.v. bolus dose of either butorphanol tartrate, nalbuphine hydrochloride, morphine or placebo was administered and enflurane MAC was redetermined. A higher dose of the same drug was then administered and enflurane MAC was redetermined up to a total of 4 doses in each animal. The successive doses for morphine and nalbuphine were 0.5, 1.5, 5.0 and 20.0 mg/kg; for butorphanol, 0.1, 0.3, 1.0 and 4.0 mg/kg; lactated Ringer''s solution was used as a placebo. Both butorphanol and nalbuphine produced significant reductions of enflurane MAC (11 and 8%, respectively) at their lowest doses. No futher reductions were produced by 3- to 40-fold larger doses of either agonist-antagonist. Morphine produced a 17% reduction of enflurane MAC at the lowest dose with progressive decreases of enflurane MAC up to 63% at a dose of 5 mg/kg morphine. A 4-fold increase in the morphine dose did not further decrease MAC. No change in enflurane MAC occurred in the animals given placebo. There is evidently a ceiling to the potency of butorphanol and nalbuphine as anesthetic supplements. There is a limit to the anesthetic sparing effect of morphine, but it is considerably greater than that of the agonist-antagonist narcotic analgesics.