The enflurane sparing effect of morphine, butorphanol, and nalbuphine

Area of Science:

• Anesthesiology
• Pharmacology
• Veterinary Medicine

Background:

• Narcotic analgesics are used to reduce anesthetic requirements.
• Agonist-antagonist drugs like butorphanol and nalbuphine have different pharmacological profiles than pure agonists like morphine.
• Understanding dose-response relationships is crucial for optimizing anesthetic management.

Purpose of the Study:

• To compare the potencies of morphine, butorphanol, and nalbuphine in reducing enflurane minimum alveolar concentration (MAC) in dogs.
• To investigate the dose-response characteristics of these agents as anesthetic supplements.
• To determine if a ceiling effect exists for the anesthetic-sparing properties of butorphanol and nalbuphine.

Main Methods:

• Enflurane MAC was determined in dogs before and after intravenous administration of escalating doses of morphine, butorphanol, nalbuphine, or placebo.
• Dose-response curves were constructed to assess the reduction in MAC.
• Statistical analysis was performed to evaluate the significance of MAC reductions.

Main Results:

• Butorphanol and nalbuphine produced significant reductions in enflurane MAC at low doses (11% and 8%, respectively), with no further decrease at higher doses, indicating a ceiling effect.
• Morphine demonstrated a dose-dependent reduction in enflurane MAC, reaching up to 63% at 5 mg/kg.
• A plateau in morphine's effect was observed at higher doses, but its overall anesthetic-sparing potential was greater than that of the agonist-antagonists.

Conclusions:

• Butorphanol and nalbuphine exhibit a limited anesthetic-sparing effect, reaching a ceiling potency when used as enflurane supplements.
• Morphine possesses a more substantial anesthetic-sparing capacity compared to butorphanol and nalbuphine, although it also has a dose limit.
• These findings have implications for selecting appropriate adjuncts to general anesthesia in clinical practice.