Objective: To test whether naltrexone, an opioid receptor antagonist, affects the minimum alveolar concentration (MAC) of isoflurane in cats, a species that is relatively resistant to the general anesthetic sparing effects of most opioids. Study design: Randomized, crossover, placebo-controlled, blinded experimental design. Animals: Six healthy adult cats weighing 4.9 ± 0.7 kg. Methods: The cats were studied twice. In the first study, baseline isoflurane MAC was measured in duplicate. The drug (saline control or 0.6 mg kg-1 naltrexone) was administered IV every 40-60 minutes, and isoflurane MAC was re-measured. In the second study, cats received the second drug treatment using identical methods 2 weeks later. Results: Isoflurane MAC was 2.03 ± 0.12% and was unchanged from baseline following saline or naltrexone administration. Conclusion and clinical relevance: Minimum alveolar concentration was unaffected by naltrexone. Because MAC in cats is unaffected by at least some mu-opioid agonists and antagonists, spinal neurons that are directly modulated by mu-opioid receptors in this species cannot be the neuroanatomic sites responsible for immobility from inhaled anesthetics.
- Minimum alveolar concentration
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