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Naltrexone does not affect isoflurane minimum alveolar concentration in cats.

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.