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.