Naltrexone does not affect isoflurane minimum alveolar concentration in cats

Abstract

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 two weeks later. Results Isoflurane MAC was 2.03 ± 0.12% and was unchanged from baseline following saline or naltrexone administration. Conclusion and clinical relevance MAC 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.

Phenylpiperidine opioid effects on isoflurane minimum alveolar concentration in cats

“…To test whether opioid drug effects on isoflurane MAC were due to actions on opioid receptors, the opioid infusion was continued, and cats were administered 0.6 mg/kg naltrexone through the saphenous vein once hourly until the end of the study. Naltrexone administered at this dose and frequency is sufficient to antagonize remifentanil infusions equipotent to the phenylpiperidines used in the present study (Pypendop, Brosnan, & Ilkiw, 2011) but does not affect isoflurane MAC (Brosnan, Pypendop, Majewski‐Tiedeken, Shilo‐Benjamini, & Ilkiw, 2013). Fifteen minutes after naltrexone administration, isoflurane MAC measurements using the tail‐clamp procedure began.…”

Comparison of an infrared anaesthetic agent analyser (Datex-Ohmeda) with refractometry for measurement of isoflurane, sevoflurane and desflurane concentrations

“…; Brosnan et al. ). Many of these MAC studies impact on clinical standards and become widely accepted textbook information.…”

“…Ideally, such calibration is performed with the use of multiple standard gas mixtures containing anaesthetic agent concentrations overlapping the range of desired measurements (Brosnan et al. ). Unfortunately, this is not always the case (Shaughnessy & Hofmeister ), and many investigators may not calibrate their IR analyser at all, or may use a single calibration gas mixture provided by the manufacturer that usually contains only a low anaesthetic agent concentration.…”