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Use of naltrexone to antagonize high doses of remifentanil in cats: a dose-finding study


Bruno H Pypendop, Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, One Shields Avenue, Davis, CA 95616, USA. E-mail:


Objective  To determine the dose of naltrexone necessary to fully antagonize a high dose of remifentanil in cats.

Study design  Prospective experimental study.

Animals  Six healthy adult cats weighing 4.9 ± 0.7 kg.

Methods  In a first phase, remifentanil (200 μg kg−1 followed by 60 μg kg−1 minute−1) was administered intravenously to two cats, causing an increase in locomotor activity. Naltrexone (100 μg kg−1) was then administered intravenously every minute until the increase in locomotor activity had been reversed. In a second phase, six cats were used. Baseline thermal threshold was determined, naltrexone (600 μg kg−1) was administered intravenously and 30 minutes later thermal threshold determination repeated. Remifentanil (200 μg kg−1 followed by 60 μg kg−1 minute−1) was administered intravenously and thermal threshold determination repeated at 60, 120, 180, and/or 240 minutes after naltrexone administration. Thermal threshold determinations were started shortly after the start of the continuous rate infusion (CRI) of remifentanil and this CRI was discontinued immediately after thermal threshold determination. If an increase in thermal threshold was found, naltrexone administration was repeated at decreasing intervals in the next experiment (all cats were not used for all dosing intervals). Experiments were repeated until a naltrexone dosing interval was found that prevented increases in thermal threshold for 4 hours in all six cats.

Results  In the first phase, both cats became severely dysphoric following remifentanil administration. A cumulative naltrexone dose of 300 μg kg−1 was necessary to restore normal behavior in both cats. In the second phase, hourly administration of naltrexone (600 μg kg−1) prevented increases in thermal threshold associated with hourly administration of remifentanil for 4 hours. Less frequent administration did not prevent increases in thermal threshold consistently.

Conclusions  Hourly administration of naltrexone (600 μg kg−1) antagonizes the behavioral and antinociceptive effects of a high dose of remifentanil in cats.

Clinical relevance  Naltrexone may be useful for the treatment of opioid overdose in cats.