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±0.7kg. Methods In a first phase, remifentanil (200μgkg -1 followed by 60μgkg -1minute -1) was administered intravenously to two cats, causing an increase in locomotor activity. Naltrexone (100μgkg -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 (600μgkg -1) was administered intravenously and 30minutes later thermal threshold determination repeated. Remifentanil (200μgkg -1 followed by 60μgkg -1minute -1) was administered intravenously and thermal threshold determination repeated at 60, 120, 180, and/or 240minutes 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 4hours in all six cats. Results In the first phase, both cats became severely dysphoric following remifentanil administration. A cumulative naltrexone dose of 300μgkg -1 was necessary to restore normal behavior in both cats. In the second phase, hourly administration of naltrexone (600μgkg -1) prevented increases in thermal threshold associated with hourly administration of remifentanil for 4hours. Less frequent administration did not prevent increases in thermal threshold consistently. Conclusions Hourly administration of naltrexone (600μgkg -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.
ASJC Scopus subject areas