Antagonistic Activities of Atipamezole, 4-aminopyridine and Yohimbine Against Medetomidine Ketamine-induced Anesthesia in Cats

The objectives of this trial were to determine the ability of atipamezole, 4-aminopyridine and yohimbine to reverse the anaesthetic effects of a combination of medetomidine and ketamine in cats. Forty healthy cats were anaesthetised with 80 mu-g/kg medetomidine combined with 5 mg/kg ketamine. Thirty minutes later atipamezole (200 or 500 mu-g/kg), 4-aminopyridine (500 or 1000 mu-g/kg) or yohimbine (250 or 500 mu-g/kg) were injected intramuscularly. The doses of antagonists were randomised, so that each dose was administered to five cats, and 10 cats were injected only with physiological saline. Atipamezole clearly reversed the anaesthesia and bradycardia induced by medetomidine and ketamine. The mean (+/- sd) arousal times were 28 (+/- 4.7), 5.8 (+/- 1.8) and 7 (+/- 2.1) minutes in the placebo group, and the groups receiving 200 and 500 mu-g/kg atipamezole, respectively. The heart rates of the cats receiving 200 mu-g/kg atipamezole rapidly returned to values close to the initial ones, but 15 minutes after the injection of 500 mu-g/kg atipamezole a significant tachycardia was observed. All the cats showed moderate signs of ataxia during the recovery period. A dose of 500 mu-g/kg yohimbine also clearly reversed the anaesthetic effects of medetomidine/ketamine but 250 mu-g/kg was not effective. The dose of 500 mu-g/kg allowed a smooth recovery with no particular side effects except for some signs of incomplete antagonism of the ketamine effects, ie, ataxia and muscular incordination. With 4-aminopyridine there were no statistically significant effects on the recovery, or the heart and respiratory rates of the cats anaesthetised with medetomidine/ketamine.