Reduction of isoflurane anesthetic requirement by medetomidine and its restoration by atipamezole in dogs.

The isoflurane-sparing effect of the alpha 2-adrenergic agonist medetomidine (30 micrograms/kg of body weight, IV) was tested in 7 dogs, using a blinded, randomized-block study design. The baseline minimal alveolar concentration (MAC) of isoflurane was 1.18 vol% (95% confidence interval [0.97,1.39]). Medetomidine significantly (P < 0.003) reduced isoflurane MAC by 47.2%. Atipamezole (0.3 mg/kg, IV), an alpha 2-adrenergic antagonist, completely reversed the effect of medetomidine on isoflurane MAC. Atipamezole alone did not significantly alter isoflurane MAC. After medetomidine administration, marked bradycardia developed in all dogs and persisted for more than 2 hours. Mean arterial blood pressure increased acutely, but later decreased, and hypotension persisted for more than 2 hours. Atipamezole reversed the bradycardic and hypotensive effects of medetomidine. Results of this study indicate that medetomidine may be useful in clinical cases in which isoflurane MAC-reduction is desirable and that atipamezole might be used to reverse desirable and undesirable effects of medetomidine during isoflurane anesthesia.