Cardiopulmonary effects of a two hour medetomidine infusion and its antagonism by atipamezole in horses and ponies

The cardiopulmonary effects of an intravenous (iv) medetomidine injection (5 μg/kg) followed 5 min later by its infusion at 3.5 μg/kg/h for 115 rnin were studied in 9 horses and ponies. Five minutes after the end of infusion 60 μg/kg atipamezole were given. Physiological data during infusion were compared with pre-sedation values. Stroke volume was reduced significantly 5 min after initial medetomidine injection. Cardiac index was reduced significantly and systemic vascular resistance increased significantly for the first 20 min, but returned towards pre-sedation values after this time. Arterial blood pressures were reduced significantly from 30 min until the end of the procedure (minimum MAP was 102.4 ± 9.61 mmHg). Mixed venous oxygen tension was reduced significantly during the infusion. Respiratory rate fell and PaCO₂- rose significantly from 40 min onward. Other variables showed no significant changes. The horses recovered rapidly after atipamezole was injected. Arterial blood pressures remained significantly lowered, but other cardiovascular variables returned towards pre-sedation values. It is concluded that the infusion of medetomidine at 3.5 μg/kg/h causes minimum cardiopulmonary depression once the effects of an initial 5 μg/kg injection have waned, and so could prove suitable as part of an anaesthetic technique in equidae.