A pharmacokinetic/clinical approach to postulate a local action of intra-articular xylazine administration in the horse: a preliminary study

The study aims to evaluate whether the analgesic effect of intra-articular (IA) route of xylazine administered to horses following arthroscopic surgery is due to a local or a systemic action. Two connected studies were performed. In the first, 1mg/kg b.w. of xylazine was injected IA, and blood samples were taken to assess drug systemic absorption. In addition, systemic effects of the drug (sedation, ataxia or reduction of respiratory and cardiac rate) were registered. Control horses injected with saline IA were included in the study to exclude the influence of anaesthesia in the occurrence of these manifestations. In the second study, 1mg/kg b.w. of xylazine was administered intravenously (i.v.) in healthy horses. Blood samples were collected to determine the concentrations of xylazine, and the same signs of systemic effects of the drug were recorded. By correlating these parameters, a systemic no effect' concentration was defined. Pharmacokinetic data after IA administration resulted in some xylazine absorption (bioavailability equal to 58.12%) with values above the systemic no effect' concentration. The occurrence of some signs related to systemic effects in horses receiving IA xylazine was significant compared with horses receiving saline. In conclusion, a systemic action of the drug after IA administration cannot be excluded.