Evaluation of xylazine as a sedative and preanesthetic agent in horses

Xylazine administered intramuscularly (IM) to horses at the dose level of 2 mg/kg was an effective sedative and preanesthetic for thiamylal sodium narcosis or thiamylal sodium and halothane anesthesia. Evaluation of response of cardiovascular, respiratory, and hepatic function did not indicate serious untoward effects, although cardiac and respiratory rate decreased, calculated vigor of left ventricular contraction decreased, calculated peripheral vascular resistance increased, and transient innocuous cardiac arrhythmias occurred. Effects of the anesthetics used on respiratory function (blood gases and pH), using xylazine as a preanesthetic, were comparable with those observed when promazine was used. The onset of action of xylazine given IM was at least as rapid as that occurring when promazine was given intravenously; e.g., 5 minutes for first observable effects, and 15 to 20 minutes for maximal effect. Recovery, times from anesthesia when using xylazine administered IM as a preanesthetic agent were comparable with those reported after promazine was given intravenously; moreover, horses given xylazine were more calm during recovery and seldom tried to stand before they were able.