The primary classes of drugs used for pain control in horses include the non-steroidal anti-inflammatory drugs (NSAIDS), the alpha-2 agonists, and the opiates, but each of these drug classes has significant adverse effects. Ketamine hydrochloride is a non-competitive antagonist at N-methyl D-aspartate (NMDA) receptors in the spinal cord and has been used in both dogs and humans to provide analgesia. The purpose of this study was to identify a safe administration rate for a continuous infusion of ketamine to awake horses and describe the clinical effects of this infusion. Six horses were administered a continuous infusion of ketamine hydrochloride at a low dose (0.4 mg/kg/hr) and high dose (0.8 mg/kg/hr). Each infusion took place over 6 hours and the horses were monitored during infusion as well as for the 6 hours post-infusion. The following parameters were evaluated: heart rate, blood pressure, respiratory rate, activity level, and analgesic thresholds. Both the low dose and high dose rates were tolerated well by all horses with no signs of excitation or discomfort. Post-infusion heart rate and mean arterial blood pressure were significantly decreased as compared to pre-infusion values (p=0.02 and p=0.02, respectively) A significant increase in analgesic threshold was not identified.
Subanesthetic infusions of ketamine to awake horses appear to be well tolerated and cause only mild changes in clinical parameters. Further research is needed to evaluate the analgesic potential of these infusions.