The epidural administration of hydromorphone in horses produces fast onset of action and moderate analgesia up to 4 hours. No major adverse reaction is expected when epidural hydromorphone is administered in horses undergoing general anesthesia with isoflurane. For a little over a century now, epidural anesthesia and analgesia have been used in both human and veterinary medicine. It has been most beneficial during surgery as both an anesthetic tool and as a valuable instrument for pain management. The epidural administration of a drug in horses is achieved by placing a 5- to 7.5-cm, 18-gauge spinal needle with stylet into the epidural space between the first and second coccygeal vertebrae, using strict aseptic technique. Epidural analgesia is obtained by injecting opioids (i.e., morphine, buprenorphine, hydromorphone), alpha-2-adrenergic agonists (i.e., xylazine, detomidine), and/or dissociative agents (i.e. ketamine) into the epidural space. Analgesics such as opioid agonists must reach the subarachnoid space and bind to the appropriate receptors, e.g., kappa-opioid receptors, for the desired effect to occur.
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