Objective: To determine the analgesic, sedative, and cardiopulmonary effects of epidural ketamine in the horse.
Animal population: Six healthy horses (three males and three females) weighing between 350 and 450 kg.
Methods: Three doses of ketamine were selected (0.5, 1, 2 mg/kg). Two months before the beginning of experiments, the carotid artery was exteriorized, and 1 week before experiments began, an epidural catheter was placed percutaneously in all animals with the tip located 12 cm cranially in the midsacrum. One week later, either saline (control) or one of three doses of ketamine was injected epidurally. Each animal received each ketamine dose and saline in random order at 1-week intervals. Ketamine was diluted in saline 0.9% before the experiment, and the volume used was adjusted to horse size and correlated to clinically used volumes. All the animals received a standard noxious stimulus consisting of needle insertion into the skin and deep muscle using a 3-point scale for scoring the response. A second scale was used to score the degree of sedation. The response to a noxious stimulus, the degree of sedation, and arterial blood pressure were assessed at previously determined intervals: before drug and 2, 5, 10, and 15 minutes and every 15 minutes to 210 minutes after ketamine or saline administration. Arterial blood samples were drawn for blood gas analysis before drug and at 15, 30, 60, and 90 minutes.
Results: All the tested doses of ketamine were effective in producing analgesia of the tail, perineum, and upper hindlimb. Total tail and perineal analgesia times were similar depending on dosage (30 minutes for 0.5 and 1.0 mg/kg and 75 minutes for 2.0 mg/kg). A sedative effect of ketamine was also observed in a dose-response manner with a peak effect between 15 and 30 minutes postadministration. No cardiopulmonary effects were observed with any dose of ketamine.
Conclusions: Results indicate that epidurally administered ketamine in the horse produces local spinal and central nervous system effects with analgesia and sedation but minimal cardiopulmonary effects.
Clinical relevance: Further studies are required to determine whether the analgesia is sufficient for surgery.