Xylazine and Morphine Caudal Epidural Analgesia in Horses: Influence of Volume on Location of Effect

Abstract: Background: Caudal epidural analgesia is an important modality in the treatment of pain in the horse, though the influence of dilution volume on the location of effect requires further elucidation.

Hypothesis/Objectives: Objectives of the study were to describe the influence of dilution on the regional effects of xylazine and morphine epidural analgesia, with the hypothesis that increasing volumes produce different patterns and cranial spread of analgesia as determined by wireless thermal threshold (TT) testing.

Animals: Six university-owned research/teaching horses (two mares, four geldings), aged 6-19 years and weighing 420-560 kg, that were deemed healthy on physical examination and basic lameness evaluation.

Methods: Prospective, randomized, blinded, cross-over experimental study. Following routine placement of a caudal epidural catheter, all animals were subsequently instrumented with a TT testing system at the left and right withers (Location A) cranial (Location B) and caudal (Location C) abdominal area, over the tuber coxae (Location D), and the hind limb dorsal pasterns (Location E). All horses underwent five testing cycles with 0.2mg/kg morphine and 0.2mg/kg xylazine diluted to 20, 35, 50, 75, and 100mL with a randomized order of treatment. TT testing was performed at 2, 4, 6, 8, and 10 hours by a blinded investigator.

Results: Timepoints were compared via paired t-test to the baseline control (p< 0.05). Results are summarized in Table 1.

Conclusions and clinical importance: Increasing volume influences the regional effects of caudal epidural analgesia in horses, though efficacy and reliability of analgesia may be reduced when treating more cranial loci of pain.