Effects of epidural morphine on gastrointestinal transit in unmedicated horses

Objective To evaluate the effect of epidural morphine on gastrointestinal (GI) motility in horses. Study design Randomly ordered crossover design. Animals Six healthy adult horses weighing 585 ± 48 kg (mean ± SD). Methods Horses were randomly assigned to receive either 0.2 mg kg⁻¹ morphine or an equal volume (0.04 mL kg⁻¹) of saline epidurally (the first inter coccygeal space) with 2 weeks between treatments. The horses were stabled, fed a standardized diet and allowed water ad libitum throughout the duration of the study. Radiopaque spheres were administered by stomach tube. Xylazine 0.2 mg kg⁻¹ intravenously was administered prior to epidural injection. Heart rate, respiratory rate, GI sounds score and behavior score were recorded before drug administration and after epidural injection at 4, 8, 12, 18, 24 hours and every 12 hours thereafter for 6 days. Feces were weighed, radiographed and the number of spheres counted. Data were analyzed using a mixed effect model. Results At no time did horses exhibit signs of colic or show significant differences between treatments regarding heart rate, respiratory rate, GI sounds score, behavior score, or cumulative number of spheres. The concentration of spheres per kg of feces was significantly lower (p < 0.05) for the morphine group at 18 and 24 hours. Using the centroid of the curves (spheres kg⁻¹ plotted versus time) the average transit time after saline epidural was 38 hours and after morphine it was 43 hours. The weight of feces hour⁻¹ was significantly lower (p < 0.05) at only 4 and 8 hours after morphine. Conclusions and clinical relevance Epidural morphine, at a dose of 0.2 mg kg⁻¹, temporarily reduced GI motility but did not cause ileus or colic in this small group of healthy unfasted horses. Care should be taken when extrapolating these data to situations in which other factors may also affect GI motility.