Effect of Fentanyl on Visceral and Somatic Nociception in Conscious Horses

Background : Transdermal fentanyl is used clinically in horses based on pharmacokinetic data and antinociceptive effects documented in other species. Hypothesis : Fentanyl IV administration increases both visceral and somatic nociceptive threshold in conscious horses. Animals : Six clinically normal horses, each fitted with a permanent gastric cannula. Methods : Visceral nociception was evaluated with 2 methods of threshold detection—colorectal distention and duodenal distention. Somatic nociception was assessed by measurement of thermal threshold. Fentanyl was administered as an increasing stepwise infusion followed by a continuous‐rate infusion for a total of 2 hours. There were 4 doses of fentanyl and 1 dose each of saline and xylazine administered to each horse. Serum fentanyl concentrations were measured and the resulting data were used to determine pharmacokinetic parameters for each horse. All data were analyzed by means of a 3‐factor analysis of variance followed by either a simple t test or a Bonferroni t test for multiple comparisons. Results : Fentanyl administration did not result in significant changes in duodenal or colorectal distention threshold. Thermal threshold showed an increased trend at the 15‐minute time point for the highest fentanyl group only, with a corresponding mean serum fentanyl concentration of 7.82 ± 2.10 ng/mL. Two horses in this group became agitated and tachycardic during the first 15 minutes of the infusion. Conclusions and Clinical Importance : Fentanyl did not produce a significant antinociceptive effect at the doses used, 2 of which resulted in serum concentrations above the nociceptive threshold in other species.