Sacrococcygeal epidural administration of morphine and fentanyl inhorses: antinociceptive and physiological effects

Abstract

Opioids possess antinociceptive properties, and they are considered safe when administered via the epidural route. However, theyare seldom utilized in equine medicine. This study aimed to evaluate the physiological and antinociceptive effects of morphine and fentanyl, either individually or in combination, when administered via sacrococcygeal epidural catheterization in healthy horses. Five horses underwent sacrococcygeal epidural catheterization and received morphine (0.1 mg/kg) [M1], fentanyl (2 μg/kg) [M2], and a combination of morphine (0.15 mg/kg) and fentanyl (1 μg/kg) [M3]. Heart rate (HR), respiratory rate (RR), intestinal motility, rectal temperature (RT), sedation level, behavior, ataxia, and antinociceptive scores were recorded in response to thermals and mechanicals stimuli applied to the ischial and coxal tuberosities, as well as lumbar, tibial, and thoracic regions, at T-20’ (baseline), 15 min, 1 h, 4 h, 12 h, and 20 h. HR and RT remained within normal ranges. RR increased with [M1] (24.00 ± 2.83) and [M2]  (25.20 ± 3.35) at T20h compared to baseline. Intestinal motility was modulated, notably reduced with morphine and its combination up to 1 h post administration, while fentanyl maintained stable bowel movements. Morphine alone or combined with fentanyl induced mild sedation up to 1 h post administration. Ataxia was notable for up to 4 h postadministration, especially with morphine, which extended ataxia up to 12 h. Antinociceptive scores increased notably between T1 and T4, with higher scores observed in the ischiatic, coxal, and lumbar dermatomes under cold stimulation, whereas all dermatomes exhibited increased scores under hot stimulation. Mechanical stimulation elicited antinociception in the coxal and lumbar dermatomes. In conclusion, fentanyl exhibited fewer physiological effects, while morphine demonstrated more potent antinociceptive effects during the initial 12-h period, with the combination showing intermediate analgesia.