Epsilon Open Archive

Abstract

Background: In treatment of nociceptive pain with opioids the full μ-agonists have highest efficacy. Methadone is a synthetic full μ-agonist and also an N-methyl-d-aspartate (NMDA) receptor antagonist used in horses. Opioids might also cause excitement in the horse and respiratory depression especially when used together with other drugs affecting the respiration. The α2-agonists are used in horses for sedation and analgesia both separately and in combination with opioids. A combination of methadone and the α2-agonist detomidine may be useful in pain management but may also cause effects on behaviour and respiratory rate. Objectives: The aims were to investigate the effects on behavior and respiratory rate after treatment with methadone IV or methadone IV in combination with detomidine IM in order to evaluate and possibly diminish adverse effects in the treatment of horses. Materials and methods: The study was randomized, blinded and placebo controlled with cross-over design. Eight Standardbred horses were treated with (1) methadone IV (0.2 mg/kg) in a total volume of 20 mL over 5 min together with placebo (saline) IM or (2) 20 mL methadone IV (0.1 mg/kg) over 5 min together with detomidine IM (0.01 mg/kg) or (3) equivalent volumes saline IV and IM. The effects on behavior and respiratory rate were examined. Results: After methadone administration the horses displayed behavioral changes such as staggering for a short period, head tremors and looking vigilantly around. The behaviors licking, nodding head, picking hay, tail flapping, skin twitching and scraping with front leg were more frequent after treatment with methadone compared to the control. There were no differences in respiratory rate between methadone and saline treatments. After methadone/detomidine administration the horses showed drowsiness, snored, stood with dropped head or head supported by the wall or the crib during the first one to three hours. In addition, a few horses were sweating. The respiratory rate was lowered between one and three hours after administration of the drug combination compared with placebo (P = 0.02). Conclusions: When the horses were treated with methadone they showed drug-related agitation which has been described in horses treated with different opioids. The combination of methadone/detomidine seems to prevent excitement caused by opioid exposure. A lowered respiratory rate is also a known non-wanted effect of opioids but in this study no such adverse effect was observed after solely methadone administration but the combination methadone/detomidine induced decreased respiratory rate. Acknowledgements: Supported by The Swedish-Norwegian Foundation for Equine Research.