Sedative and antinociceptive effects of different detomidine constant rate infusions, with or without methadone in standing horses

Summary Background Standing surgery avoids the risks of general anaesthesia in horses. Objectives To assess sedation, antinociception and gastrointestinal motility in standing horses after a detomidine loading dose and 2‐h constant rate intravenous (i.v.) infusion, with or without methadone. Study design Blinded, randomised, crossover with seven healthy adult cross‐bred horses, three geldings and four females (404 ± 22 kg). Methods Five i.v. treatments were administered to all horses with 1‐week washout period: saline (SAL), detomidine low (2.5 μg/kg bwt + 6.25 μg/kg bwt/h) (DL) and high doses (5 μg/kg bwt + 12.5 μg/kg bwt/h) (DH) alone or combined with methadone (0.2 mg/kg bwt + 0.05 mg/kg bwt/h), (DLM) and (DHM), respectively. Height of head above the ground (HHAG), electrical (ET), thermal (TT) and mechanical (MT) nociceptive thresholds and gastrointestinal motility were evaluated at predetermined times between 5 and 240 min. A mixed effect model and Kruskal–Wallis test were used to analyse normally and non‐normally distributed data, respectively. Results Sedation (