Effects of constant rate infusion of a fixed-dose combination of medetomidine and vatinoxan on sedation, cardiorespiratory variables, and gastrointestinal activity in standing Thoroughbred horses.

BACKGROUND: A fixed-dose combination of medetomidine (MED) and vatinoxan (VTX), a peripherally selective α2-adrenoceptor antagonist, is commercially available, but its constant rate infusion (CRI) has not been evaluated. AIMS/OBJECTIVES: To compare the sedative effects, cardiorespiratory variables, and gastrointestinal activity of MED-VTX and MED-atipamezole (ATIP) with MED alone in Thoroughbred horses. METHODS: Six healthy Thoroughbreds underwent a blinded three-treatment, three-period crossover study. Horses received MED (7 μg/kg IV; 5 μg/kg/h CRI for 60 min) alone (gM), with VTX (140 μg/kg IV; 100 μg/kg/h CRI; gMV), or with ATIP at the same dose (gMA). Sedation-related variables, heart rate (HR), respiratory rate (RR), second-degree atrioventricular block, borborygmi scores, and defecation were assessed. RESULTS: Compared with gM, gMV had greater head height and lower ataxia scores, whereas stimulus response scores did not differ. gMA had greater head height and stimulus response scores and lower ataxia scores than gM, consistent with reduced sedation. RR was lower in both gMV and gMA than in gM, whereas HR did not differ in prespecified pairwise comparisons. Second-degree atrioventricular block occurred in one, five, and six horses in gMV, gM, and gMA, respectively. Borborygmi scores were higher in gMV than in gM at most time points, and all horses defecated in gMV (median: 2.5 times, range: 1-4 times), whereas none did in gM or gMA. CONCLUSION: In Thoroughbred horses, MED-VTX was associated with maintained sedation, fewer observed second-degree atrioventricular blocks and less suppression of gastrointestinal activity in this small experimental cohort.