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.

Overview

• This study evaluated the effects of a continuous infusion of a combination of medetomidine (a sedative) and vatinoxan (an α2-adrenoceptor antagonist) on sedation, heart and respiratory function, and gut activity in standing Thoroughbred horses, comparing it to medetomidine alone and medetomidine with atipamezole.

Background and Objective

• Medetomidine (MED) is a sedative commonly used in horses but can cause cardiovascular and gastrointestinal side effects.
• Vatinoxan (VTX) is a peripherally selective α2-adrenoceptor antagonist that can counteract some peripheral side effects without reversing sedation.
• A combination of MED and VTX exists, but its effects when given as a constant rate infusion (CRI) had not been studied thoroughly.
• The objective was to compare sedation quality, cardiac and respiratory variables, and gastrointestinal (GI) function between three treatments in Thoroughbred horses: MED alone (gM), MED with VTX (gMV), and MED with another antagonist atipamezole (ATIP) (gMA).

Study Design and Methods

• Six healthy Thoroughbred horses participated in a blinded, three-treatment, three-period crossover design.
• Treatments given intravenously included:
• gM: Medetomidine alone (loading dose 7 μg/kg IV, then 5 μg/kg/h CRI for 60 minutes)
• gMV: Medetomidine plus vatinoxan (loading 140 μg/kg IV, then 100 μg/kg/h CRI, combined with MED doses as above)
• gMA: Medetomidine plus atipamezole at the same dose as vatinoxan
• Measured variables during and after infusion:
• Sedation-related metrics: head height (as a measure of sedation depth), ataxia (motor coordination), and response to stimuli
• Cardiorespiratory variables: heart rate (HR), respiratory rate (RR), and incidence of second-degree atrioventricular (AV) block
• Gastrointestinal activity: borborygmi scores (gut sounds) and defecation frequency

Key Findings

• Sedation:
• Compared with medetomidine alone (gM), the combination with vatinoxan (gMV) resulted in a greater head height, meaning less deep sedation, but ataxia (uncoordinated movements) was lower, suggesting better motor control.
• Stimulus response scores did not differ significantly between gMV and gM, indicating that sedation depth was relatively maintained.
• For the medetomidine plus atipamezole group (gMA), head height and stimulus response scores were higher and ataxia scores were lower than in gM, indicating reduced sedation.
• Cardiorespiratory variables:
• Respiratory rate (RR) was lower in both gMV and gMA groups compared with gM, indicating some respiratory depression with the antagonists.
• Heart rate (HR) did not differ significantly among groups in pairwise comparisons.
• Second-degree atrioventricular (AV) block, a conduction abnormality in the heart:
• Occurred in 1 horse with gMV
• Occurred in 5 horses with gM
• Occurred in 6 horses with gMA
• This suggests that vatinoxan reduced the incidence of AV block compared to medetomidine alone and atipamezole.
• Gastrointestinal activity:
• Borborygmi scores, indicative of gut motility, were higher in the gMV group compared to gM at most time points.
• All horses in gMV defecated during the study period (median 2.5 times), whereas none defecated in gM or gMA groups.
• This indicates that vatinoxan preserved or improved GI function compared to the other treatments, which showed suppression of GI activity.

Conclusions

• Using medetomidine in combination with vatinoxan given as a constant rate infusion in standing Thoroughbred horses maintained sedation effectively while improving safety parameters.
• Specifically, this combination was associated with fewer cardiac conduction abnormalities (second-degree AV blocks) and less suppression of gastrointestinal activity compared to medetomidine alone or medetomidine with atipamezole.
• This suggests that the MED-VTX combination could be a superior sedation protocol for horses, balancing sedation quality with improved cardiorespiratory and GI safety.
• However, results are from a small experimental group, so further studies are needed to confirm the findings.