A constant rate infusion (CRI) of medetomidine is used to balance equine inhalation anesthesia, but its cardiovascular side effects are a concern. This experimental crossover study aimed to evaluate the effects of vatinoxan (a peripheral α2-adrenoceptor antagonist) on cardiorespiratory and gastrointestinal function in anesthetized healthy horses. Six horses received medetomidine hydrochloride 7 μg/kg IV alone (MED) or with vatinoxan hydrochloride 140 μg/kg IV (MED + V). Anesthesia was induced with midazolam and ketamine and maintained with isoflurane and medetomidine CRI for 60 min. Heart rate, carotid and pulmonary arterial pressures, central venous pressure, cardiac output and arterial and mixed venous blood gases were measured. Selected cardiopulmonary parameters were calculated. Plasma drug concentrations were determined. Fecal output was measured over 24 h. For statistical comparisons, repeated measures analysis of covariance and paired t-tests were applied.

Heart rate decreased slightly from baseline in the MED group. Arterial blood pressures decreased with both treatments, but significantly more dobutamine was needed to maintain normotension with MED + V (P = 0.018). Cardiac index (CI) and oxygen delivery index (DO2I) decreased significantly more with MED, with the largest difference observed at 20 min: CI was 39 ± 2 and 73 ± 18 (P = 0.009) and DO2I 7.4 ± 1.2 and 15.3 ± 4.8 (P = 0.014) mL/min/kg with MED and MED + V, respectively. Fecal output or plasma concentrations of dexmedetomidine did not differ between the treatments. In conclusion, premedication with vatinoxan induced hypotension, thus its use in anesthetized horses warrants further studies. Even though heart rate and arterial blood pressures remained clinically acceptable with MED, cardiac performance and oxygen delivery were lower than with MED + V.