2000 Volume 62 Issue 10 Pages 1025-1032
The cardiovascular effects of medetomidine, detomidine, and xylazine in horses were studied.Fifteen horses, whose right carotid arteries had previously been surgically raised to a subcutaneous position during general anesthesia were used.Five horses each were given the following 8 treatments:an intravenous injection of 4 doses of medetomidine(3, 5, 7.5, and 10 μg/kg), 3 doses of detomidine(10, 20, and 40 μg/kg), and one dose of xylazine(1 mg/kg).Heart rate decreased, but not statistically significant.Atrio−ventricular block was observed following all treatments and prolonged with detomidine.Cardiac index(CI)and stroke volume(SV)were decreased with all treatments.The CI decreased to about 50% of baseline values for 5 min after 7.5 and 10 μg/kg medetomidine and 1 mg/kg xylazine, for 20 min after 20 μg/kg detomidine, and for 50 min after 40 μg/kg detomidine.All treatments produced an initial hypertension within 2 min of drug administration followed by a significant decrease in arterial blood pressure(ABP)in horses administered 3 to 7.5 μg/kg medetomidine and 1 mg/kg xylazine.Hypertension was significantly prolonged in 20 and 40 μg/kg detomidine.The hypotensive phase was not observed in 10 μg/kg medetomidine or detomidine.The changes in ABP were associated with an increase in peripheral vascular resistance.Respiratory rate was decreased for 40 to 120 min in 5, 7.5, and 10 μg/kg medetomidine and detomidine.The partial pressure of arterial oxygen decreased significantly in 10 μg/kg medetomidine and detomidine, while the partial pressure of arterial carbon dioxide did not change significantly.Medetomidine induced dose−dependent cardiovascular depression similar to detomidine.The cardiovascular effects of medetomidine and xylazine were not as prolonged as that of detomidine.