Cardiovascular effects of medetomidine, detomidine and xylazine in horses

The cardiovascular effects of medetomidine, detomidine, and xylazine in hor ses were studied. Fifteen horses, whose right carotid arteries had previous ly been surgically raised to a subcutaneous position during general anesthe sia were used. Five horses each were given the following 8 treatments: an i ntravenous injection of 4 doses of medetomidine (3, 5, 7.5, and 10 mug/kg), 3 doses of detomidine (10, 20, and 40 mug/kg), and one dose of xylazine (1 mg/kg). Heart rate decreased, but not statistically significant. Atrio-ven tricular block was observed following all treatments and prolonged with det omidine. Cardiac index (CI) and stroke volume (SV) were decreased with all treatments. The CI decreased to about 50% of baseline values for 5 min afte r 7.5 and 10 mug/kg medetomidine and I mg/kg xylazine, for 20 min after 20 mug/kg detomidine, and for 50 min after 40 mug/kg detomidine. All treatment s produced an initial hypertension within 2 min of drug administration foll owed by a significant decrease in arterial blood pressure (ABP) in horses a dministered 3 to 7.5 mug/kg medetomidine and 1 mg/kg xylazine. Hypertension was significantly prolonged in 20 and 40 mug/kg detomidine. The hypotensiv e phase was not observed in 10 mug/kg medetomidine or detomidine. The chang es in ABP were associated with an increase in peripheral vascular resistanc e. Respiratory rate was decreased for 40 to 120 min in 5, 7.5, and 10 mug/k g medetomidine and detomidine. The partial pressure of arterial oxygen decr eased significantly in 10 mug/kg medetomidine and detomidine, while the par tial pressure of arterial carbon dioxide did not change significantly. Mede tomidine induced dose-dependent cardiovascular depression similar to detomi dine. The cardiovascular effects of medetomidine and xylazine were not as p rolonged as that of detomidine.