Cardiovascular effects of detomidine, a new alpha 2-adrenoceptor agonist, in the conscious pony

The cardiovascular effects of detomidine and xylazine were compared in six chronically instrumented, conscious ponies. Ponies were instrumented with a micromanometer in the left ventricular chamber, a Doppler flow probe on a coronary artery and sonomicrometer crystals in the left ventricular free wall. Heart rate, ventricular systolic pressure, stroke work, dP/dtmax, minute work and coronary blood flow were measured for 4 h following intravenous injection of detomidine at several doses or xylazine at 1.1 mg/kg. Both drugs caused a profound hypertensive response at 15 s post-injection. The magnitude of the pressure change did not increase with detomidine doses greater than 20 micrograms/kg. There was a dose-dependent effect on the duration of the hypertension. Bradycardia and A-V blockade of similar magnitude followed the hypertension at all drug doses. Both drugs caused a negative inotropic effect on the heart at all doses. Minute work, a mechanical index of myocardial O2 demand, and coronary flow decreased to a similar extent following all drug treatments. With the exception of a greater hypertensive response, detomidine at the dosages studied, produced cardiovascular effects that were very similar to those of the recommended dosage of xylazine.