Effects of clenbuterol hydrochloride on certain respiratory and cardiovascular parameters in horses performing treadmill exercise.

Five standardbred geldings received intravenous clenbuterol hydrochloride and saline in a crossover experiment to evaluate the effects of clenbuterol on certain cardiorespiratory parameters during and after treadmill exercise. The exercise test consisted of four steps. Step 1 at a speed of 76 m per minute, step 2 at 129 m per minute, step 3 at 190 m per minute and step 4 at 236 m per minute. The duration of each step was two minutes, except step 4 which was four minutes. The treadmill was set at a grade of 19 per cent. Before exercise, 30 minutes after clenbuterol or saline administration, arterial and mixed venous blood gases, mean arterial pressure, mean pulmonary artery pressure, telemetric electrocardiogram and respiratory rate were measured. The measurements were repeated immediately before completion of step 2, step 4 and after one minute, 10 minutes and 30 minutes of recovery from exercise. After clenbuterol administration there was a transient fall in mean arterial pressure, which had returned to control values by 90 seconds. There was no change in mean pulmonary artery pressure. During exercise the heart rate was significantly higher at the end of steps 1 and 4 and after 10 minutes recovery when horses given clenbuterol were compared with those given saline. No adverse effects on the electrocardiogram were demonstrated. Effects of clenbuterol on blood gases included an increase in PaO2 and decrease in PaCO2 one minute after exercise, when compared to saline. It was concluded from the parameters examined that there was no evidence that clenbuterol had any significant untoward effect on the circulatory system of exercising horses.