Aerosol pirbuterol: bronchodilator activity and side effects in ponies with recurrent airway obstruction (heaves).

The dose of aerosol pirbuterol that could be administered safely to ponies (weight approximately 200 kg) was determined by observation for sweating, trembling and excitement and measurement of heart and respiratory rates during cumulative administration of the drug. Sweating, trembling and excitement were first observed following a dose of 2,400 micrograms and became more severe at 3,200 micrograms. These effects were accompanied by an increase in heart rate but not a change in respiratory rate. When 3200 micrograms was administered without prior administration of lower doses, side effects were trivial. This dose was therefore tested for its bronchodilator activity. Pulmonary function was evaluated in ponies that developed airway obstruction ('heaves') when housed in a barn and fed hay. Measurements were made when ponies were in clinical remission (Period A) and during an acute attack of airway obstruction (Period B). At Period A, pirbuterol had no effect on pulmonary function. Barn housing increased pulmonary resistance and decreased dynamic compliance. At Measurement Period B, pirbuterol administration significantly reduced pulmonary resistance and increased dynamic compliance and minute ventilation. These changes were significant 5 min after drug administration and lasted for the 30 min duration of the study. Vehicle administration had no effect on pulmonary function. It was concluded that aerosol pirbuterol has few undesirable side effects and is an effective bronchodilator in ponies with recurrent airway obstruction.