Pulmonary vascular pressures of strenuously exercising Thoroughbred horses after administration of phenylbutazone and frusemide.

The present study was carried out to examine the effects of phenylbutazone treatment on the pulmonary haemodynamic effects of frusemide in strenuously exercising horses. Using catheter mounted manometers, whose in vivo signals were referenced at the point of the shoulder, heart rate, right atrial, right ventricular and pulmonary vascular pressures were measured in 3 different sets of experiments. Seven Thoroughbreds were subjected to 1) control (no medications), 2) frusemide control and 3) phenylbutazone + frusemide. The experiments were carried out in random order and were separated by 7 days. Measurements were made at rest and during incremental exercise performed on a treadmill set at 3.5% uphill grade. In the frusemide control experiment, horses received frusemide 250 mg i.v., 4 h pre-exercise. In the phenylbutazone + frusemide experiment, horses received 4 i.v. injections of phenylbutazone (4.4 mg/kg bwt) at 12 h intervals. Twenty-four hours after the last phenylbutazone injection, horses received frusemide 250 mg i.v. and exercise was performed 4 h later. This latter regimen mimics prevailing veterinary practice at Illinois racetracks. The highest work intensity (14.2 m/s, 3.5% uphill grade) elicited maximal heart rate of horses. Significant right atrial, as well as pulmonary arterial, capillary and venous hypertension occurred with exertion in all 3 experiments. However, in the frusemide-control and the phenylbutazone + frusemide studies, the exercise induced rise in mean right atrial and pulmonary vascular pressures was significantly (P<0.05) attenuated in comparison with that in the control experiments. Statistically significant differences were not found between the frusemide control study and the phenylbutazone + frusemide study either at rest or during any level of exertion. Therefore, it was concluded that the phenylbutazone treatment in our study did not mitigate the pulmonary haemodynamic effects of frusemide in strenuously exercising Thoroughbred horses.