Furosemide premedication of horses 4 h prior to exercise significantly attenuates exercise-induced pulmonary capillary
hypertension which may help diminish the severity of exercise-induced pulmonary haemorrhage. As pulmonary hemodynamic effects of
furosemide may be mediated via a reduction in plasma volume (which is most pronounced 15-30 min postfurosemide administration, with plasma volume recovering thereafter), we hypothesized that administration of
furosemide at intervals shorter than 4 h before exertion may be more effective in attenuating the exercise-induced rise in pulmonary capillary blood pressure. Thus, our objective was to determine whether
furosemide-induced attenuation of exercise-induced pulmonary arterial, capillary and venous
hypertension would be enhanced when the
drug is administered at intervals shorter than 4 h before exercise. Using established techniques, right atrial, and pulmonary arterial, capillary and wedge (venous) pressures were ascertained in seven healthy, sound, exercise-trained Thoroughbred horses in a randomized split-plot experimental design. Measurements were made at rest and during exercise performed at maximal heart rate (217 +/- 3 beats/min) in the control (no medications) experiments and following
furosemide administration (250 mg intravenously (i.v.)) at 1, 2, 3 and 4 h before exercise. Sequence of treatments was randomized and 7 days were allowed between experiments on each horse. Although
furosemide administration in the four treatment groups caused only insignificant changes in the pulmonary arterial, capillary and wedge pressures of standing horses,
furosemide-induced reduction in mean right atrial pressure achieved statistical significance in the 2 h postfurosemide experiments. In the control studies, exercise was attended by statistically significant increments in mean right atrial, as well as pulmonary arterial, capillary and wedge pressures. Although exercise in each of the four
furosemide experiments was also attended by significant increments in right atrial as well as pulmonary vascular pressures, in the 1, 2 and 3 h postfurosemide experiments, mean right atrial pressure increased to a significantly lower value than in the control study. Exercise-induced changes in pulmonary vascular pressures in the 1 h postfurosemide experiments were not different from the pressures in the control study. There was a significant attenuation of exercise-induced pulmonary capillary and venous
hypertension in the 2, 3 and 4 h postfurosemide experiments, but significant differences among these treatments were not found. Thus, these data did not support the contention that administration of
furosemide at intervals shorter than 4 h before exercise is more effective in attenuating exercise-induced pulmonary capillary or venous
hypertension in Thoroughbred horses.