PULMONARY VASCULAR PRESSURES OF STRENUOUSLY EXERCISING THOROUGHBREDS AFTER ADMINISTRATION OF VARYING DOSES OF FRUSEMIDE

The frusemide dose-response for attenuation of exercise induced pulmon ary capillary hypertension was studied in 7 healthy, exercise-conditio ned Thoroughbred horses using previously described haemodynamic proced ures. Four different doses of frusemide were tested: 250 mg regardless of bodyweight (amounting to 0.56 +/- 0.03 mg/kg bwt), 1.0 mg/kg bwt, 1.5 mg/kg bwt and 2.0 mg/kg bwt, Frusemide was administered i.v., 4 h before exercise, Haemodynamic data were obtained at rest and during tr eadmill exercise performed at 14.2 m/s on a 3.5% uphill grade; this wo rkload elicited maximal heart rate of horses, Airway endoscopy was per formed post exercise to detect exercise-induced pulmonary haemorrhage (EIPH), In standing horses, frusemide administration resulted in a sig nificant (P < 0.05) decrease in mean pulmonary arterial, pulmonary cap illary and pulmonary artery wedge pressures, but significant differenc es among the various frusemide doses were not observed, In the control experiments, exercise caused significant increments in the right atri al as well as pulmonary arterial, wedge, and capillary pressures, and all horses experienced EIPH. Following frusemide administration, the e xercise-induced rise in right atrial and pulmonary vascular pressures was significantly attenuated, but significant differences between the frusemide doses of 250 mg, 1.0 mg/kg, and 1.5 mg/kg were not discerned and all horses remained positive for EIPH, Although a further signifi cant (P < 0.05) attenuation of the exercise-induced rise in pulmonary capillary blood pressure occurred when frusemide dose increased from 2 50 mg to 2.0 mg/kg bwt, all horses still experienced EIPH. It is concl uded that a linear response to increasing frusemide dosage in terms of attenuation of the pulmonary capillary hypertension does not exist in strenuously exercising Thoroughbred horses.