We hypothesised that frusemide would decrease pulmonary capillary pressure in horses during strenuous exercise. Seven horses were tested after receiving saline or frusemide (2 mg/kg bwt) in random order with an interval of at least one week. Measurements were made with the horses standing, exercising at 75, 90 and 100% HRmax (maximal heart rate), and then walking 2 min after cessation of 100% HRmax. The exercise tests lasted for ˜3 min with an interval of walking between them. Pulmonary artery and oesophageal pressures were recorded continuously and subsequent analysis of the pulmonary artery pressure signal was carried out after subtraction of the oesophageal pressure signal. Pulmonary arterial pressure, pulmonary capillary pressure, pulmonary artery wedge pressure, breathing rate, heart rate and arterial blood gas tensions were recorded at each level of exercise. Pulmonary arterial wedge and pulmonary capillary pressures were determined from the pulmonary arterial waveform after dynamic occlusion of a branch of the pulmonary artery. The resulting decay in pressure was submitted to exponential curve fitting and the amplitude on this curve at the moment of occlusion was recorded as pulmonary capillary pressure. When adjusted for horse and exercise intensity, horses receiving frusemide had lower pulmonary capillary and wedge pressures (adjusted least-squares means=36 mmHg and 28 mmHg, respectively) when compared with control values (adjusted least-squares means=41 mmHg (P=0.042) and 35 mmHg (P = 0.002), respectively). Pulmonary arterial pressure, breathing rate, heart rate and arterial blood gas tensions did not differ between treatments at any exercise intensity.
We conclude that frusemide reduces pulmonary capillary and wedge pressures. This is compatible with reduced transcapillary filtration and, therefore, reduced accumulation of lung water at exercise. It may also account for the putative protective effect of frusemide against exercise-induced pulmonary haemorrhage.