Pulmonary haemodynamics in the exercising horse and their relationship to exercise-induced pulmonary haemorrhage.

Exercise-induced pulmonary haemorrhage (EIPH) is a common occurrence in race horses. Although blood in cases of EIPH has been suspected to originate from the bronchial circulation, which receives approximately 1% of the left ventricular output, physiological evidence has recently emerged to indicate that the pulmonary circulation, which receives the entire output of the right ventricle, is a more likely source. High transmural pulmonary capillary pressures have been shown to cause breaks in the capillary endothelium, basement membrane as well as in the alveolar epithelium. Blood constituents escape into the interstitium and alveoli through such breaks in the blood-gas barrier--a phenomenon referred to as stress failure of pulmonary capillaries. Concomitant measurement of pulmonary arterial and venous pressures in strenuously exercising horses have revealed that both of these variables increased dramatically such that the intravascular pulmonary capillary pressure during exertion at 14 m/s (heart rate of 214 beats/min) approached 105 cm H2O (79 mmHg). Alveolar pressure during peak inhalation is likely to be negative; therefore, it is probable that transmural (intravascular minus perivascular) pulmonary capillary pressure of maximally exercising horses may be greater than 105 cm of water. Thus, the pulmonary blood-gas barrier, which has to be thin to provide for adequate diffusion of O2, is exposed to very high transmural forces associated with high cardiac output during exercise. Recent evidence suggests that the alveolar-capillary membrane may not be able to withstand the high transmural forces during maximal exertion, and that stress failure of pulmonary capillaries occurs, leading to EIPH. Intravenous furosemide premedication 4 h before exercise attenuates the exercise-induced rise in pulmonary arterial, capillary and venous pressures and, therefore, may be efficacious in reducing or limiting the extent of EIPH in race horses.