EIPH is a common respiratory condition of horses that race at top speed. During galloping, massive pressure differentials and tissue shear forces develop in the lung tissue that ruptures small blood vessels. These bleed into the small airways. Blood moves up into the windpipe and in a relatively small number of horses, out the nostrils.

Some horses show mild to severe bleeding at the nostrils after galloping. Many more bleed but do not show obvious blood coming from the nostrils and may not be diagnosed with this problem. EIPH is a common performance limiting problem on the racetrack. Bleeding into the airway reduces racing performance.

Horses are treated routinely with furosemide, a diuretic, in order to reduce the incidence of bleeding. The drug increases urine output, decreases circulating volume of blood, and thus lowers blood pressure. This reduced blood pressure in the lungs is likely what causes the lessening of signs of disease. Reduced bleeding is probably associated with better lung function and improved racing performance.