Exercise Induced Pulmonary Hemorrhage in Thoroughbred Horses is a Primary Cardiac Problem

The equine cardiovascular and pulmonary systems are critical to performance outcome in thoroughbred horse racing. Exercise induced pulmonary hemorrhage is a well recognized problem in these equine athletes. High level exercise performance in these horses can produce capillary hemorrhage which can have both short and long term sequelae for the thoroughbreds who have this issue. Previous research has focused on this being a primary lung issue, thought to be due to pulmonary hypertension from high cardiac output through the pulmonary vascular bed during maximal exercise. Given that this occurs in other mammalian species such as human endurance athletes and triathletes and canine athletes it seems unlikely that there is a different pathophysiologic mechanism of injury in equine athletes than in other mammalian species. In the setting of congenital cardiac surgery and congenital heart disease, supra systemic pulmonary artery pressures can be sustained for extended periods of time, even many months without any evidence of pulmonary hemorrhage, hence the idea that pulmonary hypertension is the cause is unlikely to be correct, essentially eliminating the inflow side of the equation. Pulmonary outflow problems in the operating room due to high left atrial pressures can, and most often do cause acute pulmonary hemorrhage. Increases in left atrial pressure can be caused by anatomic abnormalities but they can also because by left ventricular diastolic dysfunction. Extreme exercise training can result in left ventricular hypertrophy with outstanding systolic function but a decrease in diastolic function, especially at high heart rates. In conclusion, we believe that exercise induced pulmonary hemorrhage in equine athletes is due to left ventricular diastolic dysfunction at high heart rates raising left atrial pressure causing a physiologic pulmonary outflow obstruction that results in the pathologic bleeding. Exercise induced pulmonary hemorrhage in thoroughbred race horses a primary cardiac problem.