Exercise-Induced Pulmonary Hemorrhage
Basic Information 
Clinical Presentation
Disease Forms/Subtypes
• Blood visualized in trachea with endoscope. This is the most common method of diagnosing EIPH. The severity of EIPH is quantified by scores from 0 to 4.
• Red blood cells (RBCs) seen microscopically in bronchoalveolar lavage (BAL) fluid in the absence of blood visible endoscopically in the trachea
• Epistaxis: Uncommonly observed (<1% of race starts); some association with higher bleeding scores
Etiology and Pathophysiology
• Horses undergoing strenuous exercise have very high pulmonary vascular and left atrial pressures. This predisposes the smaller vessels in the lungs to damage.
• Stress failure of pulmonary capillaries results when transmural forces exceed the strength of the capillary walls.
• Increased transmural forces can occur from both elevated pulmonary vascular pressures or increased swings in airway pressures during intense exercise.
• When this occurs, protein-rich fluid and then RBCs (hemorrhage) leak into the interstitium and alveoli.
• The blood in the alveoli eventually migrates to the trachea, where it may be seen with endoscopy. This blood can be engulfed and cleared by alveolar macrophages, which are seen on cytologic evaluation of BAL fluid as hemosiderophages. Very rarely, blood is seen at the nose (epistaxis). It may also be coughed up and swallowed.
• The RBCs remaining in the interstitium likely set up an inflammatory response in the lung parenchyma and may be responsible for ongoing damage and fibrosis in the lung.
• The capillaries in the dorsocaudal region of the lung are the most prominently affected, particularly in early episodes of EIPH.
• With time, the vessels in more cranial portions of the lung are affected, especially noted at the periphery of prior lesions. The ventral aspect of the lung is much less commonly involved.
• Other factors, including airway inflammation, limb loading and hoof impact, and cardiac disease, are thought to modify the extent or frequency of hemorrhage.
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