The equine respiratory system can be a major cause of poor performance or premature retirement from competition. Let’s look at some of the key factors:
The Nose: Horses are "obligate nasal breathers" which means that during intensive exercise the nose is the only way air reaches the lungs. A significant portion of the horse’s nasal passage is unsupported by bone or cartilage. This portion of the nasal passage collapses in all horses when breathing-in during exercise, reducing the size of the airway and greatly increasing resistance to airflow.
At rest, more than 50% of resistance to air flow comes from the nasal passages. This percentage increases to 80% during exercise. In addition, pathological upper airway obstructions (haryngeal hemiplegia, or roaring, for example) and functional obstructions (such as significant head flexion) create additional increases in the work of breathing. This makes it more difficult to move air to the lungs.
The Lungs: Resistance to airflow and high pulmonary blood pressure are known to cause lung bleeding. The lung tissue that separates the airways from the blood vessels is extremely thin–about 1/100 the thickness of a human hair. This ultra-thin membrane makes for efficient oxygen and carbon dioxide transfer, however, it also makes the membrane very fragile and it can rupture when exposed to high blood pressures and the enormous suction-like airway pressures generated during intensive exercise. This is referred to as exercise induced pulmonary hemorrhage (EIPH).
Numerous studies show that essentially all exercising horses experience some degree of