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

This research investigates the occurrence of Exercise Induced Pulmonary Haemorrhage (EIPH), a common problem in race horses. The study suggests that high pressure in pulmonary capillaries during vigorous exercise causes them to rupture, leading to EIPH, and indicates that furosemide premedication could help manage this condition by reducing exercise-induced pressure in the lungs.

Pulmonary Hemodynamics in Exercising Horses

• Exercise-induced pulmonary haemorrhage (EIPH) is a common issue in race horses. Traditionally, the source of this bleeding was thought to be the bronchial circulation which receives around 1% of the horse’s cardiac output. The current study, however, identifies the pulmonary circulation that accommodates the entire output from the right ventricle as the more probable source.
• The researchers theorise that the high transmural (across the wall) pressure in the capillaries in horse’s lungs during intense exercise causes damage to blood vessels and allows blood components to escape in surrounding tissues. This occurrence, mainly referring to the disruption of the blood-gas barrier in pulmonary capillaries, is known as stress failure.
• In line with this theory, measurements captured in the study show significant rises in both arterial and venous pressures in the lungs of horses during strenuous exercise, to a point that pulmonary capillary pressure can reach 105 cm H2O (79 mmHg).
• The study also suggests that due to negative forces during peak inhalation, the actual transmural capillary pressure in the lungs of exercising horses could even be greater than 105 cm of water.

High Transmural Forces and the Blood-Gas Barrier

• The researchers identify the blood-gas barrier in the lungs as being a probable point of failure during maximum exertion in horses. This barrier needs to be thin to allow sufficient diffusion of oxygen. However, during high-intensity exercise, this thin barrier experiences high transmural forces, resulting from elevated cardiac output, which it may not be able to withstand.
• Accordingly, the paper points to stress failure of pulmonary capillaries as a likely cause of EIPH, where high transmural forces may cause ruptures in the blood-gas barrier, allowing blood constituents to escape into adjacent tissues.”

Potential Medication Intervention

• The research also indicates potential intervention methods. An intravenous dose of furosemide administered 4 hours before exercise appears to mitigate the rise in pulmonary arterial, capillary, and venous pressures during exercise.
• Researchers suggest that this could be an effective way to reduce or control the extent of EIPH in race horses, as lower lung pressure may decrease the likelihood of capillary rupture and consequent hemorrhage.