Stress failure of pulmonary capillaries as a mechanism for exercise induced pulmonary haemorrhage in the horse.

The research studies the cause of Exercise Induced Pulmonary Hemorrhage (EIPH) in Thoroughbred horses, suggesting that the primary cause is the mechanical failure of the thin walls of pulmonary capillaries due to high internal pressure experienced during intense exercise.

Pulmonary Capillary Stress Failure as the Cause of EIPH

• The primary focus of this study is to understand the mechanism behind Exercise Induced Pulmonary Hemorrhage (EIPH), a condition that widely affects the Thoroughbred horse industry. The researchers found that EIPH is caused by the mechanical failure of the walls of pulmonary capillaries due to very high pressure levels within them during intense exercise.
• The pulmonary capillaries have extremely thin walls, which enable the rapid exchange of gases. However, these thin walls undergo considerable stress when the internal – or transmural – pressure within the capillary increases.
• The researchers analyzed damage to the capillaries’ walls in anesthetised rabbits, which showed ultrastructural damage at pressures of 40 mmHg and above. Interestingly, this damage was found to be rapidly reversible if the capillary pressure was lowered.

The Role of Lung Volume and Pressure on EIPH

• Increased lung volume during intense exercise also significantly contributes to the incidence of stress failure in pulmonary capillaries. This is likely due to the heightened pressure and work demanded of the lungs and heart in such scenarios.
• Thoroughbred horses, specifically bred over centuries for short bursts of rapid speed, tend to have high oxygen consumptions and high cardiac outputs. This results in considerable filling pressures in the left ventricle of the heart, which in turn leads to high pulmonary vascular pressures – up to 120 mmHg in the pulmonary artery and 70 mmHg in the left atrium.
• It’s hypothesized that the strength of the capillary wall is largely due to Type IV collagen found in the extracellular matrix, which possibly helps in protecting the capillary during periods of high pressure.

Implications of The Study

• Understanding the cause of EIPH paves the way for possible treatments or preventative measures to be taken in the future, to mitigate the risks associated with this common issue within the Thoroughbred horse industry.
• Knowing that the high incidences of EIPH are in large part due to selective breeding practices also invites further consideration of breeding and training practices in order to minimize these health risks.