Do blood clots in the lungs cause PH?

Blood clots in the lungs (also known as pulmonary embolism) can cause or contribute to PH in many patients.

In some patients, PH is directly due to pulmonary embolism. These patients are said to have WHO Group IV PH, or chronic thromboembolic PH (CTEPH).

• The risk of PH after pulmonary embolism is quite small. In most patients with pulmonary embolism, the body is able to break up the clot, leaving no evidence of PH
• However, PH will develop within 2 years of pulmonary embolism in a small number of patients (up to 4%). The risk is highest in patients with multiple episodes of pulmonary embolism
• Patients with pulmonary embolism who also develop PH have a higher risk of dying

In many other patients with PH, blood clots in the lungs are not the cause of PH. However, even in these patients, PH itself causes damage to the innermost lining cells (also known as endothelial cells) of the pulmonary arteries. Damage to endothelial cells can lead to blood clots forming (also known as thrombosis) in the pulmonary arteries.

• This thrombosis in the pulmonary arteries narrows the pulmonary arteries
• This thrombosis in the pulmonary arteries can further increase the blood pressure in the pulmonary arteries (pulmonary artery pressure)

This thrombosis in the pulmonary arteries can lead to the worsening of PH over time