Pulmonary arterial hypertension (PAH) is a rare, progressive disorder characterised by high blood pressure in the arteries of the lungs, due to thickening and stiffening of the walls of the smaller branches of the pulmonary arteries. Over time, PAH damages the heart, which has to work harder to overcome this arterial resistance.

PAH can be associated with a number of conditions, including connective tissue diseases (such as scleroderma), portal hypertension, HIV, and certain medications/drugs. The cause may be unknown (idiopathic PAH).

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