Pulmonary arterial hypertension (PAH) is a rare and severe disease
that continues to progress over time. It is caused by the narrowing
of the pulmonary arteries, which connect the right side of the heart to
the lungs. PAH may occur with no identifiable cause or may be
associated with one or more underlying conditions. As PAH develops,
blood flow through the pulmonary arteries is restricted and the right
side of the heart becomes enlarged due to the increased strain of
pumping blood through the lungs.