The cause of primary pulmonary hypertension is unknown.
Pulmonary hypertension develops as a response to increased resistance to blood flow.
Diffuse narrowing of the pulmonary arterioles occurs. The right side of the heart becomes enlarged due to the increased work load of pumping blood against this resistance. Eventually, progressive heart failure develops. People at risk are women between 20 and 40 years old, although the disease can affect any sex and age. The incidence is 8 out of 100,000 people.