Pulmonary arterial hypertension (PAH) is characterized by high blood pressure in the pulmonary artery, and occurs when the small arteries throughout the lungs narrow in diameter. This increases the resistance to blood flow through the lungs, causing hypertension in the pulmonary artery as the right ventricle compensates to preserve pulmonary blood flow. Progressive heart failure develops when the right ventricle can no longer compensate for the higher resistance. Initial symptoms include dyspnea, fatigue, syncope, chest pain, palpitation, and edema. Idiopathic PAH is diagnosed clinically by pulmonary arterial hypertension (>25 mmHg at rest or >30 mmHg during exercise) in the absence of other known causes of pulmonary hypertension. The average age of diagnosis is approximately 36 years. Genetic anticipation has been reported, with earlier ages of diagnosis in successive generations in families with PAH, but the mechanism remains unclear.