Pulmonary arterial hypertension: Pulmonary arterial hypertension (PAH) is a disease primarily affecting the small precapillary pulmonary vessels. It is characterized by sustained elevation of the pulmonary vascular resistance (PVR). Without therapy, right heart failure and death eventually occur. PAH occurs in an idiopathic form, primary pulmonary hypertension (PPH), and in association with other disorders such as connective tissue diseases or congenital heart disease (CHD). PAH remains a disease of unknown etiology. Until recently, prognosis for PAH was poor, with a median survival of less than 3 years.