The hallmark of pulmonary arterial hypertension is an increase in pulmonary vascular resistance, which may be 20 times normal, leading to mean pulmonary artery pressures exceeding 25 mm Hg at rest and 30 mm Hg during exercise. If untreated, the condition leads to right ventricular failure and ultimately death. All layers of the pulmonary vessel walls are affected. Proliferation of the adventitia limits vascular elasticity, while hypertrophy of the medial smooth muscle promotes vasoconstriction in what is normally a high-flow, low-pressure system. Intimal proliferation and in situ thrombosis may eventually occlude the vascular lumen. Endothelial cell dysfunction is part of the disease process, and mounting evidence shows that endothelin-1, a strong endogenous vasoconstrictor, has a key pathogenic role. According to the 1998 World Health Organization (WHO) classification, subtypes of pulmonary arterial hypertension include primary pulmonary hypertension and pulmonary arterial hypertension associated with collagen vascular diseases (especially scleroderma), congenital heart disease with a left-to-right shunt, portal hypertension, HIV infection, and assorted drugs and toxins.