TABLE 1

World Health Organization (WHO) classification of pulmonary hypertension

WHO classification	Etiology
Group I: pulmonary arterial hypertension	Idiopathic; drug- or toxin-related; associated with connective tissue disease, human immunodeficiency virus infection, portal hypertension, congenital heart disease, schistosomiasis; persistent pulmonary hypertension of the newborn; pulmonary arterial hypertension with venous or capillary involvement
Group II: pulmonary hypertension associated with left heart disease	Heart failure, valvular heart disease, congenital or acquired heart conditions leading to postcapillary pulmonary hypertension
Group III: pulmonary hypertension associated with lung disease, hypoxia, or both	Obstructive lung disease or emphysema, restrictive lung disease, lung disease with mixed pattern, hypoventilation syndromes, hypoxia without lung disease, developmental lung disease
Group IV: pulmonary hypertension associated with pulmonary artery obstruction	Chronic thromboembolic pulmonary hypertension, other pulmonary artery obstructions (malignant tumors, sarcomas)
Group V: pulmonary hypertension with unclear or multifactorial mechanisms	Hematologic disorders, systemic disorders, metabolic disorders, chronic renal failure with or without dialysis, fibrosing mediastinitis, pulmonary tumor thrombotic microangiopathy