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Normally, the pulmonary vascular system is a high-flow, low-resistance circuit, with a mean pulmonary arterial pressure that constitutes approximately 15% to 20% of the systemic circulation.1 Normal pulmonary arterial systolic pressures range from 15 to 30 mm Hg, whereas diastolic pulmonary arterial pressures range from 4 to 12 mm Hg.1 Pulmonary hypertension is defined as a mean pulmonary arterial pressure >25 mm Hg at rest or >30 mm Hg during exertion.1,2

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Pulmonary hypertension is classified based on measurements of pulmonary capillary wedge pressure (PCWP) and pulmonary vascular resistance. Patients with pulmonary arterial hypertension have a mean pulmonary arterial pressure >25 mm Hg, a pulmonary vascular resistance >240 dynes/s/cm5, and a PCWP <15 mm Hg.2 In contrast, patients with pulmonary hypertension caused by left heart disease, the most common cause of pulmonary hypertension, have a PCWP >15 mm Hg.2 Although echocardiography can estimate pulmonary arterial pressure in a patient with suspected pulmonary hypertension, definitive diagnosis requires right heart catheterization.

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The World Health Organization classifies pulmonary hypertension into five categories based on cause and response to treatment (Table 1).1,3 Some patients have features of multiple categories, but the majority have one predominant type of pulmonary hypertension.4 Accurate classification of pulmonary hypertension is key to directing treatments, which vary among the categories. Pulmonary venous hypertension is the most common cause of pulmonary hypertension, affecting almost 4 million patients in the United States.4 In contrast, pulmonary arterial hypertension is the least common cause of pulmonary hypertension, with an estimated prevalence of 15 patients per million.4,5 Pulmonary hypertension develops in up to 4% of patients with chronic thromboembolic disease.4,6,7 Regardless of the cause, patients with pulmonary hypertension have high morbidity and mortality rates,4,8 with a 5-year mortality rate for patients with idiopathic pulmonary arterial hypertension exceeding 30%.5

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Table 1 World Health Organization Classification of Pulmonary Hypertension
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The exact pathophysiology of all forms of pulmonary hypertension remains unknown. In pulmonary arterial hypertension, the initial abnormality is endothelial dysfunction, which results in an imbalance between endogenous vasodilators (e.g., prostacyclin) and vasoconstrictors (e.g., endothelin-1). The ...

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