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In page Atrial septal defect:

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If pulmonary hypertension is present, the evaluation may include a right heart catheterization. This involves placing a catheter in the venous system of the heart and measuring pressures and oxygen saturations in the superior vena cava, inferior vena cava, right atrium, right ventricle, and pulmonary artery, and in the wedge position. Individuals with a pulmonary vascular resistance (PVR) less than 7 wood units show regression of symptoms (including NYHA functional class). However, individuals with a PVR greater than 15 wood units have increased mortality associated with closure of the ASD.[citation needed]