a. idiopathic or heritable PAH b. PAH associated with: i. connective tissue disease (e.g., limited scleroderma, diffuse scleroderma, mixed connective tissue disease, systemic lupus erythematosus, or overlap syndrome) ii. drugs or toxins iii. Human Immunodeficiency Virus (HIV) infection iv. congenital heart defects repaired greater than 1 year prior to screening (i.e., atrial septal defects, ventricular septal defects, and patent ductus arteriosus) NB: subjects with portopulmonary hypertension and pulmonary veno-occlusive disease are NOT eligible for the study