Due to complex study protocol, no patients have been enrolled in the study.

It has been hypothesized that one of the benefits of bosentan relates to pulmonary vascular remodeling. The investigators believe that this study will help document the nature of beneficial changes that occur in patients with Pulmonary Arterial Hypertension (PAH) in response to bosentan therapy. In turn, demonstrating that changes in pulmonary vascular structure and function accompany clinical improvement.

Subjects will have Optical coherence tomography (OCT) and Intravascular Ultrasound (IVUS) imaging during the right heart catheterization that diagnoses them as having PAH. Subjects will start their bosentan and will be followed by monthly clinic visits for 4 months. At 4 months they will have repeat OCT and IVUS imaging.

Inclusion Criteria:

1. WHO GROUP 1
2. AGE 18-75
3. Baseline 6-min walk distance (6MWD) between 200 and 450 m
4. Diagnosed with pulmonary artery hypertension during right heart catheterization (Mean Pulmonary Artery Pressure > 25mmHg)

General Exclusion Criteria

1. Pregnant or nursing
2. Acute or chronic illness other than those associated with PAH (collagen vascular disease, human immunodeficiency virus, or anorexigen use)
3. Previously received any investigational medications, prostanoids, or phosphodiesterase inhibitors
4. eGFR < 60
5. Angina
6. Syncope
7. Failing right ventricle
8. Hemoptysis