Inhaled Treprostinil (Tyvaso Nebulizer) For COPD Patients With Hypoxemia

Inclusion Criteria * Outpatients of either gender, age ≥ 40. * Clinical evidence of chronic bronchitis (a productive cough lasting more than 3 months within a 2-year period). * Current or former cigarette smokers with a smoking history of 20 or more pack-years. * Clinical diagnosis of chronic obstructive pulmonary disease (COPD) confirmed by spirometry demonstrating forced expiratory volume in 1 second (FEV1) to forced vital capacity (FVC) ratio \< 0.70, pre-bronchodilator FEV1 ≥ 30% predicted, and diffusing capacity of the lungs for carbon monoxide (DLCO) ≥ 45% predicted. * On stable maintenance COPD medications including inhaled corticosteroids (ICS), long-acting beta agonists (LABA), long-acting muscarinic antagonists (LAMA), combination inhalers, azithromycin, or roflumilast (with no changes over the past 3 months). * Patients with hypoxemia, evidenced by the use of supplemental oxygen at rest or during exercise (≤ 4 liters/minute). * Willing and able to give informed consent and adhere to visit and protocol schedules (consent must be obtained prior to any study procedures). * Women of childbearing potential must have a negative serum pregnancy test, confirmed prior to participation in this investigational protocol. Exclusion Criteria * Upper respiratory tract infection within 6 weeks (participants may be rescreened after this period). * History of lung resection surgery or pleural decortication. * Previous history of pneumothorax. * Imaging evidence of interstitial lung disease, occupational lung disease, or chronic infectious lung disease. * History of asthma. * History of exposure to occupational or environmental hazards known to cause lung disease. * Positive pregnancy test for women of childbearing potential. * Major chronic illness that, in the judgment of the study physician, would interfere with participation in the study. For Magnetic Resonance Imaging (MRI): * Contraindications to MRI based on screening questionnaire responses. * Participant is pregnant or lactating. * Respiratory illness of bacterial or viral etiology within 6 weeks prior to MRI. * Known cardiac arrhythmia. * Participant does not fit into the hyperpolarized xenon-129 (\^129Xe) vest coil used for MRI. * Participant cannot hold their breath for 10 seconds. * Participant is unlikely to comply with instructions during imaging.