All subjects enrolled will be assigned to receive midodrine. The initial starting dose will be midodrine 5mg orally three times a day. After 7-10 days, patients will increase the dose to 10mg three times a day as tolerated if no adverse effects occur. Treatment duration will be 6 months.

Midodrine is an oral alpha-1 agonist that increases vascular tone. It is administered orally. The initial starting dose will be midodrine 5mg orally three times a day. After 7-10 days, patients will increase the dose to 10mg three times a day as tolerated if no adverse effects occur. Treatment duration will be 6 months.

Outcome will be defined by the incidence of adverse events (AEs) that occur during the study period. An adverse event is defined as any symptom, sign, illness or experience that develops or worsens in severity during the course of the study. Abnormal results of diagnostic procedures are considered to be AEs if the abnormality results in study withdrawal, is associated with a serious AE, is associated with clinical signs or symptoms, leads to additional treatment or further diagnostic tests or is considered by the investigator to be of clinical significance. Each adverse event will be further characterized by severity and relationship to the study drug. Adverse event are classified as serious if they are: fatal, life-threatening, require or prolongs hospital stay, lead to persistent or significant disability or incapacity or a congenital anomaly or birth defect.

Describe the effect of midodrine on:

• arterial oxygenation (PaO2 and A-a gradient )(mmHg)

Describe the effect of midodrine on:

• percent predicted diffusion capacity for carbon monoxide (Range 0-100%)