Subjects will have 2D-Echocardiogram measuring left ventricular strain and strain rate using speckle tracking techniques, have 6 minute walk test, World Health Organization functional class (I-IV)assignment, and BNP lab result at baseline and at 3 months. Subjects will be started on sildenafil at 20 mg by mouth three times per day at the baseline visit. Each individual will serve as his/her own control.

Subjects will be started on sildenafil at 20 mg PO TID at the baseline visit. Each individual will serve as his/her own control. Subjects will be evaluated at 3 months, with participation completed at that time.

Subjects will have 6 minute walk testing prior to sildenafil therapy and 3 months post-initiation of sildenafil therapy. Improvement in submaximal exercise capacity on sildenafil therapy is defined as ≥ 30 M increase in 6 minute walk testing.

Echo parameters will be collected pre-therapy vs 3 months on sildenafil therapy. Changes in ventricular wall motion will be documented with left ventricular (LV) septal and lateral wall longitudinal strain and strain rate, right ventricular strain and strain rate, and LV lateral wall/septal strain and strain rate utilizing speckle tracking techniques values determined by 2D-Echo.

World Health Organization functional class (I-IV) will be determined at baseline and at 3 months

Blood will be collected to get BNP (brain natriuretic peptide)lab result at baseline and at 3 months