Sildenafil for Early Pulmonary Vascular Disease in Scleroderma

Inclusion Criteria: * Previous documentation of mean pulmonary artery pressure between 21 and 24 mm Hg with a pulmonary capillary wedge pressure (or left ventricular end-diastolic pressure) ≤ 15 mm Hg within six months before study entry. * Diagnosis of SSc according to 2013 American College of Rheumatology/ European League Against Rheumatism (ACR/EULAR) classification criteria. * Pulmonary function tests with forced expiratory volume in one second/forced vital capacity (FEV1/FVC) \>50% AND either a) total lung capacity (TLC) or forced vital capacity (FVC) \> 70% predicted or b) TLC or FVC between 60% and 70% predicted with no more than mild interstitial lung disease on computerized tomography scan of the chest on studies obtained within 6 months of enrollment. * Ventilation perfusion scan or computed tomography with intravenous contrast (CT angiogram) without evidence of chronic thromboembolism at anytime before study entry. * Ability to perform six minute walk testing without significant limitations in musculoskeletal function or coordination. * Informed consent. Exclusion Criteria: * World Health Organization (WHO) Class IV functional status. * Systolic blood pressure less than 90 mmHg at screening visit prior to enrollment. * Clinically significant untreated sleep apnea. * Left-sided valvular disease (more than moderate mitral valve stenosis or insufficiency or aortic stenosis or insufficiency), pulmonary artery or valve stenosis, or ejection fraction \< 45% on most recent echocardiography (within 1 year). * Use of Pulmonary Arterial Hypertension (PAH) therapy (prostacyclin analogues, endothelin-1 receptor antagonists,phosphodiesterase-5 inhibitors, riociguat, selexipag) within the past 3 months. * Hospitalized or acutely ill. * Renal failure (creatinine above 2.0) at screening visit. * Enrollment in a clinical trial or concurrent use of another investigational drug (non FDA approved) or device therapy within 30 days of screening visit. * Age \< 18. * Currently pregnant. * Current use of nitrates.