A randomized, placebo controlled, single center clinical trial for evaluation of efficacy and safety of Sildenafil administration in the cardiac ICU following Mitral Valve (MV) Surgery in patients with pre-operative Significant Pulmonary Hypertension.

Inclusion Criteria:

1. Age ≥ 22
2. Undergoing mitral valve surgery (either repair or replacement) with or without coronary revascularization, aortic valve replacement (AVR) or repair; or tricuspid valve surgery.
3. Pre-operative pulmonary arterial systolic pressure >50 mm Hg as determined by resting echocardiography and post-operative sPAP > 45 mmHg as obtained from invasive hemodynamics measurements.
4. Willing and able to give written informed consent prior to the procedure

Exclusion Criteria:

1. Hypersensitivity to study drug
2. Women of child-bearing potential
3. Expected need to administer nitrates that are clinically indicated peri-operatively
4. Post-operative hypotension (systolic blood pressure (BP) <80) or evidence of shock (postoperative evidence of any kinds of shock)
5. Cardiac or systemic amyloidosis
6. Active malignancy other than BCC (basal cell carcinoma)
7. Stable kidney dysfunction with Creatine clearence (CrCl) <30 mL/min during the screening period or hepatic failure other than mild
8. Significant anemia (hemoglobin <8 mg/dl) preoperative.
9. Known drug or alcohol dependence or any other factors which will interfere with the study conduct or interpretation of the results or in the opinion of the investigator are not suitable to participate;
10. Any illness other than cardiac which might reduce life expectancy to less than 1 year from screening