• To determine the efficacy of 12.5mg oral sildenafil to decrease the mPAP in patients presenting with a mPAP >/= 25mmHg after cardiopulmonary bypass.
  

• To determine the efficacy of a second dose of oral sildenafil 12.5mg to decrease the mPAP in those patients who do not respond with a 20% decrease in mPAP after the initial administration of study medication.
  
• To determine the safety of oral sildenafil to treat increased mPAP after cardiac surgery.
  

Inclusion Criteria:

• 18 years of age or older
• Scheduled for or has recently undergone a primary or re-do cardiac surgical procedure including coronary artery bypass graft surgery (CABG), heart valve replacement or repair, ascending aortic replacement or repair or any combinations of these procedures with CPB
• No documented allergy to sildenafil citrate
• No clinical or laboratory evidence on routine blood work of significant renal disease or failure. (requires dialysis or a creatinine >/= 200umol/L)
• No clinical or documented laboratory evidence on routine blood work of hepatic disease or failure. (ALT or AST 5x upper limit of normal (ULN) or jaundice)
• The patient if female and of child bearing age is not known to be pregnant.
• No documented history of severe chronic respiratory disease defined as an FEV/VC1< 50% predicted.
• Not currently enrolled as an active participant in another clinical trial of a medical therapy or device.
• No documented stroke or transient ischemic attack within 6 months of study participation
• No documented critical carotid artery stenosis (>70%)
• No retinitis pigmentosa.
• The patient has authorized his/her consent to participate in this trial pre-operatively OR consent to participation is granted on the patient's behalf by a substitute decision maker pre or post operatively.

POST-OPERATIVELY

• A pulmonary arterial catheter (swan-ganz catheter) is insitu.
• The patient has a mPAP measurement of >/= 25mmHg for at least 1 hour.
• The patient has a mean arterial pressure (MAP) of >/= 65mmHg.
• The patient has a heart rate of greater than 40 and less than 130 beats/minute.
• The patient is intubated and mechanically ventilated as per standard ventilation protocol at St. Michael's Hospital.
• The patient has not received intravenous nitroglycerin or nitroprusside within 1 hour before treatment with study medication.

Exclusion Criteria:

POST-OPERATIVELY

• The patient requires nitroglycerin based medications continuously (topical/oral/intravenous)
• The patient has an arterial pH of < 7.30 or ≥ 7.47
• The patient has clinical and or documented laboratory evidence from routine blood work of renal dysfunction or failure. (Doubling of pre-operative creatinine or requiring dialysis is indicative of renal dysfunction.)
• The patient has clinical and or documented laboratory evidence from routine blood work of significant hepatic disease or failure. (ALT or AST 5 times ULN or obvious jaundice will be indicative of liver dysfunction)
• The patient's current medical condition in the opinion of the investigator and/or the patient's attending ICU physician makes him/her inappropriate for participation in this study.
• The patient is currently a participant in a clinical trial of an investigational therapy including a drug or medical device.
• The patient has clinical indications of sepsis defined as 2 of the following five criteria: i) leukocytosis (or) leukopenia: wbc >12 x 109/L (or) <4x109/L (>10% bands also, ii) fever (or) hypothermia (temp >38.5C or <36C), iii) tachycardia, hr > 90 beats/minute, iv) tachypnea, respiratory rate (RR) >18 breaths/minute, v) hypotension, SBP <90 mmHg