Does Oral Sildenafil (Viagra) Decrease Mean Pulmonary Artery Pressure 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