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