Does Sildenafil Protect Against Pulmonary Related Complications Following Cardiopulmonary Bypass?

Inclusion Criteria: Paediatric patients undergoing open heart surgery utilising cardiopulmonary bypass to correct acyanotic congenital heart disease. Age \> 3 months Parents that show a good understanding of their child's condition and are happy for their child to participate in the study. Exclusion Criteria: Patients with known organ dysfunction prior to surgery (pulmonary, renal or hepatic) Communication barrier resulting in poor basic comprehension of the proposed study (e.g. language barrier) Patients with cyanotic heart disease Patients undergoing heart surgery without the use of cardiopulmonary bypass Patients who do not tolerate oral Sildenafil (e.g. Vomiting) or whose surgery is subsequently cancelled.