The use of heparin and aspirin for their respective anticoagulation and antiplatelet effects is routine in intensive care units around the country and world in children who have undergone surgery for congenital heart disease (CHD) who are at risk for thrombosis after repair. Most common protocols recommend heparin infusions after surgery with transition to oral aspirin when oral intake begins. Patients at risk for thrombosis after congenital heart surgery requiring long-term antiplatelet therapy include those undergoing shunt placement for single-ventricle palliation or establishment of pulmonary blood flow, valve replacements, or coronary artery manipulation. Post-operative thrombosis after congenital heart surgery is rare, but thrombosis can result in death. The routine use of aspirin in these patients is intended to prevent the potentially catastrophic consequences of thrombosis. However, there is no routine clinical assessment of the platelet inhibitory effect of aspirin; nor have there been any prospective studies to assess the effectiveness of aspirin's antiplatelet effect in children after congenital heart surgery. No data exists to suggest that the first dose of aspirin transition from heparin infusion is either sufficient or insufficient in its antiplatelet effect.

In this study, the investigators will evaluate the effectiveness of the first aspirin dose in its antiplatelet effects after surgery for congenital heart disease. Degree of antiplatelet effect will be evaluated with thromboelastograph (TEG) and TEG-platelet mapping study analysis. Thromboelastography is a clinical laboratory method of assessing whole blood hemostasis and allows for assessment of clot formation, strength, and stability. TEG is most commonly used in patients at risk for bleeding or thrombosis and also to monitor antiplatelet therapies (such as aspirin).

The investigators hypothesize: the first dose of aspirin transitioned from heparin infusion after congenital heart surgery is sufficient in its antiplatelet effect as tested by TEG and TEG-platelet mapping studies. Confirmation of the antiplatelet effect of aspirin will help support the current practice of empiric aspirin therapy to prevent post-operative thrombosis in children after congenital heart surgery.

Inclusion Criteria:

• Age 0-18
• Undergoing surgery for congenital heart disease requiring oral antiplatelet therapy

Exclusion Criteria:

• Strict: personal or family history of bleeding/clotting disorder, patients requiring anticoagulation rather than antiplatelet therapy after surgery.
• Relative: patients with significant post-operative bleeding precluding the use of anticoagulation and/or antiplatelet therapy within the first 48-72 hours (eg. clinical instability preventing the initiation of oral aspirin therapy), patients requiring additional fresh platelet transfusion after the initiation of post-operative heparin infusion.

The study population includes children aged 0-18 with congenital heart disease (CHD) in the immediate post- operative period who receive heparin infusion prior to initiation of long-term antiplatelet therapy. The current study population includes patients who have undergone one of these surgeries requiring aspirin therapy. The following are the surgical procedures included in this study:

1. Arterio-venous shunts: classic and modified Blalock-Taussig (BT) shunts, and central shunts.
2. Veno-venous shunts: partial cavo-pulmonary connections (Glenn anastomosis), total cavo-pulmonary shunts (Fontan), and right ventricle to pulmonary artery conduits.
3. Patients requiring coronary artery re-implantation: arterial switch operation for transposition of the great arteries and repair of anomalous left coronary artery arising from the pulmonary artery.
4. Pulmonary valve replacement.