The goal of this proposal is to prospectively collect data from a series of 200 patients (all ages) undergoing complex cardiac surgical procedures involving cardiopulmonary bypass (CPB) to:

1. Measure the number of blood activated circulating monocytes before, during and after cardiac surgery and serum GABA and pro-inflammatory cytokine levels
2. Understand the correlation between GABA and inflammatory cytokines (and/or activated monocytes) and
3. Assess the correlation between thrombosis and monocyte activation in patients undergoing cardiac surgery under CPB and at risk of thrombosis.

Often when people undergo heart surgery, they are placed on cardiopulmonary bypass (CPB). Sometimes clotting of the blood occurs during and after heart surgery with CPB, which can lead to multiple complications. In order to try to avoid abnormal/excessive clotting that may lead to complications, it is important to understand the specific causes of why the blood sometimes clots abnormally during and after heart surgery.

Currently, tests looking at how the blood clots (coagulation assessment) are usually performed as standard laboratory testing during and after surgery. However, these tests are not able to predict the future development of abnormal clots.

This study will be looking at the role that immune cells (monocytes) have in the formation of abnormal clots after heart surgery. It is possible that these immune cells produce signals that can contribute to the formation of clots. Using experimental tests (monocyte analysis and mediator analysis), the hope is to understand if these cells participate in forming abnormal clots in any meaningful way. Although these tests are not yet approved for use in children, this study plans to use these new methods to understand this phenomenon by analyzing the blood at different times during the surgery and recovery period.

Overall, the purpose of this study is to look at these tests in people undergoing heart surgery to help the investigators understand why the blood sometimes forms clots abnormally. Results from this study may help treat future patients who experience abnormal clotting during and after surgery.

Inclusion Criteria:

• Patients (all ages) undergoing complex cardiac surgical procedures with CPB.

Exclusion Criteria:

• undergoing an emergent procedure and do not have more than 1 hour ahead of time to review the consent
• patient in a moribund condition American Society of Anesthesiology (ASA 5) -patient with a hematological and/or oncological disease
• Jehovah witnesses.

Patients (all ages) undergoing complex cardiac surgical procedures with cardiopulmonary bypass (CPB) at Boston Children's Hospital.