Cardiac surgery requiring use of a heart-lung machine, also known as cardiopulmonary bypass (CPB) can induce abnormalities in blood coagulation (clotting) that lead to excessive blood loss during and after operation. While of significant concern in adults, this problem takes on even greater importance in children due to the simple fact that they have a smaller blood volume. It is well known that if a child has previously undergone a heart operation, they are at increased risk of bleeding should more cardiac surgery be required ("reoperation"). The processes regulating blood coagulation are extraordinarily complex, and little is known about the exact mechanisms that contribute to the increased bleeding associated with cardiac reoperation in children. Rotational thromboelastometry (ROTEM®) is a technology that can provide, at the bedside, detailed information about coagulation abnormalities. While not currently approved for general use in the United States, in Europe ROTEM® has been used to guide administration of the blood products in surgery based upon determination of specific coagulation abnormalities. Importantly, there is now evidence that guidelines for transfusion therapy based on ROTEM® reduce transfusion requirements thus decreasing patient exposure to blood products. Whether the use of ROTEM® has potential benefit for pediatric cardiac surgical patients has not been studied. Accordingly, the present study was designed to provide preliminary data comparing coagulation profiles between children undergoing cardiac reoperation to those having primary procedure.