Tranexamic axid is routinely used as an antifibrinolytic agent in cardiac surgery to reduce the risk of blood loss and transfusion. However, there is no consensus regarding the dosage regimen of tranexamic acid that should be administered. The purpose of this study is to compare different dosages of tranexamic acid in cardiac surgery using cardiopulmonary bypass regarding the duration of inhibition of fibrinolysis as measured by the ClotPro test. Blood samples will be taken from the arterial line placed in the patient at specified time points in order to perform viscoelastic tests (ClotPro, TPA test), to detect successful inhibition of fibrinolysis and to measure tranexamic acid levels in the patient's blood. In case the action of tranexamic acid stops early postoperatively an additional dose of the medication will be administered to the patient.