For the intervention group, a heparin dose (HDR) response assay will be performed prior to surgical incision by the Hepcon instrument. The hepcon machine performs a patient-specific HDR assay based on body surface area, and determines a projected heparin concentration required by that patient to achieve an ACT of 480 seconds. The HDR assay provides the initial dose of heparin to give the patient as well as the amount of heparin to be added to the CPB circuit. Additional heparin doses as determined by the hepcon instrument will be administered as needed if the patient's heparin concentration falls below the projected reference concentration. The hepcon instrument will also calculate the protamine dose needed to reverse heparin at the end of CPB.