he advantages of unfractionated heparin are its rapid onset of action, clinical efficacy, rapid neutralization by protamine, safety, and low cost. The dose of heparin used to prevent blood clotting during CPB is 300–400 U kg−1 plus additional doses to achieve and maintain an activated clotting time (ACT) of greater than 480 s (Table 1). However, the individual response to a fixed dose of heparin varies. Higher doses of heparin may result in better thrombin inhibition, thereby preserving coagulation factors on CPB.3,4