None of the extracorporeal perfusion systems equals the perfection of the human heart and lung. Therefore, physiological changes occur during and after surgery which may affect the patient's postoperative course and treatment. There are significant changes in the patient's clotting mechanism. This occurs, in part, because the patient is heparinized before he is placed on the pump-oxygenator. Heparinization prevents the clotting of blood in the extracorporeal circuit. Also, fresh, heparinized blood is used to replace blood loss as well as to prime the pump-oxygenator, although many institutions now use non-blood solutions for priming. After surgery is completed, the heparin is neutralized by administration of protamine sulfate. However, heparin is metabolized approximately half as fast as protamine. Therefore, reversal of anticoagulation is often imperfect, resulting in a "heparin rebound" effect which requires the administration of additional protamine( 1,2). Other changes in the patient's clotting mechanism are due to the mechanical trauma to blood cells by the pump and suction systems. Platelets are damaged and decreased, as are some of the plasma clotting factors (prothrombin and fibrinogen). Hemolysis of red blood cells occurs because of mechanical trauma and the direct exposure of blood to oxygen. Red blood cells may be hemolyzed more rapidly than the liver and kidneys can metabolize and excrete the damaged cells. If significant hemolysis occurs, a mannitolinduced diuresis is usually effective, but "sludging" in the renal tubules may lead to acute tubular necrosis and renal failure.