Thrombin and cardiopulmonary bypass – A paradigm for evaluation of the regulation of hemostasis

Extracorporeal circulation for the purpose of cardiopulmonary bypass (CPB) is used extensively for repair of cardiac defects. Complex mechanisms, many of which involve the generation of thrombin, are initiated by the institution of CPB. Thrombin is generated during CPB in spite of high doses of heparin. The tissue factor/factor VII pathway is also activated and is probably responsible for generation of most of the thrombin seen during CPB. Plasma proteins (humoral phase) along with platelets, endothelium and white cells (cellular phase) are also activated by contact of blood with synthetic surfaces. Small amounts of thrombin activate endothelial cells which then generate tissue plasminogen activator (t-PA). Plasmin is generated by the action of t-PA and fibrinolysis ensues. The blood becomes a mix of vasoactive substances that alter vascular smooth muscle tone and endothelial cell contraction. The sum of these reactions is called the 'whole body inflammatory response' and is responsible for postoperative morbidity including bleeding and organ system dysfunction.