Antithrombin III. Key factor in extracorporeal circulation.

Antithrombin III plays many different roles during cardiac operations with cardiopulmonary bypass. Basically, it acts as the natural inhibitor of thrombin, which, in presence of heparin, blocks the thrombin action and avoids gross thrombus formation inside the extracorporeal circulation circuit. By acting as a "suicide substrate", antithrombin III is consumed during cardiopulmonary bypass; at the same time, thrombin is extensively formed during and after cardiopulmonary bypass. As a result, at the end of the operation there is a potential imbalance in the antithrombin III-thrombin interaction, leading to a prothrombotic status. Moreover, patients pre-treated with heparin reach the operating theater with reduced levels of circulating antithrombin III; this may lead to the heparin resistance phenomenon and may further increase the risk for postoperative thrombotic complications. Finally, the anti-inflammatory properties of antithrombin III in the setting of the "whole body inflammatory reaction" induced by the cardiopulmonary bypass represents a new and unexplored field of research.