In vivo inactivation of antithrombin III is promoted by heparin during cardiopulmonary bypass.

To study the in vivo effect of heparin on antithrombin III (AT3) when elastase is elevated, the blood of 20 patients undergoing cardiopulmonary bypass (CPB) was assayed for elastase and AT3. The model was chosen because CPB is known to increase plasma elastase and the patients were heparinized. The blood of 20 patients undergoing cardiac surgery was assayed for elastase and AT3 one day preoperatively, every half hour during CPB, and one day postoperatively. Elastase increased significantly and AT3 decreased significantly during CPB. There was a direct correlation between the rise in elastase and decrease in AT3. AT3 decreased even further when elastase was elevated and patients were heparinized (AT3/elastase = 0.04 + 0.07), compared with the drop with elevated elastase alone (AT3/elastase = 0.11 + 0.14) (P less than .0015). These data indicate that (1) CPB is associated with an increase in plasma elastase, (2) elevated plasma elastase is associated with a reduction in AT3, and (3) heparin promotes the inactivation of AT3 when serum elastase is increased. These data confirm the in vitro observation that heparin accelerates the inactivation of AT3 in the presence of elastase.