There have been a variety of bleeding syndromes associated with the use of cardiopulmonary bypass procedures attributed to such things as platelet function defects, activation of plasmin, thrombocytopenia and consumption coagulopathy. One other cause of bleeding, particularly in the postoperative period, has been that of hyperheparinemia or so-called "heparin rebound". Usually high doses of heparin are used during bypass to prevent not only thrombosis in the patient, but to avoid clot formation

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... within the machine. At the end of the pump run, the heparin is neutralized with either protamine sulfate or protamine chloride. In the United States, protamine sulfate is the only neutralizing agent being used at the present time. By definition then, heparin rebound is the resurgence of anticoagulant or antithrombin activity in a heparinized patient whose blood has been neutralized with protamine. There have been many explanations for this phenomenon offered in the literature1). On the one side, there is an increase in heparin concentration in the blood due to release from various sites of storage in the vessels such as red cells or endothelial surfaces, or the return of heparin into the circulation from extravascular spaces. An additional possibility is a naturally occurring protaminase which will destroy protamine, thereby releasing heparin from a heparin protamine complex. Over the past several years, I have been investigating the effect on antithrombin of the interaction of heparin and protamine2)-5). In most of these studies, we have used a tritium labeled porcine mucosal heparin supplied by Abbott Laboratories. When tritiated heparin is added to defibrinatted plasma and fractionated on a sephadex G200 column, we find a small peak of radioactivity in the macromolecular area and then a larger amount between the globulin and albumin peaks with another high peak in the postprotein area representing the excess heparin. In the fractions between the albumin and globulin area, there is a high peak of immediate antithrombin activity as measured by the inhibition of the thrombin clotting time of a fibrinogen substrate. If, however, plasma is used as the substrate for measuring a thrombin clotting time, inhibition is found in not only this area, but in the macromolecular fractions as well as in the postprotein fractions of excess heparin. These peaks represent primarily activation of the antithrombin III (AT III) in the substrate plasma. The inhibitory activity promoted by the macromolecular fractions was thought to be due to heparin complexed with some other protein than AT III. Marciniak6) had shown that a large amount of heparin was found in this area when she fractionated heparinized plasma but had ruled out lipoproteins and immunoglobulins as the carrier protein. We considered other macromolecules and decided that since alpha-2-macroglobulin is, by itself, antithrombin, it might also complex with heparin even though it is not activated by it. This proved to be true. It was also found that the heparin complexed to alpha-2-macroglobulin had the capacity to activate purified AT III). If the heparinized plasma is first neutralized with protamine sulfate, a different pattern is obtained in the fractions subjected to chromatography. There is now a large peak of radioactivity in the macromolecular area, very little among the other protein fractions and a smaller peak of radioactivity in the postprotein fractions. These latter fractions represent nonactive heparin which is present in the commercial product. No immediate antithrombin activity is found in any of the fractions. In the albumin peak, we found progressive antithrombin activity which can be converted to immediate inhibitory activity by the addition of fresh heparin. If, however, plasma is used as the substrate for the thrombin clotting time, it is found that there is a