Comparison of the in vivo hemorrhagic and antithrombotic effects of a low antithrombin-III affinity heparin fraction.

Antithrombin III (ATIII) affinity chromatography of commercial grade heparin yields fractions of high and low affinity for ATIII. In vitro, the high affinity fraction accounts for most of the anticoagulant effect while there is evidence that the low affinity material interferes with platelet function. We have studied the relative antithrombotic and hemorrhagic effects of low affinity heparin. The low affinity heparin fraction, specific activity 43 USP units/mg, was compared with standard heparin (150 USP units/mg) in rabbit experimental models. A residual 12.5% by weight of this low affinity heparin showed high affinity for ATIII. Inhibition of thrombosis in a stasis-hypercoagulability model was directly related to the weight (mg) of high affinity material in each of the heparins. In the bleeding model, when similar weights (mg) of high affinity material were infused, significantly more bleeding was demonstrated with the low affinity fraction which contained a 5-fold excess by weight of low affinity material. We have demonstrated that a low affinity heparin depleted of in vitro anticoagulant and in vivo antithrombotic activity significantly contributes to hemorrhage.