The effect of unfractionated vs. low molecular weight heparin on tissue factor pathway inhibitor levels in hospital inpatients

Although heparin is widely used as an antithrombotic agent, its multiple me chanisms of action an not fully defined. Recent work has suggested that tis sue factor pathway inhibitor (TFPI) may contribute to the antithrombotic ac tivity of heparin by inhibiting the extrinsic pathway of coagulation. We ha ve investigated the effect of heparin on TFPI and have found that when unfr actionated heparin is given by continuous intravenous infusion to hospitali zed inpatients. TFPI levels increase 2.3-fold and remain high as long as he parin is continued, but return to baseline levels soon after the infusion i s stopped. In contrast, therapeutic doses of the low molecular weight hepar in, dalteparin, resulted in significantly less TFPI induction. Given the in creasing number of studies establishing the clinical efficacy of low molecu lar weight heparins as antithrombotic agents, these results suggest that TF PI may not be a major contributor to the antithrombotic effect of heparin.