Why low molecular weight heparin?

The introduction of LMW heparins has opened new avenues for the management of patients with thromboembolic diseases. In contrast to regular heparin, the mean molecular weights are smaller and represent about half that of regular heparin. Yet, they are still somewhat heterogeneous and seem to differ from preparation to preparation in some of their biologic properties. No two LMW heparins are alike. In spite of these differences, certain commonalities do exist, which may represent an advantage of unfractionated heparin, especially in their use as a prophylactic antithrombotic modality: 1. They are at least as effective, if not more effective, as antithrombotic agents when compared with unfractionated heparin. 2. Their rate of absorption from the subcutaneous depots is more complete, leading to a longer efficacy in vivo. This translates to a once a day injection instead of the twice a day use of unfractionated heparin. 3. There are suggestions that their use is associated with less bleeding than unfractionated heparin, although the final outcome awaits completion of many ongoing clinical trials. LMW heparins are not free of bleeding complications, but hemorrhages seem to occur that are dose dependent. 4. The overall incidence of heparin-induced thrombocytopenia seems to be markedly reduced with LMW heparins. It is still encountered in an occasional patient and may be related to the preparation used. There is reason to believe that heparin fractions free of heparin-induced thrombocytopenia will be forthcoming. As LMW heparins are used therapeutically, new dosage schemes and new laboratory tests will have to be developed to assure maximum safety for the patients.