Low molecular weight heparins.

Low molecular weight heparins are derived from unfractionated heparin by chemical or enzymatic depolymerization; as a result, the mean molecular weight of unfractionated heparin is reduced by about one third and its biochemical and pharmacologic properties are improved. Demonstrated advantages of low molecular weight heparins over unfractionated heparin are the greater bioavailability at low doses, the longer half-life, and the more predictable dose response, which allows for fixed doses to be administered without laboratory monitoring; a potential advantage is the reduced hemorrhagic-to-thrombotic ratio observed in experimental animals. Clinical studies in the prevention of venous thromboembolism have shown that whereas the advantages offered by low molecular weight heparin over unfractionated heparin are modest in general surgery, they are substantial when compared with these and other agents in orthopedic surgery. In addition, low molecular weight heparins are at at least as safe and effective as unfractionated heparin in the treatment of established deep vein thrombosis, but have the advantage that they can be administered once or twice daily without laboratory monitoring and can be used to treat uncomplicated deep venous thrombosis on outpatient basis.