Overview of low molecular weight heparins and heparinoids: basic and clinical aspects.
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Low molecular weight heparins (LMWHs) are fragments derived by enzymatic or chemical depolymerization of standard heparin (SH). They are approximately one-third the size of SH, with a mean molecular weight of 4,000 to 6,000. LMWHs produce less bleeding for equivalent antithrombotic effects than SH in experimental animals. Clinically LMWHs exhibit different pharmacokinetics than SH; they have much better bioavailability at low doses, a longer half life than SH and clearance pattern which is dose independent. In addition, LMWHs have a more predictable dose response than SH. These differences in pharmacokinetic properties occur because in contrast to SH, LMWHs show minimal binding to endothelium and plasma proteins. Clinical trials have demonstrated that LMWHs are effective and safe for the prevention and treatment of venous thromboembolism. In patients having orthopaedic surgery, LMWHs are more effective than low dose SH, more effective than dextran or warfarin. They are also more effective than SH in preventing venous thrombosis in stroke patients and those suffering spinal injury. In addition, recent studies suggest that LMWHs administered by subcutaneous injection in fixed weight-adjusted doses are at least as effective and safe as adjusted dose SH given by continuous intravenous infusion for the treatment of venous thrombosis.
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