An Introduction to Low-Molecular-Weight Heparins and Their Use in the Treatment of Deep Venous Thrombosis

Low-molecular-weight heparins (LMWHs) are fractions of standard heparin (SH) with an average molecular weight of 4000--5000 daltons. They have the ability to inhibit the activity of factor Xa in the coagulation cascade but are unable to significantly change the activity of thrombin (factor IIa). Therefore, LMWHs have antithrombotic properties, but they do not prolong the activated partial thromboplastin time (APTT). In contrast to SH, the pharmacokinetic profiles of LMWHs are different. LMWHs exhibit minimal binding with plasma proteins, endothelial cells and platelet factor IV, a longer half-life, and linear elimination. Due to these properties, the clinical response is more predictable than that seen with SH. In addition, monitoring of clotting factors and subsequent dosage adjustment are not necessary to ensure efficacy when using a LMWH. These agents may have a decreased risk of thrombocytopenia and bleeding. LMWHs have been used in the initial treatment of deep venous thrombosis (DVT) and have been found to be safe and efficacious. When compared to SH, the LMWHs studied have been more effective in improving venographic scores and short-term treatment, but further evaluation is needed to determine if a benefit exists in preventing recurrent DVT.