Pharmacology of second generation low molecular weight heparins.

There is agreement in the literature regarding the pharmacological inequivalence of LMWHs and there is evidence that differences in pharmacological profiles affect clinical outcomes. Unfractionated heparin (UFH) produces its anticoagulation effects by complexing with antithrombin III (AT III) to inactivate both factor IIa (thrombin) and factor Xa. Inhibition of factor IIa requires a polysaccharide chain of at least 18 polysaccharides, while inactivation of factor Xa does not depend completely on the length of the heparin molecule. The shorter saccharide chains of LMWHs allow for inhibition of activated factor Xa but are not long enough to complex with thrombin and inactivate it as fully as UFH [1]. This preferential effect of LMWHs on factor Xa over thrombin reduces the bleeding potential while prolonging the anticoagulant effect.