Clinical meaning
Venous thromboembolism (VTE) encompasses deep vein thrombosis (DVT) and pulmonary embolism (PE), representing a spectrum of the same pathological process. Thrombus formation in the venous system is governed by Virchow's triad: venous stasis, endothelial injury, and hypercoagulability.
At the molecular level, venous stasis in valve pockets of deep veins creates a hypoxic microenvironment. Endothelial cells respond to hypoxia by upregulating P-selectin (stored in Weibel-Palade bodies) and expressing tissue factor on their surface. P-selectin recruits monocytes and neutrophils, which release tissue factor-bearing microparticles. Tissue factor activates Factor VII, initiating the extrinsic coagulation cascade and generating thrombin. Thrombin converts fibrinogen to fibrin, activates Factor XIII for cross-linking, and activates platelets via PAR-1 and PAR-4 receptors.
Simultaneously, stasis impairs the natural anticoagulant mechanisms. Normally, flowing blood dilutes activated clotting factors and delivers antithrombin III to neutralize them. Thrombomodulin on intact endothelium binds thrombin, converting it from a procoagulant to an activator of Protein C (which inactivates Factors Va and VIIIa). Stasis disrupts all these protective mechanisms.
Low-molecular-weight heparins (LMWHs) like enoxaparin exert their anticoagulant effect by binding to antithrombin III via a specific pentasaccharide sequence, inducing a conformational change that accelerates antithrombin's inhibition of Factor Xa by 100-1000 fold. Unlike unfractionated heparin, LMWHs preferentially inhibit Factor Xa over Factor IIa (thrombin) due to their shorter chain length, resulting in a more predictable dose-response, longer half-life, and reduced risk of heparin-induced thrombocytopenia.