About Heparin A highly-sulfated glycosaminoglycan, Anticoagulant. Mechanism of Action of Heparin The anticoagulant action of heparin is exerted through affecting conversion of fibrinogen to fibrin mediated by factors Xa as well as IIa. It acts by activating plasma antithrombin III (AT III). The heparin-AT III complex then binds to and inactivates clotting factors -.( Xa, IIa, IXa, XIa, and XIIIa) of the intrinsic and common pathways but not factor VIIa operative in the extrinsic pathway. At lower concentrations it selectively inhibits factor Xa mediated conversion of Prothrombin to thrombin. At lower concentrations it inhibits intrinsic pathway of coagulation but at higher concentration it also interfere with common pathway. At higher doses it prevents platelet aggregation. Indications for Heparin 1. Deep vein thrombosis 2. Pulmonary embolism 3. Intravascular coagulation 4. Peripheral arterial embolism 5. Atrial fibrillation with embolism 6. Venous thrombosis