Under normal circumstances, antithrombin III (ATIII) inactivates thrombin (factor IIa) and factor Xa. This process occurs at a slow rate. Administered heparin binds reversibly to ATIII and leads to almost instantaneous inactivation of factors IIa and Xa The heparin-ATIII complex can also inactivate factors IX, XI, XII and plasmin. The mechanism of action of heparin is ATIII-dependent. It acts mainly by accelerating the rate of the neutralization of certain activated coagulation factors by antithrombin, but other mechanisms may also be involved. The antithrombotic effect of heparin is well correlated to the inhibition of factor Xa. Heparin is not a thrombolytic or fibrinolytic. It prevents progression of existing clots by inhibiting further clotting. The lysis of existing clots relies on endogenous thrombolytics.
Unfractionated heparin is a highly acidic mucopolysaccharide formed of equal parts of sulfated D-glucosamine and D-glucuronic acid with sulfaminic bridges. The molecular weight ranges from 3000 to 30, 000 daltons. Heparin is obtained from liver, lung, mast cells, and other cells of vertebrates. Heparin is a well-known and commonly used anticoagulant which has antithrombotic properties. Heparin inhibits reactions that lead to the clotting of blood and the formation of fibrin clots both in vitro and in vivo. Small amounts of heparin in combination with antithrombin III, a heparin cofactor, ) can inhibit thrombosis by inactivating Factor Xa and thrombin. Once active thrombosis has developed, larger amounts of heparin can inhibit further coagulation by inactivating thrombin and preventing the conversion of fibrinogen to fibrin. Heparin also prevents the formation of a stable fibrin clot by inhibiting the activation of the fibrin stabilizing factor. Heparin prolongs several coagulation tests. Of all the coagulation tests, activated partial prothrombin time (aPTT) is the most clinically important value.
Liver and the reticulo-endothelial system are the sites of biotransformation. The metabolic fate of heparin is not well understood.
In mouse, the median lethal dose is greater than 5000 mg/kg. Another side effect is heparin-induced thrombocytopenia (HIT syndrome). Platelet counts usually do not fall until between days 5 and 12 of heparin therapy. HIT is caused by an immunological reaction that makes platelets form clots within the blood vessels, thereby using up coagulation factors. It can progress to thrombotic complications such as arterial thrombosis, gangrene, stroke, myocardial infarction and disseminated intravascular coagulation. Symptoms of overdose may show excessive prolongation of aPTT or by bleeding, which may be internal or external, major or minor. Therapeutic doses of heparin give for at least 4 months have been associated with osteoporosis and spontaneous vertebral fractures. Osteoporosis may be reversible once heparin is discontinued. Although a causal relationship has not been established, administration of injections preserved with benzyl alcohol has been associated with toxicity in neonates. Toxicity appears to have resulted from administration of large amounts (i.e., about 100Ð400 mg/kg daily) of benzyl alcohol in these neonates. Its use is principally associated with the use of bacteriostatic 0.9% sodium chloride intravascular flush or endotracheal tube lavage solutions.