Heparin and low molecular weight heparin

Heparin and low molecular weight heparin

Heparin is found in the secretory granules of mast cells. Heparin is a catalytic template for the interaction of antithrombin and thrombin. Heparin produces a conformational change in antithrombin making it more reactive to thrombin. Heparin prolongs activated partial thromboplastin time. Heparin causes release of lipoprotein lipase into the circulation and clears lipemic plasma. Heparin is degraded and cleared by the reticuloendothelial system.

Protamine sulphate is the heparin antidote. 1 mg for 100 units of heparin left is needed. Dose of protamine for neutralisation can be calculated taking into account the initial dose administered, time elapsed and the half life of heparin. Protamine is routinely used for reversal of anticoagulation when coming off cardiopulmonary bypass in open heart surgery. Protamine is also useful during interventional procedures when there is a cardiac tamponade or access site hematoma. But in case of cardiac tamponade, reversal is deferred till pericardiocentesis is completed. Otherwise blood in the pericardial cavity will clot and further drainage by aspiration becomes impossible.

Low Molecular Weight Heparin has more anti factor Xa activity than heparin. The incidence of heparin induced thrombocytopenia (HIT) is lesser with LMWH. But LMWH cannot be used as an anticoagulant in a person who has already developed heparin induced thrombocytopenia syndrome. The bioavailability of LMWH is more predictable than that of unfractionated heparin. LMWH activity can be measured by factor Xa activity. No effective antidote is available for neutralization of LMWH though protamine sulphate may give partial neutralization.