Unfractionated heparin (better known as just heparin) is a naturally occuring molecule while low molecular weight heparins are synthetic analogues. Low molecular weight heparins have the convenience of a smaller volume of injection and a longer half life and hence ease of administration.
Unfractionated Heparin
Heparin is found in the secretory granules of mast cells. Heparin is a catalytic template for the interaction of anti thrombin and thrombin. Heparin produces a conformational change in anti thrombin making it more reactive to thrombin. Heparin prolongs activated partial thromboplastin time (aPTT). 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.
Low molecular weight heparin (LMWH)
Low molecular weight heparin has more anti factor Xa activity than heparin. The incidence of heparin induced thrombocytopenia is lesser with low molecular weight heparin. But LMWH cannot be used as an anticoagulant in a person who has already developed heparin induced thrombocytopenia syndrome (HIT). The bioavailability of LMWH is more predictable than that of unfractionated heparin. LMWH activity can be measured by factor Xa activity. No anti dote is available for neutralization of LMWH.