Heparin, prepared commercially from animal tissue, is an anti-thrombolytic agent used to treat and prevent clot formation. Be-cause it doesn’t affect the synthesis of clotting factors, heparin can’t dissolve already-formed clots.
The two types of heparin are unfractionated heparin (UFH) and low-molecular-weight heparin (LMWH). LMWHs, such as dal-teparin, enoxaparin, and tinzaparin, were developed to prevent deep vein thrombosis (DVT) (a blood clot in the deep veins, usual-ly of the legs) in surgical patients.
Because heparin isn’t absorbed well from the GI tract, it must be administered parenterally. UFH is administered I.V. by continuous infusion or by subQ injection. LMWHs, because of their prolonged circulating half-life, can be administered once or twice daily by subQ injection.
After I.V. administration, the distribution of heparin is immedi-ate; however, distribution isn’t as predictable following subQ in-jection.
Heparin isn’t given I.M. because of the risk of localized bleeding. Heparin is metabolized in the liver, and its metabolites are excret-ed in urine.
Heparin prevents the formation of new thrombi.Here’s how it works:
§ Heparin inhibits the formation of thrombin and fibrin by activating antithrombin III.
§ Antithrombin III then inactivates factors IXa, Xa, XIa, and XIIa in the intrinsic and common pathways. The end result is prevention of a stable fibrin clot.
§ In low doses, heparin increases the activity of an-tithrombin III against factor Xa and thrombin and in-hibits clot formation.
§ Much larger doses are necessary to inhibit fibrin
§ formation after a clot has been formed. This relationship between dose and effect is the rationale for using low-dose heparin to pre-vent clotting.
§ Whole blood clotting time, thrombin time, and partial thrombo-plastin time (PTT) are prolonged during heparin therapy. Howev-er, these times may be only slightly prolonged with low or ultra-low preventive doses.
Heparin may be used in a number of clinical situations to prevent the formation of new clots or the extension of existing clots. These situations include:
· preventing or treating venous thromboemboli, characterized by inappropriate or excessive intravascular activation of blood clot-ting treating disseminated intravascular coagulation, a complication of other diseases, resulting in accelerated clotting
· treating arterial clotting and preventing embolus formation in patients with atrial fibrillation, an arrhythmia in which ineffective atrial contractions cause blood to pool in the atria, increasing the risk of clot formation
· preventing thrombus formation and promoting cardiac circula-tion in an acute myocardial infarction (MI) by preventing further clot formation at the site of the already formed clot.
Heparin can be used to prevent clotting whenever the patient’s blood must circulate outside the body through a machine, such as the cardiopulmonary bypass machine or hemodialysis machine, and during blood transfusions. (See Monitoring PTT in heparintherapy.)
Heparin is also useful for preventing clotting during intra-abdominal or orthopedic surgery. (These types of surgeries, in many cases, activate the coagulation mechanisms excessively.) In fact, heparin is the drug of choice for orthopedic surgery.
· Because heparin acts synergistically with all oral anticoagu-lants, the risk of bleeding increases when the patient takes both drugs together. The prothrombin time and International Normal-ized Ratio (INR), used to monitor the effects of oral anticoagu-lants, may also be prolonged.
· The risk of bleeding increases when the patient takes nons-teroidal anti-inflammatory drugs (NSAIDs), iron dextran, cilosta-zol, or an antiplatelet drug, such as aspirin, clopidogrel, ticlopi-dine, or dipyridamole, while receiving heparin.
§ Drugs that antagonize or inactivate heparin include antihista-mines, cephalosporins, digoxin, neomycin, nicotine, nitroglycerin, penicillins, phenothiazines, quinidine, and tetracycline.
§ Nicotine may inactivate heparin; nitroglycerin may inhibit the effects of heparin.
§ Administration of protamine sulfate and fresh frozen plasma counteract the effects of heparin. (See Adverse reactions to he-parin.)