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Information |
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Drug Groups
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approved; investigational |
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Description
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Unfractionated heparin (UH) is a heterogenous preparation of anionic, sulfated glycosaminoglycan polymers with weights ranging from 3000 to 30,000 Da. It is a naturally occurring anticoagulant released from mast cells. It binds reversibly to antithrombin III (ATIII) and greatly accelerates the rate at which ATIII inactivates coagulation enzymes thrombin (factor IIa) and factor Xa. UH is different from low molecular weight heparin (LMWH) in the following ways: the average molecular weight of LMWH is about 4.5 kDa whereas it is 15 kDa for UH; UH requires continuous infusions; activated partial prothrombin time (aPTT) monitoring is required when using UH; and UH has a higher risk of bleeding and higher risk of osteoporosis in long term use. Unfractionated heparin is more specific than LMWH for thrombin. Furthermore, the effects of UH can typically be reversed by using protamine sulfate. |
| Indication |
Unfractionated heparin is indicated for prophylaxis and treatment of venous thrombosis and its extension, prevention of post-operative deep venous thrombosis and pulmonary embolism and prevention of clotting in arterial and cardiac surgery. In cardiology, it is used to prevent embolisms in patients with atrial fibrillation and as an adjunct antithrombin therapy in patients with unstable angina and/or non-Q wave myocardial infarctions (i.e. non-ST elevated acute coronary artery syndrome) who are on platelet glycoprotein (IIb/IIIa) receptor inhibitors. Additionally, it is used to prevent clotting during dialysis and surgical procedures, maintain the patency of intravenous injection devices and prevent in vitro coagulation of blood transfusions and in blood samples drawn for laboratory values. |
| Pharmacology |
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. |
| Toxicity |
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 100–400 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. |
| Affected Organisms |
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Humans and other mammals |
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| Biotransformation |
Liver and the reticulo-endothelial system are the sites of biotransformation. The metabolic fate of heparin is not well understood. |
| Absorption |
Heparin must be given parenterally as it is not absorbed through the gastrointestinal mucosa. It is usually given by iv infusion or deep sc injection. The onset of action is immediate after iv injection but can be delayed 20 to 60 minutes following sc injection.
Plasma heparin concentrations may be increased and activated partial thromboplastin times (aPTTs) may be more prolonged in geriatric adults (older than 60 years of age) compared with younger adults. |
| Half Life |
1.5 hours.
The plasma half-life of heparin increases from about 60 minutes with a 100 unit/kg dose to about 150 minutes with a 400 unit/kg dose. |
| Protein Binding |
Very high, mostly to low-density lipoproteins. It is also extensively bound by globulins and fibrinogens. |
| Elimination |
The drug appears to be removed mainly by the reticuloendothelial system. A small fraction of unchanged heparin also appears to be excreted in urine. Heparin cannot be eliminated by hemodialysis. |
| Distribution |
40-70 mL/min (approximately the same as blood volume)
Although heparin does not distribute into adipose tissues, clinicians should use actual body weight in obese patients to account for extra vasculature.
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| Clearance |
Adult Clearance = 0.43 ml/kg/min
25-28 weeks gestation = 1.49 ml/kg/min |
| References |
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Linhardt RJ, Gunay NS: Production and chemical processing of low molecular weight heparins. Semin Thromb Hemost. 1999;25 Suppl 3:5-16.
[Pubmed]
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Ferro DR, Provasoli A, Ragazzi M, Casu B, Torri G, Bossennec V, Perly B, Sinay P, Petitou M, Choay J: Conformer populations of L-iduronic acid residues in glycosaminoglycan sequences. Carbohydr Res. 1990 Jan 15;195(2):157-67.
[Pubmed]
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Mulloy B, Forster MJ, Jones C, Davies DB: N.m.r. and molecular-modelling studies of the solution conformation of heparin. Biochem J. 1993 Aug 1;293 ( Pt 3):849-58.
[Pubmed]
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Hirsh J, Raschke R: Heparin and low-molecular-weight heparin: the Seventh ACCP Conference on Antithrombotic and Thrombolytic Therapy. Chest. 2004 Sep;126(3 Suppl):188S-203S.
[Pubmed]
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Petitou M, Herault JP, Bernat A, Driguez PA, Duchaussoy P, Lormeau JC, Herbert JM: Synthesis of thrombin-inhibiting heparin mimetics without side effects. Nature. 1999 Apr 1;398(6726):417-22.
[Pubmed]
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American Academy of Pediatrics Committee on Fetus and Newborn and Committee on Drugs. Benzyl alcohol: toxic agent in neonatal units. Pediatrics. 1983; 72:356-8. [PubMed] [IDIS 175725] |
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Anon. Benzyl alcohol may be toxic to newborns. FDA Drug Bull. 1982; 12:10-1. [PubMed] |
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Anon. Neonatal deaths associated with use of benzyl alcohol—United States. MMWR Morb Mortal Wkly Rep. 1982; 31:290-1. [PubMed] [IDIS 150868] |
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Abbott Laboratories. Heparin lock flush solution, USP 100 USP units/mL prescribing information. North Chicago, IL; 1998 Jul. |
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Spruill WJ, Wade WE, Huckaby WG, Leslie RB: Achievement of anticoagulation by using a weight-based heparin dosing protocol for obese and nonobese patients. Am J Health Syst Pharm. 2001 Nov 15;58(22):2143-6.
[Pubmed]
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McDonald MM, Jacobson LJ, Hay WW Jr, Hathaway WE: Heparin clearance in the newborn. Pediatr Res. 1981 Jul;15(7):1015-8.
[Pubmed]
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