Heparin (also known as calciparine {United States, United Kingdom, Canada}, liquaemin{United States}, calcilean/hepalean/heparin leo {Canada}) is an injectable anticoagulant, nowadays usually made synthetically. It is used both as an anticoagulant in people, and in various medical devices such as test tubes and extracorporal circulation devices such as renal dialysis machines.
Native heparin is a glycosaminoglycan with a molecular weight ranging from 6 to 40 kDa. The average molecular weight of most commercial heparin preparations is in the range of 12 to 15 kDa. Heparin consists of alternating units of sulfated D-glucosamine and D-glucuronic acid . Because of its ester and amide groups of sulfuric acid it exists as the anion at physiologic pH and is usually administered as the sodium salt.
History
Heparin was originally isolated from liver cells, hence its name (hepar is Latin for "liver"). Scientists were looking for an anticoagulant that could work safely in humans, and a second-year medical student from Johns Hopkins University found a compound extracted from liver that could work.
Mechanism of action
Heparin works by potentiating the action of antithrombin III, as it is similar to the heparan sulfate proteoglycans which are naturally present on the cell membrane of the endothelium. Because antithrombin III inactivates many coagulation proteins, the process of coagulation will slow down.
The effects of heparin are measured in the lab by the partial thromboplastin time (APTT), (the time it takes the blood plasma to clot).
Administration
Heparin has to be adminstered parenterally: it is digested when taken by mouth. It can be injected intravenously, into a muscle, or subcutaneously (under the skin). Because of its short biologic half-life of approximately one hour, heparin must be given frequently or as a continuous infusion.
If long-term anticoagulation is required, heparin is often only used to commence anticoagulation therapy until the oral anticoagulant warfarin is working effectively.
Medical use
When given parenterally, heparin acts as an anticoagulant, preventing the formation of clots or extension of existing clots within the blood. While heparin does not break down clots that have already formed, it allows the body's natural clot lysis mechanisms to work normally to break down clots that have already formed.
Other uses
Test tubes, Vacutainers, and capillary tubes that use lithium heparin as an anticoagulant are usually marked with green stickers and green tops. Heparin has the advantage over EDTA as an anticoagulant, as it does not affect levels of ions (such as calcium). Heparin can interfere with some immunoassays, however. As lithium heparin is usually used, a person's lithium levels cannot be obtained from these tubes; for this purpose royal-blue topped Vacutainers containing sodium heparin are used.
Unfractioned heparin is under many circumstances being replaced by low molecular weight heparin preparations, including prophylaxis against thrombosis. This is due to the fact that monitoring of the APTT is not required and there is a reduced risk of osteoporosis and heparin induced thrombocytopenia (HIT).
Treatment of overdose
In case of overdose, protamine sulfate can be given to counteract the action of heparin.
Adverse reactions
A serious side effect of heparin is heparin induced thrombocytopenia (HIT syndrome). HIT is caused by an immunological reaction that makes platelets form clots within the blood vessels, thereby using up coagulation factors. Formation of platelet clots can lead to thrombosis, while the loss of coagulation factors and platelets may result in bleeding. HIT can (rarely) occur shortly after heparin is given, but also when a person has been on heparin for a long while. Immunologic tests are available for the diagnosis of HIT. There is also a benign form of thrombocytopenia associated with heparin use.
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Last updated: 05-13-2005 07:56:04
