Heparin

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 mucopolysaccharide 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 exits 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 researchers in 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.

Because of the serious side effects and because heparin must be injected, it 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 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 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