Heparin works very quickly, but has a short half-life of roughly 1 hour. It is for this reason it must be given frequently or administered as a continuous infusion. An exception to this is low-molecular-weight heparin (enoxaparin), which acts more on factor Xa and has a longer half-life and bioavailability.
Heparin binds to the enzyme inhibitor antithrombin III, causing its activation. Antithrombin, now activated, leads to the inactivation of thrombin (factor IIa) and other proteases involved in blood clotting, most notable factor Xa.
Activated antithrombin, leads to the inactivation of thrombin (factor IIa) and other proteases involved in blood clotting, most notable factor Xa. Antithrombin's effects are increased 1000-fold due to the activation via heparin, leading to vastly decreased coagulability.
Bleeding is a side effect seen with (unfractionated) heparin if administered in too large a dose. High dose administration of heparin should be monitored by following the PTT. Patients taking low-molecular weight heparin have a lesser risk of bleed and do not need the PTT to be followed.
A serious side effect seen 5-10 days after heparin administration is HIT, or heparin-induced thrombocytopenia. This is described as an immunological reaction leading to pathological platelet activation and clearance, causing thrombocytopenia.
For rapid reversal of heparin, protamine sulfate is used as an antidote. It is highly positively charged, and binds to negatively charged heparin. Low-molecular-weight heparin is not easily reversible.
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