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Anaesthesia => Drug => Anticoagulant
Anticoagulant
Anticoagulant, substance that prevents the process of coagulation (clotting) of blood. Anticoagulants are distinct from fibrinolytic drugs, which cause blood clots to dissolve, and from anti-platelet drugs, which prevent blood platelet aggregation.
Anticoagulants are mainly used to prevent thromboembolism, the abnormal clotting of blood within the blood vessels, in patients at increased risk, and to prevent further thromboembolism in cases where clot formation has already occurred. Risk factors associated with thromboembolism include atrial fibrillation, or irregular heart rate; myocardial infarction, that is, the death of heart muscle leading to heart attack; surgery; major trauma; presence of artificial heart valves; prolonged immobility; obesity; and high blood pressure during pregnancy. Anticoagulants also include those chemicals that are used to prevent clotting of blood that is taken from patients for investigation or transfusion purposes.
Anticoagulants may be administered by injection, such as heparin, or by mouth, as with warfarin. Heparin is a complex polysaccharide (a form of carbohydrate) that exists as a natural anticoagulant in humans. It carries a strong electronegative charge and consequently inhibits almost every sequence in the blood clotting cascade mechanism. It must be administered by injection because it cannot be absorbed from the gut. It is usually used to initiate rapid anticoagulation, or in circumstances where oral anticoagulants cannot be administered or are contraindicated owing to increased risk of adverse effects, such as in pregnancy. Heparin's main adverse effect is haemorrhage, which may be reversed by the administration of a positively-charged protein, protamine.
Oral anticoagulants interfere with clotting that involves the synthesis of the vitamin K. As a consequence the full anticoagulant effect does not take place until 72 to 96 hours after the dose is taken. Haemorrhaging is also the main adverse effect with these drugs and is countered by withdrawal of the drug and intravenous administration of vitamin K. In emergencies, it may be necessary to supply the missing coagulating factors by giving a blood transfusion. Warfarin and related anticoagulants are generally not prescribed for pregnant women because of the risk of congenital abnormalities in early pregnancy and the risk of haemorrhage in late pregnancy. In some circumstances, however, the benefits from continued satisfactory maternal health outweigh these risks and women are carefully monitored and counselled accordingly.
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