With regards to anticoagulants in microsurgery:a. Heparin and antithrombin III have similar mechanisms of action
Unfractionated and fractionated heparins have different proportions of anti-Factor Xa and antithrombin activity. Heparin exerts its anticoagulant activity principally by binding antithrombin III. This causes increased exposure of the antithrombin III active site that, in turn, inactivates the coagulation enzymes, Factor IIa, IXa and Xa. Fractionated heparins also bind antithrombin III but have greater anti-Factor Xa activity rather than greater antithrombin activity. The frequency of heparin-induced thrombocytopenia varies greatly depending on, amongst other factors, the type of heparin administered and the patient population receiving it. In a large clinical trial, serologically-confirmed heparin-induced thrombocytopenia was approximately 1% at 7 days and 3% at 14 days in patients receiving prophylactic unfractionated heparin and 0% in patients receiving prophylactic fractionated heparin. Aspirin is contraindicated in patients under the age of 16 years when used other than as an antiplatelet agent (due to the risk of Reye’s syndrome).
1. Warkentin TE, Levine MN, Hirsh J, Horsewood P, Roberts RS, Gent M, Kelton JG. Heparin-induced thrombocytopenia in patients treated with low-molecular-weight heparin or unfractionated heparin. N Engl J Med 1995; 332: 1330.