الفهرس 
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Abstract
Blood coagulation is part of an important host defense mechanism. Upon vessel injury, platelets adhere to macromolecules in subendothelial tissues at the site of injury and then aggregate to form the primary hemostatic plug. Platelets stimulate the local activation of plasma coagulation factors, which leads to the generation of a fibrin clot that reinforces the platelet aggregate.
For more than 50 years, vitamin K antagonists (e.g. warfarin) were the only available oral anticoagulants. Oral direct thrombin inhibitors (e.g Dabigatran) inhibit thrombin by directly binding to the active site of thrombin. Factor Xa is also an attractive target for the design of new anticoagulants (e.g Rivaroxaban, Apixaban).
LMWH and fondaparinux are monitored by anti-Xa level assay. There is no proven method for neutralizing LMWH but, protamine sulfate neutralizes a portion of the anti-Xa activity of LMWH. Recombinant factor VIIa may be effective with fondaparinux.
With major surgeries ,VKAs should be stopped 5 days before surgery and bridged with LMWH SC in patients with a mechanical heart valve, atrial fibrillation, or VTE at high risk for thromboembolism. If LMWH is used for bridging ,it should be withhold 12 hours before surgery and resumed within 24 hours postoperatively.
UFH should be stopped 4 hours before CNB and resumed after at least 1 hour after catheter removal . LMWH should be stopped 12 hours before CNB and resumed 24 hours after catheter removal. With VKAs ,CNB and/or catheter removal should only be performed when the PT is at least at 50% or the INR equal or below 1.4 .