New oral anticoagulants for thromboprophylaxis in patients having hip or knee arthroplasty

The newest oral anticoagulants, dabigatran etexilate (hereafter referred to as dabigatran) and rivaroxaban, have been approved in more than 70 countries for prevention of venous thromboembolism after elective hip or knee arthroplasty. Dabigatran targets thrombin (factor IIa) and rivaroxaban targets factor Xa1 (figure⇓). Sites of action of the new oral anticoagulants. By targeting thrombin, thrombin inhibitors (such as dabigatran) block thrombin mediated conversion of fibrinogen to fibrin, which leads to clot formation, as well as thrombin mediated feedback activation of coagulation factors V and VIII. Factor Xa inhibitors (such as rivaroxaban and apixaban) block the conversion of prothrombin to thrombin by the prothrombinase complex (the complex of factor Xa and factor Va bound to the surface of the activated platelet). Heparin targets thrombin and factor Xa equally well, whereas low molecular weight heparin targets factor Xa to a greater extent than thrombin, and fondaparinux targets only factor Xa. Vitamin K antagonists, such as warfarin, reduce the levels of factors II, VII, IX, and X as well as protein C and S #### Case scenario A 78 year old woman has had an elective left hip arthroplasty and on discharge her orthopaedic surgeon prescribes a further, 28 day postoperative course of anticoagulant prophylaxis. She sees her general practitioner a week later, concerned that these “blood thinners” are tablets, whereas she had daily injections after her knee replacement three years ago. Her general practitioner explains that the tablets have recently become available and that guidelines recommend them as an alternative to injections as they are as effective, more convenient, and safe. Evidence based guidelines recommend anticoagulant thromboprophylaxis with subcutaneous agents such as low molecular weight heparin or fondaparinux or oral agents such as warfarin, dabigatran, or rivaroxaban for at least 10 days after knee arthroplasty and for up to 35 days after …