During the first 3 months after aortic or mitral valve replacement with a bioprosthesis, the guidelines indicate that it is reasonable to give warfarin to achieve an INR of 2.0 to 3.0 (Class IIa; Level of Evidence C). In the first 3 months after bioprosthetic valve implantation, aspirin is recommended for aortic valves; the combination of aspirin and clopidogrel is recommended if the aortic valve is transcatheter; and vitamin K antagonist therapy with a target INR of 2.5 is recommended for mitral valves. After 3 months, aspirin is recommended.459 A meta-analysis of 46 studies comprising 13088 patients who received mechanical mitral or aortic valve prostheses reported an incidence of valve thrombosis or embolism in the absence of antithrombotic therapy of 8.6 per 100 patient-years (95% CI, 7.0 10.4). Risk of embolism was lower in patients with tilting disk and bileaflet valves than in those with caged ball valves (no longer used).470 Antithrombotic therapy with a vitamin K antagonist reduced the risk of thromboembolic events to 1.8 per 100 patient-years (95% CI, 1.7 1.9). Even among anticoagu- lated patients, the risk of embolism is higher among those with mechanical mitral valves than mechanical aortic valves.471,472 ACC/AHA guidelines for the management of patients with val- vular heart disease recommend warfarin (INR, 2.0 to 3.0) after aortic valve replacement with bileaflet mechanical or Medtronic Hall prostheses in patients with no risk factors (Class I; Level of Evidence B), warfarin (INR, 2.5 to 3.5) in patients with risk factors (Class I; Level of Evidence B), and warfarin (INR, 2.5 to 3.5) after mitral valve replacement with any mechanical valve (Class IIa; Level of Evidence C).460 The addition of low- dose aspirin to warfarin is recommended for all patients with mechanical valves (Class IIa; Level of Evidence C). 