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). The novel oral anticoagulants (factor Xa inhibitors and direct thrombin inhibitors) are not indicated for the prevention of thromboembolism associated with mechanical heart valves. 