Heparin is indicated to replace warfarin in patients with valve disease requiring antithrombotic treatment. Its use is thus necessary for short periods during which warfarin is contraindicated, but the thromboembolic risk persists. These circumstances, which are common in patients with mechanical prostheses, include: hemorrhagic risk or event complicating an existing thromboembolic risk (heart or extracardiac surgery, severe hemorrhage, end of pregnancy); when an unstable situation develops and imposes the rapid diminution or interruption of anticoagulants (stroke, infectious endocarditis); when immediate efficacy is required, rather than the delayed action of warfarin (onset of atrial fibrillation); and when warfarin is contraindicated (early pregnancy). Regardless of whether