Randomized on-X Anticoagulation Trial

Eligibility Details: Inclusion Criteria: - Patients requiring isolated aortic valve replacement (AVR), or isolated mitral valve replacement (MVR). - AVR patients receiving low dose or antiplatelet only anticoagulation will be divided into groups at low risk and high risk for thromboembolism with all patients being in the low risk group except for patients with the following conditions which place a patient in the high risk group: - Chronic atrial fibrillation - Left ventricular ejection fraction < 30 % - Enlarged left atrium >50mm diameter - Spontaneous echo contrasts in the left atrium - Vascular pathology - Neurological events - Hypercoagulability - Left or right ventricular aneurysm - Lack of platelet response to aspirin or clopidogrel - Women receiving estrogen replacement therapy - Concomitant cardiac surgery is allowed - Adult patients Exclusion Criteria: - Right side valve replacement - Double (aortic plus mitral) valve replacement - Patients with active endocarditis at the time of implant - Previous confirmed or suspected thromboembolic event or thrombophlebitis - Other terminal illness - Patients who are in an emergency state - Inability to return for required follow-ups - Patients with an On-X valve implanted within the study and subsequently explanted - Patients who are known to be pregnant, plan to become pregnant or are lactating - Patients with acquired immunodeficiency syndrome or know to be HIV positive - Patients who are prison inmates or known drug or alcohol abusers - Patients unable to give adequate informed consent.