This trial will include patients who have a heart condition called atrial fibrillation. Atrial fibrillation is an abnormal rhythm (irregular beat) in the heart. Patients with atrial fibrillation have an increased chance for a blood clot to form in the heart and move to other blood vessels in the body and cause obstruction. This obstruction may damage tissue. For example, a blood clot plugging a vessel in the brain could cause a stroke. Therefore, patients with atrial fibrillation may be given anticoagulant (blood-thinning) tablets such as warfarin or acenocoumarol.