The average annual rate of ischemic stroke among patients with nonvalvular atrial fibrillation is 5%.2-7 The rate of stroke in atrial-fibrillation patients increases with age and the presence of other risk factors such as a prior stroke or transient ischemic attack (TIA), hypertension, diabetes mellitus, and congestive heart failure.8,9Since the clotting factor cascade plays a critical role in thr