Atrial fibrillation is a very common abnormal heart rhythm affecting 3 million Americans.  The most dangerous complication of atrial fibrillation is stoke, which can happen when a blood clot forms in the fibrillating heart chambers and travels to the brain. Blood thinners have been the mainstay of treatment for atrial fibrillation.  They reduce the risk of stroke by preventing blood clots.  Warfarin (marketed under the brand name Coumadin) is the most effective available oral blood thinner, but...