Coumadin v. Aspiring for atrial fib + mitral valve prolapse

[craddle]

My father is 86 y.o. and has been on coumadin therapy for several years for tachycardia + atrial fibrillation + mitral valve prolapse. My brother wants to get him off of the coumadin therapy and use aspirin instead, but based on my research, I can't find any studies that would show how to deal with such a transition or if it is even possible for someone in such a high risk category. He also suffered at TIA two weeks ago, even on coumadin with an INR between two and three. We are now trying to keep him between 2.5 and 3 now. Any input on this case? Thanks a million.