Couple of comments on beta blockers and on blood thinners.
Beta blockers work by 2 routes. #1 they reduce the ability of the heart to beat faster - decreasing the workload and oxygen demand of the heart (negative chronotrope). #2 they reduce the ability of the heart to beat forcefully - again decreasing the workload and oxygen demand of the heart and the blood pressure in the system (negative inotrope). They are effective medicines, but do prevent increasing oxygen delivery under higher physical workloads. In plain English, they significantly decrease exercise tolerance.
Blood thinners for atrial fibrillation: The heart is 2 separate pumps side by side; one pumps blood to the body (left side) and the other pumps blood to the lungs (right side). Each pump has 2 chambers; the atria - helps fill the ventricles; and the ventricles - pumps the blood into the respective circulations. Fibrillation is a ineffective quivering of the chamber and pumps no blood. If the ventricle fibrillates, then the person dies without immediate treatment. This is the most common cause of cardiac arrest. If the atria fibrillates, not much happens unless the whole heart beats too fast. People live normal lives with the atria in constant fibrillation (atrial fibrillation or a.fib for short). A.fib has 2 dangers. First is the entire heart beating too fast. This is where the beta blockers come in. They are used for heart rate control. The second danger is having a blood clot or clots form in the poorly emptying atria. Since the blood in the atria isn't emptying well, it stagnates. Stagnant blood tends to clot. Clots in the heart are then sent out to either the lungs or the body (including the brain). A clot in the lung can cause death with a pulmonary embolus (prevents oxygenation of the blood) or a clot to the brain causes a stroke. How much damage depends on the size of the clot and where it lodges. Blood thinners (coumadin, warfarin) are used to reduce the risk of a blood clot. They have their own dangers - but not really and issue here.
As for Barracuda2's question: 3 episodes in 7 years, I can see his doctor advising either way. But blood thinners by themselves shouldn't change his exercise tolerance. Beta blockers certainly can.