Your cardiologist stopped the beta-blocker (Coreg) because it could have interfered with obtaining a useful result from your exercise nuclear stress test. Information derived from this test is considered significant only if a certain heart rate is achieved and beta-blockers blunt the effects of exercise on heart rate. Its impossible to say whether stopping Coreg before the stress test was directly involved in the development of your subsequent problems. Control of high heart rates would certainly be lessened, but beta-blockers themselves do not suppress atrial fibrillation. You may have experienced atrial fibrillation prior to the stress test, but were not aware of it. Not taking the Coreg simply allowed the rate to be higher.