meron,
thanks for the post.
If you have persistent atrial fibrillation you should be on coumadin regardless of ejection fraction. The problem with fibrillaiton is in the atrium and not the ventricle where the atria do not squeeze normally leading to blood clots.
good luck
People with very low ejection fractions are often given blood thinners due to increased risk for clotting. 55% is normal.