Atrial Fibrillation – a heart rhythm disorder that results in the top chambers of the heart (atria) having disorganized atrial activity which is fast and irregular.  Atrial fibrillation is the most common cardiac rhythm disturbance.  The upper chambers of the heart (atria) fibrillate (quiver) resulting in uncoordinated contractions. This may also result in the bottom chamber (ventricle) also to increase its rate.