In atrial fibrillation (AFib), the atria fibrillate, or quiver, instead of contracting in a coordinated and rhythmic manner. Atrial Fibrillation by Carl Juan. Title Atrial Fibrillation. Because the abnormal heart rhythm can trigger the production of blood clots in the atria, which can then migrate to other regions of the body, including the brain, AFib is associated with an increased risk of consequences, notably stroke.