Atrial fibrillation is a disorder of heart rate and rhythm. Also commonly abbreviated as AF or Afib, it occurs when the heart's two small, upper chambers (atria) quiver rapidly and empty blood into the heart's lower chambers (ventricles) in a disorganized manner instead of beating effectively. Blood that isn't pumped completely out of the atria when the heart beats may pool and clot. If a piece of a clot enters the bloodstream, it may lodge in the brain causing a stroke. Causes of atrial fibrillation include dysfunction of the sinus node (the heart's pacemaking area in the right atrium), coronary artery disease, rheumatic heart disease, hypertension and hyperthyroidism.