Atrial fibrillation is a disorder found in about 2 million Americans. In it the two small upper chambers of the heart, the atria, quiver instead of beating effectively. Blood isn't pumped completely out of them when the heart beats, so it may pool and clot. If a piece of the blood clot in the atria becomes lodged in an artery in the brain, a stroke results. About 15 percent of strokes occur in people with atrial fibrillation.