The heart usually beats with a regular rhythm controlled by electrical signals. These signals move like a wave from the top chambers of the heart (the atria) to the bottom chambers (the ventricles). In some people, however, the electrical signals become disorganized and make the muscles of the atria quiver rather than beat in an organized fashion. This condition, called atrial fibrillation, produces an irregular heart rhythm that increases the risk for forming blood clots inside the chambers of the heart. These blood clots can be pumped out into the vessels that send blood to the brain, causing a stroke. For this reason, doctors usually treat patients who have atrial fibrillation with blood thinners. Unfortunately, people receiving blood thinners are at risk for bleeding inside the skull (intracranial hemorrhage, or ICH). Elderly people are thought to be more susceptible to ICH when they are being treated with blood thinners. Recently, guidelines have been issued for doctors who treat patients with atrial fibrillation, suggesting that to avoid ICH, patients older than age 75 years who have never had a stroke be treated with lower doses of blood thinners than younger patients.