This article discusses the following five causes of sudden death: deadly arrhythmias, acute myocardial infarction, intracranial bleeding, major stroke (cerebrovascular accident), massive pulmonary embolism, and acute aortic catastrophe. Arrhythmias are very common in people who are not aware they are having a heart attack (no symptoms). The most common type of arrhythmia is called atrial fibrillation. The patient with atrial fibrillation may have no symptoms, or he or she may have symptoms of an irregular heartbeat (e.g., chest pain, shortness of breath, dizziness). A patient with atrial fibrillation is usually treated with one of two drugs: amiodarone or flecainide. These medications can be effective in preventing future episodes of atrial fibrillation or other related problems. Atrial fibrillation can be a sign of another problem called cardiomyopathy. Cardiomyopathy is a disease of the heart muscle that causes it to become weak and lose its ability to pump blood effectively. Some forms of cardiomyopathy are caused by problems with the structure of some of the heart's chambers (e.g., dilated cardiomyopathy) while others are due to problems with the heart's electrical system (e.g., arrhythmogenic cardiomyopathy).