A life-threatening arrhythmia usually develops in a person with a pre-existing heart condition, such as: • Coronary artery disease. Most cases of sudden cardiac arrest occur in people who have coronary artery disease. In coronary artery disease, your arteries become clogged with cholesterol and other deposits, reducing blood flow to your heart. This can make it harder for your heart to conduct electrical impulses smoothly. • Heart attack. If a heart attack occurs, often as a result of severe coronary artery disease, it can trigger ventricular fibrillation and sudden cardiac arrest. In addition, a heart attack can leave behind areas of scar tissue. Electrical short circuits around the scar tissue can lead to abnormalities in your heart rhythm. • Enlarged heart (cardiomyopathy). This occurs primarily when your heart's muscular walls stretch and enlarge or thicken. In both cases, your heart's muscle is abnormal, a condition that often leads to heart tissue damage and potential arrhythmias. • Valvular heart disease. Leaking or narrowing of your heart valves can lead to stretching or thickening of your heart muscle or both. When the chambers become enlarged or weakened because of stress caused by a tight or leaking valve, there's an increased risk of developing arrhythmia. • Congenital heart disease. When sudden cardiac arrest occurs in children or adolescents, it may be due to a heart condition that was present at birth (congenital heart disease). Even adults who've had corrective surgery for a congenital heart defect still have a higher risk of sudden cardiac arrest. • Electrical problems in the heart. In some people, the problem is in the heart's electrical system itself instead of a problem with the heart muscle or valves. These are called primary heart rhythm abnormalities and include conditions such as Brugada's syndrome and long QT syndrome.