context: Sudden cardiac death ( SCD) is a sudden, unexpected death caused by a lethal heart rhythm (sudden cardiac arrest). It is a leading cause of ''natural deaths'' in the U. S., causing about 295,000 adults to die each year. SCD is responsible for half of all heart disease deaths. SCD occurs most frequently in adults in their mid-30s to mid-40s, and affects men twice as often as it does women. SCD is rare in children, although the exact prevalence is not known. Sudden cardiac arrest can be treated if addressed within the first few minutes. The American Heart Association promotes the following four steps, called "the chain of survival:" Early Access to Care. Quick contact with emergency care is essential. Call 911 (in most communities) or your local emergency number immediately. Early Cardiopulmonary Resuscitation ( CPR). Learning CPR is the greatest gift you can give your family and friends. If performed properly (see below), CPR can help save a life until emergency medical help arrives. Early Defibrillation. In most adults, sudden cardiac death is related to ventricular fibrillation. Quick defibrillation (delivery of an electrical shock) is necessary to return the heart rhythm to a normal heartbeat. Many public places, such as malls, golf courses, and airports, have automated external defibrillators ( AEDs; see below) available for use in emergency situations. Early Advanced Care. After successful defibrillation, most patients require hospital care to treat and prevent future events. These four steps can increase survival as much as 90% if initiated within the first minutes after sudden cardiac arrest. Survival decreases by about 10% each minute longer. CPR (cardiopulmonary resuscitation) is an emergency technique used to help someone whose heart and/or breathing has stopped. When a person's heart stops, blood stops circulating throughout the body. If a person stops breathing, the blood can't get oxygen. Therefore, it is vital for people in this emergency situation to receive medical treatment, such as CPR, within the first few minutes of the event. By administering a combination of manual chest compressions and artificial, or "mouth-to-mouth," respiration, the rescuer can breathe for the victim and help circulate some of the blood throughout their body. But even without mouth-to-mouth, hands only CPR can be very effective. CPR does not restart a heart that has stopped, but it can keep a victim alive until more aggressive treatment (defibrillation) can be administered. CPR is not difficult to learn, and many organizations offer courses in CPR, including the American Heart Association, the American Red Cross, and local community centers, health clubs, and YMCAs. An AED, an automatic external defibrillator, is a device used to administer an electric shock through the chest wall to the heart. The device has built-in computers that assess the victim's heart rhythm, judge whether defibrillation is needed, and then administer the shock. Audible and/or visual prompts guide the user through the process. Most AEDs are designed to be used by non-medical people such as fire department personnel, police officers, lifeguards, flight attendants, security guards, teachers, and even family members of high-risk persons. The goal is to provide access defibrillation when needed as quickly as possible. CPR along with AEDs can dramatically increase survival rates for sudden cardiac arrest. No, the AED treats only a heart in an abnormal rhythm suitable for defibrillation. If a person is in cardiac arrest without such a rhythm, the heart will not respond to electric currents. CPR should be administered until medical help arrives.