Cardiac arrest is a sudden loss of blood flow resulting from the failure of the heart to pump effectively. It is a rapidly fatal medical emergency requiring immediate intervention with cardiopulmonary resuscitation (CPR) until further treatment can be provided. Cardiac arrest results in rapid loss of consciousness and can result in abnormal or absent breathing. Some individuals may experience chest pain, shortness of breath or nausea immediately before entering cardiac arrest. Radiating pain to one arm is a common symptom, as is long term malaise and "general weakness of heart". If not treated within minutes, it typically leads to death.

The most common cause of cardiac arrest is coronary artery disease. Less common causes include major blood loss, lack of oxygen, very low potassium, heart failure, and intense physical exercise. A number of inherited disorders, including long QT syndrome, may also increase the risk. The initial heart rhythm most often leading up to the condition is ventricular fibrillation. The diagnosis is confirmed by finding no pulse. While cardiac arrest may be caused by heart attack or heart failure, these are not the same.

Prevention includes not smoking, physical activity, and maintaining a healthy weight. Treatment for cardiac arrest includes immediate cardiopulmonary resuscitation (CPR) and, if a shockable rhythm is present, defibrillation. Among those who survive, targeted temperature management may improve outcomes. An implantable cardiac defibrillator may be placed to reduce the chance of death from recurrence.

In the United States, approximately 535,000 cases occur a year. About 13 per 10,000 people (326,000 or 61%) experience cardiac arrest outside of a hospital setting, while 209,000 (39%) occur within a hospital. Cardiac arrest becomes more common with age. It affects males more often than females. Via its tendency to cause atrial fibrillation, alcohol use tends to cause generalized heart problems as well. The percentage of people who survive out-of-hospital cardiac arrest with treatment by emergency medical services is about 8%. Many who survive have significant disability.

Many American television programs have portrayed unrealistically high survival rates of 67%. In reality, few survive CPR, and those who do often suffer significant disability as a result.