Sudden cardiac death (SCD) is a sudden, unexpected death caused by loss of heart function (sudden cardiac arrest). It is the largest cause of natural death in the U.S., causing about 325,000 adult deaths in the United States 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, affecting only 1 to 2 per 100,000 children each year.

How Is Sudden Cardiac Arrest Different from a Heart Attack?

Sudden cardiac arrest is not a heart attack (myocardial infarction) but can occur during a heart attack. Heart attacks occur when there is a blockage in one or more of the arteries to the heart, preventing the heart from receiving enough oxygen-rich blood. If the oxygen in the blood cannot reach the heart muscle, the heart becomes damaged.

In contrast, sudden cardiac arrest occurs when the electrical system to the heart malfunctions and suddenly becomes very irregular. The heart beats dangerously fast. The ventricles may flutter or quiver (ventricular fibrillation), and blood is not delivered to the body. In the first few minutes, the greatest concern is that blood flow to the brain will be reduced so drastically that a person will lose consciousness. Death follows unless emergency treatment is begun immediately.

Emergency treatment includes cardiopulmonary resuscitation (CPR) and defibrillation. CPR is a manual technique using repetitive pressing to the chest and breathing into the person's airways that keeps enough oxygen and blood flowing to the brain until the normal heart rhythm is restored with an electric shock to the chest, a procedure called defibrillation. Emergency squads use portable defibrillators and frequently there are public access defibrillators (AEDs, ambulatory external defibrillators) in public locations that are intended to be available for use by citizens who observe cardiac arrest.

What Are the Symptoms of Sudden Cardiac Arrest?

Some people may experience symptoms of sudden cardiac arrest, such as a racing heartbeat or they may feel dizzy, alerting them that a potentially dangerous heart rhythm problem has started. In over half of the cases, however, sudden cardiac arrest occurs without prior symptoms.

What Causes Sudden Cardiac Death?

Most sudden cardiac deaths are caused by abnormal heart rhythms called arrhythmias. The most common life-threatening arrhythmia is ventricular fibrillation, which is an erratic, disorganized firing of impulses from the ventricles (the heart's lower chambers). When this occurs, the heart is unable to pump blood and death will occur within minutes, if left untreated.

What Are the Risk Factors of Sudden Cardiac Arrest?

There are many risk factors that can increase a person's risk of sudden cardiac arrest and sudden cardiac death, including the following:

Previous heart attack with a large area of the heart damaged (75 percent of SCD cases are linked to a previous heart attack).
A person's risk of SCD is higher during the first six months after a heart attack.
Coronary artery disease (80 percent of SCD cases are linked with this disease).
Risk factors for coronary artery disease include smoking, family history of heart disease, and high cholesterol.

Other risk factors of sudden cardiac arrest include:

Ejection fraction of less than 40 percent, combined with ventricular tachycardia.
Prior episode of sudden cardiac arrest.
Family history of sudden cardiac arrest or SCD.
Personal or family history of certain abnormal heart rhythms, including long or short QT syndrome, Wolff-Parkinson-White syndrome, extremely low heart rates, or heart block.
Ventricular tachycardia or ventricular fibrillation after a heart attack.
History of congenital heart defects or blood vessel abnormalities.
History of syncope (fainting episodes of unknown cause).
Heart failure: a condition in which the heart's pumping power is weaker than normal. Patients with heart failure are 6 to 9 times more likely than the general population to experience ventricular arrhythmias that can lead to sudden cardiac arrest.
Dilated cardiomyopathy (cause of SCD in about 10 percent of the cases): a decrease in the heart's ability to pump
Hypertrophic cardiomyopathy: a thickened heart muscle that especially affects the ventricles.
Significant changes in blood levels of potassium and magnesium (from using diuretics, for example), even if there is not underlying heart disease.
Obesity.
Diabetes.
Recreational drug abuse.
Taking drugs that are "pro-arrhythmic" may increase the risk for life-threatening arrhythmias.