What Causes Sudden Cardiac Arrest?

Most cases of sudden cardiac arrest (SCA) are due to an arrhythmia called ventricular fibrillation (v-fib). In v-fib, the ventricles (the large pumping chambers of the heart) quiver very rapidly and irregularly instead of beating normally. When this happens, the heart pumps little or no blood to the body. Death results if the arrhythmia is not treated within a few minutes.

Other electrical problems that can cause SCA are extreme slowing of the rate of the heart's electrical signals or when heart muscle stops responding to the electrical signals.

Several factors can cause the electrical problems that lead to SCA. These factors include:

• Coronary artery disease (CAD)
• Severe physical stress
• Inherited disorders
• Structural changes in the heart

Coronary Artery Disease

CAD occurs when the arteries that supply blood to the heart muscle (the coronary arteries) become hardened and narrowed, causing less blood to flow to the muscle. The arteries harden and narrow because a material called plaque (plak) builds up on their inner walls. As the plaque increases in size, the insides of the coronary arteries narrow, and less blood flows through them to the heart muscle. Eventually, the heart muscle is not able to receive the amount of blood and oxygen that it needs. Reduced blood flow or no blood flow to the heart muscle can result in a heart attack. During a heart attack, some heart tissue dies and turns into scar tissue. This can damage the heart's electrical system, increasing the risk for dangerous arrhythmias and SCA.

Physical Stress

Certain physical stresses can cause the heart's electrical system to fail. The physical stresses that cause this to happen include:

• Major blood loss.
• Severe lack of oxygen.
• Very low blood levels of potassium or magnesium.
• Intense exercise. The hormone adrenaline released during intense exercise can trigger SCA in people who have other heart problems.

Inherited Disorders

A tendency to develop arrhythmias runs in some families. This tendency is inherited, which means it is passed from parents to children. Members of these families might have an increased chance of having SCA. Other people are born with inherited structural defects in their hearts that may increase their chance of having SCA.

Structural Changes in the Heart

Changes in the heart's normal size or structure can affect its electrical system. Such changes include a heart enlarged by blood pressure or advanced heart disease. Heart infections also can cause structural changes in the heart.

Several research studies are under way to try to understand these possible causes of SCA and to find other causes.