1. Quiz: Sudden Cardiac Arrest!
2. Visual Guide to Heart Disease Slideshow
3. Heart Symptoms Never to Ignore Slideshow

Most cases of sudden cardiac arrest (SCA) are due to ventricular fibrillation (v-fib). V-fib is a type of arrhythmia. In v-fib, the ventricles (the heart's lower chambers) don't beat normally. Instead, they quiver very rapidly and irregularly.

When this happens, the heart pumps little or no blood to the body. V-fib is fatal if not treated within a few minutes.

Other electrical problems in the heart also can cause sudden cardiac arrest. For example, sudden cardiac arrest can occur if the rate of the heart's electrical signals becomes very slow and stops. sudden cardiac arrest also can occur if the heart muscle doesn't respond to the heart's electrical signals.

Several factors can cause the electrical problems that lead to sudden cardiac arrest. These factors include:

• Coronary artery disease (CAD), which reduces blood flow to the heart muscle
  
  
• Severe physical stress, which raises the risk for abnormal electrical activity in the heart
  
  
• Inherited disorders that disrupt the heart's electrical activity
  
  
• Structural changes in the heart that cause electrical signals to spread abnormally

Several research studies are under way to try to find the exact causes of sudden cardiac arrest and how to prevent them.

Coronary Artery Disease

CAD occurs when a fatty material called plaque (plak) builds up in the coronary arteries. These arteries supply your heart muscle with oxygen-rich blood.

Plaque narrows the arteries and reduces blood flow to your heart muscle. Eventually, an area of plaque can rupture, causing a blood clot to form on the surface of the plaque.

A blood clot can mostly or completely block the flow of oxygen-rich blood to the part of the heart muscle fed by the artery. This causes a heart attack.

During a heart attack, some heart cells die and are replaced by scar tissue. This damages the heart's electrical system. The scar tissue may cause electrical signals to spread abnormally throughout the heart. These changes increase the risk for dangerous ventricular arrhythmias and sudden cardiac arrest.

CAD seems to be the cause of most cases of sudden cardiac arrest in adults. Many of these adults, however, have no signs or symptoms of CAD before having sudden cardiac arrest.

Physical Stress

Certain types of physical stress can cause your heart's electrical system to fail. Examples include:

• Intense physical activity. The hormone adrenaline is released during intense physical activity. This hormone can trigger sudden cardiac arrest in people who have other heart problems.
  
  
• Very low blood levels of potassium or magnesium. These minerals play an important role in your heart's electrical signaling.
  
  
• Major blood loss.
  
  
• Severe lack of oxygen.

Inherited Disorders

A tendency to have arrhythmias runs in some families. This tendency is inherited, which means it's passed from parents to children. Members of these families may be at increased risk for sudden cardiac arrest.

An example of an inherited disorder that makes you more likely to have arrhythmias is long QT syndrome(LQTS). LQTS is a disorder of the heart's electrical activity due to problems with tiny pores on the surface of heart muscle cells. LQTS can cause sudden, uncontrollable, dangerous heart rhythms.

People who inherit structural heart problems also may be at increased risk for sudden cardiac arrest. Many cases of sudden cardiac arrest in children are due to these problems.

Structural Changes in the Heart

Changes in the heart's normal size or structure may affect its electrical system. Examples of such changes include an enlarged heart due to high blood pressure or advanced heart disease. Heart infections also may cause structural changes in the heart.

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