For more information, visit http://www.nhlbi.nih.gov/health/health-topics/topics/scda/

What Causes Sudden Cardiac Arrest?

Ventricular fibrillation (v-fib) causes most sudden cardiac arrests (SCAs). V-fib is a type of arrhythmia.

During 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 problems with the heart's electrical system also can cause SCA. For example, SCA can occur if the rate of the heart's electrical signals becomes very slow and stops. SCA also can occur if the heart muscle doesn't respond to the heart's electrical signals.

Certain diseases and conditions can cause the electrical problems that lead to SCA. Examples include coronary heart disease (CHD), also called coronary artery disease; severe physical stress; certain inherited disorders; and structural changes in the heart.

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

Coronary Heart Disease

CHD is a disease in which a waxy substance called plaque (plak) builds up in the coronary arteries. These arteries supply oxygen-rich blood to your heart muscle.

Plaque narrows the arteries and reduces blood flow to your heart muscle. Eventually, an area of plaque can rupture (break open). This may cause a blood clot to form on the plaque's surface.

A blood clot can partly or fully block the flow of oxygen-rich blood to the portion of heart muscle fed by the artery. This causes a heart attack.

During a heart attack, some heart muscle cells die and are replaced with scar tissue. The scar tissue damages the heart's electrical system. As a result, electrical signals may spread abnormally throughout the heart. These changes to the heart increase the risk of dangerous arrhythmias and SCA.

CHD seems to cause most cases of SCA in adults. Many of these adults, however, have no signs or symptoms of CHD before having SCA.

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 SCA in people who have 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 through the genes. Members of these families may be at higher risk for SCA.

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. Problems with tiny pores on the surface of heart muscle cells cause the disorder. LQTS can cause sudden, uncontrollable, dangerous heart rhythms.

People who inherit structural heart problems also may be at higher risk for SCA. These types of problems often are the cause of SCA in children.

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.

Who Is at Risk for Sudden Cardiac Arrest?

The risk of sudden cardiac arrest (SCA) increases with age. The risk also is higher if you have underlying heart disease. Men are two to three times more likely to have SCA than women.

SCA rarely occurs in children unless they have inherited problems that make them likely to have SCA. Only a very small number of children have SCA each year.

Major Risk Factors

The major risk factor for SCA is coronary heart disease (CHD). Most people who have SCA have some degree of CHD. However, these people may not know that they have CHD until SCA occurs.

Their CHD is "silent"—that is, it has no signs or symptoms. Because of this, doctors and nurses have not detected it. Many SCAs happen in people who have silent CHD and no known heart disease prior to SCA.

Many people who have SCA also have silent, or undiagnosed, heart attacks before SCA happens. These people have no clear signs of heart attack, and they don't even realize that they've had one. The chances of having SCA are higher during the first 6 months after a heart attack.

For more information about CHD risk factors, go to the Health Topics Coronary Heart Disease Risk Factors article.

Other Risk Factors

Other risk factors for SCA include:

• A personal or family history of SCA or inherited disorders that make you prone to arrhythmias
• A personal history of arrhythmias
• Heart attack
• Heart failure
• Drug or alcohol abuse

What Are the Signs and Symptoms of Sudden Cardiac Arrest?

Usually, the first sign of sudden cardiac arrest (SCA) is loss of consciousness (fainting). At the same time, no heartbeat (or pulse) can be felt.

Some people may have a racing heartbeat or feel dizzy or light-headed just before they faint. Within an hour before SCA, some people have chest pain, shortness of breath, nausea (feeling sick to the stomach), or vomiting.

How Is Sudden Cardiac Arrest Diagnosed?

Sudden cardiac arrest (SCA) happens without warning and requires emergency treatment. Doctors rarely diagnose SCA with medical tests as it's happening. Instead, SCA often is diagnosed after it happens. Doctors do this by ruling out other causes of a person's sudden collapse.

Specialists Involved

If you're at high risk for SCA, your doctor may refer you to a cardiologist. This is a doctor who specializes in diagnosing and treating heart diseases and conditions. Your cardiologist will work with you to decide whether you need treatment to prevent SCA.

Some cardiologists specialize in problems with the heart's electrical system. These specialists are called cardiac electrophysiologists.

Diagnostic Tests and Procedures

Doctors use several tests to help detect the factors that put people at risk for SCA.

EKG (Electrocardiogram)

An EKG is a simple, painless test that detects and records the heart's electrical activity. The test shows how fast the heart is beating and its rhythm (steady or irregular). An EKG also records the strength and timing of electrical signals as they pass through each part of the heart.

An EKG can show evidence of heart damage due to coronary heart disease (CHD). The test also can show signs of a previous or current heart attack.

Echocardiography

Echocardiography, or echo, is a painless test that uses sound waves to create pictures of your heart. The test shows the size and shape of your heart and how well your heart chambers and valves are working.

Echo also can identify areas of poor blood flow to the heart, areas of heart muscle that aren't contracting normally, and previous injury to the heart muscle caused by poor blood flow.

There are several types of echo, including stress echo. This test is done both before and after a cardiac stress test. During this test, you exercise (or are given medicine if you're unable to exercise) to make your heart work hard and beat fast.

Stress echo shows whether you have decreased blood flow to your heart (a sign of CHD).

MUGA Test or Cardiac MRI

A MUGA (multiple gated acquisition) test shows how well your heart is pumping blood. For this test, a small amount of radioactive substance is injected into a vein and travels to your heart.

The substance releases energy, which special cameras outside of your body can detect. The cameras use the energy to create pictures of many parts of your heart.

Cardiac MRI (magnetic resonance imaging) is a safe procedure that uses radio waves and magnets to create detailed pictures of your heart. The test creates still and moving pictures of your heart and major blood vessels.

Doctors use cardiac MRI to get pictures of the beating heart and to look at the structure and function of the heart.

Cardiac Catheterization

Cardiac catheterization is a procedure used to diagnose and treat certain heart conditions. A long, thin, flexible tube called a catheter is put into a blood vessel in your arm, groin (upper thigh), or neck and threaded to your heart. Through the catheter, your doctor can do diagnostic tests and treatments on your heart.

Sometimes dye is put into the catheter. The dye will flow through your bloodstream to your heart. The dye makes your coronary (heart) arteries visible on x-ray pictures. The dye can show whether plaque has narrowed or blocked any of your coronary arteries.

Electrophysiology Study

For an electrophysiology study, doctors use cardiac catheterization to record how your heart's electrical system responds to certain medicines and electrical stimulation. This helps your doctor find where the heart's electrical system is damaged.

Blood Tests

Your doctor may recommend blood tests to check the levels of potassium, magnesium, and other chemicals in your blood. These chemicals play an important role in your heart's electrical signaling.

How Is Sudden Cardiac Arrest Treated?

Emergency Treatment

Sudden cardiac arrest (SCA) is an emergency. A person having SCA needs to be treated with a defibrillator right away. This device sends an electric shock to the heart. The electric shock can restore a normal rhythm to a heart that's stopped beating.

To work well, defibrillation must be done within minutes of SCA. With every minute that passes, the chances of surviving SCA drop rapidly.

Police, emergency medical technicians, and other first responders usually are trained and equipped to use a defibrillator. Call 9–1–1 right away if someone has signs or symptoms of SCA. The sooner you call for help, the sooner lifesaving treatment can begin.

Automated External Defibrillators

Automated external defibrillators (AEDs) are special defibrillators that untrained bystanders can use. These portable devices often are found in public places, such as shopping malls, golf courses, businesses, airports, airplanes, casinos, convention centers, hotels, sports venues, and schools.

AEDs are programmed to give an electric shock if they detect a dangerous arrhythmia, such as ventricular fibrillation. This prevents giving a shock to someone who may have fainted but isn't having SCA.

You should give cardiopulmonary resuscitation (CPR) to a person having SCA until defibrillation can be done.

People who are at risk for SCA may want to consider having an AED at home. Currently, one AED, the Phillips HeartStart Home Defibrillator, is sold over-the-counter for home use.

A 2008 study by the National Heart, Lung, and Blood Institute and the National Institutes of Health found that AEDs in the home are safe and effective. However, the benefits of home-use AEDs are still debated.

Some people feel that placing these devices in homes will save many lives because many SCAs occur at home.

Others note that no evidence supports the idea that home-use AEDs save more lives. These people fear that people who have AEDs in their homes will delay calling for help during an emergency. They're also concerned that people who have home-use AEDs will not properly maintain the devices or forget where they are.

When considering a home-use AED, talk with your doctor. He or she can help you decide whether having an AED in your home will benefit you.

Treatment in a Hospital

If you survive SCA, you'll likely be admitted to a hospital for ongoing care and treatment. In the hospital, your medical team will closely watch your heart. They may give you medicines to try to reduce the risk of another SCA.

While in the hospital, your medical team will try to find out what caused your SCA. If you're diagnosed with coronary heart disease, you may have angioplasty or coronary artery bypass grafting. These procedures help restore blood flow through narrowed or blocked coronary arteries.

Often, people who have SCA get a device called an implantable cardioverter defibrillator (ICD). This small device is surgically placed under the skin in your chest or abdomen. An ICD uses electric pulses or shocks to help control dangerous arrhythmias. (For more information, go to "How Can Death Due to Sudden Cardiac Arrest Be Prevented?")

How Can Death Due to Sudden Cardiac Arrest Be Prevented?

Ways to prevent death due to sudden cardiac arrest (SCA) differ depending on whether:

• You've already had SCA
• You've never had SCA but are at high risk for the condition
• You've never had SCA and have no known risk factors for the condition

For People Who Have Survived Sudden Cardiac Arrest

If you've already had SCA, you're at high risk of having it again. Research shows that an implantable cardioverter defibrillator (ICD) reduces the chances of dying from a second SCA.

An ICD is surgically placed under the skin in your chest or abdomen. The device has wires with electrodes on the ends that connect to your heart's chambers. The ICD monitors your heartbeat.

If the ICD detects a dangerous heart rhythm, it gives an electric shock to restore the heart's normal rhythm. Your doctor may give you medicine to limit irregular heartbeats that can trigger the ICD.

Implantable Cardioverter Defibrillator

The illustration shows the location of an implantable cardioverter defibrillator in the upper chest. The electrodes are inserted into the heart through a vein.

An ICD isn't the same as a pacemaker. The devices are similar, but they have some differences. Pacemakers give off low-energy electrical pulses. They're often used to treat less dangerous heart rhythms, such as those that occur in the upper chambers of the heart. Most new ICDs work as both pacemakers and ICDs.

For People at High Risk for a First Sudden Cardiac Arrest

If you have severe coronary heart disease (CHD), you're at increased risk for SCA. This is especially true if you've recently had a heart attack.

Your doctor may prescribe a type of medicine called a beta blocker to help lower your risk for SCA. Other treatments for CHD, such as angioplasty or coronary artery bypass grafting, also may lower your risk for SCA.

Your doctor also may recommend an ICD if you're at high risk for SCA.

For People Who Have No Known Risk Factors for Sudden Cardiac Arrest

CHD seems to be the cause of most SCAs in adults. CHD also is a major risk factor for angina (chest pain or discomfort) and heart attack, and it contributes to other heart problems.

Following a healthy lifestyle can help you lower your risk for CHD, SCA, and other heart problems.

Healthy Diet and Physical Activity

A healthy diet is an important part of a healthy lifestyle. Choose a variety of fruits, vegetables, and grains; half of your grains should come from whole-grain products.

Choose foods that are low in saturated fat, trans fat, and cholesterol. Healthy choices include lean meats, poultry without skin, fish, beans, and fat-free or low-fat milk and milk products.

Choose and prepare foods with little sodium (salt). Too much salt can raise your risk for high blood pressure. Studies show that following the Dietary Approaches to Stop Hypertension (DASH) eating plan can lower blood pressure.

Choose foods and beverages that are low in added sugar. If you drink alcoholic beverages, do so in moderation.

Aim for a healthy weight by staying within your daily calorie needs. Balance the calories you take in with the calories you use for physical activity. Be as physically active as you can.

Some people should get medical advice before starting or increasing physical activity. For example, talk with your doctor if you have a chronic (ongoing) health problem, are on medicine, or have symptoms such as chest pain, shortness of breath, or dizziness. Your doctor can suggest types and amounts of physical activity that are safe for you.

For more information about following a healthy diet, go to the National Heart, Lung, and Blood Institute's (NHLBI's) Aim for a Healthy Weight Web site, "Your Guide to a Healthy Heart," and "Your Guide to Lowering Your Blood Pressure With DASH." All of these resources provide general information about healthy eating.

For more information about physical activity, go to the Health Topics Physical Activity and Your Heart article and the NHLBI's "Your Guide to Physical Activity and Your Heart."

Other Lifestyle Changes

Other lifestyle changes also can help lower your risk for SCA. Examples include:

• Quitting smoking. Talk to your doctor about programs and products that can help you quit. Also, try to avoid secondhand smoke.
• Losing weight if you're overweight or obese.
• Treating other health problems, such as high blood pressure, high blood cholesterol, and diabetes.

Clinical Trials

The National Heart, Lung, and Blood Institute (NHLBI) is strongly committed to supporting research aimed at preventing and treating heart, lung, and blood diseases and conditions and sleep disorders.

NHLBI-supported research has led to many advances in medical knowledge and care. Often, these advances depend on the willingness of volunteers to take part in clinical trials.

Clinical trials test new ways to prevent, diagnose, or treat various diseases and conditions. For example, new treatments for a disease or condition (such as medicines, medical devices, surgeries, or procedures) are tested in volunteers who have the illness. Testing shows whether a treatment is safe and effective in humans before it is made available for widespread use.

By taking part in a clinical trial, you can gain access to new treatments before they're widely available. You also will have the support of a team of health care providers, who will likely monitor your health closely. Even if you don't directly benefit from the results of a clinical trial, the information gathered can help others and add to scientific knowledge.

If you volunteer for a clinical trial, the research will be explained to you in detail. You'll learn about treatments and tests you may receive, and the benefits and risks they may pose. You'll also be given a chance to ask questions about the research. This process is called informed consent.

If you agree to take part in the trial, you'll be asked to sign an informed consent form. This form is not a contract. You have the right to withdraw from a study at any time, for any reason. Also, you have the right to learn about new risks or findings that emerge during the trial.

For more information about clinical trials related to sudden cardiac arrest, talk with your doctor. You also can visit the following Web sites to learn more about clinical research and to search for clinical trials:

• http://clinicalresearch.nih.gov
• www.clinicaltrials.gov
• www.nhlbi.nih.gov/studies/index.htm
• www.researchmatch.org

For more information about clinical trials for children, visit the NHLBI's Children and Clinical Studies Web page.

Links to Other Information About Sudden Cardiac Arrest

NHLBI Resources

• Arrhythmia (Health Topics)
• Automated External Defibrillator (Health Topics)
• Coronary Heart Disease/Coronary Artery Disease (Health Topics)
• Heart Attack (Health Topics)
• How the Heart Works (Health Topics)
• Implantable Cardioverter Defibrillator (Health Topics)
• "Your Guide to Living Well With Heart Disease"

Non-NHLBI Resources

• Cardiac Arrest (MedlinePlus)

Clinical Trials

• Children and Clinical Studies
• Clinical Trials (Health Topics)
• Current Research (ClinicalTrials.gov)
• NHLBI Clinical Trials
• NIH Clinical Research Trials and You (National Institutes of Health)
• ResearchMatch (funded by the National Institutes of Health)