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What Is Atrial Fibrillation?

Atrial fibrillation (A-tre-al fi-bri-LA-shun), or AF, is the most common type of arrhythmia (ah-RITH-me-ah). An arrhythmia is a problem with the rate or rhythm of the heartbeat. During an arrhythmia, the heart can beat too fast, too slow, or with an irregular rhythm.

AF occurs if rapid, disorganized electrical signals cause the heart's two upper chambers—called the atria (AY-tree-uh)—to fibrillate. The term "fibrillate" means to contract very fast and irregularly.

In AF, blood pools in the atria. It isn't pumped completely into the heart's two lower chambers, called the ventricles (VEN-trih-kuls). As a result, the heart's upper and lower chambers don't work together as they should.

People who have AF may not feel symptoms. However, even when AF isn't noticed, it can increase the risk of stroke. In some people, AF can cause chest pain or heart failure, especially if the heart rhythm is very rapid.

AF may happen rarely or every now and then, or it may become an ongoing or long-term heart problem that lasts for years.

Understanding the Heart's Electrical System

To understand AF, it helps to understand the heart's internal electrical system. The heart's electrical system controls the rate and rhythm of the heartbeat.

With each heartbeat, an electrical signal spreads from the top of the heart to the bottom. As the signal travels, it causes the heart to contract and pump blood.

Each electrical signal begins in a group of cells called the sinus node or sinoatrial (SA) node. The SA node is located in the right atrium. In a healthy adult heart at rest, the SA node sends an electrical signal to begin a new heartbeat 60 to 100 times a minute. (This rate may be slower in very fit athletes.)

From the SA node, the electrical signal travels through the right and left atria. It causes the atria to contract and pump blood into the ventricles.

The electrical signal then moves down to a group of cells called the atrioventricular (AV) node, located between the atria and the ventricles. Here, the signal slows down slightly, allowing the ventricles time to finish filling with blood.

The electrical signal then leaves the AV node and travels to the ventricles. It causes the ventricles to contract and pump blood to the lungs and the rest of the body. The ventricles then relax, and the heartbeat process starts all over again in the SA node.

For more information about the heart's electrical system and detailed animations, go to the Health Topics How the Heart Works article.

Understanding the Electrical Problem in Atrial Fibrillation

In AF, the heart's electrical signals don't begin in the SA node. Instead, they begin in another part of the atria or in the nearby pulmonary veins. The signals don't travel normally. They may spread throughout the atria in a rapid, disorganized way. This can cause the atria to fibrillate.

The faulty signals flood the AV node with electrical impulses. As a result, the ventricles also begin to beat very fast. However, the AV node can't send the signals to the ventricles as fast as they arrive. So, even though the ventricles are beating faster than normal, they aren't beating as fast as the atria.

Thus, the atria and ventricles no longer beat in a coordinated way. This creates a fast and irregular heart rhythm. In AF, the ventricles may beat 100 to 175 times a minute, in contrast to the normal rate of 60 to 100 beats a minute.

If this happens, blood isn't pumped into the ventricles as well as it should be. Also, the amount of blood pumped out of the ventricles to the body is based on the random atrial beats.

The body may get rapid, small amounts of blood and occasional larger amounts of blood. The amount will depend on how much blood has flowed from the atria to the ventricles with each beat.

Most of the symptoms of AF are related to how fast the heart is beating. If medicines or age slow the heart rate, the symptoms are minimized.

AF may be brief, with symptoms that come and go and end on their own. Or, the condition may be ongoing and require treatment. Sometimes AF is permanent, and medicines or other treatments can't restore a normal heart rhythm.

The animation below shows atrial fibrillation. Click the "start" button to play the animation. Written and spoken explanations are provided with each frame. Use the buttons in the lower right corner to pause, restart, or replay the animation, or use the scroll bar below the buttons to move through the frames.

The animation shows how the heart's electrical signal can begin somewhere other than the sinoatrial node. This causes the atria to beat very fast and irregularly.

Outlook

People who have AF can live normal, active lives. For some people, treatment can restore normal heart rhythms.

For people who have permanent AF, treatment can help control symptoms and prevent complications. Treatment may include medicines, medical procedures, and lifestyle changes.




Types of Atrial Fibrillation

Paroxysmal Atrial Fibrillation

In paroxysmal (par-ok-SIZ-mal) atrial fibrillation (AF), the faulty electrical signals and rapid heart rate begin suddenly and then stop on their own. Symptoms can be mild or severe. They stop within about a week, but usually in less than 
24 hours.

Persistent Atrial Fibrillation

Persistent AF is a condition in which the abnormal heart rhythm continues for more than a week. It may stop on its own, or it can be stopped with treatment.

Permanent Atrial Fibrillation

Permanent AF is a condition in which a normal heart rhythm can't be restored with treatment. Both paroxysmal and persistent AF may become more frequent and, over time, result in permanent AF.




Other Names for Atrial Fibrillation

  • A fib
  • Auricular fibrillation



What Causes Atrial Fibrillation?

Atrial fibrillation (AF) occurs if the heart's electrical signals don't travel through the heart in a normal way. Instead, they become very rapid and disorganized.

Damage to the heart's electrical system causes AF. The damage most often is the result of other conditions that affect the health of the heart, such as high blood pressure and coronary heart disease.

The risk of AF increases as you age. Inflammation also is thought to play a role in causing AF. 

Sometimes, the cause of AF is unknown.




Who Is at Risk for Atrial Fibrillation?

Atrial fibrillation (AF) affects millions of people, and the number is rising. Men are more likely than women to have the condition. In the United States, AF is more common among Whites than African Americans or Hispanic Americans.

The risk of AF increases as you age. This is mostly because your risk for heart disease and other conditions that can cause AF also increases as you age. However, about half of the people who have AF are younger than 75.

AF is uncommon in children.

Major Risk Factors

AF is more common in people who have:

AF also is more common in people who are having heart attacks or who have just had surgery.

Other Risk Factors

Other conditions that raise your risk for AF include hyperthyroidism (too much thyroid hormone), obesity, diabetes, and lung disease.

Certain factors also can raise your risk for AF. For example, drinking large amounts of alcohol, especially binge drinking, raises your risk. Even modest amounts of alcohol can trigger AF in some people. Caffeine or psychological stress also may trigger AF in some people.

Some data suggest that people who have sleep apnea are at greater risk for AF. Sleep apnea is a common disorder that causes one or more pauses in breathing or shallow breaths while you sleep.

Metabolic syndrome also raises your risk for AF. Metabolic syndrome is the name for a group of risk factors that raises your risk for CHD and other health problems, such as diabetes and stroke.

Research suggests that people who receive high-dose steroid therapy are at increased risk for AF. This therapy is used for asthma and some inflammatory conditions. It may act as a trigger in people who have other AF risk factors.

Genetic factors also may play a role in causing AF. However, their role isn't fully known.




What Are the Signs and Symptoms of Atrial Fibrillation?

Atrial fibrillation (AF) usually causes the heart's lower chambers, the ventricles, to contract faster than normal.

When this happens, the ventricles can't completely fill with blood. Thus, they may not be able to pump enough blood to the lungs and body. This can lead to signs and symptoms, such as:

  • Palpitations (feelings that your heart is skipping a beat, fluttering, or beating too hard or fast)
  • Shortness of breath
  • Weakness or problems exercising
  • Chest pain
  • Dizziness or fainting
  • Fatigue (tiredness)
  • Confusion

Atrial Fibrillation Complications

AF has two major complications—stroke and heart failure.

Stroke

During AF, the heart's upper chambers, the atria, don't pump all of their blood to the ventricles. Some blood pools in the atria. When this happens, a blood clot (also called a thrombus) can form.

If the clot breaks off and travels to the brain, it can cause a stroke. (A clot that forms in one part of the body and travels in the bloodstream to another part of the body is called an embolus.)

Blood-thinning medicines that reduce the risk of stroke are an important part of treatment for people who have AF.

Atrial Fibrillation and Stroke

The illustration shows how a stroke can occur during atrial fibrillation. If a clot (thrombus) forms in the left atrium of the heart, a piece of it can dislodge and travel to an artery in the brain, blocking blood flow through the artery. The lack of blood flow to the portion of the brain fed by the artery causes a stroke.

The illustration shows how a stroke can occur during atrial fibrillation. A blood clot (thrombus) can form in the left atrium of the heart. If a piece of the clot breaks off and travels to an artery in the brain, it can block blood flow through the artery. The lack of blood flow to the portion of the brain fed by the artery causes a stroke.

Heart Failure

Heart failure occurs if the heart can't pump enough blood to meet the body's needs. AF can lead to heart failure because the ventricles are beating very fast and can't completely fill with blood. Thus, they may not be able to pump enough blood to the lungs and body.

Fatigue and shortness of breath are common symptoms of heart failure. A buildup of fluid in the lungs causes these symptoms. Fluid also can build up in the feet, ankles, and legs, causing weight gain.

Lifestyle changes, medicines, and procedures or surgery (rarely, a mechanical heart pump or heart transplant) are the main treatments for heart failure.




How Is Atrial Fibrillation Diagnosed?

Atrial fibrillation (AF) is diagnosed based on your medical and family histories, a physical exam, and the results from tests and procedures.

Sometimes AF doesn't cause signs or symptoms. Thus, it may be found during a physical exam or EKG (electrocardiogram) test done for another purpose.

If you have AF, your doctor will want to find out what is causing it. This will help him or her plan the best way to treat the condition.

Specialists Involved

Primary care doctors often are involved in the diagnosis and treatment of AF. These doctors include family practitioners and internists.

Doctors who specialize in the diagnosis and treatment of heart disease also may be involved, such as:

  • Cardiologists. These are doctors who diagnose and treat heart diseases and conditions.
  • Electrophysiologists. These are cardiologists who specialize in arrhythmias.

Medical and Family Histories

Your doctor will likely ask questions about your:

  • Signs and symptoms. What symptoms are you having? Have you had palpitations? Are you dizzy or short of breath? Are your feet or ankles swollen (a possible sign of heart failure)? Do you have any chest pain?
  • Medical history. Do you have other health problems, such as a history of heart disease, high blood pressure, lung disease, diabetes, or thyroid problems?
  • Family's medical history. Does anyone in your family have a history of AF? Has anyone in your family ever had heart disease or high blood pressure? Has anyone had thyroid problems? Does your family have a history of other illnesses or health problems?
  • Health habits. Do you smoke or use alcohol or caffeine?

Physical Exam

Your doctor will do a complete cardiac exam. He or she will listen to the rate and rhythm of your heartbeat and take your pulse and blood pressure reading. Your doctor will likely check for any signs of heart muscle or heart valve problems. He or she will listen to your lungs to check for signs of heart failure.

Your doctor also will check for swelling in your legs or feet and look for an enlarged thyroid gland or other signs of hyperthyroidism (too much thyroid hormone).

Diagnostic Tests and Procedures

EKG

An EKG is a simple, painless test that records the heart's electrical activity. It's the most useful test for diagnosing AF.

An EKG shows how fast your heart is beating and its rhythm (steady or irregular). It also records the strength and timing of electrical signals as they pass through your heart.

A standard EKG only records the heartbeat for a few seconds. It won't detect AF that doesn't happen during the test. To diagnose paroxysmal AF, your doctor may ask you to wear a portable EKG monitor that can record your heartbeat for longer periods.

The two most common types of portable EKGs are Holter and event monitors.

Holter and Event Monitors

A Holter monitor records the heart's electrical activity for a full 24- or 48-hour period. You wear small patches called electrodes on your chest. Wires connect these patches to a small, portable recorder. The recorder can be clipped to a belt, kept in a pocket, or hung around your neck.

You wear the Holter monitor while you do your normal daily activities. This allows the monitor to record your heart for a longer time than a standard EKG.

An event monitor is similar to a Holter monitor. You wear an event monitor while doing your normal activities. However, an event monitor only records your heart's electrical activity at certain times while you're wearing it.

For many event monitors, you push a button to start the monitor when you feel symptoms. Other event monitors start automatically when they sense abnormal heart rhythms.

You can wear an event monitor for weeks or until symptoms occur.

Stress Test

Some heart problems are easier to diagnose when your heart is working hard and beating fast. During stress testing, you exercise to make your heart work hard and beat fast while heart tests are done. If you can't exercise, you may be given medicine to make your heart work hard and beat fast.

Echocardiography

Echocardiography (echo) uses sound waves to create a moving picture 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.

This test sometimes is called transthoracic (trans-thor-AS-ik) echocardiography. It's painless and noninvasive (no instruments are inserted into the body). For the test, a device called a transducer is moved back and forth over your chest. The device sends special sound waves through your chest wall to your heart.

The sound waves bounce off the structures of your heart, and a computer converts them into pictures on a screen.

Transesophageal Echocardiography

Transesophageal (trans-e-SOF-ah-ge-al) echo, or TEE, uses sound waves to take pictures of your heart through the esophagus. The esophagus is the passage leading from your mouth to your stomach.

Your heart's upper chambers, the atria, are deep in your chest. They often can't be seen very well using transthoracic echo. Your doctor can see the atria much better using TEE.

During this test, the transducer is attached to the end of a flexible tube. The tube is guided down your throat and into your esophagus. You'll likely be given medicine to help you relax during the procedure.

TEE is used to detect blood clots that may be forming in the atria because of AF.

Chest X Ray

A chest x ray is a painless test that creates pictures of the structures in your chest, such as your heart and lungs. This test can show fluid buildup in the lungs and signs of other AF complications.

Blood Tests

Blood tests check the level of thyroid hormone in your body and the balance of your body's electrolytes. Electrolytes are minerals that help maintain fluid levels and acid-base balance in the body. They're essential for normal health and functioning of your body's cells and organs.




How Is Atrial Fibrillation Treated?

Treatment for atrial fibrillation (AF) depends on how often you have symptoms, how severe they are, and whether you already have heart disease. General treatment options include medicines, medical procedures, and lifestyle changes.

Goals of Treatment

The goals of treating AF include:

  • Preventing blood clots from forming, thus lowering the risk of stroke.
  • Controlling how many times a minute the ventricles contract. This is called rate control. Rate control is important because it allows the ventricles enough time to completely fill with blood. With this approach, the abnormal heart rhythm continues, but you feel better and have fewer symptoms.
  • Restoring a normal heart rhythm. This is called rhythm control. Rhythm control allows the atria and ventricles to work together to efficiently pump blood to the body.
  • Treating any underlying disorder that's causing or raising the risk of AF—for example, hyperthyroidism (too much thyroid hormone).

Who Needs Treatment for Atrial Fibrillation?

People who have AF but don't have symptoms or related heart problems may not need treatment. AF may even go back to a normal heart rhythm on its own. (This also can occur in people who have AF with symptoms.)

In some people who have AF for the first time, doctors may choose to use an electrical procedure or medicine to restore a normal heart rhythm.

Repeat episodes of AF tend to cause changes to the heart's electrical system, leading to persistent or permanent AF. Most people who have persistent or permanent AF need treatment to control their heart rate and prevent complications.

Specific Types of Treatment

Blood Clot Prevention

People who have AF are at increased risk for stroke. This is because blood can pool in the heart's upper chambers (the atria), causing a blood clot to form. If the clot breaks off and travels to the brain, it can cause a stroke.

Preventing blood clots from forming is probably the most important part of treating AF. The benefits of this type of treatment have been proven in multiple studies.

Doctors prescribe blood-thinning medicines to prevent blood clots. These medicines include warfarin (Coumadin®), dabigatran, heparin, and aspirin.

People taking blood-thinning medicines need regular blood tests to check how well the medicines are working.

Rate Control

Doctors can prescribe medicines to slow down the rate at which the ventricles are beating. These medicines help bring the heart rate to a normal level.

Rate control is the recommended treatment for most patients who have AF, even though an abnormal heart rhythm continues and the heart doesn't work as well as it should. Most people feel better and can function well if their heart rates are well-controlled.

Medicines used to control the heart rate include beta blockers (for example, esmolol, propranolol, metoprolol and atenolol), calcium channel blockers (diltiazem and verapamil), and digitalis (digoxin). Several other medicines also are available.

Rhythm Control

Restoring and maintaining a normal heart rhythm is a treatment approach recommended for people who aren't doing well with rate control treatment. This treatment also may be used for people who have only recently started having AF. The long-term benefits of rhythm control have not been proven conclusively yet.

Doctors use medicines or procedures to control the heart's rhythm. Patients often begin rhythm control treatment in a hospital so that their hearts can be closely watched.

The longer you have AF, the less likely it is that doctors can restore a normal heart rhythm. This is especially true for people who have had AF for 6 months or more.

Restoring a normal rhythm also becomes less likely if the atria are enlarged or if any underlying heart disease worsens. In these cases, the chance that AF will recur is high, even if you're taking medicine to help convert AF to a normal rhythm.

Medicines. Medicines used to control the heart rhythm include amiodarone, sotalol, flecainide, propafenone, dofetilide, and ibutilide. Sometimes older medicines—such as quinidine, procainamide, and disopyramide—are used.

Your doctor will carefully tailor the dose and type of medicines he or she prescribes to treat your AF. This is because medicines used to treat AF can cause a different kind of arrhythmia.

These medicines also can harm people who have underlying diseases of the heart or other organs. This is especially true for patients who have an unusual heart rhythm problem called Wolff-Parkinson-White syndrome.

Your doctor may start you on a small dose of medicine and then gradually increase the dose until your symptoms are controlled. Medicines used for rhythm control can be given regularly by injection at a doctor's office, clinic, or hospital. Or, you may routinely take pills to try to control AF or prevent repeat episodes.

If your doctor knows how you'll react to a medicine, a specific dose may be prescribed for you to take on an as-needed basis if you have an episode of AF.

Procedures. Doctors use several procedures to restore a normal heart rhythm. For example, they may use electrical cardioversion to treat a fast or irregular heartbeat. For this procedure, low-energy shocks are given to your heart to trigger a normal rhythm. You're temporarily put to sleep before you receive the shocks.

Electrical cardioversion isn't the same as the emergency heart shocking procedure often seen on TV programs. It's planned in advance and done under carefully controlled conditions.

Before doing electrical cardioversion, your doctor may recommend transesophageal echocardiography (TEE). This test can rule out the presence of blood clots in the atria. If clots are present, you may need to take blood-thinning medicines before the procedure. These medicines can help get rid of the clots.

Catheter ablation (ab-LA-shun) may be used to restore a normal heart rhythm if medicines or electrical cardioversion don't work. For this procedure, a wire is inserted through a vein in the leg or arm and threaded to the heart.

Radio wave energy is sent through the wire to destroy abnormal tissue that may be disrupting the normal flow of electrical signals. An electrophysiologist usually does this procedure in a hospital. Your doctor may recommend a TEE before catheter ablation to check for blood clots in the atria.

Sometimes doctors use catheter ablation to destroy the atrioventricular (AV) node. The AV node is where the heart's electrical signals pass from the atria to the ventricles (the heart's lower chambers). This procedure requires your doctor to surgically implant a device called a pacemaker, which helps maintain a normal heart rhythm.

Research on the benefits of catheter ablation as a treatment for AF is still ongoing. (For more information, go to the "Clinical Trials" section of this article.)

Another procedure to restore a normal heart rhythm is called maze surgery. For this procedure, the surgeon makes small cuts or burns in the atria. These cuts or burns prevent the spread of disorganized electrical signals.

This procedure requires open-heart surgery, so it's usually done when a person requires heart surgery for other reasons, such as for heart valve disease (which can increase the risk of AF).

Approaches To Treating Underlying Causes and Reducing Risk Factors

Your doctor may recommend treatments for an underlying cause of AF or to reduce AF risk factors. For example, he or she may prescribe medicines to treat an overactive thyroid, lower high blood pressure, or manage high blood cholesterol.

Your doctor also may recommend lifestyle changes, such as following a healthy diet, cutting back on salt intake (to help lower blood pressure), quitting smoking, and reducing stress.

Limiting or avoiding alcohol, caffeine, or other stimulants that may increase your heart rate also can help reduce your risk for AF.




How Can Atrial Fibrillation Be Prevented?

Following a healthy lifestyle and taking steps to lower your risk for heart disease may help you prevent atrial fibrillation (AF). These steps include:

  • Following a heart healthy diet that's low in saturated fat, trans fat, and cholesterol. A healthy diet includes a variety of whole grains, fruits, and vegetables daily.
  • Not smoking.
  • Being physically active.
  • Maintaining a healthy weight.

If you already have heart disease or other AF risk factors, work with your doctor to manage your condition. In addition to adopting the healthy habits above, which can help control heart disease, your doctor may advise you to:  

  • Follow the DASH eating plan to help lower your blood pressure.
  • Keep your cholesterol and triglycerides at healthy levels with dietary changes and medicines (if prescribed).
  • Limit or avoid alcohol.
  • Control your blood sugar level if you have diabetes.
  • Get ongoing medical care and take your medicines as prescribed.

For more information about following a healthy lifestyle, visit the National Heart, Lung, and Blood Institute's Aim for a Healthy Weight Web site, "Your Guide to a Healthy Heart," "Your Guide to Lowering Your Blood Pressure With DASH," and "Your Guide to Physical Activity and Your Heart."




Living With Atrial Fibrillation

People who have atrial fibrillation (AF)—even permanent AF—can live normal, active lives. If you have AF, ongoing medical care is important.

Keep all your medical appointments. Bring a list of all the medicines you're taking to every doctor and emergency room visit. This will help your doctor know exactly what medicines you're taking.

Follow your doctor's instructions for taking medicines. Be careful about taking over-the-counter medicines, nutritional supplements, and cold and allergy medicines. Some of these products contain stimulants that can trigger rapid heart rhythms. Also, some over-the-counter medicines can have harmful interactions with heart rhythm medicines.

Tell your doctor if your medicines are causing side effects, if your symptoms are getting worse, or if you have new symptoms.

If you're taking blood-thinning medicines, you'll need to be carefully monitored. For example, you may need routine blood tests to check how the medicines are working. Also, talk with your doctor about your diet. Some foods, such as leafy green vegetables, may interfere with warfarin, a blood-thinning medicine.

Ask your doctor about physical activity, weight control, and alcohol use. Find out what steps you can take to manage your condition.




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. However, many questions remain about various diseases and conditions, including atrial fibrillation (AF).

The NHLBI continues to support research aimed at learning more about AF. For example, NHLBI-supported research on AF includes studies that explore:

  • Ways to improve catheter ablation procedures in people who have AF
  • Whether fish oil supplements can lower the risk of repeat episodes of AF
  • Whether genetic factors can help determine the best dosing strategies for warfarin, a blood-thinning medicine
  • Ways to predict the risk of complications, such as stroke, for people with AF
  • Whether an implantable cardiac monitor provides the ability to remotely and continuously evaluate a person for repeat episodes of AF
  • The effectiveness of a new drug therapy for people with AF who must stop taking their blood thinning drug prior to surgery

Much of this research depends 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.

Some clinical trials compare two current treatments. For example, the NHLBI is supporting a trial that compares catheter ablation with rate control or rhythm control medicines in people who have AF.

The study results will help researchers understand which of these treatments is best, and whether one is better than another in certain situations. For more information about this study, go to https://www.cabanatrial.org.

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 AF, talk with your doctor. You also can visit the following Web sites to learn more about clinical research and to search for clinical trials:




Links to Other Information About Atrial Fibrillation

NHLBI Resources

Non-NHLBI Resources

Clinical Trials

 
September 18, 2014 Last Updated Icon

The NHLBI updates Health Topics articles on a biennial cycle based on a thorough review of research findings and new literature. The articles also are updated as needed if important new research is published. The date on each Health Topics article reflects when the content was originally posted or last revised.