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What Is Heart Block?

Heart block is a problem that occurs with the heart's electrical system. This system controls the rate and rhythm of heartbeats. ("Rate" refers to the number of times your heart beats per minute. "Rhythm" refers to the pattern of regular or irregular pulses produced as the heart beats.)

With each heartbeat, an electrical signal spreads across the heart from the upper to the lower chambers. As it travels, the signal causes the heart to contract and pump blood.

Heart block occurs if the electrical signal is slowed or disrupted as it moves through the heart.

Overview

Heart block is a type of arrhythmia (ah-RITH-me-ah). An arrhythmia is any problem with the rate or rhythm of the heartbeat.

Some people are born with heart block, while others develop it during their lifetimes. If you're born with the condition, it's called congenital (kon-JEN-ih-tal) heart block. If the condition develops after birth, it's called acquired heart block.

Doctors might detect congenital heart block before or after a baby is born. Certain diseases that may occur during pregnancy can cause heart block in a baby. Some congenital heart defects also can cause heart block. Congenital heart defects are problems with the heart's structure that are present at birth. Often, doctors don't know what causes these defects.

Acquired heart block is more common than congenital heart block. Damage to the heart muscle or its electrical system causes acquired heart block. Diseases, surgery, or medicines can cause this damage.

The three types of heart block are first degree, second degree, and third degree. First degree is the least severe, and third degree is the most severe. This is true for both congenital and acquired heart block.

Doctors use a test called an EKG (electrocardiogram) to help diagnose heart block. This test detects and records the heart's electrical activity. It maps the data on a graph for the doctor to review.

Outlook

The symptoms and severity of heart block depend on which type you have. First-degree heart block may not cause any severe symptoms.

Second-degree heart block may result in the heart skipping a beat or beats. This type of heart block also can make you feel dizzy or faint.

Third-degree heart block limits the heart's ability to pump blood to the rest of the body. This type of heart block may cause fatigue (tiredness), dizziness, and fainting. Third-degree heart block requires prompt treatment because it can be fatal.

A medical device called a pacemaker is used to treat third-degree heart block and some cases of second-degree heart block. This device uses electrical pulses to prompt the heart to beat at a normal rate. Pacemakers typically are not used to treat first-degree heart block.

All types of heart block may increase your risk for other arrhythmias, such as atrial fibrillation (A-tre-al fih-brih-LA-shun). Talk with your doctor to learn more about the signs and symptoms of arrhythmias.




Understanding the Heart's Electrical System and EKG Results

Doctors use a test called an EKG (electrocardiogram) to help diagnose heart block. This test detects and records the heart's electrical activity. An EKG records the strength and timing of electrical signals as they pass through the heart.

The data are recorded on a graph so your doctor can study your heart's electrical activity. Different parts of the graph show each step of an electrical signal's journey through the heart.

EKG

The image shows the standard setup for an EKG. In figure A, a normal heart rhythm recording shows the electrical pattern of a regular heartbeat. In figure B, a patient lies in a bed with EKG electrodes attached to his chest, upper arms, and legs. A nurse monitors the painless procedure.

The image shows the standard setup for an EKG. In figure A, a normal heart rhythm recording shows the electrical pattern of a regular heartbeat. In figure B, a patient lies in a bed with EKG electrodes attached to his chest, upper arms, and legs. A nurse monitors the painless procedure.

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 (AY-tree-um), which is the upper right chamber of the heart. (Your heart has two upper chambers and two lower chambers.)

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.

From the SA node, the signal travels through the right and left atria. This causes the atria to contract, which helps move blood into the heart's lower chambers, the ventricles (VEN-trih-kuls). The electrical signal moving through the atria is recorded as the P wave on the EKG.

The electrical signal passes between the atria and ventricles through a group of cells called the atrioventricular (AV) node. The signal slows down as it passes through the AV node. This slowing allows the ventricles enough time to finish filling with blood. On the EKG, this part of the process is the flat line between the end of the P wave and the beginning of the Q wave.

The electrical signal then leaves the AV node and travels along a pathway called the bundle of His. From there, the signal travels into the right and left bundle branches. The signal spreads quickly across your heart's ventricles, causing them to contract and pump blood to your lungs and the rest of your body. This process is recorded as the QRS waves on the EKG.

The ventricles then recover their normal electrical state (shown as the T wave on the EKG). The muscle stops contracting to allow the heart to refill with blood. This entire process continues over and over with each new heartbeat.

The animation below shows how your heart's electrical system works and how an EKG records your heart's electrical activity. 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 an electrical signal moves through your heart and how an EKG records your heart's electrical activity.

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




Types of Heart Block

Some people are born with heart block (congenital), while others develop it during their lifetimes (acquired). Acquired heart block is more common than congenital heart block.

The three types of heart block are first degree, second degree, and third degree. First degree is the least severe, and third degree is the most severe. This is true for both congenital and acquired heart block.

First-Degree Heart Block

In first-degree heart block, the heart's electrical signals are slowed as they move from the atria to the ventricles (the heart's upper and lower chambers, respectively). This results in a longer, flatter line between the P and the R waves on the EKG (electrocardiogram).

First-degree heart block may not cause any symptoms or require treatment.

Second-Degree Heart Block

In this type of heart block, electrical signals between the atria and ventricles are slowed to a large degree. Some signals don't reach the ventricles. On an EKG, the pattern of QRS waves doesn't follow each P wave as it normally would.

If an electrical signal is blocked before it reaches the ventricles, they won't contract and pump blood to the lungs and the rest of the body.

Second-degree heart block is divided into two types: Mobitz type I and Mobitz type II.

Mobitz Type I

In this type (also known as Wenckebach's block), the electrical signals are delayed more and more with each heartbeat, until the heart skips a beat. On the EKG, the delay is shown as a line (called the PR interval) between the P and QRS waves. The line gets longer and longer until the QRS waves don't follow the next P wave.

Sometimes people who have Mobitz type I feel dizzy or have other symptoms. This type of second-degree heart block is less serious than Mobitz type II.

The animation below shows how your heart's electrical system works. It also shows what happens during second-degree Mobitz type I heart block. 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 first part of this animation is the same as the animation in the section on understanding the heart's electrical system and EKG results. If you want to skip directly to the frames on Mobitz type I heart block, click the "skip intro" link above the start, pause, and replay buttons.

The first part of the animation shows how an electrical signal moves through your heart and how an EKG records your heart's electrical activity. The second part of the animation shows how a pause in the heart's electrical signal can delay or block the contraction of the ventricles.

Mobitz Type II

In second-degree Mobitz type II heart block, some of the electrical signals don't reach the ventricles. However, the pattern is less regular than it is in Mobitz type I. Some signals move between the atria and ventricles normally, while others are blocked.

On an EKG, the QRS wave follows the P wave at a normal speed. Sometimes, though, the QRS wave is missing (when a signal is blocked).

Mobitz type II is less common than type I, but it's usually more severe. Some people who have type II need medical devices called pacemakers to maintain their heart rates.

Third-Degree Heart Block

In this type of heart block, none of the electrical signals reach the ventricles. This type also is called complete heart block or complete AV block.

When complete heart block occurs, special areas in the ventricles may create electrical signals to cause the ventricles to contract. This natural backup system is slower than the normal heart rate and isn't coordinated with the contraction of the atria. On an EKG, the normal pattern is disrupted. The P waves occur at a faster rate, and it isn't coordinated with the QRS waves.

Complete heart block can result in sudden cardiac arrest and death. This type of heart block often requires emergency treatment. A temporary pacemaker might be used to keep the heart beating until you get a long-term pacemaker.





What Causes Heart Block?

Heart block has many causes. Some people are born with the disorder (congenital), while others develop it during their lifetimes (acquired).

Congenital Heart Block

One form of congenital heart block occurs in babies whose mothers have autoimmune diseases, such as lupus. With autoimmune diseases, the body makes proteins called antibodies that attack and damage tissues or cells.

In pregnant women, antibodies can cross the placenta. (The placenta is the organ that attaches the umbilical cord to the mother's womb.) These proteins can damage the baby's heart and lead to congenital heart block.

Congenital heart defects also can cause congenital heart block. These defects are problems with the heart's structure that are present at birth. Often, doctors don't know what causes congenital heart defects.

Acquired Heart Block

Many factors can cause acquired heart block. Examples include:

Other diseases may increase the risk of heart block. Examples include sarcoidosis (sar-koy-DOE-sis) and the degenerative muscle disorders Lev's disease and Lenegre's disease.

Certain types of surgery also may damage the heart's electrical system and lead to heart block.

Exposure to toxic substances and taking certain medicines—including digitalis, beta blockers, and calcium channel blockers—also may cause heart block. Doctors closely watch people who are taking these medicines for signs of problems.

Some types of heart block have been linked to genetic mutations (changes in the genes).

An overly active vagus nerve also can cause heart block. You have one vagus nerve on each side of your body. These nerves run from your brain stem all the way to your abdomen. Activity in the vagus nerve slows the heart rate.

Sometimes acquired heart block will go away if the factor causing it is treated or resolved. For example, heart block that occurs after a heart attack or surgery may go away during recovery.

If medicine is causing heart block, the disorder may go away if the medicine is stopped or the dosage is lowered. (Always talk with your doctor before you change the way you take your medicines.)




Who Is at Risk for Heart Block?

The risk factors for congenital and acquired heart block are different.

Congenital Heart Block

If a pregnant woman has an autoimmune disease, such as lupus, her fetus is at risk for heart block.

Autoimmune diseases can cause the body to make proteins called antibodies that can cross the placenta. (The placenta is the organ that attaches the umbilical cord to the mother's womb.) These antibodies may damage the baby's heart and lead to congenital heart block.

Congenital heart defects also can cause heart block. These defects are problems with the heart's structure that are present at birth. Most of the time, doctors don't know what causes congenital heart defects.

Heredity may play a role in certain heart defects. For example, a parent who has a congenital heart defect might be more likely than other people to have a child with the condition.

Acquired Heart Block

Acquired heart block can occur in people of any age. However, most types of the condition are more common in older people. This is because many of the risk factors are more common in older people.

People who have a history of heart disease or heart attacks are at increased risk for heart block. Examples of heart disease that can lead to heart block include heart failure, coronary heart disease, and cardiomyopathy (heart muscle diseases).

Other diseases also may raise the risk of heart block, such as sarcoidosis and the degenerative muscle disorders Lev's disease and Lenegre's disease.

Exposure to toxic substances or taking certain medicines, such as digitalis, also can raise your risk for heart block.

Well-trained athletes and young people are at higher risk for first-degree heart block caused by an overly active vagus nerve. You have one vagus nerve on each side of your body. These nerves run from your brain stem all the way to your abdomen. Activity in the vagus nerve slows the heart rate.




What Are the Signs and Symptoms of Heart Block?

Signs and symptoms depend on the type of heart block you have. First-degree heart block may not cause any symptoms.

Signs and symptoms of second- and third-degree heart block include:

  • Fainting
  • Dizziness or light-headedness
  • Fatigue (tiredness)
  • Shortness of breath
  • Chest pain

These symptoms may suggest other health problems as well. If these symptoms are new or severe, call 9–1–1 or have someone drive you to the hospital emergency room. If you have milder symptoms, talk with your doctor right away to find out whether you need prompt treatment.




How Is Heart Block Diagnosed?

Heart block might be diagnosed as part of a routine doctor's visit or during an emergency situation. (Third-degree heart block often is an emergency.)

Your doctor will diagnose heart block based on your family and medical histories, a physical exam, and test results.

Specialists Involved

Your primary care doctor might be involved in diagnosing heart block. However, if you have the condition, you might need to see a heart specialist. Heart specialists include:

  • Cardiologists (doctors who diagnose and treat adults who have heart problems)
  • Pediatric cardiologists (doctors who diagnose and treat babies and children who have heart problems)
  • Electrophysiologists (cardiologists or pediatric cardiologists who specialize in the heart's electrical system)

Family and Medical Histories

Your doctor may ask whether:

  • You have any signs or symptoms of heart block
  • You have any health problems, such as heart disease
  • Any of your family members have been diagnosed with heart block or other health problems
  • You're taking any medicines, including herbal products and prescription and over-the-counter medicines
  • You smoke or use alcohol or drugs

Your doctor also may ask about other health habits, such as how physically active you are.

Physical Exam

During the physical exam, your doctor will listen to your heart. He or she will listen carefully for abnormal rhythms or heart murmurs (extra or unusual sounds heard during heartbeats).

Your doctor also may:

  • Check your pulse to find out how fast your heart is beating
  • Check for swelling in your legs or feet, which could be a sign of an enlarged heart or heart failure
  • Look for signs of other diseases that could be causing heart rate or rhythm problems (such as coronary heart disease)

Diagnostic Tests and Procedures

EKG (Electrocardiogram)

Doctors usually use an EKG (electrocardiogram) to help diagnose heart block. This simple test detects and records the heart's electrical activity.

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

The data are recorded on a graph. Different types of heart block have different patterns on the graph. (For more information, go to "Types of Heart Block.")

A standard EKG only records the heart's activity for a few seconds. To diagnose heart rhythm problems that come and go, your doctor may have you wear a portable EKG monitor.

The most common types of portable EKGs are Holter and event monitors. Your doctor may have you use one of these monitors to diagnose first- or second-degree heart block.

Holter and Event Monitors

A Holter monitor records the heart's electrical signals for a full 24- or 48-hour period. You wear one 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.

You may wear an event monitor for 1 to 2 months, or as long as it takes to get a recording of your heart during symptoms.

Electrophysiology Study

For some cases of heart block, doctors may do electrophysiology studies (EPS). During this test, a thin, flexible wire is passed through a vein in your groin (upper thigh) or arm to your heart. The wire records your heart's electrical signals.

Other Tests

To diagnose heart block, your doctor may recommend tests to rule out other types of arrhythmias (irregular heartbeats). For more information, go to "How Are Arrhythmias Diagnosed?"




How Is Heart Block Treated?

Treatment depends on the type of heart block you have. If you have first-degree heart block, you may not need treatment.

If you have second-degree heart block, you may need a pacemaker. A pacemaker is a small device that's placed under the skin of your chest or abdomen. This device uses electrical pulses to prompt the heart to beat at a normal rate.

If you have third-degree heart block, you will need a pacemaker. In an emergency, a temporary pacemaker might be used until you can get a long-term device. Most people who have third-degree heart block need pacemakers for the rest of their lives.

Some people who have third-degree congenital heart block don't need pacemakers for many years. Others may need pacemakers at a young age or during infancy.

If a pregnant woman has an autoimmune disease, such as lupus, her fetus is at risk for heart block. If heart block is detected in a fetus, the mother might be given medicine to reduce the fetus' risk of developing serious heart block.

Sometimes acquired heart block goes away if the factor causing it is treated or resolved. For example, heart block that occurs after a heart attack or surgery may go away during recovery.

Also, if a medicine is causing heart block, the condition may go away if the medicine is stopped or the dosage is lowered. (Always talk with your doctor before you change the way you take your medicines.)




Living With Heart Block

First-degree heart block may not cause any symptoms or require treatment. However, some research has shown that people who have first-degree heart block might be at higher risk for atrial fibrillation (AF) in the future.

AF is a type of arrhythmia. It occurs if rapid, disorganized electrical signals cause the heart's upper chambers to contract very fast and irregularly.

If you've been diagnosed with first-degree heart block, ask your doctor whether you need to take any special steps to control it.

Your doctor can tell you whether you need ongoing care or whether you need to change the way you take certain medicines.

If you have second-degree heart block that doesn't require a pacemaker, talk with your doctor about keeping your heart healthy. Your doctor will tell you whether you need ongoing care for your condition.

Living With a Pacemaker

People who have third-degree heart block and some people who have second-degree heart block need pacemakers. These devices use electrical pulses to prompt the heart to beat at a normal rate.

If you have a pacemaker, you should take special care to avoid things that may interfere with it. Avoid close or prolonged contact with electrical devices and devices that have strong magnetic fields. These objects can keep your pacemaker from working properly.

Let all of your doctors, dentists, and medical technicians know that you have a pacemaker. You also should notify airport screeners.

Your doctor can give you a card that states what kind of pacemaker you have. Carry this card in your wallet. You may want to wear a medical ID bracelet or necklace that states that you have a pacemaker.

Certain medical procedures can disrupt pacemakers. Examples include MRI (magnetic resonance imaging), electrocauterization during surgery, and shock-wave lithotripsy to get rid of kidney stones.

Your doctor may need to check your pacemaker several times a year to make sure it's working well. Some pacemakers must be checked in the doctor's office, but others can be checked over the phone.

Ask your doctor about what types of physical activity are safe for you. A pacemaker usually won't limit you from doing sports and physical activity. But you may need to avoid full-contact sports, such as football, that can damage the pacemaker.

For more information about living with a pacemaker, go to the Health Topics Pacemaker article.




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

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




Links to Other Information About Heart Block

NHLBI Resources

Non-NHLBI Resources

Clinical Trials

 
July 09, 2012 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.

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