Heart Block

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.


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.


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 - 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. 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.


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 - Heart Block

There are many types of conduction disorders that can occur anywhere along the cardiac conduction system: at the sinoatrial (SA) node, the atrioventricular (AV) node, or the bundle branches. The problem can be with signal generation, how the signal travels through the heart, or both. Disorders can occur on a much smaller level within ion channels in a heart muscle cell that work together to make the heart cell contract.

Read about other types of conduction disorders not covered in this topic, including atrial fibrillation and arrhythmia.

Learn more about the cardiac conduction system in our How the Heart Works Health Topic.

Illustration of the cardiac conduction system.
Cardiac conduction system. This image shows the cardiac conduction system. The signal starts with pacemaker cells in the SA node. This causes your right and left atria to contract. The signal travels down to your AV node and bundle branches, causing your right and left ventricles to contract. Medical Illustration Copyright © 2019 Nucleus Medical Media, All rights reserved.


Sick sinus syndrome
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Sick sinus syndrome (SSS), also known as sinus node disease, describes problems that can occur when the SA node does not work properly. The SA node controls the rate and rhythm of your heartbeat. SSS includes conditions in which the following occurs:

  • The heart rate cannot be increased when needed, such as with exercise or stress.
  • The signal is not conducted from the SA node.
  • There are pauses or stops during the generation of the electrical signal in the SA node.
  • There is a slow or fast heart rate.

Atrioventricular block
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There are three main types of AV block, depending on how severe the conduction disorder is: first-, second-, and third-degree AV block.

  • First-degree AV block occurs when the electrical signal for your heartbeat moves too slowly. This may lead to a slower heart rate, but no heartbeats are blocked.
  • Second-degree AV block occurs when some electrical signals from the atria do not reach the ventricles, leading to dropped beats. More severe cases of second-degree AV block can turn into third-degree AV block.
  • Third-degree AV block, also called complete heart block, occurs when no signals reach the ventricles, resulting in serious symptoms such as a very slow heart rate, fainting, and chest pain. This may be discovered at birth, or it may be due to other heart conditions, drugs, or injury to the heart during heart surgery. Children who have congenital complete heart block often have mothers who have an autoimmune disease such as lupus.

Bundle branch blocks
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Usually, the electrical signal in the heart travels down the left and right bundle branches at the same speed, causing the ventricles to contract or squeeze at the same time. Sometimes one side travels more slowly than the other, causing that ventricle to contract slightly after the other ventricle. This is known as a bundle branch block and is called a left bundle branch block (LBBB) or a right bundle branch block (RBBB), depending on which side the block occurs. Bundle branch blocks may also be more or less severe.

Ion channel disorders
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On the surface of each heart muscle cell are tiny pores called ion channels. Ion channels open and close to let electrically charged sodium, calcium, and potassium ions flow into and out of each cell. This generates the heart’s electrical activity, which triggers the contracting and relaxing of the ventricles to pump blood and refill. Ion channels that do not work properly may cause problems with the heart’s electrical activity.

The most common type of ion channel disorder is long QT syndrome. Other types include Brugada syndrome, catecholaminergic polymorphic ventricular tachycardia, and short QT syndrome.

Causes - Heart Block

Conduction disorders may be caused by structural problems in the heart or other medical conditions. Some people have a conduction disorder at birth, while others develop it later in life.

Structural problems in the heart
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A congenital heart defect—one that is present at birth—can cause structural damage even when the heart appears normal. This can prevent the cardiac conduction system from working properly. For example, the conduction system may be damaged during fetal development. This is seen in people who have a genetic condition called Holt-Oram syndrome, who have different degrees of cardiac conduction system problems such as AV block.

Other medical conditions
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Other conditions that cause conduction disorders include the following:

  • Electrolyte problems such as high or low blood levels of potassium, calcium, and magnesium
  • Fever in Brugada syndrome
  • Heart attack

Look for
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  • Treatment will discuss lifestyle changes that your doctors may recommend if you are diagnosed with a conduction disorder.

Risk Factors - Heart Block

You may have an increased risk for conduction disorders because of your age, family history and genetics, medical conditions, medicines, race or ethnicity, or sex. These factors can differ by the type of conduction disorder you have.

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Conduction disorders can happen at any age, but some are more common at certain ages. For example, SSS and bundle branch blocks are more common in older adults due to fibrosis and changes in the conduction system that happen as a result of aging.

Family history and genetics
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Your family history can increase your risk for a conduction disorder. Even when it is not known exactly how a conduction disorder is inherited, some disorders are more common among families, including:

  • Brugada syndrome
  • Complete heart block or third-degree AV block, in children born to mothers who have a connective tissue disease
  • Wolff-Parkinson-White syndrome

Other medical conditions
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Some medical conditions may increase your risk for a conduction disorder, including the following.

  • Autoimmune diseases, such as scleroderma, systemic lupus erythematosus, and rheumatoid arthritis
  • Cardiomyopathy
  • Complications of a heart surgery or procedure, including heart valve replacement, procedures to treat ischemic heart disease, surgery to correct a congenital heart defect at birth, and radiation treatment for cancer. The damage to the conduction system may be caused by physical damage during the surgery or procedure, or from damage due to swelling, lack of blood flow, or hematoma.
  • Diseases in which there are deposits of abnormal substances in the heart, such as amyloidosis, hemochromatosis, and sarcoidosis
  • Endocrine conditions, such as hypothyroidism
  • Heart inflammation, such as in myocarditis and pericarditis
  • High blood pressure
  • Neuromuscular disorders such as muscular dystrophy
  • Sleep apnea

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Some medicines may increase your risk for developing a conduction disorder, including the following.

  • Antiarrhythmics to treat abnormal heart rhythms, such as quinidine, flecainide, sotalol, and amiodarone
  • Antidepressants and antipsychotic medicines
  • Diuretics, which are medicines that remove excess water from your body
  • Heart medicines, such as beta blockers, calcium channel blockers, and digoxin
  • High blood pressure medicines, including clonidine and methyldopa
  • Lithium
  • Muscle relaxants and sedatives

Race or ethnicity
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Brugada syndrome is especially common in people of Asian descent, particularly those of Japanese, Filipino, and Thai ancestry.

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The following conduction disorders are more common in men than in women:

  • Brugada syndrome
  • Catecholaminergic polymorphic ventricular tachycardia

Signs, Symptoms, and Complications - Heart Block

Many people who have conduction disorders, such as first-degree AV block, may have no obvious signs or symptoms. Others may have signs and symptoms that occur in specific situations, such as when you experience physical or emotional stress or when you sleep. Conduction disorders can lead to life-threatening arrhythmias if undiagnosed or untreated.

Signs and symptoms
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You may experience the following signs or symptoms if you have a conduction disorder:

  • Fatigue
  • Dizziness or lightheadedness
  • Slow or fast heart rate
  • Shortness of breath
  • Heart palpitations
  • Chest pain or discomfort
  • Abdominal pain
  • Nausea
  • Fainting or loss of consciousness
  • Gasping or labored breathing during sleep
  • Seizures

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Complications of conduction disorders may be serious or life-threatening, and they include the following:

Look for
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  • Diagnosis will discuss tests and procedures used to detect signs of a conduction disorder and help rule out other conditions that may mimic conduction disorders.
  • Treatment will explain approaches to preventing or alleviating problems caused by conduction disorders.

Diagnosis - Heart Block

To diagnose a conduction disorder, your doctor will ask about your medical history, any signs and symptoms, and your family’s medical history, and he or she will perform a physical exam. Your doctor may also recommend tests to look at your heart’s electrical activity and structure and to determine if you have genetic changes that may signal a conduction disorder.

Medical history
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Your doctor may ask questions about your personal medical history using questions such as the following:

  • Do you experience heart palpitations?
  • Do you experience unusual shortness of breath or fatigue with exercise?
  • Do you have high blood pressure?
  • Do you have symptoms of sleep apnea?
  • Have you ever fainted or almost fainted without a known reason?
  • Have you experienced chest pain or discomfort with exertion or exercise?
  • What medicines are you taking?

Your doctor may ask about your family’s medical history, including:

  • Complications from heart disease in a close relative
  • Premature death of a relative, either sudden or unexpected
  • Specific knowledge of heart conditions in family members

Physical exam
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As part of your physical exam, your doctor may listen to your heart, record your heart rate, and measure your blood pressure.

Diagnostic tests and procedures
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To diagnose a conduction disorder, your doctor may order an EKG, which records your heart’s electrical activity. Each conduction disorder has a specific pattern that can be seen on an EKG.

If the diagnosis is unclear from the EKG or your doctor would like more information, your doctor may order additional tests, such as one or more of the following:

  • Blood tests to look for potentially reversible causes, including high or low blood electrolyte levels or endocrine disorders such as hypothyroidism, or to check medicine levels.
  • Holter or event monitor to record your heart’s electrical activity over long periods of time while you do your normal activities.
  • Implantable loop recorder to look at the heart’s electrical activity over a longer period of time or to find a diagnosis if earlier tests do not rule out a conduction disorder. The recorder can transmit data to the doctor’s office to help with monitoring. An implantable loop recorder helps doctors figure out why a person may be having palpitations or fainting spells, especially if these symptoms do not happen very often.
  • Stress test to look at changes in your heart’s activity that occur with an increase in heart rate and during recovery after exercise.
  • Cardiac magnetic resonance imaging (MRI) or other types of imaging to look at the heart’s structure.
  • Echocardiography to look at the heart’s structure and how well it is working.
  • Electrophysiology study (EPS) to look at the electrical activity of the heart and to find the source of an abnormal heartbeat. The study uses a wire to stimulate your heart electrically. The information from an EPS may help guide treatment.
  • Genetic testing to confirm a genetic diagnosis, as in some ion channel disorders. If you have certain genes known to be risk factors for a conduction disorder, your doctor may also recommend testing other family members.

If a conduction disorder is diagnosed while you are asleep, your doctor may also request a test to find out if you have sleep apnea, which is a common complication.

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Treatment - Heart Block

If you are diagnosed with a conduction disorder, your doctor may recommend lifestyle changes, medicines, surgery, or another procedure to treat your condition. Conduction disorders can be a medical emergency that requires immediate treatment in a hospital emergency department.

Healthy lifestyle changes
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Different conduction disorders have different triggers. Depending on your disorder, your doctor may ask you to do one or more of the following.

  • Avoid medicines that may trigger your conduction disorder or make it worse.
  • Limit exposure to sudden noises, such as in your occupation.
  • Limit physical activity or use precautions if exercise puts you at higher risk of dangerous heart rhythms. For example, swimming may be risky for people who have catecholaminergic polymorphic ventricular tachycardia.
  • Make heart-healthy lifestyle changes, including heart-healthy eating, managing stress, and limiting alcohol.

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Your doctor may recommend one or more of the following types of medicines to treat your conduction disorder:

  • Atropine to increase heart rate in severe types of AV block.
  • Beta blockers to decrease the activity of the stress hormones and make your heart beat more slowly. Examples of beta blockers include nadolol, propranolol, and metoprolol.
  • Other medicines, such as antiarrhythmics, may be used to treat certain types of arrhythmias.

Surgery and other procedures
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Your doctor may recommend one of the following procedures to treat a conduction disorder or implant a medical device.

  • Pacemakers to help your heart beat at a normal rate. Several conduction disorders, including SSS and more severe types of AV block, are treated with a pacemaker. Pacemakers may prevent fainting and improve survival. With research, new, more effective pacemaker technologies are emerging. Complications from pacemakers may include infection, bleeding, and damage to the heart or heart valves.
  • Radiofrequency catheter ablation to help prevent abnormal electrical activity. This type of catheter ablation uses radiofrequency energy to destroy a small section of heart tissue that is causing the abnormal electrical activity or arrhythmia.
  • Other procedures, such as placing an ICD—a type of defibrillator—or wearing away or removing nerves that are overstimulating the heart, called left cardiac sympathetic denervation, may be used to treat arrhythmias. Read more about these procedures in Living With.

Look for
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  • Research for Your Health will explain how we are using current research and advancing research to treat people who have conduction disorders.
  • Participate in NHLBI Clinical Trials will discuss our ongoing clinical studies that are investigating treatments for conduction disorders.
  • Living With will discuss what your doctor may recommend, including lifelong lifestyle changes and medical care to prevent your condition from recurring, getting worse, or causing complications.

Living With - Heart Block

If you have been diagnosed with a conduction disorder, it is important that you continue your treatment. Follow-up care may depend on your type of conduction disorder, your risk of a complication, and your response to treatment. If you have a conduction disorder whose cause is genetic, talk to your doctor and family members about possible genetic testing for your family members.

Receive routine follow-up care
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Unless caused by a medicine or blood electrolyte imbalance, conduction disorders are often lifelong conditions for which you will need to continue receiving care.

  • Continue to take all medicines prescribed for your condition to help prevent symptoms and complications.
  • Continue with any precautions that your doctor has advised.
  • Keep any appointments and tests recommended by your doctor.
  • Tell your doctor if you have any symptoms.

Return to Treatment to review possible treatment options for your conduction disorder.

Monitor your condition
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Your doctor may recommend the following to monitor your condition:

  • Blood tests. If the cause of your conduction disorder is due to abnormal electrolyte blood levels or a metabolic disorder, your doctor may request follow-up blood tests.
  • Follow-up EKGs to monitor your conduction disorder and to check for any new complications. Having one conduction disorder may put you at higher risk for having another conduction disorder. For example, AV block is more likely in people who have bundle branch blocks.
  • Regular check-ups to monitor the data collected by your pacemaker or ICD. If your ICD delivers a shock to restore your heart’s normal rhythm, contact your doctor right away.

Learn about other precautions to help you stay safe
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To help prevent complications such as dangerous arrhythmias, your doctor may recommend the following.

  • Avoid alcohol if it may trigger an event.
  • Avoid any known triggers for your conduction disorder, including emotional or stressful situations.
  • Avoid competitive sports and contact sports, if advised by your doctor.
  • Avoid medicines or illegal drugs that can make your conduction disorder worse. Ask your doctor about specific medicines you take or plan to take.
  • Follow any instructions regarding exercise. Some people are specifically advised not to swim.
  • Make recommended changes in what you eat. Your doctor may recommend watching how much potassium you eat.

Learn the warning signs of serious complications and have a plan
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Even with treatment for a conduction disorder, a repeat event or arrhythmia may occur. Know the signs and symptoms of your conduction disorder and how to recognize other complications, such as sudden cardiac arrest.

Usually, the first sign of sudden cardiac arrest is fainting. At the same time, no heartbeat can be felt. Some people may have a racing heartbeat or feel dizzy or light-headed just before they faint. Within an hour before cardiac arrest, some people have chest pain, shortness of breath, nausea, or vomiting. Call 9-1-1 right away if someone has signs or symptoms of sudden cardiac arrest. Look for an automated external defibrillator (AED) nearby and follow the instructions.

Prevent complications over your lifetime
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If you have a conduction disorder, your doctor may recommend medicines or procedures to help prevent or stop an arrhythmia. These include the following.

  • Antiarrhythmics to help prevent an arrhythmia. This class of medicines includes quinidine to treat Brugada syndrome.
  • ICD placement to stop an arrhythmia and help prevent sudden cardiac arrest. Survivors of sudden cardiac arrest caused by ventricular fibrillation are at high risk of another arrhythmia. Your doctor may recommend an ICD on its own or with medicines.
  • Isoproterenol medicine to suppress a life-threatening arrhythmia.
  • Left cardiac sympathetic denervation procedure to reduce the chance of arrhythmia in high-risk patients and those who do not respond to or cannot take certain medicines. This procedure can reduce the release of hormones that stimulate the heart and also make it harder for the heart’s ventricles to produce a dangerous arrhythmia. It reduces the risk of fainting and cardiac arrest.

Participate in NHLBI Clinical Trials

The National Heart, Lung, and Blood Institute (NHLBI) leads or sponsors many studies aimed at preventing, diagnosing, and treating heart, lung, blood, and sleep disorders.

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Other Resources
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Non-NHLBI Resources