Conduction Disorders

Also known as Heart Block
A conduction disorder is a problem with the electrical system that makes your heart beat and controls its rate and rhythm. This system is called the cardiac conduction system. Normally, the electrical signal that makes your heart beat travels from the top of your heart to the bottom. The signal triggers your heart muscle, causing your heart to beat and pump blood to your lungs and body. In conduction disorders, this electrical signal either does not generate properly, or it does not travel properly through the heart, or both.

The types of conduction disorders, such as atrioventricular (AV) blocks and bundle branch blocks, vary depending on where they occur in the conduction system. Certain medicines can cause conduction disorders, as can conditions such as ischemic heart disease or heart attack, or your genetics. Your doctor may be able to diagnose a conduction disorder with an electrocardiogram (EKG), which is a reading of your heart’s electrical activity.

Conduction disorders may be treated with medicines, pacemakers, implantable cardioverter defibrillators (ICDs), and surgical procedures. Treatment depends on the location, type, and severity of your conduction disorder. Your doctor will consider how the disorder affects your heart and symptoms, which can differ from one person to the next. Sometimes an irregular heartbeat, called an arrhythmia, is the first sign of a conduction disorder. If left untreated, severe conduction disorders can lead to sudden cardiac arrest, in which the heart suddenly stops beating.

Explore this Health Topic to learn more about conduction disorders such as sick sinus syndrome, AV block, and bundle branch blocks, as well as our role in research and clinical trials to improve health, and where to find more information.

Types - Conduction Disorders

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
- Conduction Disorders

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
- Conduction Disorders

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
- Conduction Disorders

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
- Conduction Disorders

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 - Conduction Disorders

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
- Conduction Disorders

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 genetics condition called Holt-Oram syndrome, who have different degrees of cardiac conduction system problems such as AV block.

Other medical conditions
- Conduction Disorders

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
- Conduction Disorders

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

Risk Factors - Conduction Disorders

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.

- Conduction Disorders

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
- Conduction Disorders

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
- Conduction Disorders

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

- Conduction Disorders

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
- Conduction Disorders

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

- Conduction Disorders

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

  • Brugada syndrome
  • Catecholaminergic polymorphic ventricular tachycardia

Screening and Prevention - Conduction Disorders

If you or your child has known risk factors for a conduction disorder, the doctor may recommend screening, which may include genetic testing. Screening tests for conduction disorders may also be suggested for competitive athletes. Depending on the cause of your conduction disorder, heart-healthy lifestyle changes and other precautions may help decrease the risk of developing a conduction disorder.

Screening tests
- Conduction Disorders

High school and college athletes may be screened by their own physician. The doctor will ask about personal history and family history, in addition to performing a physical exam. EKGs or echocardiograms are used by many colleges to screen athletes—either all athletes or those who are at high risk. Learn more about EKGs and echocardiograms in the Diagnosis section.

When one family member has an inherited conduction disorder, close relatives may need a physical exam and medical tests, including genetic testing. This screening may help find conduction disorders early and lead to prompt treatment or monitoring.

If you are scheduled for surgery or a procedure, your doctor may recommend an EKG to screen for any heart problems including conduction disorders. Heart monitoring can also help detect problems that may emerge after the procedure.

Prevention strategies
- Conduction Disorders

While many conduction disorders cannot be prevented, your doctor may recommend the following steps to lower your chances of developing a conduction disorder.

  • Following a heart-healthy lifestyle to lower the chance of ischemic heart disease, limiting alcohol, and avoiding illegal drug use.
  • Managing conditions and medicines that change the normal levels of potassium, calcium, and magnesium.
  • Taking an angiotensin converting enzyme (ACE) inhibitor, used to treat high blood pressure. This may lower the risk of developing a conduction disorder in people who have high blood pressure.

Look for
- Conduction Disorders

  • Diagnosis will explain tests and procedures that your doctor may use to diagnose types of conduction disorders.
  • Living With will explain what your doctor may recommend to prevent your conduction disorder from recurring, getting worse, or causing complications.
  • Research for Your Health will explain how we are using current research and advancing research to prevent conduction disorders.
  • Participate in NHLBI Clinical Trials will discuss our open and enrolling clinical studies that are investigating prevention strategies for conduction disorders.

Signs, Symptoms, and Complications - Conduction Disorders

Many people who have conduction disorders, such as first-degree AV block, may have no obvious signs or symptom. 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
- Conduction Disorders

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

- Conduction Disorders

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

Look for
- Conduction Disorders

  • 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 - Conduction Disorders

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
- Conduction Disorders

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
- Conduction Disorders

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
- Conduction Disorders

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.

- Conduction Disorders

Treatment - Conduction Disorders

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
- Conduction Disorders

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.

- Conduction Disorders

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
- Conduction Disorders

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
- Conduction Disorders

  • 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 - Conduction Disorders

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
- Conduction Disorders

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
- Conduction Disorders

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
- Conduction Disorders

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
- Conduction Disorders

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
- Conduction Disorders

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.

Research for Your Health

The NHLBI is part of the U.S. Department of Health and Human Services’ National Institutes of Health (NIH)—the Nation’s biomedical research agency that makes important scientific discoveries to improve health and save lives. We are committed to advancing science and translating discoveries into clinical practice to promote the prevention and treatment of heart, lung, blood, and sleep disorders, including heart conduction disorders. Learn about current and future NHLBI efforts to improve health through research and scientific discovery.

Improving health with current research
- Conduction Disorders

Learn about the following ways the NHLBI continues to translate current research into improved health for people who have conduction disorders. Research on this topic is part of the NHLBI’s broader commitment to advancing heart and vascular disease scientific discovery.

  • Funding advances in emergency response. The NHLBI-sponsored Resuscitation Outcomes Consortium (ROC) is a clinical trial network that tested treatments to address high rates of injury and death from out-of-hospital cardiac arrest and severe traumatic injury. In 2018, ROC data helped show that more patients survive sudden cardiac arrest in public spaces when bystanders use an AED while waiting for a standard emergency response. ROC followed people who had cardiac arrest from ventricular fibrillation and ventricular tachycardia, which are complications of many conduction disorders.
  • The NHLBI provides recommendations on the research, diagnosis, and treatment of cardiac membrane channel diseases. In collaboration with the NIH Office of Rare Diseases, we developed a consensus report to provide recommendations on research about the diagnosis and management of conduction disorders such as Brugada syndrome.
  • The NHLBI supports the evaluation of new therapies and techniques to improve cardiovascular disease outcomes. The Cardiothoracic Surgical Trials Network (CTSN) is an international network that studies heart valve diseases, arrhythmias, heart failure, ischemic heart disease, and the complications of surgery. Since it was established, CTSN has developed a portfolio of trials spanning from early translation to the completion of six randomized clinical trials and three large observational studies.
  • Medicines May Prevent Conduction System Disease in Some Individuals. The Antihypertensive and Lipid-Lowering Treatment to Prevent Heart Attack Trial (ALLHAT) was a large study that enrolled people who have high blood pressure and high blood cholesterol, two major risk factors for heart disease. Through this study, researchers found that an ACE inhibitor commonly used to treat high blood pressure, lisinopril, may decrease the risk of developing a conduction disorder in patients who have cardiac risk factors. This is the first report that medicines may prevent conduction system disease.

Advancing research for improved health
- Conduction Disorders

In support of our mission, we are committed to advancing conduction disorder research, in part through the following ways.

  • We perform research. Our Division of Intramural Research and its Cardiovascular Branch perform research on diseases that affect the heart and blood vessels, including conduction disorders. NHLBI investigators in the Laboratory of Cardiovascular Intervention have developed a more precise method of catheter ablation that is used to treat arrhythmias, which are often the result of conduction disorders. The investigators showed that the new method was as successful and faster in some cases than the method currently in use.
  • We fund research. The research we fund today will help improve our future health. Our Division of Cardiovascular Sciences and its Heart Failure and Arrhythmias Branch oversee research aimed at improving the diagnosis, treatment, and prevention of arrhythmias, which often result from conduction disorders. Search the NIH RePORTer to learn about research the NHLBI is funding on conduction disorders.
  • We stimulate high-impact research. Our Trans-Omics for Precision Medicine (TOPMed) Program now includes participants who have conduction disorders, which may help us understand how genes contribute to differences in disease severity and how patients respond to treatment. The NHLBI Strategic Vision highlights ways we may support research over the next decade.

Learn about exciting research areas the NHLBI is exploring about heart conduction disorders.

Participate in NHLBI Clinical Trials

We lead or sponsor many studies on conduction disorders. See if you or someone you know is eligible to participate in our clinical trials.

Does your developing baby have a heart defect? Have you had a stillborn baby?

This study tests a new technology to measure fetal heart activity and find possible problems early. These problems include fetal arrhythmia and conduction disorders such as Brugada syndrome. To participate in this study, you must be at least 18 years old and pregnant, and have one of five high-risk complications of pregnancy: a fetus with a major congenital heart defect, hydrops, or gastroschisis; a previous unexplained stillbirth; or twins who share a placenta. This study is located in Madison and Milwaukee, Wisconsin.

Have you been diagnosed with orthostatic intolerance?

This study aims to find out how the body’s regulation of basic functions, such as heart rate and blood pressure, is altered in people who have orthostatic intolerance. This condition, which has an unknown cause, is characterized by a racing heartbeat, dizziness, lightheadedness, and other symptoms that occur when a person stands up. To participate in this study, you must be between 18 and 80 years old and have orthostatic intolerance. This study is located in Nashville, Tennessee.

Are you an adult living in Beijing, China, with a heart conduction disorder?

This study will investigate health outcomes among people with a heart conduction disorder that is sometimes treated with an implantable cardioverter defibrillator. To participate in this study, you must be between 18 and 60 years old and have complete left bundle branch block. The study takes place in Beijing, China.

Do you have a ventricular arrhythmia?

This study aims to evaluate the safety and effectiveness of low-energy shocks for the treatment of ventricular tachycardia and ventricular fibrillation. This approach will be tested during a catheter ablation or placement of an implantable cardioverter defibrillator (ICD). To participate in this study, you must be between 18 and 75 years old. This study is located in Minneapolis, Minnesota, and Austin, Texas.

More Information

After reading our Conduction Disorders Health Topic, you may be interested in additional information found in the following resources.

Non-NHLBI resources
- Conduction Disorders