Bundle Branch Block: Symptoms, Causes, and Treatment
Updated: 3 days ago
Medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: October 2026
Quick answer
Bundle branch block is a delay or blockage in the heart's electrical pathway to either the left or right ventricle. Most people have no symptoms, though it rarely causes fainting. See a doctor after any fainting spell. Most people need no treatment; when needed, care targets the cause and may include medicines or a pacemaker. [1] [2]
What the research says
According to PubMed:
Cardiac resynchronization therapy (CRT) helped people with left bundle branch block, not right. A patient-level meta-analysis of 8 randomized trials (6,264 patients) found CRT lowered the risk of heart failure hospitalization or death in people with QRS of 150 ms or more and left bundle branch block (HR 0.56, CrI 0.48 to 0.66), but not with right bundle branch block (HR 0.97, CrI 0.68 to 1.34). (Friedman et al., DOI)
Non-left bundle branch block patterns showed no clear CRT benefit. A meta-analysis of 5 randomized trials (6,523 participants, 1,766 with non-left bundle branch block QRS morphology) found CRT was not associated with a reduction in death or heart failure hospitalization in that group (HR 0.99, 95% CI 0.82 to 1.20). (Cunnington et al., DOI)
Device therapy reduced deaths in heart failure, with more benefit for left bundle branch block. A network meta-analysis of 13 randomized trials (12,638 patients) found CRT with a defibrillator reduced the rate of death by 42% versus medical therapy alone (95% credible interval 32 to 50%), and patients with QRS of 150 ms or more, left bundle branch block, and female sex benefited more from CRT. (Woods et al., DOI)
Study details are from PubMed abstracts.
What is bundle branch block?
Bundle branch block is a condition in which there is a delay or blockage along the pathway that electrical impulses travel to make the heart beat. It sometimes makes it harder for the heart to pump blood to the rest of the body. [1]
The heart's rhythm begins as an electrical signal. Impulses that cause the heart to beat start in the upper right chamber (the right atrium) and travel down to the lower chambers (the ventricles). That pathway includes two branches, called the left bundle and the right bundle. [1]
The delay or blockage can occur on either side: the pathway to the left ventricle or the pathway to the right ventricle. If one bundle is damaged, the ventricles rely on the other bundle to receive and respond to the signals from the upper chamber. If both bundles are blocked, the heart may beat very slowly, which can require a pacemaker. [1]
The good news: bundle branch block might not need treatment at all. When treatment is needed, it involves managing the underlying health condition, such as heart disease, that caused the block in the first place. [1]
What are the symptoms of bundle branch block?
In most people, bundle branch block does not cause symptoms at all. Some people with the condition do not know they have it. [1]
Rarely, symptoms may include fainting (called syncope) or feeling as if you are going to faint (called presyncope). These signs deserve a proper checkup because fainting can have other serious causes. [1]
When to see a doctor
If you have fainted, see a healthcare provider to rule out serious causes. [1]
If you have heart disease or have already been diagnosed with bundle branch block, ask your provider how often you should have follow-up visits. [1]
What causes bundle branch block?
Electrical impulses within the heart muscle cause it to beat. These impulses travel along a pathway that includes two branches, called the right bundle and the left bundle. If one or both of these bundles are damaged, due to a heart attack for example, the electrical impulses can become blocked, and the heart beats irregularly. [1]
Sometimes there is no known cause. The likely cause also differs depending on whether the left or the right bundle is affected. [1]
Side affected | Possible causes |
Left bundle branch block | Heart attack (myocardial infarction); high blood pressure (hypertension); inflammation of the heart muscle (myocarditis); thickened, stiffened, or weakened heart muscle (cardiomyopathy) [1] |
Right bundle branch block | Blood clot in the lungs (pulmonary embolism); heart attack (myocardial infarction); heart structure problems present at birth (congenital heart defects), such as a hole in the wall between the upper chambers (atrial septal defect); high blood pressure in the lung arteries (pulmonary hypertension); inflammation of the heart muscle (myocarditis) [1] |
Notice that inflammation of the heart muscle (myocarditis) and heart attack appear on both lists. Either bundle can be affected by damage to the heart muscle itself. [1]
What are the risk factors for bundle branch block?
Two factors stand out: [1]
Increasing age. Bundle branch block is more common in older adults than in younger people. [1]
Underlying health problems. Having high blood pressure or heart disease increases the risk of having bundle branch block. [1]
What complications can bundle branch block cause?
If both the right and the left bundles are blocked, the main complication is a complete blockage of the electric signaling from the upper chambers to the lower chambers of the heart. The lack of signaling can slow the heart rate. A slowed heart rate may lead to fainting, irregular heart rhythms, and other serious complications. [1]
There is a second, quieter complication worth knowing about. Because bundle branch block affects the electrical activity of the heart, it can sometimes complicate the accurate diagnosis of other heart conditions, especially heart attacks. It may lead to delays in the proper management of those heart conditions. [1]
Complication | What happens |
Complete heart block | Both bundles blocked stops signaling from upper to lower chambers; the heart rate slows [1] |
Fainting and irregular rhythms | A slowed heart rate can cause fainting and abnormal heart rhythms [1] |
Masked heart attacks | The altered electrical pattern can make heart attacks harder to diagnose on an ECG [1] |
How is bundle branch block diagnosed?
The approach depends on which side is affected. If you have a right bundle branch block and you are otherwise healthy, you might not need a full medical checkup. If you have a left bundle branch block, you will need a thorough medical exam. [2]
Two tests do most of the work: [2]
Electrocardiogram (ECG or EKG). This quick and painless test measures the electrical activity of the heart. During the test, sensors called electrodes are attached to the chest and sometimes to the arms or legs. An ECG shows how well the heart is beating. It can show signs of a bundle branch block and which side of the heart is being affected. [2]
Echocardiogram. This test uses sound waves to produce detailed images of the heart and the heart valves. It can show the structure and the thickness of the heart muscle, and it helps the provider pinpoint a condition that caused the bundle branch block. [2]
What are the treatment options for bundle branch block?
Most people with bundle branch block have no symptoms and need no treatment. For example, left bundle branch block is not treated with medications on its own. Instead, treatment depends on the specific symptoms and on other heart conditions. [2]
Medications
If you have a heart condition causing the bundle branch block, treatment might involve medications to reduce high blood pressure or to reduce symptoms of heart failure. [2]
Pacemaker
If you have bundle branch block and a history of fainting, your provider might recommend a pacemaker. A pacemaker is a small device implanted under the skin of the upper chest. Two wires connect it to the right side of the heart. The pacemaker releases electrical impulses when needed to keep the heart beating regularly. [2]
Cardiac resynchronization therapy
If you have bundle branch block with low heart-pumping function, you may need cardiac resynchronization therapy, also called biventricular pacing. This treatment is similar to having a pacemaker implanted, but you get a third wire connected to the left side of the heart so the device can keep both sides in proper rhythm. Cardiac resynchronization therapy helps the heart's chambers squeeze (contract) in a more organized and efficient way. [2]
Treatment | When it is used | How it works |
No treatment | Most people with no symptoms [2] | |
Medications | A heart condition is causing the block [2] | Reduce high blood pressure or heart failure symptoms [2] |
Pacemaker | Bundle branch block with a history of fainting [2] | Small device under the skin of the upper chest; two wires connect to the right side of the heart and release impulses when needed [2] |
Cardiac resynchronization therapy (biventricular pacing) | Bundle branch block with low heart-pumping function [2] | Like a pacemaker but with a third wire to the left side, keeping both sides in rhythm [2] |
How should you prepare for a bundle branch block appointment?
You are likely to start by seeing your primary care doctor. You might then be referred to a doctor trained in heart disorders, called a cardiologist. [2]
Be aware of pre-appointment restrictions. When you make the appointment, ask if there is anything you need to do in advance. For example, you may need to limit or avoid caffeine before having heart function tests. [2]
Make a list that includes: your symptoms, including any that seem unrelated to the reason you scheduled the appointment, when they began, and how often they occur; key personal information, including major stresses or recent life changes; all medications, vitamins, and supplements you take, including the doses; and the questions you want to ask your provider. [2]
Ask a family member or friend to come with you, if possible, to help you remember the information you receive. [2]
Useful questions to ask your provider: What are the most likely causes of my symptoms? What tests do I need? What treatments are available, and which do you recommend? Will the bundle branch block return after treatment? What side effects might I expect from treatment? I have other health conditions, so how can I best manage them together? Do you have brochures or other printed material, and what websites do you recommend? [2]
What to expect from your doctor: Your provider will likely ask whether anything seems to improve your symptoms, what worsens them, and whether a healthcare provider has ever told you that you have a bundle branch block. [2]
Conclusion
Bundle branch block is a delay or blockage in the electrical pathway that makes the heart beat, affecting either the left or the right bundle branch that feeds the lower chambers. Most people never feel a thing; the condition is usually discovered on an ECG during a routine or diagnostic visit. Causes differ by side: the left bundle points toward heart attack, high blood pressure, or a weakened heart muscle, while the right bundle often points to a lung clot, congenital heart defects, or lung artery pressure problems. Treatment, when needed, targets the underlying cause through blood pressure or heart failure medicines, a pacemaker for people with a history of fainting, or cardiac resynchronization therapy when pumping function is low. Anyone who has fainted, or who lives with heart disease or a bundle branch block diagnosis, should keep regular follow-up care with their provider.
Next steps: Talk with your healthcare professional after any fainting spell, and ask how often you should follow up if you have heart disease or bundle branch block.
Frequently asked questions
Does bundle branch block cause symptoms?
In most people, bundle branch block causes no symptoms at all, and some people do not know they have it. Rarely, it can cause fainting or the feeling that you are about to faint. [1]
What causes a left bundle branch block?
A left bundle branch block can be caused by a heart attack, high blood pressure, inflammation of the heart muscle (myocarditis), or a thickened, stiffened, or weakened heart muscle (cardiomyopathy). Sometimes there is no known cause. [1]
What causes a right bundle branch block?
A right bundle branch block can be caused by a blood clot in the lungs (pulmonary embolism), a heart attack, congenital heart defects present at birth (such as a hole between the upper chambers), high blood pressure in the lung arteries, or inflammation of the heart muscle. [1]
When is a pacemaker used for bundle branch block?
A pacemaker might be recommended if you have bundle branch block and a history of fainting, or cardiac resynchronization therapy (biventricular pacing) if you also have low heart-pumping function. [2]
Is bundle branch block dangerous?
Many people never need treatment. The main risk comes if both bundle branches are blocked, which can slow the heart rate, cause fainting and irregular rhythms, and make other heart conditions such as heart attacks harder to diagnose. [1]
This article is for general education and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional about your individual situation.
What is the difference between left and right bundle branch block?
Each bundle branch carries signals to one of the heart's lower chambers. Right bundle branch block is often found in otherwise healthy people, while left bundle branch block more often needs a full heart checkup.
How is bundle branch block diagnosed?
It is found on an electrocardiogram (ECG), a quick, painless test of the heart's electrical activity. An echocardiogram may also be done to look for a heart condition that caused it.
Can I exercise with bundle branch block?
Many people with bundle branch block and no other heart disease can stay active. Ask your provider before starting hard exercise, especially if you have heart disease or have fainted.
Does bundle branch block make a heart attack harder to detect?
Yes. It changes the ECG pattern, which can make a heart attack harder to spot. Tell emergency staff you have it, and call 911 for chest pain or other heart attack signs.
Is bundle branch block more common with age?
Yes. It becomes more common as people get older, and high blood pressure or heart disease also raises the risk.
Trusted external resources
Related Reading
Understanding Left Bundle Branch Block (LBBB): Causes, Symptoms, and Support takes a closer look at left bundle branch block and its care.
Heart Block: Understanding Types, Symptoms, and Treatment explains heart block types, from mild delays to complete block.
Electrocardiogram (EKG or ECG): Why It's Done, How to Prepare, What Happens and Results describes the quick test that shows how the heart's signals travel.
Cardiac Resynchronization Therapy (CRT): Procedure and Results explains the pacing treatment used for some people with weak hearts.
References
Mayo Clinic Staff. Bundle branch block - Symptoms and causes. Mayo Clinic. June 11, 2022. https://www.mayoclinic.org/diseases-conditions/bundle-branch-block/symptoms-causes/syc-20370514
Mayo Clinic Staff. Bundle branch block - Diagnosis and treatment. Mayo Clinic. June 11, 2022. https://www.mayoclinic.org/diseases-conditions/bundle-branch-block/diagnosis-treatment/drc-20370518
Friedman DJ, Al-Khatib SM, Dalgaard F, et al. Cardiac resynchronization therapy improves outcomes in patients with intraventricular conduction delay but not right bundle branch block: a patient-level meta-analysis of randomized controlled trials. Circulation. 2023;147(10):812-823. PMID 36700426. https://doi.org/10.1161/CIRCULATIONAHA.122.062124
Cunnington C, Kwok CS, Satchithananda DK, et al. Cardiac resynchronisation therapy is not associated with a reduction in mortality or heart failure hospitalisation in patients with non-left bundle branch block QRS morphology: meta-analysis of randomised controlled trials. Heart. 2015;101(18):1456-1462. PMID 25678498. https://doi.org/10.1136/heartjnl-2014-306811
Woods B, Hawkins N, Mealing S, et al. Individual patient data network meta-analysis of mortality effects of implantable cardiac devices. Heart. 2015;101(22):1800-1806. PMID 26269413. https://doi.org/10.1136/heartjnl-2015-307634
American Heart Association. Heart conduction disorders. American Heart Association. September 24, 2024. https://www.heart.org/en/health-topics/arrhythmia/about-arrhythmia/conduction-disorders
U.S. National Library of Medicine. Heart pacemaker. MedlinePlus. July 14, 2024. https://medlineplus.gov/ency/article/007369.htm
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