Left Anterior Fascicular Block (LAFB): Causes, Symptoms, and Heart Health
Updated: 2 days ago
Medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: October 2026
Left anterior fascicular block (LAFB) happens when the heart's electrical signal is delayed at the left anterior fascicle, so the left ventricle contracts later than the right. It usually causes no symptoms and often needs no treatment on its own, but it can go along with underlying heart disease, so follow-up focuses on protecting heart health and treating the cause [1].
Quick Answer: What is left anterior fascicular block?
Left anterior fascicular block (LAFB) is a delay of the heart's electrical impulse in the left anterior fascicle, so the left ventricle contracts slightly later than the right. An estimated 1% to 6% of people have it, mostly older adults. It usually causes no symptoms and needs no specific treatment, but it raises heart failure risk, so regular checkups matter. [1]
What the research says
In people who already have heart failure, LAFB can signal a worse outlook. In a prospective cohort of 291 hospitalized heart-failure patients (78 with LAFB and 213 without), the absence of LAFB was associated with a lower risk of death or heart-failure readmission at 6 months (adjusted hazard ratio 0.48, 95% CI 0.23–0.99), and the group without LAFB had greater improvement in left ventricular ejection fraction (between-group difference 2.94%, 95% CI 0.55–5.33). Patients with LAFB were older (70.5 vs 65.0 years) and had a lower baseline ejection fraction (30.9% vs 33.8%) (Liu et al., Pacing Clin Electrophysiol, 2026 [2]).
Conduction problems are a recognized complication of TAVR, one of the known causes of LAFB. In a systematic review of 11 risk-prediction models, the incidence of permanent pacemaker implantation after transcatheter aortic valve replacement ranged from 7.3% to 31.0%, and right bundle branch block, PR interval, QRS interval, and atrioventricular block were among the frequent predictors. This review looked at pacemaker need after TAVR in general, not LAFB specifically (Mao et al., Front Cardiovasc Med, 2025 [3]).
What is left anterior fascicular block (LAFB)?
The heart's electrical system controls the timing of each beat. In a healthy heart, signals travel through specialized pathways called bundle branches so that the ventricles contract in a coordinated way. In LAFB, something interferes with the signal when it reaches the left anterior fascicle, a branch of the left bundle branch, so the signal must take an alternative route and the left ventricle contracts later than the right [1].
The heart's conduction system: in LAFB, the signal is delayed on its way through the left ventricle.
LAFB is relatively common in older adults but rare in younger people [1]. When it does appear, it can be a sign that the heart's electrical pathways are being affected by structural changes, scarring, or underlying cardiovascular disease.
What causes left anterior fascicular block, and who is at risk?
LAFB can result from a range of factors, from an inherited tendency to complications of medical procedures. Finding the underlying cause is important, because treatment focuses on that cause rather than on the block itself [1].
Cause category | Specific examples and conditions |
|---|---|
Medical procedures | Complications of transcatheter aortic valve replacement (TAVR) or of surgical septal myectomy. |
Heart diseases | Coronary artery disease (CAD), dilated cardiomyopathy, and possibly myocarditis (inflammation of the heart muscle). |
Structural changes | Fibrosis (scarring) of the heart tissue or sclerodegenerative disease of the bundle branch. |
Genetic factors | Researchers think some people may have an inherited tendency toward abnormal electrical signals in the lower chambers of the heart. |
What are the symptoms of LAFB, and when should you seek care?
Left anterior fascicular block (LAFB) usually causes no noticeable symptoms and is often found on an EKG done for another reason. In severe cases, or when other heart conditions are present, people may notice [1]:
Dizziness: feeling lightheaded or unsteady.
Tiredness: unusual fatigue.
Fainting (syncope): a brief loss of consciousness.
Contact your provider if you notice changes in how you feel, especially if you are out of breath when you walk. Call your local emergency number right away for chest pain, discomfort or pressure, shortness of breath, upper body pain, nausea, or lightheadedness, as these can be signs of a heart attack [1].
How is left anterior fascicular block (LAFB) diagnosed?
LAFB is diagnosed with an electrocardiogram (EKG), which records the heart's electrical activity and shows the delay at the left anterior fascicle [1]. Because LAFB can accompany other heart problems, your provider may suggest further tests:
Echocardiogram: an ultrasound of the heart to look for structural problems.
Stress test: monitoring the heart during physical exertion.
Cardiac MRI or nuclear imaging: detailed imaging to look for scarring or blood-flow problems.
An EKG records the heart's electrical activity and is the main test used to diagnose LAFB.
What is the outlook for left anterior fascicular block, and how is it managed?
There is no specific treatment for LAFB, but you will most likely need treatment for the heart disease that caused it [1]. The outlook is good for young people with LAFB, although they rarely have it. The finding itself may be benign, but researchers have found that people with LAFB have a higher risk of congestive heart failure, atrial fibrillation, and death [1].
Because LAFB can be an indicator of other heart problems, the best approach is to take care of your heart [1]:
Nutrition: eat foods low in saturated fat.
Physical activity: get regular exercise.
Chronic condition control: manage high blood pressure and high cholesterol, and attend regular checkups so your provider can monitor them.
Stress: keep your stress level down.
Healthy food and regular exercise help protect the heart.
What is the bottom line on left anterior fascicular block?
A diagnosis of left anterior fascicular block can sound alarming, but it is often a manageable finding that prompts better attention to heart health. By identifying the underlying cause and keeping up with regular checkups, people with LAFB can lower the risks linked with this conduction delay. Small, steady steps such as improving diet, exercising, and managing stress and blood pressure support a healthier heart.
Your next step: if you have been diagnosed with LAFB, work with your cardiologist or doctor to monitor your heart. Ask whether you should be checked for coronary artery disease, how to estimate your heart attack risk, and what else you can do to keep your heart healthy [1].
Frequently Asked Questions
What is LAFB?
Left anterior fascicular block (LAFB) is a delay in the heart's electrical signal that causes the left ventricle to contract later than normal.
Is LAFB a heart attack?
No. LAFB is a conduction delay, not a heart attack. It is sometimes associated with underlying heart disease, which is why your provider may check your heart further.
What are the symptoms of LAFB?
Most people have no symptoms. Severe cases may cause dizziness, fatigue, or fainting.
How is LAFB diagnosed?
LAFB is primarily diagnosed using an electrocardiogram (EKG), which tracks the heart's electrical signals.
Does LAFB require treatment?
The block itself usually doesn't need treatment, but the underlying heart disease causing it will require medical management.
Who is most at risk for LAFB?
LAFB is most common in senior adults and is rarely found in younger people.
Can LAFB lead to heart failure?
In older adults, LAFB is associated with a higher risk of developing congestive heart failure and atrial fibrillation.
What causes the electrical signal to be blocked?
Causes include coronary artery disease, heart tissue scarring, myocarditis, and complications from certain heart procedures such as TAVR or septal myectomy.
How can I manage my heart health with LAFB?
Focus on regular exercise, a heart-healthy diet, managing blood pressure and cholesterol, and keeping stress low.
When should I go to the ER?
Call your local emergency number if you have chest pain, discomfort or pressure, shortness of breath, upper body pain, nausea, or lightheadedness, which can be signs of a heart attack.
Related Reading
Bundle Branch Block: Symptoms, Causes, and Treatment explains related delays in the heart's electrical wiring and their treatment.
Left Posterior Fascicular Block (LPFB): Understanding Heart Conduction explains the less common partner block of the left bundle branch.
Electrocardiogram (EKG or ECG): Why It's Done, How to Prepare, What Happens and Results describes the heart test used to detect fascicular blocks.
Echocardiogram: Types, What to Expect, Results and Risks describes the heart ultrasound used to look for underlying heart disease.
References
Cleveland Clinic. Left Anterior Fascicular Block. Cleveland Clinic. May 24, 2022. https://my.clevelandclinic.org/health/diseases/23212-left-anterior-fascicular-block
Liu J, Guo F, Zuo S, et al. Left anterior fascicular block as a predictor of outcomes and reduced cardiac functional improvement in heart failure. Pacing Clin Electrophysiol. 2026;49(3):347-357. PMID 41457851. https://doi.org/10.1111/pace.70101
Mao Y, Liu Q, Fan H, et al. Risk prediction models for permanent pacemaker implantation following transcatheter aortic valve replacement: a systematic review and meta-analysis. Front Cardiovasc Med. 2025;12:1563597. PMID 41079589. https://doi.org/10.3389/fcvm.2025.1563597
Medical disclaimer
This article is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. The clinical information in this guide reflects source material last reviewed in May 2022.
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