Left Bundle Branch Block: Your Questions Answered

What is a left bundle branch block?

A left bundle branch block (LBBB) is an abnormality in the way electrical signals travel through your heart. Your heart has a wiring system that carries each heartbeat signal from the upper chambers to the lower chambers. That system splits into a right and left branch. When the left branch is blocked, the electrical signal takes a slower, indirect route to reach the left ventricle (your heart's main pump). This means the two lower chambers no longer contract together in perfect sync. On an ECG, LBBB shows up as a widened QRS complex of 120 milliseconds or more with a distinctive broad, notched pattern.

Is left bundle branch block serious?

It depends on the context. In some patients, LBBB is an isolated electrical finding with a normal heart structure and no symptoms, which carries a relatively favorable outlook. In others, LBBB signals underlying heart disease such as coronary artery disease, cardiomyopathy, or long-standing high blood pressure. Even isolated LBBB reduces the left ventricle's pumping efficiency and has been shown to be an independent risk factor for developing heart failure over time. That's why every patient with LBBB should have a thorough cardiac evaluation, regardless of how they feel.

What causes LBBB?

The most common causes include age-related degeneration of the conduction system (fibrosis and calcification), long-standing high blood pressure that thickens the heart muscle, coronary artery disease and prior heart attacks, cardiomyopathy (weakened heart muscle from any cause), and aortic valve disease or procedures like TAVR. In a small percentage of patients, no cause is identified (idiopathic LBBB). Importantly, cardiac MRI has revealed subclinical heart muscle disease in about one-third of patients whose echocardiograms appeared normal.

What tests will my cardiologist order?

An echocardiogram is always the first step. This ultrasound measures heart size, wall thickness, valve function, and ejection fraction (pumping strength). Blood work checks for thyroid issues, electrolyte imbalances, and heart failure markers like BNP. If coronary artery disease is a concern, I'll order a nuclear stress test or stress echocardiogram rather than a standard treadmill test, because LBBB can produce misleading results on a regular stress ECG. In selected cases, cardiac MRI provides the most detailed look at the heart muscle and can detect subtle abnormalities that echocardiography misses.

Can LBBB be treated?

Treatment depends on what's causing the LBBB and whether heart function is affected. If LBBB is isolated with normal heart function, the main approach is monitoring with periodic echocardiograms, managing cardiovascular risk factors, and watching for new symptoms.

If LBBB is associated with reduced heart function and heart failure, cardiac resynchronization therapy (CRT) is one of the most effective treatments in cardiology. CRT uses a specialized pacemaker to pace both ventricles simultaneously, restoring the synchronized contraction that LBBB disrupted. Major clinical trials have shown CRT reduces heart failure events by 53% and death by 25% in LBBB patients, with benefits lasting over a decade. Newer approaches called conduction system pacing (His bundle pacing and left bundle branch area pacing) engage the heart's own wiring directly and are increasingly available as alternatives.

Can I exercise with LBBB?

In most cases, yes. If your echocardiogram shows normal heart function and you have no symptoms, there are typically no exercise restrictions. Regular physical activity is actually beneficial for cardiovascular health. However, if your LBBB is associated with reduced heart function, significant valve disease, or exercise-induced symptoms like shortness of breath or lightheadedness, your cardiologist may recommend a modified exercise program. Always get clearance from your cardiologist before starting or changing an exercise regimen when LBBB is present.

How often should I follow up?

I recommend an echocardiogram within the first 6-12 months after LBBB is discovered, then every 1-2 years if the baseline is normal. Annual ECGs track whether the pattern is stable or changing. If you develop new symptoms such as shortness of breath, fatigue, swelling in your legs, or lightheadedness, contact your cardiologist promptly rather than waiting for your next scheduled visit.

Should I see a cardiologist?

Yes. Every patient with LBBB should have at least one thorough cardiology evaluation, including an echocardiogram. If you're in the San Diego area, I welcome you to schedule an appointment at San Diego Cardiovascular Associates in Encinitas, where we can review your ECG, perform the appropriate testing, and build a follow-up plan specific to your situation.

References

Kusumoto, Fred M., et al. "2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients with Bradycardia and Cardiac Conduction Delay." Journal of the American College of Cardiology 74, no. 7 (2019): e51-e156.

Auffret, Vincent, et al. "Idiopathic/Iatrogenic Left Bundle Branch Block-Induced Reversible Left Ventricle Dysfunction." Journal of the American College of Cardiology 72, no. 24 (2018): 3177-3188.

Goldenberg, Ilan, et al. "Survival with Cardiac-Resynchronization Therapy in Mild Heart Failure." New England Journal of Medicine 370, no. 18 (2014): 1694-1701.

Sapp, John L., et al. "Long-Term Outcomes of Resynchronization-Defibrillation for Heart Failure." New England Journal of Medicine 390, no. 3 (2024): 212-220.

Thein, Aung S., et al. "Left Bundle Branch Block as a Risk Factor for Heart Failure." JAMA Network Open (2025).

Tan, Nicholas Y., et al. "Left Bundle Branch Block: Current and Future Perspectives." Circulation: Arrhythmia and Electrophysiology 13, no. 4 (2020): e008239.

Glikson, Michael, et al. "ESC Clinical Consensus Statement on Indications for Conduction System Pacing." Europace (2025).