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Left Bundle Branch Block: Causes, Symptoms & Treatment

Left bundle branch block is a cardiac conduction disorder in which the electrical impulse is delayed or blocked in the left branch of the heart conduction system.

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Things worth knowing about "Left Bundle Branch Block"

Left bundle branch block is a cardiac conduction disorder in which the electrical impulse is delayed or blocked in the left branch of the heart conduction system.

What is Left Bundle Branch Block?

Left bundle branch block (LBBB) is a disorder of the cardiac conduction system. Under normal circumstances, the electrical impulse triggering a heartbeat is conducted simultaneously through the right and left bundle branches to both ventricles. In LBBB, this conduction is delayed or completely blocked in the left bundle branch, causing the left ventricle to be activated with a delay. A distinction is made between complete LBBB (QRS duration on ECG greater than 120 ms) and incomplete LBBB (QRS duration between 110 and 120 ms).

Causes

Left bundle branch block is rarely found in individuals with a healthy heart. In most cases it reflects an underlying cardiac condition. Common causes include:

Coronary artery disease (CAD) and myocardial infarction
Cardiomyopathy (disease of the heart muscle)
Arterial hypertension (high blood pressure)
Heart valve disease, particularly of the aortic valve
Myocarditis (inflammation of the heart muscle)
Degenerative changes of the conduction system (e.g., Lenegre disease)
Rarely: congenital heart defects

Symptoms

Left bundle branch block often does not cause any symptoms on its own. Symptoms that do occur are usually related to the underlying heart disease. Possible complaints include:

Heart palpitations or irregular heartbeat
Dizziness and lightheadedness
Shortness of breath (dyspnea), especially during physical exertion
Fatigue and reduced exercise tolerance
In rare cases: fainting (syncope)

Newly occurring symptoms combined with a freshly diagnosed LBBB should always be evaluated by a physician, as they may indicate an acute cardiac event.

Diagnosis

The diagnosis of left bundle branch block is primarily established by electrocardiogram (ECG). Typical ECG features include:

Widened QRS complex (greater than 120 ms in complete LBBB)
Absent or small R wave in leads V1 and V2
Broad, notched, or slurred R wave in left-sided leads I, aVL, V5, and V6
Discordant ST-segment and T-wave changes

Further diagnostic workup to identify the underlying cause may include:

Echocardiography to assess cardiac structure and function
Holter monitoring to detect intermittent block
Stress ECG (exercise testing)
Coronary angiography if CAD is suspected
Laboratory tests (e.g., cardiac enzymes, BNP)

Treatment

Left bundle branch block itself usually does not require direct treatment. The primary focus is on managing the underlying condition. Depending on the severity and clinical situation, the following approaches may be considered:

Optimized pharmacological therapy for heart failure, hypertension, or CAD
Cardiac resynchronization therapy (CRT): A specialized pacemaker that stimulates both ventricles synchronously. It is mainly used in patients with reduced cardiac pump function (impaired left ventricular ejection fraction) and LBBB.
Implantable cardioverter-defibrillator (ICD): For patients at elevated risk of life-threatening arrhythmias
Treatment of structural heart disease (e.g., valve replacement)

Prognosis

The prognosis in left bundle branch block depends strongly on the underlying condition. A newly diagnosed LBBB is an important clinical finding that requires timely cardiological evaluation, as it can be associated with increased rates of heart failure and mortality. With optimal treatment of the underlying disease and, where appropriate, CRT, quality of life and survival rates can be significantly improved.

References

Pschyrembel Clinical Dictionary. 268th Edition. De Gruyter, Berlin 2020.
Fuster V. et al. (eds.): Hurst's The Heart. 14th Edition. McGraw-Hill Education, New York 2017.
European Society of Cardiology (ESC): Guidelines for Cardiac Pacing and Cardiac Resynchronization Therapy. European Heart Journal, 2021.

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