What does a left bundle branch block (LBBB) indicate?

From the Guidelines

A left bundle branch block (LBBB) indicates a delay or obstruction in the electrical conduction system of the heart, specifically in the left bundle branch, which can be a sign of underlying heart disease such as coronary artery disease, hypertension, cardiomyopathy, or valvular heart disease. This conduction abnormality prevents the normal, synchronized electrical activation of the left ventricle, causing it to contract later than the right ventricle. LBBB appears on an electrocardiogram (ECG) as a widened QRS complex (typically greater than 120 milliseconds), with specific morphology changes in leads V1 and V6.

Key Points to Consider

• While LBBB itself doesn't require specific treatment, it often signals underlying heart disease that may need to be addressed.
• In some cases, LBBB may be a normal variant, particularly in older individuals.
• The clinical significance of LBBB depends on the context - new-onset LBBB with chest pain raises concern for acute myocardial infarction and requires immediate medical attention, as noted in the 2013 ACCF/AHA guideline for the management of ST-elevation myocardial infarction 1.
• LBBB can also complicate the diagnosis of heart attacks on ECG and may contribute to heart failure by reducing cardiac efficiency through dyssynchronous ventricular contraction.
• Patients with LBBB should undergo cardiac evaluation to identify any underlying heart condition requiring treatment, which may include echocardiography, stress testing, and potentially ambulatory monitoring or electrophysiological studies (EPS) as recommended by the 2018 ACC/AHA/HRS guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay 1 and the eligibility and disqualification recommendations for competitive athletes with cardiovascular abnormalities 1.

Recommendations for Management

• Cardiac evaluation is essential for patients with LBBB to determine the underlying cause and to guide management decisions.
• The presence of LBBB on ECG markedly increases the likelihood that left ventricular systolic dysfunction will be diagnosed by echocardiogram, as highlighted in the 2018 ACC/AHA/HRS guideline 1.
• Cardiac resynchronization therapy (CRT) may be beneficial for patients with LBBB and heart failure, as indicated by the 2018 ACC/AHA/HRS guideline 1 and further supported by studies such as the REVERSE trial.

From the Research

Left Bundle Branch Block Indications

A left bundle branch block (LBBB) may indicate several conditions, including:

• Conduction system degeneration or myocardial pathology 2
• Aortic valve disease or cardiac procedures 2
• Underlying cardiac disease, which may play a role in the pathogenesis of cardiomyopathy through delays in interventricular conduction, leading to dyssynchrony 3
• A poorer prognosis in comparison to normal intraventricular conduction or right bundle branch block 4
• Abnormalities in left ventricular performance due to abnormal asynchronous contraction patterns 4
• Latent cardiomyopathy, characterized by normal left ventricular dimensions and normal ejection fraction at rest, but abnormal increase in pulmonary artery pressure during exercise, production of lactate during high-rate pacing, signs of ischemia on myocardial scintigrams, and abnormal ultrastructural findings on myocardial biopsy 4
• Mechanical dyssynchrony in heart failure patients, which can be assessed using left ventricular hemodynamic forces calculated from 4D flow MRI 5
• Electrical and mechanical ventricular dyssynchrony, which can influence ischemia detection on the surface electrocardiogram, stress testing, and imaging modalities dependent on wall motion and thickening 6

Clinical Significance

The clinical significance of LBBB includes:

• Association with a low mortality rate and incidence of cardiomyopathy in patients without structural heart disease, ventricular dysfunction, or coronary artery disease 3
• Potential response to cardiac resynchronization therapy, which can improve symptoms and longevity in carefully selected patients 2, 6
• Importance of considering the risks imposed by conduction abnormalities in heart failure patients and predicting response to CRT using unique flow-specific measures of mechanical dyssynchrony 5