What is Left Bundle Branch Block (LBBB)?

What is Left Bundle Branch Block (LBBB)?

LBBB is a cardiac conduction abnormality where activation of the left ventricle is delayed, causing the left ventricle to contract later than the right ventricle, resulting in electrical and mechanical dyssynchrony. 1

Electrocardiographic Definition

LBBB is diagnosed on ECG by specific criteria that have evolved to improve diagnostic accuracy:

Conventional ECG Criteria

• QRS duration ≥120 ms 1
• Slurred predominant R wave in left precordial leads (V5, V6) 1
• Slurred predominant S wave in right precordial leads 1
• Absence of septal q waves in leads I, V5, and V6 due to abnormal right-to-left septal activation 2

Strict ECG Criteria (Higher Specificity)

More recent strict criteria improve diagnostic specificity, particularly important for cardiac resynchronization therapy (CRT) patient selection:

• QRS duration ≥140 ms in men or ≥130 ms in women 3
• Mid-QRS notching in at least two leads among I, aVL, V1, V2, V5, and/or V6 3
• Monophasic notched or plateau-topped R waves in leads I, aVL, V5, and V6 1

The strict criteria achieve 100% sensitivity and 100% specificity, whereas conventional criteria have only 48% specificity in the presence of left ventricular hypertrophy or dilatation 3.

Pathophysiology and Clinical Significance

Immediate Effects

LBBB causes instant electrical and mechanical dyssynchrony that immediately reduces left ventricular ejection fraction to approximately 55% in an otherwise normal heart. 4

The conduction abnormality creates variable delays in the cardiac cycle 5:

• Delayed mitral valve closure (Q-MC >0.08 seconds) occurs as the major site of delay in 23% of patients 5
• Prolonged left ventricular isovolumetric contraction time (>0.06 seconds) is the most common abnormality, occurring in 59% of patients 5
• Delayed onset of ventricular contraction (Q-VC >0.07 seconds) occurs in 26% of patients 5

Progressive Consequences

LBBB is rarely benign and typically represents underlying structural cardiovascular disease or leads to progressive cardiac dysfunction over time. 1

The natural history involves 4:

• Initial presentation as heart failure with preserved ejection fraction (HFpEF ≥50%)
• Progression to heart failure with moderately reduced ejection fraction (HFmrEF 35-50%)
• Eventually advancing to heart failure with reduced ejection fraction (HFrEF ≤35%) over 5-21 years
• LBBB shortens median survival in patients with HFmrEF by 5.5 years 4

Underlying Etiologies

LBBB is a strong ECG marker of underlying structural cardiovascular disease and may occur as an early manifestation of ischemic heart disease or cardiomyopathy, many years before structural changes can be detected. 1

LBBB may develop from 1:

• Primary degenerative lesions of specialized conducting tissue 6
• Ischemic and hypertensive heart disease 1
• Cardiomyopathies 1
• Myocarditis 1
• Channelopathies 1
• Cardiac tumors, sarcoidosis, Chagas' disease 1
• Congenital heart disease (operated and unoperated) 1
• Aortic valve disease or following cardiac procedures 6

Epidemiology

LBBB is uncommon in healthy individuals, particularly in younger age groups, with prevalence of 0.3% in females and 0.6% in males under age 40. 1 In athletes, complete bundle branch blocks occur in less than 2% of ECGs and represent a potential marker of serious underlying cardiovascular disease 1.

Clinical Evaluation

Mandatory Workup

In patients with newly detected LBBB, transthoracic echocardiography to exclude structural heart disease is recommended. 1

Complete bundle branch block in an athlete should lead to comprehensive cardiological evaluation including exercise testing, 24-hour ECG monitoring, and cardiac imaging. 1

Genetic Considerations

An ECG should be obtained in siblings of young patients with bifascicular block patterns (including LBBB) to exclude genetically determined progressive cardiac conduction disease (Lenegre disease). 1 Lenegre disease is an autosomal dominant condition linked to SCN5A gene mutations affecting cardiac sodium ion channels 1.

Important Clinical Pitfalls

Asymptomatic Presentation

Because of the heart's functional redundancy, patients with LBBB may not notice reduced cardiac efficiency immediately, with a variable latent period from days to decades before becoming symptomatic. 4 This "latent cardiomyopathy" may present with 2:

• Abnormal pulmonary artery pressure increase during exercise
• Lactate production during high-rate pacing
• Signs of ischemia on myocardial scintigraphy without coronary narrowing
• Abnormal ultrastructural findings on myocardial biopsy

Diagnostic Challenges

The distinction between "complete" LBBB and left ventricular hypertrophy with intraventricular conduction delay can be difficult, as QRS duration in LVH can exceed 120 ms without a localized left bundle lesion. 1 A QRS duration >155 ms combined with left atrial P-wave abnormality and precordial voltage criteria have relatively high specificity for LVH in the presence of LBBB 1.

Rate-Dependent LBBB

Intermittent, rate-dependent LBBB may occur as a precursor and carries the same clinical and prognostic significance as stable LBBB. 1

Therapeutic Implications

Cardiac resynchronization therapy (CRT) is currently indicated in guidelines for heart failure with reduced ejection fraction (HFrEF) and LBBB. 4 Randomized controlled trials have shown CRT improves echocardiographic indices in HFmrEF with LBBB, and emerging evidence suggests benefit in HFpEF with LBBB (approximately 6% of all heart failure patients) 4. Left bundle branch area pacing is an emerging conduction system pacing strategy that may reverse and forestall the deleterious effects of LBBB 4, 6.