What does Left Bundle Branch Block (LBBB) signify in the context of cardiac function?

What Does Left Bundle Branch Block (LBBB) Signify?

LBBB is a marker of underlying structural heart disease that warrants immediate echocardiographic evaluation, as it rarely occurs in healthy individuals and may represent the first manifestation of cardiomyopathy, ischemic heart disease, or other serious cardiac pathology. 1

Primary Clinical Significance

LBBB indicates delayed or blocked conduction in the left bundle branch of the His-Purkinje system, resulting in abnormal ventricular activation that causes both electrical and mechanical dyssynchrony. 1 Unlike right bundle branch block (which can be benign), LBBB is strongly associated with underlying cardiovascular disorders and carries important prognostic implications. 1

Key Pathophysiologic Consequences

• Mechanical dyssynchrony: LBBB causes asynchronous left ventricular contraction, which immediately reduces ejection fraction (typically to approximately 55% even in otherwise normal hearts) and creates regional differences in myocardial workload. 2, 3

• Progressive structural damage: The abnormal activation pattern leads to asymmetric hypertrophy and left ventricular dilatation, particularly in late-activated regions, which can progress from heart failure with preserved ejection fraction (HFpEF) to reduced ejection fraction (HFrEF) over 5-21 years. 4

• Cardiac dyssynchrony: LBBB may cause symptoms particularly in the setting of reduced left ventricular function, though isolated fascicular and bundle branch blocks are rarely symptomatic on their own. 1

Underlying Etiologies to Consider

LBBB develops from multiple potential causes that must be systematically evaluated:

• Ischemic heart disease: LBBB may occur as an early and isolated manifestation of coronary disease, appearing many years before structural LV changes become detectable. 1

• Cardiomyopathies: Including dilated cardiomyopathy, hypertrophic cardiomyopathy, and infiltrative diseases like sarcoidosis. 1

• Degenerative conduction system disease: Primary fibrosis or degeneration of the specialized conducting tissue. 1

• Other cardiac pathologies: Myocarditis, cardiac tumors, Chagas disease, congenital heart disease, valvular disease, and hypertensive heart disease. 1

Mandatory Evaluation

In patients with newly detected LBBB, a transthoracic echocardiogram to exclude structural heart disease is required (Class I recommendation). 1

Additional Diagnostic Steps Based on Clinical Context

• If structural heart disease is suspected but echocardiogram is unrevealing: Advanced imaging with cardiac MRI, CT, or nuclear studies is reasonable, as cardiac MRI detects subclinical cardiomyopathy in one-third of patients with asymptomatic LBBB and normal echocardiograms. 1

• If symptoms suggest intermittent bradycardia (lightheadedness, syncope): Ambulatory ECG monitoring is useful to document atrioventricular block or establish symptom-rhythm correlation. 1

• If ischemic heart disease is suspected: Stress testing with imaging may be considered, though LBBB makes ischemic ECG changes difficult to interpret, necessitating an imaging component. 1

• In syncope with LBBB: Electrophysiology study is reasonable when conduction system disease is identified but no atrioventricular block is demonstrated, as a prolonged HV interval ≥70 ms predicts higher risk for complete heart block. 1

Prognostic Implications

LBBB is generally associated with poorer prognosis compared to normal conduction or right bundle branch block. 2 The specific prognostic impact depends on:

• Presence of underlying structural disease: LBBB may be the first manifestation of diffuse myocardial disease. 2

• T-wave concordance pattern: Discordant LBBB (negative T waves in leads I, V5-V6) is associated with worse clinical profile, lower LVEF, greater LV dimensions, higher natriuretic peptides, and worse prognosis compared to concordant LBBB (positive T waves). 5

• "Latent cardiomyopathy": Some patients with LBBB have normal LV dimensions and ejection fraction at rest but demonstrate abnormal pulmonary artery pressure increases during exercise, lactate production during pacing, and signs of ischemia without coronary narrowing. 2

Critical Diagnostic Pitfall

The ECG diagnosis of left ventricular hypertrophy should not be attempted when LBBB is present, as LBBB itself alters QRS patterns and voltage criteria. 1 The prevalence of anatomic LVH in patients with LBBB may exceed 90%, making specificity estimates unreliable. 1

Therapeutic Considerations

• Cardiac resynchronization therapy (CRT): Currently indicated for heart failure with reduced ejection fraction (HFrEF) and LBBB, CRT can restore ventricular synchrony and improve symptoms and longevity in carefully selected patients. 3, 4

• Emerging therapies: Conduction system pacing (left bundle branch area pacing) is an emerging strategy that may reverse and forestall the deleterious effects of LBBB. 4