What is a left bundle branch block (LBBB)?

Left Bundle Branch Block (LBBB)

Left bundle branch block (LBBB) is a cardiac conduction abnormality characterized by delayed activation of the left ventricle due to impaired conduction through the left bundle branch, resulting in QRS prolongation >120 ms, monophasic notched or plateau-topped R waves in leads I, aVL, V5, and V6, and slurred predominant S waves in the right precordial leads. 1

Electrocardiographic Characteristics of LBBB

• QRS duration >120 ms (typically 140-160 ms)
• Monophasic notched or plateau-topped R waves in leads I, aVL, V5, and V6
• Absence of septal Q waves in leads I, V5, and V6 (due to abnormal septal activation from right to left)
• Slurred predominant S waves in the right precordial leads
• Left axis deviation is commonly associated
• Secondary ST-T wave changes (ST depression and T-wave inversion in leads with dominant R waves)

Pathophysiology

LBBB causes an abnormal pattern of cardiac activation that affects:

• Ventricular depolarization sequence: With LBBB, the right ventricle is activated first, followed by delayed activation of the left ventricle
• Mechanical dyssynchrony: The abnormal electrical activation leads to uncoordinated contraction between the ventricles
• Regional myocardial function: Areas of the left ventricle contract at different times, reducing overall cardiac efficiency 2

Clinical Significance and Implications

LBBB may be:

• Primary: Due to idiopathic fibrosis or degeneration of the conduction system
• Secondary: Associated with underlying cardiac conditions such as:
  • Hypertension
  • Coronary artery disease
  • Cardiomyopathy
  • Valvular heart disease (especially aortic valve disease)
  • Cardiac surgery complications 3

Diagnostic Implications

• Masks ECG diagnosis of LVH: The diagnosis of left ventricular hypertrophy should not be attempted when LBBB is present, except in specific cases with left atrial P-wave abnormality, QRS duration >155 ms, and certain precordial lead voltage criteria 1
• Interferes with ischemia detection: LBBB can mimic or mask signs of myocardial ischemia on ECG
• Affects stress testing interpretation: Wall motion abnormalities may be present due to dyssynchrony rather than ischemia 2

Prognostic Implications

• LBBB is generally associated with a poorer prognosis compared to normal intraventricular conduction or right bundle branch block 4
• May be the first manifestation of a more diffuse myocardial disease
• Can lead to "latent cardiomyopathy" - patients with initially normal left ventricular function who develop abnormal cardiac function over time 4

Therapeutic Implications

• Cardiac Resynchronization Therapy (CRT): Indicated for patients with LBBB, LVEF ≤35%, and QRS ≥150 ms despite optimal medical therapy 5
• Monitoring: Regular follow-up with clinical evaluation and ECG is recommended to detect progression of conduction disease or development of heart failure 5
• Underlying disease management: Treatment of any underlying cardiac condition is essential

Complications and Long-term Effects

• Progression of conduction disease: May develop into higher-degree AV blocks
• Mechanical effects: Asynchronous ventricular contraction may lead to:
  • Reduced cardiac output
  • Mitral regurgitation
  • Ventricular remodeling
  • Development or worsening of heart failure 4
• Structural changes: Regional differences in workload may lead to asymmetric hypertrophy and left ventricular dilatation 4

Key Differences from Other Conduction Abnormalities

• Unlike Right Bundle Branch Block (RBBB), LBBB is rarely benign and often indicates underlying structural heart disease
• LBBB has greater implications for cardiac resynchronization therapy compared to other conduction abnormalities
• LBBB more significantly affects left ventricular function due to the dominant role of the left ventricle in cardiac output

LBBB requires careful evaluation to identify underlying causes and appropriate management to prevent progression of cardiac dysfunction and improve outcomes.