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 and dyssynchronous left ventricular contraction. 1

Diagnostic Criteria and ECG Features

LBBB is diagnosed based on specific ECG characteristics with high-level evidence:

• QRS duration >120 ms
• Monophasic notched or plateau-topped R waves in leads I, aVL, V5, and V6
• Slurred predominant S waves in the right precordial leads
• Absence of septal q waves in leads I, V5, and V6 (due to abnormal septal activation from right to left) 1, 2

Pathophysiology and Clinical Significance

LBBB causes immediate electrical and mechanical dyssynchrony of the left ventricle with several important consequences:

• Reduces left ventricular ejection fraction instantly (to approximately 55% in an otherwise normal heart)
• Causes regional differences in myocardial workload leading to asymmetric hypertrophy
• May lead to left ventricular dilatation over time
• Creates inefficient ventricular contraction patterns 3, 2

LBBB is generally associated with a poorer prognosis compared to normal intraventricular conduction or right bundle branch block. It often indicates underlying cardiac pathology rather than being a benign finding. 1, 2

Progression and Clinical Course

LBBB can lead to a progressive decline in cardiac function:

• Initially, patients may be asymptomatic due to the heart's functional redundancy
• Over time (ranging from days to decades), patients may develop heart failure symptoms
• Disease progression typically follows this pattern:
  1. Heart failure with preserved ejection fraction (HFpEF) ≥50%
  2. Heart failure with moderately reduced ejection fraction (HFmrEF) 35-50%
  3. Heart failure with reduced ejection fraction (HFrEF) ≤35% 3

Etiologies of LBBB

LBBB can result from:

• Underlying cardiomyopathy or heart disease
• Conduction system degeneration
• Aortic valve disease
• Following cardiac procedures
• In rare cases, it may occur in structurally normal hearts (as in painful LBBB syndrome) 4, 5

Clinical Management

Evaluation

• LBBB on ECG warrants further cardiac evaluation in all cases as it represents an independent risk factor for cardiovascular mortality 4
• Baseline 12-lead ECG is essential for patients undergoing elevated-risk surgeries 1

Diagnostic Testing

• Exercise stress testing should be avoided due to poor specificity (33%) and diagnostic accuracy (36-60%)
• Pharmacologic stress testing is preferred:
  • Vasodilator stress (dipyridamole or adenosine) perfusion imaging has superior diagnostic accuracy (88-92%)
  • Dobutamine stress echocardiography is also appropriate with diagnostic accuracy of 87% 1

Treatment Options

• Treatment of underlying cardiac disease is essential

• Cardiac Resynchronization Therapy (CRT) is strongly recommended (Level I evidence) for patients with:

  • LBBB
  • Left ventricular ejection fraction ≤35%
  • QRS duration ≥150 ms
  • Despite optimal medical therapy 1, 3

• Emerging treatment approaches:

  • Conduction system pacing, particularly left bundle branch area pacing, shows promise in reversing LBBB effects
  • His bundle pacing may normalize wide QRS patterns in selected patients 3, 6

Special Considerations

• LBBB masks ECG diagnosis of left ventricular hypertrophy
• LBBB interferes with ischemia detection on ECG
• Regular follow-up with clinical evaluation and ECG is recommended to detect progression of conduction disease or development of heart failure 1
• Some patients with LBBB and normal ventricular dimensions/function at rest may have abnormal exercise responses (termed "latent cardiomyopathy") 2