What is the clinical significance and management of Left Bundle Branch Block (LBBB) and Right Bundle Branch Block (RBBB) in patients?

Clinical Significance and Management of Left Bundle Branch Block (LBBB) and Right Bundle Branch Block (RBBB)

Left bundle branch block (LBBB) and right bundle branch block (RBBB) have different clinical implications, with LBBB generally associated with higher morbidity and mortality and requiring more aggressive evaluation and management than RBBB.

Diagnostic Criteria

Left Bundle Branch Block (LBBB)

• Complete LBBB is defined by: 1
  • QRS duration ≥120 ms in adults
  • Broad notched or slurred R wave in leads I, aVL, V5, and V6
  • Absent Q waves in leads I, V5, and V6
  • R peak time >60 ms in leads V5 and V6
  • ST and T waves usually opposite in direction to QRS

Right Bundle Branch Block (RBBB)

• Complete RBBB is defined by: 1
  • QRS duration ≥120 ms in adults
  • RSR' pattern in leads V1 and V2
  • S wave of greater duration than R wave in leads I and V6
  • Normal R peak time in leads V5 and V6 but >50 ms in lead V1

Clinical Significance

Left Bundle Branch Block (LBBB)

• LBBB is frequently associated with underlying structural heart disease and predicts higher rates of morbidity and mortality 2
• Exercise-induced LBBB (but not exercise-induced RBBB) is associated with increased risk of death and cardiac events 1
• LBBB may cause abnormal asynchronous contraction patterns leading to: 3
  • Regional differences in ventricular workload
  • Asymmetric hypertrophy
  • Left ventricular dilatation
  • Potential development of cardiomyopathy

Right Bundle Branch Block (RBBB)

• RBBB is generally considered more benign than LBBB in the absence of underlying cardiac disorder 3
• RBBB has lower association with structural heart disease compared to LBBB 4
• Mortality rate of approximately 2.5%/year compared to 4.5%/year for LBBB 5

Evaluation Approach

For LBBB:

1. Transthoracic echocardiogram is recommended for all patients with newly detected LBBB to exclude structural heart disease 1
2. If echocardiogram is unrevealing but structural heart disease is still suspected, advanced imaging (cardiac MRI, CT, or nuclear studies) is reasonable 1
3. In patients with symptoms suggestive of intermittent bradycardia with LBBB, ambulatory electrocardiographic monitoring is useful 1
4. In patients with syncope and LBBB, electrophysiologic study (EPS) is reasonable to assess for HV interval prolongation or infranodal block 1

For RBBB:

1. Echocardiography is reasonable if structural heart disease is suspected 1
2. Less aggressive evaluation is generally warranted compared to LBBB unless symptomatic 1
3. In patients with symptoms suggestive of intermittent bradycardia, ambulatory monitoring is indicated 1

Management Recommendations

Permanent Pacing Indications:

1. Permanent pacing is recommended in patients with:

   • Syncope and bundle branch block with HV interval ≥70 ms or evidence of infranodal block at EPS 1
   • Alternating bundle branch block (QRS complexes with alternating LBBB and RBBB morphologies) 1

2. Permanent pacing is reasonable in patients with:

   • Kearns-Sayre syndrome and conduction disorders 1
   • Anderson-Fabry disease with QRS prolongation >110 ms 1

3. Cardiac resynchronization therapy (CRT) may be considered in patients with:

   • Heart failure with mildly to moderately reduced LVEF (36%-50%)
   • LBBB with QRS ≥150 ms 1

4. Permanent pacing is NOT indicated in:

   • Asymptomatic patients with isolated conduction disease and 1:1 atrioventricular conduction 1

Risk Stratification:

• Patients with LBBB should be considered at higher short-term risk requiring prompt evaluation when presenting with: 1
  • Syncope
  • Heart failure symptoms
  • Family history of sudden cardiac death
  • Structural heart disease

Long-term Prognosis

• Isolated LBBB without structural heart disease has a 10-year mortality rate of 22% and incidence of cardiomyopathy of 9.1% 6
• Patients with LBBB and normal stress echocardiogram have similar mortality to those without LBBB 5
• Patients with LBBB and abnormal stress echocardiogram have more than twice the risk of all-cause mortality (HR = 2.4) 5

Clinical Pearls and Pitfalls

• Don't miss: Alternating bundle branch block implies unstable conduction disease in both bundles and is a high-risk finding requiring permanent pacing 1
• Don't miss: Exercise-induced LBBB is associated with increased risk of death and cardiac events and warrants thorough evaluation 1
• Avoid: Assuming RBBB and LBBB have similar clinical implications; LBBB generally carries worse prognosis 3, 5
• Remember: The presence of LBBB on ECG markedly increases the likelihood of left ventricular systolic dysfunction being diagnosed by echocardiogram 1