What is the difference between right bundle branch block (RBBB) and left bundle branch block (LBBB)?

Differences Between Right Bundle Branch Block and Left Bundle Branch Block

Right bundle branch block (RBBB) and left bundle branch block (LBBB) differ significantly in their ECG patterns, clinical implications, and prognostic significance, with LBBB generally having worse outcomes due to its association with more extensive cardiac disease and greater impact on ventricular synchrony. 1

ECG Characteristics

Left Bundle Branch Block (LBBB)

• Characterized by QRS duration >120 ms with monophasic notched or plateau-topped R waves in leads I, aVL, V5, and V6 1
• No septal Q waves in leads I, V5, and V6 due to abnormal septal activation from right to left 2
• Slurred predominant R waves in left precordial leads and slurred predominant S waves in right precordial leads 1
• Marked ST-segment depression during exercise that doesn't reliably indicate ischemia 1

Right Bundle Branch Block (RBBB)

• QRS duration >120 ms with characteristic RSR' pattern in right precordial leads (V1-V3) 1
• Prominent S waves in leads I, aVL, V5, and V6 1
• ST depression in anterior precordial leads (V1, V2, V3) that increases with exercise even without coronary obstruction 1
• Preserves normal diagnostic criteria for ischemia in inferolateral leads 1

Clinical Implications

Diagnostic Considerations

• LBBB makes diagnosis of myocardial ischemia from exercise ECG usually impossible 1
• RBBB does not invalidate interpretation of exercise ECG, except in anterior precordial leads (V1-V3) 1
• LVH diagnosis should generally 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
• In RBBB, LVH diagnosis can still be made using modified criteria, including S wave depth in left precordial leads 1

Cardiac Function Impact

• LBBB causes more significant ventricular dyssynchrony, leading to abnormal asynchronous contraction patterns 2
• LBBB may induce asymmetric hypertrophy and left ventricular dilatation due to regional differences in workload 2
• RBBB primarily affects right ventricular activation with less impact on overall cardiac function 3
• In heart failure patients, both blocks show significant LV activation delay, but RBBB patients have larger right-sided conduction delay 3

Prognostic Significance

Mortality Risk

• Both LBBB and RBBB are associated with increased mortality risk 4
• LBBB that develops during exercise predicts higher risk of death and major cardiac events 1
• LBBB occurs commonly in patients with nonischemic cardiomyopathies 1
• RBBB has been associated with underlying coronary artery disease (CAD), particularly obstructive disease affecting the left anterior descending artery 1
• Newly acquired LBBB carries a 10-fold increase in mortality compared to preexisting LBBB 5

Cardiac Resynchronization Therapy (CRT)

• LBBB pattern predicts favorable outcome with CRT 1
• RBBB is associated with non-favorable outcomes with CRT, with particularly high event rates 1
• Prolonged PR interval and RBBB were identified as predictors of poor response to CRT 1

Clinical Associations

• LBBB is often associated with more extensive myocardial disease and carries worse prognosis 5
• RBBB development during exercise is less common than LBBB in populations with high prevalence of heart disease 1
• In heart failure patients, RBBB has been shown to have a significantly worse clinical and hemodynamic profile compared to LBBB 3
• Benign LBBB (without underlying heart disease) is rare; usually disease becomes manifest over time 5

Important Considerations for Clinical Practice

• The development of either bundle branch block during exercise testing should prompt careful evaluation 1
• Both LBBB and RBBB can exist before exercise, develop during exercise, or (rarely) disappear during exercise 1
• Rate-dependent intraventricular blocks that develop during exercise often precede the appearance of chronic blocks 1
• When evaluating patients with bundle branch blocks, consider associated conditions like coronary artery disease, cardiomyopathy, and valvular heart disease 1, 5