Can Right Bundle Branch Block (RBBB) and Left Bundle Branch Block (LBBB) coexist?

Can RBBB and LBBB Coexist?

No, right bundle branch block (RBBB) and left bundle branch block (LBBB) cannot truly coexist simultaneously in the same heart because they represent mutually exclusive conduction patterns. 1

Physiological and Electrocardiographic Explanation

• Complete bundle branch blocks are characterized by QRS duration >120 ms and represent interruption of conduction through specific pathways of the cardiac conduction system 1
• RBBB represents a block in the right bundle branch, causing delayed activation of the right ventricle, while LBBB represents a block in the left bundle branch, causing delayed activation of the left ventricle 1
• By definition, these conduction patterns produce opposite and incompatible ECG morphologies:
  • LBBB creates a dominant R wave in leads I and aVL with absence of S waves
  • RBBB creates a terminal R wave in lead V1 2

Conditions That May Appear Similar to Coexisting RBBB and LBBB

1. Alternating Bundle Branch Block

• This refers to RBBB and LBBB appearing on successive ECGs at different times (not simultaneously) 1
• Indicates severe conduction system disease affecting both bundle branches
• Associated with rapid progression to complete heart block 1
• Requires immediate pacemaker implantation even without symptoms (Class I recommendation) 1

2. Bifascicular Block

• Combination of RBBB with either left anterior hemiblock or left posterior hemiblock 1
• Represents conduction disease in two of the three main fascicles
• Carries increased risk of progression to complete AV block 1
• Requires cardiological workup including exercise testing, 24h ECG, and imaging 1

3. Masquerading Bundle Branch Block

• A rare ECG pattern showing RBBB pattern in precordial leads and LBBB pattern in limb leads 3
• Indicates severe and diffuse conduction system disease
• Associated with poor prognosis and severe underlying heart disease 3
• Often seen in patients with advanced coronary artery disease 3

Clinical Implications and Management

• Bundle branch blocks may develop from:

  • Primary degenerative lesions of specialized conducting tissue
  • Ischemic or hypertensive heart disease
  • Cardiomyopathies
  • Myocarditis
  • Channelopathies
  • Cardiac tumors
  • Sarcoidosis
  • Chagas' disease
  • Congenital heart disease 1

• Patients with complex conduction abnormalities require:

  • Comprehensive cardiac evaluation 1
  • Consideration for pacemaker therapy in specific scenarios (alternating BBB, syncope with BBB and positive electrophysiology study) 1
  • Assessment for underlying structural heart disease 1

• Special considerations for specific patient populations:

  • In athletes, complete bundle branch blocks are uncommon (<2%) and warrant thorough investigation 1
  • In patients with suspected acute myocardial infarction, both RBBB and LBBB can obscure ST-segment analysis 1
  • In heart failure patients, RBBB produces less mechanical dyssynchrony than LBBB and may respond differently to cardiac resynchronization therapy 4

Important Caveats

• The term "bilateral bundle branch block" may be used in literature but refers to advanced conduction disease affecting both bundles, not simultaneous RBBB and LBBB 2
• Combinations of bundle branch block and hemiblock reflect extensive involvement of the specialized conduction system and carry increased risk of clinically significant AV block 1
• ECG interpretation in patients with complex conduction abnormalities requires careful analysis and correlation with clinical findings 1