What do Right Bundle Branch Block (RBBB) and Left Anterior Fascicular Block (LAFB) on an electrocardiogram (EKG) indicate?

What RBBB and LAFB Mean on EKG

RBBB (Right Bundle Branch Block) and LAFB (Left Anterior Fascicular Block) are electrical conduction abnormalities in the heart that indicate delayed or blocked transmission through specific parts of the cardiac conduction system, and when found together—especially in younger individuals—they warrant comprehensive cardiac evaluation to exclude underlying structural heart disease or progressive conduction disorders. 1

Understanding the Individual Components

Right Bundle Branch Block (RBBB)

RBBB represents a conduction delay or block through the right bundle branch of the heart's electrical system. 1

ECG Characteristics:

• QRS duration ≥120 ms in adults (>100 ms in children 4-16 years, >90 ms in children <4 years) 2
• Characteristic rsr', rsR', or rSR' pattern in leads V1 or V2 3
• S wave of greater duration than R wave or >40 ms in leads I and V6 3
• R peak time >50 ms in lead V1 but normal in V5 and V6 3

Clinical Significance:

• Complete RBBB is uncommon in healthy individuals and athletes (<2% prevalence) 1
• May represent idiopathic, isolated, clinically benign conduction interruption in some cases 1
• Can indicate underlying cardiac pathology including ischemic heart disease, hypertensive heart disease, cardiomyopathies, myocarditis, channelopathies, cardiac tumors, sarcoidosis, Chagas' disease, or congenital heart disease 1

Left Anterior Fascicular Block (LAFB)

LAFB (also called left anterior hemiblock) represents a conduction block through the anterior fascicle of the left bundle branch. 1

ECG Characteristics:

• Left axis deviation, typically marked (-45° to -90°) 4
• qR pattern in lead aVL 4
• R-peak time in lead aVL ≥45 ms 4
• QRS duration <120 ms (when isolated) 4
• Small q waves in inferior leads 1

Clinical Significance:

• More common in men and increases with age 1
• Prevalence in general population under 40 years: 0.5-1.0% 1
• Usually an incidental finding in subjects without structural heart disease 1
• Can be associated with various cardiovascular disorders 1

The Combined Pattern: RBBB + LAFB (Bifascicular Block)

When RBBB and LAFB occur together, this is called bifascicular block and represents more extensive involvement of the specialized conduction system. 1

Critical Clinical Implications

This combination carries increased risk of:

• Progression to complete atrioventricular block 1, 5
• Underlying structural heart disease 1
• Genetically determined progressive cardiac conduction disease (Lenegre disease), particularly in young individuals 1

Lenegre disease is an autosomal dominant condition linked to SCN5A gene mutations affecting cardiac sodium channels, characterized by various conduction defects occurring in young individuals. 1

Special ECG Pattern: "Masquerading Bundle Branch Block"

In some cases, RBBB with LAFB can mimic LBBB on ECG due to:

• Absence of S wave in lead I (standard masquerading) 6
• Absence of S wave in leads V5 and V6 (precordial masquerading) 6
• This occurs when predominant left ventricular forces are present, often with left ventricular hypertrophy, scar tissue from myocardial infarction, or cardiomyopathy 6

Mandatory Workup Algorithm

When RBBB and/or LAFB (especially combined) are identified, the European Heart Journal guidelines mandate the following evaluation: 1

1. Exercise stress testing to assess for rate-dependent conduction changes and ischemia 1

2. 24-hour Holter ECG monitoring to evaluate for:

   • Progression to higher-degree AV block 1
   • Intermittent complete heart block 1
   • Arrhythmias 1

3. Cardiac imaging (echocardiography) to exclude:

   • Structural heart disease 1
   • Cardiomyopathies 1
   • Ischemic heart disease 1
   • Arrhythmogenic right ventricular cardiomyopathy (ARVC) 1
   • Congenital heart disease 1

4. Family screening: Obtain ECG in siblings of young patients with bifascicular block to screen for Lenegre disease 1

Critical Pitfalls to Avoid

Do not dismiss these findings as benign without proper evaluation, particularly in:

• Young individuals (<40 years) where prevalence should be very low 1
• Athletes, where these findings are uncommon (<2%) and may indicate serious underlying disease 1
• Patients with symptoms (syncope, palpitations, dyspnea) 1

Do not confuse RBBB+LAFB with LBBB:

• Check for the characteristic rsr' pattern in V1-V2 (indicates RBBB) 3, 6
• Verify left axis deviation and qR pattern in aVL (indicates LAFB) 4
• The "masquerading" pattern can be misleading, especially with concurrent left ventricular pathology 6

Do not overlook the possibility of progressive conduction disease:

• Bifascicular block may progress to trifascicular block (complete AV block) 5
• Prognosis is generally poor in patients with coronary or hypertensive heart disease 5
• May necessitate pacemaker implantation 5