What are the electrocardiogram (ECG) criteria for right bundle branch block (RBBB) and left anterior hemiblock (LAHB)?

ECG Criteria for RBBB and LAHB

The diagnosis of complete RBBB requires QRS ≥120 ms in adults with an rsr', rsR', or rSR' pattern in V1/V2, prolonged S waves in leads I and V6 (>40 ms or longer than R wave), and R peak time >50 ms in V1, while LAHB is diagnosed by left axis deviation between -45° and -90°, qR pattern in aVL, R-peak time ≥45 ms in aVL, and QRS <120 ms. 1

Complete Right Bundle Branch Block (RBBB)

The AHA/ACCF/HRS standardization guidelines provide definitive criteria that must be systematically applied 1:

Required Criteria (All 3 Must Be Present):

1. QRS Duration:

   • Adults: ≥120 ms
   • Children 4-16 years: >100 ms
   • Children <4 years: >90 ms

2. Precordial Lead Morphology (V1/V2):

   • Classic patterns: rsr', rsR', or rSR'
   • The terminal R' or r' deflection is typically wider than the initial R wave
   • Minority variant: Wide, notched R wave pattern may appear instead

3. Lateral Lead Changes:

   • Leads I and V6: S wave duration greater than R wave OR S wave >40 ms in adults
   • Lead V1: R peak time >50 ms
   • Leads V5-V6: Normal R peak time maintained

Critical Diagnostic Point: All three criteria must be satisfied to diagnose complete RBBB. When a dominant R wave (with or without notching) appears in V1, ensure criterion 3 is met 1.

Left Anterior Fascicular Block (LAHB)

Also termed left anterior hemiblock, LAHB diagnosis requires all four criteria 1:

Required Criteria (All 4 Must Be Present):

1. Frontal Plane Axis: Between -45° and -90° (marked left axis deviation)

2. Lead aVL Morphology: qR pattern

3. R-Peak Time in aVL: ≥45 ms

4. QRS Duration: <120 ms (this distinguishes it from complete bundle branch blocks)

Important Caveat: These criteria do NOT apply to patients with congenital heart disease who have left axis deviation present from infancy 1.

Clinical Pearls and Pitfalls

RBBB Considerations:

• Incomplete RBBB exists when QRS is 110-119 ms in adults with otherwise typical RBBB morphology 1
• In children, an rsr' pattern in V1-V2 with normal QRS duration is a normal variant and should not be diagnosed as RBBB 1
• RBBB can mask or be masked by other conduction abnormalities—when LAHB coexists with RBBB, the LAHB may obscure typical RBBB features, particularly abolishing the terminal S wave in leads I and aVL 2, 3
• Recording high V1 and V2 leads or right-sided chest leads can unmask hidden RBBB when LAHB is present 3

LAHB Considerations:

• The axis deviation must be marked (beyond -45°) to diagnose LAHB; moderate left axis deviation (-30° to -45°) alone is insufficient 4
• QRS duration remaining <120 ms is essential—if ≥120 ms, consider complete LBBB or nonspecific intraventricular conduction delay instead 1
• When LAHB occurs with RBBB (bifascicular block), the combination may have clinical significance in acute MI settings, though isolated bifascicular block from LAHB obscuring RBBB typically has a benign course 3

Differential Diagnosis Alerts:

• Distinguish RBBB from normal rsr' variant, particularly important when evaluating for bifascicular block 1
• RBBB pattern can appear with higher electrode placement—verify proper V1/V2 positioning 1
• In wide complex tachycardias, RBBB with LAHB morphology may represent ventricular tachycardia rather than aberrancy 5