What are the EKG criteria for diagnosing bifascicular block (Bundle Branch Block)?

EKG Criteria for Bifascicular Block

Bifascicular block on EKG demonstrates impaired conduction in two of the three fascicles of the ventricular conduction system, most commonly appearing as right bundle branch block (RBBB) combined with left anterior fascicular block (LAFB). 1

Most Common Pattern: RBBB + LAFB

RBBB Component Criteria

The right bundle branch block component requires all of the following 1:

• QRS duration ≥120 ms 1
• rsr', rsR', or rSR' pattern in leads V1-V2 (the R' or r' deflection is typically wider than the initial R wave) 1
• Prolonged S waves in leads I and V6 (>40 ms or longer than the R wave) 1
• R peak time >50 ms in V1 but normal in V5-V6 1

LAFB Component Criteria

The left anterior fascicular block component requires all four mandatory criteria 2:

• Left axis deviation between -45° and -90° 2
• qR pattern in lead aVL (small q wave with tall R wave) 2
• R-peak time in lead aVL ≥45 ms 2
• QRS duration <120 ms (when isolated, but this is superseded by RBBB duration when combined) 2

Key Morphologic Features of LAFB

When LAFB is present, the QRS vector shifts posteriorly and superiorly, producing 3, 1:

• Larger R waves in leads I and aVL 3
• Smaller R waves but deeper S waves in leads V5 and V6 3

Less Common Pattern: RBBB + LPFB

LPFB Component Criteria

Left posterior fascicular block requires all four mandatory criteria 4:

• Frontal plane QRS axis between 90° and 180° in adults 4
• rS pattern in leads I and aVL 4
• qR pattern in leads III and aVF 4
• QRS duration <120 ms (when isolated) 4

Trifascicular Block Pattern

When first-degree AV block (PR interval >200 ms) accompanies bifascicular block, this suggests trifascicular involvement and carries significantly higher mortality risk. 3, 1 This pattern indicates disease in all three fascicles with the remaining fascicle conducting slowly 3.

Alternating Bundle Branch Block

The most severe form shows block in all three fascicles on successive ECGs, such as 3:

• RBBB and LBBB on different ECGs, or
• RBBB with LAFB on one ECG and RBBB with LPFB on another ECG

Critical Diagnostic Pitfalls

Exclusions for LAFB Diagnosis

Do not diagnose LAFB when left axis deviation is due to 2:

• Congenital heart disease with left axis deviation present in infancy 2
• Left ventricular hypertrophy causing left axis deviation 2
• Age-related leftward axis shift in elderly patients 2

Impact on LVH Detection

R-wave amplitude in leads I and aVL becomes unreliable for diagnosing left ventricular hypertrophy when LAFB is present 3, 1. Instead, criteria incorporating S-wave depth in left precordial leads (V5, V6) improve LVH detection in this setting 3.

Clinical Significance

Bifascicular blocks reflect extensive involvement of the specialized conduction system and carry increased risk of clinically significant AV block. 1 The risk of progression varies:

• Annual rate of progression to complete heart block is approximately 4% per year in patients with heart disease 5
• Patients with trifascicular block (bifascicular block + first-degree AV block) have high mortality rates and substantial incidence of sudden death 3
• The presence of incomplete LBBB pattern with bifascicular block identifies patients at particularly high risk, with 22.7% developing complete heart block during follow-up 6