ECG Characteristics of Bifascicular Block
Bifascicular block on ECG shows evidence of 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), or less commonly as RBBB with left posterior fascicular block (LPFB). 1
Most Common Pattern: RBBB + LAFB
Right Bundle Branch Block Component:
- QRS duration ≥120 ms 2, 3
- rsr', rsR', or rSR' pattern in leads V1-V2 2, 3
- Prolonged S waves in leads I and V6 (>40 ms or longer than the R wave) 2
- R peak time >50 ms in V1 but normal in V5-V6 2
Left Anterior Fascicular Block Component:
- QRS vector shifts posteriorly and superiorly 1, 2
- Larger R waves in leads I and aVL 1, 2
- Smaller R waves but deeper S waves in leads V5 and V6 1, 2
- Left axis deviation (typically more negative than -30°) 4
Less Common Pattern: RBBB + LPFB
- Same RBBB features as above 2, 3
- Right axis deviation in the frontal plane 5
- Left posterior fascicular block is very rare and usually associated with RBBB 2
Alternating Bundle Branch Block
This represents the most severe form and indicates block in all three fascicles on successive ECGs: 1
- RBBB and LBBB appearing on different ECGs, OR
- RBBB with LAFB on one ECG and RBBB with LPFB on another ECG 1
- This pattern is a Class I indication for permanent pacemaker implantation 3
Complete LBBB as Bifascicular Block
LBBB itself represents bifascicular block (involving both left anterior and posterior fascicles): 2
- QRS duration ≥120 ms 2
- Broad notched or slurred R waves in leads I, aVL, V5, and V6 2
- Absent Q waves in leads I, V5, and V6 2
- R peak time >60 ms in V5-V6 2
Clinical Context and Pitfalls
When first-degree AV block accompanies bifascicular block, this suggests trifascicular involvement and carries higher risk: 1
- PR interval prolongation is common but the delay is often at the AV node level, not necessarily indicating third fascicle involvement 1
- Patients with this combination and symptomatic advanced AV block have high mortality rates 1
The progression from bifascicular block to complete heart block is generally slow (approximately 4% per year in patients with heart disease), but certain patterns carry higher risk: 6, 7
- Bifascicular block with incomplete LBBB pattern has 22.7% risk of developing complete heart block versus 3.2% without this pattern 7
- Sudden cardiac death occurred in 15.9% with the incomplete LBBB pattern versus 3.9% without it 7
LBBB is very rare in healthy individuals and strongly suggests underlying structural cardiovascular disease, requiring cardiology evaluation even when asymptomatic. 2