What is Bifascicular Block?
Bifascicular block refers to ECG evidence of impaired conduction below the AV node affecting two of the three fascicles of the His-Purkinje system 1, 2.
ECG Patterns
Bifascicular block manifests in specific combinations on successive ECGs:
- Right bundle branch block (RBBB) + left anterior fascicular block (LAFB) - most common pattern
- Right bundle branch block + left posterior fascicular block (LPFB)
- Alternating bundle branch block (also called bilateral bundle branch block) - when clear ECG evidence shows block in all 3 fascicles on successive tracings, such as:
The left bundle branch itself divides into anterior and posterior fascicles, while the right bundle remains as a single branch. Left bundle branch block (LBBB) is also technically considered a bifascicular block since it affects both left fascicles 3.
Clinical Significance
Natural History
The rate of progression from bifascicular block to complete (third-degree) AV block is slow 1, 4. Although third-degree AV block is most often preceded by bifascicular block, no single clinical or laboratory variable identifies patients at high risk of death from future bradyarrhythmia 1.
Syncope Considerations
Syncope is common in patients with bifascicular block, but recurrent syncope alone is NOT associated with increased sudden death 1, 4. However, critical distinctions exist:
- When permanent or transient third-degree AV block accompanies syncope, sudden death risk increases significantly regardless of electrophysiology study results 1, 4
- Pacing relieves neurological symptoms but does not reduce sudden death occurrence 1
High-Risk Scenario
Patients with first-degree AV block PLUS bifascicular block PLUS symptomatic advanced AV block have high mortality rates and substantial sudden death incidence 1.
Diagnostic Evaluation
When syncope occurs with bifascicular block and the cause cannot be determined with certainty, or when treatments (such as drugs) may worsen AV block, prophylactic permanent pacing is indicated, especially if syncope may have been due to transient third-degree AV block 1, 2.
Electrophysiological study may be helpful to evaluate and direct treatment of inducible ventricular arrhythmias, which are common in patients with bifascicular block 1.
Predictive Variables
- PR interval prolongation is common but often reflects AV nodal delay; no correlation exists between PR interval length and progression to third-degree AV block or sudden death 1
- HV interval ≥100 milliseconds warrants consideration for permanent pacing in asymptomatic patients, though HV prolongation often accompanies advanced cardiac disease and death is typically from underlying heart disease rather than AV block 1
Common Pitfall
Do not confuse bifascicular block with trifascicular block. Trifascicular block implies involvement of all three fascicles and typically manifests as complete AV block (the "peripheral form"), whereas bifascicular block maintains 1:1 AV conduction through the remaining fascicle 3.