What is a Bifascicular Block?
Bifascicular block refers to ECG evidence of impaired conduction in two of the three fascicles of the intraventricular conduction system below the AV node—specifically involving the right bundle branch and one of the two divisions of the left bundle branch. 1
Anatomical Definition and ECG Patterns
The most common manifestations of bifascicular block include:
- Right bundle branch block (RBBB) with left anterior fascicular block (left axis deviation) — this is the most frequently encountered pattern 1, 2
- Right bundle branch block with left posterior fascicular block (right axis deviation or vertical axis) — less common due to the larger size of the posterior fascicle 2, 3
- Complete left bundle branch block — though technically involving the left bundle system, this is functionally considered a bifascicular block as it affects both left fascicles 1, 4
Alternating Bundle Branch Block (Trifascicular Involvement)
A critical variant is alternating bundle-branch block, which demonstrates clear ECG evidence of block in all three fascicles on successive ECGs — examples include RBBB and LBBB on different tracings, or RBBB with left anterior fascicular block on one ECG and RBBB with left posterior fascicular block on another. 1, 5 This pattern carries the highest risk and is a Class I indication for permanent pacemaker implantation. 1, 6
Clinical Significance and Natural History
The primary concern with bifascicular block is the potential progression to complete heart block, though the rate of progression from bifascicular block to third-degree AV block is generally slow. 1, 5
No single clinical or laboratory variable, including bifascicular block itself, reliably identifies patients at high risk of death from future bradyarrhythmia. 1, 5 This is a critical point—the presence of bifascicular block alone does not predict sudden death risk.
Prognostic Factors
When assessing patients with bifascicular block, the following factors are most important:
- Underlying heart disease and congestive heart failure are the most significant predictors of mortality, not the conduction abnormality itself 7, 5
- Coronary artery disease is a major cause of bifascicular block, particularly when left anterior fascicular block is present with significant LAD lesions 2
- Syncope in the setting of bifascicular block may represent transient complete heart block and carries increased sudden death risk 7, 5
- HV interval prolongation ≥100 ms does not reliably identify high-risk patients for sudden death from bradyarrhythmia 7
Common Pitfall: First-Degree AV Block
PR interval prolongation commonly coexists with bifascicular block, but the delay is often at the AV node level and does not correlate with progression to complete heart block or sudden death. 5, 6 Do not assume that first-degree AV block plus bifascicular block automatically indicates "trifascicular disease" requiring pacing—this is a Class III indication (not recommended) in asymptomatic patients. 1, 6
When Pacing is NOT Indicated
Permanent pacemaker implantation is not indicated for fascicular block without AV block or symptoms, nor for fascicular block with first-degree AV block without symptoms. 1, 6 Routine prophylactic pacing does not reduce mortality in asymptomatic patients with bifascicular block. 7, 6