No, a left posterior fascicular block is NOT the same as a bundle branch block—they are distinct types of intraventricular conduction abnormalities.
A left posterior fascicular block (also called left posterior hemiblock) represents a conduction delay in only one subdivision of the left bundle branch system, whereas a bundle branch block involves the entire right or left bundle branch. These are fundamentally different anatomical and electrical entities 1.
Understanding the Anatomical Distinction
The intraventricular conduction system has a hierarchical structure:
- The His bundle divides into right and left bundle branches
- The left bundle branch further subdivides into:
- Anterior fascicle
- Posterior fascicle
- The right bundle branch remains as a single structure (though it has fascicular subdivisions that are rarely clinically relevant) 1, 2
A fascicular block affects only one subdivision of the left bundle branch, while a bundle branch block affects the entire right or left bundle branch 3.
Clinical and Electrocardiographic Differences
Left Posterior Fascicular Block (LPFB):
- Extremely rare as an isolated finding in both general and specific patient populations 4
- Causes right axis deviation (typically beyond +90° to +120°)
- QRS duration usually remains <120 ms (normal or minimally prolonged)
- Diagnosis requires excluding right ventricular hypertrophy, lateral MI, or vertical heart position 4
- Often associated with right bundle branch block (forming a bifascicular block) 5
Bundle Branch Block:
- QRS duration >120 ms is the defining feature 1
- Left bundle branch block (LBBB) is rare in healthy individuals and represents a strong marker of underlying structural heart disease 5
- Right bundle branch block (RBBB) may be benign but warrants evaluation 5
Important Clinical Context
When LPFB occurs with RBBB, this combination is called "bifascicular block" 5, 6. This terminology can be confusing because it involves both a fascicular block AND a bundle branch block, but they remain distinct entities affecting different parts of the conduction system.
The 2010 European Heart Journal guidelines emphasize that combinations of bundle branch block and hemiblock (fascicular block) reflect more extensive involvement of the specialized conduction system and carry increased risk of clinically significant AV block 5.
Practical Implications
Both fascicular blocks and bundle branch blocks warrant cardiological evaluation including exercise testing, 24-hour ECG monitoring, and imaging to exclude underlying pathological causes 5. However, the diagnostic approach and prognostic implications differ:
- Isolated LPFB is so rare that its presence should prompt thorough evaluation for alternative diagnoses
- Complete LBBB strongly suggests structural heart disease and requires echocardiography 6
- The combination (bifascicular block) requires more aggressive monitoring due to risk of progression to complete heart block 5
Common Pitfall to Avoid:
Don't confuse the term "bifascicular block" with two fascicular blocks. Bifascicular block typically refers to RBBB plus either left anterior or left posterior fascicular block—meaning one complete bundle branch block plus one fascicular block 5, 6.