What is Left Posterior Fascicular Block (LPFB) on EKG?
Left Posterior Fascicular Block is a rare intraventricular conduction disturbance characterized by a rightward QRS axis shift (between 90° and 180°), an rS pattern in leads I and aVL, and a qR pattern in leads III and aVF, with QRS duration remaining less than 120 ms. 1
Diagnostic Criteria
The American Heart Association/American College of Cardiology/Heart Rhythm Society established four mandatory criteria for LPFB diagnosis 1:
- Frontal plane QRS axis between 90° and 180° in adults 1
- rS pattern in leads I and aVL 1
- qR pattern in leads III and aVF 1
- QRS duration less than 120 ms (distinguishing it from complete bundle branch block) 1
Important Pediatric Consideration
In children up to 16 years of age, the rightward axis criterion should only be applied when a distinct rightward change in axis is documented, since children normally have more rightward axes than adults 1. This prevents overdiagnosis in the pediatric population where physiologic right axis deviation is common.
Clinical Significance and Associations
LPFB is an extremely rare finding in both the general population and specific patient groups 2, with prevalence estimated at only 0.24% in unselected ECG series 3. The European Society of Cardiology emphasizes that LPFB is usually associated with right bundle branch block (bifascicular block) 4.
Strong Association with Coronary Disease
When isolated LPFB is identified, it serves as a reliable marker for inferior myocardial infarction and severe three-vessel coronary artery disease 3:
- In patients with old inferior MI, 5.5% showed LPFB pattern, with eight of nine having three-vessel disease 3
- LPFB generally masks the ECG findings of inferior MI, leading to underdiagnosis 3
- The presence of LPFB reliably indicates the need for invasive coronary investigation 3
When LPFB occurs with RBBB and acute inferior MI, PR interval prolongation is very frequent, suggesting more extensive conduction system involvement 2.
Critical Diagnostic Pitfalls
A definite diagnosis of LPFB cannot be made in the presence of right ventricular hypertrophy (from COPD/emphysema), extensive lateral myocardial infarction, or an extremely vertical heart 2. These conditions can mimic the rightward axis deviation seen in LPFB, making clinical correlation essential.
The diagnosis requires careful consideration because:
- Right ventricular hypertrophy produces similar rightward axis deviation 2
- Anatomically vertical hearts in thin individuals may have rightward axes without conduction disease 2
- Extensive lateral MI can alter axis orientation 2
Vectorcardiographic Insights
Vectorcardiography provides additional diagnostic precision 2:
- In isolated LPFB, 20% of the QRS loop is located in the right inferior quadrant 2
- When LPFB is associated with RBBB, ≥40% of the QRS loop is in the right inferior quadrant 2
Completeness of Block
Intermittent LPFBs are never complete blocks (representing transient or second-degree LPFB), and even in permanent LPFB, complete block cannot be definitively confirmed 2. Rate-independent transient LPFB has been documented, occurring randomly without relationship to heart rate 5.
Clinical Management Implications
The European Society of Cardiology states that bifascicular blocks (including RBBB with LPFB) reflect more extensive involvement of the specialized conduction system and carry increased risk of clinically significant AV block 4. When first-degree AV block accompanies bifascicular block, this suggests trifascicular involvement with higher mortality risk 4.