What is a Left Posterior Fascicular Block (LPFB)?
Left posterior fascicular block is an intraventricular conduction disturbance characterized by marked right-axis deviation (between 120° and 180°) on ECG, resulting from impaired conduction through the posterior fascicle of the left bundle branch. 1, 2
Diagnostic Criteria
According to the AHA/ACCF/HRS standardized criteria, LPFB is diagnosed when all four of the following ECG findings are present 2:
- Frontal plane QRS axis between 90° and 180° in adults (or documented rightward axis shift in children up to 16 years, who normally have more rightward axes)
- rS pattern in leads I and aVL (small r wave, deep S wave)
- qR pattern in leads III and aVF (small q wave, tall R wave)
- QRS duration less than 120 ms (normal QRS width)
Important Pediatric Consideration
In children up to 16 years of age, this criterion should only be applied when a distinct rightward change in axis is documented, since children normally have more rightward axes than adults 2.
Clinical Significance and Underlying Pathology
LPFB is an extremely rare finding, occurring in only 0.24% of general ECG populations and 5.5% of patients with inferior myocardial infarction 3, 4. When isolated LPFB is identified, it carries important clinical implications:
- Reliably associated with inferior myocardial infarction (IMI) - LPFB often masks the typical ECG findings of IMI 4
- Strong marker for severe three-vessel coronary artery disease - approximately 80-90% of patients with LPFB have three-vessel CAD 4
- May indicate ventricular septal rupture - mechanical disruption of the posterior fascicle from septal rupture complicating inferior MI can produce isolated LPFB 5
Anatomical Basis for Rarity
The posterior fascicle is less vulnerable to conduction disturbances than the anterior fascicle or right bundle branch because 3, 5:
- It receives dual blood supply
- It has a thicker, more robust anatomical structure
- It requires more extensive myocardial damage to produce conduction block
Diagnostic Pitfalls and Differential Diagnosis
A definitive diagnosis of LPFB cannot be made in the presence of 3:
- Right ventricular hypertrophy (RVH)
- Chronic obstructive pulmonary disease (COPD) or emphysema
- Extensive lateral myocardial infarction
- Extremely vertical heart position
These conditions can produce similar right-axis deviation patterns and must be excluded clinically before diagnosing LPFB.
Transient and Intermittent Forms
LPFB can occur transiently or intermittently 6, 3:
- Rate-dependent forms (phase 3 tachycardia-dependent or phase 4 bradycardia-dependent)
- Rate-independent intermittent LPFB (random occurrence)
- Intermittent LPFBs are never complete blocks - they represent transient or second-degree blocks 3
Associated Conduction Abnormalities
When LPFB occurs with right bundle branch block (RBBB) and acute inferior MI, PR interval prolongation is very frequent 3. In vectorcardiographic analysis, isolated LPFB shows 20% of the QRS loop in the right inferior quadrant, increasing to ≥40% when associated with RBBB 3.
Clinical Action Required
Given the strong association with severe CAD and inferior MI, identification of LPFB mandates invasive investigation to evaluate for significant coronary disease, particularly in the setting of acute or recent cardiac symptoms 4.