Left Posterior Fascicular Block ECG Findings
Left posterior fascicular block (LPFB) is diagnosed on ECG by four key criteria: frontal plane axis between +90° and +180°, rS pattern in leads I and aVL, qR pattern in leads III and aVF, and QRS duration less than 120 ms. 1
Diagnostic Criteria (Adults)
The AHA/ACCF/HRS standardized criteria for LPFB include 1:
- Frontal plane axis: +90° to +180° (right axis deviation/marked right axis deviation)
- rS pattern in leads I and aVL (small r wave, deep S wave in lateral leads)
- qR pattern in leads III and aVF (small q wave followed by tall R wave in inferior leads)
- QRS duration <120 ms (narrow QRS complex - this distinguishes it from bundle branch blocks)
Special Considerations in Children
In pediatric patients up to age 16, apply these criteria only when a distinct rightward axis shift is documented, as children normally have more rightward axes that shift leftward with age 1. Between ages 8-16, the normal QRS axis can extend to +120°, making LPFB diagnosis challenging without prior ECGs showing axis change 1.
Clinical Context and Diagnostic Pitfalls
Critical Exclusions Required
LPFB cannot be definitively diagnosed in the presence of 2:
- Right ventricular hypertrophy (COPD/emphysema patients)
- Extensive lateral myocardial infarction
- Extremely vertical heart position
Distinguishing LPFB from Inferior MI
The key differentiating feature is R wave amplitude in inferior leads 3:
- LPFB: R amplitude increases in leads II, III, aVF (tall R waves)
- Inferior MI: R amplitude decreases in leads II, III, aVF (small or absent R waves)
Both conditions produce Q waves in inferior leads, but the R wave behavior differs fundamentally. LPFB can mask concomitant inferior MI by producing tall R waves that obscure infarction patterns 4.
Associated Findings
When LPFB occurs with right bundle branch block (RBBB), expect 2:
- Enhanced superior displacement of initial QRS forces
- More anterior and rightward shift of main QRS forces
- ≥40% of the vectorcardiographic QRS loop located in the right inferior quadrant
PR interval prolongation is very frequent when LPFB coexists with RBBB and acute inferior MI 2.
Clinical Significance
Isolated LPFB is extremely rare in both general and specific patient populations 2. When present, particularly as left posterior fascicular block in arrhythmogenic left ventricular cardiomyopathy, it serves as a strong independent predictor of major arrhythmic events (OR: 4.7) 5.
Transient/intermittent LPFB patterns suggest the block is incomplete even when permanent 2. Rate-independent intermittent LPFB can occur without structural heart disease 6.
Practical Approach
Always correlate ECG findings with clinical context - the diagnosis requires excluding alternative explanations for right axis deviation. Compare with prior ECGs when available to document axis shift, particularly in younger patients where physiologic right axis is normal.