EKG Criteria for Left Anterior Fascicular Block
The diagnosis of left anterior fascicular block (LAFB) requires four specific criteria: frontal plane axis between -45° and -90°, qR pattern in lead aVL, R-peak time in lead aVL of 45 ms or more, and QRS duration less than 120 ms. 1
Diagnostic Criteria
The AHA/ACCF/HRS guidelines establish the following mandatory criteria for LAFB diagnosis:
- Left axis deviation between -45° and -90° (marked left axis deviation) 1
- qR pattern in lead aVL (small initial q wave followed by dominant R wave) 1
- R-peak time (intrinsicoid deflection) in lead aVL ≥45 ms 1
- QRS duration <120 ms (normal or only slightly prolonged) 1, 2
Key Morphologic Features
LAFB causes a characteristic shift of the QRS vector in a posterior and superior direction, resulting in specific patterns across the 12-lead ECG 2:
- Larger R waves in leads I and aVL 2
- Smaller R waves but deeper S waves in leads V5 and V6 2
- Small q waves in leads I and aVL with tall R waves 1
- Small r waves in leads II, III, and aVF with deep S waves 1
Critical Diagnostic Pitfalls
Left axis deviation alone should NOT be considered synonymous with LAFB 3. The correlation between degree of left axis deviation and delayed intrinsicoid deflection in aVL is imperfect—only 41% of cases with axis at -30° show delayed aVL activation, increasing to 69% at -45° and 82% at -60° 3.
These criteria do not apply to patients with congenital heart disease in whom left axis deviation is present in infancy 1.
Differential Considerations
Left axis deviation may result from alternative causes that must be excluded:
- Left ventricular hypertrophy itself (not LAFB) 1, 2
- Age-related leftward axis shift in elderly patients 1, 2
- Congenital heart defects such as atrioventricular canal defects 2
Impact on Other ECG Interpretations
R-wave amplitude in leads I and aVL are NOT reliable criteria for left ventricular hypertrophy when LAFB is present 1, 2. In this setting, criteria incorporating S-wave depth in left precordial leads (V5, V6) improve LVH detection 1, 2.
Clinical Significance
- Isolated LAFB is generally benign 2
- When associated with other conduction abnormalities, there is increased risk of developing clinically significant AV block 2
- In acute myocardial infarction, LAFB may indicate more severe coronary disease and warrants vigilant monitoring for progression to higher-grade AV block 2