What are the electrocardiographic (ECG) criteria for diagnosing left anterior fascicular block (LAFB)?

Left Anterior Fascicular Block ECG Criteria

The diagnostic criteria for left anterior fascicular block (LAFB) include: 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

Essential Diagnostic Criteria

According to the AHA/ACCF/HRS recommendations for standardization and interpretation of the electrocardiogram, the following four criteria must be met for the diagnosis of LAFB:

1. Frontal plane axis between -45° and -90°
2. qR pattern in lead aVL
3. R-peak time in lead aVL of 45 ms or more
4. QRS duration less than 120 ms 1

Clinical Significance and Interpretation

Left anterior fascicular block represents a conduction delay or block in the anterior fascicle of the left bundle branch. When evaluating for LAFB, it's important to consider:

• The degree of left axis deviation correlates with the likelihood of LAFB. Research shows that as the axis becomes more negative, the probability of finding delayed intrinsicoid deflection in aVL increases significantly 2:

  • -30° axis: 41% have delayed intrinsicoid deflection
  • -45° axis: 69% have delayed intrinsicoid deflection
  • -60° axis: 82% have delayed intrinsicoid deflection
  • -75° or greater: 100% have delayed intrinsicoid deflection

• Important caveat: Left axis deviation alone is not sufficient for diagnosing LAFB. The delayed R-peak time in aVL is a critical supplemental criterion 2.

Differential Diagnosis

When evaluating left axis deviation on ECG, consider these alternative diagnoses:

• Left ventricular hypertrophy
• Inferior myocardial infarction
• Ventricular pre-excitation (WPW)
• Hyperkalemia
• Emphysema
• Normal variant (especially in obese patients)

Additional Diagnostic Pearls

• In cases where the diagnosis remains uncertain, an additional criterion that may help is the duration of qR in aVL being at least 40 ms longer than in V6 3.

• LAFB must be distinguished from non-specific intraventricular conduction delay, which can sometimes produce similar ECG patterns but typically has a longer QRS duration 4.

• These criteria do not apply to patients with congenital heart disease in whom left-axis deviation is present in infancy 1.

• When LAFB occurs with other conduction abnormalities (such as RBBB), the diagnosis becomes more complex and requires careful evaluation of all ECG criteria.

• LAFB is one of the most common causes of left axis deviation and should prompt evaluation for underlying structural heart disease 5.