What are the ECG (electrocardiogram) findings in left anterior fascicular block?

ECG Findings in Left Anterior Fascicular Block

Left anterior fascicular block (LAFB) is characterized by four key ECG criteria: frontal plane axis between -45° and -90°, qR pattern in lead aVL, R-peak time in lead aVL of ≥45 ms, and QRS duration less than 120 ms. 1

Diagnostic Criteria for LAFB

The American Heart Association/American College of Cardiology/Heart Rhythm Society guidelines define LAFB by the following specific criteria:

1. Frontal plane axis between -45° and -90° (marked left axis deviation)
2. qR pattern in lead aVL (small r, tall R)
3. R-peak time in lead aVL of ≥45 ms or more
4. QRS duration less than 120 ms (to distinguish from bundle branch blocks)
5. rS pattern (small r, deep S) in leads II, III, and aVF 1

Pathophysiological Basis

LAFB occurs when there is a block in the left anterior fascicle of the left bundle branch. This results in:

• Delayed activation of the anterosuperior region of the left ventricle
• Initial forces directed inferiorly and to the right (producing the small q wave in aVL)
• Terminal forces directed superiorly and to the left (producing the prominent R wave in aVL)
• Deep S waves in the inferior leads (II, III, aVF) 1

Important Distinctions and Pitfalls

• Not all left axis deviation indicates LAFB: Left axis deviation alone is insufficient for diagnosis. The presence of a qR pattern in aVL with appropriate R-peak time is essential 2
• QRS duration must be <120 ms: This distinguishes LAFB from complete left bundle branch block
• Congenital heart disease: These criteria do not apply to patients with congenital heart disease in whom left-axis deviation is present in infancy 1
• Differential diagnosis: Left ventricular hypertrophy, extensive lateral myocardial infarction, and vertical heart position can mimic some features of LAFB 3

Clinical Significance

• LAFB may be an isolated finding or may occur in combination with other conduction abnormalities
• When LAFB is combined with right bundle branch block (RBBB), it suggests more extensive conduction system disease
• LAFB has been associated with an increased risk of progression to complete heart block, particularly when combined with other conduction abnormalities 1

Vectorcardiographic Considerations

In LAFB, the initial QRS forces are directed inferiorly and to the right, while the terminal forces are directed superiorly and to the left. This creates the characteristic pattern of small q and tall R in aVL, along with the rS pattern in leads II, III, and aVF 4.

Left anterior fascicular block should be distinguished from left posterior fascicular block, which presents with right axis deviation (90° to 180°), rS pattern in leads I and aVL, and qR pattern in leads III and aVF 1, 3.