What is Left Anterior Fascicular Block (LAFB) on an Electrocardiogram (EKG)?

Left Anterior Fascicular Block (LAFB) on ECG

Left Anterior Fascicular Block (LAFB) is a conduction abnormality characterized by a QRS vector shift in a posterior and superior direction, resulting in larger R waves in leads I and aVL and smaller R waves but deeper S waves in leads V5 and V6. 1

Diagnostic Criteria for LAFB

According to the American Heart Association/American College of Cardiology/Heart Rhythm Society guidelines, the following criteria should be present to diagnose LAFB:

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

ECG Characteristics of LAFB

• QRS Axis Deviation: Marked left axis deviation between -45° and -90°
• Lead I and aVL: Prominent R waves with qR pattern in aVL
• Leads V5 and V6: Smaller R waves with deeper S waves compared to normal
• QRS Duration: Normal (<120 ms), distinguishing it from bundle branch blocks
• R-peak Time: Prolonged in lead aVL (≥45 ms)

Clinical Significance

LAFB has important clinical implications:

• Association with Cardiac Disease: LAFB is associated with increased risk of heart failure 2
• Mortality Risk: LAFB is an independent risk factor for all-cause death (HR = 1.552) and cardiac death (HR = 2.287) 3
• Coronary Artery Disease: Patients with LAFB have higher prevalence of pathological coronary artery disease (66.3% vs 54.6%) and myocardial infarction (53.3% vs 37.9%) 3
• Cardiac Structure: LAFB is associated with heavier hearts and thicker left ventricular walls 3

Impact on Other ECG Diagnoses

LAFB can affect the interpretation of other cardiac conditions:

• Left Ventricular Hypertrophy (LVH): R-wave amplitude in leads I and aVL are not reliable criteria for LVH in the presence of LAFB. Criteria that include the depth of the S wave in left precordial leads improve detection of LVH when LAFB is present 1
• Myocardial Infarction: LAFB may mask or mimic myocardial infarction patterns 4
  • Small Q waves in V2 may simulate anteroseptal myocardial infarction
  • Both inferior and anterior myocardial infarctions may be masked by R waves replacing Q waves
• Right Bundle Branch Block: In the presence of a recent anterior infarction, RBBB may be masked by LAFB 4

Common Pitfalls in LAFB Diagnosis

• Relying solely on axis deviation: Left axis deviation alone should not be considered synonymous with LAFB. Recognition of delayed inscription of the intrinsicoid deflection in aVL is a useful supplemental criterion 5
• Misdiagnosis of CAD: Among patients with LAFB, 58.1% of patients with CAD and 30.2% of patients with MI were clinically misdiagnosed 3
• Confusing with other conduction disorders: LAFB must be distinguished from other causes of left axis deviation and from other intraventricular conduction disorders

Differential Diagnosis

• Left ventricular hypertrophy
• Inferior myocardial infarction
• Left bundle branch block (which has longer QRS duration)
• Normal variant in some individuals (especially children with congenital heart disease)

Remember that LAFB is not merely an ECG finding but has significant prognostic implications that warrant careful clinical evaluation and follow-up.