What does a left anterior fascicle block (LAFB) on an electrocardiogram (EKG) mean?

Left Anterior Fascicular Block (LAFB) on ECG

Left anterior fascicular block (LAFB) on an electrocardiogram represents a conduction delay or block in the left anterior fascicle of the left bundle branch, causing the QRS vector to shift in a posterior and superior direction.

Characteristic ECG Findings of LAFB

• LAFB causes a shift of the QRS vector 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
• Left axis deviation (typically -30° to -90°) is a primary feature of LAFB, though left axis deviation alone is not sufficient for diagnosis 2
• Delayed intrinsicoid deflection in lead aVL (asynchronous to V6) is an important supplemental criterion for diagnosis 2
• QRS duration is usually normal or only slightly prolonged (less than 120 ms) 1

Diagnostic Criteria

• The correlation between left axis deviation and delayed intrinsicoid deflection in aVL increases with more extreme axis deviation 2:
  • At -30°: 41% show delayed intrinsicoid deflection
  • At -45°: 69% show delayed intrinsicoid deflection
  • At -75° or greater: 100% show delayed intrinsicoid deflection

Clinical Implications

• LAFB may be an isolated, benign finding or may indicate underlying cardiovascular disease 1
• LAFB can occur in association with various cardiac conditions including:
  • Ischemic heart disease (especially proximal left anterior descending artery obstruction) 3
  • Cardiomyopathies (including Chagas' disease in Latin America) 3
  • Hypertensive heart disease 1
  • Congenital heart defects (such as atrioventricular canal defects) 1

Impact on Other ECG Diagnoses

• LAFB affects the diagnosis of left ventricular hypertrophy (LVH):

  • R-wave amplitude in leads I and aVL are not reliable criteria for LVH in the presence of LAFB 1
  • Criteria that include the depth of the S wave in left precordial leads improve detection of LVH when LAFB is present 1

• LAFB can mask or mimic other conditions:

  • May decrease voltage in chest leads and increase voltage in limb leads 4
  • Can produce small Q waves in V2 that may simulate anteroseptal myocardial infarction 4
  • May mask inferior or anterior myocardial infarctions by replacing Q waves with R waves 4
  • Can mask right bundle branch block in the setting of acute anterior myocardial infarction 4

Differential Diagnosis

• Left axis deviation may result from:

  • LAFB
  • Left ventricular hypertrophy itself
  • Age-related changes (tendency toward leftward axis with increasing age) 1

• LAFB-like ECG patterns may also be caused by:

  • Impaired conduction in the ventricular myocardium (with differences primarily in QRS duration and wave amplitude) 5

Clinical Evaluation

• When complete bundle branch block or hemiblock is found in an athlete, a thorough cardiological work-up is recommended, including:

  • Exercise testing
  • 24-hour ECG monitoring
  • Cardiac imaging to evaluate for underlying pathology 1

• ECG screening of siblings is recommended when bifascicular block patterns are identified in young athletes 1

Prognostic Significance

• Isolated LAFB is generally considered to have a benign prognosis 1
• When associated with other conduction abnormalities (forming bifascicular or trifascicular block), there is an increased risk of developing clinically significant AV block 1
• LAFB has been associated with an increased risk of heart failure in some studies 5