Left Anterior Fascicular Block


  • In left anterior fascicular block (aka left anterior hemiblock), impulses are conducted to the left ventricle via the left posterior fascicle, which inserts into the infero-septal wall of the left ventricle along its endocardial surface.
  • On reaching the left ventricle, the initial electrical vector is therefore directed downwards and rightwards (as excitation spreads outwards from endocardium to epicardium), producing small R waves in the inferior leads (II, III, aVF) and small Q waves in the left-sided leads (I, aVL).
  • The major wave of depolarisation then spreads in an upwards and leftwards direction, producing large positive voltages (tall R waves) in the left-sided leads and large negative voltages (deep S waves) in the inferior leads.
  • This process takes about 20 milliseconds longer than simultaneous conduction via both fascicles, resulting in a slight widening of the QRS.
  • The impulse reaches the left-sided leads later than normal, resulting in a increased R wave peak time (the time from onset of the QRS to the peak of the R wave) in aVL.

Left Anterior Fascicular Block

Diagnostic Criteria for LAFB

  • Left axis deviation (usually between -45 and -90 degrees)
  • Small Q waves with tall R waves (= ‘qR complexes’) in leads I and aVL
  • Small R waves with deep S waves (= ‘rS complexes’) in leads II, III, aVF
  • QRS duration normal or slightly prolonged (80-110 ms)
  • Prolonged R wave peak time in aVL > 45 ms
  • Increased QRS voltage in the limb leads

qR complexes in leads I and aVL, rS complexes in II, III and aVF

late intrinsicoid deflection

Prolonged R-wave peak time (= the time from onset of the QRS to the peak of the R wave) in aVL > 45 ms

Handy Tips

  • In LAFB, the QRS voltage in lead aVL may meet voltage criteria for LVH (R wave height > 11 mm), but there will be no LV strain pattern.

Related Topics

Further Reading

Author Credits


  • Surawicz B, Knilans T. Chou’s Electrocardiography in Clinical Practice (6th edition), Saunders 2008.
  • Wagner, GS. Marriott’s Practical Electrocardiography (11th edition), Lippincott Williams & Wilkins 2007.
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  1. Tom Clark says

    I have been diagnosed as having left anterior fascicular block. Being 72 yrs old, what does that mean? And what does it indicate?

    • Edward Ringis says

      It means you have evidence of impairment in your conduction system, by itself it is not significant and won’t impair your heart function but may indicate you have coronary disease which led to the impairment, or that in the future you are at a higher risk for other regions of conduction impairment including risk to progress to left bundle branch block.