Left Anterior Fascicular Block (LAFB)

Left Anterior Fascicular Block (LAFB)

In left anterior fascicular block (previously 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.

  • In 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.

ECG Criteria for Left Anterior Fascicular Block (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

Example ECG

Left Anterior Fascicular Block (LAFB)

Left Anterior Fascicular Block (LAFB) qR rS complexes
  • qR complexes in leads I and aVL
  • rS complexes in II, III and aVF

Left Anterior Fascicular Block (LAFB) Left axis LAD

Left Axis Deviation (LAD)

  • Leads I and aVL are POSITIVE;
  • Leads II, III and aVF are NEGATIVE

Prolonged R-wave peak time aVL

Prolonged R-wave peak time

  • 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


LITFL Further Reading


Advanced Reading


ECG LIBRARY 700

ECG LIBRARY

Electrocardiogram

Emergency Medicine Specialist MBChB FRCEM FACEM. Medical Education, Cardiology and Web Based Resources | @jjlarkin78 | LinkedIn |

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