- 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.
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
- 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.
- 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.