Left Anterior Fascicular Block

It helps to remember how the current is conducted.  The posterior fascicle originates inferiorly and posteriorly on the endocardium.  Thus the initial flow of current in LV will be rightward and down, as current moves to the epicardium.  Additionally, depolarization of the right ventricle is progressing normally.  As depolarization progresses, it moves from inferior, posterior to anterior, superior giving you the  patter noted below.  Check out LITFL's page on LAFB.
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• Left axis deviation
• Small q waves with tall R waves in leads I and aVL
• Small r waves with deep S waves in leads II, III, aVF
• QRS duration is normal or slightly prolonged (80-110 ms)
• Prolonged R wave peak time in aVL > 45 ms (measure from start of Q to peak of R)
• Increased QRS voltage in the limb leads

Dr. Burns also provides a beautiful pearl that in LAFB you might see LVH voltage criteria in aVL (R wave height > 11 mm), but there will be no LV strain pattern.

Copa Cardiogram #66