Left anterior fascicular block
QRS duration < 0.10 sec
QRS vector shows left axis deviation (LAD) in frontal plane > -30 degrees.
QRS morphology
Limb leads
qR in leads I and aVL.
rS in leads II, III, and aVF
R peaks in aVL before aVR
Precordial leads
Delayed R wave transition
Persistent S wave in V5-V6
Normal QRS morphology when precordial leads recorded in 2-3 interspaces higher
Orthogonal leads
qR in lead X
rS in lead Y
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