A left anterior-superior fascicular block (hemiblock) has a number of electrocardiographic features. Because the inferior-posterior fascicle is intact, activation of the myocardium starts in an inferior and posterior direction.
Left axis deviation (at least -45 degrees, usually -60 degrees)
The QRS duration is normal if the fascicular block is pure. It may be increased up to 0.20 seconds if another process is present such as bundle branch block or ventricular hypertrophy.
Lead I: small Q, prominent and tall R
Lead II: small R, prominent and deep S
Lead III: small R, prominent and deep S
Lead aVL: small Q, prominent and tall R, late intrinsicoid deflection
Lead aVF: small R, prominent and deep S
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