Fascicular Blocks

Left Anterior Fascicular Block

1. Left Axis Deviation: Frontal plane axis between −45° and −90°.
2. qR pattern in lead aVL. i.e. Small Q waves with tall R waves in aVL
3. R-peak time in lead aVL of 45 ms or more. i.e. Prolonged R wave peak time in aVL
4. QRS duration less than 120 ms. QRS duration normal or slightly prolonged (80-110 ms).

These criteria do not apply to patients with congenital heart disease in whom left-axis deviation is present in infancy.

Left Posterior Fascicular Block

1. Frontal plane axis between 90° and 180° in adults. Owing to the more rightward axis in children up to 16 years of age, this criterion should only be applied to them when a distinct rightward change in the axis is documented.
2. rS pattern in leads I and aVL.
3. qR pattern in leads III and aVF.
4. QRS duration less than 120 ms.