23-21: Left Anterior Fascicular Block

ECG Findings

• QRS duration normal or slightly prolonged

• Left axis deviation (without any other reasons for leftward axis on ECG)

• Small R wave and large S wave in the inferior leads (II, III, aVF)

• Small Q with a large R wave in the lateral leads (I, aVL)

Pearls

1. In left anterior fascicular block (LAFB) the conduction through the left anterior fascicle is blocked, which causes the conduction of the high lateral portion of the left ventricle to occur in a delayed fashion, as conduction spreads from the intact left posterior fascicle and the right bundle branch. This causes a typical leftward axis deviation.

2. This finding is more common than left posterior fascicular block and in isolation is generally considered a normal variant and is not considered a bad prognostic finding. It can be seen in anterior MI as well.