Left anterior fascicular block

A left anterior fascicular block (LAFB) also known as a left anterior hemiblock (LAHB) occurs when the anterior fascicle of the left bundle branch is no longer able to conduct action potentials.

The criteria to diagnose a LAFB is as follows:

  1. Left axis deviation of at least -450
  2. The presence of a qR complex in lead I and a rS complex in lead III.
  3. Usually a rS complex in lead II and aVF as well (not always).

A LAFB can occur simultaneously with a right bundle branch block. This is referred to as a bifascicular block.

Note that the presence of a qR complex in the inferior leads makes concomitant diagnosis of an old inferior myocardial infarction impossible in the setting of a LAFB.

Insert Images: LAFB1.jpg, LAFB2.jpg, LAFB3.jpg, BifascicularBlock1.jpg


Left anterior fascicular block.



Left anterior fascicular block.



Left anterior fascicular block.