Left anterior fascicular block electrocardiogram

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Cafer Zorkun, M.D., Ph.D. [2]

• Left axis deviation (usually between -45° and -90°), some consider -30° to meet criteria
• QRS interval < 0.12 seconds
• qR complex in the lateral limb leads (I and aVL)
• rS pattern in the inferior leads (II, III, and aVF)
• Delayed intrinsicoid deflection in lead aVL (> 0.045 s)^[1]

It is important not to call LAFB in the setting of a prior inferior wall myocardial infarction which may also demonstrate left axis deviation due to the initial forces (Q wave in a Qr complex) in leads II, III, and aVF. As opposed to LAHB, the left axis shift is due to terminal forces (i.e., the S wave in an rS complex) being directed superiorly.

LAHB may be a cause of poor R wave progression across the precordium causing a pseudoinfarction pattern mimicking an anteroseptal infarction. It also makes the electrocardiographic diagnosis of LVH more complicated, because both may cause a large R wave in lead aVL. Therefore to call LVH on an EKG in the setting of an LAHB you should see the presence of a “strain” pattern when you are relying on limb lead criteria to diagnose LVH.^[2]

Shown below is an example of an EKG demonstrating left axis deviation with rS pattern in lead III and QRS complex < 0.12 seconds indicating a left anterior hemiblock. A qR complex is also seen in lead aVL.

Copyleft image obtained courtesy of ECGpedia, http://en.ecgpedia.org/wiki/Main_Page

Shown below is an example of an EKG showing rS pattern in lead III, qR complex in lead aVL and QRS complex < 0.12 seconds along with left axis deviation indicating left anterior fascicular block.

Copyleft image obtained courtesy of ECGpedia, http://en.ecgpedia.org/wiki/Main_Page

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