Left Anterior Fascicular Block - ECG

Left anterior fascicular block (LAFB), also known as left anterior hemiblock is an abnormal condition of the left ventricle of the heart, related to, but distinguished from, left bundle branch block (LBBB). It is caused by only the anterior half of the left bundle branch being defective. It is manifested on the ECG by left axis deviation.

ECG Findings
• QRS complex widening, usually 90 to 120 ms
• Left axis deviation beyond minus 45 degrees with no other cause (such as inferior myocardial infarction)
• Small R wave and large S wave in the inferior leads
• Slurred S wave in V5 and V6

Points To Remember:

1. The signal exiting the AV node is carried rapidly to the inferior aspect of the LV and all of the RV through the intact left posterior fascicle and right bundle, where quick depolarization occurs. However, conduction to the high lateral and upper portions of the left ventricle is slower and must proceed cell-to-cell due to the blocked left anterior fascicle. Therefore, the latter portion of the QRS

depolarizes toward the upper lateral myocardium, manifested as strong left axis deviation.

2. Left anterior fascicular block is the most common intraventricular conduction disturbance associated with acute anterior myocardial infarction, with the left anterior descending artery usually involved.

Small R waves, large S waves in all inferior leads (arrows), with QRS axis deviated left beyond minus 45 degrees