A left anterior fascicular block, also known as left anterior hemiblock, occurs when the anterior fascicle of the left bundle branch is no longer able to conduct action potentials.
The criteria to diagnose a LAFB, or LAHB, on ECG include the following:
Left axis deviation of at least -45 degrees
The presence of a qR complex in lead I and a rS complex in lead III
Usually a rS complex in lead II and III (sometimes aVF as well)
A very quick way to diagnose a left anterior fascicular block on ECG is to first identify the left axis deviation; see image below. If the QRS complex is up in lead I and down in lead aVF while also down in lead II, then left axis deviation is present. Then look at lead III. If there is an rS complex, then you have a LAFB on ECG. This method should take just a few seconds.
Note: An old inferior wall myocardial infarction is not able to be diagnosed in the setting of a left anterior fascicular block due to the inferior Q waves present from the LAFB.
A left anterior fascicular block can also occur in the setting of a bifascicular or trifascicular block. Below is a bifascicular block with a LAFB on ECG. Note there is also a right bundle branch block.
1. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric, 6e
2. Surawicz B, et al. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram. Circulation. 2009; doi:10.1161/CIRCULATIONAHA.108.191095.