Left Anterior Fascicular Block: Who Cares?

We see this all the time: A routine ECG shows a left anterior fascicular block (also known as LAFB or left anterior hemiblock). Who cares? Does this mean anything, and is it any cause for concern? Let’s take a look.

LAFB occurs when the anterior fascicle of the left bundle branch can no longer conduct action potentials. Review how to diagnose this on an ECG here.

Here is what it looks like:

The LAFB itself does not cause any symptoms. If action potentials do not go through the normal LAFB to the left ventricular myocardium to induce contraction of the heart muscle cells, then the action potentials will eventually get there by traveling through the posterior fascicle and right bundle branch. They will ultimately reach the muscle cells that the left anterior fascicle normally supplies by spreading electrical activity slowly from myocyte to myocyte through “intercolated disks.”

The question is, does LAFB signal a heart problem that we need to worry about? Maybe and maybe not.

Some LAFBs are a result of age-related conduction system disease, aka Lev's disease. Others are, indeed, from structural heart problems. These can include coronary artery disease, prior myocardial infarction, left ventricular hypertrophy from hypertensive heart disease, cardiomyopathies or valvular heart disease.

Having said all that, in the absence of any symptoms suggestive of heart disease, no testing needs to be done just because LAFB is seen on the ECG. Nevertheless, LAFB is not often a "normal variant," so have a low threshold for cardiac testing if potential heart disease symptoms are present.

Therefore, we shouldn't simply ignore the LAFB seen on ECG, but at the same time there is no need to call 911.

by Steven Lome, DO, RVT