This ECG provides an example of LEFT ANTERIOR FASCICULAR BLOCK (LAFB). It is from a 71-year-old woman for whom we have no other history. She also has first-degree AV block and right bundle branch block. RBBB and LAFB together are called bifascicular block. It is not uncommon to see this type of bifascicular block, as the right bundle branch and the anterior fascicle of the left bundle share a blood supply.
The conduction system below the AV node consists of the Bundle of His, the left bundle branch, and the right bundle branch. While there is some variation among individuals, most of us have two main fascicles, or branches, of the left bundle. The ANTERIOR-SUPERIOR fascicle carries the electrical impulse to the anterior wall of the left ventricle, and the POSTERIOR - INFERIOR fascicle carries the impulse to the inferior area of the left ventricle.
Blocks can occur at any level in the conduction system, including left bundle branch block, right bundle branch block, left anterior fascicular block, left posterior block, and bi-fascicular blocks. LAFB can have many causes, including myocardial infarction, cardiomyopathies, fibrosis of the cartilagenous ring, and aortic valve disease. Left anterior fascicular block is much more common than left posterior fascicular block. Both are also called hemiblocks.
When LAFB is present, the initial septal depolarization forces are still left to right, providing a small initial q wave in Lead I and a small r wave in Lead III. After septal depolarization is complete, the activation vector moves inferiorly and to the right as the electrical wavefront moves through the left posterior hemifascicle and right bundle branch. The impulse finally makes its way to the left and superiorly via slow conduction through myocardium normally depolarized by the left anterior hemifascicle, which is blocked. It is because the terminal left ventricular activation moves upward and toward the left that the inferior leads have negative deflections.