Alila Academy

The left bundle branch that delivers electrical signals to the left ventricle has 2 main fascicles: left anterior and left posterior, which conduct impulses to the anterior and posterior walls, respectively. There is also a small septal fascicle that activates the left septal surface.

In normal conduction, the 3 fascicles start to depolarize simultaneously. 

In the first 10ms, the anterior and posterior vectors move in nearly opposite directions, cancelling each other, so the initial overall electrical direction is mainly determined by the septal fascicle, which conducts from the left septal surface to the right. 

The major vector that follows is the combination of activation waves from the anterior and posterior fascicles. The net movement is downward and slightly left. Because this vector is responsible for depolarization of the main mass of the larger left ventricle, it represents the cardiac axis, defined as the net direction of electrical activity during depolarization.

Left anterior fascicular block (LAFB) and left posterior fascicular block (LPFB) refer to an obstruction in the left anterior or left posterior fascicle, respectively. They are also known as hemiblocks. 

The hallmark of LAFB and LPFB is a deviation in the cardiac axis. 

Subscribe to one of the courses below to continue!