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 10 ms,
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.
The hallmark of LAFB and LPFB is a deviation in the cardiac axis.
In LAFB, the anterior branch is blocked, and the posterior fascicle must activate the entire
left ventricle. At first, the impulses follow the posterior fascicle downward, toward the inferior
leads, resulting in a small positive deflection, small r wave, in these leads. The signals then
move up and leftward, through the myocardium, to the rest of the ventricle, producing a large
negative deflection, large S wave, in inferior leads. The reverse is observed in lateral leads,
which show small q, big R patterns. The cardiac axis is skewed left compared to normal conduction.
Also, as one fascicle is doing the job of two, it takes slightly more time than usual,
resulting in a small widening of the QRS complex, not more than 0.12 secs.
LAFB may occur as an isolated finding, or in association with cardiovascular diseases. Isolated
LAFB was thought to be benign but has recently been linked to higher risks for heart diseases.
Left posterior fascicular block, LPFB, is much less common than LAFB, probably due to the fact
that it is bigger in diameter, has better blood supply, and runs through a more protected area.
In LPFB, the initial vector follows the intact anterior fascicle, which goes upward and leftward,
producing a small positive deflection, small r, in lateral leads; and a small negative deflection,
small q, in inferior leads. As impulses spread to the rest of the ventricle,
the major vector directs downward and to the right, producing a big R in inferior leads,
big S in lateral leads; and a right-ward shift in the cardiac axis. LPFB is almost always associated
with coronary artery disease, especially myocardial infarction of the inferior wall.
Bifascicular block is blockage of any 2 of the 3 fascicles: left anterior, left posterior and
right bundle branch. The term is usually used to describe right bundle branch block plus LAFB,
or LPFB. The ECG patterns reflect both conditions. The term “trifascicular block” means blockage of
all 3 fascicles, which would equal to a complete heart block. In practice, however, it is used to
refer to a variety of situations, including: a bifascicular block plus a first-degree
or second-degree AV block; a right bundle branch block plus alternating LAFB and LPFB;
alternating right and left bundle branch blocks; and bifascicular block plus third-degree AV block.