Left bundle branch block
In a left bundle branch block, activation of the right ventricle is normal whereas that of the left ventricle is delayed since its slowly depolarized by an activation wavefront originating from the right bundle branch. The fundamental abnormality is the reversal of the direction of septal activation. Unlike the right bundle branch block where the vectors of the first half of the QRS are normal, the left bundle branch block is characterized by a disturbance of all ventricular depolarization vectors. The activation of the right ventricle, typically masked by activation of the left ventricle, becomes abnormally visible since the depolarization of the left ventricle is delayed. In leads facing the left ventricle, only the positive potentials are recorded whereas in leads facing the right ventricle, a majority of negative potentials are recorded: the pattern in V1 is of the QS or rS type and that of V6 of the exclusive wide R type.
In a left bundle branch block, the first portion of the septum to be activated is the right side near the base of the anterior papillary muscle of the right ventricle; the activation wave then propagates through the middle and inferior right septal mass from right to left toward the apex of the heart; the activation of the septum being reversed, the septal activation vector has a direction opposite to the usual direction; the first QRS vectors correspond to the activation of the right portion of the septum and part of the right ventricle: the vectors move frontward, to the left and a little downward and yield a small initial positive deflection both in V1 and in V6 thereby eliminating the q wave in V6.
The ensuing vectors correspond to the inverted portion of the left part of the septum and thereafter to the delayed and prolonged activation of the left ventricle; this delay (crossing of the "septal barrier" + slow conduction in the left ventricle) conditions the prolongation of the QRS duration; the vectors are directed to the left and backward and are reflected by prominent, wide and notched positive deflections in V6 and prominent negative deflections in V1 and V2.