Generation of the QRS complex

Ventricular depolarization can be divided into two major phases, each represented by a vector.
Panel A: The first phase (arrow 1) denotes depolarization of the ventricular septum, beginning on the left side and spreading to the right. This process is represented by a small "septal" r wave in the anterior-posterior lead V1 and a small septal q wave in the left-right lead V6.
Panel B: Simultaneous depolarization of the left and right ventricles (LV and RV) constitutes the second phase. This second vector (arrow 2) is oriented to the left and posteriorly, reflecting the electrical predominance of the LV. The net effect is an S wave in lead V1 and an R wave in lead V6.