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Figure 5 Examples of right and left ventricular activation in left bundle branch block. In panel A, septal breakthrough of the activation front coming from the right ventricle occurs in the superior part of the intraventricular septum. This results in desynchronized contraction of both ventricles with the right ventricle contracting in an apicobasal direction and the left ventricle in a basoapical direction. In panel B, septal breakthrough takes place in the inferior portion of the intraventricular septum. Right and left ventricular contraction is desynchronized but occurs in both in apicobasal direction.