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Figure 6 A cross-section of the right ventricle (RV) and left ventricle (LV) in the horizontal plane. As shown during left bundle branch block, the site of septal breakthrough may affect desynchronized contraction of the two papillary muscles (PMs) resulting in mitral incompetence. As shown in panel A, desynchronization of the PMs is most marked in posteroseptal breakthrough. More synchronized activation of the PMs occurs in case of anteroseptal breakthrough (panel B).