Left bundle branch block and echocardiography in the era of CRT

Left ventricular (LV) dyssynchrony is a key pathophysiology in the era of cardiac resynchronization therapy (CRT). Left bundle branch block (LBBB) is the main substrate for CRT, and understanding the electrical pathophysiology is important in assessing the effects of CRT. Three-dimensional voltage mapping systems clearly demonstrate the typical propagation pattern characterized as propagation from the mid or apical septum to the lateral or posterior wall through the apex, which appears as a U shape. The electrical characteristics in LBBB closely associate with mechanical dyssynchrony, which is visualized as a septal flash motion. This rapid motion can be detected well by M-mode, tissue Doppler, and speckle tracking imaging. However, intraventricular discoordination between the septum and free wall is also a key to the response to CRT. We classified M-mode septum images into 10 patterns and septal strain pattern into two patterns. Through detailed analysis, we found that septal contraction contributes to intraventricular coordination. Therefore, in addition to septal flash, subsequent analysis of wall motion patterns also provides additional information about myocardial contractibility and the severity of electrical dyssynchrony. Recently, 3-dimensional speckle tracking imaging was introduced and used as a novel method to image electromechanical coupling. Because activation imaging by 3-dimensional speckle tracking can visualize similar U-shaped propagation images to those by 3-dimensional voltage mapping systems, it is hoped that this method will contribute to further research. Until now, it has not been fully understood how electrical dyssynchrony is expressed as mechanical abnormalities; therefore, continuous study will be required in the future.