Left bundle branch block and the evolving role of QRS morphology in selection of patients for cardiac resynchronization

The clinical significance of left bundle branch block (LBBB) has recently expanded with the discovery of a strong association with better outcomes in patients receiving cardiac resynchronization therapy. Several milestones have contributed to the current understanding on the role of LBBB in clinical practice. Sunao Tawara described the arrangement of components of what he called the cardiac conduction system from the atrioventricular node to the terminal Purkinje fibers that connect to the working myocardium, and his hypotheses on how it functions remain current. Mauricio Rosenbaum and colleagues developed the bifascicular model of the left-sided conduction system that explains the characteristic electrocardiographic changes associated with propagation disturbances in its components. Andres Ricardo Perez-Riera and others have disputed the bifascicular model as oversimplified and have emphasized the role of the left septal fascicle. Marcelo Elizari and colleagues have explained the importance of masquerading bundle branch block. Elena Sgarbossa and colleagues developed a scheme to recognize ST elevation myocardial infarction in patients with left bundle branch block which remains current after more than 20 years. Enrique Cabrera and others identified electrocardiographic signs of remote myocardial infarction. Substantial progress has been made in the understanding of LBBB, yet its role in clinical practice continues to evolve and important gaps remain to which research should be directed.