Clinical, Structural and Mechanical Features in Patients With Left Bundle Branch Block: an Observational and Prospective Study.

Left bundle-branch block (LBBB) is an electrocardiographic marker often associated with underlying cardiac disease (such as coronary artery disease, dilated cardiomyopathy, etc) but in some cases it is also found in patients with isolated LBBB defined as "none apparent structural heart disease".

Several investigations in animal and human study have showed a different electroanatomical activation in isolated LBBB when compared to LBBB in heart failure patients mostly due to coronary artery disease. The sequence of endocardial activation through the left ventricle has also proven to be heterogeneous in heart failure occuring at different septal regions. This electrical and mechanical variability seems to be related to location of LBBB block or lesion that could be proximal, distal or diffuse, consequently influencing also electrocardiographic aspects.

These considerations explain and justify the great variability in the proposed "true or typical LBBB" definitions (derived from from Strauss, Perrin and Marican definitions and from Heart Association/American College Cardiology Foundation/ European Society of Cardiology guidelines) but also shift attention to relatively new concept of atypical LBBB definitions with extreme left or right axis deviation and of non specific intraventricular conduction delay defined as a wide QRS (≥110 ms) without the appearance of left and right bundle-branch block. In fact, several investigations have shown that QRS axis deviation is a predictor of unfavorable outcome after cardiac resynchronization therapy and LBBB. The less favorable outcome of cardiac resynchronization therapy candidates with LBBB and left axis deviation seems to be associated to the presence of a shorter activation delay, scar tissue and left ventricular hypertrophy. Moreover, actual LBBB definitions recommended by guidelines in cardiac resynchronization therapy patients differ significantly and only individual ECG criteria (such as electrocardiographic intervals QS or RS in lead V1, notch and no q wave in lead V5, V6, I and avL) seems to be associated to clinical outcome when compared with combining criteria.

Finally, a frequent dyssynchrony of ventricular activation due to LBBB may be responsible for a dyssynchrony-related cardiomyopathy that needs to be identified early and differentiated from other forms of dilated cardiomyopathy through a multiparametric approach such as genetic, clinical data, electrocardiography and cardiac magnetic resonance. In fact, in isolated LBBB undergone cardiac magnetic resonance it was observed a greater left ventricular volumes and lower global left ventricular Ejection Fraction despite the absence of cardiovascular disease as compared with control subject. Despite this, comparison between isolated LBBB and structural heart disease shows a different myocardial mechanism all the more unfavourable the more significant is the presence of underlying myocardial scarring. In the other hand, isolated LBBB, dyssynchrony-related cardiomyopathy and myocardial scarring could present different temporal phase in the clinical history of the same patient and that must be early identified.

The aim of this present large-scale prospective observational study is to evaluate 1) clinical, electrocardiographic and imaging features in a population of patients with typical and atypical LBBB and " non specific intraventricular conduction delay" pattern during a long-term follow up and 2) association among eventual different LBBB patterns (typical, atypical (LBBB and extreme left and/or right axis deviation, fragmentation) and " non specific intraventricular conduction delay " pattern and tissue characterization, mechanical myocardial activation and perfusion. 3) a possible time-related evolution from "non specific intraventricular conduction delay" patterns to typical or atypical LBBB.