After some confusion, the characteristic QRS patterns corresponding to specific blocks in the conduction system have been defined and related to individual anatomical parts of the conduction system, defining the bundle branch and fascicular blocks, respectively. The His-Purkinje conduction system in the ventricles represents well defined and visible structures, and its role in ventricular impulse propagation was early recognized. They refer to abnormalities in the ventricular activation sequence, and, as stated in the scientific statement from the AHA/ACCF/HRS recommendations, “they may be caused by structural abnormalities in the His-Purkinje conduction system or ventricular myocardium that result from necrosis, fibrosis, calcification, infiltrative lesions, or impaired vascular supply”. The ECG diagnostic classification of intraventricular conduction disturbances (IVCD) is currently generally well-defined and accepted and represents a standard diagnostic ECG classification.
Recognizing the underlying mechanisms of the LBBB patterns and the involvement of the ventricular working myocardium is of utmost clinical importance, considering a patient’s prognosis and indication for cardiac resynchronization therapy. This opinion paper is focused on the alterations of the ventricular working myocardium leading to the most frequent IVCD pattern-the left bundle branch block pattern (LBBB). The involvement of the working ventricular myocardium is only marginally mentioned or not considered. However, the current understanding of the IVCD’s underlying mechanism is mostly attributed to HP structural or functional alterations. They are characterized by defined QRS complex patterns that reflect the abnormalities in the intraventricular sequence of activation that can be caused by pathology in the His-Purkinje conduction system (HP) or ventricular myocardium. Intraventricular conduction disturbances (IVCD) are currently generally accepted as ECG diagnostic categories.
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