["Left bundle branch block" pattern induced by premature right atrial stimulation. Electrogenetic and clinical considerations (author's transl)].

Although in the last years very important knowledges have been obtained in the field of the intraventricular conduction disturbances, many electrocardiographic-vectorcardiographic findings concerning the left bundle branch block (LBBB) and their electrogenesis are still controversial. With the purpose to clarify this problem we have reviewed and analysed 23 cases in whom BCG-VCG patterns of "complete" or "incomplete" LBBB were induced by the premature right atrial stimulation (PRAS) during an electrophysiologic study in man. The analysis of these cases have demonstrated that: 1) the same LBBB pattern can be caused by a slowed conduction or block at different sites of left intraventricular conduction system i.e. not only in the main stem of the left bundle branch (LBB) (tronkular LBBB) but also within the bundle of His (intra-His LBBB) or in all the three fascicles of the LBB distally to its subdivision (divisional LBBB); 2) ECG-VCG are not able to distinguish the anatomical or functional site of slowed conduction or block; 3) the right or left axis deviation in the LBBB is not due to LBBB per se but it represents or a block at two different sites, i.e. a tronkular or intra His LBBB plus a block in the anterior or posterior subdivision of the LBB, or a block at only one side i.e. a tronkular, intra-His or divisional LBBB but with prevalent involvement of one fascicle of the LBB; 4) the ECG-VCG "incomplete" LBBB pattern is similar to that of the left ventricular enlargement; 5) figures of "atypical LBBB" are not specific of myocardial necrosis; 6) the criteria for the diagnosis of "complete" LBBB are not reliable. We conclude that, since LBBB pattern does not always correspond to a slowed conduction or block in the main stem of the LBB, the current terminology of LBBB is inappropriate and could be changed with another which considers the site of delayed activation and not the site of slowed conduction. Therefore we propose the following terminology: 1) generalized left ventricular activation delay instead of LBBB without axis deviation; 2) generalized left ventricular activation delay superiorly predominant instead of LBBB with left axis deviation; 3) generalized left ventricular activation delay inferiorly predominant instead of LBBB with right axis deviation.