Incomplete left bundle-branch block. The view from transseptal intraventricular leads.

SUMMARY The direction of the initial 20 to 30 msec forces of multiple unipolar and transseptal bipolar intraventricular electrocardiograms was examined in five patients with and five without incomplete left bundle-branch block (LBBB) and in one patient with complete LBBB. The data obtained were consistent with preservation of the normal left-to-right and apex-to-base direction of septal depolarization in patients with apparent, incomplete LBBB. Furthermore, no evidence of electromechanical delay in the left ventricle could be ascribed to this conduction defect. Until the minute details of normal and abnormal human septal excitation become known, it is suggested that the more descriptive and less mechanistic term "initial force abnormality" be substituted for "incomplete LBBB." Additional Indexing Words: Intracavitary electrocardiograms Intraventricular conduction defect I NCOMPLETE left bundle-branch block (LBBB) continues to be a controversial electrocardiographic interpretation despite the accumulation of a considerable number of clinical and experimental observations concerning its existence.