In search of left septal fascicular block

Background In most humans, the left bundle branch divides into 3 fascicles. Electrocardiographic changes resulting from conduction abnormalities of the left anterior and left posterior fascicles are well described and commonly diagnosed. Existence of conduction defects of the left septal fascicle is controversial, without generally accepted criteria for diagnosis. Methods Approximately 26,000 electrocardiograms were reviewed to find examples consistent with left septal fascicle block according to criteria derived from review of the literature. Results Electrocardiograms meeting proposed criteria were found in about 0.5% of tracings. Illustrative examples are shown with discussion of reasons for their selection. Conclusions Left septal fascicle block seems to exist, is polymorphic, and may explain some previously inadequately understood electrocardiographic abnormalities.