Incomplete left bundle-branch block. A definite electrocardiographic entity.

SUMMARY Rate-dependent intermittent incomplete and complete left bundle-branch block (LBBB) is described in a patient without other evidence of heart disease. Continuous electrocardiographic tracings revealed QRS complexes intermediate in configuration between normal conduction and complete LBBB during the transitional phase from normal to abnormal conduction. This provided an opportunity to study the various grades of incomplete LBBB in man. During the incomplete LBBB phase, the morphology of the QRS complexes bore a close resemblance to the QRS form in the Wolff-Parkinson-White syndrome (type B). The differences between these two entities are emphasized. Additional Indexing Words: Critical rate phenomenon Intrinsicoid deflection Septa INCOMPLETE left bundle-branch block (LBBB) has been produced and studied in experimental animals,' but its occurrence in man has been conclusively established only recently. Prior to 1964, when Schamroth and Bradlow2 described a patient with intermittent incomplete LBBB of gradual onset and disappearance, this abnormality received little documentation.24 Schamroth and Bradlow's report provided for the first time convincing evidence of the existence of incomplete LBBB. The purpose of the present communication is to report the same phenomenon in an additional patient who exhibited various grades of incomplete LBBB, thus presenting further evidence of the existence of such an entity in man.