Incomplete left bundle-branch block. A definite electrocardiographic entity

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.