Electrical, endocardial mapping of 5 patients with typical ECG of left middle fascicular block

Five patients that met the electrocardiographic criteria for left middle fascicular block underwent a cardiac electrophysiologic study and electrical endocardial mapping of both right and left ventricles. All patients also fulfilled the electrocardiographic criteria for left anterior fascicular block but no one showed right bundle branch block. The electrical endocardial mapping used the technical procedures described by Josephson et al. elsewhere. Comparisons between the points of endocardial activation to the right and left ventricle were made. The differences between these points are shown in Fig. 1 below. Notice that there are great differences of the activation time of the free walls of both ventricles. This is the result of a conduction delay in the regions 9, 10, 11, and 12 of the electrical endocardial mapping confirming the presence of that left middle fascicular block.