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Fourteen patients with conduction defects were analyzed by using bundle of His (BH) recordings. The BH electrograms were validated by BH and right atrial pacing (AP). In 12 patients with Mobitz type II A-V block, failure in impulse transmission for the dropped beats was localized distal to the recording site of the BH. Three of these 12 patients showed normal QRS complexes. In two of these three, the QRS complexes remained unchanged during intermittent periods of complete heart block (CHB), and thus represent His bundle rhythm with subsidiary pacemaker arising above the bifurcation of the BH. The A-H time in this group of 12 patients ranged from 60 to 160 msec and the H-V time ranged from 40 to 90 msec. At any atrial rate (NSR or AP) conduction time through the A-V node (A-H), and His-Purkinje system (H-V) remained constant. With increasing atrial (A) rates during AP, the number of impulses blocked distal to the BH increased. At high AP rates Wenckebach phenomenon between A and BH occurred concomitantly with block distal to the BH.Of the two additional patients studied, the one with Wenckebach type 2° A-V block during NSR and a narrow QRS complex had the delay localized between two recorded and validated BH potentials (“split” BH). The other patient, with 1:1 A-V conduction during NSR and left bundle-branch block, developed Wenckebach cycles during AP at 110 beats/min. The progressive delay in the P-R interval was localized in the His-Purkinje system (HPS) probably in the right bundle.Our data support the clinical observations that Mobitz II A-V blocks are associated with bilateral bundle-branch block as well as with BH lesions. The ECG is of limited value in the localization of the delays occurring at two sites simultaneously: namely, in the A-V node and the HPS. Furthermore, demonstration of the Wenckebach cycles within the HPS (BH or either bundle branch), which cannot be determined from the surface ECG, has important clinical implications.

Wenckebach and Mobitz Type II A-V Block Due to Block Within the His Bundle and Bundle Branches