Retrograde conduction in complete atrioventricular block Study using His bundle recordings.

Using His bundle recordings and ventricular stimulation retrograde conduction was studied in 57 cases of complete AV block. VA conduction was observed in 13 patients. Of the 13, 12 had AV block distal to H, and during cardiac pacing 5 of them showed a retrograde His bundle potential. In all cases the AV node-His pathway was implicated in the transmission of the retrograde impulse. There was no retrograde atrial response in the remaining patients. VA block usually occurred at the same level as AV block. Nevertheless, two cases of AV block distal to H were associated with concealed retrograde conduction which stopped within the AV node.     

Accelerated A-V conduction associated with complete A-V block

HBE's were recorded in a 70-year-old female patient with complete heart block, narrow QRS, and past episodes of supraventricular tachycardia. The HBE revealed split His potentials with intra-His block. Atrial pacing at increased rates failed to increase the AH1 interval, suggesting an accelerated pathway between the atria and the proximal His bundle bypassing the A-V node. Below the site of block, the ventricles were paced by a distal His bundle (H2) with a resultant normal QRS interval.     

Mobitz II AV block within the His bundle, with progression to complete heart block.

A case of a 48-year-old woman with frequent syncopal episodes is reported. The electrocardiogram showed high degree AV block with narrow QRS complexes. The His bundle electrogram displayed a split His deflection indicating impairment of conduction within the His bundle of the Mobitz II type. The AH interval was prolonged and Wenckebach phenomenon occurred at the same atrial pacing rate before and after atropine administration. During spontaneous or induced high grade AV block an escape rhythm originating in the distal His bundle was observed. A secondary study performed one year later showed progression to complete AV block. Both His potentials were present, one following the atrial and the other preceding the ventricular deflection. The H'V interval was prolonged and a further lengthening was seen after ajmaline. All these findings indicated proximal, mid, and distal disease of the His trunk.     

Failure to recognize a His bundle potential in complete atrioventricular block.

This report concerns a patient with complete heart block, in whom electrophysiological studies showed at times an escape rhythm with narrow QRS complexes preceded by His potentials with normal HV intervals (35--40 msec) and at other times an escape rhythm of similar rate, having wide QRS complexes of left bundle branch block configuration with no preceding His bundle activity. Complexes intermediate in width and configuration and preceded by His potentials with an HV interval inversely proportional to QRS width were also recorded. These observations are explained by a site of block proximal to the His bundle and competition between two pacemaker foci having similar discharge rates, one situated in the junctional region below the site of block and the other more distally in the right bundle branch or right ventricle. It is proposed that the combination of a proximal site of block and a distally situated dominant pacemaker may be a common reason for failure to record a His potential in patients with complete heart block.     

His bundle recordings in atypical A-V nodal Wenckebach block during cardiac pacing

The features of the classic atrioventricular (A-V) nodal Wenckebach phenomenon are attributed to a conduction delay in the A-V nodal region. Changes in the P-R intervals merely represent the variations of the H-H intervals. The characteristic abnormalities of the latter consist of: (1) a gradual increase in P-H intervals; (2) a progressive decrease of the P-H increments; (3) a progressive diminution of the H-H intervals; (4) the long H-H interval produced by the nonconducted P wave is equal to the sum of the increments subtracted from twice the P-P intervals; and (5) the H-H interval after the intermission is longer than the H-H interval preceding the pause.