Ventriculoatrial Conduction in Complete Atrioventricular Block Due to AH Block

In 2 patients with complete heart block, characteristics of retrograde conduction and the localization of orthograde conduction block were studied. In patient 1, retrograde conduction was demonstrated during both A-V junctional rhythm and ventricular pacing. The sinus and retrograde P waves seemed to utilize and compete for the A-V conduction system. The time from QRS complexes to retrograde P waves was longer during ventricular pacing than during A-V junctional rhythm by approximately 65 to 110msec. 1:1 ventriculoatrial conduction could be elicited at the ventricular pacing rate between 70 and 100 beats per minute. The time interval between the pacemaker impulse and the retrograde P wave became progressively longer as the pacing rate increased. The V-A conduction showed Wenckebach phenomenon with 5:4 conduction ratio at the rate of 110/min, and that with 3:2 conduction at 120/min, respectively. The His bundle electrogram demonstrated a complete heart block due to AH block. In patient 2, electrocardiogram during a Stokes-Adams attack showed complete heart block and an escape rhythm with retrograde P waves. The QRS complexes of the escape rhythm were normal in configuration and duration, indicating an A-V junctional origin. At the time of His bundle recording, ECG showed 1:1 atrioventricular conduction. Although PA, AH and HV times were all within normal limits without any intervention, AH time developed a Wenckebach periodicity in response to atrial pacing rate of 90 beats per minute or higher. Therefore, it was presumed that the complete heart block demonstrated during a Stokes-Adams attack had been due to AH block. These studies indicate that a retrograde V-A conduction could occur in the presence of orthograde A-V block due to AH block. Possible mechanisms of retrograde V-A conduction in the presence of complete orthograde AH block were discussed, and unidirectional block at the A-V junction seemed to be the most probable mechanism operating in our patients.