Pseudo A-V Block Secondary to Premature Nonpropagated His Bundle Depolarizations

His and right bundle electrograms were recorded in a patient with unexplained P-R prolongations and periods suggestive of both type I and II second degree A-V block. The mechanism of these conduction disturbances was shown to be due to multiple nonpropagated premature His bundle depolarizations (H') which produced their effects on the subsequent cycle by virtue of concealed conduction into the A-V junction. The effects on A-V conduction were related to the time of occurrence of H' in relation to the subsequent P wave: If H'-P was short, a more profound conduction defect occurred. H' was generally undetectable on the ECG because of both antegrade and retrograde block, but a few propagated H' produced typical junctional premature contractions. Propagation of H' depended on a short preceding cycle length and a late occurrence of H'. The site of origin of H' appeared to be in the bundle of His. The mechanism of production of H' did not appear to be either reentry or parasystole. The term pseudo A-V block is used to describe the arrhythmia because there was no evidence of an intrinsic abnormality of A-V conduction. It is expected that other unusual electrocardiographic phenomena may be explainable with intracardiac recordings of specialized conduction tissue potentials.