A new type of ventriculo-atrial gap phenomenon in man.

A new type of ventriculo-atrial (V-A) gap phenomenon was observed in a patient who underwent a cardiac electrophysiology study and had complete antegrade infranodal A-V block but intact V-A conduction. During ventricular extrastimulus testing, a split retrograde His potential emerged from the ventricular electrogram. As the prematurity of the extrastimulus was increased, the later of the two retrograde His deflections disappeared suddenly at a critical coupling interval and V-A conduction was interrupted. More premature extrastimuli resulted in progressive delay in the His-Purkinje system and delayed appearance of the earlier retrograde His deflection. As a result of this delay, conduction through the His bundle eventually resumed, the second (split) retrograde His reappeared and conduction to the atria resumed. Thus, the initial site of block during the V-A gap phenomenon in this patient was located in the His bundle, with proximal delay occurring in the distal His-Purkinje system. The results indicate that the classic mechanism of gap phenomenon is operative. To our knowledge, this type of V-A gap phenomenon has not been previously described in man.