[Gap phenomenon in the bundle of his conduction].

His bundle electrograms recorded during atrial stimulation with the extrastimulus technique permitted the analysis of the so called gaps in bundle branch conduction. Three different types of gaps were identified each with a distinct electrophysiological mechanism. In type I gap a CLBBB pattern disappeared at shorter coupling (A1-A2) intervals because the more premature impulses encountered a greater degree of the delay at the A-V node therefore reaching the left bundle branch after its effective refractory period had ended. During this part of the cycle the H1-H2 were longer and H2-V2 intervals shorter than when CRBBB was present. In type II gap a CRBBB pattern disappeared at shorter coupling intervals because the premature impulses met a greater delay in the proximal portions of the His-Purkinje system. In consequence, they arrived at the right bundle branch when its effective refractory period had expired. When this occurred the H1-H2 interval were shorter and the H2-V2 longer than those at which CRBBB had been present. The patient with type III gap had CLBBB. A-V conduction was possible through the right bundle branch very late in the cycle. However, it could not occur at shorter coupling intervals finally reappearing at even shorter coupling intervals. True supernormal conduction or vagal pulsatile discharges could have been present. Yet, we favor, as the most likely possibility, the existence of 2:1 phase 4 block with latent or abortive escapes not recorded in the surface leads.