Type I, Type II, and Type III Gaps in Bundle-Branch Conduction

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Abstract
"Gaps" in bundle-branch conduction were observed in three patients using the combined technic of premature atrial stimulation and His bundle recordings. In type I gap a complete LBBB pattern disappeared at shorter coupling intervals because the premature atrial impulses encountered enough delay at the A-V node to reach the left bundle branch after the end of its effective refractory period. When this occurred, the H1-H2 intervals were longer and the H2-V2 intervals shorter than that at which complete LBBB had been present. In type II gap a complete RBBB pattern disappeared at shorter coupling intervals because the premature atrial impulses were so delayed within the proximal His-Purkinje system that they reached the right bundle branch after the end of its effective refractory period. When this occurred the H1-H2 intervals were shorter and the H2-V2 longer than that at which complete RBBB had been present. In the patient with type III gap and complete LBBB, conduction to the ventricles through the right branch failed at long coupling intervals but was resumed at shorter coupling intervals while the H1-H2 intervals were shorter, and the H2-V2 intervals similar, to those at which block had occurred. True supernormal conduction was excluded in the first two cases. Nevertheless, this phenomenon, as well as a longitudinal dissociation, varying pulsatile vagal discharges acting on an area of depressed conductivity, and phase 4 diastolic depolarization in the right branch coexisting with complete block in the left branch, could have been responsible for type III gap.