Gap phenomenon in "the right and left bundle branch systems" during retrograde conduction in man.

Gap phenomenon in right and left bundle branch systems during retrograde conduction is described in two patients with manifest reentry within the His-Purkinje System (V3 phenomenon). In this form of gap the premature impulse (S2) initially blocked in the right bundle branch system and conducted retrogradely via the left bundle branch system as manifested by sudden prolongation of S2H2 interval and appearance of V3. At close coupling intervals S2 impulse encountered retrograde block in the left bundle branch system and resumed retrograde conduction via the right bundle branch system with S2H2 intervals shorter than critical value and was not followed by V3. However, on further shortening the S1S2 intervals S2 impulse blocked again in right bundle branch system and resumed conduction via the left bundle branch system with S2H2 intervals longer than critical values and V3 reappeared. The mechanism of these gaps is not clear but we believe is similar to the one proposed in Types I and II gaps in antegrade bundle branch conduction and involves proximal delay allowing distal recovery. The similarities and differences between the gap phenomenon in bundle branches during antegrade and retrograde conduction are discussed.