The Gap Phenomena During Retrograde Conduction in Man

Ventricular refractory period studies were performed in 12 consecutive and unselected patients using the ventricular extrastimulus method (V 2 ) at basic ventricular cycle lengths (V 1 V 1 ). In six of 12 patients two types of retrograde gaps occurred. At relatively long V 1 V 2 intervals, ventriculo-atrial (V-A) conduction failed and then resumed at shorter V 1 V 2 intervals. The initial sites of retrograde block were the A-V node in two patients and the His-Purkinje system in four patients. In both groups, resumption of V-A conduction at shorter V 1 V 2 intervals occurred because of retrograde delay within the His-Purkinje system. Retrograde gaps differ from previously described antegrade gaps in A-V conduction: The site of initial block in A-V gaps is the His-Purkinje system and resumption of A-V conduction occurs at shorter A 1 A 2 intervals because of proximal delay in the A-V node (type I) or delay in the proximal His-Purkinje system (type II). In V-A gaps the site of initial block may be either the A-V node or the His-Purkinje system and resumption of V-A conduction always occurs due to delay within the distal His-Purkinje system. A common feature of both A-V and V-A gaps is the fact that delay of the more premature impulses allows time for previously refractory areas to recover excitability and both can be functional in nature. Only one of the 12 patients had both A-V and V-A gaps in conduction. Retrograde gaps in V-A conduction are more commonly observed than A-V gaps.