The Gap Phenomenon

The gap phenomenon, sometimes confused with supernormal conduction, was first described in 1965 by Moe in dogs and later by Ahktar in humans as a condition in which premature impulses fail to conduct but conduction resumes with even earlier premature extrastimuli. The mechanism has been well studied and is related to the inherent electrophysiological properties of conduction tissue responsible for functional and effective refractory periods of the tissues involved. The effective refractory period is the longest premature coupling interval during fixed rate pacing that fails to activate tissue. The functional refractory period is the shortest coupling interval that can result in conduction after delivery of premature extrastimuli during a fixed rate pacing.
The gap phenomenon occurs when the functional refractory period of tissue proximal in the conducting system is shorter than the effective refractory period of distal conducting tissue. The functional refractory period of proximal conducting tissue occurs at longer coupling intervals than the effective refractory period of the same tissue. During delivery of progressively more premature extrastimuli, block first occurs in the distal conduction system, but with progressive prematurity, conduction will resume in the distal tissue due to proximal conduction delay. The conduction delay that occurs in proximal tissue must exceed the refractory period of the distal tissue for conduction to be restored. We show an example of the gap phenomenon in Figures 1-3.

Types of Gap Phenomena

Several types of gap phenomenon during anterograde conduction have been described. Type 1 has a proximal site of block at the AV node and the distal site of block is the His-Purkinje system (HPS). Type 2 has a proximal site of block at the HPS (proximal) and a distal site of block at the HPS (distal). Type 3 has a proximal site of block at the His bundle and distal site of block at the HPS. Type 4 has a proximal site of block in the atrium and a distal site of block at the HPS or the AV node. Type 5 gap phenomenon has a proximal site of block at the AV node (proximal) and a distal site of block at the AV node (distal). In type 6 gap phenomenon, similar to type 2 gap phenomenon, there is conduction at a proximal site and block at a distal site. This differs from type 2 as there is no conduction delay or block noted at the proximal site.

Supernormal Conduction