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 Supernormal conduction occurs during delivery of premature extrastimuli such that conduction is unexpected and/or more rapid than anticipated when block is predicted. The supernormal conduction may masquerade as gap phenomenon, but the mechanism is completely different. Supernormal conduction is due to cellular depolarization during a brief period after cellular repolarization following phase 3 of the action potential. The presence of supernormal excitability has been observed in vitro and in whole animal experiments, but its presence as an important clinical phenomenon is disputable, except perhaps, in ischemic myocardium. Instead, what is suspected to be supernormal conduction clinically is likely, in most cases, due to another mechanism such as the gap phenomenon or the peeling back of the refractory period. The peeling back effect is due to pre-excitation of the AV node by a ventricular or junctional beat that shortens the absolute refractory period of the AV node or the His-Purkinje system and allows conduction of a supraventricular impulse. Other potential explanations for apparent supernormal conduction are summation of subthreshold responses and phasic autonomic influences whereby increased sympathetic tone may be responsible for rapid tissue conduction where block existed previously. The gap phenomenon remains an important physiologic mechanism that can explain an apparent paradoxical AV conduction. It is important for electrophysiologists to understand these phenomena in order to gain insight into the response to premature extrastimuli.