Nature of the Gap Phenomenon in Man

Abstract

The atrioventricular (AV) gap phenomenon occurs when the effective refractory period of a distal site is longer than the functional refractory period of a proximal site and when closely coupled stimuli are delayed enough at the proximal site to allow distal site recovery. According to previous studies, in type 1 gap, the distal site of block is distal to the His bundle (ventricular specialized conduction system) and the proximal site of block is in the AV node. In type 2 gap, both the proximal and the distal sites of conduction block are within the ventricular specialized conduction system. Using His bundle recordings and atrial extra-stimulus techniques in man, we observed three previously undescribed types of gaps between (1) the AV node (distal) and the atrium (proximal), (2) the His bundle (distal) and the AV node (proximal), and (3) the ventricular specialized conduction system or a bundle branch (distal) and the His bundle (proximal). The delays at the His bundle in the second and third types of gaps seen in this study were demonstrated as splitting of His bundle potentials. Gaps between the AV node or the His bundle and the ventricular specialized conduction system were more easily demonstrated at long cycle lengths, but gaps between the atrium and the AV node were more easily demonstrated at short cycle lengths. Therefore, the previous subdivision of gaps into two types is an oversimplification, because gaps can occur between multiple sites in the conduction system. The gap phenomenon may be potentiated by both long and short cycle lengths; long cycle lengths increase the effective refractory period of a distal site, e.g., the His bundle and the ventricular specialized conduction system, and the short cycle lengths decrease the functional refractory period of a proximal site, e.g., the atrium and the AV node. KEY WORDS atrial extra stimuli functional block cycle length His bundle electrogram supernormal conduction refractory period split His bundle potentials intra-atrial conduction delay• One type of supernormal conduction, i.e., the paradoxical propagation of closely coupled stimuli when stimuli at longer coupling intervals are blocked, reflects a gap phenomenon (1-6). This phenomenon occurs when the refractory period of a distal conduction site limits conduction. With closely coupled stimuli, enough proximal delay occurs so that the impulse arrives at the distal conducting site late enough to be conducted. Gallagher and co-workers (5) have defined two types of gaps. In type 1 gap, the initial distal site of block is distal to the His bundle recording site, and the proximal site of delay is in the atrioventricular (AV) node. In type 2 gap, both the distal and the proximal site of conduction delay are distal to the His bundle recording site (5). A third kind of gap has recently been described by Agha and co-workers (6) in a patient with AV block; however, this kind of gap appears to reflect supernormal conduction, and proximal and distal sites of delay have not been defined (6).In the present st...

Gaps in anterograde conduction in patients with the short PR interval, normal QRS complex syndrome.

“…This explanation did not require the existence of proximal conduction system duality. Many examples of anterograde (Agha et al, 1973;Gallagher et al, 1973;Wu et al, 1974) and retrograde gap (Akhtar et al, 1974a, b) have now been described and classified in terms of sites of distal block and proximal delay (Damato et al, 1976) and Durrer's (1968) original hypothesis still awaits confirmation. Electrophysiological evidence of AV nodal dual conduction pathways is provided by the construction of an anterograde conduction curve (Moe et al, 1956).…”

An atrionodal and nodo-Hisian gap phenomenon.

“…Wu et al (1974) predicted that a gap between the atria and the AV node would occur more easily at higher rates. Shorter cycle lengths tend to shorten the effective refractory period of the atrium and lengthen the effective refractory period of the AV node (Wit et al,I97oa) and AV block will occur more easily.…”

“…Recently Wu et al (I974) showed that this classification was an over simplification of the gap phenomenon and that it could also occur at other levels in the conducting system from the atria to the ventricles. The purpose of this paper is to present two cases which tend to reinforce the view of Wu et al (1974).…”

A new mechanism for atrioventricular nodal gap-vagal modulation of conduction.

“…For example, abolishment of a gap between the AV node and His Purkinje system was demonstrated by decreasing the cycle length," as well as by the use of ,B-adrenergic blocking agents or digitalis.9 Alternatively, atropine, by altering refractoriness within the AV node, was shown to induce a gap involving the more distal portions of the conduction system,3 while shortening of the cycle length facilitates gap between the atrium and the AV node. 10 Although the phenomenon of a clinical gap is usually associated with conduction block, the prerequisite for the development of PGVS-dependent "gap," as shown in Figures 3 and 4, was the occurrence of a positive slope in the conduction curve (i.e., an increase in the AV nodal conduction time accompanying an increase in the coupling interval). Using this broader criterion, we examined the effect of two agents, atropine and physostigmine, on PGVS-induced "gap."…”

“…[2][3][4][5][6][7][8][9][10][11][12][13] In all cases, the gap phenomenon was explained in terms of different refractory or conductive properties or both of two consecutive units of the conduction system (e.g., atrium-atrioventricular [AV] node, AV node-bundle of His, bundle of His-His Purkinje system). The prerequisite for the development of the gap phenomenon in most instances was the existence of a distal unit with an effective refractory period longer than the functional refractory period of the proximal unit.…”