Josephson Clinical Cardiac Electrophysiology

Miscellaneous Phenomena Related to Atrioventricular Conduction


Concealed conduction, the gap phenomenon, and supernor mality are physiologic events that may be considered variants of the normal response. These phenomena are responsible for many unusual or unexpected responses of atrioventricular (A-V) conduction. This chapter addresses these separate but interrelated phenomena of cardiac conduction. CONCEALED CONDUCTION The definition of concealed conduction has been irrevocably altered by the availability of intracardiac electrophysiologic studies. The concept of concealed conduction, an explana tion for the effects of incomplete penetration of an impulse into a portion of the A-V conduction system, was introduced (and then expanded on) by Langendorf 1,2 and Katz and Pick. 3 The term was applied to unexpected phenomena observed on the surface electrocardiogram (ECG) that were compat ible with the effects of incompletely penetrating impulses that were not directly reflected on the surface ECG; hence the term concealed . Because intracardiac recordings can di rectly document the presence of these impulses during the electrophysiologic study, they are no longer truly concealed. Thus, specific consequences of incomplete penetration of impulses may be a less ambiguous term than concealed conduction of impulses to describe a variety of ECG find ings. 4 Although the A-V node is the structure with which concealed conduction has been most often associated, this phenomenon can occur in any portion of the A-V conduc tion system. The manifestations of concealed conduction (ie, the effects of incomplete penetration of an impulse) include (a) unexpected prolongation of conduction; (b) unexpected failure of propagation of an impulse; (c) unexpected facilita tion of conduction by “peeling back” refractoriness, directly altering refractoriness, and/or summation 4-6 ; and (d) unex pected pauses in the discharge of a spontaneous pacemaker. Excellent reviews of the ECG manifestations of concealed conduction are available. 7-12 Concealed conduction may result from antegrade or ret rograde penetration of an impulse into a given structure. The impulse-producing concealment may originate anywhere in the heart, including the sinus node, an ectopic atrial site, the A-V junction, the fascicles, or the ventricles. 7 The most common site manifesting the effects of concealed conduction

is the A-V node. The effects of retrograde concealment in the A-V node under different circumstances are shown in Figures 5.1 through 5.4 . Impulses from any subnodal site can produce concealed conduction. The ability of ventricu lar premature complexes (VPCs) to produce concealment in the A-V node depends on intact retrograde His-Purkinje conduction. In Figure 5.4 , similarly coupled VPCs, manifest ing different patterns of retrograde His-Purkinje conduction, have totally different effects on the A-V nodal conduction of the sinus complex that follows. The effect of His bundle, fas cicular, or ventricular extrasystoles on subsequent A-V nodal conduction is inversely related to the coupling interval of the premature depolarization. In patients with dual A-V nodal pathways, VPCs, fascicular premature complexes, and His bundle complexes can shift conduction from the fast to the slow pathway. Slow pathway conduction can be maintained by retrograde invasion into the fast pathway (see Chapter 7). Retrograde concealment at multiple levels of the A-V con duction system may also occur ( Figure 5.5 ). The levels of concealment depend on the relative timing of antegrade and retrograde impulses. The most frequent clinical circumstances in which con cealed conduction is operative are (a) atrial fibrillation dur ing which the irregular ventricular response is due to the varying depth of penetration of the numerous wavefronts bombarding the A-V node 8 ; (b) prolongation of the P-R (A-H) interval or production of A-V nodal block by a pre mature depolarization of any origin; (c) reset of a junctional (His bundle) pacemaker by atrial or subjunctional premature depolarizations; and (d) perpetuation of aberrant conduction during tachyarrhythmias. In the latter circumstance, retro grade penetration of the blocked bundle branch subsequent to transeptal conduction perpetuates aberration. 13,14 This is the most common mechanism of perpetuation of aberration during supraventricular tachycardia observed in our labora tory (~70% of cases). Wellens et al 15 have found a similar in cidence of retrograde concealment, producing perpetuation of aberration. Concealed His bundle depolarizations can produce many unusual patterns of conduction, including simulation of Type II second-degree A-V block (see Chapters 3 and 6). His bun dle depolarizations are frequently not recognized because they must conduct antegrade and/or retrograde to have any

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