Josephson Clinical Cardiac Electrophysiology

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Chapter 5: Miscellaneous Phenomena Related to Atrioventricular Conduction ■

SR

SR

APD

SVT

SVT

SVT

SVT

RP of RBB

VPD ∗

FIGURE 5.7 Mechanism of normalization of aberration during supraventricular tachycardia (SVT) by a ventricular premature depolarization (VPD, asterisk). Parallelograms represent the refractory period (RP) of the right bundle branch (RBB). During sinus rhythm (SR), an atrial premature depolarization (APD) initiates SVT with RBB block because the APD occurred during the RP of the RBB. The RBB aberration persists, owing to continued retrograde concealment during the tachycardia. A VPD normalizes antegrade conduction by prematurely depolarizing the RBB, thereby “peeling back” its RP and allowing time for it to recover when engaged by the next impulse. In addition, the RP itself is shortened (decreased duration of parallelogram).

GAP PHENOMENON The term gap in A-V conduction was originally used by Moe and his associates 17 to define a zone in the cardiac cycle during which premature atrial impulses failed to evoke ventricular responses, while atrial complexes of greater and lesser prematurity con ducted to the ventricles. The gap phenomenon was attributed to functional differences of conduction and/or refractoriness in

two or more regions of the conducting system. The physiologic basis of gap phenomenon, in most instances, depended on a distal area with a long refractory period and a proximal site with a shorter refractory period. During the gap phenomenon, initial block occurs distally. With earlier impulses, proximal de lay is encountered, which allows the distal site of early block to recover excitability and resume conduction.

1 2

3

V1

V5

310

HRA

AVJ

190 S

RV

T

A

1 2 3 V1

V5

350

AVJ

265 S

RV

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T

B FIGURE 5.8 Abrupt termination of aberration by ventricular premature complex (VPC). A and B. From top to bottom, leads 1, 2, 3, V1, and V5, along with electrograms from the high right atrium (HRA), atrioventricular junction (AVJ), and right ventricle (RV). A. Atrial pacing at a cycle length of 310 msec produces right bundle branch block aberration. A VPC is introduced at a coupling interval of 190 msec, which immediately normal izes the QRS. B. Pacing at a longer cycle length of 350 msec produces left bundle branch block aberration. A premature VPC is introduced at 265 msec, which abruptly normalizes the QRS as pacing is continued. In both instances, the VPC interrupted retrograde penetration of the blocked bundle branch, which was the cause of perpetuation of aberration. See text for discussion. T, time line.