Josephson Clinical Cardiac Electrophysiology

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Chapter 6: Ectopic Rhythms and Premature Depolarizations ■

1

aVF V1

A

A

HRA

A

A

A

CS

H

H

H

A-V

A-V

A-V

HBE

45

45

45

RVA

T

FIGURE 6.8 His bundle rhythm with retrograde conduction. Electrocardiogram (ECG) leads 1, aVF, and V1 are displayed with electrograms from the high right atrium (HRA), coronary sinus (CS), His bundle region (HBE), and right ventricular apex (RVA). A His bundle deflection (H) precedes each ventricular depolarization by an H-V interval identical to sinus rhythm. In this case, the H-V interval is 45 msec, and retrograde atrial activation precedes ventricular depolarization. A-V, atrioventricular; T, time line.

can occur in these circumstances in the presence of heightened symptomatic tone that can reverse digitalis A-V nodal blocking effects and enhances its ability to produce enhanced impulse for mation. The site of origin of His bundle rhythms may differ. Ha riman et al, 1 using high gain, specially filtered unipolar signals, have detected Phase 4 slope in such rhythms. They suggested that such junctional rhythms may arise in the node because overdrive suppression by atrial pacing did not depend on the impulse reaching the His bundle (ie, during A-V nodal block) ( Figure 6.11 ). 1 However, electrotonic interactions within the node at the N-H region could alter the resulting escape rhythm. Theoretically, an automatic His bundle rhythm should demon strate a greater degree of overdrive suppression than the A-V

node because it is lower in the hierarchy of “pacemaker” activity and is sodium dependent. This should result in greater over drive suppression than the calcium-dependent A-V node since overdrive suppression is based on the Na/K exchanger. The speed of retrograde conduction to the atrium has also been sug gested as a means to help localize the site of the pacemaker, with a short retrograde conduction time suggesting an A-V nodal origin. This reasoning, however, is not valid, because retrograde conduction may be rapid (even more rapid than anterograde) if a “fast” A-V nodal pathway is used in the retrograde direction (see Chapter 7). Further proof of this is provided by the simi larly short H-A times that can be observed during ventricular stimulation (see Chapter 1).

Atria

AVN

His

Ventricle

H-V - sinus Serial activation

H-V - extrasystole Parallel activation

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FIGURE 6.9 His bundle depolarizations with an H-V shorter than recorded in sinus rhythm. In the left panel, surface electrocardiogram (ECG) leads and intracardiac recordings from a mapping catheter (Carto), His bundle and RV apex are shown during sinus rhythm with ectopic beats that occurred in a bigeminal pattern. The QRS morphology of the ectopic beats is almost identical to sinus rhythm, and the QRS is preceded by His bundle activation. The H-V during the ectopic beats is shorter than that recorded in sinus (62 vs 82 msec). This indicates that the site of origin is distal to the site of the recording of the His bundle electrogram. The right panel presents a ladder diagram that explains that the shorter H-V during the ectopic beat is parallel activation of the His recording site and the V, whereas during sinus rhythm, the H and V are recorded sequentially. AVN, atrioventricular node; RV, right ventricle. (Courtesy of Ramanan Kumareswaran, MD.)