Josephson Clinical Cardiac Electrophysiology

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■ Josephson’s Clinical Cardiac Electrophysiology

PR 140 msec

PR 165 msec

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A

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HRA

A-H 80

A-H 80

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A H

A H

A H

A H V

HBE

TL

FIGURE 6.6 Different P-R intervals with identical A-H intervals, which is due to altered atrial activation and input into the atrioventricular (A-V) node. Electrocardiogram (ECG) leads 1, 2, and 3 are shown with a high right atrial (HRA) and a His bundle electrogram (HBE). The panel on the left was taken during a low right atrial rhythm (A in HBE is earliest and precedes the onset of the surface P wave). The panel on the right was taken during sinus rhythm. Note that despite a difference in P-R intervals, A-V nodal conduction remains the same (A-H = 80 msec). See text for explanation. TL, time line.

reentrant phenomena to conduction delay and the specific patterns of atrial activation associated with these echoes are discussed in Chapters 7 and 8.

( Figure 6.7 ). The response of the A-V node to pacing and pre mature atrial stimulation from different sites suggests that in some instances, atrial activation originating in the coronary sinus seems to bypass part of the node, leading to shorter A-V nodal conduction and Wenckebach cycles. Finally, it is important to distinguish atrial echoes that are due to reentry in the sinus node, A-V node, or via a con cealed A-V bypass tract from APDs. The relationship of these

JUNCTIONAL (HIS BUNDLE OR ATRIOVENTRICULAR NODAL) DEPOLARIZATIONS

His bundle depolarizations can be definitively recognized only by intracardiac recordings, especially if their manifestations are concealed (see Chapter 5). Junctional (most commonly, His bundle) depolarizations most frequently take the form of escape rhythms in the presence of sinus node dysfunction (see Chapter 2) or A-V nodal block (see Chapter 3). These escape rhythms usually have a QRS morphology identical to that during sinus rhythm. However, the hallmark of these rhythms is a His bun dle deflection that precedes ventricular depolarization; the H-V interval can be identical, longer (in the case of bradycardia- dependent intra- or infra-His conduction disturbances), or shorter (in the case of a site of origin distal to the His electro gram recording site) than that recorded in sinus rhythm H-V interval ( Figures 6.8 and 6.9 ). When the H-V is longer than in sinus (ie, intra- or infra-His delay), the QRS is usually aberrant. Retrograde atrial activation may or may not accompany His bundle rhythms, and it depends on the ability of the A-V node to conduct impulses retrogradely at the rate of the His bundle rhythm. Although rapid conduction to the atrium is demon strated in Figure 6.8 , in which atrial activation precedes ven tricular activation, variable retrograde conduction patterns may be presented, producing unusual rhythms. One such rhythm is a bigeminal pattern produced by a His bundle rhythm that is due to retrograde dual A-V nodal pathways and A-V nodal echoes ( Figure 6.10 ; see Chapter 7). Retrograde conduction during His bundle escape rhythms (or those due to triggered activity) is uncommon in the presence of digitalis intoxication, in the absence of catecholamines, because of coexistent impairment of A-V nodal conduction by digitalis. Retrograde conduction

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aVF V1

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500

HRA

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HBE

A-H 70

RV

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aVF V1

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LAA

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HBE

A-H 55

RV

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FIGURE 6.7 Effect of stimulation site on A-H interval. The A-H in tervals during pacing from the high right atrium (HRA) and left atrial appendage (LAA) at a cycle length of 500 msec are shown. The A-H interval during left atrial pacing is shorter (55 msec) than that during HRA pacing (70 msec). This is likely due to left atrial input to the atrio ventricular (A-V) node. HBE, His bundle electrogram; RV, right ventricle.