Josephson Clinical Cardiac Electrophysiology

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■ Josephson’s Clinical Cardiac Electrophysiology

V1

A

A2

A1

A2

A1

S2

A1

A1

S2

A1

S1

S1

S1

S1 A1

500 S1

A1

S1

400

500 S1

HRA

700

700

700

400

HRA

A

V1

A2

V1

V1

V2

V1

A1

V1

V1

V2

A

V

V1

A1

A1

V

A1

A1

H

A1 H1 A2

A1 H1

H1

H

H1

H1

H1

H1

H2

H2

HB

HB

245

190

190

90

190

90

40

40

40

40

40

155

210

155

40

155

40

155

40

40

40

40

T

T

A

C

A

A1

A1

A1

A1

S2

A1

S1

S2

A2

S1

S1 A1

S1

A2

S1 A1

S1

500

500

400

700

700

700

400

A

V1

V1

V2

V1

V1

V1

A2

A1

A1

A2

A1

A1

A1

V2

V

H2

H

H1

H2

H1

H1

VS

190

200

90 40

190

VS

90

40

40

40

195

155

155

155 40

40

40

40

40

B

D

FIGURE 5.9 Effect of ventricular premature depolarization on atrioventricular (A-V) nodal conduction. A to D. From top to bottom are surface electrocardiogram (ECG) lead, high right atrial (HRA), His bundle (HB) electrograms, and time line (T). A and B. At the longer basic cycle length (S1-S1) of 700 msec and an S1-S2 of 400 msec, ( A ) the S2H2 is 210 msec with standard stimulation (method I). B. With a preceding VPD simultaneous with the last paced beat (method II), the S2-H2 is 195 msec. C and D. At the shorter basic cycle length of 500 msec and an S1-S2 of 400 msec, ( C ) the S2-H2 interval is 245 msec with method I and ( D ) 200 msec with method II. The magnitude of shortening in S2-H2 with method II is greater at shorter cycle lengths ( C and D ) than at longer cycle lengths ( A and B ). All measurements are in milliseconds; pertinent deflections and intervals are labeled. VS is ventricular beat introduced simultaneously with the last A1. (From Shenasa M, Denker S, Mahmud R, Lehmann M, Gilbert CJ, Akhtar M. Atrioventricular nodal conduction and refractoriness after intranodal collision from antegrade and retrograde impulses. Circ . 1983;67:651-660.)

Gaps in A-V conduction most commonly occur during programmed stimulation, although they may occur spontane ously. 18 The major significance of the gap phenomenon is its contribution to the understanding of conduction and refracto riness of the A-V–conducting system. In particular, the resump tion of conduction at shorter coupling intervals has frequently been interpreted as a form of “supernormal” conduction. In fact, most cases of the so-called supernormal conduction can be ex plained physiologically by the gap phenomenon. Observation of all types of gaps demonstrates the physiology that was predicted by Moe; that block initially occurs distal to the stimulation site and that conduction resumes when earlier impulses result in proximal delay, allowing the initial site of block to recover. 17 Gaps may occur during either antegrade or retrograde stim ulation. Any pair of structures in the A-V conduction system that have the appropriate physiologic relationship to one another can participate in gap phenomena. Six different types of ante grade gap and two types of retrograde gap have been described ( Table 5.1 ). 19-21 In antegrade gap Types I, II, and III, the His Purkinje system is the site of initial distal block with the A-V node (Type I, Figure 5.10 ), proximal His-Purkinje system (Type II, Figure 5.11 ), and His bundle (Type III, Figure 5.12 ), respec tively, as the site of proximal delay. These are, in descending or der of frequency, by far the most common forms of antegrade gap. These three types, and all others in which the His-Purkinje system is the site of initial block, are most commonly observed during long drive cycle lengths, at which times His-Purkinje refractoriness is greatest. Multiple gaps may be recorded in the same patient. This is due to multiple levels of block and delay

TABLE 5.1 Classification of Gap Phenomena in the Human Heart Type Distal Site (Initial Block) Proximal Site (Delay) Antegrade I HPS A-V node II HPS (distal) HPS (proximal) III HPS His bundle IV HPS or A-V node Atrium V A-V node (distal) A-V node (proximal) VI HPS (?) True supernormality Retrograde I A-V node HPS II HPS (proximal) HPS (distal) A-V, atrioventricular; HPS, His-Purkinje system. Adapted from Damato AN, Akhtar M, Ruskin J, et al. Gap phenomena: antegrade and retrograde. In: Wellens HJJ, Lie KI, Janse MJ, eds. The Con duction System of the Heart: Structure, Function and Clinical Implications . Lea & Febiger; 1976:504. in the A-V–conducting system. One such example is shown in Figure 5.13 , in which distal block in the His-Purkinje system initially recovers because of delay in the proximal His-Purkinje system. Earlier coupling intervals again block, but dual A-V nodal pathways observed at even shorter coupling intervals pro duce enough A-V nodal delay to allow the His-Purkinje system

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