Preexcited Tachycardias
Introduction
Preexcited tachycardias refer collectively to tachycardias associated with antegrade conduction over an accessory pathway (AP). The AP can either 1) be a passive bystander serving a subsidiary role to the tachycardia mechanism (bystander preexcitation) or 2) be an integral part of the tachycardia mechanism (antidromic reciprocating tachycardia [ART]).
The purpose of this chapter is to:
Discuss the electrophysiologic features of bystander preexcited tachycardias.
Discuss the electrophysiologic features of ART and methods to differentiate ART from preexcited tachycardia atrio-ventricular nodal reentrant tachycardia (AVNRT).
BYSTANDER PREEXCITED TACHYCARDIAS
ATRIAL FIBRILLATION
The most common bystander preexcited tachycardia is preexcited atrial fibrillation. Atrial fibrillation with bystander conduction over an AP produces an irregular, wide-complex tachycardia with variable QRS morphologies representing different degrees of fusion over the His-Purkinje system and AP (Fig. 18-1). The shortest preexcited RR interval is a measure of functional refractoriness of the AP, and a value <250 ms identifies an AP capable of causing rapid ventricular rates that could potentially degenerate into ventricular fibrillation.1
ATRIAL FLUTTER/TACHYCARDIA
Atrial flutter/tachycardia with 1:1 bystander conduction over an AP produces a regular, wide-complex tachycardia. The presence of 2:1 AP conduction excludes ART (Fig. 18-2).
ATRIO-VENTRICULAR NODAL REENTRANT TACHYCARDIA
AVNRT with bystander conduction over an AP results in a regular, wide-complex tachycardia. Preexcited QRS complexes represent fusion between His-Purkinje and AP conduction. During typical (slow-fast) AVNRT following fast pathway (FP) activation of the atrium, antegrade AP conduction competes with slow pathway (SP)-His-Purkinje activation, and therefore, QRS complex might appear maximally preexcited and mimic ART. During atypical (fast-slow) AVNRT following SP activation of the atrium, antegrade AP conduction completes with FP-His-Purkinje activation and QRS complex might appear less preexcited which excludes ART. Loss of AP conduction normalizes QRS complexes without affecting tachycardia.2
Electrophysiologic Features
The characteristic electrophysiologic features of preexcited AVNRT are 1) regular, wide-complex tachycardia; 2) foreshortened (positive) or negative HV intervals; 3) antegrade His-right bundle (RB) activation sequence; and 4) concentric atrial activation pattern (Fig. 18-3).3,4 QRS complexes represent fusion between His-Purkinje and AP conduction but might appear fully preexcited. Absence of maximal preexcitation excludes ART. When His bundle potentials are visible, HV intervals are short (positive) or negative and inversely related to the degree of preexcitation. The His-Purkinje system is activated antegradely, giving rise to an antegrade His-RB activation sequence (unless the AP inserts into the RB [e.g., atrio-fascicular/nodo-fascicular AP] in which case retrograde RB activation could occur nearly simultaneously with antegrade His bundle activation). The atrial activation pattern is midline and earliest at the His bundle region (typical [slow-fast] AVNRT) or coronary sinus os region (atypical [fast-slow] AVNRT) unless retrograde conduction occurs over left atrio-nodal inputs.
Zones of Transition
Induction of preexcited AVNRT (similar to its non-preexcited counterpart) requires that a premature impulse fall into the tachycardia window defined as the difference in refractory periods between the FP and SP of the atrio-ventricular (AV) node. The impulse 1) fails to conduct over one pathway (unidirectional block) and 2) conducts exclusively over the counterpart pathway with sufficient delay (slow conduction) to allow
the previously blocked pathway to recover excitability, conduct retrogradely, and initiate tachycardia. Oscillations in cycle length at tachycardia onset can reveal variable preexcitation, which excludes ART (Figs. 18-3 and 18-4). Block in either the FP or SP but not AP terminates tachycardia. Because the AP is a bystander, tachycardia persists despite loss of preexcitation (Fig 18-5).
the previously blocked pathway to recover excitability, conduct retrogradely, and initiate tachycardia. Oscillations in cycle length at tachycardia onset can reveal variable preexcitation, which excludes ART (Figs. 18-3 and 18-4). Block in either the FP or SP but not AP terminates tachycardia. Because the AP is a bystander, tachycardia persists despite loss of preexcitation (Fig 18-5).
 FIGURE 18-1 Preexcited atrial fibrillation. Top: Right posteroseptal AP. The shortest preexcited RR measures ˜230 ms (circled). Bottom: Left free wall AP. The shortest preexcited RR intervals measure ˜160 ms (circled). |
Pacing Maneuvers
Both preexcited AVNRT and ART are regular, wide-complex preexcited tachycardias that can appear similar but differentiated by pacing maneuvers in the atrium (inverse rule).
AVJ-Refractory APD
During preexcited AVNRT, a critically timed atrial extrastimulus delivered near the atrial insertion site of the AP when the septal atrium is refractory (“committed”) advances or delays the ventricle over the AP but does not reset or terminate tachycardia (failure to affect the subsequent atrium).5 (The AVJ-refractory atrial premature depolarization [APD] that advances the ventricle over the AP is equivalent to delivery of a late-coupled ventricular premature depolarization (VPD) from the ventricular insertion site of the AP, which would fail to affect AVNRT because AVNRT has a large preexcitation index (> 100 ms)).6 Moreover, advancement of the ventricle over
the AP might show a greater degree preexcitation indicating that QRS complexes during tachycardia are not maximally preexcited further excluding ART. (In contrast, during ART, an AVJ-refractory APD 1) advances or delays the ventricle over the AP and 2) resets tachycardia [advances or delays the subsequent atrium], indicating that both the ventricle and atrium are integral components of the circuit (obligatory 1:1 AV/VA relationship, see below).)
the AP might show a greater degree preexcitation indicating that QRS complexes during tachycardia are not maximally preexcited further excluding ART. (In contrast, during ART, an AVJ-refractory APD 1) advances or delays the ventricle over the AP and 2) resets tachycardia [advances or delays the subsequent atrium], indicating that both the ventricle and atrium are integral components of the circuit (obligatory 1:1 AV/VA relationship, see below).)
 FIGURE 18-2 Preexcited atrial flutter with 1:1 (top) and 2:1 (bottom) conduction over a left posterior AP. |
 FIGURE 18-3 Preexcited AVNRT. Programmed atrial extrastimulation induces an atypical (fast-superior slow) AVNRT with bystander conduction over a left posteroseptal AP (earliest ventricular activation at CS px). His bundle potentials precede QRS onset with short HV intervals (HV = 8 ms). Absence of maximal preexcitation excludes ART. |
 FIGURE 18-4 Preexcited AVNRT. Burst atrial pacing induces an atypical (fast-superior slow) AVNRT with bystander conduction over a left posteroseptal AP (earliest ventricular activation at CS px). Greater preexcitation during atrial pacing and variable preexcitation during tachycardia exclude ART. Preexcited QRS complexes represent fusion between His-Purkinje and AP conduction. |
 FIGURE 18-5 Preexcited AVNRT. Preexcited tachycardia persists for one beat despite loss of preexcitation excluding ART. It terminates with AV block excluding preexcited atrial tachycardia (AT). Loss of preexcitation reveals a short VA interval (65 ms) and earliest atrial activation at CS 4 (left atrio-nodal input). A subsequent junctional escape complex coincides with a sinus beat. |
Rapid Atrial Pacing (Maximal Preexcitation)
Rapid Ventricular Pacing/Entrainment (ΔHA Value/PPI)
During AVNRT, the HA interval reflects simultaneous activation of the His bundle and atrium over the AV node (pseudointerval), but during right ventricular (RV) entrainment (or pacing at the tachycardia cycle length [TCL]), it represents sequential activation over the AV node (true interval). Therefore, the HA(preexcited AVNRT) < HA(RV pacing).7 In contrast, during both ART and RV entrainment (or pacing at TCL), the HA interval reflects sequential activation of the His bundle and atrium over the AV node. Therefore, the HA(ART) = HA(RV pacing).7 During entrainment of preexcited AVNRT from the ventricle, the post-pacing interval (PPI) is long similar to its non-preexcited counterpart.8
ANTIDROMIC REENTRANT TACHYCARDIA
MECHANISM
In contrast to bystander preexcitation, the AP during ART is an integral component of the tachycardia mechanism. During typical (true) ART, the antegrade and retrograde limbs of the circuit are the AP and His-Purkinje-AV node axis, respectively, both of which demonstrate short refractory periods.9 Atypical ART uses two APs for antegrade and retrograde conduction (“pathway to pathway” tachycardia) with or without fusion (antegrade or retrograde) over the AV node.10,11 (Alternatively, atypical ART with antegrade fusion over the AV node might be considered orthodromic reciprocating tachycardia [ORT] with antegrade bystander AP conduction depending on whether the AV node or antegrade AP forms the dominant antegrade limb of the circuit.)
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