Key Teaching Points
•
Rapid tachycardia can originate from an intra-nodoventricular (NV) focal source. The
arrhythmogenic mechanism is due to triggered activity.
•
This form of NV tachycardia can masquerade as antidromic reciprocating tachycardia
with atrioventricular dissociation or as a passive bystander pathway in atrioventricular
nodal reentry.
•
The distinction among these 3 entities is challenging, but the algorithm outlined
in the Table facilitates the diagnosis. In addition, adenosine termination of a presumed
NV-related tachycardia (antidromic or passive bystander) that precedes conduction
block in the NV pathway or slow atrioventricular nodal pathway is consistent with
a diagnosis of an intra-NV tachycardia.
Introduction
Decremental accessory pathways (APs) have long been the focus of considerable interest
because of their unusual and complex modes of presentation as well as for their proclivity
for participating in antidromic reciprocating tachycardia (ART) or to act as passive
bystanders in supraventricular tachycardia.
1
Initially described by Mahaim as fibers originating from the atrioventricular (AV)
node and inserting into the basal ventricular myocardium, decremental APs, often referred
to generically as “Mahaim” pathways, are now classified into at least 3 subtypes:
(1) long AV APs that insert into the right bundle branch (atriofascicular) or anterior
right ventricular myocardium, (2) short AV APs that insert into peritricuspid ventricular
muscle, and (3) nodoventricular (NV) or nodofascicular (NF) pathways that are linked
to the AV node and usually emerge from the slow AV nodal pathway.2, 3, 4, 5 With some
exceptions, NV/NF pathways are right-sided and, when associated with a regular wide
complex tachycardia (WCT), may show AV dissociation, since the atria are not integral
to the circuit, making AV dissociation a hallmark for differentiating this form of
ART from other forms of decremental AP-mediated ART.
We present a case of a patient who had presumed NV-dependent ART with AV dissociation.
However, during electrophysiologic evaluation, we demonstrate that the tachycardia
originated from an intra-Mahaim pathway focus, highlighting the potential of decremental
APs to develop rapid de novo arrhythmias that may masquerade as ART, passive bystanders,
or ventricular tachycardia.
Case report
A 41-year-old woman with a history of a right-sided AP ablated in China at age 19
presented to an outside hospital with recurrent palpitations and a regular WCT. The
arrhythmia terminated with adenosine (6 mg). One week later, she presented to our
hospital with a similar tachycardia. The tachycardia had a left bundle branch block
pattern with a left superior axis and a cycle length of 280 ms. In the emergency room,
she was given intravenous amiodarone, which terminated tachycardia. An electrocardiogram
in sinus rhythm showed no evidence of preexcitation. Her subsequent work-up, including
an echocardiogram and cardiac magnetic resonance imaging, was normal.
During electrophysiologic study, baseline AH and HV intervals were 65 ms and 37 ms,
respectively. Dual AV nodal pathways were demonstrated. Rapid pacing from the proximal
coronary sinus resulted in a QRS morphology that reproduced the patient's clinical
arrhythmia. Retrograde conduction was concentric and adenosine resulted in ventriculoatrial
block. During atrial pacing at a cycle length of 370 ms, conduction proceeded initially
over the fast AV nodal pathway, resulting in a narrow QRS complex. However, when conduction
abruptly switched to the slow AV nodal pathway (AH increased from 105 to 194 ms),
the QRS complex showed fusion for 1 beat (Figure 1A). All subsequent beats were fully
preexcited as the AH interval further increased and the His bundle potential was displaced
into the ventricular electrogram. Incremental atrial pacing resulted in progressive
prolongation of the stimulus-delta interval, findings consistent with a decremental
AP.
Figure 1
A: Preexcitation linked to conduction over the slow atrioventricular pathway. During
atrial pacing at a cycle length of 370 ms from the proximal coronary sinus (CSp),
a fusion beat (*) occurred coincident with abrupt prolongation of the AH interval
(from 105 to 194 ms), which was followed by fully preexcited complexes. Surface leads
1, aVF, and V1 are shown, as well as intracardiac recordings from the distal His bundle
(Hisd), CSp, and right ventricular apex (RVA). A = atrial activation; H = His. B:
Effect of adenosine on preexcitation. Adenosine (12 mg) caused prolongation of atrioventricular
(AV) nodal conduction, which was associated with AV prolongation, shortening of the
HV interval, and reversal of relative ventricular activation recorded from the posteroseptal
tricuspid annulus (TAps) and RVA. During the first 2 beats, conduction proceeds over
the fast AV nodal pathway, the QRS complex is narrow, and the RVA is activated before
TAps ventricular excitation. This relationship reverses as conduction switches to
the slow AV nodal pathway and preexcitation becomes manifest. ΔV = relative ventricular
activation of TA and RVA (ms); positive value indicates that TA ventricular activation
precedes RVA; negative value indicates that RVA activation precedes TA activation.
In the absence of preexcitation, right ventricular apex activation preceded tricuspid
annulus (TA) ventricular activation (Figure 1B). However, this relationship reversed
with the onset of preexcitation, coincident with a shift in conduction from the fast
to the slow AV nodal pathway. (Figure 1B). Greater degrees of preexcitation caused
progressively earlier TA ventricular activation relative to the right ventricular
apex.
Atrial pacing during concurrent infusion of isoproterenol (2 μg/min) consistently
induced WCT with AV dissociation (Figure 2A and B). The QRS morphology of the clinical
tachycardia and the conducted QRS complexes during atrial pacing were identical. The
tachycardia terminated with rapid ventricular pacing or adenosine. Of note, fusion
beats were observed during induction of tachycardia with atrial pacing and during
tachycardia (the latter were due to spontaneous atrial beats) (Figure 2A). Although
fusion beats during atrial induction were due to parallel conduction over the NV pathway
and AV node, such a mechanism for producing fusion beats cannot occur during ART,
owing to collision of anterograde and retrograde wave fronts within the AV node–His-Purkinje
system. These data therefore provide incontrovertible evidence that the tachycardia
was not due to ART.
Figure 2
A: Initiation of wide complex tachycardia with atrial pacing. The preexcited beats
during atrial pacing (220 ms) have the same morphology as the tachycardia (240 ms).
Abbreviations are as previously defined. * fusion beat during atrial pacing resulting
from conduction over Mahaim pathway and AV node; ** fusion beat during tachycardia.
B: Wide complex tachycardia with AV dissociation. Abbreviations as defined in Figure 1.
Also informative was the differential timing of the response of the Mahaim pathway
and tachycardia to adenosine. Immediately following termination of tachycardia with
adenosine, AV node conduction prolonged between the first and second sinus beats,
although conduction still proceeded over the NV pathway through activation of the
slow AV nodal pathway (Figure 3A). During the third sinus beat, conduction blocked
in both the Mahaim pathway and AV node; however, by the fourth sinus beat fast AV
nodal pathway conduction recovered and the impulse proceeded over the His-Purkinje
system, not the Mahaim pathway. Conduction over the Mahaim pathway was therefore linked
to conduction over the slow AV nodal pathway. Since tachycardia terminated before
conduction block occurred in the slow AV nodal pathway or NV pathway, adenosine's
effects on tachycardia occurred independently of its effects on the AV node, thus
eliminating ART involving an NV pathway or AV nodal reentry with bystander conduction
as possibilities. An alternative interpretation is that the tachycardia was due to
NV-dependent ART and that adenosine terminated tachycardia by blocking conduction
in the retrograde limb, ie, retrograde fast AV nodal pathway. However, this alternative
scenario is unlikely, since the anterograde slow AV nodal pathway is notably more
sensitive to adenosine than the retrograde fast AV nodal pathway.6, 7, 8 Therefore,
in response to adenosine, NV-dependent ART would be expected to terminate in the anterograde
limb (slow AV nodal pathway), not the retrograde limb. Accordingly, because of persistence
of conduction over the slow AV nodal and NV pathways following termination of tachycardia,
and because of the presence of fusion beats during tachycardia, we deduce that the
tachycardia had an intra-Mahaim pathway origin and that termination of tachycardia
with adenosine was due solely to its direct effects on the Mahaim pathway.
Figure 3
A: Termination of wide complex tachycardia with adenosine. Adenosine's effects on
tachycardia are manifest before its effects on Mahaim pathway conduction or its abolition
of conduction in the slow AV nodal pathway. Conduction over the Mahaim pathway via
the slow pathway of the AV node is maintained during the first 2 sinus beats (labeled
1 and 2) following tachycardia termination. The third beat blocks in both the AV node
and Mahaim pathway before conduction resumes over the fast AV nodal pathway without
evidence of prexcitation (fourth beat). Abbreviations as defined in Figure 1. B: The
best match during ventricular pace mapping (97%) was recorded from the ventricular
insertion site of the Mahaim pathway, in the region of the posteroseptal tricuspid
annulus. Note that there is also a near-identical QRS match when comparing the morphology
during atrial pacing (from proximal coronary sinus) and the tachycardia morphology.
Activation maps were performed during atrial pacing and tachycardia to identify the
earliest site of ventricular activation. Both maps localized the ventricular insertion
site to the posteroseptal TA. Ablation at this site during tachycardia terminated
the arrhythmia within 2 seconds. Although anterograde dual pathways were present post-ablation,
AP conduction was not.
Discussion
Our initial observations, which included the presence of a decremental NV pathway,
AV dissociation during WCT, and linkage of the NV pathway to the slow AV nodal pathway,
suggested the possibility of NV-mediated ART (which was atypical, since the pathway
inserted at the base of the right ventricle) (Figure 1A and B). Also consistent with
this diagnosis is that the morphology of the tachycardia was reproduced with atrial
pacing and by pacing at the pathway's ventricular insertion site (Figure 3B). Despite
these findings, the presence of fusion beats during tachycardia suggested other potential
mechanisms for the patient's WCT. This includes reentrant ventricular tachycardia
originating from ventricular muscle contiguous to the pathway's insertion site. However,
this is an unlikely explanation, since the tachycardia was sensitive to adenosine,
a finding that virtually rules out ventricular reentry.
9
Focal triggered activity originating from the ventricular aspect of the TA annulus
is another possibility
10
; however, this is improbable, as it would require a circumstance whereby conduction
over the patient's Mahaim pathway exactly replicated the morphology of an unrelated
focal tricuspid annular ventricular tachycardia, which also originated at the Mahaim
pathway's precise exit site (Figure 2A).
Another consideration is tachycardia originating from within the Mahaim pathway. Although
automaticity is known to originate from Mahaim pathways, these arrhythmias usually
occur in response to catecholamine stimulation or ablation, are transient, occur at
substantially slower rates than that observed in the present study, are not inducible
with programmed stimulation, and transiently slow but fail to terminate in response
to adenosine.11, 12 Therefore, initiation of tachycardia with programmed stimulation
and its termination with adenosine in this study excluded AP automaticity as an etiology
(Table 1).13, 14
Table 1
Wide complex tachycardia associated with a Mahaim pathway
AVNRT with decremental bystander AP∗
Mahaim-dependent ART
Intra-Mahaim tachycardia due to triggered activity
Intra-Mahaim tachycardia due to automaticity
Reentrant VT originating contiguous to ventricular insertion of Mahaim pathway∗
Initiation with atrial pacing
+
+
+
-
+
Fusion during atrial pacing
-
-
+
+
+
Adenosine termination
+
+
+
-†
-
Progressive and fixed fusion during RVP at ≥ 2 CLs
+
+
-
-
+
CL dependence of tachycardia on VH interval
-
+
-
-
-
PPI-TCL < 30 ms (pacing from ventricular insertion site)
-
+
+
+
+
ART = antidromic reciprocating tachycardia; AVNRT = atrioventricular nodal reentrant
tachycardia; CL = cycle length; PPI = post-pacing interval; RVP = rapid ventricular
pacing; TCL = tachycardia cycle length; VT = ventricular tachycardia.
∗
Tachycardia associated with but not dependent on Mahaim pathway conduction.
†
Transient slowing without termination.
The weight of evidence suggests that the clinical tachycardia is due to adenosine-sensitive
triggered activity originating from a focal site within the Mahaim pathway. Supportive
of this diagnosis is that adenosine-mediated termination of tachycardia preceded conduction
block in both the Mahaim pathway and slow AV nodal pathway, thus dissociating the
time course of adenosine's effects on Mahaim tachycardia from its effects on Mahaim
pathway conduction or slow AV node pathway conduction. Moreover, the presence of fusion
beats during tachycardia conclusively eliminates ART as a consideration (Figure 2A).
Although this report represents the first example of an intra-Mahaim pathway tachycardia
due to triggered activity, we suspect this entity may be more common than is presently
appreciated.
This case presents an unusual variation on the spectrum of arrhythmias associated
with NV pathways and the criteria for establishing the etiology of WCT in these patients.
The presence of AV dissociation during tachycardia indicates that the atrium is not
an obligatory component of the reentrant tachycardia circuit and is consistent with
ART due to an NV pathway. However, the distinction between this diagnosis and that
of focal triggered activity originating from the Mahaim pathway is not readily delineated,
given the multiple electrophysiological features they share. As a means for distinguishing
among these entities, we propose the criteria outlined in the Table 1.
Finally, we suggest that conceptually, the tachycardia described in this study can
be considered synonymous with ventricular tachycardia. Although the focal source of
the tachycardia originates from the Mahaim pathway, the arrhythmia does not manifest
until it exits from its insulated pathway to the ventricle, which then presents as
a tachycardia clinically indistinguishable from ventricular tachycardia. This is an
inversion of the usual circumstance of bystander tachycardia involving a Mahaim pathway,
where it can be passively activated during supraventricular tachycardia. In contrast,
in the present case, we demonstrate a circumstance where the Mahaim pathway is the
active source of the arrhythmia and the ventricles serve as passive bystanders. Nonetheless,
whether the arrhythmia is classified by its site of origin, ie, Mahaim pathway tachycardia,
or by its exit (ventricular insertion) site, ie, ventricular tachycardia, is clinically
inconsequential, since the arrhythmia's hemodynamic consequences and ablation target
are the same, regardless of designation, making further refinement of the clinical
diagnosis akin to a distinction without a difference.