Commentary:
12 lead ECG showed narrow QRS tachycardia with retrograde VA
wenkebaching, hence Atrio-ventricular reciprocating tachycardia (AVRT)
and atrial tachycardia (AT) could be excluded. Intracardiac electrogram
during spontaneous induction showed classical 1:2 AV nodal conduction
followed by initiation of tachycardia (Fig 1B). Several features favor
AV nodal re-entrant tachycardia (AVNRT) in this case: VA dissociation,
response to adenosine, 1:2 AV nodal conduction, concentric atrial
activation pattern and fixed HV interval (44ms) during tachycardia. The
possibility of automatic junctional tachycardia was excluded based on
response to premature atrial extrastimuli and intravenous adenosine.
Intracardiac electrogram showed variation in A-A, V-V, H-H and HA
intervals. Atypical AVNRT of slow – slow type with decremental
conduction of upper common pathway (UCP) could explain the variation in
A-A and H-A intervals (Fig 3A, B and C). The H-H and V-V interval
variations could be explained by presence of multiple slow pathways as
below
- Three slow pathways with different electrophysiological properties
- Initial re-entry between slow pathway-1 (S1-antegrade) and slow
pathway-2 (S2-retrograde) at a cycle length of 410ms with concealed
conduction into slow pathway-3 (S3) and decremental conduction in
upper common pathway (fig 3A)
- Decremental conduction of S1 resulting in gradual prolongation of H-H
interval followed by block which favored the transient re-entry
between S3 (antegrade) and S2 (retrograde) (cycle length 379ms) and
subsequent resumption of S1 conduction (fig 3B & C)
Fig 3: A – Atypical slow- slow AVNRT with re-entry between S1 and
S2 with concealed conduction into S3. Upper common pathway showed
decremental conduction. B – Transient block in S1 due to decremental
conduction favored re-entry between S3 and S2 with concealed conduction
into S1. C – Ladder diagram showing the same phenomenon. The
decremental conduction in S1 prolonged H-H/V-V interval from 410ms to
478ms. S3 could conduct in the next beat with H-H interval of 379ms as
S1 was transiently blocked. In the subsequent beat S1 conduction resumed
with minimal prolongation of H-H interval due to concealed conduction.
Atrial conduction occurred decrementally through upper common pathway
(UCP). S1,2,3 – slow pathways 1,2 and 3, F – fast pathway, LCP -Lower
common pathwayThe possibility of two antegrade slow pathway conduction has been
described previously as a cause for cycle length alteration during
AVNRT1. Multiple slow pathways are required for the
initiation of re-entrant tachycardia after 1:2 AV response which
otherwise would not have induced. In our patient the cycle length
alteration was noted in both atrium (A-A) and ventricle (V-V). This
could be explained by the rare combination of decremental conduction in
both upper common pathway and antegradely conducting slow pathway (S1).
The second pathway (S2) has different electrophysiological properties
(conduction velocity and refractory period) as evidenced by the
intracardiac electrogram and it is unlikely to be a fast pathway as the
HA interval is long. The fourth beat in the figure 2C has slightly
longer cycle length (422ms) due to concealed conduction of previous
impulse into S1. These findings emphasize the fact that both atrium and
the ventricle are not the part of circuit in AVNRT. Electroanatomic
mapping (ENSITE Velocity, Abbott, Plymouth, MN) confirmed earliest
atrial activation at coronary sinus ostium (fig 2B) Slow pathway was
ablated (medium curve catheter; 60W, 60⁰C) which rendered the
tachycardia non-inducible. Since there were three slow pathways involved
in this patient, this rare variety could be labelled as atypical
slow-slow-slow AVNRT.
The mechanism of re-entry in typical as well as atypical AVNRT remains
elusive2. There has been electrophysiologic evidence
of multiple superior atrial inputs to the AV node3that could explain multiple sites of early atrial activation during
tachycardia. Cycle length alternans can occur during AVNRT due to either
antegrade conduction via two slow pathways or junctional
bigeminism4. The decremental conduction properties of
antegrade slow pathway (S1) and upper common pathway were the reasons
for variation in A-A, H-H, H-A and V-V intervals in our case of atypical
slow-slow-slow AVNRT.References
- Maury P, Raczka F, Piot C, Davy JM. QRS and cycle length alternans
during paroxysmal supraventricular tachycardia: What is the mechanism?
J Cardiovasc Electrophysiol 2002;13:92-3
- Katritsis DG, Camm AJ. Atriovenricular nodal reentrant tachycardia.
Circulation 2010;122:831-40.
- Wu J, Wu J, Olgin J, Miller JM, Zipes DP. Mechanisms underlying the
reentrant circuit of atrioventricular nodal re-entrant tachycardia in
isolated canine atrioventricular nodal preparation using optical
mapping. Circ Res 2001;88:1189-95.
- Surawicz B, Fisch C. Cardiac Alternans: Diverse mechanisms and
clinical manifestations. J Am Coll Cardiol 1992;20:483-499