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\begin{document}

\title{A long RP tachycardia with the earliest atrial activation at the His
bundle region : What is the mechanism?}



\author[1]{Atsushi Doi}%
\author[1]{Wataru Nakano}%
\author[1]{Tasuku Ashida}%
\author[1]{Takayuki Yamada}%
\affil[1]{Department of Cardiovascular Medicine Otemae Hospital}%


\vspace{-1em}



  
  \date{May 22, 2024}


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\selectlanguage{english}
\begin{abstract}
A 72-year-old female with frequent palpitation was referred for
radiofrequency ablation. The baseline 12-lead electrocardiogram and
echocardiography results were normal. At baseline, the atrio-His (AH)
and His-ventricular (HV) intervals were 90 and 41 ms, respectively. Dual
atrioventricular (AV) nodal physiology or ventriculoatrial (VA)
conduction was not observed during programmed atrial and ventricular
stimulation. After isoproterenol infusion, VA conduction became
decremental and concentric, with the earliest atrial activation seen at
the His bundle (HB) region during ventricular pacing. A supraventricular
tachycardia with a long RP interval (SVT) was induced by atrial
extra-stimulation, without any jump-up in the AH interval. During the
SVT, the AH and HV intervals were 180 and 180 ms, respectively, and the
earliest atrial activation was recorded in the HB region (Figure 1A).
During the SVT, transient 2:1AV conduction was observed (Figure 1B).
Ventricular overdrive pacing at a pacing cycle length (CL) of 360 ms was
performed during the SVT with a CL of 390ms (Figures 2A and B). Based on
these observations, what is the mechanism of this tachycardia?%
\end{abstract}%



\sloppy


A long RP tachycardia with the earliest atrial activation at the His
bundle region: What is the mechanism?

Atsushi Doi, MD, PhD; Wataru Nakano, MD; Tasuku Ashida, MD; Takayuki
Yamada, MD.

Department of Cardiovascular Medicine, Otemae Hospital, 1-5-34 Otemae,
Chuo-ku, Osaka, 540-0008, Japan.

Correspondence to Atsushi Doi, MD, PhD.

Department of Cardiovascular Medicine, Otemae Hospital, 1-5-34 Otemae,
Chuo-ku, Osaka, 540-0008, Japan.

E-mail; adoi7abl@gmail.com

\textbf{Short title:} A long RP tachycardia with the earliest atrial
activation at the His bundle region

\textbf{Keywords:} atrioventricular nodal reentrant tachycardia, His
bundle, nodofascicular/nodoventricular bypass tract, superior slow
pathway, ventricular overdrive pacing

The authors have no funding sources to disclose.

The authors have no conflicts of interest to disclose.

\textbf{EP ROUNDS}

\textbf{Case presentation}

A 72-year-old female with frequent palpitation was referred for
radiofrequency ablation. The baseline 12-lead electrocardiogram and
echocardiography results were normal. At baseline, the atrio-His (AH)
and His-ventricular (HV) intervals were 90 and 41 ms, respectively. Dual
atrioventricular (AV) nodal physiology or ventriculoatrial (VA)
conduction was not observed during programmed atrial and ventricular
stimulation. After isoproterenol infusion, VA conduction became
decremental and concentric, with the earliest atrial activation seen at
the His bundle (HB) region during ventricular pacing. A supraventricular
tachycardia with a long RP interval (SVT) was induced by atrial
extra-stimulation, without any jump-up in the AH interval. During the
SVT, the AH and HV intervals were 180 and 180 ms, respectively, and the
earliest atrial activation was recorded in the HB region (Figure 1A).
During the SVT, transient 2:1AV conduction was observed (Figure 1B).
Ventricular overdrive pacing at a pacing cycle length (CL) of 360 ms was
performed during the SVT with a CL of 390ms (Figures 2A and B). Based on
these observations, what is the mechanism of this tachycardia?

\textbf{Commentary}

The differential diagnosis of a long RP tachycardia, with the earliest
atrial activation at the HB region, includes atypical atrioventricular
(AV) nodal reentry tachycardia (AVNRT), atrial tachycardia (AT), and
orthodromic reciprocating tachycardia (ORT) using a slowly conducting AV
accessory pathway or a concealed nodofascicular/nodoventricular (NF/NV)
bypass tract (BT). The persistence of the SVT with a 2:1 AV block
excluded the possibility of an ORT with a slowly conducting AV accessory
pathway and a concealed NF/NV BT. As shown in Figure 2A, ventricular
burst pacing did not accelerate the atrium to the pacing cycle length,
and the SVT terminated without conduction to the atrium after the third
fully-paced QRS complex. This finding is consistent with AVNRT and ruled
out the possibility of an AT. Figure 2B shows the response to
ventricular burst pacing with a delayed timing during the SVT. The SVT
terminated without conduction to the atrium after the first fused paced
QRS complex. The fused-paced QRS complex indicates a His-refractory
timing. Therefore, this response to the ventricular pacing indicates the
presence of a concealed NF/NV BT, but not necessarily its participation
in the tachycardia circuit. The responses to these pacing maneuvers can
be explained as follows: when ventricular burst pacing with a delayed
timing was performed during the SVT (Figure 2B), the first
His-refractory ventricular pacing impulse was conducted over the NF/NV
AP and penetrated the slow pathway (SP). The conduction block in the SP
resulted in tachycardia termination owing to the decremental properties
of the SP. When ventricular burst pacing with an earlier timing was
performed during the SVT (Figure 2A), the connection of the NF/NV BT to
the SP was blocked, but the SVT persisted. The timing of the third
ventricular pacing impulse being earlier than the His-refractory period
resulted in tachycardia termination without conduction to the atrium,
due to the conduction block in the SP via the right bundle branch-His
conduction. These findings established the diagnosis of an atypical
AVNRT, with a bystander NF/NV BT inserting into the retrograde SP.

We performed electroanatomical mapping to identify the earliest site of
atrial activation during the SVT, and radiofrequency ablation was
performed in the noncoronary aortic cusp of Valsalva just superior to
the HB region, thereby eliminating the inducible tachycardias (Figure
3). Kaneko et al. reported that the mechanism of an atypical AVNRT,
incorporated a superior-SP, located above Koch's triangle as the
retrograde limb.\textsuperscript{1} Thus, we diagnosed the SVT as an
atypical AVNRT, with a bystander NF/NV BT inserting into the retrograde
superior SP.

NF/NV BTs are rare accessory pathway variants connecting the AV node to
the right bundle branch or right ventricle. Most of NF/NV BTs are
reportedly inserted into the SP; hence, concealed NF/NV BT-associated
supraventricular tachycardias can be eliminated by the ablation of the
right or left inferior extensions of the AV node, regardless of their
mechanism.\textsuperscript{2,3} In this case, the nodal pathway of the
NF/NV BT is the superior SP, because the successful SP ablation site was
the noncoronary aortic cusp of Valsalva just superior to the HB region.
Termination of the SVT without conduction to the atrium after the fused
paced QRS complex favors an NV BT over an NF BT, because the collision
site between antidromic and orthodromic wavefronts during the QRS fused
period occurs in the ventricular myocardium. The present case
highlighted that the precise mechanism of AVNRT, in which NVBT was
attached to the superior SP, could be elucidated by adjusting the timing
of ventricular burst pacing during tachycardia.

\textbf{References}

\begin{enumerate}
\tightlist
\item
  Kaneko Y, Naito S, Okishige K, Morishima I, Tobiume T, Nakajima T,
  Irie T, Ota M, Iijima T, Iizuka T, Tamura M, Tamura S, Saito A, Igawa
  O, Kato R, Matsumoto K, Suzuki F, Kurabayashi M. Atypical Fast-Slow
  Atrioventricular Nodal Reentrant Tachycardia Incorporating a
  ''Superior'' Slow Pathway: A Distinct Supraventricular
  Tachyarrhythmia. Circulation 2016;133:114-123.
\item
  Ho RT, Ortman M, Levi SA. The various manifestations of concealed
  nodofascicular/nodoventricular bypass tracts. Heart Rhythm
  2020;171280-1290.
\item
  Cardona-Guarache R, Han FT, Nguyen DT, Chicos AB, Badhwar N, Knight
  BP, Johnson CJ, Heaven D, Scheinman MM. Ablation of Supraventricular
  Tachycardias From Concealed Left-Sided Nodoventricular and
  Nodofascicular Accessory Pathways. Circ Arrhythm Electrophysiol
  2020;13:e007853.
\end{enumerate}

\textbf{FIGURE LEGENDS}

\textbf{Figure 1}(A) Intracardiac electrogram during the tachycardia
with the earliest atrial activation at the His bundle region. (B)
Intracardiac electrogram during the tachycardia with 2:1
atrioventricular conduction. I, II, V1 = surface electrogram; HRA d and
HRS p= distal and proximal high right atrium; His d and His p = distal
and proximal His bundle region; CS d and CS p = distal and proximal
coronary sinus; RV d and RV p = distal and proximal right ventricle; H =
His bundle electrogram.\textbf{Figure 2}Intracardiac electrograms during
ventricular burst pacing at a pacing cycle length of 360ms during the
tachycardia with a cycle length of 390ms.

With an early timing.

With a delayed timing.

The numbers indicate the cycle lengths in ms. VP = ventricular pacing.
The other abbreviations are as in Figure 1.\textbf{Figure 3}

Intracardiac electrogram at a successful ablation site. Radiofrequency
ablation in the noncoronary aortic cusp of Valsalva just superior to the
His bundle region terminated the tachycardia.

(B) Fluoroscopy image at a successful ablation site.

ABL = ablation catheter. The other abbreviations are as in Figure 1.\selectlanguage{english}
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