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\begin{document}

\title{Leadless Pacemaker Implantation in a 4-year-old, 16-kg Child}



\author[1]{Arjun Mahendran}%
\author[1]{Sara Bussey}%
\author[1]{Philip Chang}%
\affil[1]{University of Florida Health}%


\vspace{-1em}



  
  \date{May 07, 2024}


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\selectlanguage{english}
\begin{abstract}
Leadless pacemakers have an accepted role with demonstrable benefit in
adults. In contrast, implant and follow-up experience in pediatric
patients is quite limited. We present our implant experience in a
4-year-old for treatment of high-grade AV block. Implant considerations
in small patients and follow-up assessment of the vessel used for
implantation are discussed.%
\end{abstract}%



\sloppy


\textbf{Case Report}

A 4-year-old male (16.1 kg, 0.67 m\textsuperscript{2}) infant heart
transplant recipient presented with unexplained syncope. Historically,
despite aggressive immunosuppression, he experienced chronic
antibody-mediated rejection and markedly elevated panel reactive
antibodies precluded re-transplantation candidacy. Long-term treatment
has prioritized palliative care and maximization of quality of life.

Inpatient telemetry captured episodes of second-degree, Mobitz type II
AV block with symptoms of tiredness and dizziness (Fig 1). Episodes
lasted for several minutes with spontaneous resolution. Heart block was
considered the mechanism for syncope and a sequela of chronic graft
rejection. Permanent pacing was indicated and traditional and
non-traditional pacing options were considered. Implications of each
approach including potential impact on vascular access, relative ease of
future endomyocardial biopsies, and post-procedure recovery were
discussed. Collective agreement between the parents, transplant team,
cardiac surgeons, and electrophysiologist was reached to pursue leadless
pacemaker implantation. With the recent availability of the FDA-approved
Micra\textsuperscript{TM} AV pacemaker at our institution, this was
selected for its potential benefit in addressing the patient's specific
pacing indication (ie. heart block).

Conventional right femoral venous implantation was performed under
general anesthesia. Ultrasound-guided venous access was obtained and a
6-French (Fr) sheath was inserted. Contrast injection through the sheath
was performed to assess femoral and iliac vein caliber. Given the
vessels' capacity for distension, slow and serial dilation was performed
over an exchange length stiff wire with a series of dilators culminating
in the dilator of the Micra introducer sheath (IS), followed by
insertion of the complete IS set (27-Fr outer diameter). Insertion of
the IS proceeded smoothly without resistance.

The Micra delivery catheter (DC) was prepped and advanced through the
IS. To cross the tricuspid valve (TV), active flexion of the DC was
applied with counterclockwise torque. Mid-septal implantation was
initially attempted, however adequate septal orientation of the delivery
cup could not be achieved. Furthermore, given the dimensions of the
chamber at this location, mid-septal deployment was felt to potentially
obstruct the TV apparatus proximally and the RV outflow and pulmonary
valve superiorly (Fig 2A). Therefore, an apical septal location was
pursued. The TV was crossed again more inferiorly. Clockwise torque was
applied after crossing the valve to direct the DC tip toward the apical
septum (Fig 2B). The device was deployed with at least 2 splines
exhibiting appropriate fixation. Testing demonstrated acceptable
functional values (R waves \textgreater{}10 mV, pacing threshold 0.63 V
at 0.24 ms, impedance 570 ohms). Transesophageal imaging confirmed the
implant location and showed no pericardial effusion or significant
tricuspid interference. The anchoring tether was removed with
preservation of device position (Fig 2C). Following DC withdrawal and
before IS removal, an 0-Ethibond pursestring suture was placed around
the skin entry site for closure and hemostasis. The site was covered
with a pressure dressing. Post-implant interrogation showed preserved
functional values and reasonable accelerometer-derived signals
corresponding to atrial contraction (Fig 3).

Right lower extremity (LE) vascular ultrasound performed 2 days
post-implant showed a near-occlusive thrombus in the common femoral vein
(CFV). The pressure dressing and skin suture were removed on
post-implant day \#4 without any evidence of bleeding or hematoma. Given
the luminal thrombus, subcutaneous heparin was initiated without
bleeding recurrence. Follow-up assessment at 6 weeks post-implant showed
normal device function and interval reduction in thrombus size with
normal vessel flow pattern on follow-up LE scanning. The patient had
returned to normal activities without syncope recurrence.

\textbf{Discussion}

To our knowledge, this case represents the smallest pediatric patient
implanted with the Micra leadless pacemaker and the first implanted with
the newest model with VDD functionality. The patient's pacing
indication, coupled with avoidance of more invasive implant procedures
and complications with leads (transvenous or epicardial) and
subcutaneous device pockets, made leadless pacing preferable, especially
in light of his long-term palliative care strategy.

Published implant and follow-up experience with leadless pacemakers in
pediatric patients has been limited, with fewer than 20 total implants.
The largest single-center cohort included 9 patients (median age 13
years and median weight 37 kg).\textsuperscript{1} A conventional
femoral venous approach has generally been taken, though all patients
were \textgreater{}11 years of age and \textgreater{}30 kg in
weight.\textsuperscript{1-3} Alternatively, Gallotti et al and Cortez
presented separate cases using right internal jugular venous access in
smaller patients.\textsuperscript{4-5} Post-implant assessment of the
vessels used for implantation has not been previously reported.

Our case demonstrates several important considerations with a
conventional femoral venous approach in very small patients. Slow and
serial vessel dilation should be performed. The IS itself with its
hydrophobic outer surface advanced very smoothly even in the small
vessel used in our case. Catheter flexion and device positioning in the
RV cavity were easily accomplished. The application of counterclockwise
torque with flexion was extremely effective for TV crossing. A
mid-septal implant location with Micra may not be ideal in small
patients owing to the smaller RV chamber size and potential risk of
inflow and outflow interference. In addition, adequate septal
orientation of the DC tip may not be achievable at this more proximal
location on the septum. Post-implant development of a near-occlusive
thrombus in the CFV, while not entirely unexpected, was unfortunate. It
is encouraging that venous patency was preserved with interval reduction
of the luminal clot on short-term anticoagulation therapy.

While we hope the VDD pacing capability of Micra AV can be useful for
our patient's primary pacing indication, its use in young patients may
be limited due to its relatively low programmable upper tracking rate
(115 bpm). Given an anticipated low pacing burden and the sporadic
nature of Mobitz II AV block in our patient, we were unable to record
significant VDD pacing over the short follow-up period. However, manual
testing of atrial sensing demonstrates reasonable signals corresponding
to atrial contraction and we would anticipate that synchronous pacing
can be effectively applied within programmable parameters.

Device retrievability is of particular interest in pediatric patients
and has been successfully performed up to 4 years
post-implant.\textsuperscript{6} Given the smaller sizes of RV chambers
in pediatric patients, the Micra's footprint may be proportionately
substantial and abandonment of a battery-depleted Micra may limit space
for new device implantation. Retrievability was not considered to be a
limiting factor in our case given the device's anticipated long battery
longevity in comparison to the patient's long-term prognosis.

In summary, conventional femoral Micra implantation is feasible even in
small sized patients. The lower limits for age and size for safe and
effective Micra pacemaker implantation have not been established but
appear smaller than previously published. Assessment of implant vessel
size and its capacity for dilation and large sheath accommodation must
be performed. Vessel and cardiac chamber size can pose limitations to
implantation in smaller patients. Synchronous pacing expands the
potential use of leadless pacing even in pediatric patients, though
upper tracking rates may be a limitation.

\textbf{References}

\begin{enumerate}
\tightlist
\item
  Breatnach CR, Dunne L, Al-Alawi K, Oslizlok P, Kenny D, Walsh KP.
  Leadless Micra Pacemaker Use in the Pediatric Population: Device
  Implantation and Short-Term Outcomes. Pediatr Cardiol 2019, Epub:
  https://doi.org/10.1007/s00246-019-02277-y.
\item
  Tejman-Yarden S, Nof E, Beinart R, Ovadia N, Goldshmit Y, Buber J,
  Yonath H, Keizman E, Glikson M. Leadless Pacemaker implantation in a
  Pediatric Patient with Prolonged Sinus Pauses. Pediatr Cardiol
  2018;39:844-847.
\item
  McCanta AC, Morchi GS, Tuozo F, Berdjis F, Starr JP, Batra AS.
  Implantation of a Leadless Pacemaker in a Pediatric Patient with
  Congenital Heart Disease. HeartRhythm Case Rep 2018;4:506-509.
\item
  Gallotti RG, Biniwale R, Shannon K, Russell M, Moore JP. Leadless
  Pacemaker Placement in an 18-Kilogram Child: Procedural Approach and
  Technical Considerations. HeartRhythm Case Rep 2019;5:555-558.
\item
  Cortez D. Innovative Implantation of a Leadless Pacemaker in a 19 kg
  Paediatric Patient via the Right Internal Jugular Vein. Europace
  2019;21:1542.
\item
  Kiani S, Merchant FM, El-Chami MF. Extraction of a 4-year-old Leadless
  Pacemaker with a Tine-based Fixation. HeartRhythm Case Rep
  2019;5:424-425.
\end{enumerate}

\textbf{Figures:}

Figure 1: Telemetry-captured second-degree, Mobitz II AV block
(transient complete AV block shown). The patient had normal 1:1 AV
conduction before and following the block event.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

Figure 2A-C: A) Attempted mid-septal implantation: The DC cup extended
toward the outflow tract and pulmonary valve (black arrow). B) Apical
septal implantation: Contrast opacification seen at the septal RV apex
(black arrow). C) Anterior-posterior fluoroscopic projection
demonstrating the final implant position of the Micra AV device.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

Figure 3: Post-implant interrogation with assessment of accelerometer
signals: Sharp A1 and rounded A2 signals correspond to tricuspid valve
(A1) and pulmonary valve (A2) closure during ventricular contraction.
The A3 signal corresponds to passive ventricular filling. The A4 signal
corresponds to atrial contraction.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\selectlanguage{english}
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\end{document}

