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).1 A conventional
femoral venous approach has generally been taken, though all patients
were >11 years of age and >30 kg in
weight.1-3 Alternatively, Gallotti et al and Cortez
presented separate cases using right internal jugular venous access in
smaller patients.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.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.