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.