Subcutaneous ICDs in Children and Adolescents: Indications and Outcomes

Sudden cardiac death in the young, while uncommon, leaves an enduring imprint on families and clinical teams. Across Australian paediatric cardiology services, the recognition of inherited arrhythmia syndromes, congenital heart disease, and cardiomyopathies has steadily grown, and so has the demand for reliable, low-complication defibrillator protection. Implantable cardioverter-defibrillators remain the most effective therapy for terminating life-threatening ventricular arrhythmias, but the choice of system in a growing child carries distinctive trade-offs that adult practice does not always anticipate.

For decades, transvenous systems were the default, threading leads through the venous system to the right ventricle. In children, those leads must navigate small vessels, survive decades of growth, and withstand an active life that often includes sport. Lead-related complications — fracture, insulation failure, venous occlusion, and infective endocarditis — accumulate over time and are not trivial in a 10-year-old expected to live with the device for 70 years or more. The subcutaneous implantable cardioverter-defibrillator was developed precisely to address these long-term lead concerns by placing the electrode entirely outside the vascular space.

In Australia, paediatric electrophysiology is concentrated in a small number of tertiary centres, including the Royal Children's Hospital in Melbourne, The Children's Hospital at Westmead in Sydney, and the Queensland Children's Hospital in Brisbane. Decisions about device selection in these units are made within multidisciplinary teams that weigh anatomical factors, underlying diagnosis, and family preferences. The arrival of a fully extrathoracic system has, for many Australian clinicians, reframed the discussion about who should receive an ICD and at what age.

The remainder of this piece walks through the indications, implantation considerations, and reported outcomes for the subcutaneous defibrillator in children and adolescents, drawing on registry data, single-centre series, and the practical experience of Australian teams managing this population.

Why subcutaneous rather than transvenous in young patients

The anatomical argument is straightforward. Children are not small adults. Their subclavian and axillary veins are narrow, and as they grow, mechanical stress on a transvenous lead increases the risk of conductor fracture. Subcutaneous systems sit in the anterior chest wall, following a path along the sternum and lower left rib margin, and the lead is not subject to the same cyclic flexion as an intravascular conductor. This single feature underpins much of the appeal in paediatrics, where device longevity is measured in decades.

There is also the matter of systemic infection. Transvenous leads that become colonised usually require extraction, a procedure that carries meaningful risk in any patient and is especially hazardous in those with limited vascular access or complex congenital anatomy. By keeping hardware outside the vasculature, subcutaneous devices reduce the risk of bacteraemia and device-related endocarditis, although they do not eliminate pocket infection. For families and adolescent patients who have already endured multiple cardiac procedures, the prospect of avoiding long-term intravascular hardware carries psychological as well as clinical weight.

A further consideration is future imaging. Many young patients with inherited arrhythmia syndromes will, over a lifetime, require repeated cardiac magnetic resonance imaging. While modern transvenous ICDs from Australian suppliers listed on the Therapeutic Goods Administration register are largely MRI-conditional, the subcutaneous system was designed with MRI compatibility as a core feature, simplifying follow-up imaging pathways and reducing the need for repeated generator changes simply to access newer MRI-approved hardware.

Patient selection and indications in paediatrics

Indications in children broadly mirror those in adults: survivors of cardiac arrest without a reversible cause, patients with sustained ventricular tachycardia, and those with cardiomyopathy or inherited channelopathies deemed at high arrhythmic risk. The 2022 PACES expert consensus statement, widely used by Australian paediatric electrophysiologists, provides age- and weight-specific thresholds, generally accepting implantation in children weighing above 25 kg, though several centres have reported successful use in smaller patients.

Hypertrophic cardiomyopathy remains the most frequent substrate in Australian paediatric practice, followed by congenital long QT syndrome, catecholaminergic polymorphic ventricular tachycardia, and arrhythmogenic right ventricular cardiomyopathy. Dilated cardiomyopathy with severe left ventricular dysfunction is also a common referral indication, although many such children eventually receive a transvenous system capable of cardiac resynchronisation or antitachycardia pacing. Patient selection therefore hinges not only on arrhythmic risk but on whether pacing support is anticipated. Subcutaneous devices do not provide bradycardia support or ATP, which limits their use in those with concomitant conduction disease.

The conversation with families is rarely straightforward. Paediatric electrophysiologists at Westmead and the Royal Children's describe lengthy counselling sessions that address generator visibility in a developing body, the likelihood of appropriate and inappropriate shocks, and the implications for sport. Discussions also cover the small but important risk of needing a second procedure should the patient later require pacing, at which point a transvenous system may need to be added or the subcutaneous device converted.

Implantation technique considerations in small patients

The standard subcutaneous implantation technique uses three incisions — at the xiphoid, at the manubrium or upper sternum, and at the lateral pocket for the generator. In a small child, the distance between these points is reduced, and the lead may need to be trimmed or tunnelled more carefully to avoid redundancy. Most Australian implanters use the two-incision technique, omitting the superior parasternal incision when anatomy permits, to minimise visible scarring in young patients.

Generator positioning is also adjusted. The conventional left lateral position may overhang the ribcage or sit awkwardly in slim adolescents, and many implanters now favour a more anterior or even inframammary placement in females. The goal is to avoid erosion, particularly in patients with limited subcutaneous tissue, and to keep the device beneath clothing. Pocketing the generator over the lower ribs rather than the lateral chest wall can also reduce the risk of discomfort when wearing a school bag strap or playing contact sport.

Screening for suitable candidates requires a pre-implant surface ECG to confirm a stable, reliably sensed QRS, as well as a screening chest measurement to ensure the lead-to-can vector is feasible. The screening tool used by manufacturers estimates a suitable sensing vector in roughly 90 per cent of adult candidates; paediatric series have reported similar success rates, with the small proportion failing screening usually redirected to transvenous systems.

Acute and short-term outcomes from registries

Pooled data from the international S-ICD registries and from national datasets, including a small but growing Australian contribution via ANZCTR-listed studies, suggest a peri-procedural complication rate comparable to that of transvenous devices. Infection rates sit around 1–2 per cent, with pocket haematoma and lead dislodgement the most commonly reported early events. Inappropriate shocks, driven largely by T-wave oversensing, are more frequent in the first year, particularly in children with hypertrophic cardiomyopathy or those with hypertrophied R waves.

Conversion efficacy for ventricular fibrillation at implant testing is excellent, with reported first-shock success above 95 per cent in paediatric cohorts. Energy requirements in children are sometimes lower than in adults because of smaller thoracic volumes, and most devices are programmed to standard outputs with discretion to reduce if appropriate. Australian implanters tend to follow the conservative 80-joule standard for the first shock, escalating as required, mirroring adult practice.

Hospital length of stay is typically overnight, and children return to school within one to two weeks. Activity restrictions are usually limited to the first month, after which most activities, including non-competitive sport, are permitted. Paediatric teams in Melbourne and Sydney report strong family satisfaction with the cosmetic and psychological outcomes, even when a shock has occurred.

Long-term performance and device longevity

Long-term data in children remain limited, as the technology has only been widely used in patients under 18 since around 2015. Adult data suggest a generator longevity of roughly seven to nine years, with longer intervals achievable depending on shock frequency and pacing requirements — the latter being absent in subcutaneous systems, which favours battery preservation. In children, this translates into fewer generator changes across a lifetime, an important consideration given that each replacement carries its own pocket-related risks.

Lead performance over a decade appears favourable, with very low rates of conductor fracture. The lead is, however, not immune to mechanical stress, and there are case reports of fracture at the xiphoid anchoring suture in active adolescents. The use of two anchoring sutures, now standard practice at several Australian centres, appears to mitigate this risk. Device-related infection rates after the first year remain low, although they are not zero, and families are counselled to seek early review for any unexplained fever or pocket erythema.

Inappropriate shocks continue to be the most common late adverse event, particularly in younger children and in those with structural heart disease. Dual-zone programming, careful attention to the sensing vector at implant, and the use of conditional zone discrimination have all contributed to reducing these events. For families and patients, the avoidance of shocks — appropriate or otherwise — remains a major determinant of quality of life, and ongoing refinements in detection algorithms are an area of active research.

Special considerations: sport, growth, and MRI access

Sport is central to the lives of most Australian children, and the question of returning to physical activity is among the first asked by families. Subcutaneous devices are broadly compatible with non-contact sport, and most paediatric electrophysiologists clear patients for activities such as swimming, netball, and cricket after a brief recovery. The position of the generator, however, can be a limiting factor in contact sports, and many clinicians advise against collision activities such as rugby league or Australian rules football, or recommend protective padding where participation is non-negotiable.

Growth is a more subtle consideration. As the chest wall expands during adolescence, the lead may become relatively tighter, and the vector between lead and generator can shift. Annual review with chest measurement and intermittent device interrogation is standard in Australian paediatric clinics, with device reprogramming or, rarely, lead revision considered if sensing becomes unreliable. The lack of intravascular hardware means that growth does not pose the same risk of lead tension or venous stenosis that is seen with transvenous systems.

MRI access, increasingly important for surveillance of cardiomyopathy and congenital heart disease, is well supported by current-generation subcutaneous systems. For Australian patients referred for cardiac MRI, the lack of need for specific reprogramming before and after the scan simplifies the pathway and reduces hospital visits. In the broader context of arrhythmia management, AI ECG interpretation is also beginning to inform earlier detection of subclinical disease in young relatives of probands, potentially shortening the path to defibrillator consideration.

The Australian and regional experience

Australian paediatric centres have contributed to the international literature, though the absolute numbers remain modest. Case series from Melbourne and Sydney have consistently shown feasibility in patients above 25 kg, with low complication rates and high family satisfaction. Multidisciplinary clinics, often linked to inherited cardiac conditions services and supported by charities such as HeartKids, provide a coordinated pathway from genetic testing and family screening through to device implantation and long-term follow-up.

Funding for subcutaneous devices in children has, at times, been a point of negotiation with private health insurers and, in public patients, with state-based procurement. The Therapeutic Goods Administration has approved the current generation of devices for paediatric use, and Medicare rebates apply under defined indications, though the gap between cost and reimbursement continues to influence device choice in some centres. For patients with concurrent conditions such as atrial fibrillation, broader management — including the role of weight loss outcomes in reducing AF recurrence — is increasingly part of the holistic cardiovascular care offered to adolescents transitioning to adult services.

Looking ahead, the priorities for the field are clear: longer-term follow-up to confirm lead durability over decades, refinement of screening and detection algorithms to reduce inappropriate shocks, and equitable access across both public and private systems. For families weighing the options, the conversation begins with a careful assessment of the underlying arrhythmic risk, the likelihood of future pacing needs, and the child's life ahead, and ends with a shared decision grounded in the best available evidence.

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