Subcutaneous ICDs in Young Patients With Congenital Heart Disease

Subcutaneous implantable cardioverter-defibrillator technology has matured into a mainstream option for adults at risk of sudden cardiac death, but its role in pediatric and congenital heart disease populations continues to evolve. Avoiding transvenous leads removes a long-term source of venous thrombosis, endocarditis and lead fracture, which matters considerably when the recipient is expected to live for decades with the device.

Evidence in young patients with structural heart disease remains thinner than in adults with ischaemic or dilated cardiomyopathy, leaving clinicians to extrapolate selectively from adult cohorts. Australian centres such as the Royal Children's Hospital in Melbourne and the Queensland Children's Hospital in Brisbane manage some of the largest arrhythmia practices in the country and have begun contributing experience that clarifies where it fits within local care pathways.

Evolution and rationale for subcutaneous defibrillation

The original concept of an entirely extravascular defibrillator was driven by the complications of transvenous systems, particularly in young recipients who faced a lifetime of lead-related morbidity. As the technology progressed, refinements in biphasic waveform delivery, capacitor charge time and the automated screening tool made the device viable for a much broader range of body sizes and anatomies.

For patients with congenital heart disease the rationale is even stronger. Those who have undergone Fontan completion, repair of tetralogy of Fallot, or palliation for single-ventricle physiology frequently have obstructed or absent venous access on one side of the chest, and any repeat intervention through those vessels risks progressive occlusion. Placing the generator laterally and routing the parasternal lead along the sternum keeps the central venous circulation untouched, preserving access for future catheter procedures or pacing leads if they become necessary later in life.

Patient selection and pre-implant screening

Pre-implant vector screening is the single most important step before committing a young patient to subcutaneous defibrillation. Using the automated screening tool applied at the surface of the chest, clinicians test the candidate's R-wave amplitude, QRS morphology and T-wave ratios in supine and standing positions to predict whether the device will reliably distinguish ventricular tachycardia from background noise.

In children with congenital heart disease, additional factors influence suitability. Patients with right-sided chamber dilation, prior sternotomy or scoliosis may not meet the screening thresholds, and careful attention is needed in those with hypertrophic cardiomyopathy or with conduit-driven right bundle branch block. Published studies now available through the Journal of Arrhythmia archive outline how screening success rates in adolescents compare with adults and which anatomical patterns tend to predict failure.

Australian clinicians working within the framework of the Medical Benefits Schedule generally refer candidates through a multidisciplinary heart rhythm clinic that includes a paediatric electrophysiologist, a congenital cardiologist, an anaesthetist familiar with thoracic block techniques and a cardiac device nurse. This team structure helps standardise decisions and supports families navigating a non-trivial procedure.

Shared decision making is particularly important because the procedure is prophylactic and the family must weigh an intervention today against potential benefits decades into the future. Discussions commonly cover the risk of lead failure over a projected lifetime, the likelihood of needing additional cardiac surgery, and the small but real risk of device infection that accompanies any implanted hardware. Documented consent processes in Australian centres increasingly include age-appropriate materials for adolescents, who may themselves be old enough to participate meaningfully in the conversation.

Procedural technique and device selection

Implant technique in children and adolescents is broadly similar to adults, but several modifications deserve emphasis. The generator is usually placed in a submuscular or intermuscular pocket along the mid-axillary line rather than subcutaneously, which reduces pocket erosion in thin patients and provides better cosmesis once growth is complete. The parasternal lead is tunnelled to the xiphoid and then superiorly along the left parasternal border, and its distal tip sits near the manubriosternal junction.

Anaesthetic planning differs from adult practice. General anaesthesia is standard, and many centres now use ultrasound-guided serratus anterior plane or transversus thoracic muscle plane blocks to reduce opioid requirements. For smaller children, weight-adjusted defibrillation safety margin testing is sometimes performed at implant, although paediatric-specific thresholds remain a topic of active investigation.

Patient preparation also deserves attention. Pre-operative counselling usually includes a demonstration of the device, an explanation of how alerts and remote transmissions work, and a clear description of activity restrictions during the early healing phase. For school-aged patients, liaison with the school nurse or welfare officer helps ensure that any post-discharge event, including inappropriate shocks that warrant attention, is managed appropriately.

Device selection should also account for future growth. A generator that is appropriately sized for a ten-year-old may become marginal by late adolescence, and surgeons at the Women's and Children's Hospital in Adelaide have described staged pocket revisions when required. The latest generation of devices with extended longevity, remote monitoring capability and MRI conditional labelling also influences implant choice for younger recipients.

Outcomes, complications and inappropriate therapy

Real-world registries suggest that inappropriate shocks occur in roughly 8 to 12 percent of adult S-ICD recipients within the first three years, with the majority driven by T-wave oversensing or supraventricular arrhythmias above the discrimination zone. In paediatric and congenital cohorts the reported rates are more variable, partly because of small sample sizes and heterogeneous underlying anatomy.

Appropriate shock efficacy for ventricular fibrillation termination is high and comparable to transvenous systems, although defibrillation threshold testing is not routinely performed in children because of concerns about repeated high-energy discharges in young myocardium. Lead-related issues, including insulation breach and migration, appear less common than with transvenous leads but can be more challenging to manage because of the longer extraction path through soft tissue.

Longer term, registry data from European and North American cohorts suggest that appropriate therapy rates in young recipients with congenital heart disease mirror those seen in adults with ischaemic cardiomyopathy, particularly among patients with tetralogy of Fallot and those with systemic right ventricles. Careful programming of the conditional shock zone, dual-zone detection and post-shock pacing parameters has been shown to reduce inappropriate therapy by a meaningful margin.

Patient-reported outcomes, including cosmetic satisfaction and freedom from activity restriction, are consistently favourable. This is relevant in Australia where adolescent patients often expect to participate fully in school sport, surf lifesaving and recreational activities, and any device that limits those pursuits is poorly accepted.

Integration into Australian clinical practice

Regulatory and funding arrangements shape access. The Therapeutic Goods Administration has approved the current S-ICD platform, and listing on the Prostheses List enables private health insurers to cover the device within their hospital agreements. For public hospital patients, access depends on state-level health technology assessments, and there remains variability between jurisdictions in how quickly paediatric indications are adopted.

Families in regional and remote parts of the country face particular challenges. Children from the Northern Territory, western Queensland or far-west New South Wales typically travel to a tertiary centre in Sydney, Melbourne or Perth for implantation and follow-up, and the financial and emotional cost of that travel is significant. Embedding structured remote monitoring into post-implant care, supported by the Australasian Telehealth Society's guidelines, helps reduce some of this burden, although reimbursement for asynchronous review remains inconsistent.

Future directions include dedicated paediatric leads under development, algorithms to reduce inappropriate shocks in sinus tachycardia during exertion, and combined platforms that may integrate leadless pacing with defibrillation. Clinicians interested in following the evidence base as it expands can browse themed issues and ongoing calls for papers through the specialised journal collections section of the publisher's website, which collates manuscripts by topic and makes them easier to retrieve for busy practitioners.

Local investigators are also contributing to international registries that track S-ICD performance in congenital heart disease, and participation in these efforts offers Australian centres a chance to benchmark their own outcomes against global peers. As evidence matures, the conversation is likely to shift from whether the device works in this population to how its capabilities can be tailored to the specific physiology of each lesion.

For anyone engaged in this area, the conversation continues to develop rapidly. Practitioners and researchers are encouraged to review the latest peer-reviewed material, including related clinical reading that has been curated alongside other educational content, and to contribute case reports, registry analyses and review articles to the open-access literature so that the international community, including those caring for Australian children with congenital heart disease, can learn from each patient encounter.