T-Wave Oversensing In Subcutaneous Defibrillators

A subcutaneous implantable cardioverter-defibrillator (S-ICD) protects patients from sudden cardiac death without placing a transvenous lead inside the heart. Its sensing system, however, interprets surface-like electrical signals through electrodes positioned under the skin. When a T wave is detected as a second ventricular event, the device may classify the signal as rapid ventricular tachycardia or fibrillation and deliver an inappropriate shock.

T-wave oversensing is therefore a clinical and technical problem rather than a simple programming nuisance. Prevention depends on suitable screening, careful sensing-vector selection, recognition of changing electrocardiographic conditions, and prompt assessment after symptoms or therapy. For Australian electrophysiology teams, these decisions also sit within a health system that spans major metropolitan centres, regional services, device clinics, and the regulatory framework overseen by the Therapeutic Goods Administration (TGA).

How The Device Misreads Repolarisation

An S-ICD detects cardiac activity through subcutaneous electrodes rather than an intracardiac lead. The QRS complex is intended to represent ventricular depolarisation, while the T wave reflects repolarisation. If the T wave has sufficient amplitude, an unusual morphology, or a favourable relationship to the QRS, the sensing algorithm may count both signals as ventricular beats. The perceived rate can then cross the programmed detection zone.

This is often described as double counting. The risk is influenced by the ratio between T-wave and R-wave amplitude, QRS width, signal polarity, filtering, and the selected sensing vector. A patient may pass pre-implant screening in one posture but produce an unsafe signal during exercise, after a change in autonomic tone, or in another body position.

The electrocardiographic substrate can change over time. Ischaemia, myocardial infarction, ventricular hypertrophy, bundle branch block, electrolyte disturbance, cardiomyopathy, pacing-related changes, and antiarrhythmic therapy may alter repolarisation. Sinus tachycardia can further increase the chance that a broad or prominent T wave is interpreted as a second beat. These factors make a previously reliable sensing configuration vulnerable to later oversensing.

Patient And Device Factors

Pre-implant screening remains a central safeguard. Surface ECG screening is performed in several sensing vectors and, where appropriate, during different postures or at an elevated heart rate. The aim is to identify whether the QRS-T pattern remains suitable for the device algorithm, rather than relying on a single resting tracing. Screening should be revisited when the patient develops a new conduction abnormality, experiences a major change in ventricular function, or requires treatment likely to affect repolarisation.

Body habitus and generator position can affect signal quality. Subcutaneous fat, a deep or displaced generator, lead angulation, and inadequate electrode contact may reduce the R wave or distort the far-field electrogram. A small R wave makes the T wave appear proportionally larger, increasing the likelihood of double counting. Lead integrity problems, connector issues, or changes in the pocket can create noise that complicates interpretation, although noise and T-wave oversensing are distinct mechanisms.

Activity matters in everyday life. A patient who is stable while seated in a Sydney clinic may show a different electrogram while cycling, lifting, swimming, or walking briskly in hot weather. Postural change and exertion can alter heart rate, respiratory movement, myopotential interference, and the relative amplitude of cardiac signals. Device follow-up should therefore relate stored episodes to what the patient was doing at the time, rather than treating every inappropriate therapy as an isolated programming event.

The Australian market includes established S-ICD systems and locally available programmer software, but the precise sensing options depend on the model, firmware, and device generation. Clinicians should verify current manufacturer instructions and Australian product information rather than extrapolating settings from an older system or another jurisdiction.

Recognising Oversensing Before A Shock

A stored electrogram is essential for diagnosis. True ventricular arrhythmia generally shows a rapid rhythm with consistent ventricular morphology and an appropriate clinical context, whereas T-wave oversensing often displays alternating large and small sensed events or a pattern in which each QRS is followed by a similar repolarisation signal. The surface ECG, marker channels, symptoms, posture, exercise history, and device counters should be reviewed together.

Patients may report palpitations, a sudden warning tone, presyncope, or an inappropriate shock. Some have no warning because the device interprets the signal rapidly. Recurrent shocks require urgent clinical assessment, including exclusion of ventricular arrhythmia, myocardial ischaemia, electrolyte abnormalities, heart failure deterioration, and lead or generator complications. A magnet may temporarily inhibit shock therapy in selected emergency circumstances, but its use should follow local protocols and expert direction; it is not a substitute for device interrogation.

Remote monitoring can shorten the interval between an episode and specialist review. This is especially valuable for people living outside Melbourne, Sydney, Brisbane, Perth, or other tertiary centres, where travel to an electrophysiology service may be difficult. Telehealth and coordinated local hospital care can support triage, while an in-person assessment remains important when a shock has occurred or the electrogram is uncertain.

Documentation should identify the sensed vector, posture, rhythm rate, morphology, programmed zones, therapies delivered, and any recent illness or medication change. Recording the circumstances of an episode—such as exercise, fever, vomiting, diarrhoea, or a new prescription—can reveal a reversible trigger. Electrolyte correction and treatment of the underlying condition may resolve the problem without an invasive procedure.

Programming And Procedural Solutions

Modern S-ICD algorithms use filtering and morphology analysis to reduce inappropriate detection. A dynamic high-pass filter, such as a SMART Pass-type function in compatible systems, can reduce sensitivity to low-frequency T-wave components while preserving the faster QRS signal. Its status should be checked during follow-up, because changes in sensed amplitude or signal characteristics may affect whether the filter remains active or appropriate.

Reprogramming to a different sensing vector is often the simplest effective intervention. The primary, secondary, and alternate configurations capture different electrical projections, so one may provide a larger R wave and a more favourable QRS-T relationship. The chosen vector should be assessed in multiple postures and, when clinically justified, at an increased heart rate. Detection zones and conditional discrimination settings may also be reviewed, but raising a therapy threshold without understanding the rhythm can create an unacceptable risk of delayed treatment.

If oversensing persists, repositioning the lead or generator may improve signal quality. The procedural decision depends on imaging, pocket anatomy, lead position, body habitus, and the reason for the original problem. Lead revision is not a routine response to every inappropriate shock, since it carries infection, bleeding, anaesthetic, and procedural risks. A specialist team should first establish whether programming, medical treatment, or a correctable signal issue can solve the problem.

In some patients, persistent sensing limitations or a new need for bradycardia pacing, antitachycardia pacing, or cardiac resynchronisation may favour a change in defibrillator strategy. The choice between continued S-ICD therapy and a transvenous or alternative system must reflect the patient’s arrhythmic risk, venous access, infection risk, age, comorbidities, and anticipated pacing needs. Shared decision-making should include the implications of future procedures and device follow-up.

Follow-Up In Australian Practice

A structured follow-up pathway should combine routine interrogation with event-driven review. The first post-implant assessment confirms wound healing, sensing vectors, therapy settings, and patient education. Later checks should examine trends in sensed amplitude, stored episodes, filter status, battery performance, and any changes in the ECG. A new T-wave pattern on a 12-lead ECG deserves attention even when no shock has occurred.

Australian geography makes continuity particularly important. A patient in regional New South Wales, northern Queensland, or Western Australia may depend on a local cardiology service for initial assessment before a tertiary electrophysiologist reviews the recordings. Clear escalation pathways, compatible remote-monitoring arrangements, and access to an emergency department familiar with S-ICD care can prevent delays. Device identification cards and written instructions are useful when a patient presents to an unfamiliar hospital while travelling.

The regulatory setting also matters. S-ICD devices supplied in Australia are therapeutic goods subject to the Therapeutic Goods Act 1989 and inclusion requirements administered by the TGA. Clinicians should use the Australian-approved indications, safety notices, and manufacturer guidance relevant to the implanted model. When remote data are transmitted, services must also manage patient information in accordance with applicable Australian privacy obligations and local health-service policies.

For clinicians and trainees, peer-reviewed reports can clarify how sensing algorithms behave across different substrates and device generations. The journal’s special issue archive provides a focused route to themed material on electrophysiology, implantable devices, and related clinical questions. Reviewing such evidence alongside manufacturer advisories helps teams distinguish a recognised algorithm limitation from an individual patient’s changing physiology.

A practical patient pathway includes education to report shocks promptly, review of medications and intercurrent illness, assessment after vigorous activity or unexplained symptoms, and regular communication between implanting centres and community providers. Patients should understand that an S-ICD shock is clinically important even when they feel well afterwards. The event may represent oversensing, a treatable ventricular arrhythmia, or both.

Research Priorities And Clinical Learning

Future work should improve discrimination under real-world conditions rather than relying only on resting pre-implant ECGs. Exercise testing, posture-specific assessment, ambulatory recordings, and longitudinal analysis may identify patients whose QRS-T relationship changes outside the clinic. Machine-learning approaches could help classify far-field electrograms, although safety validation and transparent performance data remain essential before broad adoption.

Research is also needed in groups who may be under-represented in device trials, including people with obesity, congenital heart disease, inherited arrhythmia syndromes, advanced kidney disease, and diverse skin and body types. Australian registries could contribute valuable data from metropolitan and regional services, especially where long-distance follow-up and differing access to device expertise influence outcomes.

Clinical education should emphasise that inappropriate therapy is a signal to investigate, not merely an inconvenience to suppress. A careful review can reveal silent ischaemia, evolving cardiomyopathy, electrolyte disturbance, or a change in arrhythmic risk. The broader evidence base is accessible through curated arrhythmia collections, supporting continuing education for electrophysiologists, cardiologists, device nurses, and fellows.

T-wave oversensing can usually be approached systematically: confirm the electrogram, identify the mechanism, correct reversible factors, test alternative sensing configurations, and reassess the need for procedural intervention. That sequence protects patients from avoidable shocks while preserving the S-ICD’s principal purpose—reliable protection from life-threatening ventricular arrhythmia.

Explore the Journal of Arrhythmia’s clinical and research resources to strengthen S-ICD assessment, share device-management experience, and support safer electrophysiology practice across Australia and the wider Asia-Pacific region.