Next-generation ICDs: Individualised Selection for Improved Cardiac Outcomes

Cardiovascular disease remains a leading cause of mortality globally, accounting for approximately 30% of all deaths worldwide.¹ Among these, sudden cardiac death (SCD) constitutes 40–50% of cardiovascular-related fatalities, with nearly 80% resulting from ventricular tachyarrhythmia.¹ In the UK, an estimated 60,000 out-of-hospital cardiac arrests occur annually.^2,3 Alarmingly, approximately one individual under the age of 35 years of age dies from cardiac causes each day in the UK.⁴ Despite the advances in emergency response and the use of optimal medical therapies, survival rates following out-of-hospital cardiac arrests in the UK remain low, ranging from 2% to 12%.^5,6 Even among patients receiving state-of-the-art treatments, the recurrence rate of arrhythmias remains significant, with 40–50% experiencing a relapse within 5 years post-intervention.⁷ ICDs have been developed as a life-saving intervention, offering protection to individuals deemed at elevated risk of SCD.

Contemporary ICDs

Since the first human implant in 1980, ICDs have undergone significant advancements in multiple aspects, including implantation techniques, device positioning, lead technology, and overall size and functionality, particularly in arrhythmia detection and therapeutic response.⁸ These continuous innovations reflect a sustained commitment to improving cardiovascular care and enhancing the efficacy of interventions aimed at preventing sudden cardiac events (Figure 1 ).

Transvenous ICDs

The first clinical implantation of a transvenous ICD (TV-ICD) lead occurred in 1987, and commercial availability followed in 1993. This innovation enabled ICD implantation without the need for open-heart surgery, representing a significant simplification compared to earlier thoracotomy-based approaches. Over time, TV-ICD leads have undergone iterative improvements in materials, structural design and durability testing to mitigate complications such as lead failure and infection. The clinical efficacy of ICDs has since been firmly established, demonstrating superiority over medical therapy alone for both primary and secondary prevention of SCD.^9–12 Nonetheless, concerns persist regarding the long-term reliability and infection risk associated with TV-ICD leads. Notably, the annual incidence of lead-related complications requiring intervention increases with time with up to 20% of leads requiring attention by their 10th year.¹³ Reported lead survival rates are approximately 85% at five years and declined to 60% by eight years post-implantation.¹³

Subcutaneous ICDs

The advent of the entirely subcutaneous ICDs (S-ICDs) marked a significant advancement in device-based therapy for SCDs, offering an alternative to traditional TV-ICDs. Unlike TV-ICDs, the S-ICD system avoids transvenous access, thereby eliminating the risks associated with intravascular lead placement and direct endocardial contact. By remaining outside the heart and vascular system, the S-ICD reduces complications such as lead dislodgement, endocarditis and vascular damage.^14,15

The first fully subcutaneous ICD received Food and Drug Administration (FDA) approval in 2012.¹⁶ The size of an S-ICD is 60 cm³, which is roughly the size of a deck of cards. Prior to implantation, patients must undergo a screening process to determine anatomical and electrocardiographic eligibility. This involves recording 10-second surface ECGs using configurations that mimic the intended sensing vectors of the implanted device. Each vector is assessed independently, and eligibility is confirmed if at least one lead demonstrates stable and acceptable QRS morphology across different postures.

Implantation typically occurs in the left lateral thoracic region near the fifth to sixth intercostal spaces along the mid-axillary line. The device is placed in a pocket created through an inframammary incision, either over the fascia of the serratus anterior muscle, or between the serratus anterior and latissimus dorsi muscles, depending on patient anatomy. The pulse generator connects to a parasternal subcutaneous electrode used for both sensing and shock delivery. Implantation can be performed under general anaesthesia, monitored anaesthesia care or moderate sedation, most operators currently favour general anaesthesia.¹⁷

The S-ICD can deliver biphasic shocks up to 80 J and has a median estimated battery life of approximately 8.7 years.¹⁸

Extravascular ICDs

While the S-ICD offers a significant advantage over TV-ICDs by reducing complications related to intravascular leads, it has distinct limitations that impact its clinical versatility. One of the most prominent drawbacks is its lack of anti-tachycardia pacing (ATP) and long-term pacing support features that are often necessary for patients with frequent monomorphic ventricular tachycardias or bradyarrhythmias.^15,19,20 Additionally, the S-ICD is a high-energy system, requiring larger shocks (up to 80 J), which contributes to a bulkier device. This size, combined with the high energy demands, typically results in reduced patient comfort and shorter device longevity compared to transvenous systems.

To overcome these limitations and still avoid the risks associated with transvenous leads, the extravascular ICD (EV-ICD) was developed. Approved by the FDA in 2023, the EV-ICD offers an innovative middle ground. It supports ATP and post-shock pacing while eliminating the need for leads within the vasculature or heart chambers.²¹

The EV-ICD is comparable in size to traditional TV-ICDs, with a volume of approximately 33 cm³. It delivers shocks of up to 40 J, sufficient for terminating life-threatening arrhythmias while preserving device compactness. The system includes a pulse generator implanted subcutaneously over the serratus anterior muscle along the left midaxillary line, with the lead position in the substernal space.²¹

One of the key advantages of the EV-ICD is its improved battery performance, with an estimated average device longevity of 11.7 years.²²

Limitations of Contemporary ICDs

Implantation Procedure and Periprocedural Management

The implantation procedures for the various types of ICDs differ significantly in terms of anaesthetic requirements, procedural complexity, and postoperative care.

TV-ICD implantation is typically performed under local anaesthesia, often with light sedation. It is a well-established and relatively straightforward procedure that, on average, takes about 1 hour to complete. Due to the minimally invasive nature of the procedure and the routine use of venous access, most patients can be discharged on the same day, provided there are no complications.

S-ICD implantation can also be conducted under local anaesthesia with conscious sedation or under general anaesthesia, depending on the patient’s condition and the implanter’s preference. Like TV-ICD procedures, S-ICD implantation typically takes about one hour, and many patients can also be discharged the same day, although some centres may keep patients for overnight observation.¹⁹

In contrast, EV-ICD implantation is notably more complex. It is currently performed under general anaesthesia and is limited to cardiothoracic surgical centres due to the technical demands of substernal lead placement and the need for surgical expertise.²³ The EV-ICD procedure usually takes a few hours to complete, and the patients are generally admitted overnight for postoperative observation and pain management.

These differences in anaesthetic approach, procedure duration, and post-procedure management underscore the need to tailor device choice not only to the clinical characteristics of the patient but also to institutional capabilities and patient preferences.

Patient Characteristics

Due to the unique characteristics of contemporary ICDs, certain device types may be more suitable for specific patient populations and less ideal for others. The implantation of S-ICD or EV-ICD can be technically challenging in individuals with high BMI due to anatomical limitations. Conversely, patients with low BMI may find the size of the S-ICD device uncomfortable, which can limit its use in this group.

However, in certain scenarios, such as patients with a high risk of infection or those with difficult/prohibitive venous anatomy, S-ICD and EV-ICD may offer significant advantages over TV-ICDs. Conversely, prior analyses have demonstrated that S-ICDs are associated with a higher rate of inappropriate shocks in dialysis patients, which should be taken into consideration when selecting the most suitable device for this population.²⁴

The S-ICD and EV-ICD systems are also considered more favourable in younger patients, where the long-term risks associated with transvenous leads are a major concern.

Nevertheless, specific conditions may necessitate the use of transvenous systems. For example, in patients with hypertrophic obstructive cardiomyopathy, a dual-chamber ICD may be preferable. Such devices can reduce left ventricular outflow tract obstruction or facilitate medical therapy with β-blockers and/or verapamil (Class IIb, Level C recommendation).²⁵

The use of EV-ICDs is also limited in patients with prior sternotomy or other surgical or medical interventions that may have resulted in adhesions within the anterior mediastinal space, which could compromise substernal lead placement.²²

Pacing and Programming Capability

When a decision is made to implant a TV-ICD, the operator must choose between a single-chamber and a dual-chamber ICD. The primary indication for adding an atrial pace-sense lead in the atrium is the presence of sinus node dysfunction. Some studies have demonstrated that dual-chamber systems offer improved discrimination between supraventricular tachycardia and VT, resulting in fewer inappropriate therapies.²⁶ However, dual-chamber ICDs are associated with a higher rate of device-related complications.²⁷ Although pacing and defibrillator leads are generally durable, approximately 1–3% of patients require lead extraction secondary to endocarditis or lead failure within 10 years of implantation.²⁸ While earlier studies favoured dual-chamber devices for superior supraventricular tachycardia discrimination, more recent data suggest that single- and dual-chamber ICDs deliver comparable rates of inappropriate shocks when using contemporary built-in waveform discrimination algorithms.²⁹ Therefore, the prophylactic implantation of an atrial lead is strongly discouraged in the absence of a clear indication for atrial pacing.

The S-ICD, by design, offers only post-shock pacing and is thus not suitable for patients requiring bradycardia pacing, ATP, or CRT. To address this limitation, the MODULAR-ATP system has been developed, combining S-ICD with a leadless pacemaker capable of delivering ATP and ventricular pacing support. The MODULAR-ATP system has been proposed as a promising solution. In the MODULAR-ATP trial, ATP successfully terminated 61.3% of arrhythmic episodes at 6-month follow-up; however, long-term outcomes remain to be determined.³⁰ This system is not yet commercially available, and clinical trials are ongoing. Further, the S-ICD is currently limited in treating slower VT with a tachycardia cycle length below 170 bpm, as its programmable shock zone ranges from 170 to 250 bpm.

The EV-ICD incorporates pacing capabilities that include ATP, pause-prevention pacing at 40 bpm for up to 30 seconds, and post-shock pacing at the same rate. While this represents an advantage over the S-ICD, pacing thresholds for EV-ICD are higher than those typically seen in transvenous systems, which may negatively impact battery longevity in patients who require frequent pacing therapy.³¹ Hence, it is effective in managing unexpected asystole but is not appropriate for individuals requiring sustained pacing for bradycardia or resynchronisation therapy.^22,23 The EV-ICD demonstrated a 77% success rate for ATP. While TV-ICDs deliver pacing to endocardial tissue, EV-ICDs stimulate the heart from a substernal position, which may theoretically result in slight skeletal muscle stimulation. A small proportion of patients in the pivotal study (2.8%) had ATP therapy programmed “off” at two years due to pacing sensation discomfort experienced during in-clinic electrical testing; however, no patient who received successful ambulatory ATP subsequently had the therapy deactivated.³¹ Further research is required to optimise pacing parameters and to better define the tolerability and clinical efficacy of these systems. Another important area for further development is the integration of EV-ICD systems with pacing technologies. At present, clinical experience with combining EV-ICDs and leadless pacemakers, such as the Medtronic Micra system, remains limited. However, this hybrid approach could be particularly beneficial for patients who require both defibrillation therapy and bradycardia pacing. Ongoing technological advancements may enable more seamless device-to-device communication, thereby expanding therapeutic options and improving clinical outcomes.

Rate of Complications

According to the US National ICD Registry, from 2010 and 2011 the total adverse event rate associated with TV-ICDs was 2.2%, with an infection rate of 1.47%.³² In the CARAT prospective multi-centre international observational post-market study, the two-year rate of inappropriate shocks was reported at 1.9% for single-chamber and 2% in dual-chamber systems.³³ In contrast, the UNTOUCHED trial evaluating S-ICD demonstrated a complication-free rate of 92.7% at 18 months, with an infection-related device explantation rate of 1.1% and an inappropriate shock rate of 5%.³⁴ More recently, the pivotal study on EV-ICDs reported a major complication rate of 9.2% with 1.3% of patients requiring explantation due to infection. Notably, the rate of inappropriate therapy in the EV-ICD group was 17.5% at three-year follow-up and the long-term outcomes of the EV-ICD remain undetermined, as extended follow-up data are currently limited.³¹

The latest advancements in S-ICD and EV-ICD device technology and programming – such as newer device generations, the Smart Pass filter, and Smart Sense algorithm – were not available during earlier studies. Therefore, the reported rates of inappropriate shocks in those studies may not accurately reflect real-world experience with current devices.

An under-recognised complication of ICD implantation is phantom shocks, characterised by patient-reported sensations of shock in the absence of recorded device therapy. They are predominantly associated with psychological factors, such as anxiety, depression, post-traumatic stress disorder, and a prior history of true ICD shocks, rather than device malfunction. Reported prevalence ranges from 2% to 25% of ICD recipients.³⁵ Although not harmful from a device perspective, phantom shocks can substantially impair quality of life and increase healthcare usage. Management should therefore prioritise patient reassurance, education, and psychological support rather than device-based interventions.

Practical Workflow for Individualised Selection of Appropriate ICD

When a patient is being considered for ICD implantation, several clinical factors must guide device selection to ensure optimal outcomes. These include left ventricular ejection fraction, pacing requirements, history of VT and response to therapy, prior sternotomy, risk of infection, and vascular access. The following stepwise workflow provides a practical approach for individualised ICD selection (Figure 2 ):

Step 1: If the patient has an LVEF ≤35% and meets additional criteria for CRT, CRT with defibrillator (CRT-D) is the device of choice (Class I, Level A recommendation). Even if CRT criteria are not fully met, CRT-D is still indicated in patients who require ventricular pacing, such as those with high-degree atrioventricular block (Class I, Level A recommendation).

Step 2: For patients with chronic pacing requirements (e.g. complete or high-grade atrioventricular block), TV-ICD is the most appropriate option due to its pacing capabilities.

Step 3: In patients with a history of slower VT (<170 bpm) or VT previously terminated by ATP therapy, both TV-ICD and EV-ICD offer ATP functionality and are preferred over S-ICD.

Step 4: Patients with a previous sternotomy or potential anterior mediastinal adhesions may not be suitable for EV-ICD. In such cases, TV-ICD or S-ICD may be considered, provided the patient passes S-ICD screening.

Step 5: For patients with a high risk of infection, patients with previous endocarditis or device infection, patients with difficult or limited vascular access, cardiac transplant candidates or young patients, S-ICD or EV-ICD are preferable to avoid long-term complications of transvenous leads.³⁶

Qualification: In selected patients with hypertrophic obstructive cardiomyopathy, TV-ICDs may provide atrial or ventricular pacing that can help reduce left ventricular outflow tract gradient or facilitate β-blockers or verapamil therapy.

Conclusion

The transvenous single-chamber ICD remains the standard modality for ICD therapy, providing both ATP and bradycardia pacing through a single right ventricular lead. For patients with additional requirements for atrial pacing, the dual-chamber ICD serves as an appropriate alternative. In patients with heart failure and significant conduction delay, CRT-D devices have been shown to improve symptoms and survival outcomes. For patients who do not require pacing support, both S-ICD and EV-ICD offer protection from SCD while avoiding the long-term risks associated with transvenous leads. However, despite the existence of general guideline recommendations, the choice of ICD must be individualised and take into account the patient’s clinical condition, comorbidities, lifestyle factors and personal preferences, as well as the operator’s experience and institutional expertise. Ongoing advances in ICD technology – including leadless pacing, modular ATP systems, and extravascular lead designs – continue to broaden therapeutic options and facilitate personalised device selection to better align with individual patient needs.

Medical indications for ICD therapy continue to evolve, particularly with increasing attention focused on their role in patients with non-ischaemic cardiomyopathy. As new evidence emerges, clinical guideline recommendations may be refined, potentially resulting in either broader or more selective use of ICDs within different patient populations.