Leadless Defibrillator: S-ICD, EV-ICD, and Modular Systems

A fully leadless defibrillator, in the sense of a standalone implant that can both pace and shock without any wires, does not yet exist as a single commercially available device. What does exist are systems designed to keep electrical leads out of the heart’s veins and chambers, eliminating many of the complications that come with threading wires through the bloodstream. The subcutaneous implantable cardioverter-defibrillator (S-ICD), the newer extravascular ICD (EV-ICD) with a lead placed behind the breastbone, and a modular system that wirelessly pairs a leadless pacemaker inside the heart with an S-ICD outside it represent the current landscape. Each takes a different approach to solving the same problem, and none is a perfect replacement for a traditional transvenous ICD in every patient.

Why Getting Leads Out of the Bloodstream Matters

Conventional transvenous ICDs have been saving lives since the 1980s. A wire is fed through a vein into the right side of the heart, where it can sense dangerous rhythms and deliver shocks directly to the heart muscle. The approach works well, but those intravascular leads carry real risks. They can fracture, erode through vessel walls, develop insulation breaks, or serve as a highway for bacteria to reach the heart itself. A secondary analysis of the PRAETORIAN trial found that when device infections occurred in the transvenous ICD group, over 60% were systemic, meaning the infection had spread beyond the pocket into the bloodstream. Two of those patients died within a year. In the subcutaneous ICD group, all four infections that occurred stayed local, none became systemic, and none were fatal.¹ That difference captures the core motivation behind lead-free designs: keeping hardware out of the veins dramatically changes the infection profile.

Lead removal, when it becomes necessary, is itself a high-risk procedure. Scar tissue grows around leads over time, bonding them to vessel walls and heart tissue. Extracting them can tear a vein or puncture the heart. A device that never puts a lead inside the vasculature in the first place sidesteps that entire problem.

The Subcutaneous ICD

The S-ICD is the most mature of the lead-avoidance technologies. The generator sits along the left side of the ribcage, and a single electrode runs under the skin alongside the breastbone. No part of the system enters a vein or touches the heart directly. It senses the heart’s rhythm through subcutaneous signals and delivers a shock through the chest wall when it detects a life-threatening arrhythmia.

In the landmark PRAETORIAN trial, the S-ICD proved noninferior to the traditional transvenous ICD. Over a median follow-up of about four years, roughly 15% of patients in each group experienced a primary endpoint event, with a hazard ratio of 0.99, meaning the two technologies performed essentially identically in preventing device-related complications and inappropriate shocks combined.² That result was a turning point for the field: it showed that you could get equivalent protection without ever entering the bloodstream.

The tradeoff is that the S-ICD cannot pace the heart. Traditional transvenous ICDs can deliver antitachycardia pacing, a technique where the device fires a rapid series of small electrical pulses to interrupt a dangerous rhythm before it escalates, often sparing the patient a painful shock. They can also provide backup pacing if the heart rate drops too low. The S-ICD can do neither. For patients who need pacing, whether for slow heart rates or to terminate fast rhythms without a shock, the S-ICD alone is not enough.

The Extravascular ICD

The extravascular ICD was designed to bridge the gap between the S-ICD and the transvenous ICD. Instead of placing a lead under the skin or inside a vein, the EV-ICD threads a single electrode behind the breastbone, into the space between the sternum and the heart’s outer lining. That location is close enough to the heart to allow both pacing and defibrillation, but the lead never enters the bloodstream.

Preclinical modeling showed that a substernal electrode needed only about 22% more energy to defibrillate than a traditional intracardiac lead, compared with roughly double the energy required by a subcutaneous lead.³ In practical terms, that means the EV-ICD can defibrillate at energies much closer to what transvenous systems use, while the S-ICD needs to push harder through more tissue.

The pivotal clinical trial of the EV-ICD showed encouraging results. Among patients who had spontaneous arrhythmias during follow-up, antitachycardia pacing (ATP) successfully terminated the dangerous rhythm in about 77% of episodes, and the use of ATP increased significantly over time as clinicians gained experience programming the device.⁴ That pacing capability is the EV-ICD’s central advantage over the S-ICD. A patient whose ventricular tachycardia can be painlessly paced away rather than shocked has a meaningfully better day.

Early feasibility work on substernal pacing showed that roughly 69% of patients had successful ventricular capture in at least one tested configuration, though failed capture was sometimes linked to suboptimal catheter placement or air trapped in the space during the procedure.⁵ The technology has matured since those early studies, but substernal lead placement remains technically more involved than a subcutaneous approach.

The Modular System Approach

Perhaps the most ambitious concept in the leadless defibrillator space is the modular cardiac rhythm management system. The idea is elegant: take a tiny leadless pacemaker implanted directly inside the heart and a subcutaneous ICD outside it, and have the two devices talk to each other wirelessly. The pacemaker handles antitachycardia pacing and bradycardia support, while the S-ICD handles high-energy shocks. Together, they provide full ICD functionality without a single transvenous lead.⁶

A 2024 study in the New England Journal of Medicine reported results from a trial of this communicating system. Wireless communication between the leadless pacemaker and the S-ICD succeeded in nearly 99% of communication tests. Over 97% of patients had acceptable pacing thresholds. When antitachycardia pacing was needed, it terminated the arrhythmia in about 61% of episodes, and no episodes were missed because of a communication failure between the two devices.⁷ Those numbers are promising, though the ATP success rate is lower than what the EV-ICD achieved in its pivotal trial. The modular system is still relatively early in its clinical validation, and programming strategies will likely evolve.

Inappropriate Shocks and Oversensing

One of the most feared complications for any ICD patient is the inappropriate shock: a painful, jarring jolt delivered when the heart is actually fine. Because the S-ICD reads electrical signals from outside the heart, it has to work harder to distinguish a genuine arrhythmia from noise. Two main sources of confusion plague these devices.

The first is T-wave oversensing, where the device mistakes the T-wave (a normal part of every heartbeat) for an extra heartbeat and concludes the heart rate is dangerously fast. The second is myopotential oversensing, where electrical signals from skeletal muscles, especially the chest and abdominal muscles, fool the device. In one systematic study, muscle-related electrical interference could be provoked in about 90% of S-ICD patients through maneuvers like abdominal clenching or push-ups.⁸ Whether that noise actually triggers a shock depends on the device’s filtering algorithms, but the vulnerability is clearly there.

A Japanese study found that inappropriate shocks occurred in about 10% of S-ICD patients, and myopotential oversensing was the most common cause, ahead of T-wave issues and atrial fibrillation.⁹ A European analysis categorized inappropriate shocks into three patterns: high-frequency oversensing was the most common, followed by T-wave oversensing, and QRS undersensing that paradoxically triggered the device by making it amplify everything too aggressively.¹⁰

An analysis of FDA-reported adverse events over a two-year period, during which an estimated 15,000 S-ICDs were in service, counted 550 inappropriate shocks, with roughly 69% attributed to oversensing. The same dataset recorded 542 infection reports and 137 cases of system migration, where the device or lead shifted from its implanted position.¹¹ These numbers represent reported events, not rates per patient, but they underscore that the S-ICD is not complication-free despite avoiding intravascular leads.

Patient Screening Before Implantation

Not every patient can receive a subcutaneous or extravascular ICD. Because these systems read the heart’s electrical activity from a distance, the surface ECG pattern has to meet certain criteria. If a patient’s T-waves are unusually tall relative to their QRS complexes, the device may chronically confuse T-waves for heartbeats. A screening ECG is performed before implantation to check that at least one sensing configuration will work reliably.

Research has shown that routine preoperative screening does a solid job identifying good candidates. In one study, all patients who passed the ECG screening had at least one usable sensing vector after the device was implanted, and most had the same number or more vectors available than the screening had predicted.¹²

Screening becomes more complex for patients who already have a leadless pacemaker. One study found that while about 94% of patients with leadless pacemakers passed the S-ICD screening during their own intrinsic heart rhythm, only about 24% passed when the pacemaker was actively pacing.¹³ The paced rhythm changes the shape of the electrical signal enough to confuse the S-ICD’s sensing algorithms. This is a real limitation for the modular system concept: the very patients who need both pacing and defibrillation may be the hardest to screen for S-ICD compatibility.

Switching From a Transvenous to a Subcutaneous System

A common clinical scenario involves patients who already have a transvenous ICD but need it removed because of infection or lead failure. Can they safely receive an S-ICD as a replacement? The evidence says yes. One study found that the rate of major infection after S-ICD implantation was low across all groups, including patients whose transvenous systems had been removed specifically because of infection.¹⁴ A separate analysis of patients who underwent transvenous lead extraction followed by S-ICD implantation found that about 8% received appropriate therapy from their new device during follow-up, while complications including inappropriate shocks, pocket erosion, and ineffective therapy occurred in about 19% of patients.¹⁵ Those complication rates are somewhat higher than what is seen in first-time S-ICD implants, which makes sense given that these patients have already had one device fail or become infected.

Children and Patients With Congenital Heart Disease

Younger patients and those with congenital heart defects stand to benefit enormously from avoiding transvenous leads. A child who receives a lead at age 10 may need that lead to function for decades, and the longer a lead sits inside a growing body, the higher the chance of failure or the need for a risky extraction. A multicenter review by the Pediatric and Congenital Electrophysiology Society found that the S-ICD had comparable complication rates, inappropriate shock rates, and conversion success to what has been reported for transvenous systems in similar populations. Appropriate therapy was delivered in about 11% of patients, with first-shock conversion succeeding in about 93% of those cases. Inappropriate shocks occurred in roughly 16% of patients.¹⁶

A European registry focusing on younger patients with congenital heart defects reported no defibrillation failures, with 17% receiving appropriate shocks and 13% receiving inappropriate ones. Complications requiring surgical revision occurred in about 9% of patients, with higher risk in those with a body mass index under 20 or those who had undergone the older three-incision implant technique.¹⁷ For thinner patients, the subcutaneous hardware can sit closer to the skin surface, making erosion and discomfort more likely. Reprogramming and appropriate drug therapy eliminated further inappropriate shocks in the patients who experienced them.

Quality of Life and Anxiety

Getting an ICD is psychologically complicated. The device exists because you have a heart condition that could kill you without it, and it protects you by delivering a shock that patients often describe as being kicked in the chest. That psychological weight shows up as measurable anxiety and depression in many ICD recipients. Whether the lead-free design changes that experience matters for real people living with these devices.

A study comparing quality of life, depression, and anxiety between S-ICD and transvenous ICD patients found that physical and mental quality of life and depression levels were similar in both groups. But S-ICD patients reported significantly lower anxiety and a greater reduction in depressive symptoms over the first year after implant.¹⁸ The reasons are speculative, but knowing that no wire threads through your veins into your heart may offer some reassurance. Both groups improved most in the first three months after implantation, which likely reflects the psychological adjustment of simply having the device in place and functioning.

MRI Compatibility

MRI scans are a frequent concern for any patient with implanted metal. The S-ICD is now conditionally approved for MRI at 1.5 Tesla under specific conditions, but what actually happens to image quality matters as much as whether the scan is safe. Studies have found that brain, knee, lumbar spine, and cervical spine scans are unaffected by the S-ICD. Cardiac MRI is a different story: the device generator creates an artifact that interferes with imaging parts of the left ventricle, though the right ventricle can still be assessed.¹⁹ The artifact comes from the metal generator housing itself, not the electrode.

A more detailed cardiac MRI study found that standard cine images were impaired in the left ventricular anterior, lateral, and inferior wall segments, but switching to a different imaging sequence recovered usable images in about 74% of patients. Late gadolinium enhancement imaging, used to detect scar tissue in the heart, was also affected but could be improved with a wideband technique in about 65% of cases. Aortic flow measurements and 3D angiography were unaffected. Perfusion imaging, however, was largely unusable in at least half of patients, and about 52% permanently lost their device’s audible beeper after the scan.²⁰ That beeper loss is a minor but real nuisance: the audible alert is how the device tells the patient something needs attention.

Lead Fracture and Hardware Recalls

Avoiding intravascular leads does not mean avoiding all lead problems. In February 2021, the FDA issued a Class I recall, its most serious category, after 27 reports of lead fracture in one S-ICD model. All fractures occurred at the same spot: just past the proximal sensing ring, where a manufacturing notch in the lead created a structural weak point. The reported fracture rate was about 0.2% at 41 months, with a risk of life-threatening harm estimated at 0.004% over 10 years.²¹ Those are low numbers, but for a device whose entire job is to save your life during a cardiac arrest, any failure mode that could disable sensing or shocking capability is taken seriously.

The S-ICD lead sits in a mechanically active environment: it runs along the sternum, where breathing, coughing, and body movement flex it continuously. Lead positioning during implantation matters. The electrode should be placed as deep as possible, close to the sternum and the pectoralis fascia, with subcutaneous tissue and fat sitting above it. Keeping it close to bone reduces the flexion stress and optimizes the electrical pathway for shock delivery.²²

Where the Technology Is Headed

The trajectory is clear: the field is moving away from intravascular leads. Each generation of lead-avoidance technology addresses a limitation of the previous one. The S-ICD eliminated vascular access but could not pace. The EV-ICD restored pacing by finding a middle ground anatomically, getting close to the heart without entering the bloodstream. The modular system takes the most radical approach, combining two separate implants that coordinate wirelessly, and early data show it works, though the device-to-device communication adds a layer of complexity that will need long-term validation.

Battery life, device size, and the ability to deliver cardiac resynchronization therapy (pacing both ventricles to improve heart-pump efficiency) remain open challenges. Current S-ICD generators are larger than transvenous ICD generators because they need to push more energy through tissue, which can be an issue in smaller or thinner patients. Whether the EV-ICD’s substernal position will prove durable over 10 to 15 years of continuous flexion against the back of the breastbone is something only time will answer. And the modular system’s reliance on wireless communication between two battery-powered devices raises questions about what happens when one device reaches end of life before the other. These are engineering problems, not fundamental barriers, but they will shape which patients receive which technology for years to come.