What makes the S-ICD different
A traditional ICD relies on a lead — a long insulated wire — that runs from the device, through a vein under the collarbone, and into the heart. That lead is what makes the device powerful: it can sense beats directly, pace, and deliver anti-tachycardia pacing as well as shocks. But that lead is also where most of the long-term trouble with ICDs comes from. Leads can fracture, infect, and become very difficult to remove years later because they scar into the vein and heart wall.
The S-ICD was designed to solve that problem by keeping everything outside the bloodstream. The generator sits in a pocket on the left side of the chest, under the arm. A single lead is tunneled in a Z-shape under the skin — across to the breastbone, then up alongside it. When the device detects a dangerous rhythm, it delivers a shock between a coil on that lead and the generator itself, sending the energy across the chest and through the heart.
Nothing ever enters a vein or heart chamber.
The currently FDA-approved subcutaneous ICD system in the United States is the Boston Scientific EMBLEM MRI S-ICD.
The trade-off: no pacing, no ATP
The S-ICD’s biggest strength is also its biggest limitation. Because the lead never touches the heart, the device cannot reliably pace from outside the chest wall for any length of time — it can only deliver very brief external-style pacing immediately after a shock, and it cannot deliver anti-tachycardia pacing (ATP), the painless pacing burst that often stops VT without a shock.
That means the S-ICD is best for patients who:
- Need protection against sudden cardiac death.
- Do not need long-term pacing for slow heartbeats.
- Have a type of VT that doesn’t tend to respond to ATP, or have not had documented VT at all (most primary-prevention patients).
If you have known slow-rhythm problems, a high pacing need, or VT that has been treated successfully by ATP in the past, a transvenous or EV-ICD is usually the better fit.
Why we screen with an EKG first
Because the S-ICD watches the heart from the surface — not from inside it — the signal it sees is smaller and noisier than a wire’s view from within the heart. To make sure the device can reliably tell the difference between a normal beat and a dangerous one, we do a special screening EKG before implant. We record from several body positions (sitting, standing, sometimes during light exercise) and check that the signal is clean in at least one of the device’s possible sensing vectors. A small number of patients — often those with very unusual T-waves on their EKG — fail screening, and in those cases we recommend a transvenous or EV-ICD instead.
Who tends to do especially well with an S-ICD
- Younger patients with many decades of device-life ahead, where avoiding lead-related complications matters most.
- Patients with prior device infections — a clean, vein-free system reduces re-infection risk.
- Patients on dialysis or with limited venous access, where preserving veins is critical.
- Patients with congenital heart disease whose anatomy makes transvenous lead placement difficult.
What a shock feels like
Because the shock energy crosses more tissue (skin, muscle, then the heart) than a transvenous shock, S-ICD shocks tend to feel more substantial — most patients describe a hard thump or a kick to the chest. The duration is the same: a single, brief moment. Most appropriate shocks are followed by an immediate sense of feeling unwell for a few seconds, then relief that the rhythm is back to normal. If you receive a shock and feel okay afterward, call us within 24 hours. Multiple shocks or feeling unwell after a shock should prompt a 911 call.
Living with an S-ICD
The device is larger and more lateral than a transvenous ICD — some patients can feel a slight bulge on the side of the chest, particularly when lying on that side. Remote monitoring works the same way as with a transvenous system: data is sent from home automatically, and we usually see episodes before you would think to call.
Battery life is somewhat shorter than transvenous ICDs because the device works harder to sense and to shock from outside the heart. When the time comes, only the generator is replaced through the same incision — the lead is left in place if it’s still working well.
Manufacturer reference
For technical specifications, indications, and the latest official information on the EMBLEM MRI S-ICD system from its maker, see Boston Scientific’s product page: EMBLEM S-ICD System on bostonscientific.com. (External link — content there is Boston Scientific’s and may be technical.)