A standard ICD uses leads that travel through a vein into the heart chambers, where they sense the heart’s rhythm and deliver shocks if needed. A subcutaneous ICD (S-ICD) does not enter the blood vessels or the heart at all. Instead, the lead is tunnelled just under the skin, running alongside the breastbone, and the device generator is implanted under the skin on the left side of the chest, below the armpit.
Because there are no leads inside the heart or blood vessels, an S-ICD avoids several risks associated with transvenous leads: lead fracture within the bloodstream, damage to heart valves, and bloodstream infections (endocarditis). It is also generally easier to remove if the device ever needs replacing or explanting.
The most important limitation of an S-ICD is that it cannot deliver ongoing pacing therapy or anti-tachycardia pacing (ATP). It can only detect a dangerous rhythm and deliver a shock. If you need continuous pacing — because your heart rate is too slow, or because ATP is part of your arrhythmia management — an S-ICD would not be appropriate on its own. This distinction is assessed before a device is chosen.