A subcutaneous implantable cardioverter defibrillator (S-ICD or SICD) is a device that monitors heart rhythm and delivers a shock to correct life-threatening arrhythmias, without placing any leads (wires) inside the heart or blood vessels. All components sit beneath the skin: the generator is implanted under the skin at the left side of the chest, and a sensing lead runs just under the skin along the breastbone.
A conventional transvenous ICD has leads that run through a vein into the heart chambers. The S-ICD avoids this entirely: because no hardware enters the cardiovascular system, the risk of bloodstream infection and lead-related complications is significantly lower. The implant procedure is also generally quicker and does not require X-ray guidance of leads inside the heart.
The S-ICD is particularly suitable for people who need protection against ventricular fibrillation or fast ventricular tachycardia but do not need pacing. This includes younger patients who may need a device for many decades, people with small or difficult veins, and those who have had previous device infections. It is not appropriate where regular pacing or anti-tachycardia pacing (ATP) is required, as the S-ICD cannot deliver these therapies.
The device continuously analyses signals from the subcutaneous lead using algorithms to distinguish dangerous rhythms from normal ones. When VF or fast VT is detected, the S-ICD charges and delivers an 80-joule shock. Battery life is typically eight to eleven years. The device is checked at regular clinic appointments, with remote interrogation available, and the generator is replaced when the battery nears depletion.
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