The Subcutaneous Defibrillator: A Review of the Literature
The following are 10 important points to remember about the subcutaneous implantable cardioverter-defibrillator (ICD):
1. The subcutaneous ICD (S-ICD) is comprised of a subcutaneous pulse generator and a single subcutaneous electrode containing both sensing and defibrillating components. The generator sits over the fifth intercostal space between the mid- and anterior-axillary lines; the lead lies parallel to the left side of the sternum, extending from the sternal notch to the xiphoid process.
2. Seven clinical trials enrolling a total 745 patients have proven S-ICD to be effective in detecting and treating ventricular fibrillation and ventricular tachycardia (VT).
3. The potential advantages of S-ICD include elimination of complications related to venous access, no physical stress on leads associated with cardiac motion, less morbidity associated with device extraction, and a potential reduction in endovascular infection risk to patients with dialysis access or endovascular prostheses.
4. The S-ICD system delivers energy to the heart in a more homogenously distributed pattern as compared to the endocardial shock delivered by the transvenous ICD (T-ICD). The uneven distribution of energy across the myocardium after an endocardial shock from T-ICD can produce voltage gradients and electroporation, resulting in myocardial stunning and damage. Whether this translates into a survival benefit remains to be determined.
5. The limitations of the S-ICD include its inability to provide antitachycardia pacing (ATP) for VT, the relatively large size of the pulse generator, and the absence of endovascular monitoring capabilities for heart failure. Since 80% of spontaneous VT episodes respond to painless ATP, patients with prior history of VT are likely to benefit more from a traditional ICD.
6. The rate of inappropriate shocks in the seven clinical trials of S-ICDs ranged from 5% to 16%, mostly due to T-wave and myopotential oversensing. The recent trials of traditional ICDs show that newer algorithms reduce the rate of inappropriate shocks to <5%.
7. The rate of infection with S-ICD ranges from 5.6% to 10% of implants. The rate of pocket infection with the S-ICD exceeds that with the T-ICD, which is approximately 3%. The three incisions required for S-ICD implant provide a greater probability for bacterial entry.
8. The longevity of the S-ICD battery is estimated at 5 years, compared with the most recently introduced single-lead T-ICD that may exceed 10 years.
9. The S-ICD system in its current version lacks remote monitoring capability, a feature that improves patient outcomes and simplifies follow-up.
10. There are no data on long-term performance of S-ICD. No study to date directly compared the T-ICD and S-ICD.
Keywords: Follow-Up Studies, Morbidity, Ventricular Fibrillation, Electric Countershock, Myocardial Stunning, Electroporation, Equipment Failure, Prostheses and Implants, Renal Dialysis, Sternum, Tachycardia, Ventricular, Cardiac Pacing, Artificial, Endocardium, Heart Failure, Xiphoid Bone, Defibrillators, Implantable
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