Subcutaneous ICDs

Much of the morbidity associated with the present ICD systems is related to the presence of transvenous defibrillation and sensing leads. An ICD that uses sensing and shock electrodes only in the subcutaneous (or submuscular) tissue would eliminate specific problems associated with transvenous leads. Further, such an ICD might be implanted without fluoroscopy in any surgical suite, thus simplifying the implant procedure. Early studies indicate that defibrillation can be achieved in pediatric patients80-83 and the majority of adults84'85 using subcutaneous electrodes with shock strengths that are in the range of present technology. To date, this has been achieved by using high-capacitance waveforms with maximum voltages near 800 V, although higher voltage/lower capacitance waveforms would likely be more efficient for high-resistance defibrillation pathways. Subcutaneous ICDs must rely on subcutaneous EGMs for sensing and detection of VF. In one version, correlation waveform analysis, using two different channels of subcutaneous EGMs, is used to detect fast VT/VF and avoid inappropriate detection of myopotential noise and electromagnetic interference.86 There are no published data on sensing and detection performance of algorithms in subcutaneous ICDs under investigation, but there is a substantial literature on subcutaneous EGMs based on implantable loop recorders.87'88

It is too early to know what fraction of patients will be defibrillated reliably by subcutaneous ICDs or how well long-term sensing and detection will perform using subcutaneous electrodes. Such ICDs also have important inherent limitations: They cannot perform bradycardia pacing or antitachycardia pacing. In contrast to transvenous ICD, which treat more than 70% of ventricular tachyarrhythmias with painless pacing, all therapies for subcutaneous ICDs are shocks. The reliability with which VF can be induced to test defibrillation efficacy is unknown. If induction of VF requires a temporary transvenous electrode, fluoroscopy will be required for the implant.

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