EP Review: The Subcutaneous ICD

Since FDA approval in 1985, traditional ICDs have become the standard of care for treating sudden cardiac arrest and have prevented many premature deaths. 

However, in the United States, sudden cardiac death still accounts for 300,000 to 400,000 deaths annually.¹ Additionally, sudden cardiac death is responsible for approximately 50% of the mortality from cardiovascular disease, both in the United States and in other developed countries.¹ Early clinical research determined that cause of death for many sudden death patients was the onset of ventricular fibrillation. Furthermore, patients with previous myocardial infarction (MI) and left ventricular dysfunction who have non-sustained ventricular tachycardia were found to have a two-year mortality rate in the range of 30 percent.^2-4 The Multicenter Automatic Defibrillator Implantation Trial⁵ (MADIT) results, published in 1996, demonstrated that prophylactic therapy with an ICD, compared with conventional medical therapy, improved patient survival in this high-risk patient group. The following year, the ICD was shown to be superior to antiarrhythmic drug therapy (predominantly amiodarone) for treatment of ventricular fibrillation in the AVID Trial⁶ in this population of patients. 

In 2002, MADIT II⁷ demonstrated a significant reduction in all-cause patient mortality when ICD therapy was prophylactically implanted in patients with prior MI and advanced left ventricular dysfunction. 

Despite the strong ICD clinical trial evidence of these studies and the high risk of sudden cardiac death in patients with a prior MI and reduced LV function, questions have been raised about the number of potentially eligible patients not receiving ICD therapy. One study reviewed hospital claims and clinical data for an eight-month window during 2007 at a tertiary care center.⁸ Although reviewers found the rate of ICD underuse may be “substantially lower than previous estimates”, they noted that 17% of patients in this data review refused the device. Additionally, another recent study analyzing how use of a screening tool might improve patient referral to an EP for ICD implant found that only 41% of patients accepted referral.⁹ The authors commented that “another barrier to [ICD] use may be patient willingness to undergo evaluation, demonstrated by the number of patients declining referral. Possible explanations for this finding may include misunderstanding of the technologically advanced intervention, media coverage of some of the recalls associated with ICDs, or fear of defibrillation, among others.”⁹ 

One factor contributing to these refusals may also be patient concern regarding transvenous defibrillation lead failures, including those failures highlighted by the press during recent lead recalls or advisories, but also non-recalled lead failure rates over time. U.S. manufacturers’ most current product performance reports for non-advisory ICD leads with ten-year data show lead survival rates ranging from 95.2%¹⁰ to a high of 98.66%¹¹ (Figure 1). Survival data for leads under advisory show seven-year survival data as low 80.7%.¹² Further confirmation of these ICD lead survival rates was shown in a large single-center experience in Leiden, the Netherlands, published in 2009, which documented an overall 16.4% failure rate at 10 years.¹³ 

Considering ICD lead survival rates relative to patient survival rates help assess clinical impact. A 2005 study demonstrated ICD patient survival rates of over 40% at ten years.¹⁴ More recently, a retrospective study published in 2010 of 69,556 ICD patients showed five-year ICD patient survival rates of 75% for individuals with remote monitoring,¹⁵ which could lead to a ten-year patient survival rate easily exceeding 50%. Patient survival rates examined in view of ICD lead survival poses a challenge for many transvenous lead technologies — the greater the long-term patient survival, the more likely they will experience a transvenous lead failure. 

Additionally, management of failed leads has shown increased cost and burden. A study published in 2012 summarized a retrospective review from two medical centers in Ireland¹⁶ for patients requiring ICD lead replacement due to lead failure. This review of 23 patients showed procedure-related complications of 13% with a median cost per lead replacement of €7,660 ($10,500 US). The cost was significantly higher if a new ICD generator was also required. A second study looked at patients who underwent lead revision for failed Sprint Fidelis Defibrillator Leads,¹⁷ and found the cost for new lead implant and abandoning the failed one was approximately $33,802, while new lead implantation with lead extraction averaged $45,077. 

These studies review replacement procedures for failed ICD leads, but a more common issue is lead extraction due to infection. A single-center study in Australia that reviewed 1,006 lead removals from 510 patients determined the indication for lead removal included systemic infection in 25% of cases and pocket infection in 40% of cases, while lead failure was the indication in 26%.¹⁸ 

While the risk of complication for initial ICD system implant is substantive, device replacements have a notably higher risk. A large study from Leiden University Medical Center¹⁹ reviewed pocket-related complications requiring surgical re-intervention after ICD implantation or replacement. This analysis of 3,161 ICDs found 145 surgical re-interventions were required. They experienced a three-year cumulative incidence for first surgical re-intervention of 4.7%. Additionally, “ICD replacement is associated with a doubled risk for pocket-related surgical re-interventions. Furthermore, the need for re-intervention increases with every consecutive replacement.”¹⁹ 

ICD therapy has been proven successful and long-term patient survival is high. The clinical challenge for physicians and patients is the choice and management of transvenous ICD system technology to effectively reduce lead failures, and reduce device replacements due to battery depletion so that infection and complications can significantly be reduced. As these challenges have become evident, the concept of an ICD system implanted subcutaneously was investigated for patients not requiring bradycardia pacing. 

The S-ICD System

One advantage of a completely subcutaneous system is that the S-ICD lead has almost no lead motion compared to transvenous leads inside the beating heart. Additionally, the lead body itself can be thicker than a transvenous lead, providing more insulation and potential resistance to abrasion, particularly in the device surgical pocket. The most significant advantage, however, is that any system revision does not require removing hardware from the fragile heart and vasculature. Two engineering challenges for the subcutaneous ICD system concept were the need for higher energy outputs (resolved by increasing the output to 80 Joules delivered) to effectively terminate ventricular fibrillation, and the difficultly of having only a surface-like EKG signal for arrhythmia detection (new sensing algorithms and methods were developed).

Clinical short-term defibrillation testing to determine optimum electrode configurations started in 2001 and continued through 2004.²⁰ Using these trial results, a second trial was initiated comparing defibrillation thresholds from a conventional transvenous system to the optimum subcutaneous lead and device placement. These results were used for further development and refinement of the subcutaneous system. Pilot human trials were started in 2008 with final device clinical evaluation being tested in a protocol designed to enroll 330 patients at multiple centers in numerous countries including the United States. Based on the results of this clinical trial, Boston Scientific’s S-ICD System was approved by the Food and Drug Administration in September 2012.

The S-ICD System has two main components: 1) the pulse generator, which powers the system, monitors heart activity, and delivers a shock if needed, and 2) the electrode, which enables the device to sense the cardiac rhythm and serves as a pathway for shock delivery when necessary. An illustration of the implanted S-ICD system is shown in Figure 2.

With this new alternative to the traditional ICD becoming available, the S-ICD System leaves the heart and vasculature completely untouched. Like traditional ICDs, it uses a pulse generator implanted under the skin; however, it is placed by the 5th or 6th intercostal space on the left lateral wall of the chest to detect and treat ventricular arrhythmias. Unlike traditional ICDs, the S-ICD does not require any wires to be placed in the heart. Instead, a single-coil electrode is tunneled subcutaneously from the device pocket to the xiphoid process and up to the sternal notch using anatomical landmarks, thus eliminating the need for image-guided implants. The S-ICD eliminates the potential of acute complications such as pneumothorax, lead dislodgement, perforation, and endocarditis. Long-term complications such as lead failures and subsequent extractions thereof, as well as full system extractions due to infection, can largely be avoided. 

Indications for the S-ICD System are identical to the primary and secondary indications for a traditional system, provided that the patient does not require pacing for bradycardia or heart failure, or have well-documented monomorphic ventricular tachycardia that would be better treated with a traditional system that can provide anti-tachycardia pacing. Analysis of the commercial implants found that 25% of patients who received the S-ICD have had previous transvenous systems.²¹ This highlights a unique aspect of the S-ICD therapy application as a novel solution following extractions for traditional ICD lead failure or systemic infections. 

Experience at Deborah Heart and Lung Center

A patient was recently referred to the Deborah Heart and Lung Center for complete system extraction after presenting to a community hospital ER stating he started to develop fever several days earlier, with swelling and warmth over the left-sided AICD pocket. The 51-year-old man was originally implanted with an ICD in 2008 for secondary prevention: dilated nonischemic cardiomyopathy with a preserved ejection fraction, history of VT with syncopal episodes. Additionally, he had congenital thrombocytopenia and a history of acute renal failure. In 2010 his ICD lead failed, and he underwent an RV lead revision and generator change. The original RV lead was retained and capped. In 2012 he developed Group B Strep Bacteremia, and underwent full system extraction and reimplantation. He now presents with recurrent bacteremia positive for Group B Strep. Our recommended plan of action was to extract all of his current hardware (Figure 3) and reimplant with an S-ICD. 

It is recommended that extractions should always be done by an experienced extractor. At Deborah Heart and Lung Center, the EP department extracts more than 200 leads per year. This extraction was performed in our new hybrid OR. Post-extraction procedure, a portion of the existing RV lead remained and was later removed via a femoral snare approach (Figure 4). Another advantage of the S-ICD in these situations is that whereas it is normally recommended to wait two weeks prior to re-implantation, the S-ICD does not require this two-week wait. I prefer not to combine the two procedures due to the vastly different preparation areas and procedures. Instead, we bring the patients back the following day for S-ICD implantation. Figure 5 shows the result after S-ICD implant.

Summary

The S-ICD is a less invasive option that should be considered at the original time of implant. Additionally, it represents an option with significant advantages that should be strongly considered after complications associated with a traditional system resulting in extraction. In either case, it represents a less invasive option that avoids invading the vascular space until it may be needed in the future. Additionally, future technology advancements such as leadless pacing options with device-to-device communication may be available to further supplement the S-ICD System. 

Disclosures: Dr. Corbisiero has no conflicts of interest to report. Mr. Stahl is an employee of Boston Scientific. 

References

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