The Subcutaneous ICD: Application to Clinical Practice

Since the first regular implantations of cardioverter-defibrillators (ICDs) in 1985, there has been a steady progression of technological advances in device hardware and software that has improved the function of these complex systems for the benefit of our patients. The devices began as large, robust units that were so bulky they required placing the generator in the anterior abdominal wall. Leads were all epicardial: two screw-ins for the rate-sense portion and one or two large patches placed epicardially for a shock vector. Unquestionably, they worked, and even in the era of non-programmable rate cut-offs, they saved lives. However, the device and implantation technique were cumbersome.

Steady improvements led to transvenous systems, with a variety of leads, and with generators of ever smaller size to be accommodated in the pre-pectoral or sub-pectoral space. Programmability, biphasic shocks, electrogram storage, and wireless connectivity now allow us to more efficiently care for our patients and to more accurately diagnose.

However, the transvenous ICD lead is not foolproof. With an average of 100,000 heartbeats each day, the lead is continuously stressed, in a body fluid environment, in active patients who move their shoulders every day during the daily activities of living. The stress is multiplied in young patients who are more physically active, leading to a higher fracture rate. Lead failure rates for transvenous ICD leads may be as high as 10–20% over a 10-year period.¹ The potential for lead failure has become especially evident in recent years through major ICD lead recalls due to structural failure, all this with teams of engineers who dedicate their lives to the improvement of the complex device called an ICD lead. Moreover, young patients scar more heavily, making a fractured lead more difficult to remove through lead extraction techniques. Nevertheless, these young patients may need the protection of an ICD for many years, and they may potentially require repeated lead revisions or extractions.

Infection of transvenous leads poses another risk. Although infection rates for primary device implants are on the order of 1%, these rates are higher for generator replacements, and increase further when patients need lead revision at the same time. With patients living longer, due to the devices themselves and due to better medications for treatment of their underlying cardiac condition, they are more prone to require repeated surgeries for device replacement or revision. Even local erosion of a device mandates complete removal of the system and potentially lead extraction. Systemic infections can lead to endocarditis with intracardiac vegetations, which can lead to in-hospital mortality as high as 10%, and are associated with an increased one-year mortality approaching 30%.²

Silicone backfilling of the high energy coils, or Gore-Tex coating, clearly improves the safety of extraction of ICD transvenous high energy leads, but these technologies do not reduce the risk to zero.³ Lead extraction morbidity and mortality in the most experienced centers remain at best 1.4% and 0.28%.⁴ In most centers, these numbers are higher, especially since patients are living longer with their devices, and with younger patients being implanted who scar more heavily. Additionally, it is now clear that the SVC coil adds the greatest danger to extraction, with a markedly greater chance for complications from lead removal than with single coil ICD leads.⁵ For this reason, many implanters have converted to the primary use of single coil ICD leads, with an eye toward the potential need for lead removal in the future.

Concept of the Subcutaneous ICD (S-ICD)

The subcutaneous ICD provides a solution for many of these problems. However, it too does not solve all of the issues. 

The system incorporates much of the best technology that has been gleaned from generations of overall improvement in ICDs.^6,7 Electronics, battery, and capacitors have the ability to accurately sense ventricular tachy-arrhythmias and to deliver 80 Joules trans-thoracically for conversion. The higher energy is needed because the shock vector is trans-thoracic, not directly intracardiac as with transvenous ICDs. Because of the larger capacitors needed to charge to 80 Joules, the generator itself is larger than its match in the transvenous arena; however, it is well accepted by patients, of mostly any body size. We have implanted systems in patients who would be considered relatively thin to one weighing 460 lbs. The database for worldwide S-ICD implantation includes a wide span of body mass indexes (BMIs). (Figure 1) In short, the device size is well accepted by patients, partly due to the implant location in the left sub-axillary region, and it rarely limits mobility. Device downsizing is inevitable as well.

Lead design is both complex and straightforward. (Figures 2 and 3) The silicone-based lead has two sensing electrodes positioned at the ends of a high energy coil. Conductor cables run the length of the lead in internal conduits and are individually insulated. The proximal end is established as a single pin plug in type connector to fit into the single port of the ICD generator. An anchoring sleeve is used to secure the lead body to fascia at the lower sternum. The distal end of the lead has a suture hole in it to secure the end of the lead to fascia at the upper end of the sternum. This hole is also used to pull the lead through the subcutaneous tissues from the ICD generator pocket in the left sub-axillary area to the left side of the sternum. The lead is tunneled up the side of the sternum, also using the tunneling tool.

Implanting the S-ICD

The entire implant may be performed anatomically, without the use of fluoroscopy. However, imaging may be useful for patients at either extreme in the BMI scale or those with COPD to ensure the appropriate positioning of the pulse generator.

Before draping, the proper positions for the pulse generator and lead should be drawn on the chest, since anatomical landmarks may be more difficult to find after the thorax is covered. The pocket for the pulse generator is made first using an incision at about the mid-axillary line on the left side at the level of the nipple. This is carried through the subcutaneous tissues to the muscular fascia, where a pocket is fashioned large enough to accommodate the generator in as posterior a position as possible. (Figure 4)

Small incisions are then placed at the lower and upper borders of the left side of the sternum. A tunneling toll is passed from the lower sternal incision to the generator pocket, and the distal end of the lead is tied with a suture to the tunneling tool and pulled to the sternum. The lead anchor is placed at the lower sternum, and the lead is then tunneled to the upper sternal incision, where it is secured to fascia with a non-absorbable suture.

The pulse generator is connected to the lead, and the generator is secured to fascia with a suture. After flush, the incisions are closed with layers of absorbable suture. (Figure 5)

The device tests three vectors for the best sensing configuration and auto-programs to that configuration. (Figure 6) Induction of ventricular fibrillation (VF) is easily accomplished through the device for defibrillation threshold testing. We usually perform a single test at 60 Joules to ensure an adequate safety margin for the patient.

Patient Selection

Candidates for implantation of the S-ICD need to have standard indications for ICD placement by established criteria. Moreover, because the device does not provide backup pacing beyond 30 seconds of trans-thoracic pacing after a shock if required, patients must not require systematic pacing. This means that patients who require biventricular pacing for LV resynchronization are also not candidates for an S-ICD.

Though the lack of bradycardia pacing at first seems a limitation, many patients who receive ICDs, in fact, do not need a backup pacing system. Thus, a large percentage of patients may be considered candidates for the S-ICD.

One other exclusion for use of the S-ICD involves patients with frequent monomorphic ventricular tachycardia (VT) where anti-tachycardia may be, or has been shown to be, effective for termination of the VT. Because the S-ICD does not provide a pacing capability, using the device in such a patient could lead to many unnecessary shocks, which could be avoided if anti-tachycardia pacing were used.

Each patient needs to be screened to ensure that the generator and lead system in that patient will accurately sense the native electrical activity of the heart from the subcutaneous location of the device. The sensing vectors essentially act like a surface ECG. So before implantation, each patient is screened in a supine and sitting position with surface electrodes that mimic the location of the permanent device to be sure that the QRS complex and T wave fall within template parameters for accurate sensing, and not for oversensing of the T wave. Nearly all patients pass the template test, but the couple percent who do not should not receive the subcutaneous device.

Besides the patients with standard indications for use of the S-ICD, implanters have deemed that a few patient populations warrant special attention for the use of this groundbreaking technology. (Table 1) Teenagers and young adults who need an ICD may be especially good candidates for the S-ICD because of their high level of activity that can lead to an increased rate of fracture of endovascular leads, as described above.⁸

It is clear that end-stage renal patients on hemodialysis have an increased risk of endovascular infection, especially with indwelling catheters, but also due to repeated invasions of their vascular systems. Use of the S-ICD in these patients may reduce the risk of systemic infection and the subsequent need to remove an endovascular ICD due to that infection.⁹

Finally, a substantial number of patients with indwelling endovascular ICDs will develop erosion of the pulse generator or systemic infection; these devices need to be extracted to cure the infection. Reimplantation of an S-ICD in these patients may be done even before they have completed their antibiotic course for infection, even endocarditis with cardiac vegetations, since the S-ICD is extravascular. This could potentially shorten their hospital course while providing them with needed protection.¹⁰

All of these newer indications are being comprehensively evaluated for their real utility in each patient population.

What are the Data?

Currently, the S-ICD has been implanted worldwide in over 1,900 patients. (Figure 7) An IDE clinical trial established the safety and effectiveness of the device in the U.S. prior to its approval by the FDA. A post-market surveillance study is also underway to follow implanted devices for a longer period of time.

After FDA approval, demand for the device has surpassed the expectations of the manufacturer and the implanting community. It has been used in patients across the spectrum of ages and with a wide range of BMI. (Figures 1 and 8)

Implant success is high, and the ability to sense VF and to terminate it successfully is well established. Patients have received successful treatment from the device.

Because there is no atrial lead for discrimination of atrial vs. ventricular arrhythmias, the device uses standard single chamber discrimination algorithms, including morphology templates, to differentiate the arrhythmias. Nevertheless, inappropriate shocks can occur, as with any single chamber ICD. Differentiation rates for supraventricular vs. ventricular arrhythmias match or exceed the results for single chamber transvenous ICD systems.¹¹

Overall infection rates for implantation and device revision for the S-ICD are about the same as for operations involving endovascular systems. However, with the S-ICD, these are local, subcutaneous infections, with a much reduced incidence of systemic bloodstream infection. This means that, if necessary, an infection can be treated with device removal with a reduced course of antibiotics compared with the treatment of bacteremia or endocarditis. Further, the device is easier to remove since endovascular lead extraction is not required.

Conclusions

A breakthrough technology, the S-ICD will establish itself as a viable and strong alternative to the use of a transvenous defibrillator. Avoiding the need to place an endovascular lead reduces potential complications of lead fracture and systemic bloodstream infection. Removal of the device, if required, does not entail lead extraction, with its attendant risks.

Patients accept the implant location and have few limitations to mobility. The device has proven success to differentiate VF and to save lives. It has applicability to a wide range of patients, where some may especially benefit from having a subcutaneous, rather than an endovascular, lead system. The rapid acceptance of the S-ICD for clinical practice, its straightforward implantation technique, and its proven ability to treat malignant ventricular arrhythmias all indicate that the S-ICD will remain as a strong, viable part of the selection of defibrillators we have to offer our patients.  

Disclosure: Dr. Kutalek reports consultancy, honoraria, and travel/accommodations expenses covered or reimbursed from Boston Scientific and Spectranetics. He also reports grants/grants pending from Boston Scientific. 

References

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