|Year : 2016 | Volume
| Issue : 2 | Page : 50-55
Subcutaneous implantable cardioverter defibrillator: Where do we stand?
Arindam Pande, Soumya Patra, Debabrata Bera, Prakas Chandra Mondal, Rabin Chakraborty
Department of Cardiology, Apollo Gleneagles Hospital, Kolkata, West Bengal, India
|Date of Web Publication||6-Jun-2016|
Department of Cardiology, Apollo Gleneagles Hospital, 58, Canal Circular Road, Kolkata - 700 054, West Bengal
Source of Support: None, Conflict of Interest: None
The indications for implantable cardioverter defibrillators (ICDs) for the prevention of sudden cardiac death (SCD) have rapidly expanded over the past 15 years. Clinical trial data have quickly been implemented into guidelines. The recently introduced subcutaneous implantable cardioverter defibrillator (S-ICD) uses a completely subcutaneous electrode configuration to treat potentially lethal ventricular tachyarrhythmia. The device is now commercially available in India. Clinical trials have proven its effectiveness in detecting and treating ventricular fibrillation (VF) and tachycardia. The S-ICD offers the advantage of eliminating the need for intravenous and intracardiac leads and their associated risks and shortcomings. However, major disadvantages of this device include inability to provide bradycardia rate support and antitachycardia pacing to terminate ventricular tachycardia. As seen with other early examples of evolutionary technology, we hope improvements in design and manufacture will improve some of the drawbacks of the current generation device.
Keywords: Implantable cardioverter defibrillator, subcutaneous, sudden cardiac death
|How to cite this article:|
Pande A, Patra S, Bera D, Mondal PC, Chakraborty R. Subcutaneous implantable cardioverter defibrillator: Where do we stand?. Heart India 2016;4:50-5
|How to cite this URL:|
Pande A, Patra S, Bera D, Mondal PC, Chakraborty R. Subcutaneous implantable cardioverter defibrillator: Where do we stand?. Heart India [serial online] 2016 [cited 2021 Jan 18];4:50-5. Available from: https://www.heartindia.net/text.asp?2016/4/2/50/183519
| Introduction|| |
Since the first descriptions of external defibrillation in the 1960s and the first human implantable cardioverter defibrillator (ICD) in 1980 by Mirowski et al., the paradigm for the prevention of sudden cardiac death (SCD) shifted away from antiarrhythmic drugs. The indications of ICDs for the prevention of SCD have rapidly expanded over the past 15 years. However, lead malfunction caused by conductor failure or insulation breach occurs in up to 40% of indwelling transvenous leads at 8 years after implantation.  Failure occurs more commonly in active young patients or in patients with longer life expectancy who expose the leads to greater cumulative physical stress. Longer living ICD patients may also undergo several generator exchanges, each with an associated risk of pocket infection reaching up to 3%. Because lead malfunction may necessitate and device infection usually requires extraction of the lead, the use of transvenous pacing and defibrillating leads introduces the potential risk of extraction-associated morbidity and mortality  to patients with chronically present transvenous leads.
The need to completely avoid venous access issues, endovascular mechanical stress-producing lead malfunction, and extraction-associated risks led to the development of the entirely subcutaneous ICD (S-ICD). Its unique design avoids endovascular leads, thus eliminating many of the complications associated with the traditional tranvenous ICD (T-ICD). The novel device, developed and tested over the past decade, gained approval as an accepted therapy for the detection and termination of ventricular arrhythmias. The European Union approved its use in 2009; the U.S. Food and Drug Administration approved it in 2012. Till date, six such devices have been implanted in India.
| Fundamental principles and subcutaneous implantable cardioverter defibrillator device system|| |
The S-ICD system includes a dedicated external programmer, a subcutaneous pulse generator enclosed in a titanium case, and a single subcutaneous electrode containing both sensing and defibrillating components. The lead is composed of proximal and distal sensing electrodes positioned adjacent to either end of a 7 cm defibrillation coil electrode. The recommended position for the pulse generator involves a subcutaneous pocket created over the fifth intercostal space between the mid and anterior axillary lines. The subcutaneous lead should lie parallel to the left side of the sternum, with its upper pole anchored at the level of the sternal notch and the lower electrode anchored just below the level of the xiphoid process. The electrode then makes a right angle turn laterally to enter the pulse generator pocket [Figure 1]a and b. Implantation of the device relies exclusively on anatomical landmarks, with the option to confirm defibrillating electrode position by fluoroscopy. The procedure can be performed with minimal requirement of fluoroscopy, thereby reducing the exposure to the physician and the laboratory staff. The implant procedure has a learning curve and as time passes, the fluoroscopy exposure can certainly be minimized. However, fluoroscopy cannot be totally eliminated as the positions of the device and the electrode need to be confirmed before commencement of the procedure. There are reports of unsuccessful defibrillation threshold (DFT) that warranted alternate electrode positioning, which may require fluoroscopy as well.
|Figure 1: PA (a) and lateral (b) chest radiographs of a patient with S-ICD|
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The S-ICD system detects changes in the ventricular rate by using modified subsurface electrocardiography through either a primary, secondary, or alternate vector. The device uses proprietary algorithms to automatically determine the optimal sensing vector based on an R- to T-wave ratio that avoids double QRS counting or T-wave oversensing. It measures the heart rate as the rolling average of four consecutive sensed intervals, recognizing ventricular fibrillation (VF) when 18 of the 24 consecutive sensed events exceed a predetermined nonprogrammable detection zone limit. The device then charges its capacitors to deliver a biphasic waveform defibrillating pulse of up to 80 J. The S-ICD can provide postshock bradycardia ventricular pacing support for 30 s. The current pulse generator weighs 145 g and has a volume of 69 mL [Figure 2]a and b. The manufacturer estimates longevity of the battery to be 5 years. 
|Figure 2: (a) Subcutaneous ICD device (model SQ-RX 1010, Cameron Health, Inc., San Clemente, California, USA), (b) Schematic positioning of the same device in human body|
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| Clinical evidence for subcutaneous implantable cardioverter defibrillator|| |
The first acute evaluation identified the optimal defibrillation configuration as a combination of parasternal electrode and left lateral thoracic pulse generator.  A comparison of the S-ICD defibrillation system with a T-ICD in 2004 (n = 49) found that the S-ICD equally terminated induced VF although at a higher DFT than that of the T-ICD (36.6 ± 19.8 J vs 11.1 ± 8.5 J, respectively). A pilot study in 2008 (n = 6) showed that the implanted S-ICD effectively detected and terminated two consecutive episodes of induced VF acutely with no inappropriate shock or device complication during a 488 ± 2 day follow-up. 
Dabiri Abkenari et al.  reported a single-center European experience (n = 31) with the S-ICD in which the device detected and terminated 100% of induced VF episodes. Additionally, four patients with spontaneous VF/ventricular tachycardia (VT) received successful therapy during follow-up, five patients received inappropriate shocks, and two patients experienced lead migration that required operative repositioning. A multicenter trial from the Netherlands (n = 118) conducted between 2008 and 2011 reported a 177-patient-year follow-up.  The S-ICD successfully detected nine episodes of spontaneous VT and 36 episodes of spontaneous VF in eight patients. It successfully treated 98% of these episodes (1 VT episode accelerated into VF and terminated before delivery of a second shock). However, 13% of the patients experienced inappropriate shocks for multiple reasons; three developed lead migration, creating the need to develop a sleeve that anchored the lead as it turned laterally at the subxiphoid level, two patients developed skin erosion over the device pocket, and infection occurred in seven patients, requiring removal of the device. The highest rate of inappropriate shocks and device-related complications occurred in the first 15 implanted devices in each of the centers, suggesting the presence of a learning curve associated with this new technology. 
The early UK S-ICD experience (n = 111) included a group mean age of 33 years (range: 10-87 years) with primary cardiac electrical heart disease (43%), hypertrophic cardiomyopathy (20%), and ischemic cardiomyopathy (14%).  The device detected and treated all induced episodes and all 10 spontaneous episodes of VF and 14 of VT. Complications included device-related infection or skin erosion requiring reoperation in 17% of the patients. A total of 15% of the patients received inappropriate shocks and younger patients experienced a higher rate of inappropriate shocks because of T-wave oversensing. Another multicenter evaluation of the S-ICD that included a mixed pediatric and adult population also demonstrated inappropriate shocks due to T-wave oversensing in younger patients. 
The largest multicenter clinical evaluation of the safety and efficacy of the S-ICD enrolled 330 individuals with established indications for an ICD.  Nine patients withdrew before device implantation, and 17 patients did not undergo DFT testing; 304 enrolled subjects underwent successful implantation and DFT testing. The S-ICD terminated all induced VF episodes. Twenty-one subjects experienced 119 episodes of spontaneous VT/VF, 38 as isolated events and 81 as part of a VT storm. The device successfully terminated 37 of 38 isolated episodes; one VT terminated as the device was charging to deliver a second shock. The S-ICD successfully treated all VT storm events. Forty-one patients (13.1%) received an inappropriate shock; the cause was treatment for supraventricular tachycardia in 16 patients and oversensing in the other 25 patients. Eighteen subjects developed pocket infections (5.6%), four required device explantation, and 1 required pocket revision (1.56% rate of intervention for infection).
Another report of a multicenter case-control study (n = 69) compared defibrillation efficacy in patients receiving an S-ICD with an age- and sex-matched cohort receiving T-ICD.  The S-ICD first shock efficacy in terminating induced VF using a 15-J safety margin was 89.5% compared to 90.8% with the T-ICD, at a 10-J safety margin (P = 0.8). The success rate with the S-ICD increased to 95.5% after a second shock by using reverse electrode polarity. Another small study (n = 40) reported a first shock efficacy of 58% with an overall shock efficacy of 96% (for induced and spontaneous VF/VT) after additional shocks. 
| Limitations of subcutaneous implantable cardioverter defibrillator|| |
The limitations of the current S-ICD include its inability to provide antitachycardia pacing for VT, limited bradycardia pacing support, relatively large size and bulk of the pulse generator, and absence of endovascular monitoring capabilities for collateral data gathering such as impedance monitoring for chronic heart failure.
Another concern with the S-ICD system is the rate of inappropriate shocks, which is observed to be 5-25% in different trials, a frequency similar to the observed rate reported in earlier trials of the T-ICD.  However, more recent T-ICD trials show that newer algorithms reduce the rate of inappropriate shocks to less than 5%,  suggesting an advantage of T-ICDs over the current S-ICD. Ideally, greater user programming experience and improvements in S-ICD technology may reduce the rate of inappropriate shocks.  An increased ventricular rate during atrial arrhythmia constitutes the major cause of inappropriate shocks delivered by T-ICD systems. However, oversensing T-waves or myopotential signals produce most inappropriate S-ICD shocks. ,, Inappropriate shocks occur more frequently in younger, physically active patients, the group most likely to benefit from the features of the S-ICD system. , The addition of a second tachycardia zone to S-ICD programming may significantly reduce the rate of inappropriate shocks. 
No study has prospectively addressed the use of ICD therapy as primary prevention of SCD in the dialysis population; all the clinical trials actively excluded enrolling dialysis patients. The S-ICD clinical evaluation protocols also excluded the enrollment of patients with chronic kidney disease requiring dialysis; yet it may provide a safer approach in this group of patients with a greater risk for infection due to access catheters, limited venous access due to scarring, and greater lead extraction-related complications due to increased calcification around implanted leads. Use of the S-ICD should be avoided in patients with either known monomorphic VT or with conditions (sarcoidosis or arrhythmogenic right ventricular cardiomyopathy) likely to result in VT amenable to antitachycardia pacing. 
The rate of pocket infection with the S-ICD exceeds that with the T-ICD. The three incisions required for S-ICD implantation provide a greater probability for bacterial entry. Also, the increased bulk of the S-ICD may exert more pressure on the skin and increase the risk of tissue necrosis and erosion. The infection rate may decrease with more operator experience, introduction of smaller pulse generators, and use of a two-incision technique for system implantation. 
Most of the limitations of the S-ICD typify the first-generation nature of the current device: Lack of continuous demand and antitachycardia pacing contraindicates the use of the S-ICD in patients with sinus node dysfunction, atrioventricular block, or an indication for cardiac resynchronization. Because 80% of spontaneous VT episodes respond to painless antitachycardia pacing,  patients with a history of VT benefit more from T-ICD. , The longevity of the S-ICD battery is estimated to be 5 years compared with the most recently introduced single-lead T-ICD that may exceed 10 years. In addition, the S-ICD system lacks remote monitoring capability, a feature that improves patient outcomes and simplifies follow-up. , The limitations of S-ICD system have been summarized in [Table 1].
| Potential advantages of subcutaneous implantable cardioverter defibrillator|| |
Development of the S-ICD represents a quantum step in the evolution of ICD technology to prevent SCD. Data from the S-ICD clinical trials support its efficacy and safety in detecting and terminating VT. Although the current generation device experience included adverse events such as sensing issues that led to inappropriate shocks, lead migration, and device infection, their frequency appears to be within the bounds of clinical experience with T-ICD, and the preponderance of device infection and lead migration early in a center's experience is consistent with a learning curve. The advantages of a nontransvenous ICD system 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.
One estimate of potential candidates for the S-ICD might include every patient indicated for primary SCD prevention without a pacing indication. However, the limitations of the current system and the relative paucity of data on long-term performance compared to that of the T-ICD might temper that view.  The S-ICD appears to be an attractive alternative in relatively young patients (i.e., age <40 years), those at high-risk for bacteremia (due to indwelling catheters/hardware or immune-compromised states), and patients lacking venous access. Without the use of transvenous leads, most major complications associated with their use are avoided. Given that the duration of implanted leads greatly influences the probability of malfunction, the S-ICD presents an attractive alternative in younger patients with greater longevity such as those with hypertrophic cardiomyopathy and inherited ion channel abnormalities. [Table 2] compares the different aspects of T-ICD and S-ICD systems.
The S-ICD system delivers energy to the heart in a more homogenously distributed pattern than the endocardial shock delivered by the T-ICD.  The uneven distribution of energy across the myocardium after an endocardial shock can produce voltage gradients and electroporation resulting in myocardial stunning and damage. , Endocardial shocks produce significant troponin release; shocks delivered from subcutaneous electrodes do not. , Myocardial injury and stunning associated with ICD discharge might explain the increased mortality seen in heart failure patients receiving multiple shocks. , Whether the lack of significant troponin release after a subcutaneous shock is an advantage of the S-ICD and whether it translates into a survival benefit is yet to be determined. [Table 3] has categorized the candidates for S-ICD according to suitability. The literature mentions a positive screening of around 80-95%, leaving about 5-15% patients unsuitable for implant. The study by Olde Nordkamp et al.  found that increased body weight, presence of hypertrophic cardiomyopathy, prolonged QRS duration, and R: T ratio of 3 on the surface ECG lead with the largest T-wave predicted screening failure. Only QRS duration was a predictor of failure in the study by Randles et al.  Unlike these studies, another study by Christopher A Groh et al.,  identified a readily available and easily identified ECG feature that appears to be a very strong predictor of screening failure, specifically, the presence of T-wave inversion (TWIs).
| Future perspectives|| |
At present, most of the patients are implanted with transvenous ICD (TV-ICD) devices for primary prevention. Studies are continuing to evaluate subcutaneous implantable defibrillator (EMBLEM S-ICD System by Boston Scientific, Minnesota, USA) for primary prevention of SCD in the setting of severely reduced cardiac function (left ventricular ejection fraction ≤35%). In order to further demonstrate the usefulness of the S-ICD system in this particular patient population, the UNTOUCHED study will compare outcomes during an 18-month follow-up period to objective performance criteria derived from the multicenter automatic defibrillator implantation trial-reduce inappropriate therapy (MADIT-RIT) study. The landmark MADIT-RIT study evaluated the shock rates in 1,500 patients implanted with TV-ICD devices and is one of the largest randomized trials to assess shock reduction strategies for TV-ICD devices.  The MADIT-RIT trial demonstrated that standardized programming using higher rate cutoffs and longer delays to therapy reduces the incidence of inappropriate shocks for TV-ICDs. The UNTOUCHED trial will examine the incidence of all-cause shocks when using the EMBLEM S-ICD system with standardized therapy settings similar to MADIT-RIT. As of December 2015, six subcutaneous ICDs have been implanted in India. All the devices were implanted in and around the National Capital Region of Delhi. There was no complication related to the procedure reported in any of the cases. The procedure time varied between 45 min and 1 h. The center where the procedures were repeated took a progressively lesser time in the second procedure or subsequent procedures.
| Conclusions|| |
The clinical experience from the introduction of the S-ICD system underscores its role as a reliable alternative for preventing SCD. The exclusive use of a subcutaneous lead for sensing and defibrillation represents the greatest advantage of this novel technology; the S-ICD eliminates the drawbacks associated with endovascular electrodes. However, the lack of demand bradycardia or antitachycardia pacing limits its utility in patients with conduction system disease or pace-terminable VT. The first-generation device raises concerns about an increased risk of pocket infection, battery longevity, and inappropriate shocks compared to the newest T-ICD systems. No study to date has directly compared T-ICD and S-ICD in patients indicated for ICD therapy as a primary prevention of SCD. The clinical experience does suggest that its use be considered in relatively younger patients (i.e., age <40 years), those at an increased risk for bacteremia, patients with indwelling intravascular hardware at risk for endovascular infection, or in patients with compromised venous access. As seen with other early examples of evolutionary technology, improvements in the design and manufacture will improve some of the drawbacks of the current generation device.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3]