TY - JOUR
AU1 - Bögeholz,, Nils
AU2 - Willy,, Kevin
AU3 - Niehues,, Philipp
AU4 - Rath,, Benjamin
AU5 - Dechering, Dirk, G
AU6 - Frommeyer,, Gerrit
AU7 - Kochhäuser,, Simon
AU8 - Löher,, Andreas
AU9 - Köbe,, Julia
AU1 - Reinke,, Florian
AU1 - Eckardt,, Lars
AB - Abstract Subcutaneous ICD (S-ICD™) therapy has been established in initial clinical trials and current international guideline recommendations for patients without demand for pacing, cardiac resynchronization, or antitachycardia pacing. The promising experience in ‘ideal’ S-ICD™ candidates increasingly encourages physicians to provide the benefits of S-ICD™ therapy to patients in clinical constellations beyond ‘classical’ indications of S-ICD™ therapy, which has led to a broadening of S-ICD™ indications in many centres. However, the decision for S-ICD™ implantation is still not covered by controlled randomized trials but rather relies on patient series or observational studies. Thus, this review intends to give a contemporary update on available empirical evidence data and technical advancements of S-ICD™ technology and sheds a spotlight on S-ICD™ therapy in recently discovered fields of indication beyond ideal preconditions. We discuss the eligibility for S-ICD™ therapy in Brugada syndrome as an example for an adverse and dynamic electrocardiographic pattern that challenges the S-ICD™ sensing and detection algorithms. Besides, the S-ICD™ performance and defibrillation efficacy in conditions of adverse structural remodelling as exemplified for hypertrophic cardiomyopathy is discussed. In addition, we review recent data on potential device interactions between S-ICD™ systems and other implantable cardio-active systems (e.g. pacemakers) including specific recommendations, how these could be prevented. Finally, we evaluate limitations of S-ICD™ therapy in adverse patient constitutions, like distinct obesity, and present contemporary strategies to assure proper S-ICD™ performance in these patients. Overall, the S-ICD™ performance is promising even for many patients, who may not be ‘classical’ candidates for this technology. Subcutaneous implantable cardioverter-defibrillator, Sudden cardiac death Introduction During the past decade, entirely subcutaneous ICD (S-ICD™) systems have evolved as a valuable alternative to transvenous implantable cardioverter-defibrillator (ICD) systems.1–3 The promising initial experiences have recently led to a Class IIa (level of evidence C) recommendation for S-ICD™ systems in patients without demand for antibradycardia pacing, cardiac resynchronization therapy (CRT), or antitachycardia pacing (ATP).4 The ‘ideal’ S-ICD™ candidates are young, have a long life expectancy and are prone to high-rate ventricular tachyarrhythmias such as ventricular fibrillation (VF). Indeed, in this specific cohort of patients, S-ICD™ systems may be considered as a potential therapy of first choice. As illustrated in Figure 1, the spectrum of underlying cardiac disease entities within the cohort of S-ICD™ patients in our tertiary care centre is not limited anymore to inherited arrhythmia syndromes, but also includes patients with coronary artery disease (19%) or dilated cardiomyopathy (16%). In fact, the increasing experience with S-ICD™ technology goes along with a progressive broadening of indications for S-ICD™ implantation. Yet, as with all young technologies, exploration of novel indication fields entails several uncertainties and open questions that require evidence-based clarification and cause controversy among clinicians. Figure 1 View largeDownload slide Distribution of underlying cardiac disease entities within a cohort of 240 S-ICD™ patients implanted between 2010 and 2018 in our tertiary care centre. In contrast to transvenous implantable cardioverter-defibrillator (ICD) patient cohorts, S-ICD™ patient cohorts are characterized by a smaller fraction of patients with underlying coronary artery disease (CAD) and dilated cardiomyopathy (DCM), but exhibit higher fractions of patients with electrical heart diseases (ED), hypertrophic cardiomyopathy (HCM), congenital heart disease (CHD), and idiopathic ventricular fibrillation (IVF). S-ICD™, subcutaneous implantable cardioverter-defibrillator. Figure 1 View largeDownload slide Distribution of underlying cardiac disease entities within a cohort of 240 S-ICD™ patients implanted between 2010 and 2018 in our tertiary care centre. In contrast to transvenous implantable cardioverter-defibrillator (ICD) patient cohorts, S-ICD™ patient cohorts are characterized by a smaller fraction of patients with underlying coronary artery disease (CAD) and dilated cardiomyopathy (DCM), but exhibit higher fractions of patients with electrical heart diseases (ED), hypertrophic cardiomyopathy (HCM), congenital heart disease (CHD), and idiopathic ventricular fibrillation (IVF). S-ICD™, subcutaneous implantable cardioverter-defibrillator. The major focus of current debates around S-ICD™ technology is on the sensing of S-ICD™ systems in case of adverse electro-anatomic conditions. In contrast to transvenous ICD systems that use a near-field electrogram sensing technique with sharp deflections, S-ICD™ systems rely on a far-field sensing technology due to the extrathoracic lead placement. Since far-field sensing involves a larger amount of myocardium, the voltage deflections are less sharp in S-ICD™ systems and approximate to surface ECG recordings comprising the atrial excitation as well as the transmural repolarization pattern. Thus, S-ICD™ systems are more exposed to the risk of T-wave- or even P-wave-oversensing.5,6 Hence, sensing and detection algorithms in S-ICD™ systems face a distinct higher technical challenge as compared to transvenous ICD systems. Several inherited arrhythmia syndromes are characterized by an adverse electro-anatomy predisposing to inappropriate S-ICD™ shocks. Unfortunately, the affected patients are often young and do not have a demand for antibradycardia pacing, ATP, or cardiac resynchronization, thus virtually representing optimal S-ICD™ candidates. Hence, it is of deep clinical interest to broaden the spectrum of eligibility for S-ICD™ implantation even in case of a certain degree of adverse electro-anatomy to prevent transvenous lead implantation. Beyond that, electrocardiographic features of potential S-ICD™ candidates may be altered due to continuous or even discontinuous pacing for antibradycardia or cardiac resynchronization purposes, which represents another technical challenge to proper sensing and detection in S-ICD™ systems. Since the combination of transvenous or leadless pacemaker systems with the S-ICD™ may be beneficial in various clinical constellations, it is desirable to broaden the deployability of S-ICD™ systems in this field,7 although there is uncertainty regarding technical interaction of both systems. Next to electric features, distinct anatomic peculiarities may also hamper a proper S-ICD™ therapy, such as marked obesity that may cause adverse mobility of the S-ICD™ lead. Thus, both the individual electric and anatomic (‘electro-anatomic’) eligibility has to be taken into account prior to implantation. Decision pro or against S-ICD™ implantation is fiddly in such clinical constellations, because the experience is limited with this rather young technology. This review intends to shed the spotlight on the patient selection process, relevant outcome data, and gaps in evidence to facilitate the physician’s decision-making in these difficult clinical constellations. Current data on the clinical outcome of S-ICD™ systems Milestone empirical data on the performance of S-ICD™ systems stem from early reports that have been published short-term after introduction of the first S-ICD™ generation,1,8,9 short- and mid-term outcomes of the European EFFORTLESS registry and the US-American IDE study2,3,10 and further large clinical investigations that have been published afterwards (Table 1).12–20 Up to now, results of large-scaled randomized controlled trials (RCT) are awaited that compare S-ICD™ systems directly with transvenous ICD systems (PRAETORIAN trial; first posted: 15 February 2011; active, not recruiting; ClinicalTrials.gov ID NCT01296022)21 or that investigate the additional prognostic benefit of S-ICD™ implantation in patients with diabetes mellitus, prior myocardial infarction, older age, and relatively preserved left ventricular ejection fraction (LVEF: 36–50%) (MADIT-S-ICD trial; first posted: 1 June 2016; active, not recruiting; ClinicalTrials.gov ID NCT02787785).22 Furthermore, the global, multicentre, prospective, non-randomized UNTOUCHED study (first posted: 5 May 2015; active, not recruiting; ClinicalTrials.gov ID NCT02433379)23 investigates the clinical outcome of S-ICD™ implantation for primary prevention of sudden cardiac death in patients with a LVEF < 35%. Table 1 Comparison of the recent publications on large-scaled studies on the outcome of S-ICD™ therapy in the general population Database Number (n) of patients Primary prevention Follow-up duration Appropriate therapy Inappropriate shocks Efficacy of defibrillation conversion test EFFORTLESS study mid-term outcomes Boersma et al.10 n = 985 patients (72% male) 65% 3.1 ± 1.5 years 1-year rate 5.8% 5-year-rate 13.5% 1-year rate 8.1% 3.1-year rate 11.7% 99.5% IDE study Weiss et al.1 n = 314 successfully implanted patients (74.1% male) 79.4% 180 days 6.7% 13.1% 100% (acute induced VT/VF conversion results) PAS trial Gold et al.11 n = 1637 patients (68.6% male) 76.7% 30 days n.a. 0.2% 98.7% Database Number (n) of patients Primary prevention Follow-up duration Appropriate therapy Inappropriate shocks Efficacy of defibrillation conversion test EFFORTLESS study mid-term outcomes Boersma et al.10 n = 985 patients (72% male) 65% 3.1 ± 1.5 years 1-year rate 5.8% 5-year-rate 13.5% 1-year rate 8.1% 3.1-year rate 11.7% 99.5% IDE study Weiss et al.1 n = 314 successfully implanted patients (74.1% male) 79.4% 180 days 6.7% 13.1% 100% (acute induced VT/VF conversion results) PAS trial Gold et al.11 n = 1637 patients (68.6% male) 76.7% 30 days n.a. 0.2% 98.7% Note the diverse follow-up periods with regard to the referring outcome data. Younger studies demonstrate a more favourable safety outcome as compared to the initial IDE study reflecting technical advancements. S-ICD™, subcutaneous implantable cardioverter-defibrillator; VT/VF, ventricular tachycardias/ventricular fibrillation. View Large Table 1 Comparison of the recent publications on large-scaled studies on the outcome of S-ICD™ therapy in the general population Database Number (n) of patients Primary prevention Follow-up duration Appropriate therapy Inappropriate shocks Efficacy of defibrillation conversion test EFFORTLESS study mid-term outcomes Boersma et al.10 n = 985 patients (72% male) 65% 3.1 ± 1.5 years 1-year rate 5.8% 5-year-rate 13.5% 1-year rate 8.1% 3.1-year rate 11.7% 99.5% IDE study Weiss et al.1 n = 314 successfully implanted patients (74.1% male) 79.4% 180 days 6.7% 13.1% 100% (acute induced VT/VF conversion results) PAS trial Gold et al.11 n = 1637 patients (68.6% male) 76.7% 30 days n.a. 0.2% 98.7% Database Number (n) of patients Primary prevention Follow-up duration Appropriate therapy Inappropriate shocks Efficacy of defibrillation conversion test EFFORTLESS study mid-term outcomes Boersma et al.10 n = 985 patients (72% male) 65% 3.1 ± 1.5 years 1-year rate 5.8% 5-year-rate 13.5% 1-year rate 8.1% 3.1-year rate 11.7% 99.5% IDE study Weiss et al.1 n = 314 successfully implanted patients (74.1% male) 79.4% 180 days 6.7% 13.1% 100% (acute induced VT/VF conversion results) PAS trial Gold et al.11 n = 1637 patients (68.6% male) 76.7% 30 days n.a. 0.2% 98.7% Note the diverse follow-up periods with regard to the referring outcome data. Younger studies demonstrate a more favourable safety outcome as compared to the initial IDE study reflecting technical advancements. S-ICD™, subcutaneous implantable cardioverter-defibrillator; VT/VF, ventricular tachycardias/ventricular fibrillation. View Large Given the extrathoracic lead and can position of S-ICD™ systems, which involves musculoskeletal structures and lung tissue in the shock field, a careful evaluation is mandatory, whether the doubled shock energy (80 J) in S-ICD™ systems is indeed sufficient to terminate tachyarrhythmias reliably in the clinical setting as compared to contemporary transvenous ICD systems that apply around 35–40 J via the intracardiac shock coil. The sensitivity to detect induced ventricular tachycardias (VT)/VF during intraoperative testing was found as 99.8%.1 Overall, large-scaled studies consistently reported successful defibrillation of induced sustained VT/VF during implantation (applying up to 80 J shock energy) in >98% of patients.1,2,9–11,13–15 These efficacy rates on induced tachyarrhythmias during implantation are comparable to those of transvenous ICD systems.15,18 The majority of studies report that the initial 65 J shock is sufficient to terminate induced tachyarrhythmias during implantation in >90% of patients,2,10,13 although lower rates (88.5% of patients) have also been reported in a smaller study.18 A minor fraction of patients (≤5%) needs intraoperative repositioning of the can and/or electrode due to ineffective defibrillation testing.2,10,18 These data suggest that intraoperative defibrillation testing is reasonable in S-ICD™ systems as its shock efficacy may depend more on the anatomic position of the system than the shock efficacy of transvenous ICD systems. However, it has been demonstrated that the use of defibrillation conversion tests on induced sustained VT/VF has significantly decreased between 2012 and 2015 from 82.4% to 71.4% in the US ICD registry.13 Major reasons to omit conversion testing during S-ICD™ implantation are concerns with regard to prolonged resuscitation, stroke, or death. Alike, conversion testing is one of the most important reasons to apply general anaesthesia. The SIMPLE Trial24 led to a release of the canonical conversion testing in the majority of patients who receive a transvenous ICD system. Given the high conversion rates of S-ICD™ systems (>98% if up to 80 J are applied), it is questionable, whether conversion testing is mandatory for each S-ICD™ patient. Recently, the PRAETORIAN score has been introduced to predict the defibrillation success based on determinants like subcoil or subgenerator fat tissue and anterior positioning of the S-ICD™ can.25 Based upon a scoring system, patients could be retrospectively stratified to low, intermediate, and high risk for conversion failure, which yielded promising positive predictive values.25 The clinical value will be prospectively validated in the randomized PRAETORIAN-DFT trial, which compares the application of the score without conversion testing vs. standard conversion testing (PRAETORIAN-DFT trial; first posted: 12 April 2018; recruiting; ClinicalTrials.gov ID NCT03495297). However, the efficacy performance of S-ICD™ systems may differ in true clinical scenarios during the patient’s daily life from the artificial conditions of induced tachyarrhythmias within the intraoperative setting in the supine position and in presence of general anaesthesia. The termination success of the first shock in ‘real-life’ ventricular tachyarrhythmias has been consistently reported around 90%.1–3,10,19 These conversion efficacy rates for the first shock are similar to contemporary studies on transvenous ICD systems.26 The overall conversion efficacy rate (if up to five shocks are applied) ranges between 96% and 100% in S-ICD™ systems.1–3,10,19 The time to therapy was significantly prolonged (+3 s) in ‘real-life’ tachyarrhythmias as compared to intraoperative conversion testing.10 Taken together, the performance of S-ICD™ systems has shown to be effective and similar to transvenous ICD systems. Next to efficacy, the safety performance of S-ICD™ systems with regard to inappropriate shocks and peri- and post-operative complications is of vital importance. Since both, the implantation procedure and the S-ICD™ sensing and detection algorithms considerably differ from transvenous ICD systems, safety of S-ICD™ systems may also differ from transvenous ICD systems. Early publications on the EFFORTLESS S-ICD registry reported a 360-day inappropriate shock rate of 7% for the first generation S-ICD™ (SQ-RX® 1010; Cameron Health).2 Likewise, early major studies reported an overall incidence of inappropriate shocks of 13.1% over an 11-month average follow-up1 or a fraction of 13% patients, who received a total of 33 inappropriate shocks during a 18 month follow-up.9 Inappropriate shocks were mainly attributed to T-wave oversensing (39%) and supraventricular tachycardia above the discrimination zone (24%).3 A meta-analysis of five case–control studies17 that compared the outcome of transvenous vs. S-ICD™ systems demonstrated no differences with regard to infection rates, system or device failures or inappropriate therapies. However, the nature of inappropriate therapies differed between both systems since they were mainly caused by supraventricular tachycardias in patients with transvenous ICDs, as opposed to S-ICD™ patients, in whom inappropriate shocks were mainly driven by oversensing of noise and T-waves.17 Fewer lead complications occurred in S-ICD™ patients. Another study found that inappropriate shocks due to supraventricular tachycardia in S-ICD™ patients can be further reduced by dual-zone programming.14 Moreover, the specific problem of S-ICD™ systems on T-wave oversensing has been addressed by implementation of an optional SMART Pass filter within the INSIGHT™ algorithm in the second generation S-ICD™ (Emblem™ A209; Boston Scientific®). After implementation of these and further advancements, contemporary studies have reported decreased inappropriate shock rates of 3.5% per year19 or 11.7% over an average follow-up of 3.1 years.10 In a recent study that used a propensity score matched comparison, there was no statistical difference between inappropriate shock rates in subcutaneous vs. transvenous ICD systems with data obtained from the SIMPLE and EFFORTLESS study.20 Moreover, the patient selection process has been refined by introduction of a novel automated screening tool (AST) (Boston Scientific®) that approximates to the INSIGHT™ algorithm in S-ICD™ systems.27–30 As inappropriate shock rate data of major registries still comprise early data prior to these technical advancements, a further reduction in inappropriate shock rates may be expected in future studies. The proneness towards device infections represents one of the major problems in transvenous ICD systems (6%/12 years),31 which not least have driven the advancements of the S-ICD™ technology. The mid-term analysis of the EFFORTLESS registry10 demonstrated that only 2.4% of patients, who had a device-related infection, required a device removal. A propensity score matched analysis with the data from the SIMPLE study20 found no significant differences with regard to device infections that needed invasive interventions as compared to contemporary transvenous ICD systems. Furthermore, no cases of endocarditis occurred so far in the mid-term follow-up of the EFFORTLESS registry.10 However, the issue on device-related infections of subcutaneous vs. transvenous ICD systems clearly needs further randomized controlled data, which is expected from the ongoing PRAETORIAN trial.21 The same comparative gap in evidence applies for further complication events such as pocket haematoma (0.9%), or the necessity for system revision due to discomfort (0.8%), electrode movement (0.7%), or can movement (0.5%) (all data related to a 3.1 ± 1.5 years median follow-up of S-ICD™ systems).10 However, a significant reduction in complication rates has been correlated to the extent of experience of the implanting physician, suggesting a ‘learning curve effect’.14 Another major controversial discussion refers to the lacking ATP availability in S-ICD™ systems, as patients with recurrent monomorphic VT may benefit from ATP and should thus preferentially receive a transvenous ICD system. The mid-term results of the EFFORTLESS registry10 found very low rates (0.5%) of S-ICD™ removal due to ATP demand, but this cohort of patients is well selected. Thus, the clinical appraisal of the a priori probability for ATP demand is challenging but of utmost importance, especially in case of primary prevention. Among patients, who received a transvenous ICD for secondary prevention, a cut-off value of <300 ms for tachycardia cycle has been demonstrated to be rather not accessible for ATP, since 82% of patients with tachycardias <300 ms were converted by a shock with or without prior unsuccessful ATP.32 Hence, those patients may benefit from an S-ICD™ system. Alike, the presence of a remote myocardial infarction (OR 2.07; 95% CI 1.08–3.97) and a LVEF ≤ 35% (OR 2.09; 95% CI 1.09–4.00) were associated with tachycardia cycle length >300 ms that were mainly converted by ATP and may argue against S-ICD™ implantation. In a study on 431 transvenous ICD patients implanted between 2011 and 2015, a history of non-sustained VT or monomorphic VT was demonstrated as predictor for appropriate ATP therapy.33 However, it has to be considered that incidence rates of appropriate ATP delivery have dropped after publication of the MADIT-RIT trial34 in 2014, which may affect these results. Nevertheless, it may be worth to perform Holter ECG recordings prior to ICD implantation for ICD system selection. However, in transvenous ICD patients with lead failure or system-related infections, who had no history of ATP demand, a change towards a S-ICD™ system could be considered. Besides, even in patients with monomorphic VT, the S-ICD™ may be considered after successful ablation or effective antiarrhythmic drug therapy with mid-term non-recurrence of ATP-accessible VT. Finally, the discussion on the a priori probability towards ATP demand may be circumvented, if the technique of ATP-capable leadless pacemakers that work under unidirectional control with S-ICD™ systems could be established for the clinical application.35–37 Update on progress in S-ICD™ technology Almost a decade has passed since the introduction of the first S-ICD™ generation (SQ-RX® 1010; Cameron Health) in 2009 in Europe. Initial ‘real-life’ data have suggested a median battery longevity of 5.6 years (IQR 5.2–6.1) of the first S-ICD™ generation, which corresponds well to the manufacturer’s prediction.19 After achieving the FDA approval in 2012, the second S-ICD™ generation has been introduced in 2015 (Emblem™ A209; Boston Scientific®). To enhance the patients’ comfort, the second generation S-ICD™ can is 20% thinner resulting in a volume reduction of 15% (in total 59.5 cm3) and a 10% weight reduction to 130 g. Besides, the shape of the second generation S-ICD™ can was further rounded. The longevity of the Li+/MnO2 battery is expected to be enhanced by 40% in the second generation S-ICD™, leading to a projected longevity of ∼7.3 years in case of three annual full energy charges. However, as with all cardiovascular implantable electronic devices, real-life data may differ considerably from the projected longevity, hence clinical data on this issue are required.38 With the second generation, S-ICD™ patients could be implemented in a telemetric monitoring follow-up (LATITUDE™; Boston Scientific®). Sensing and detection in S-ICD™ systems is based on the INSIGHT™ algorithm, which is composed of three consecutive steps. In Phase I (detection), the subcutaneous ECG signal, which approximates to the surface ECG pattern, is detected. In Phase II (certification), four double detection algorithms are applied to avoid oversensing for determination of the heart rate. In the final Phase 3 (therapy decision), three rhythm discriminators are active to confirm the therapy. A major advance of the second generation S-ICD™ is the optional availability of the SMART Pass filter, which is a high pass filter that reduces the amplitude of signals with lower frequencies (e.g. T-waves), whereas signals with higher frequencies, (e.g. R-waves or VT/VF signals) remain largely unchanged. This algorithm should reduce the probability for T-wave oversensing, especially in case of decreased R-wave/T-wave ratios. Indeed, patients with activated SMART Pass filter had a significantly reduced risk for inappropriate shocks.39 Screening for eligibility of patients, who are considered for S-ICD™ implantation has been advanced, as a novel AST (Boston Scientific®) has been introduced in 2017. This software on the manufacturer’s programmer uses the initial Phase I, but not Phases II and III of the INSIGHT™ algorithm to predict the eligibility sensing vectors.27 In contrast to the prior manual screening tool (MST) that exhibits a considerable inter-observer variability,27,40 the novel AST is an objective process on storable data. The accuracy of AST in predicting eligibility of S-ICD™ sensing vectors from implanted S-ICD™ systems has so far been investigated in patients with already implanted S-ICD™ systems.27 However, this issue clearly needs further prospective evaluation in a larger cohort.29,30 Another important advancement has been established for the implantation procedure by introducing a two-incision implantation technique in 2013 to reduce the risk for complications at the tip of the electrode.41,42 Meanwhile, even a single-incision technique has been proposed;43 however, comparative outcome data are pending. The majority of S-ICD™ implantations are performed in general anaesthesia,11 as the tunnelling and dissection procedure causes a significant peri- and post-operative pain burden and defibrillation conversion testing requires conscious sedation.44 However, during the past years, there is a transition from general anaesthesia to other multimodal, non-general anaesthesia modalities,45 since general anaesthesia carries some risks for haemodynamic compromise, risks related to endotracheal intubation, post-operative nausea or vomiting, or post-intubation pneumonia and may require pre-, intra-, and post-operative involvement of an anaesthesiologist, which increases procedure times and overall costs.44–47 Among contemporary alternative anaesthesia modalities, monitored anaesthesia care (MAC)48 or non-anaesthesiologist-administered sedation and analgesia45–47 have been proven safe and effective for S-ICD implantation. Compared to transvenous ICD systems, that require local anaesthesia for a small-sized infraclavicular region, S-ICD™ systems need coverage of the left anterolateral chest wall and the left parasternal region,45 thus approaches that apply regional anaesthesia to block thoracic nerves have been proposed.44,49,50 Studies on combined transversus thoracic muscle plane block and thoracic paravertebral nerve block,49 truncal plane blocks44 or isolated serratus anterior plane block50 yielded promising initial results. Specifically, regional anaesthesia approaches may reduce or even spare opioid use.44 Up to now, there is no consensus on the best anaesthetic approach. The preoperative decision should be made individually for each patient in dependence of the clinical constellation, patient’s preferences, experiences of the implanting physician and structural prerequisites. To guide this decision, a recently published panel consensus has provided specific recommendations.45 Since magnetic resonance imaging (MRI)-capability becomes progressively important, the second generation S-ICD™ is full-body ‘conditional MRI-capable’ and have a distinct optional MRI protection mode. The third generation S-ICD™ (Emblem™ A219; Boston Scientific®) has an additional atrial fibrillation monitoring option that uses algorithms based on the ventricular rate scatter pattern and the corresponding heart rate histogram to detect atrial fibrillation. Performance data on the accuracy of this algorithm remain to be elucidated. Concerning other extravascular ICD options that compete with the S-ICD™ technology, there is a recently proposed approach to implant a shock coil in a substernal position via a subxiphoid access.51–54 Potential benefits of this approach may arise from the availability of antitachycardia and possibly even antibradycardia pacing. Moreover, lower defibrillation thresholds are expected, which may allow a downsizing of the pulse generator and/or extension of battery longevity durations. Initial studies demonstrated defibrillation efficacy rates of 92.9%, when a commanded 35-J-shock was applied on induced VF.52 Exact defibrillation thresholds were not tested within this study. In case of failed conversion success during S-ICD™ implantation, a substernal lead placement has been demonstrated as a viable solution.53 Pacing thresholds have been reported considerably higher than for endocardial systems due to the non-excitable surrounding tissue within the anterior mediastinum, the epicardial excitation path, air entrapment, suboptimal lead placement, and open-chest configuration in some cases.51,54 Far field sensing amplitudes were reported lower than recommended for endocardial systems.51 The fraction of patients, in whom an extracardiac chest wall excitation could be detected, was low, but the intraoperative application of paralytic drugs was not documented.51 However, the numeric values for both sensing and pacing thresholds were obtained from off-labelly used diagnostic multipolar pacing catheters as a proof-of-concept approach, hence these values may be advanced, when specifically developed system components are employed. As this approach is currently on an experimental stage, several questions remain to be addressed in future studies to assess the value of this technique in comparison to established S-ICD™ systems. In particular, it remains to be elucidated, whether the substernal pacing option is indeed reliably available in the clinical setting in dependence of body positions, diaphragmatic movement, and scar pattern of diseased myocardium. Alike, the acute and chronic safety aspects of a substernal lead placement need to be considered as there is a risk for perforation of heart, lungs, or internal mammary arteries or mediastinal (device) infections that are difficult to manage. Also, ineligibility criteria, such as retroperitoneal adhesions after sternotomy need to be defined. As this technique uses far-field sensing, eligibility may depend on the patient’s individual electro-anatomy analogous to S-ICD™ systems and may thus require a pre-implantation screening. Yet, the substernal lead position may be difficult to assess via a non-invasive pre-implantation screening. Clarification of these and further questions will define the value of this approach in comparison to modular S-ICD™ systems that synergistically interact with leadless endocardial pacemakers. Spotlight on S-ICD™ therapy in specific constellations S-ICD™ therapy in inherited arrhythmia syndromes: Brugada syndrome Among inherited arrhythmia syndromes, the Brugada syndrome is characterized by several peculiarities. Various mutations of the cardiac voltage-gated Na+ channel mediate the typical electrocardiographic Brugada type-1 pattern55 consisting of the coved-typed ST-segment elevation ≥ 2 mm. The transvenous ICD therapy in the predominantly middle-aged male patients56 is complicated by higher defibrillation thresholds, higher inappropriate shock rates, and higher lead failure rates.57 Initially, the specific electrocardiographic features of the Brugada pattern raised concerns on the feasibility of S-ICD™ therapy.58 Indeed, Brugada patients exhibit distinct higher ineligibility rates for S-ICD™ implantation as compared to other cardiac diseases. Among patients with implanted transvenous ICD systems without antibradycardia pacing demand, 30% of Brugada patients were considered ineligible for S-ICD™ implantation, compared to 8% of the non-Brugada patients (P < 0.003).59 Within the group of channelopathies, Brugada patients exhibit significantly lower rates of suitable sensing vectors.60 In particular, Brugada patients are prone towards an adversely increased ratio between T-wave and QRS complex thus promoting T-wave oversensing.59 Furthermore, the angle between QRS and T axis seems to be reduced in Brugada patients,59 yet this parameter might be too inconvenient for the daily clinical practice. Tachibana et al.61 demonstrated a higher prevalence of a complete right branch bundle block and significantly prolonged QRS and QT intervals in non-eligible vs. eligible Brugada patients. It is also of utmost importance that the typical Brugada pattern is not static, but dynamic and may vary in dependence of e.g. body temperature, vagal tone, or exercise. This carries two significant implications. First, screening should be performed serially within Brugada patients as Kawabata et al.59 nicely demonstrated an example of fluctuation between eligibility and ineligibility within the same Brugada patient. Second, eligibility in Brugada patients should be determined with an exercise test. Of note, manifestation of the Brugada pattern does not necessarily occur during exercise, which rather attenuates the ST segment elevation, but rather during the recovery phase where the ST-segment elevation augments.61 Of 45 Brugada patients, who were screened eligible in resting conditions, 11 converted to ineligible by exercise testing.61 Another approach to provoke the individual Brugada morphology may be achieved by pharmacological application of Na+ channel blockers. Of 88 patients, who were screened for Brugada syndrome, 21 patients, who were eligible for S-ICD™ implantation at rest, developed a type-1 Brugada phenotype, which led to ineligibility for S-ICD™ implantation in 5 of these 21 patients.62 In a small series of six consecutive S-ICD™ patients with Brugada syndrome, the eligible resting ECG converted to ineligible by drug challenge in two of the six patients.63 One of these six patients, who had an appropriate sensing during drug challenge, received inappropriate shocks due to T-wave-oversensing during exercise. Thus, ajmaline application during S-ICD™ screening in Brugada patients is a valuable option to unmask the Brugada pattern, if the resting ECG is normal at the time of evaluation. Of note, screening in Brugada patients should include both a left and a right parasternal electrode placement, since ineligibility rates of Brugada patients could be reduced from 30% to 18%, if both electrode positions were evaluated.59 In our experience, right parasternal lead placement may be a valid alternative for a certain fraction of patients with narrow heart silhouettes.64 Generally, risk stratification in Brugada syndrome is problematic due to the absence of RCTs4 and transvenous ICD therapy carries several adverse aspects in these patients. In a meta-analysis, Brugada patients with an transvenous ICD had an inappropriate annual shock rate of 3.9% predominantly due to supraventricular tachycardia and T-wave oversensing.65 This has been opposed to the comparatively low rates of appropriate ICD therapies in case of primary prevention (0.9% per year) or secondary prevention (2.5% per year).65 Moreover, high rates of lead malfunction (annually 6.3%) apply to Brugada patients.65 Thus, the S-ICD™ technology would be a viable alternative for Brugada patients, if the number of inappropriate shocks was acceptable in these patients. In a cohort of 62 consecutive S-ICD™ patients with inherited arrhythmia syndromes and a mean follow-up of 31 months, 24 Brugada patients were included.66 Each of 10 arrhythmia episodes in 4 Brugada patients was successfully treated by the first S-ICD™ shock,66 which is important regarding the higher defibrillation thresholds in Brugada syndrome.57 Only four inappropriate shocks in two patients occurred due to extracardiac causes that were unrelated to the specific electrocardiographic Brugada pattern.66 Single case reports refer to inappropriate S-ICD™ shocks in Brugada patients that could be resolved by reprogramming the sensing vector or activation of the SMART Pass filter.63,67 Gaps in evidence and future directions Since the reported eligibility rates were obtained with the MST they may differ, when screening is conducted with the novel AST. As the filter settings in AST specifically decrease the T-wave/QRS ratio, which represents a major reason for rule-out of Brugada patients,59 high rule-out rates of Brugada patients may be reduced by AST. Following the same line, inappropriate shock rates might be reduced in Brugada patients with the novel SMART pass filter, but this needs systematic evaluation. S-ICD™ therapy in structural heart disease: hypertrophic cardiomyopathy Transvenous ICD systems are well established for hypertrophic cardiomyopathy (HCM),68 since high-risk patients may exhibit sustained monomorphic VT69 that are accessible to ATP-therapy. Besides, some patients with manifest outflow obstruction may benefit from dual chamber pacing.70,71 Shock efficacy of S-ICD™ systems should be as effective as high-energy transvenous ICD systems, as the defibrillation threshold in HCM is increased in some patients.72 Not at least, high or inverted T-waves are a common finding in HCM, thus challenging the sensing and detection algorithms of S-ICD™ systems. Figure 2 represents a distinct electrocardiographic HCM pattern, which lead to rule-out by screening in the primary and alternate vector. Strikingly, both sensing vectors work flawlessly in the implanted S-ICD™ system, which raises the question, whether the S-ICD™ algorithm may potentially work better than predicted.27 Figure 2 View largeDownload slide Tracings from the surface ECG (left), the screening ECG (middle) and from an implanted S-ICD™ system (right) obtained from the same patient suffering from hypertrophic cardiomyopathy. Characteristic adverse surface ECG properties (tall R-wave amplitudes with discordant T-waves) lead to non-eligibility of the primary and alternate vector, since the T-wave clearly exceed the shape of the template. The R-wave of the secondary vector exceeds the tallest available template on the manual screening tool and is thus formally considered ineligible. Strikingly, the implanted S-ICD™ system exhibits flawless sensing in all vectors despite predicted ineligibility. The decision to implant despite borderline ineligibility of the secondary vector was made after thorough risk/benefit consideration and due to lack of alternatives in this young patient. S-ICD™, subcutaneous implantable cardioverter-defibrillator. Figure 2 View largeDownload slide Tracings from the surface ECG (left), the screening ECG (middle) and from an implanted S-ICD™ system (right) obtained from the same patient suffering from hypertrophic cardiomyopathy. Characteristic adverse surface ECG properties (tall R-wave amplitudes with discordant T-waves) lead to non-eligibility of the primary and alternate vector, since the T-wave clearly exceed the shape of the template. The R-wave of the secondary vector exceeds the tallest available template on the manual screening tool and is thus formally considered ineligible. Strikingly, the implanted S-ICD™ system exhibits flawless sensing in all vectors despite predicted ineligibility. The decision to implant despite borderline ineligibility of the secondary vector was made after thorough risk/benefit consideration and due to lack of alternatives in this young patient. S-ICD™, subcutaneous implantable cardioverter-defibrillator. Ineligibility rates for S-ICD™ implantation considerably differ among patient cohorts of different studies, ranging from low values of 7%,73 mediocre values of 15–16%74,75 and high values of 38%.76 This variability may be attributed to specific differences of the observed patient collectives and differences in the screening process. The ineligibility rates may correlate to the calculated risk of SCD.76 A common 12-lead surface ECG predictor of screening failure in HCM is a high R-wave/T-wave ratio and the T-wave inversion itself.74,76 Interestingly, the alternate sensing vector may be favourable in HCM due to its almost perpendicular orientation to the anatomic heart axis.73 Furthermore, additional right parasternal screening seems to be worthwhile in HCM and should be performed routinely at least in case of ineligibility in the left parasternal position.73 Alike, additional exercise testing should be considered.73,75,76 A large-scaled study reported no correlation between an insufficient safety margin and HCM in a mixed cohort of 7960 S-ICD™ patients.77 In a case-report, a transvenous ICD system failed to convert VF in a patient with HCM; however, the subsequently implanted S-ICD™ effectively terminated VF.78 A pooled analysis of 872 S-ICD™ patients in the EFFORTLESS Registry and US IDE study resulted in a successful defibrillation rate in 98.9% HCM patients as compared to 98.5% in non-HCM patients.79 Based on these data, S-ICD™ shock efficacy seems to be sufficient in clinical manifest HCM. Follow-up data on the performance of S-ICD™ systems in HCM are promising. One-year post-operative complication-free rates were similar in HCM and non-HCM patients, each VT was successfully terminated with the first S-ICD™ shock and the rates of inappropriate shocks were equal between HCM and non-HCM patients.79 In a small cohort of 18 patients, we found a considerably higher fraction of patients (4 patients; 22%) with inappropriate S-ICD™ shocks due to T-wave oversensing.80 However, in all but one patients, inappropriate sensing could be revised by programming. Gaps in evidence and future directions Future studies will have to elucidate the value of S-ICD™ therapy in HCM with regard to monomorphic VT that are potentially accessible to ATP. Since HCM is a dynamic and progressive disease at both morphological and electrocardiographic level, it is conceivable that patients, who were eligible prior to implantation, may develop inappropriate sensing during follow-up. In this line, it will be of interest to observe the impact of myectomy and activation of the SMART Pass filter. Combination of S-ICD™ with other cardiac implantable electronic device Clinical constellations that favour a combination between the S-ICD™ system and a pacemaker for example consist in pacemaker patients, with adverse vascular access, who develop an ICD indication (Figure 3). To date, device combinations with the S-ICD™ system are reserved for a minority of well-selected patients [e.g. 3% (n = 13) of patients in the EFFORTLESS study].2 Patients with pacemaker had a higher likelihood for inappropriate shocks.10 However, recently published patient series have demonstrated a flawless interaction of pacemakers and S-ICD™ systems if several aspects are attended.81,82 Figure 3 View largeDownload slide Tracings from the surface ECG (left), the screening ECG (middle) and from an implanted S-ICD™ system (right) obtained from the same patient, who has a congenital heart disease and requires permanent pacemaker stimulation due to AV-Block III°. The ventricular pacemaker stimulus artefact is clearly visible in the surface and screening ECG, but interestingly not in the tracing of the implanted S-ICD™ system due to the distinct filter settings of S-ICD™ systems. The T-wave of the secondary vector is considered ineligible within the screening process, since the T-wave exceeds the templates shape; however, the implanted S-ICD™ system exhibits a flawless sensing on each vector. S-ICD™, subcutaneous implantable cardioverter-defibrillator. Figure 3 View largeDownload slide Tracings from the surface ECG (left), the screening ECG (middle) and from an implanted S-ICD™ system (right) obtained from the same patient, who has a congenital heart disease and requires permanent pacemaker stimulation due to AV-Block III°. The ventricular pacemaker stimulus artefact is clearly visible in the surface and screening ECG, but interestingly not in the tracing of the implanted S-ICD™ system due to the distinct filter settings of S-ICD™ systems. The T-wave of the secondary vector is considered ineligible within the screening process, since the T-wave exceeds the templates shape; however, the implanted S-ICD™ system exhibits a flawless sensing on each vector. S-ICD™, subcutaneous implantable cardioverter-defibrillator. There are two major possibilities for cross-talk between pacemakers and S-ICD™ systems. First, the pacemaker stimulus may be detected by the S-ICD™ system, which may result in double counting together with the QRS complex or even triple counting in case of sequentially atrioventricular pacing.83 Second, in case of low-amplitude VT/VF, detection of stimulation spikes may decrease the S-ICD™ sensitivity, which may lead to undersensing. The risk for both adverse events can be reduced by programming. In general, a bipolar stimulation configuration should be selected, since the stimulation spike amplitude is lower than in a unipolar configuration.82 The stimulation output should be kept as low as reasonable.83 Endocardial leads may be more suited than epicardial leads,84 although even unipolar or epicardial leads may be feasible.82,84 Pacemakers that can automatically switch from a bipolar into a unipolar stimulation mode should not be combined with S-ICD™ systems.82 A dual-detection zone is recommended, as the conditional shock zone contains longer blanking periods than the non-conditional shock zone, which reduces the risk for oversensing of stimulation spikes. Whenever compatible with the patient’s needs, the upper rate limit of pacemaker systems should not exceed 100 b.p.m. and the conditional shock zone should start at 210–220 b.p.m. to avoid inappropriate shocks in case of double counting.83 Dual chamber pacemaker algorithms that reduce ventricular pacing may also reduce the risk for inappropriate shocks. Post-shock pacing of S-ICD™ systems should be deactivated.85 Sensitivity of pacemaker systems should be kept high for a proper sensing of ventricular tachyarrhythmia and withholding of pacing in VT due to undersensing. The S-ICD™ sensing vector with the lowest or absent stimulation spike should be favoured.82 Septal pacing may be more suitable due to a narrower QRS complex. Apart from these general recommendations after implantation, pre-implantation assessment for eligibility should be performed on the intrinsically conducted and the stimulated QRS complex. Since pacemaker patients are prone to rate-dependent bundle branch block patterns, an exercise test or high-rate atrial pacing should be performed.85 The ventricular defibrillation conversion should be performed with asynchronous ventricular pacing (V00 or D00) with maximum stimulation output.81 Most promising is the combination of leadless pacemakers and S-ICD™ systems due to the absence of intracardiac leads—however, experiences are so far very limited. Mondesert et al.35 reported in 2015 a patient without accessible subclavian vein, who first received S-ICD™ implantation for primary prevention and secondly developed a third degree atrioventricular block. The subsequently implanted leadless pacemaker system maintained flawless pacing after maximum energy S-ICD™ shock and no oversensing occurred during maximum stimulation output. These encouraging initial experiences were further supported by Tjong et al.36,37 in an animal model and a small human series. Beyond that, the latter group reported pre-clinical data on the combination of a leadless pacer that is able to deliver ATP under unidirectional control of the S-ICD™ system, which could potentially bridge the gap to patients with monomorphic VT, who may benefit from ATP.37 Some patients develop a demand for antibradycardia pacing or cardiac resynchronization after S-ICD™ implantation.10 Yet, the dynamic alteration of the QRS morphology especially in CRT patients with non-permanent biventricular pacing should be considered carefully. The biventricular pacing QRS morphology may be more suitable for proper S-ICD™ sensing than LV- or RV-only pacing, potentially due to the broader QRS morphology in univentricular stimulation.86,87 S-ICD™ and other cardiac implantable electronic device Heart failing S-ICD™ patients may benefit from implantation of a baroreflex activation system.88 Promisingly, initial clinical experiences demonstrate only a negligible interaction of both devices.89,90 Moreover, positive experiences have been published on the combined use of S-ICD™ systems and cardiac contractility modulation devices81,91,92 and even left ventricular assist devices,93 suggesting a robust function of S-ICD™ systems in combinations with other cardiac implantable electronic devices (CIED) and vice versa, although this needs further confirmation. S-ICD™ therapy in obesity and small body build Obesity is independently associated with an inadequate safety margin during VF testing in S-ICD™ systems, but not in transvenous ICD systems.77 Moreover, within 30 days after S-ICD™ implantation, obesity was found as a predictor for device- and procedure-related complications.11 One specific problem of obesity in S-ICD™ therapy is the risk for inconstancy of the S-ICD™ can and lead position in relation to the heart. Figure 4 illustrates the chest X-ray of a male patient with a body mass index of 41 kg/m2. Since the S-ICD™ implantation is performed in the supine position, the S-ICD™ position changed considerably post-implantation in the erect position, since the pre-thoracic fat tissue moves caudally together with the S-ICD™ system. Hence, the electric field of the sensing vectors between can and coil was below the heart, which caused inappropriate sensing. Thus, in case of obesity, it is crucial to insert the S-ICD™ system as close as possible to the thoracic cage to avoid S-ICD™ system movement. A too peripheral system placement may also be reflected by high intraoperative impedance values.94 Droghetti et al.94 even proposed an entire submuscular S-ICD™ placement in case of obesity. Figure 4 View largeDownload slide Anterior–posterior X-ray in erect position of a patient with advanced obesity (BMI: 41 kg/m2) and implanted S-ICD™ system. The S-ICD™ system was implanted in supine position. The post-operative X-ray in erect position reveals that the S-ICD™ system critically shifted caudal together with the prethoracic fat tissue resulting in a shock field below the anatomical apicobasal heart axis thus requiring revision. BMI, body mass index; S-ICD™, subcutaneous implantable cardioverter-defibrillator. Figure 4 View largeDownload slide Anterior–posterior X-ray in erect position of a patient with advanced obesity (BMI: 41 kg/m2) and implanted S-ICD™ system. The S-ICD™ system was implanted in supine position. The post-operative X-ray in erect position reveals that the S-ICD™ system critically shifted caudal together with the prethoracic fat tissue resulting in a shock field below the anatomical apicobasal heart axis thus requiring revision. BMI, body mass index; S-ICD™, subcutaneous implantable cardioverter-defibrillator. Oppositely, in case of a small body build (e.g. paediatric patients or Asian population), diverse and specific problems may be associated with the S-ICD™ therapy. Assessment of eligibility prior to S-ICD™ implantation yielded considerably higher rule-out rates in adolescents due to adverse electro-anatomic conditions. A prospective multicentre study on 73 patients, who are ≤18 years of age and already have a transvenous ICD for various indications implanted, found an ineligibility rate of 26%.95 This rule-out rate is high as compared to adult patients cohorts with various cardiac diseases, as they exhibited rule-out rates between 7% and 15%.40,96,97 For a small body build, the weight and volume ratio between ICD system and body is less favourable and may cause discomfort at the implantation sites. Beyond that, body image concerns may be more relevant for paediatric patients.98 Fortunately, the second generation S-ICD™ can has a rounder and thinner shape than the first generation, which may ameliorate the system-related discomfort. Furthermore, the somatic growth in paediatric patients carries the risk for S-ICD™ system dislodgement, although this risk might be lower and easier to revise as compared to transvenous ICD systems. To acknowledge these circumstances, most centres do not implant S-ICD™ systems in children below a body weight of 25–30 kg and 8–9 years of age,99,100 although successful retroperitoneal can placement has been described in very young children with a hypoplastic left heart syndrome.101 The few available efficacy data in adolescent patients consistently report a high efficacy performance of S-ICD™ systems.102,103 Given the high conversion rates of 65–80 J shocks that are delivered by the large S-ICD™ cans, it may be questioned, whether a ‘paediatric version’ of an S-ICD™ system with a smaller can and reduced shock energy may be sufficient for this specific cohort, as it has been proposed in the paediatric population for transvenous ICD systems.104 Jarman et al.105 reported early after introduction of the S-ICD™ system in 2012, considerably high rates of children, who require a re-operation due to threatened skin erosion or wound dehiscences caused by physical activities or an accident. In an age-matched comparison, we found no statistical differences between young patients with S-ICD (3.2%) and those with transvenous ICD (12.9%), who had adverse events that required reoperation or intervention (P = 0.35).106 As the superior skin incision above the tip of the subcutaneous lead is the most import predilection site for skin erosions, the two-incision implantation technique has been established, which spares the superior skin incision and should be preferred, particularly for adolescents.41,42 Conclusions This review gave an update on currently available and future milestone outcome studies, recent technical advancements in S-ICD™ systems and shed a spotlight on the performance of S-ICD™ systems in clinical fields of indication for S-ICD™ therapy beyond the classical indications. As representative examples, we have discussed the adverse ECG pattern in the Brugada syndrome, HCM with regard to shock efficacy, the risk for adverse interactions of the S-ICD™ system with other CIED and finally the feasibility of S-ICD™ therapy in distinct obesity. Overall, the performance of S-ICD™ systems seems to be promising even in delicate clinical constellations, but further research is clearly needed to improve comprehension of potential problems. These insights could thereafter be used to further improve S-ICD™ technology for a safe and effective application even beyond established fields of indication. Conflict of interest: N.B. and G.F. are supported by a fellowship and lecture honoraria of Boston Scientific Inc. and received travel grants from Biotronik, Boston Scientific, Medtronic and Abbott. K.W., P.N., B.R., D.D., S.K., A.L., J.K., F.R., and L.E. have received lecture honoraria and travel grants from Biotronik, Boston Scientific, Medtronic and Abbott. 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TI - Spotlight on S-ICD™ therapy: 10 years of clinical experience and innovation
JF - Europace
DO - 10.1093/europace/euz029
DA - 2019-07-01
UR - https://www.deepdyve.com/lp/oxford-university-press/spotlight-on-s-icd-therapy-10-years-of-clinical-experience-and-VE7Oet0xOn
SP - 1001
VL - 21
IS - 7
DP - DeepDyve
ER -