Discussion
Although the TV-ICD system has served us well over the past 30 years, having been implanted in over 1 million patients worldwide, there remain significant concerns regarding the potential problems of long-term intravascular lead complications particularly in young primary prevention patients who may face over 40 years of generator and lead revisions. This has spurred the endeavour to provide alternatives to combat what is often considered the 'Achille's heal' of the TV-ICD—the intravascular lead, at least in those patients not requiring permanent pacing or anti-tachycardia pacing (ATP). The EFFORTLESS S-ICD Registry was initiated in order to provide 'real-world' systematically collected system performance data over a suitably prolonged period beyond that normally collected in randomized controlled trials, since these are primarily interested in survival endpoints as opposed to the important details of system performance. The Registry currently demonstrates that the device is being successfully implanted in a broad spectrum of patients with 98% first procedure induced VF conversion efficacy. Furthermore, there has been 100% overall clinical conversion efficacy of discrete episodes of spontaneous VT/VF (88% first shock conversion efficacy) either immediately post-shock or within a few seconds of shock delivery. Overall, conversion efficacy of spontaneous episodes is 96.1% (90.8% CI, 100%). This is equivalent to the FDA Investigational Device Exemption (IDE) data where the conversion efficacy for spontaneous episodes was 92.1% on the first shock and 37 of 38 (97.4%) with one or more shocks. Furthermore in the context of DFT testing, the Registry data show similar efficacy to IDE 99.7% (99.2, 100%) vs. 94.7% (with a 95% lower confidence limit of 91.7%). Five of the six VT/VF storm events were successfully converted by the device. The patient with Loeffler's syndrome who did not survive the cardiac arrest is an unusual indication. As this patient had an ongoing biopsy-proven inflammatory disease process requiring steroid therapy, this may have led to elevation of the DFT.
The first shock conversion efficacy of 88% is very much in line with rates published in TV-ICD and Cardiac Resynchronisation-Defibrillator (CRT-D) cohorts which is particularly important considering the potential differences in the S-ICD patient population, including an overall average younger age and a high prevalence of non-ischaemic cardiomyopathies, congenital, and channelopathy patients—all of whom are historically more difficult to treat with the TV-ICD. The implant procedure has not been associated with the typical implant-related complications of haemo/pneumothorax and lead displacement seen in 2–6% of trial and Registry TV-ICD populations. The only significant complication has been that of procedure-related infection affecting ~4% of patients overall and resulting in explant in 2%. Infections are most probably related to procedural inexperience in terms of appropriate skin preparation, draping, and suturing associated with this new procedure which requires an unfamiliar, more surgical approach of left lateral thoracotomy skin incision and tunnelling of the lead. However, the learning curve and shared experience of optimal pre- and peri-operative technique should mean that this initial complication can be suitably addressed. Indeed in the Cameron Health IDE study, once optimal technique between centres was agreed upon there were no subsequent infections requiring explant after approximately the first 100 patients suggesting a problem related to inexperience of a new implantation technique. The relationship between infections and experience is less obvious in the EFFORTLESS Registry. It should also be recognized that infection remains a significant complication of TV-ICD implantation with acute infections in the first 30-day post-implant ranging between 2 and 4% (Entrust IDE, Canadian ICD Registry, Medicare, Canadian Advisory data) depending upon the population of patients (age, co-morbidities) and experience of the implanting centres. Infections requiring system explantation range between 1 and 2% for TV-ICDs which is compatible with the early experience with the S-ICD.
The inappropriate shock rates (7%) are comparable with the standard TV-ICD registries and trials which range from 4 to 18%. However, in contrast to TV systems, the main cause of inappropriate shocks with the S-ICD is T-wave oversensing. The S-ICD has several options for management of inappropriate shocks without the need for an invasive procedure including reprogramming of the sensing vector and exercise testing with template updates. Indeed, the more prolonged detection time and programming of a dual zone device with SVT discrimination algorithms and conditional shock zone for higher rates >220/min may have helped to minimize inappropriate shock therapy and allowed spontaneous VT episodes to self-terminate as has been recognized in recent studies of modifying VT detection criteria and delaying ATP therapies. In the PREPARE trial which prolonged VT detection to 30/40 beats inappropriate shocks were reduced to 4% as opposed to 35% over 5 years in SCD-HeFT; 35% of VT's self-terminated in PainFreeRx indicating that the strategy of prolonging detection time before committing to therapy is a reasonable approach supported by this recent TV-ICD data.
Comparison With Recent Cohort Studies
This is the largest series of S-ICD patients to be reported to date and reflects practice across multiple centres worldwide. Two recent single country series from the Netherlands and UK reported upon 118 and 111 patients, respectively. The inappropriate shock rates were higher, occurring in 13 and 15% of patients and mainly due to T wave oversensing. This is double the rate observed in this larger cohort and probably is a reflection of several issues. Firstly, many of the reported patients were implanted with the device either prior to its CE mark, or immediately after. Subsequent updates to the noise detection algorithm occurred as a result of inappropriate therapy recorded in these early patients. Secondly, with continued experience there has been an increased recognition of appropriate patient management prior to device implant including ensuring there is ideally more than one acceptable sensing vector during screening; optimising heart rate thresholds for therapy as well as ECG screening in different postures and during increased heart rates. Similarly, the higher infection rates requiring explant of 5.8 and 4% vs. 2.2% in this series are most probably a reflection of increased physician experience and optimization of implant technique. In none of these series has there been a failure to deliver therapy in the programmed shock zone for ventricular arrhythmia although there was one arrhythmic death in the UK cohort. However, the use of the S-ICD as a first line therapy in all ICD patients without the need of pacing will require confirmation in clinical trials comparing the S-ICD to the TV-ICD which are currently ongoing.
Limitations
This is a Registry designed to record the real world experience planning ultimately to recruit 1000 patients with 5 years of follow-up data. The initial results demonstrate the early outcomes in the first 12 months after system implantation. The issue of long-term device performance particularly appropriate and safe cardioversion of VF in daily life as opposed to the controlled confines of a DFT test will only become clearer with time. The fact that the system performs effectively at implant is supported by this and the IDE data. It is recognized that controversies exist regarding whether DFT testing actually is appropriate for assessing ICD efficacy and most ICD cardioversion failures occur in the real-world under conditions of major metabolic derangement, hypoxia, and ischaemia which are beyond the normal ranges of standard DFT testing when the patient is in a well oxygenated, sedated state. Despite this, successful DFT at implant has been employed as an appropriate clinical safety endpoint particularly for a new technology and an indication of safe system performance as required by the FDA.