Author + information
- Parikshit S. Sharma, MD, MPH and
- Kenneth A. Ellenbogen, MD∗ ( )()
- ↵∗Reprint requests and correspondence:
Dr. Kenneth A. Ellenbogen, Division of Cardiology, Virginia Commonwealth University, 1200 East Marshall Street, Gateway Building, Third Floor, Richmond, Virginia 23298.
Several large randomized controlled trials have demonstrated reduction in total mortality in primary and secondary prevention with the implantable cardioverter-defibrillator (ICD) (1,2). With advances in technology over the past decades, ICD systems have become smaller, and we now have a choice between the totally subcutaneous (S)-ICD and the conventional transvenous (TV)-ICD.
The TV-ICD system requires insertion of the electrode into the central venous circulation, which increases the risk of vascular obstruction, thrombosis, infection, and cardiac perforation (3,4). Lead failures or bloodstream infections often necessitate the extraction of these leads, and this can be very challenging, with major complication rates of about 1% even in experienced centers (5). There have been recalls for poorly functioning ICD leads as well as disappointing long-term ICD survival rates with some leads.
The development of the S-ICD was initially motivated because of vascular access issues in patients with congenital heart disease or limited venous access. The S-ICD system was approved by the U.S. Food and Drug Administration in 2012 on the basis of data from a regulatory investigational drug exemption study: a prospective, nonrandomized, multicenter trial by Weiss et al. (6). The main advantages of this system are that it has no leads within the heart, preserves the central venous circulation, and avoids some of the issues associated with a TV system. Another benefit is a lack of elevation of troponin or creatine phosphokinase levels in anesthetized pigs that received the S-ICD compared with those receiving the TV system (7).
The potential limitations of the S-ICD system include the need for pre-implantation screening, which is mandatory to avoid double counting of QRS complexes or T-wave oversensing; higher rates of inappropriate shocks (the majority from T-wave oversensing) in 13.1% of cases, despite appropriate programming (8); lack of pacing capability (except for post-shock pacing); a larger pulse generator with shorter anticipated battery life; and a higher cost than traditional TV-ICDs.
There has been a paucity of large studies comparing the S-ICD system with the TV-ICDs in a head-to-head manner. In this issue of the Journal, Brouwer et al. (9) provide long-term clinical outcome data from patients with S-ICDs in comparison to the TV-ICD system in a propensity-matched patient cohort. They compared 140 matched pairs balanced for 16 baseline characteristics: 140 patients with S-ICDs compared with a propensity-matched group of 140 patients with TV-ICDs from a patient cohort of 1,312 patients in a Netherlands-based health system. The main findings of this study were: 1) the overall complication rate at 5-year follow-up was similar in the 2 groups (13.7% in the S-ICD group vs. 18.0% in the TV-ICD group; p = 0.80); 2) appropriate shocks (17% in the S-ICD group and 21.3% in the TV-ICD group; p = 0.36) and inappropriate shocks (20.5% in the S-ICD group and 19.1% in the TV-ICD group; p = 0.64) were delivered at equal rates in both groups; and 3) 5-year patient survival (96.0% vs. 94.8%; p = 0.42) and need for pulse generator replacement due to battery depletion (p = 0.18) were not different at the 5-year follow-up.
The investigators should be congratulated on this important contribution to our understanding of the differences in outcomes between the TV- and S-ICD systems. In the absence of randomized data comparing these 2 implant strategies, this study represents the first of this size and kind that provides insight on this topic. It is important to review the methodology and findings more closely to help contextualize its findings and frame opportunities for future investigation. Although there were no overall differences in long-term outcomes between the 2 groups, it is important to note that there were significant differences in the type of outcomes between the 2 implant strategies.
First, in this study, use of the propensity-matching model limits the patient population to patients who were younger, were less often female, had a higher average left ventricular (LV) ejection fraction, and less often had ischemic cardiomyopathy compared with the nonmatched cohort. Also, in comparison to other S-ICD registries (8), the study population is younger and has a higher LV ejection fraction. This highly selective patient population comparison makes the results of this study difficult to generalize to most patients who are candidates for ICD therapy.
Second, although the overall complication rates were similar in the 2 groups, defibrillation lead failure was higher among the TV-ICD leads (8.5%) than the S-ICD leads (0%). This difference was offset by other types of complications (pocket erosion, defibrillation threshold testing failure, and device failure) that were higher in the S-ICD patients (9.9%) than in the TV-ICD patients (2.2%; p = 0.047). One key limitation that might confound this finding is the fact that the follow-up duration is significantly different between the 2 groups: TV-ICD patients had a longer median follow-up of 5 years compared with 3 years in the S-ICD group (p < 0.001). As noted in the Kaplan-Meier curves in the Central Illustration of the paper by Brouwer et al. (9), the number of patients in follow-up dwindled in the S-ICD group in a significant manner, so that by 3 years, the number of patients in the TV-ICD group was more than twice the number in the S-ICD group. This brings into question whether the difference in complications would be similar if there were a longer follow-up in the S-ICD group. It is also important to note that 88.6% of TV-ICDs were dual-chamber devices. Despite this fact, the risk of nonlead failure complications was lower in the TV-ICD group.
Third, there was also no difference in appropriate shocks in the 2 groups. However, a higher incidence of appropriate therapies (antitachycardia pacing [ATP] + shocks) in the TV-ICD group (31.3% vs. 17%) was noted, primarily driven by ATP in the TV-ICD group (21.8%). One would instinctively expect that the number of ICD shocks might be lower in the TV-ICD group, given the ability to pace terminate a portion of true ventricular tachycardia (VT) events; however, this was not noted in the study. It is important to note that patients in the S-ICD group were programmed to a more “liberal” unconditional zone (median of 250 beats/min) in comparison to the VF zone of 230 beats/min in the TV-ICD group. This liberal programming, along with a longer charge time in the S-ICDs, might allow more nonsustained VTs to terminate, and could explain this lack of difference in appropriate shocks. Also, the overall ability of TV-ICDs to deliver ATP after VT detection may also help explain the difference in ICD therapies (ATP + shocks) between the groups. There were no data on the number of successful ATP-terminated VT events or response to ATP; hence, it is difficult to assess this influence on VT termination or acceleration. Also, given the low percentage of patients with ischemic cardiomyopathy, the ability of ATP to pace-terminate a non–re-entrant VT mechanism or spontaneous serendipitous termination could explain some of these findings. When looking at inappropriate therapy and shocks, although there were no overall differences in the incidence of shocks, the majority of inappropriate shocks were for supraventricular tachycardia (94%) in patients with TV-ICDs and for oversensing (85%) in patients with S-ICDs. These findings are in line with prior trials, such as the START (Subcutaneous versus Transvenous Arrhythmia Recognition Testing) study (10), which demonstrated a higher specificity of supraventricular tachycardia detection by the S-ICD system.
In summary, this study by Brouwer et al. (9) provides valuable insight into the long-term outcome differences between S- and TV-ICD systems. The possible use of S-ICD as a first-line strategy or as an alternative approach to TV-ICD for specific populations needs further evaluation using large clinical trials and registries, such as the PRAETORIAN (Prospective, RAndomizEd comparison of subcuTaneOus and tRansvenous ImplANtable cardioverter-defibrillator therapy) study (11) and the EFFORTLESS S-ICD (Evaluation oF FactORs ImpacTing CLinical Outcome and Cost EffectiveneSS of the S-ICD) registry (12) that are currently ongoing.
↵∗ Editorials published in the Journal of the American College of Cardiology reflect the views of the authors and do not necessarily represent the views of JACC or the American College of Cardiology.
Dr. Ellenbogen has received honoraria and research support from Biosense Webster, Medtronic, Boston Scientific, Atricure, Biotronik, and St. Jude Medical. Dr. Sharma has reported that he has no relationships relevant to the contents of this paper to disclose.
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