Critical appraisal concerning “Wearable cardioverter defibrillators for the prevention of sudden cardiac arrest: a health technology assessment and patient focus group study”

Dear editor

We read with great interest the article “Wearable cardioverter defibrillators for the prevention of sudden cardiac arrest: a health technology assessment and patient focus group study” by Ettinger et al.1 The authors conclude that wearable cardioverter defibrillators (WCDs) seem to be fairly safe in the short-to-medium term, but the quality of the available evidence is low. They also state that – according to their study inclusion criteria – they were not able to identify studies to assess the clinical effectiveness of the WCD. Given the importance of WCD for its target population and considering our clinical expertise, we see a particular need to clarify some points of this article.

The authors consider an implantable cardioverter defibrillator (ICD) as a WCD comparator. However, it is important to clarify that a WCD is not a substitute for an ICD, since it will only be temporarily worn. Treatment with a WCD may be considered for patients with a high risk of sudden cardiac death who are currently not candidates for an ICD.

Health Technology Assessments systematically aim to evaluate the effects of a technology on different health aspects, by collecting as much evidence as possible. This includes not only randomized controlled studies, but also studies with “weaker” designs should be considered and results should be discussed, taking into consideration their hierarchical importance in terms of internal validity.2 Ettinger et al1 decided to exclude retrospective studies, but this type of study design is not only the most often applied to evaluate the WCD’s effects, it is also the one that embodies by far the largest number of patients, such as that of Epstein et al,3 (N=8,678), Zishiri et al,4 (N=4,149), and Chung et al,5 (N=3,569). Simultaneously, the authors included case series involving very few patients (Duncker et al6 [n=7 of N=12 with WCD] and Kondo et al7 [N=24]), making it questionable why small one-armed case series are preferred to large retrospective studies with several thousand patients. The narrow inclusion criteria led to the nonidentification of publications for efficacy studies on the WCD.

The authors did not account for the heterogeneity in study designs (interventional single-arm study, prospective case series, and prospective registry studies), when reporting the research results: findings were summarized for N=2,000 (Kutyifa et al8) patients together with a case series including only N=12 patients (Duncker et al6). We suggest that the study results are presented in a way that takes into account differing study methodologies as well as differing study populations.

The authors claim to have conducted a focus group, although the requirements are not fulfilled. Only 5 participants, who had all undergone heart transplantation, were included, none with practical experience in using the WCD, none with an indication for a WCD, and none with knowledge regarding its technology. Thus, participants could not meaningfully comment on the possible use of the WCD. It is highly questionable how accurately the focus group participants were informed about the devices’ functionalities: participants seem concerned that patients must decide when to receive a shock by pressing the WCD response buttons. This suggests a serious misunderstanding. It is crucial to highlight that patients can neither prevent an appropriate therapy nor trigger inappropriate shocks by pressing the WCD response buttons. Shocks must only be triggered when patients are in an unconscious state. We find it rather misleading that findings of what was apparently a single-group interview have been reported as if they were results of a qualitative study.

In our opinion, it is of high relevance to make the reader aware of the aforementioned limitations and how they influence the conclusions of the publication.