How good is your life vest in the real world?

Wearable cardioverter defibrillator (WCD) can be thought of as a life vest till the lifeboat (implantable cardioverter defibrillator - ICD) arrives [1]. It is often used as a bridge to the implantation of an ICD when the indication for the latter is not yet well established in the subject. Sometimes it may be used as definitive therapy in those with limited life expectancy due to other serious comorbidities. Yet another reason to use a WCD is when an ICD has been explanted following infection and one is waiting for the infective process to settle. In concept, the WCD is an ideal device for all these situations. We do have enough clinical trial data supporting the use of WCD for various indications [2].

Naniwadekar A and associates have reported the real world utilization of the device in a community setting, in this issue of the Journal [3]. They evaluated a total of 140 patients of which 46% had nonischemic cardiomyopathy (NICM) and 32% ischemic cardiomyopathy (ICM). There were a few with genetic predisposition for sudden cardiac death and some in whom an ICD had been explanted. Cumulatively, they have presented the data for 21 patient years, with a median usage per patient of 43 days. Daily mean use was 17.3 ± 7.5 hours. This was lower than the real world data presented by Dillon KA et al. [4], where the daily usage was 21.3 hours (range of 0–23.9) among 2105 patients.

Ethnic distribution of the patient population was grossly skewed, with 85.9% belonging to the African American race. This, the authors mention as representing the distribution in the community to which their institution caters. Skewed racial distribution may have some effect on the generalizability of their results. The predominance of NICM over ICM in their study group is also possibly related to the higher prevalence of NICM in their setting. WEARIT-II Registry on real world usage of WCD, had 46% of NICM and 40% ICM among their 2000 participants [5].

The total number of shocks, both appropriate and inappropriate, were small, due to the lower number of total subjects in the study. Out of the 6 shocks, 2 were appropriate and 4 inappropriate. This 2.8% inappropriate shock rate was much higher than the 0.5% reported from WEARIT-II Registry [5]. But the gross difference in the number subjects (140 vs 2000) prevents any meaningful comparison between the two studies. No appropriate shocks were noted in the ICM group, while 2 in the NICM group received appropriate shocks. Both of them had QRS widening and belonged to the African American race. One of them had an ICD implanted while the other recovered left ventricular function sufficiently to be moved out of ICD indication.

One patient with NICM and another with recurrent syncope and non sustained ventricular tachycardia received inappropriate shocks due to artefacts, which is of concern to us. This would indicate the need for better device algorithm and application to reduce inappropriate shocks due to artefacts. The rate of inappropriate shocks due to artefacts in the WEARIT-II Registry was 2 per 100 patient years. They comfort us by noting that none of the inappropriate shocks resulted in the induction of ventricular tachycardia or fibrillation [5]. Two NICM patients received inappropriate shocks due to supraventricular tachycardia with fast ventricular rate.

It is heartening to note that left ventricular ejection fraction improved in 32%, obviating the need for an ICD. In these patients, WCD served almost as definitive therapy rather than a bridge therapy. Percentage of patients recovering left ventricular function sufficiently were similar in both NICM (33%) and ICM (29%) groups. Forty five patients (32%) received an ICD on termination of use of WCD. This is comparatively lower than the 42% receiving an ICD in WEARIT-II Registry [5].

A special feature of WCD which is not present in ICDs is the patient response button which can prevent delivery of shocks. To some extent this is useful in avoiding shocks for nonsustained ventricular tachycardia and supraventricular tachycardia when they are hemodynamically stable. If the person does not press the two patient response buttons simultaneously, as may happen when the arrhythmia has caused syncope, the shock is delivered. Another feature in current WCDs is the long detection time, which is analogous to that in arm C of MADIT-RIT trial [6], which was shown to be effective in reducing shock therapy. This is because several sustained ventricular tachycardias terminated spontaneously during the programmed long detection time. One interesting scenario in which the therapy for sustained ventricular tachycardia was withheld by the patient and had to be terminated by external defibrillation in the emergency department was noted in the current study [3].

A recent publication has specifically addressed the use of WCD while waiting for ICD re-implantation after explant [7]. This study by Ellenbogen KA et al. retrospectively analysed the data of 8058 patients and concluded that WCD should be used as an alternative when ICD re implantation is medically delayed. They noted that the risk of ventricular tachycardia/fibrillation is around 4% in first two months 10% in one year after explantation of the ICD, which is apparently quite significant.

In conclusion, WCD is definitely to be considered as a bridge to ICD when there is a presumed risk of life threatening ventricular arrhythmia, but guideline directed ICD implantation is delayed for some reason.