Wearable cardioverter-defibrillator

INTRODUCTION — The implantable cardioverter-defibrillator (ICD) has been shown to improve survival from sudden cardiac arrest and to improve overall survival in several populations at high risk for sudden cardiac death (SCD). However, there remain situations in which implantation of an ICD is immediately not feasible (eg, patients with an active infection), may be of uncertain benefit, may not be covered by third-party payers (eg, early post-myocardial infarction, patients with limited life expectancy or new onset systolic heart failure), or when an ICD must be removed (eg, infection).

In cases where ICD implantation must be deferred, a wearable cardioverter-defibrillator (WCD) offers an alternative approach for the prevention of SCD. The WCD (LifeVest [Zoll Medical Corporation] or Assure [Kestra Medical Technologies, Inc]) is an external device capable of automatic detection and defibrillation of ventricular tachycardia and ventricular fibrillation (picture 1 and figure 1). While the WCD can be worn for years, typically the device is used for several months as temporary protection against SCD.

The indications, efficacy, and limitations of the wearable cardioverter-defibrillator will be discussed here. Detailed discussions of the roles of the ICD are presented separately. (See "Secondary prevention of sudden cardiac death in heart failure and cardiomyopathy" and "Primary prevention of sudden cardiac death in patients with cardiomyopathy and heart failure with reduced LVEF".)

DESCRIPTION AND FUNCTIONS OF THE WCD — The WCD is an external device capable of automatic detection and defibrillation of ventricular tachycardia (VT) or ventricular fibrillation (VF) [1]. The approved devices do not have pacing capabilities and therefore are unable to provide therapy for bradycardic events or antitachycardic pacing.

Wearing the WCD — The WCD is composed of dry, nonadhesive monitoring electrodes, defibrillation electrodes incorporated into a chest strap or vest assembly, and a defibrillation battery and monitor unit (picture 1). The Assure WCD garment has two styles designed for female and male body habitus and different sizes. The monitoring electrodes are positioned circumferentially around the chest and provide two to four surface electrocardiogram (ECG) leads. The defibrillation electrodes are positioned in a vest assembly for apex-posterior defibrillation. Proper fitting is required to achieve adequate skin contact to avoid noise and frequent alarms.

Detection and delivery of shocks — Arrhythmia detection by the WCD is programmed using ECG rate and morphology criteria. The system is programmed to define ventricular arrhythmias when the ventricular heart rate exceeds a preprogrammed rate threshold with an ECG morphology that does not match a baseline electrocardiographic template.

Typical programming is reflected in default device settings:

●VT detection 150 beats per minute (LifeVest) or 170 beats per minute (Assure).

●Programmable ranges for LifeVest are 120 to 250 beats per minute, not to exceed the VF detection rate; for Assure they are, 130 to the programmed VF threshold minus 10 beats per minute.

●VF detection ≥200 beats per minute.

●Programmable ranges are 120 to 250 beats per minute (LifeVest) or 180 to 220 beats per minute (Assure).

●Treatment with 150 joules (LifeVest) or 170 joules (Assure) shocks for up to five shocks.

For the Zoll LifeVest WCD, the tachycardia detection rate is programmable for VF between 120 and 250 beats per minute, and the VF shock delay can be programmed from 25 to 55 seconds. The VT detection rate is programmable between 120 bpm to the VF setting with a VT shock delay of 60 to 180 seconds. VT signals can allow synchronized shock delivery on the R wave, but if the R wave cannot be identified, unsynchronized shocks will be delivered.

For the Kestra Assure WCD, the tachycardia detection rate is programmable for VF between 180 and 220 beats per minute, and for VT detection programmable from 130 beats per minute up to the programmed VF rate: 10 beats per minute. Detection utilizes a segment-based analysis of 4.8-second segments that continuously overlap by 2.4 seconds. VF confirmation requires two out of two segments (approximately 5 seconds), and VT confirmation requires 15 out of 19 segments (approximately 45 seconds). The first and last segments must be in the programmed treatment zone.

If an arrhythmia is detected, vibration and audible alarms are initiated. A flashing red light and shock icon are activated on the Assure monitor. Although shocks may be transmitted to bystanders in physical contact with the patient being shocked by a WCD, a voice cautions the patient and bystanders to the impending shock. Patients are trained to hold a pair of response buttons on the LifeVest device or press the alert button on the Assure device during these alarms to avoid receiving a shock while awake. A patient's response serves as a test of consciousness; if no response occurs and a shock is indicated, the device charges, extrudes gel from the defibrillation electrodes, and delivers up to five biphasic shocks at preprogrammed energy levels (ranging from 75 to 150 joules for the LifeVest device and 170 joules for the Assure device). The LifeVest device includes a default sleep time from 11 PM to 6 AM, programmable in one-hour increments, which allows additional time for deep sleepers, if they awaken, to abort shocks.

Efficacy in terminating VT/VF — Shock efficacy with the WCD appears to be similar to that reported with implantable cardioverter-defibrillators (ICDs). However, sudden cardiac death may still occur in those not wearing the device, those with improper positioning of the device, due to bystander interference, due to the inability of the WCD to detect the ECG signal, or due to bradyarrhythmias. These results highlight the importance of patient education and promotion of compliance while using the WCD.

The efficacy of the WCD has been tested for induced ventricular tachyarrhythmias as well as for spontaneous events during clinical trials and postmarket studies. When worn properly, the WCD appears to be as effective as an ICD for the termination of VT and VF, with successful shocks occurring in up to 100 percent of cases [1-7].

In a study of induced VT/VF in the electrophysiology laboratory, the WCD successfully detected and terminated VT/VF with 100 percent first-shock success [2]. The following large registry studies of patients with WCDs showed high shock success rates:

●In a US postmarket study of 8453 patients who wore a WCD after myocardial infarction, 146 VT/VF events occurred in 133 patients, and the overall shock success rate for terminating VT/VF was 82 percent, with 91 percent immediate survival [6]. In this study, shock success resulting in survival was 95 percent in revascularized and 84 percent in non-revascularized patients, suggesting that lower efficacy rates may be related to ischemic events.

●In the WEARIT-II registry of 2000 patients who wore a WCD for a median of 90 days, 120 episodes of sustained VT/VF were seen in 41 patients [7]. For 90 of the episodes, patients pressed the response buttons to abort shock delivery, with the majority of sustained VT episodes terminating spontaneously following use of the response button. All of the remaining 30 VT/VF episodes in 22 different patients were successfully terminated with a single shock.

●Among 6043 German patients who wore the device between April 2010 and October 2013, 94 patients were shocked for sustained VT/VF, with the WCD successfully terminating VT/VF in 88 patients (94 percent) [8].

The WCD appears equally efficacious among patients with and without myocardial ischemia immediately prior to VT/VF detection and shock (as defined by ≥0.1 mV ST-segment changes on ECG), with first shock termination rates of 96 percent in both groups [9].

Avoiding inappropriate shocks — When electronic noise occurs, which may potentially be interpreted at VT or VF, the WCD emits a noise alarm. This electronic noise can often be minimized or eliminated by changing body position or tightening of the electrode belt, and shocks can be avoided by pushing the response buttons. While a dual-chamber ICD with an atrial lead would seemingly have greater ability to discriminate between supraventricular tachycardia (SVT) and VT, the incidence of inappropriate shocks due to atrial fibrillation, sinus tachycardia, or other supraventricular arrhythmias in clinical studies of WCDs has been low. The LifeVest WCD uses a two-channel proprietary vectorcardiogram morphology matching algorithm to prevent shocks during SVT if the QRS is unchanged, and inappropriate shocks can also be averted when the patient presses the response buttons. The Assure WCD uses a four-channel ECG with a single noise-free channel required for analysis and an algorithm that excludes noisy and low amplitude channels (figure 2). (See 'Inappropriate shocks' below.)

In a small study of the 60 patients with a permanent pacemaker, in which a variety of pacing modes (AAI, VVI, DDD) and configurations (unipolar, bipolar) were tested, unipolar DDD pacing triggered VT/VF detection in six patients (10 percent), while no other pacing modes or configurations triggered arrhythmia detection [10]. As such, patients whose pacemaker is programmed to unipolar DDD pacing should be evaluated for pacemaker reprogramming to a bipolar mode prior to WCD usage.

In a study of 130 patients with an ICD and fitted with an ASSURE WCD programmed for detection only and followed for 30 days, of 163 WCD-detected episodes, four were VT/VF and 159 were non-VT/VF with three false-positive shock alarm markers recorded, corresponding to a very low rate of inappropriate detection [11]. No ICD-recorded VT/VF episodes meeting WCD programmed criteria were missed. Median daily use was high at 23 hours.

Bradycardia/asystole — Neither of the approved WCDs deliver antibradycardic pacing, but they do record the ventricular rate when the heart rate decreases or asystole occurs:

●For the LifeVest device, asystole recordings are triggered when ventricular heart rates drop below 10 beats per minute or 16 seconds of asystole, and the device automatically records the event with 120 seconds preceding the onset. If using the secure website in conjunction with the WCD, alerts can be configured to prompt the healthcare provider that a patient is experiencing bradycardia or an asystole.

●For the Assure device, asystole is detected when there is no detected heart rate for >20 seconds (five of seven segments with heart rate 0 beats per minute or amplitude <100 uV); prolonged heart rates below 30 beats per minute may be detected as bradycardia. When asystole or bradycardia is detected, a loud alarm is triggered to attract bystanders and instruct them to call 911 and begin CPR if the patient is unconscious. The alert can be silenced by pressing the alert button or it resolves when a heart rate >30 bpm is detected for >30 seconds.

Storage of ECGs and compliance data — In addition to delivering therapeutic shocks for life-threatening ventricular arrhythmias, the WCD stores data regarding tachyarrhythmias, bradycardia/asystole (see 'Bradycardia/asystole' above), patient compliance with the device, and noise or interference with its proper functioning. Arrhythmia recordings from the WCD are available for clinician review once stored data are transmitted via a modem to the manufacturer's network. Treatments, patient compliance, ECG records, and system performance can be viewed using a secure website.

The WCD stores ECGs from arrhythmia detections, usage, and compliance trends:

●For the LifeVest system:

•The system is programmed to define ventricular arrhythmias when the ventricular heart rate exceeds a preprogrammed rate threshold with an ECG morphology that does not match a baseline ECG template. The monitoring software captures 30 seconds of ECG signal prior to the determination of VT or VF and continuously records until 15 seconds after the alarms stop.

•Patients can perform manual recordings by pressing response buttons for three seconds, which records the prior 30 seconds plus the next 15 seconds.

•Data on patient compliance, ECG signal quality, alarm history, and noise occurrence are recorded, including time/date stamps for device on/off switching, monitor connection to the electrodes, and electrode-to-skin contact. Compliance may be determined by assessing the time that the user had the device turned on, the belt connected, and at least one monitoring electrode contacting the skin.

●For the Assure system:

•Up to 120 seconds of data are recorded prior to arrhythmia onset detection, confirmation, and therapy are detected, and up to 60 seconds are detected after rate recovery or conversion.

•Patient activity is also stored, utilizing an accelerometer located in the hub component in the middle of the patient's back.

•Daily usage is recorded in one-minute increments when the sensors are in contact with the patient's skin.

INDICATIONS — The WCD is indicated as temporary therapy for patients with a high risk for sudden cardiac death (SCD) [1,12-16]. Our recommended approach is consistent with that of the 2016 science advisory from the American Heart Association (also endorsed by the Heart Rhythm Society) and the 2017 AHA/ACC/HRS guideline [16,17]. Examples of persons who may benefit from the temporary use of a WCD include:

●Patients with a permanent implantable cardioverter-defibrillator (ICD) that must be explanted, or those with a delay in implanting a newly indicated ICD (eg, due to systemic infection). (See 'Bridge to indicated or interrupted ICD therapy' below.)

●Patients with reduced left ventricular (LV) systolic function (LVEF ≤35 percent) who have had a myocardial infarction (MI) within the past 40 days. (See 'Early post-MI patients with LV dysfunction' below.)

●Patients with reduced LV systolic function (LVEF ≤35 percent) who have undergone coronary revascularization with coronary artery bypass graft (CABG) surgery in the past three months. (See 'Patients with LV dysfunction early after coronary revascularization' below.)

●Patients with newly diagnosed nonischemic cardiomyopathy with severely reduced LV systolic function (LVEF ≤35 percent) that is potentially reversible. (See 'Newly diagnosed nonischemic cardiomyopathy' below.)

●Patients with severe heart failure who are awaiting heart transplantation. (See 'Bridge to heart transplant' below.)

A 2019 systematic review and meta-analysis, which included 33,242 WCD users from 28 studies (the randomized VEST trial and 27 nonrandomized studies), assessed the likelihood of WCD therapy in a broad range of patient populations, including both primary/secondary prevention and ischemic/nonischemic cardiomyopathy patients. The incidence of appropriate shocks was 5 per 100 persons over three months (1.67 percent per month) with mortality while wearing the device noted to be 0.7 per 100 persons over three months [18].

Bridge to indicated or interrupted ICD therapy — In some patients with an indication for ICD placement, implantation of the device may be delayed due to comorbid conditions, including [16,17]:

●Infection

●Recovery from surgery

●Lack of vascular access

In addition, patients with a preexisting ICD who develop device infection or endocarditis usually require system extraction to effectively treat the infection. Unless the patient is pacemaker dependent, reimplantation in many patients is deferred until the infection is completely cleared after an appropriate course of antibiotics. The WCD may provide protection against ventricular tachyarrhythmias during these periods until an ICD can be implanted [4,5,16]. (See "Infections involving cardiac implantable electronic devices: Epidemiology, microbiology, clinical manifestations, and diagnosis".)

●In a review of 8058 patients who were prescribed the WCD after ICD removal because of infection, median time to reimplantation was 50 days, and 334 (4 percent) experienced 406 ventricular tachycardia/ventricular fibrillation (VT/VF) events, with 348 events treated by the WCD and 54 treatments averted by conscious patients [19]. The one-year cumulative event rate was 10 percent.

Early post-MI patients with LV dysfunction — Among patients with LV ejection fraction (LVEF) ≤35 percent who are less than 40 days post-MI, there are conflicting data on the benefits of a WCD for primary prevention against SCD. Following discussion of the potential benefits and risks, use of the WCD within this 40-day window could be considered among motivated patients who have LVEF ≤35 percent and in New York Heart Association (NYHA) functional class II or III, or LVEF <30 percent and in NYHA class I, as these patients would be candidates for ICD implantation after 40 days [16,17]. Patients should be reminded of the importance of compliance with the WCD in order to optimize any potential benefits on prevention of arrhythmic death. Reevaluation of LVEF should occur one to three months after the MI. If LVEF remains ≤35 percent on follow-up assessment, while the patient is taking appropriate medical therapy, ICD implantation is indicated [16]. After ICD implantation, use of the WCD would be discontinued.

Despite advances in the treatment of acute coronary syndromes with early revascularization and effective medical therapies that have reduced mortality, some residual risk of SCD remains in the early period following an MI, especially in the setting of severely reduced LVEF (2.3 percent/month for patients with LVEF ≤30 percent) [4,20]. However, there are conflicting data on the utility of an ICD in the early post-MI period.

●In an analysis of 712 patients with a history of MI who were enrolled in the SCD-HeFT trial, there was no evidence of differential mortality benefit with ICDs as a function of time after MI, indicating that the potential benefit of ICD therapy is not restricted only to remote MIs [21].

●In the DINAMIT (674 patients) and IRIS (898 patients) trials, which randomized patients with LVEF ≤35 percent to either early ICD implantation 6 to 40 days after acute MI or medical therapy alone, there was no significant improvement in overall mortality [22,23]. Despite a reduction in arrhythmic deaths among patients with an ICD, there was a higher risk of nonarrhythmic deaths during this early period, resulting in similar overall mortality rates.

Professional society guidelines do not recommend ICD implantation for primary prevention of SCD within 40 days of acute MI [16]. However, due to the risk of SCD in some patients early post-MI, the WCD has been studied in this patient population.

●In the VEST trial, 2302 patients with an acute MI and LVEF ≤35 percent were randomly assigned (within seven days of hospital discharge) in a 2:1 ratio to wear the WCD in addition to usual medical treatment (1524 patients) or to receive standard medical treatment alone (778 patients) [24]. Over an average follow-up of 84 days, patients in the WCD group had no significant improvement in the primary outcome of arrhythmic death (25 patients [1.6 percent] versus 19 patients [2.4 percent] with medical therapy alone; relative risk [RR] 0.67; 95% CI 0.37-1.21). Compliance with medical therapy was excellent in both groups, likely contributing to fewer than expected events and the trial possibly being underpowered. However, compliance with WCD usage was markedly lower than expected (median and mean daily wear times of 18 and 14 hours, respectively), with over half of patients assigned to the WCD not wearing it by the end of the 90-day study. Among 48 total deaths in the WCD group, only 12 patients (25 percent) were wearing the WCD at the time of death. Asystolic events not treated by the WCD likely also contributed to the nonsignificant primary outcome results of the trial. A subsequent as-treated and per-protocol analysis of VEST (censoring participants at the time they stopped wearing the WCD) reported a significant reduction in total and arrhythmic mortality among participants wearing the WCD compared with control participants (total mortality hazard ratio 0.25; CI 0.13-0.43; arrhythmic death hazard ratio 0.09; CI 0.02-0.39) [25].

The VEST study also demonstrates the challenges in trying to improve mortality in the post-MI population. Not all patients will survive despite initial appropriate and successful shocks for VT or VF. Of nine patients wearing the WCD with arrhythmic death in the VEST trial, four had been initially successfully treated but subsequently died. Of six patients who had an appropriate shock from the WCD but died during the study, two developed post-VT/VF asystole.

●Similar WCD shock rates (between 1.5 and 2 percent within 90 days post-MI) have been reported in observational studies [3,5,6]. In registry data from two large registries (involving 3569 and 8453 patients, respectively), similar rates of WCD shocks have been seen (1.7 and 1.6 percent of patients, respectively) [5,6].

Patients with LV dysfunction early after coronary revascularization — Among patients with LVEF ≤35 percent who have undergone coronary revascularization with coronary artery bypass graft (CABG) surgery or percutaneous coronary intervention (PCI) in the past three months, we offer a WCD to highly motivated patients for primary prevention against SCD [16]. LVEF should be reassessed three months following CABG or PCI. If a sustained ventricular tachyarrhythmia has occurred, or if the LVEF remains ≤35 percent three months after CABG or PCI, implantation of an ICD is usually indicated [16]. (See "Secondary prevention of sudden cardiac death in heart failure and cardiomyopathy" and "Primary prevention of sudden cardiac death in patients with cardiomyopathy and heart failure with reduced LVEF".)

While professional society guidelines do not specifically exclude ICD implantation for patients with LV dysfunction within three months of revascularization, reimbursement in some countries may be denied. As an example, in the United States the national coverage decision for the Centers for Medicare & Medicaid Service (CMS) excludes coverage for primary prevention ICDs if patients have had CABG surgery or PCI within the past three months. This is based upon the clinical profile of subjects included in the major ICD trials for primary prevention of SCD in ischemic cardiomyopathy [12,13,26,27]. Despite this exclusion period, patients with LV dysfunction (eg, LVEF ≤30 percent) have been shown to have significantly higher rates of mortality early after PCI or CABG based on large National Cardiovascular Data Registry (NCDR) and Society of Thoracic Surgeons (STS) Adult Cardiac Surgery Database studies, respectively [28,29].

Patients with significant LV dysfunction have higher 30-day mortality rates after coronary artery bypass graft (CABG) surgery than patients with normal LV function. While these persons have an increased risk of SCD due to ventricular arrhythmias, they are also at risk for nonarrhythmic causes of death. There are limited data on the utility of an ICD in the early post-CABG period, as several ICD studies of primary prevention have excluded patients within one to three months after coronary revascularization [12-14]. However, the CABG Patch trial did not report a survival benefit from epicardial ICD implantation at the time of CABG in patients with LVEF ≤35 percent [27]. (See "Early cardiac complications of coronary artery bypass graft surgery" and "Early noncardiac complications of coronary artery bypass graft surgery".)

Professional society guidelines do not recommend ICD implantation for primary prevention of SCD within three months of CABG [16]. However, due to the risk of SCD in some patients early post-CABG, the WCD has been studied in this patient population, in whom wearing the WCD may provide protection from SCD during healing and potential recovery of LV function [3,16,17]. The potential utility for a WCD in this setting is illustrated by the following studies:

●In a nonrandomized comparison of nearly 5000 patients with LVEF ≤35 percent from two separate cohorts who underwent revascularization with CABG or percutaneous coronary intervention (PCI) (809 patients discharged with a WCD from a national registry and 4149 patients discharged without WCD from Cleveland Clinic CABG and PCI registries), patients discharged with the WCD had significantly lower 90-day mortality rates (3 versus 7 percent) [30]. While patients using a WCD appear to have improved outcomes, only 1.3 percent of the WCD group received an appropriate therapy while wearing the device, thereby indicating that the majority of the mortality benefit was not attributable to life-saving therapies from the WCD.

●In a German cohort of 354 patients who wore the WCD, including approximately 90 patients in the early post-CABG period, 7 percent received a shock for a ventricular tachyarrhythmia during the three months of WCD use [4].

●In a study of 3569 patients in the United States using the WCD, among which 9 percent of WCD use was early post-CABG, appropriate shocks for a ventricular tachyarrhythmia occurred in 0.8 percent of these patients over a mean follow-up of 47 days [5,31].

Newly diagnosed nonischemic cardiomyopathy — In selected patients with newly diagnosed nonischemic cardiomyopathy with severely reduced LV systolic function that is potentially reversible, such as tachycardia- or myocarditis-associated cardiomyopathy, the WCD may be useful for the prevention of SCD due to ventricular arrhythmias while awaiting improvement in LV function [16,17].

While a benefit from ICD implantation has long been recognized in patients with significant LV systolic dysfunction related to underlying ischemic heart disease, an increase in SCD risk and potential benefit from an ICD has also been demonstrated in patients with a nonischemic cardiomyopathy in several studies [14,32]:

●In SCD-HeFT, which compared ICD implantation with amiodarone treatment alone or placebo for primary prevention of SCD in patients with ischemic or nonischemic heart failure and LVEF ≤35 percent, patients who received an ICD had significantly improved survival [14]. However, patients within three months of their initial heart failure diagnosis were excluded from this study.

●In DEFINITE, which compared ICD implantation with standard medical therapy to standard medical therapy alone for primary prevention of SCD in patients with a nonischemic cardiomyopathy, nonsustained VT, and LVEF ≤35 percent, there was a trend toward improved mortality in patients who received an ICD, regardless of duration since diagnosis [32].

●Following DEFINITE, another study reported similar occurrences of lethal arrhythmias irrespective of diagnosis duration in patients with a nonischemic cardiomyopathy and LVEF ≤35 percent [33].

Major society guidelines recommend implantation of an ICD for nonischemic cardiomyopathy with LVEF ≤35 percent, provided that a reversible cause of transient LV dysfunction has been excluded and that response to optimal medical therapy has been assessed [16]. The guidelines do not specify a waiting period prior to reassessing LVEF. In the United States, however, the Center for Medicare Services (CMS) requires a three-month period of optimal medical therapy prior to reimbursement for ICD placement for primary prevention (if repeat LVEF assessment continues to show LVEF ≤35 percent). (See "Primary prevention of sudden cardiac death in patients with cardiomyopathy and heart failure with reduced LVEF", section on 'Nonischemic dilated cardiomyopathy'.)

In patients felt to be at high risk of SCD while undergoing a trial of optimal medical therapy, the WCD may provide protection against SCD while awaiting improvement in LV function, although the event rates in this population appear to be lower than patients with ischemic cardiomyopathy [16].

●In a post-approval study of the WCD, 0.7 percent of patients prescribed a WCD for recently diagnosed nonischemic cardiomyopathy required shocks for a ventricular tachyarrhythmia over a mean follow-up period of 57 days [5,31].

●Among a single-center cohort of 254 patients with newly diagnosed nonischemic cardiomyopathy treated with the WCD between 2004 and 2015 (median duration of treatment 61 days, total follow-up 56.7 patient-years) who were highly compliant with using the WCD (median wear time 22 hours per day), no patients received an appropriate shock, and only three patients (1.2 percent) received an inappropriate shock [34]. This was compared with 6 of 271 patients (2.2 percent) with newly diagnosed ischemic cardiomyopathy who received an appropriate shock; in this group, two (0.7 percent) received inappropriate shocks. Of interest, 39 percent of nonischemic and 32 percent of ischemic cardiomyopathy patients experienced improvement in LVEF to >35 percent, obviating the need for an ICD.

●In a prospective study of the WCD in advanced heart failure patients (SWIFT), 75 patients hospitalized with heart failure (66 percent nonischemic cardiomyopathy) were prescribed a WCD for three months. Among the nonischemic cardiomyopathy patients, one had recurrent supraventricular tachycardia and another had multiple ventricular premature beats detected, but no WCD therapies were delivered [35].

●In the WEARIT II registry, which included 927 patients with nonischemic cardiomyopathy, over a median wear time of 90 days, the treated event rate was 1 percent, compared with 3 percent for the 805 patients with ischemic cardiomyopathy [7].

●Special populations include those with alcoholic cardiomyopathy, postpartum cardiomyopathy, or myocarditis, all of which may or may not be associated with improvement in ventricular function with optimal medical therapy and reversal or treatment of causative factors. In a study of 127 patients with alcoholic cardiomyopathy wearing the WCD a median of 51 days, 5.5 percent had appropriate shocks for VT/VF [36]. Improved LVEF occurred in 33 percent, and 23.6 percent received an ICD.

●In the PROLONG study of 156 patients (111 with nonischemic cardiomyopathy) with newly diagnosed LVEF ≤35 percent wearing a WCD for an average of 101 days, WCD shocks for VT/VF were experienced by 7.2 percent, compared with 6.7 percent in the 45 patients with ischemic cardiomyopathy [37]. The event rates were 21.1 percent in the 19 patients with postpartum cardiomyopathy, 0 percent in the six patients with myocarditis, and 4.7 percent in patients with other forms of nonischemic cardiomyopathy.

●In a separate study of 107 women with peripartum cardiomyopathy, who were matched to 159 nonpregnant women with nonischemic dilated cardiomyopathy, the event rate was 0 in the peripartum cardiomyopathy over an average WCD use of 124 days, compared with two shocks in one patient with nonperipartum nonischemic cardiomyopathy [38].

With such low event rates, the utility of the WCD for newly-diagnosed nonischemic cardiomyopathy has been debated. However, from the WEARIT II registry, the number of VT/VF events per 100 patient-years was 1.5 for treated events versus 12 for untreated events [7]. Presumably, some of the untreated events led to earlier ICD implantation and may represent a nontreatment yield from the WCD monitoring functions. As data remain limited for such patients, the decision on whether to use a WCD remains based on clinical judgment for patients assessed to have high-risk severe newly diagnosed nonischemic cardiomyopathy while undergoing optimization of medical therapy, awaiting improvement in LV function, ICD implantation, or if needed, cardiac transplantation. (See "Treatment and prognosis of myocarditis in adults", section on 'Therapy for arrhythmias'.)

Bridge to heart transplant — Patients with severe heart failure awaiting heart transplantation represent a group at particularly high risk for SCD [17]. ICD implantation is often recommended for such patients, particularly those discharged to home while awaiting transplantation. The WCD may be a reasonable noninvasive alternative approach, though data on its use in patients awaiting heart transplantation are limited:

●In one study of 91 cardiac transplant candidates discharged to home (UNOS Status 1B patients receiving home inotrope infusion), among whom 25 had an ICD and 13 used a WCD, two patients died suddenly at home, one who was not wearing his WCD and another who declined use of a WCD [39]. In the 13 patients wearing the WCD, three asymptomatic events occurred with one shock delivered for rapid atrial fibrillation.

●In a German study of 354 WCD patients, 6 percent wore the WCD while awaiting heart transplantation, with an incidence of ventricular arrhythmias of 11 percent [4].

●In the WEARIT study of WCD use in 177 patients with NYHA functional class III or IV heart failure (not listed for heart transplant but with similar functional status to patients who might be listed for heart transplant), one patient received two successful defibrillations [3].

●In a registry of 121 patients prescribed a WCD as a bridge to heart transplantation, seven patients (6 percent) received appropriate shocks over an average use of 127 days (median 39 days) [40].

The International Society for Heart and Lung Transplantation Guidelines state as a class I recommendation that an ICD or WCD should be provided for status 1B patients who are discharged home given that the wait for transplantation remains significant [41]. The WCD may also be appropriate in patients whose anticipated waiting time to transplant is short (ie, blood types A and B) if an ICD is not already present [41].

WCD in patients with VADs — The role for ICD and WCD therapy remains unclear in patients with ventricular assist devices (VADs). With VADs, circulatory support is often adequate even in the event of a ventricular tachyarrhythmia. However, one study reported the presence of an ICD was associated with improved survival in patients undergoing VAD support [42]. Whether the WCD could impart similar survival benefits in patients awaiting transplantation with VAD support has yet to be studied. (See "Treatment of advanced heart failure with a durable mechanical circulatory support device".)

WCD use in hemodialysis patients — Patients with end-stage kidney disease on hemodialysis are at high risk for SCD, but they are also at higher risk for infection, bleeding, and other complications of implantable device therapies, which may lead to underutilization of ICDs. Although the arrhythmia event rates for patients on hemodialysis wearing a WCD are not published, a study of 75 hemodialysis patients who experienced sudden cardiac arrest events while wearing a WCD reported that 78.6 percent of events were due to VT/VF and 21.4 percent were due to asystole [43]. Survival was 71, 51, and 31 percent at 24 hours, 30 days, and one year, respectively, which was reported to be improved compared with historical controls.

LIMITATIONS AND PRECAUTIONS — In spite of its overall efficacy for terminating life-threatening ventricular arrhythmias, the WCD does have some limitations. The device must be fitted to each patient, and some patients may not have a good fit due to body habitus. Its external nature does not allow for pacemaker functionality and introduces a component of patient interaction and compliance as well as the potential for external noise leading to inappropriate shocks. The device must be removed for bathing, but no protection is afforded while the device is off. Therefore, it is advisable that caregivers or other persons be nearby during these periods when the WCD is not worn. Comfort may also be an issue for some patients due to the size and weight of the device.

Patient size — The WCD can only be fitted on patients with a chest circumference less than 57 inches (144 cm); therefore, it may not be an option for morbidly obese patients. However, among 574 patients from the WCD registry, which included normal weight (body mass index [BMI] between 18 and 24.9; n = 157), overweight (BMI between 25 and 29.9; n = 186), and obese (BMI ≥30; n = 231, including 55 with BMI ≥40) patients who experienced 623 ventricular tachycardia/ventricular fibrillation (VT/VF) events while wearing the WCD, the median daily wear time (21 hours), first shock success rate (93 to 94 percent), and 24-hour post-shock survival (92 to 94 percent) were similar across all BMI groups [44].

There are also limited data on WCD use in children, in whom the device may not fit properly if the child is small. (See 'Use of the WCD in children' below.)

Lack of pacemaker functionality — Because of its external nature, the WCD is not able to function as a pacemaker, which limits the possible therapies it can deliver in two ways:

●The WCD cannot deliver pacing therapies to treat bradycardia or asystole. In the German study, two patients developed asystole while wearing the WCD, and both patients died [4]. In the US post-approval registry study, 23 of 3569 patients (0.6 percent) experienced asystole, with an associated mortality of 74 percent [5]. In the post-myocardial infarction (MI) registry of 8453 patients, 34 died (0.4 percent) with bradycardia-asystole events [6]. In the WEARIT-II registry, 6 of 2000 patients (0.3 percent) had asystole, and all three of the deaths that occurred while wearing the WCD during the study (0.2 percent) occurred following an asystole event [7].

●The WCD cannot provide antitachycardia pacing for VT, which can reduce patient shocks, when effective.

When considering these limitations, an implantable cardioverter-defibrillator (ICD) would be preferred, if indicated, in a patient who is pacemaker-dependent or in whom antitachycardia pacing is desired as the initial therapy for VT. (See "Implantable cardioverter-defibrillators: Overview of indications, components, and functions".)

Use in patients with a preexisting permanent pacemaker — With certain precautions, the WCD can be used in patients with a preexisting permanent pacemaker. The manufacturer recommends that the device not be worn if the pacemaker stimulus artifact exceeds 0.5 millivolts, as this may mask underlying ventricular fibrillation and prevent appropriate device therapy. Conversely, the VT threshold of the WCD should be set higher than the maximal pacing rate to avoid an inappropriate WCD shock due to oversensing paced beats. Following any WCD shock, the patient's pacemaker should be interrogated to ensure that there has been no damage to the pacemaker or any changes in the pacemaker setting.

Inappropriate shocks — Both the WCD and the ICD may inappropriately deliver shocks due to electronic noise, device malfunction, or detection of supraventricular tachycardia above the preprogrammed rate criteria. Studies of ICDs have reported an incidence of inappropriate shock of 0.2 to 2.3 percent of patients per month [32,45-51]. Comparable rates of inappropriate shocks have been reported among users of the WCD, with rates ranging from 0.5 to 1.4 percent per month [3-7]. In a systematic review and meta-analysis which included 33,242 patients from 28 studies (the randomized VEST trial and 27 nonrandomized studies), inappropriate shocks occurred at a rate of 2 per 100 persons over three months (0.67 percent per month) [18].

Inappropriate shocks with a WCD can be potentially reduced due to the ability to abort shocks while awake by pressing response buttons. (See 'Avoiding inappropriate shocks' above.)

Patient compliance and complaints — Patients may not comply with wearing the WCD for a variety of reasons, chief among them device size and weight, skin rash, itching, and problems sleeping. However, efficacy of the WCD in the prevention of sudden cardiac death is highly dependent on patient compliance and appropriate use of the device [3-5,7].

●In the WEARIT/BIROAD study, 23 percent of the 289 subjects withdrew before reaching a study endpoint, with size and weight of the monitor being the most frequent reason for withdrawal [3]. Skin rash and/or itching were also reported by 6 percent of patients.

●In the US postmarket study, median and mean daily use were 21.7 hours and 19.9 hours, respectively [5]. Daily use was >90 percent (>21.6 hours) in 52 percent of patients and >80 percent (>19.2 hours) in 71 percent of patients. Longer duration of monitoring correlated with higher compliance rates. WCD use was stopped prematurely in 14 percent, primarily because of comfort issues related to the size and weight of the WCD.

●In the WEARIT-II registry, median daily use was 22.5 hours [7]. Similar to the US postmarket study, longer duration of monitoring (15 or more days) was associated with higher rates of compliance.

●In the nationwide German cohort, median daily use among 6043 patients was 23.1 hours for a median of 59 days [8]. Lower rates of compliance were reported in a study of 147 patients from two academic medical centers in Boston, in which median daily use was 21 hours for a median of 50 days [52].

●In an international registry of 708 patients, appropriate WCD shock was documented in 2.2 percent, inappropriate shock in 0.5 percent, and mean wear time was 21.2±4.3 hours/day (and was lower in younger patients) [53].

●In the WEARIT-France cohort study of 1157 patients, median daily wear time was 23.4 hours, with younger age associated with lower compliance [54].

●In the VEST randomized trial after MI, median and mean daily wear times were only 18 and 14 hours, respectively, with over half of patients assigned to the WCD not wearing it by the end of the 90-day study [24]. Among 48 total deaths in the WCD group, only 12 patients (25 percent) were wearing the WCD at the time of death. In the as-treated and per-protocol analysis of VEST [25], better WCD compliance was predicted by cardiac arrest during index MI, higher creatinine, diabetes, prior heart failure, ejection fraction ≤ 25 percent, Polish enrolling center, and number of WCD alarms. Worse compliance was associated with being divorced, Asian race, higher body mass index, prior PCI, or any WCD shock.

●In a study of 130 patients with an ICD and fitted with an ASSURE WCD programmed for detection only and followed for 30 days, median daily use was high at 23 hours [11].

Rates of WCD discontinuation appear similar to reported rates of compliance with other prescribed therapies. One study reported that 15 percent of patients stop using aspirin, ACE inhibitors and beta-blockers within 30 days of a MI [55]. Improved compliance and acceptance of the WCD may be seen with newer devices, which are 40 percent smaller in size and weight or which offer multiple sizes and gender-specific fitting.

USE OF THE WCD IN CHILDREN — In December 2015, the US Food and Drug Administration (FDA) approved the WCD for use in children, although the WCD was used off-label prior to FDA approval [56]. As such, there are relatively few peer-reviewed publications documenting experience with the WCD in children [57-59].

In a retrospective review of all patients <18 years of age who were prescribed the WCD between 2009 and 2016 (n = 455 patients), median duration of use was 33 days and wear time 20.6 hours [59]. Eight patients received at least one shock (seven episodes of ventricular tachycardia/ventricular fibrillation [VT/VF] in six patients, two inappropriate shocks due to oversensing), with four of the seven episodes of VT/VF terminated with a single shock and all seven episodes successfully terminated by the WCD. There were seven deaths (1.5 percent); none were wearing the WCD at the time of death.

Children require special attention to assure compliance and correct fitting for optimal use. A variety of device harness sizes are available, but the smallest option may still be too large for smaller children. Additional data on clinical efficacy, compliance, and complications should be collected in children as WCD use increases.

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Arrhythmias in adults" and "Society guideline links: Inherited arrhythmia syndromes" and "Society guideline links: Ventricular arrhythmias" and "Society guideline links: Cardiac implantable electronic devices".)

SUMMARY AND RECOMMENDATIONS

●Introduction – The wearable cardioverter-defibrillator (WCD) is an external device capable of automatic detection and defibrillation of ventricular tachycardia (VT) or ventricular fibrillation (VF) (picture 1). In cases where the need for an implantable cardioverter-defibrillator (ICD) is felt to be temporary or implantation of the ICD must be deferred, a WCD may be an acceptable alternative approach for the prevention of sudden cardiac death (SCD). (See 'Description and functions of the WCD' above.)

●Device functions – In addition to delivering therapeutic shocks for life-threatening ventricular arrhythmias, the WCD stores data regarding arrhythmias, patient compliance with the device, and noise or interference with its proper functioning. Arrhythmia recordings from the WCD are available for clinician review once stored data are transmitted to the manufacturer's network. (See 'Storage of ECGs and compliance data' above.)

●Efficacy – When worn properly, the WCD appears to be as effective as an ICD for the termination of VT and VF, with successful shocks occurring in nearly 100 percent of cases. In addition, inappropriate shock rates from the WCD appear to be comparable to and in some studies lower than those reported for ICDs. (See 'Efficacy in terminating VT/VF' above and 'Inappropriate shocks' above.)

●Indications – The WCD is an option as temporary therapy for select patients with a high risk for SCD:

•Among patients with left ventricular ejection fraction (LVEF) ≤35 percent who are less than 40 days post-myocardial infarction (MI), we discuss the potential benefits and risks of WCD use and offer it to highly motivated patients with NYHA functional class II or III, or LVEF <30 percent and in NYHA class I, as these patients would be candidates for ICD implantation after 40 days. Reevaluation of LVEF should occur one to three months after the MI. If LVEF remains ≤35 percent on follow-up assessment, despite appropriate medical therapy, ICD implantation is indicated and should be considered. (See 'Early post-MI patients with LV dysfunction' above and "Primary prevention of sudden cardiac death in patients with cardiomyopathy and heart failure with reduced LVEF".)

•Among patients with LVEF ≤35 percent who have undergone coronary revascularization with coronary artery bypass graft (CABG) surgery in the past three months, we offer a WCD to highly motivated patients for primary prevention against SCD. LVEF should be reassessed three months following CABG. If a sustained ventricular tachyarrhythmia has occurred, or if the LVEF remains ≤35 percent three months after CABG, implantation of an ICD is usually indicated. (See 'Patients with LV dysfunction early after coronary revascularization' above and "Secondary prevention of sudden cardiac death in heart failure and cardiomyopathy" and "Primary prevention of sudden cardiac death in patients with cardiomyopathy and heart failure with reduced LVEF".)

•In selected patients with severe but potentially reversible cardiomyopathy, such as tachycardia- or myocarditis-associated cardiomyopathy, the WCD may be useful for the prevention of SCD due to ventricular arrhythmias while awaiting improvement in LV function, ICD implantation, or if needed, cardiac transplantation. (See 'Newly diagnosed nonischemic cardiomyopathy' above.)

•Patients with severe heart failure awaiting heart transplantation represent a group at particularly high risk for SCD in whom ICD implantation is often recommended. The WCD may be a reasonable noninvasive alternative approach, particularly for patients whose anticipated waiting time to transplant is short if an ICD is not already present. (See 'Bridge to heart transplant' above.)

•Some patients with an indication for an ICD may require a delay in ICD implantation due to comorbid conditions (ie, infection, recovery from surgery, lack of vascular access). Additionally, some patients who have an ICD need it removed due to infection. In such patients, the WCD may provide protection against ventricular tachyarrhythmias until an ICD can be implanted or reimplanted. (See 'Bridge to indicated or interrupted ICD therapy' above.)

●Device limitations – Limitations of the WCD (compared with a traditional ICD) include the lack of pacemaker functionality, the requirement for patient interaction and compliance, and potential discomfort due to the size and weight of the device. (See 'Limitations and precautions' above.)