The Wearable Cardioverter-Defibrillator

The wearable cardioverter-defibrillator (WCD) is a rechargeable external device that can be worn under the clothing all day long and protects the wearer from potentially life-threatening ventricular tachyarrhythmias. When a dangerous arrhythmia is detected, the WCD can deliver high-energy shocks. The WCD has been shown to be effective in accurately detecting and appropriately treating ventricular tachycardia (VT) and ventricular fibrillation (VF). It is intended for temporary use as a bridge to an implantable cardioverter-defibrillator (ICD), heart transplantation, or left ventricular assist device; patients with heart failure with reduced ejection fraction may benefit from the WCD while their condition improves. It can be used temporarily after explant of an ICD until reimplantation is deemed possible. In select patients with myocardial infarction, a WCD may be useful during the immediate period after infarction. It is indicated for use when a permanently implanted ICD must be explanted because of infection; the patient can use the WCD until the infection resolves, and a new ICD can be implanted. The role of the WCD is emerging as an important therapeutic option to protect patients at elevated risk of sudden cardiac death (SCD).


Introduction
Sudden cardiac death (SCD) is mainly due to ventricular tachyarrhythmias even though bradycardia may occur. The population at risk for SCD is heterogeneous and includes those whose risk is based on a transient arrhythmia-provoking electrical event, structural heart disease, a channelopathy, heart failure, cardiomyopathy, or other underlying conditions [1]. For patients at elevated risk for potentially lifethreatening ventricular tachyarrhythmias but with a transient contraindication for an implantable cardioverter-defibrillator (ICD) therapy, the wearable cardioverterdefibrillator (WCD) is an important therapeutic option (LifeVest 4000®, Zoll, Pittsburgh, Pennsylvania, USA). The external vest delivers high-energy rescue therapy in the event a ventricular tachyarrhythmia is detected along with electrogram storage and remote monitoring [2]. First introduced to market in 2001, the WCD is intended for short-term use, typically for a few months [3]. Patients are given a transmitter which can transmit data from the WCD directly to the clinic via a secure server. Remote transmissions do not require any patient intervention. Like cardiac implantable electronic devices, the WCD can be programmed to send out alerts when specific triggering events occur. The remote monitoring system records the number of hours per day that the patient wears the WCD, and the patient can activate the device to record an electrogram in the event of symptoms. While the WCD will attempt to make a daily remote transmission, if this is not possible, data transmission should occur at least once a week, and monthly in-clinic visits are recommended for WCD patients [5]. Reports from the WCD are shown in Figures 2 and 3.

Guidelines for the WCD
The American College of Cardiology, American Heart Association, and European Society of Cardiology (ACC/AHA/ESC) 2006 guidelines for the management of patients with ventricular arrhythmias and the prevention of sudden cardiac death was the first society-based recommendation for the use of WCD in patients at transient high risk for VF, such as patients waiting for heart transplant; patients at high risk following an acute myocardial infarction or invasive cardiac procedure; and patients whose ICD had to be temporarily explanted, for example, because of an infection [7]. The International Society for Heart and Lung Transplantation guidance made the WCD a Class I indication for Status 18 patients awaiting transplant at home [8]. In 2009, the Heart Rhythm Society expert consensus recommended that the WCD be considered as an alternative treatment for patients who needed early ICD revision following device explant in the setting of suspected continuing infection [9]. In 2013, the ACC/AHA guideline stated that the utility of the WCD in high-risk patients in the first 4 to 6 weeks after myocardial infarction was being investigated [10]. The European Heart Rhythm Association, Heart Rhythm Society, and Asia Pacific Heart Rhythm Society (EHRA/HRS/APHRS) Expert Consensus on Ventricular Arrhythmias stated that patients with heart failure Report from the WCD about a tachycardia that was detected but did not require therapy delivery.
with reduced ejection fraction after a myocardial infarction (with or without revascularization) may benefit from WCD use in weeks to months until recovery [11]. Many patients who might be potential WCD candidates are not routinely included in clinical trials, and the HRS/ACC/AHA Expert Consensus Statement of 2014 suggested that the WCD may be considered as a "bridge to ICD" in certain patients [12]. The following year, in 2015, ESC guidelines suggested that the WCD might be used in patients with transiently impaired LV function, naming certain specific conditions such as myocardial infarction, peripartum cardiomyopathy, and myocarditis, and in patients awaiting heart transplantation or a left ventricular assist device [13].
In 2016, the AHA issued a science advisory about the WCD which was endorsed by the HRS [14]. Among their key concepts: they viewed the WCD as a temporary means for preventing arrhythmic death without the need of bystander response; despite limited evidence from randomized controlled trials, observational data support the notion that the WCD can detect and terminate ventricular tachyarrhythmias; and the use of a WCD is reasonable when there is a clear ICD indication and a current, transient contraindication to ICD implantation. According to this advisory, the role of a WCD is less clear when the risk of arrhythmias is transient, but a WCD still may be appropriate. The most controversial use of the WCD is in patients in the early recovery phase after myocardial infarction or with a newly diagnosed form of nonischemic cardiomyopathy. Many of these patients will not need a permanent ICD but will experience a period of time when they are at increased risk of SCD. Evidence for use of the WCD was C-level (expert opinion) and may be summarized as: • The use of WCD is reasonable when there is an indication for an ICD, but a transient contraindication or interruption in ICD care (such as infection) temporarily prevents implantation (Class IIa).
• The use of WCD is reasonable as a bridge to more definitive therapy, such as heart transplant (Class IIa).
• The use of WCD may be reasonable if there is concern about an elevated SCD risk that is expected to resolve over time or with treatment, for example, ischemic heart disease following revascularization or nonischemic cardiomyopathy being initiated on guideline-directed medical therapy (Class IIb).
• The WCD may be appropriate as a bridge therapy when patients are at elevated risk for SCD in cases where an ICD would reduce the risk of SCD but not improve overall survival, such as within 40 days following acute myocardial infarction (Class IIb).
• WCDs should not be used when the risk for potentially life-threatening nonarrhythmic causes is expected to exceed the risk of ventricular arrhythmias, especially in those situations where longevity is not expected to exceed 6 months (Class III).

Clinical trials and other evidence
There are many observational studies about the use of the WCD, but to date only one large randomized clinical trial has been published, the Vest Prevention of Early Sudden Death (VEST) study (n = 2302) [15]. All patients had had a recent myocardial infarction and a left ventricular ejection fraction ≤35%; some but not all patients had undergone revascularization. Patients were randomized into two arms: guideline-directed medical treatment (control) or a WCD. In the first 90 days after myocardial infarction, the WCD did not result in a lower rate of arrhythmic death, but total mortality was lower in the WCD group (3.1% vs. 4.9%, p = 0.04, uncorrected) [15]. Despite the fact that the VEST study did not result in a lower rate of arrhythmic death for WCD patients, there are important aspects of this study that deserve deeper scrutiny. Unwitnessed arrhythmic death is difficult to ascertain, and five of the nine VEST subjects deemed to have died due to an arrhythmia were wearing the WCD at the time, and the WCD showed no evidence of a tachyarrhythmia. Since arrhythmic death is rare, even a small number of misinterpretations in a study like this may skew results. Moreover, the study was designed assuming patients would wear the WCD at least 70% of the time, and compliance dropped as the study progressed. Since fewer patients wearing the WCD died, it has been argued that there was not a single active treatment group in the study (WCD group) but rather two: patients randomized to the WCD group broken down into those who wore the WCD and those who did not [16]. Of the patients in the WCD arm of the study who died, 75% were not wearing the WCD at the time [15]. Early after the WCD was first cleared to market, an observational study called the Wearable Defibrillator Investigative Trial (WEARIT) enrolled 177 ambulatory patients who had New York Heart Association (NYHA) functional Class III or IV heart failure and an ejection fraction <30%. It was subsequently combined with a similar observational study, the Bridge to ICD in patients at risk of sudden arrhythmic death (BIROAD), which enrolled patients who had an AMI and needed bridge therapy for up to 3 months (n = 112). In 901 patient-months, the mean duration of wear was 3.1 months. Among the WEARIT patients, there were two appropriate and successful therapy deliveries in the same patient several days apart, and there were four appropriate, successful therapies delivered in two of the BIROAD patients. Two unsuccessful therapy deliveries occurred, both of which involved the improper wear of the WCD. Altogether, 12 patients died over the course of the study, none of whom were wearing the WCD at the time. Over the 901 patientmonths, there were 6 inappropriate therapy deliveries in 6 patients (0.7% per month) [17].
The WEARIT-II Registry enrolled 2000 patients, of whom 805 were diagnosed with ischemic cardiomyopathy, 927 with nonischemic cardiomyopathy, and 268 with congenital heart disease [18]. During the study, 41 patients experienced a total of 120 episodes of VT, of whom 54% received an appropriate shock. Inappropriate shocks occurred in 0.5% of patients. Many of the patients in WEARIT-II had improved their ejection fraction over the course of time they wore the WCD, and at the end of WCD treatment, only 42% got an ICD.
The Study of Wearable Cardioverter Defibrillator in Advanced Heart-Failure Patients (SWIFT) was a nonrandomized prospective study at two centers evaluating the use of the WCD in 75 patients hospitalized with advanced heart failure symptoms and LV dysfunction. Patients wore the WCD for 3 months after hospital discharge. Two-thirds of the patients (66%) had nonischemic cardiomyopathy. Eight arrhythmic events occurred in five patients, all successfully terminated by the WCD. No inappropriate therapies were delivered, and no patients died in the course of the study. When the study concluded, 28% were implanted with an ICD [19].
A summary of these trials appears in Table 1.

Transient contraindication for an ICD
One of the main reasons for WCD use is ICD system infection, which poses a clinical challenge in that the best course of action is to extract the device and lead(s), submit the patient to a course of antibiotic therapy, and then replace the ICD system with a new device [7,9,45]. The rate of infections associated with cardiac implantable electronic systems continues to increase, even at high-volume centers [46]. Antimicrobial therapy may last 10-14 days or longer, depending on the nature of the infection and the patient's response. During this time, the patient is without an ICD. Leaving the ICD in place while treating an infection is associated with a high mortality rate (31-66%) [47,48], but removing the device also increases the patient's mortality rate, albeit from 8-27% [49][50][51]. Thus, the clinician faces three challenges: if the device is replaced too early, the patient risks re-infection; if the patient is deprived of the device too long, there is a risk for potentially life-threatening arrhythmias; and placing the patient under close monitoring in the hospital or a long-term care facility is cost prohibitive and deleterious to the patient's quality of life. In such cases, the use of a WCD can be a valuable interim solution for arrhythmic rescue.
In a study of 97 ICD patients whose devices had to be explanted for infection, patients were prescribed a WCD for the mean antimicrobial treatment course of 21 days. As they recovered from infection, two patients experienced a total of four VT episodes, all of which could be successfully treated [52]. In a retrospective analysis of 8058 patients who received a WCD from 2002 to 2014 when an infected ICD was removed, 4% experienced ventricular tachyarrhythmias, and the rate of arrhythmic episodes was greatest in the first 3 weeks after device explantation (0.9, 0.7, and 0.7%, respectively), and the risk for ventricular tachyarrhythmias after device removal was 4% during the first 2 months and 10% at 1 year [25].

Bridge to cardiac transplantation/left ventricular assist device
Heart transplantation or the use of a left ventricular assist device is the only potentially long-term therapeutic option for some patients, but during the waiting period, patients are at high risk for dangerous arrhythmias and may have other comorbid conditions as well. In a study of 121 patients prescribed with the WCD while waiting to receive a donor heart (mean 127 days), 7 patients (5.8%) were shocked appropriately, and all survived [23]. Two inappropriate therapy deliveries occurred deemed to be caused by rapid ventricular response to atrial fibrillation. In this study, two patients died of asystole during the waiting period; asystole is not treated by the WCD because it lacks a pacing capability [23].

Low ejection fraction in reimbursement-mandated waiting period
In the USA and other parts of the world, patients with an ejection fraction ≤35% may be required by reimbursement authorities and guidelines to wait out a specific period of time before an ICD may be implanted; these time periods range from 30 to 90 days. This includes patients with cardiomyopathy.

NYHA Class IV heart failure
This group of patients meets the requirements for Class IV heart failure but is not otherwise indicated or qualified to receive an ICD. Some of these patients may be waiting for cardiac transplantation, while others may be contraindicated for ICD implant for other reasons (frailty, comorbidities, patient refusal, and so on). The Study of Wearable Cardioverter Defibrillator in Advanced Heart-Failure Patients (SWIFT) was a prospective study of 75 advanced heart failure patients at 2 centers. All patients had low ejection fraction (21.5 AE 10.4% at baseline), were prescribed a WCD, and were followed up for 3 months. In the SWIFT study, 66% of patients had nonischemic cardiomyopathy. Over the course of the study, eight arrhythmic events occurred in five patients, including three episodes of nonsustained VT and one episode of polymorphic VT; all episodes were appropriately treated. No patient in the study received inappropriate therapy delivery. At the end of the study, 28% of patients went on to permanent device implantation, and the cumulative mortality rate at 3 years in this population was 21% for patients with nonischemic cardiomyopathy compared to 21% for those with ischemic cardiomyopathy [19].

Ischemic and nonischemic cardiomyopathy
Patients with ischemic cardiomyopathy may be indicated for a primary prevention ICD if they have an ejection fraction ≤35% and NYHA functional Class II or III or if they have an ejection fraction ≤30% with NYHA Class I [53]. Nonischemic cardiomyopathy covers a range of conditions that may include inflammatory, toxic, metabolic, genetic, or autoimmunological processes, and arrhythmic activity, including SCD, may be one of the first symptoms of nonischemic cardiomyopathy [5,13]. Such patients typically fall into the reimbursement-mandated waiting period before a primary prevention ICD can be implanted, and many patients with recent-onset cardiomyopathy recover left ventricular ejection fraction and even experience reverse remodeling to the point that ICD implantation is unwarranted [5]. In cardiomyopathy patients, it is not clear if and how long patients should wait before ICD implantation is either deemed reasonable or unnecessary [5]. Ischemic cardiomyopathy patients may have higher rates of events than nonischemic cardiomyopathy patients [42]. Pharmacological therapy for cardiomyopathy may also improve the ejection fraction, and the WCD may be helpful as medical therapy is optimized [53].
In a retrospective single-center study of patients from June 2004 to May 2015, focus was placed on patients with newly diagnosed cardiomyopathy (254 nonischemic and 271 ischemic) [41]. Patients wore the WCD for a median of 61 days (interquartile range 25-102 days) and for a median of 22 h/day (17-23 h). The study produced 56.7 patient-years of data for nonischemic cardiomyopathy patients, during which no patients got appropriate shocks, but 1.2% (n = 3) were shocked inappropriately. There were 46.7 patient-years of data for ischemic cardiomyopathy, where 2.2% (n = 6) were shocked appropriately and two inappropriately (0.7%) [41].

Acute myocardial infarction
The role of defibrillation has been controversial in acute myocardial infarction (AMI) patients since the defibrillator in acute myocardial infarction trial (DINAMIT) reported that early ICD implantation failed to confer a mortality benefit in this arrhythmia-rich population [54]. Current guidelines recommend that following myocardial infarction, patients with compromised left ventricular function do not receive an ICD for a 3-month to 40-day waiting period, whether or not they have been revascularized [53]. In the weeks immediately following a myocardial infarction, patients are vulnerable to a number of potentially lethal conditions, many unrelated to ventricular tachyarrhythmias, so that the mortality rate for myocardial infarction patients with or without an ICD is roughly the same (7.2% for both, assuming linear mortality rates in the first 3 months, based on DINAMIT study data) [54]. This imposes a "waiting period" on myocardial infarction patients before a device may be implanted and during which time they may be especially vulnerable to SCD. For many patients and clinicians, this creates a tension between abiding by evidence-based guidelines and meeting reimbursement requirements yet still providing reasonable means to rescue post-AMI patients from SCD [6]. The WCD has been proposed as an interim device for this population during this waiting period before a permanent ICD may be implanted. Further complicating this picture is the fact that some myocardial infarction patients will recover left ventricular function in the weeks following their heart attack to the point that they do not require an ICD at all. Thus, it may be argued that for these patients, the use of the WCD may be to provide possible rescue during recovery from the myocardial infarction and to avoid unnecessary ICD implantation [6].
It has been observed that myocardial infarction patients prescribed a WCD and shocked appropriately and successfully to convert a ventricular tachyarrhythmia nevertheless have high mortality rates. While this remains to be elucidated, it suggests that either ventricular tachyarrhythmias in the immediate aftermath of a heart attack are indicative of poor outcomes or the arrhythmia and/or the rescue shock has a destabilizing effect on the patient [55]. The Valsartan in acute myocardial infarction trial (VALIANT) evaluated 14,609 myocardial infarction patients with low ejection fraction for SCD. VALIANT reported myocardial infarction patients with an ejection fraction ≤30% had a mortality of 2.3% per month in the first 30 days after the myocardial infarction (and that 83% of all patients who died of sudden unexpected death died within the first 30 days of hospital discharge). Every decrease of 5% in the ejection fraction was associated with a 21% increase in SCD risk in the first 30 days after myocardial infarction [56].
The Vest Prevention of Early Sudden Death (VEST) trial found that in myocardial infarction patients with low ejection fractions (≤35%), the WCD did not significantly reduce arrhythmia-associated deaths compared to the control group who did not have a WCD [15]. The rates of arrhythmic death were 1.6% in the WCD and 2.4% in the control group (relative risk 0.67, 95% confidence interval, 0.37-1.21, p = 0.18) [15]. It must be noted in this connection that arrhythmic death can be challenging to adjudicate when the patient dies without a witness. However, even comparing all-cause mortality data did not provide a significant benefit for WCD patients over those who did not have a WCD [15].

Renal failure
Compared to one SCD death per 1000 patient-years in the general population, hemodialysis patients face a 50-fold greater risk of arrhythmic death at 43 deaths per 1000 patient-years [57,58]. Patients on hemodialysis present clinical challenges in that they are often comorbid and frequently geriatric, may be frail, and are prone to infections. End-stage renal disease and hemodialysis expose these patients to a very considerable risk of arrhythmic death, but many hemodialysis patients are not appropriate candidates for ICD therapy. Compared to historical data, the WCD has been associated with improved survival in renal failure patients [24].

Takotsubo cardiomyopathy
Takotsubo cardiomyopathy, sometimes called "broken-heart syndrome," is a form of cardiomyopathy where the myocardium weakens and remodels. This condition is potentially reversible, but while patients experience the cardiomyopathy, they are at risk for potentially life-threatening ventricular tachyarrhythmias, and some develop concomitant QT interval prolongation, further increasing their risk for arrhythmia [5]. In a study based on all data from the USA involving WCD wear from 2007 through 2012, a total of 102 takotsubo patients were identified by the ICD-9 code 429.83. This population was overwhelmingly female (89%) with an initial ejection fraction of 27 AE 6% who wore the WCD for a mean duration of 44 AE 31 days with a mean follow-up of 440 AE 374 days. During the WCD wear time, 2% of patients (n = 2) received an appropriate shock, 1% (n = 1) received two inappropriate shocks, and 2% (n = 2) suffered bradyarrhythmias that required pacing. Two patients in the study died (one asystole and one from an arrhythmia while not wearing the WCD) [59].

Peripartum cardiomyopathy
Peripartum cardiomyopathy results in left ventricular dysfunction that can predispose the patient to SCD. About half of these patients will recover significantly or entirely over the course of about 6 months even without intervention; however, some will not, and all are at high risk for arrhythmias during the course of the condition [5]. In a study of 12 consecutive women with peripartum cardiomyopathy observed at a single center (of whom seven wore the WCD), four episodes of VF occurred in three of the patients wearing the WCD, all of which were successfully terminated. One patient experienced numerous alarms for inappropriate shocks but was able to abort them so that no inappropriate shocks occurred. No deaths occurred. During therapy for heart failure, over the course of the 12-month followup, ejection fractions improved significantly from 24.0 AE 11.8% at baseline to 46.6 AE 7.6%. Patients with a lower ejection fraction at baseline improved more than those with a higher ejection fraction at baseline [60].

Long QT syndrome
Long QT syndrome (LQTS) is a heritable and potentially fatal cardiac channelopathy that exposes patients to the risk of SCD. LQTS patients are typically treated with beta blockade, left cardiac sympathetic denervation, and, in some cases, a permanent ICD. It is unclear what, if any, role the WCD might play for treating LQTS. A retrospective review of 1027 LQTS patients who were prescribed a WCD as a bridge to possible ICD implantation or other treatments found no inappropriate shocks that were administered by the WCD and only 1 patient received an appropriate shock to terminate VF [61]. Since LQTS is a lifelong condition, the WCD is not an optimal permanent solution in this population, but it may be helpful as newly diagnosed patients consider their therapeutic options or for LQTS patients on medical therapy who are entering high-risk periods of life, such as having to take a medication that might prolong their QT interval further or in postpartum women [61].

Special populations
The WCD is available in different sizes and has an elasticized waistband and adjustable straps, making it suitable for use in a variety of patients, including children. The role of the WCD in certain special populations is being addressed, but there is limited evidence about these groups.

Pediatric patients
Guidance is available to schools and teachers for children prescribed the WCD. In particular, it is important that educators realize that unlike the automatic external defibrillator systems available in many schools, the WCD will detect arrhythmias and treat them without any bystander intervention [62,63]. Children seem to adjust well to the WCD. In a study of 231 pediatric WCD patients between the ages of 8 and 17 years monitored a median of 39 days with daily wear time around 21 h/day, a step-counter accelerometer device reported that activity levels for these children increased significantly over baseline in the first 3 weeks after getting the WCD (p < 0.001) [64]. This suggests that the WCD does not inhibit or curtail the children's activities and may help them achieve recommended levels of daily exercise.

Cancer patients
Some patients with cancer may be at elevated risk for dangerous arrhythmias because of chemotherapy-induced cardiomyopathy or long QT syndrome caused by drugs but may be contraindicated for device implant because of their malignancy or other reasons [65].

Geriatric patients
The prevalence of cardiovascular disease is high in the geriatric population, but there may be reluctance to consider an older patient for WCD therapy, in particular because it may be uncomfortable or feel restricting to them. In a large study of 1732 patients with ischemic and nonischemic cardiomyopathy, patients were grouped by age into younger (<65 years) and older groups (≥65 years). The older group (n = 722) wore the WCD more hours per day (median 22.8 vs. 22.3, p < 0.001) and had higher rates of events (31.95 vs. 9.82, p = 0.027). Younger patients with nonischemic cardiomyopathy had a higher rate of atrial arrhythmias (150.1 vs. 74.9, p = 0.055), and more following WCD therapy, a greater number of older than younger patients got a permanent ICD (41.8% vs. 36.5%, p = 0.034). Patients in both age groups tolerated WCD therapy well [34].

Appropriate and inappropriate therapy
The WCD has been shown to deliver appropriate high-energy therapy to convert dangerous ventricular tachyarrhythmias. In a postmarket registry of 3569 WCD patients (mean duration wear was 52.6 AE 69.9 days), first shock success occurred in 99% of cases (79/80) for all episodes of conscious VT/VF and in 100% of cases (n = 76) of unconscious VT/VF [33]. Because the WCD is an external device, it is far more exposed to sources of electromagnetic interference (noise) than implanted devices, which may result in oversensing, inappropriate arrhythmia detection, and inappropriate therapy delivery. Patients are signaled about 30 s prior to therapy delivery and may abort the shock by pressing two buttons [39,40]. For this reason, the rate of inappropriate therapy delivery with the WCD is relatively low, occurring in approximately 0.4-3.0% of patients [6,18,33,43]. See Table 1.
The WCD delivers rescue shock therapy only and has no pacing capability. Asystole, a recognized risk factor for dangerous ventricular tachyarrhythmias, may occur in patients with compromised cardiovascular function, such as low ejection fraction. While an ICD can detect and offer pacing support during an asystole episode, the WCD cannot pace such patients, and there is a risk that an untreated asystole may be fatal [66].

Patient factors
There are specific patient factors that warrant consideration when prescribing this novel therapeutic option. Many patients will have no concept of what a WCD is or how it works.

Patient education
Manufacturer's representatives may be available to help train patients in the proper function of the WCD, and, if they are not available, the clinical team should make sure the patient knows how to wear the vest, how to adjust it for proper fit, how to replace the battery, how to charge the battery, and how to transfer data from the WCD to the network. For this reason, the WCD requires the patient be able to understand and manage these tasks and be willing to do them. An initial training session should make sure the patient can put on the vest and insert batteries that may last an hour or more. It may be helpful for a second follow-up contact with the trainer over the course of the next few days to help with any questions or problems the patient may still have. The manufacturer has a 24-h technical support hotline for urgent questions [5].

Compliance
Compliance is an issue in all areas of medicine but particularly in the case of the WCD which patients may find restrictive or uncomfortable. A postmarket registry study (n = 3569) found that patients who used the WCD for a longer duration of time (days of wear) were significantly more likely to wear in more hours per day (p < 0.001) [33]. Over time, the WCD has been redesigned to make it lighter in weight and more comfortable for extended wear. Remote monitoring can alert the clinic as to actual wear time for an individual patient [67]. Compliance may be encouraged by educating the patient as to the nature of ventricular tachyarrhythmias and how the device protects them.

Psychological factors
It has been speculated that patients prescribed a WCD may experience emotional distress and view the device as a constant worrisome reminder of their own mortality. Patients may also feel isolated if they do not know anyone else who has ever worn such a device. Patients have sometimes reported that they find the device symbolic of their own vulnerability [33]. Of course, such adverse emotions may occur in all patients facing the sudden news that they have a serious cardiovascular condition regardless of whether they are prescribed a WCD or some other therapy. Psychological distress is an important clinical consideration because it is potentially modifiable. There may be ways to reduce depressive or anxious symptoms in clinically meaningful ways. Depression worsens outcomes and actually serves as a predictor for both mortality and shock therapy [68,69]. Depression has been associated with a nearly doubled risk for all-cause mortality in ICD patients [69]. Furthermore, depression and anxiety may adversely affect patient compliance, adherence to pharmacological therapy, and lifestyle.
In another study of 123 patients considered WCD candidates, at baseline 21% showed signs of clinically depressive symptoms, and 52% had anxiety. Six weeks after WCD therapy commenced, rates of depression and anxiety dropped to 7 and 25%, respectively [32]. It is not clear if patients recovered their emotional equilibrium as a result of WCD therapy or as a matter of course as they got used to their new identities as cardiac patients.

Device-device compatibility
When a patient has more than one electronic cardiac device, the potential of device-device interaction exists. The literature reports one case of a fatal devicedevice interaction between a permanent pacemaker and a WCD [70]. In this case, the patient received unipolar dual-chamber pacing, but when he developed VF, no therapy was delivered as the device inappropriately detected the large unipolar pacing spikes as cardiac signals [70].
A study sponsored by Zoll examined pacing in 60 patients testing the AAI, VVI, and DDD modes in both unipolar and bipolar device configurations to determine if the WCD would detect the pacing spikes; patients were signaled before shock delivery and could use the patient response buttons to avert the therapy delivery. Only unipolar DDD pacing was detected by the WCD's algorithm and only in 10% of patients (6/60). This study suggests that pacing may occur concomitantly with WCD use if unipolar configurations are avoided [2]. If unipolar pacing must be used in a particular patient, then the WCD is contraindicated. Another study of the concomitant use of the WCD and a pacemaker showed that double-counting and waveform alterations might also occur in certain bipolar pacing modes and in single-chamber as well as dual-chamber pacing [44]. Caution is urged in using the WCD in patients with pacing support from an implanted pacemaker system.

Costs
The WCD is "rented" to patients for a monthly fee, and reimbursement provisions vary by country. Since costs can be substantial, there is a need to better stratify patients into those who truly need a WCD for arrhythmic rescue and those who might be unlikely to benefit from it [41]. Cost-effectiveness models show that the number needed to treat to save 1 life with a WCD falls in the range of 70-110 patients over a median of 53-57 days [26]. There are situations in which the WCD poses a decided cost advantage. For example, cardiomyopathy patients who might otherwise be considered a candidate for permanent primary prevention ICD implantation may benefit from using the WCD during a recovery period; data shows that $60% of such patients will recover to the point that an ICD implantation is not necessary [18,33,41]. Thus, the costs for the temporary use of the WCD may be offset by the decision not to implant an ICD. In patients whose ICD must be removed for infection, it is sometimes necessary to keep the patient in the hospital or discharge him or her to a skilled nursing facility for weeks during antimicrobial therapy and recovery. The patient is at risk for SCD throughout this time. A cost-effectiveness analysis found that the WCD was costeffective in this situation in that it allowed the patient to be discharged home; the analysis is based on the assumption that there was a 2-week 5.6% risk of SCD in the population and the patient had to wait at least 2 weeks before ICD replacement [71].

Future directions
The WCD technology effectively treats VT/VF, but bradycardia pacing support would likely prevent SCD to an even greater extent. Adding pacing capability to the WCD would be an important and life-saving step forward.
A major obstacle in WCD therapy remains patient adherence. Unfortunately, not all patients are motivated to comply with the prescription to use the WCD, and unnecessary deaths occur because of poor compliance. Therefore, motivating the patient to adhere to therapy is of utmost importance. A combined approach with technology reminders (e.g., text messages via smartphones) and close follow-up by device professionals is crucial.
Much has been accomplished in the past 30 years to better treat the risk of SCD, and the WCD is definitely an important milestone in our advancing knowledge. Nevertheless, much more needs to be done to reduce the rates of arrhythmic death even more.
A Class II recall of the WCD occurred in January 2018, covering 33,000 devices. This problem, in which certain vests displayed a warning message to the effect that they could not charge sufficiently to deliver therapy, has been addressed.

Conclusions
The WCD is an important advancement in the armamentarium for cardiovascular disease and demonstrates safe, effective therapy, but patient compliance remains a concern. The WCD is an interim therapeutic alternative to the ICD. In some cases, the WCD may help patients recover significant systolic function to the point that an ICD is no longer necessary. Patients who need the WCD should receive individual one-on-one instruction in how to use the device, and clinicians should be prepared that there may be a degree of psychological distress. Nevertheless, these devices are important advancement in cardiac care for people at risk of dangerous arrhythmias.