Wearable Cardioverter Defibrillator
Sojin Y. Wass
Mina K. Chung
KEY POINTS
Wearable cardioverter defibrillators (WCDs) can detect and defibrillate ventricular tachycardia and ventricular fibrillation, but are unable to protect against asystole.
WCD Indications
WCDs are reasonable for patients with an implantable cardioverter defibrillator (ICD) and a history of sudden cardiac arrest or sustained ventricular arrhythmias in whom ICD removal is required, as with infection (Class IIa recommendation).
WCDs may be reasonable for patients at increased risk for sudden cardiac death but who are not eligible for ICDs (Class IIb recommendation). This includes patients awaiting heart transplant, who have left ventricular ejection fraction less than or equal to 35% and are within the first 40 days following a myocardial infarction or 90 days after revascularization, myocarditis, secondary cardiomyopathy, or systemic infection.
Effectiveness of the device relies heavily on patient adherence.
INTRODUCTION
Life-threatening ventricular tachyarrhythmias, such as sustained ventricular tachycardia (VT), torsades de pointes, and ventricular fibrillation (VF), constitute the major arrhythmias that can cause sudden cardiac death (SCD).1 VT/VF constitutes 75% to 80% of events, followed by bradyarrhythmias, which occur in 10% to 15% of cases.2 Implantable cardioverter defibrillators (ICDs) have been demonstrated to be effective in the secondary prevention of SCD in patients who have survived prior cardiac arrest, as well as for the primary prevention of SCD in certain high-risk patients, such as select patients with heart failure and ventricular dysfunction.
Despite the protective features of ICDs, there remain some situations in which patients may be at risk for SCD, but implantation of an ICD is not indicated nor feasible. The wearable cardioverter defibrillator (WCD) provides an alternative option until an ICD becomes indicated or feasible. Although WCDs can also provide monitoring capability with protection while assessing risk for SCD, this is not a Food and Drug Administration (FDA)-approved indication.
DESCRIPTION OF THE WCD
The only currently FDA-approved WCD is able to detect life-threatening VT or VF and defibrillate or cardiovert in response. This first WCD, the LifeVest manufactured by ZOLL, was approved by the FDA for commercial use in 2002.
LifeVest is a vest-like structure that contains four monitor electrodes and three defibrillating electrodes that are embedded in the straps of a vest garment and connected to a defibrillation unit, which is carried around the waist of the patient (Figure 19.1).
The four electrodes generate a two-lead electrocardiogram (ECG) strip (front to back and right to left) that is used for rhythm and morphology analysis in the detection of arrhythmias. When a shockable arrhythmia is detected, the three defibrillating electrodes extrude gel just prior to defibrillation, audible alarms are issued, and a shock is delivered to the patient, on average at 45 seconds from the time of detection. The lapse of 45 seconds allows for nonsustained arrhythmias, of less than 30 seconds in duration, to not be shocked and to allow time for patients to abort the shock by pressing a button. The latter is a test of consciousness.
The four electrodes generate a two-lead electrocardiogram (ECG) strip (front to back and right to left) that is used for rhythm and morphology analysis in the detection of arrhythmias. When a shockable arrhythmia is detected, the three defibrillating electrodes extrude gel just prior to defibrillation, audible alarms are issued, and a shock is delivered to the patient, on average at 45 seconds from the time of detection. The lapse of 45 seconds allows for nonsustained arrhythmias, of less than 30 seconds in duration, to not be shocked and to allow time for patients to abort the shock by pressing a button. The latter is a test of consciousness.
 FIGURE 19.1 Wearable cardioverter defibrillator configuration. The system consists of four monitor electrodes and three defibrillating electrodes embedded in a vest garment, connected to a defibrillation unit. A, The LifeVest consists of a garment and a monitor. The garment is composed of ECG electrodes connected to a monitor. The system continuously monitors the rhythm of the heart, detects arrhythmias, and delivers shocks when necessary. The monitor records arrhythmias and determines when defibrillation therapy is necessary. B, The vest is worn outside the body underneath clothing. The monitor can worn around the waist or on a shoulder strap. Courtesy of ZOLL. A |
As with an ICD, the WCD may be programmed to different VT or VF zones (ranging from 120 to 250 beats/min), response times (time from detection to defibrillation sequence activation, 60-180 seconds), and shock energy (between 75 and 150 J). Up to a total of five shocks can be delivered, and the typical time from detection of VT/VF to delivery of shock is less than 1 minute. The programming parameters of the device, ultimately, are at the discretion of the treating physician. Detection of arrhythmia is based on preprogrammed detection rates and deviation from baseline morphology, which is used as a template for each individual patient.
If the patient is awake at the time when shock is about to be delivered, the patient is able to press a button to abort the shock from being delivered, within 30 seconds (or up to 55 seconds) of the alarm activation by the device. Retrospective data have revealed that prolonging this period of detection to shock decreases the incidence of inappropriate shocks.
Other Functions
Besides defibrillation, the device acts as a loop recorder that continuously records and can transmit via modem, both tachyarrhythmias and bradyarrhythmias. The device stores ECG strips, up to 75 minutes, and captures events of VT/VF, 30 seconds prior to and 15 seconds following the alarm, which is activated at the time of detection of VT/VF.
Cons
WCDs are unable to protect against events of asystole, which carries a high mortality rate, and are the cause for SCD in a small number of patients. Its efficacy is dependent on user compliance and proper application.3,4 It can also be cumbersome, as it does need to be worn 24 hours a day, except during times of bathing or showering. The monitor is reported to weigh approximately 1.38 lb, battery included. Though it occurs less frequently than in ICDs, inappropriate shock delivery is also a concern (reported rates of 0.7% per month).3 Retrospective data suggest increasing the time from alarm activation to shock delivery can decrease this incidence.4
Pros
Unlike an ICD, the WCD is easily placed and removed; it does not require surgical implantation for use, nor explantation for removal. Especially when functional recovery of the myocardium is expected, the WCD allows for avoidance of risks that come with ICD placement and explant. It also carries an advantage in enabling the user to abort inappropriate shocks, a function that is not available in ICDs. Data in ICDs have suggested potential for myocardial damage and possible increase in mortality with delivered shocks, regardless of shock appropriateness
SCD AND RISK FACTORS
SCD accounts for almost 50% of cardiovascular deaths1,5 and frequently presents as sudden cardiac arrest (SCA). The American Heart Association (AHA) estimates the
annual burden of out-of-hospital cardiac arrest to be around 356 300,6 in addition to the annual in-hospital events of around 209 000.7 Survival after a SCA is heavily dependent on presence of bystander cardiopulmonary resuscitation and timely access to an external defibrillator.8
annual burden of out-of-hospital cardiac arrest to be around 356 300,6 in addition to the annual in-hospital events of around 209 000.7 Survival after a SCA is heavily dependent on presence of bystander cardiopulmonary resuscitation and timely access to an external defibrillator.8
Post-myocardial infarction (MI) and ischemic cardiomyopathy (ICM) are most frequently associated with increased risk of ventricular arrhythmias (VAs) and SCDs. Myocardial ischemia because of coronary artery disease has been identified as a primary cause of SCD, and more than half of out-of-hospital cardiac arrest events are of patients later found to have significant coronary lesions.9
Other etiologies for SCD include dilated cardiomyopathy, various subtypes of cardiomyopathies, channelopathies (such as long QT syndrome, Brugada syndrome, catecholaminergic polymorphic VT), myocarditis, developmental disorders (such as anomalous origins of coronary arteries), medication-related QT prolongation, and finally idiopathic etiologies.10
INDICATIONS FOR ICDs
ICDs are, by the 2017 American College of Cardiology (ACC), the AHA, the Heart Rhythm Society (HRS), and the European Society of Cardiology (ESC) guidelines, indicated for those at risk for SCD. This includes patients with ICM or nonischemic cardiomyopathy (NICM) with a left ventricular ejection fraction (LVEF) less than or equal to 35%.
However, because studies have shown recovery of myocardial function with optimal medical therapy in 40% or more of patients, ICDs are not recommended until optimal medical management has been sufficiently attempted. As a result, current guidelines suggest 40 days from time of acute MI, and less than 90 days after coronary artery bypass grafting (CABG) or revascularization by percutaneous coronary intervention (PCI), to allow for medical optimization and monitoring for left ventricular (LV) recovery, before an ICD is placed.
INDICATIONS FOR WCDs
The FDA has approved the WCD device for patients “at risk for SCA and are not candidates for or refuse an implantable defibrillator.”
The current indications for WCDs, as recommended by the 2017 AHA/ACC/HRS Guideline for Management of Patients with Ventricular Arrhythmias and the Prevention of Sudden Cardiac Death, are as follows11:
1. Class IIa Recommendation: The WCD is reasonable for SCD prevention in patients with an ICD and history of SCA or sustained VA in whom removal of the ICD is required (as with infection)
2. Class IIb Recommendation: The WCD may be reasonable for patients at increased risk for SCD but who are not yet eligible for an ICD, such as
a. Awaiting cardiac transplant
b. LVEF of less than or equal to 35%, and within 40 days of MI
c. Newly diagnosed NICM
d. Revascularization within the past 90 days
e. Myocarditis
f. Secondary cardiomyopathy
g. Systemic infection
RATIONALE FOR WCD
Ischemic Cardiomyopathy
Ventricular remodeling occurs most actively during the first 30 days of revascularization. Possible myocardial function recovery, in conjunction with optimal medical management, is also greatest during this period, but it is also a period of increased risk of the incidence of life-threatening arrhythmias. The Valsartan in Acute Myocardial Infarction (VALIANT) trial, a study from 2009 which aimed to predict risk factors for SCD in post-MI patients with depressed LVEF, noted that the greatest risk period for SCD was within the first 30 days following an acute MI, with a substantial proportion (51%) being from arrhythmia.12 In a secondary outcome analysis, the risk increased for SCD by 21% for every decrease in LVEF by 5%.12
This prompted the Cardiac Arrhythmia Suppression Trial (CAST), for which post-MI arrhythmia suppression was attempted using antiarrhythmic medications, with success in arrhythmia suppression but with increase in overall mortality.13,14 The Multicenter Autonomic Defibrillator Implantation Trial I (MADIT-I), subsequent to the CAST study, compared the use of ICD to conventional drug therapy and demonstrated significant survival benefit in patients with an ICD (all-cause mortality of 32% in conventional drug therapy vs 13% in the ICD group).15 The Sudden Cardiac Death in Heart Failure Trial (SCD-HeFT), one of the longest and largest ICD trials, was another landmark study that was able to demonstrate reduction in all-cause mortality, in both ICM and NICM patients with New York Heart Association (NYHA) functional Class II or III and LVEF less than or equal to 35%, randomized to ICD implantation compared to amiodarone therapy or placebo therapy.16 Similar findings were noted in the Multicenter Autonomic Defibrillator Implantation Trial II (MADIT-II) that followed patients post-MI. A relative reduction in overall mortality by 31% was noted in the ICD implantation group (14.2% mortality in ICD group vs 19.8% mortality in the conventional therapy group) in patients with post-MI ICM.17 The survival advantage of the ICD group was particularly noticeable at 9 months,17 which noticeably persisted. At 8 years of follow-up, the ICD implantation arm had an all-cause mortality rate of 49% versus the conventional therapy arm of 62%.18 These studies suggested that ICD use for primary prevention of SCD in ICM has a survival benefit.
Subsequent studies, however, suggested that for ICM after acute MI, ICD placement for primary prevention may not be necessary early for everyone. The current guidelines for 40 days of medical optimization post-MI, prior to permanent ICD placement, are derived from the findings of the Defibrillators in Acute Myocardial Infarction Trial (DINAMIT) and the Immediate Risk Stratification Improves Survival (IRIS) trial, both published in
2009. In DINAMIT, prophylactic implantation of an ICD 6 to 40 days post an acute MI, in patients with LVEF less than or equal to 35%, did not improve all-cause mortality.19 Arrhythmia-related deaths were less in the ICD group, but nonarrhythmia-related deaths were greater, offsetting the overall mortality benefit to early ICD placement for primary prophylaxis.19 The IRIS trial, completed subsequent to DINAMIT, showed similar findings congruent to DINAMIT, in which benefits of early ICD implantation for the prevention of SCD were offset by death from non-SCD events and complications.20
Based on study criteria for other ICD primary prevention trials, ICD implantation is generally deferred for 3 months after coronary revascularization. However, particularly in patients with LV dysfunction, the event rate is also higher early after CABG or PCI, compared to later times, as shown in the Intermountain Medical Center registry, the Society of Thoracic Surgeons Adult Cardiac Surgery Database,21 and the National Cardiovascular Data registry22 (Figure 19.2). A retrospective observational propensity score-matched cohort study by Zishiri et al looked at the utility of WCD wear post-CABG or PCI23 in the manufacturer registry compared to patients
undergoing CABG or PCI with LVEF less than or equal to 35% at the Cleveland Clinic (Figure 19.3). Risk of death in the WCD group was lower by 38% in the CABG cohort and was lower by 57% in the PCI cohort. In both the WCD and non-WCD groups, and in both cohorts of CABG and PCI, risk of death was highest in the early phases post-revascularization (defined as first 90 days following revascularization). The difference between the WCD and non-WCD groups, in both cohorts, CABG and PCI, were most notable during these first 90 days after revascularization as well. In addition, there was a survival difference that persisted beyond the first 90 days (Figure 19.3C); upon subgroup analyses, this difference was significant in the PCI
cohort WCD group, hazard ratio of 0.66 (P = 0.043), but was not present in the CABG cohort.23 These findings were suggestive of mortality benefits to WCD wear post-revascularization, but also suggested that mortality benefits were not entirely explained by shock therapies in the first 90 days.
undergoing CABG or PCI with LVEF less than or equal to 35% at the Cleveland Clinic (Figure 19.3). Risk of death in the WCD group was lower by 38% in the CABG cohort and was lower by 57% in the PCI cohort. In both the WCD and non-WCD groups, and in both cohorts of CABG and PCI, risk of death was highest in the early phases post-revascularization (defined as first 90 days following revascularization). The difference between the WCD and non-WCD groups, in both cohorts, CABG and PCI, were most notable during these first 90 days after revascularization as well. In addition, there was a survival difference that persisted beyond the first 90 days (Figure 19.3C); upon subgroup analyses, this difference was significant in the PCI
cohort WCD group, hazard ratio of 0.66 (P = 0.043), but was not present in the CABG cohort.23 These findings were suggestive of mortality benefits to WCD wear post-revascularization, but also suggested that mortality benefits were not entirely explained by shock therapies in the first 90 days.
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