High risk patients, who do not qualify for an implantable cardioverter-defibrillators, run a risk of ventricular arrhythmia and sudden cardiac death. However, if a new ICD cannot be implanted due to surgical contraindications, recent revascularization (within three months) or recent diagnosis of dilated cardiomyopathy (three to nine months), or an ICD has been explanted due to infection, a wearable defibrillator (WCD) provides an option for some protection against sudden cardiac death.
We describe the case of a 62-year-old man with a history of coronary artery disease, hypertension, hyperlipidemia, prior tobacco abuse, and abdominal aortic aneurysm repair, who presented to the emergency department with palpitations and dizziness. He was noted to have a wide complex tachycardia (WCT) of right bundle right axis (RBRA) morphology at a rate of 203 beats per minute. He was cardioverted into sinus rhythm with a shock of 200 Joules, after 100 J and 150 J failed in the emergency department. A 12-lead EKG following cardioversion revealed 3-4 mm ST-segment depression from leads V1 to V4 and ST segment elevation in leads III and aVF. His initial Troponin was 0.1 ng/mL, which rose to a peak of 2.2 ng/mL, with a peak CPK of 236; CK-MB was 13.1 ng/mL, and an index of 5.55. A cardiac catheterization revealed 100% occlusion of the right coronary artery, 80% stenosis of the proximal left anterior descending coronary artery, and an LVEF of 35% with an aneurysm. His left main coronary artery and left circumflex coronary arteries were patent. Successful deployment of a drug-eluting stent in the proximal left anterior descending artery reduced the stenosis to 0%.
In view of current guidelines, after a revascularization procedure a patient may not undergo ICD implantation for at least three months. Therefore, the patient was prescribed a wearable defibrillator (LifeVest, LIFECOR, Inc., Pittsburgh, PA). The following parameters were programmed:
Ventricular tachycardia (VT) rate was set at 180 beats per minute
Ventricular fibrillation (VF) rate was set at 220 beats per minute
The first shock was set to the maximum device output of 150 Joules, as were all additional shocks (biphasic waveform, up to five shocks per treatment sequence).
After 10 weeks, the patient developed sudden onset of palpitations with lightheadedness and presented to the ER of another hospital in the neighboring state. The wearable defibrillator alarm went off numerous times, prompting him to press and hold two response buttons simultaneously to avert a shock (Figure 1). Because no bystander is necessary to shock a patient with a wearable defibrillator, response buttons are incorporated into the design as a test of consciousness in order to avoid shocking a conscious patient. The resulting EKG recordings, sent via modem to the manufacturer, revealed a WCT (Figure 2). A 12-lead EKG in the ER revealed WCT with identical morphology to the index WCT with RBRA morphology at a rate of 196 beats per minute. After three doses of adenosine at 6 mg, 6 mg, and 12 mg failed, two loading doses of amiodarone of 150 mg each were administered. Because the patient remained in WCT, a lidocaine bolus was administered without success. Thereafter, the patient was cardioverted with an external defibrillator using a 100 Joule shock (synchronized biphasic waveform) into sinus rhythm. The patient had Mobitz type I, second-degree AV block and transient junctional rhythm.
He was transferred to Good Samaritan Hospital and underwent implantation of a dual-chamber ICD. After one week of implantation, the patient developed rapid ventricular tachycardia for which first sequence of antitachycardia therapy successfully converted him into sinus rhythm. After five days, the patient s antitachycardia therapy failed, a 5 Joule shock accelerated the tachycardia into VF zone and first therapy of 21 Joules successfully defibrillated the patient into sinus rhythm. He was admitted to the hospital, and started on sotalol for suppression of VT as well as lowering of defibrillation thresholds. Additionally, he underwent a cardiolite stress test, which was negative for any ongoing ischemia. Ablation for the VT focus was scheduled.
The Wearable Defibrillator
The wearable cardioverter-defibrillator components consist of:
1. A monitor that the patient wears around the waist or from a shoulder strap which weighs 1.8 pounds. When the device detects a treatable arrhythmia, an alarm sequence begins, giving a conscious patient time to stop the treatment. This keeps inappropriate arrhythmia detections from becoming inappropriate shocks a key difference between the wearable defibrillator and an implanted defibrillator.
2. Response Buttons on Alarm Module: The alarm module has pushbuttons and indicators for the user, as well as a speaker for sounding alarms and voice prompts. If the patient holds the two response buttons at any time during the treatment sequence, the alarms stop and no shocks will be delivered. If the patient does not respond or releases the response buttons, the device continues to give alarms and spoken warnings to bystanders that a treatment shock is about to be delivered.
3. ECG Electrodes are dry and non-adhesive for long-term comfort.
4. Therapy electrodes containing gel that is released just prior to delivering the treatment shock, in order to deliver the shock most efficiently.
How the Device Works
The LifeVest is worn as an undergarment (Figure 1). It senses ventricular tachyarrhythmias using ECG electrodes an average of five to six seconds after onset. The arrhythmia alarm sequence escalates from silent vibration and blinking lights, to low and high volume piercing tones, and finally to a voice warning the patient and surrounding people about the impending shock. The shock is delivered through the large posterior and smaller apical therapy electrodes.
If the patient presses on both buttons on the alarm module, he can avert the shock. If a stable tachycardia in the VT rate range continues past 30 seconds with the response buttons in use, the LifeVest will automatically reset the VT rate cutoff to 10% faster than the originally programmed value until it reaches the VF rate cutoff. If during the episode, the patient loses consciousness and lets go of the response button, a shock will be delivered.
The LifeVest delivers up to five shocks in one treatment sequence, at a maximum output of 150 Joules using a biphasic waveform. The current generation does not have antitachycardia or anti-bradycardia pacing.
The first case series of WCD demonstrating the clinical efficacy of induced ventricular tachycardia or ventricular fibrillation among survivors of cardiac arrest was published in 1998. VT or VF was successfully terminated by WCD among all 10 patients who had inducible VT/VF, by delivery of 230 Joules monophasic truncated exponential waveform shock. Seven patients were on amiodarone, and two were taking sotalol.1
In another study, all 22 electrically-induced VF episodes among 15 patients were successfully terminated by the delivery of a single 70 or 100 Joule shock in a randomized fashion.2 Only one patient was taking amiodarone, and none of the patients had an ICD or pacemaker in place. The current generation of WCDs provides a maximum output of 150 Joules in the biphasic waveform, thus providing a safety margin between 50-80 Joules.
More recently, a clinical study was published3 consisting of two disparate groups of patients. One group (WEARIT Study) consisted of 177 patients who had NYHA functional class III or IV congestive heart failure with an LVEF of ? 30% as a bridge to cardiac transplantation. The other group (BIROAD Study), which represented common clinical problems encountered in clinical practice, consisted of patients who needed a bridge for a period of four months before possible use of an ICD. The latter group included patients who had:
1. VT/VF within 48 hours of surgical revascularization or MI;
2. LVEF of
3. Cardiac arrest or syncope 48 hours post surgical revascularization but could not get an ICD;
4. ICD candidates potentially waiting for four months, for any reason;
5. Criteria for ICD was met but was refused by the patient.
Among the eight patients who had a shock, six attempts were successful (two in WEARIT patients and four in BIROAD patients) resulting in a success rate of 75%. Both unsuccessful shocks were due to incorrect wearing of the therapy electrodes, possible in earliest version of the device but now detected and warnings sent to the patient in the current version. Twelve deaths occurred, out of which six were sudden: five patients were not wearing the device and one patient was wearing it incorrectly.
Effective July 1, 2005, Medicare’s expanded coverage for the LifeVest wearable defibrillator now includes protection for patients in the following situations:
In instances where an ICD could be prescribed but can’t be used; all ICD indications are covered without demonstrating an ICD surgery contraindication.
After ICD explantation due to infection.
VF and sustained VT (spontaneous or induced) if it occurs > 48 hours post MI.
Primary prevention (EF
– Immediately post-MI.
– Pre- and post-CABG or PTCA.
– Awaiting cardiac transplantation.
– Recently diagnosed nonischemic cardiomyopathy.
Limitations of the Device
Although small and lightweight, wearable defibrillators can be cumbersome and can affect lifestyle. However, given the sick patient population, most patients would prefer it to an automatic external defibrillator, which requires the presence of a knowledgeable bystander. Inappropriate shocks could potentially cause significant anxiety among these patients, just as they do with ICDs. The total time it takes from detection to the delivery of the shock is much longer than ICDs, thus making a wearable defibrillator unsuitable as a direct substitute for an ICD. However, the longer duration may be advantageous in a situation where inappropriate arrhythmia is detected, and give a patient more time to avert a shock. Regardless, the time from detection to shock is much shorter for ICDs and wearable defibrillators than bystander defibrillation in the home or hospital, unless the patient is in an ICU environment.
Wearable defibrillators are an appropriate therapy for protecting against sudden cardiac death among high-risk patient populations who cannot get an ICD. This therapeutic approach may have certain advantages over reliance on an automatic external defibrillator, as it does not depend upon the presence of a bystander nor the bystander s willingness and ability to use an AED.