The Wearable Cardioverter-Defibrillator [chapter]

Peter Magnusson, Joseph V. Pergolizzi, Jo Ann LeQuang
2019 Sudden Cardiac Death [Working Title]  
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
more » ... 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). 2. The WCD and its function Currently, there is only one WCD, the LifeVest 4000 ® , and no other similar products are on the market. The WCD weighs 800 g and is available in a range of sizes with adjustable straps and an elasticized belt to fit snugly next to the skin under clothing (Figure 1) . The WCD has three pad-style electrodes for defibrillation and four more electrodes for arrhythmia detection (sensing). It is equipped with a battery-powered defibrillation unit capable of generating several highenergy shocks. When the WCD prepares to deliver a shock, it delivers a small amount of gel to the skin at each electrode, and a biphasic waveform of 75 or 150 J is delivered [4] . The WCD detects arrhythmias using an algorithm of heart rate (including rate stability and onset of arrhythmia) and waveform morphology. In the presence of noise or when a waveform template is not available, the detection function can work using rate alone. Once an arrhythmia is detected, the device signals the patient for about 30 s, allowing the wearer to abort the shock by manually depressing two response buttons. If the rate drops below the detection threshold during this 30-s waiting period, the detection is delayed or the shock prevented, depending on whether the slower rate was brief and temporary or persisted [5] . The WCD offers programmable parameters in that the ventricular fibrillation (VF) zone can be set between 120 and 250 beats per minute (bpm) and the ventricular tachycardia (VT) zone can be programmed from 120 bpm to the lower bound of the VF zone [6] . The clinician may also program the time from arrhythmia detection to therapy delivery from 60 to 180 s for the VT zone and 25 to 40 s for the VF zone [5] . The WCD is rechargeable and comes with two lithium-ion batteries. One battery is used at all times in the device, while the other may be charged in about 3.5 h using a proprietary charging station. Battery life is approximately 2 days, but even if the battery signals the patient that it is getting low, there is usually sufficient charge retained for 10 shocks of 150 J each. During an arrhythmic episode, the WCD will deliver up to five shocks. If the arrhythmia persists, the device detects again and repeats the cycle until the rhythm is converted or the battery is exhausted [5] . Once the WCD delivers therapy, it should be replaced.
doi:10.5772/intechopen.90663 fatcat:ubtaaqeq3vcm7hq5nlw6o3gq7a