wearable cardioverter defibrillator wearable cardioverter defibrillator a wearable cardioverter defibrillator (wcd) is a non-invasive, external device for patients at risk of sudden cardiac arrest (sca). it allows physicians time to assess their patient's arrhythmic risk and make appropriate plans. it is a leased device. a summary of the device, its technology and indications was published in 2017 and reviewed by the ehra scientific documents committee. description a wearable cardioverter defibrillator (wcd) is an external device with a built-in defibrillator. the wcd is worn directly on the body by patients who are at transient risk for sudden cardiac death (scd) for short-term risk mitigation and it does not require surgery for use. a wcd is also a temporary therapeutic option in case an implantable cardioverter defibrillator (icd) cannot be implanted immediately. the wcd enables patients to continue their normal life without constantly worrying about their risk for scd. the wcd is a non-invasive medical device. it consists of a vest, an electrode belt and a monitor. the vest with the electrode belt is worn under the clothing during the entire day. the wcd should only be removed when taking a shower or bath. the monitor can either be worn around the waist or over a shoulder strap. the electrodes and wires, placed on the inner sides of the vest and electrode belt, lie directly on the skin. the monitor constantly records heart rate and rhythm. if life-threatening cardiac arrhythmias, such as ventricular tachycardia (vt) or ventricular fibrillation (vf) are detected, the defibrillator delivers one or more treatment shock(s) in order to restore a normal heart rhythm. since the time between a cardiac arrest and defibrillation is directly linked to survival, a treatment shock must be delivered within a few minutes after an event to be effective. with every passing minute without treatment, the chances of patient survival is reduced by 7-10%. from detecting a life-threatening cardiac arrhythmia to automatically delivering a treatment shock, the wcd usually needs less than a minute. the first treatment shock success rate for resuscitating patients from scd is 98%. intervention from bystanders or emergency personnel is not required for the wcd to work. the use of the wcd is recommended for the prevention of scd in the 2006 international joint guidelines from the american college of cardiology, american heart association, and european society of cardiology (acc/aha/esc), european society of cardiology (esc) guidelines from 2015 and 2021, and american heart association, american college of cardiology, heart rhythm society (aha/acc/hrs) guidelines from 2017. the international society for heart and lung transplantation (ishlt) recommends wearable external defibrillators as a bridge therapy for patients waiting for a heart transplant in their guidelines for the care of cardiac transplant candidates. in the united kingdom (uk) the wcd lifevest® from zoll is available for temporary use on a monthly rental basis since 2017. the wcd is a temporary therapeutic option for patients waiting for an icd, patients with an icd that had to be removed (e.g., due to infection), or patients who can't have an icd but are at transient risk for scd. the wcd allows physicians time to assess their patient's cardiac arrhythmic risk, make appropriate treatment plans and monitor cardiac output before or after an invasive cardiac procedure (such as bypass surgery, stent placement or heart transplant) or in patients at high risk for scd after myocardial infarction (mi). usual wearing time of a wcd is about 3 months but depends on the patient's needs and the prescription of the treating physician. wcd showing vest and power supply wcd in situ history the use of cardiac defibrillation started in 1947 - first in an open chest and ten years later through a closed chest with high energy levels. in 1972, cardiac defibrillation with intracardiac electrodes delivering much less energy of as low as 30 joules was established, following the development of portable units delivering high energy levels of up to 1000 volt. at johns hopkins university, doctors mirowski, mower and colleagues started developing implantable cardioverter defibrillators (icd), and were able to implant an icd in the first human by 1980. over the years, the icd was further improved and is now a standard outpatient procedure. there are limiting factors for direct prophylactic implantation of an icd. for example, a diagnosed high risk for scd may be temporary, which would oppose an implantation intended for lifetime use. per current guidelines (e.g., the esc guidelines from 2015 and 2021) a patient has to wait at least 40 to 90 days after the cardiac event (e.g., myocardial infarction or newly diagnosed heart failure with reduced left ventricular function) before the decision to implant an icd should be made. an external, wearable cardioverter-defibrillator with defibrillation features similar to an icd could be a solution to be used as “bridge” to protect these patients from scd. in 1986, m. stephen heilman and larry bowling founded lifecor and started the development of the first wearable cardioverter-defibrillator (wcd). it was named lifevest®. this wcd was extensively tested for three years in multi-centre and multinational clinical trials (wearit and biroad) in the united states and europe. the results were used to improve the wcd and were also the basis for fda approval in 2001 for use of the wcd for adult patients who are at high risk for scd, who are not suitable candidates for an icd or who refuse to have one. 14 years later (2015), fda approval was received for the use of the wcd in children, who are at high risk for scd and are not candidates for an icd or do not receive one due to lack of parental consent. in 2000, prior to the fda approvals, the wcd had already received the european ce-certification. in 1986, m. stephen heilman and larry bowling founded lifecor and along with a team of former intec employees who developed the first implantable cardioverter defibrillator (icd) began development of the wcd. the wcd was extensively tested for three years in 17 major medical centers across the united states and europe. the clinical data collected from those trials allowed lifecor to obtain fda approval for use of the wcd in the united states. in 2001, the fda approved the lifevest wearable cardioverter defibrillator (model 2000). the lifecor business was acquired by zoll medical corporation in 2006 and asahi kasei in 2012. as of 2015, the lifevest was available in the united states, europe, japan, australia, israel and singapore. zoll medical corporation, meanwhile an asahi kasei group company, acquired the lifecore business in 2006. the wcd lifevest® is marketed in the united kingdom, united states, europe, japan, and several other countries worldwide. according to zoll, the lifevest® has been prescribed to more than 200,000 patients worldwide. insurance coverage in the united states the wcd is covered by most health plans in the united states, including commercial, state, and federal plans as durable medical equipment (dme) for those patients at high risk of cardiac arrest, including: primary prevention including: after recent mi (coverage during the 40-day icd waiting period) before and immediately after cabg or ptca (coverage during the 90-day icd waiting period) listed for cardiac transplant recently diagnosed nicm (coverage during the three-to-nine month icd waiting period) new york heart association (nyha) class iv heart failure terminal disease with life expectancy of less than one year icd indications when patient condition delays or prohibits icd implantation icd explantation working mechanism of the wcd the wcd consists of a comfortable, lightweight washable vest, an electrode belt and a monitor. the battery-powered monitor can be worn over a shoulder strap or in a holster around the waist (fig. 1 and 2). the monitor collects the data from the electrocardiography (ecg) electrodes and performs the appropriate calculations according to the pre-planned settings, which are programmed by the treating physician. it also contains the capacitors for the defibrillation energy. the default setting is: 1) vt detection 150-199 beats/minute; 2) vf detection ≥200 beats/minute. an alarm module on the monitor alerts the patient with an audible, visual and tactile (vibration) escalation alarm sequence, in case of a life-threatening cardiac event (arrhythmia). in the electrode belt, four dry, non-adhesive ecg electrodes continuously monitor the patient's heart rhythm. three defibrillation electrodes are placed in the vest, one on the chest (approximately at the level of the apex of the heart) and two on the back (between the shoulder blades). the ecg electrodes are placed at inside of the vest on the chest providing two independent ecg leads. prior to delivering a therapeutic shock, the dry defibrillator electrodes automatically deploy conductive gel to protect the skin from possible injury from the treatment. the wcd can deliver up to five consecutive shocks per sequence. life-saving therapy typically occurs within a minute of the onset of an arrhythmia.the patient is warned when a treating sequence has been started e.g. by siren alarms and spoken information. by pressing two response buttons on the monitor simultaneously, an unjustified shock delivery can be prevented by the patient as long as she/he is conscious. if the patient fails to respond, e.g., because the patient has lost consciousness due to an arrhythmia, gel is automatically ejected from under the defibrillation electrodes. if the arrhythmia resolves on its own, no treatment shock is delivered. action from bystanders is not required, but they are warned by voice information not to touch the patient during defibrillation and to call the emergency doctor. the patient receives two batteries for the wcd. one will be used to operate the monitor, the other will be placed in the charging station for daily replacement. the charging station contains the electronic features for data transmission over a mobile phone network to a protected database from the manufacturer. conspicuous ecg sequences or treatments are automatically transmitted to a secure server. the treating physician can view and analyze them via password-protected access. before a wcd is handed to a patient, the wcd is fitted to the patient's waist for accurate ecg signal detection and the patient receives detailed training to ensure correct handling of the wcd. the efficacy and effectiveness of the wcd has been tested in clinical trials and several international post-marketing studies. if the wcd is worn correctly and ecg signal detection is optimal, the success rate of the first shock is approximately 98%. hence, the wcd is as effective as an icd in treating vt and vf. long term follow-up studies showed that approximately 90% of all patients treated with the wcd are still alive one year after the heart failure incident. since the wcd is a non-invasive garment, no injuries or scars remain after use and shock delivery. for effective protection, the wcd should be worn 24 hours a day and should only be removed for personal hygiene. comparison of the wcd to automated external defibrillator (aed) and implantable cardioverter defibrillator (icd) 1. automated external defibrillators (aed) are portable electronic devices designed to analyse the heart rhythm and inform the operator whether defibrillation is required. they are intended for persons of the general population with an unknown risk for heart failure and are usually available in public places and first responder ambulances. aeds are designed for use by laypersons and provide simple audio and visual instructions for the operator to follow. electrode pads, placed by an operator on the chest of the patient, are for monitoring and defibrillation. in contrast to the icd and wcd, an aed needs the immediate activity of a bystander in order to prevent the scd. 2. wcds are intended for patients with a known transient risk for scd and meant for temporary use as described above.the wcd is the ideal therapeutic option to prevent scd in patients until it is clear that a patient's heart issues are indeed permanent and long-term protection with an icd must be applied. 3. implantable cardioverter-defibrillators (icd) are electronic devices implanted in the chest with a lead to the right ventricle of the heart. they are intended for patients with permanent risk for scd. an icd is, like a wcd, designed to detect and terminate cardiac arrhythmias by emergency defibrillation. an extensive invasive surgery is necessary for implantation of the icd, which is associated with a number of risks and morbidity. therefore, the decision for an icd should be carefully taken. the wcd is the ideal therapeutic option to