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A wearable defibrillator, formally a wearable cardioverter defibrillator (WCD), is a prescription device worn outside the body that monitors the heart’s rhythm continuously and can deliver a treatment shock if it detects a life-threatening rapid rhythm. This page explains what the system is made of, how its alarm and shock sequence works, how the FDA classifies it, and how it differs from an AED and an implantable defibrillator. Who uses it: physicians prescribe it, patients wear it, and physicians review the recorded data.
What a wearable defibrillator is
The FDA describes the best-known WCD, the ZOLL LifeVest, as a device “worn by patients at risk of sudden cardiac arrest.” It monitors the heart through four dry ECG electrodes and, on detecting life-threatening ventricular tachycardia (VT) or ventricular fibrillation (VF), can deliver a shock (FDA Summary of Safety and Effectiveness Data, PMA P010030/S056). The NHLBI likens a WCD to a vest worn under clothing next to the skin.
In FDA’s product-code database the category is “Wearable Automated External Defibrillator” (product code MVK), defined as a device that “monitors and treats a patient for ventricular defibrillation” and is “intended to be worn in home or in hospital settings as prescribed and overseen by a physician.”
Parts of the system
The FDA summary for the LifeVest 4000 divides the system into wearable and non-wearable components.
| Component | Worn? | What the FDA summary says |
|---|---|---|
| Monitor | Yes (carried in a holster) | Analyzes the ECG and delivers a shock when required; holds diagnostic and shock circuitry; colour touchscreen with help screens for alarms and daily functions |
| Electrode belt | Yes | Four ECG monitoring electrodes plus three therapy electrodes that release a blue gel if a shock is needed |
| Garment | Yes | Holds the electrodes in position; worn under clothing, next to the skin |
| Battery pack | Yes | Rechargeable power source that attaches to the monitor; a second charged pack is supplied |
| Holster | Yes | Carries the monitor |
| Battery charger / modem | No | Recharges the spare pack; the modem or wireless gateway sends data by cellular or standard phone line |
| LifeVest Network | No | A secure manufacturer-maintained website that collects ECG recordings for physician review, with encrypted transmission and password access |
The FDA summary names models 3000, 3100 and 4000. The NLM AccessGUDID record for the LifeVest lists it as MR Unsafe, not single-use, not packaged sterile and not labeled as containing natural rubber latex, with GMDN term 48051 “Wearable defibrillation system.”
How the alarm and shock sequence works
- The monitor records the ECG continuously. Its detection algorithm uses a baseline vectorcardiogram as a morphology template together with heart rate.
- When it detects a treatable arrhythmia, an alarm sequence begins, “giving a conscious patient time to stop the treatment.”
- The monitor has two response buttons; while they are held, the alarms stop and no shock is given.
- If the alarms are not stopped by the wearer, verbal warnings alert bystanders, gel is released from the therapy electrodes and a shock is delivered.
- A treatment sequence can include up to five shocks.
The prescribing physician sets the VT and VF rate thresholds and the pulse energy; each shock is between 75 J and 150 J. The device records the ECG for every detected arrhythmia, and the wearer can record an ECG by pressing the response buttons. According to the NHLBI, WCDs do not provide pacing therapy.
FDA classification and PMA status
| Item | Value | Source |
|---|---|---|
| Device class | Class 3, flagged as life-sustaining/life-supporting | openFDA classification, product code MVK |
| Premarket approval | PMA P010030, applicant ZOLL Manufacturing Corporation; original approval December 18, 2001 | FDA SSED; openFDA PMA |
| Pediatric indication | Supplement S056 approved December 17, 2015, a labeling expansion covering pediatric use | FDA SSED |
| Review panel | Cardiovascular | openFDA PMA |
| Later changes | Many supplements, for example S171 (November 21, 2023, models 4000 and 5100) and S195 (July 16, 2025, a 30-Day Notice for an alternate supplier of an injection-molded monitor component) | openFDA PMA |
Other PMAs also exist under product code MVK (P200037 and P230022), so the category is not limited to a single approval.
Wearable defibrillator vs AED vs ICD
The NHLBI separates the three device families by who operates them and where they sit.
| Feature | Wearable (WCD) | AED | ICD |
|---|---|---|---|
| Location | Worn externally as a vest under clothing | Portable, kept for emergencies; not worn or implanted | Surgically placed in the body (transvenous ICDs sit in the chest with leads; subcutaneous ICDs sit just under the skin) |
| Who operates it | Automatic; the wearer can stop an alert if no shock is needed | Used on someone in cardiac arrest; anyone can use it | Implanted device; monitors from inside the body |
| Monitors continuously | Yes | No; used when needed | Yes |
| Pacing | No | No | Only transvenous ICDs and CRT-Ds; subcutaneous ICDs give shocks only |
For the AED and ICD families in more detail, see what a defibrillator is, the ZOLL AED Plus, defibrillator paddles, what an ECG machine is and what a pacemaker is.
Prescription device
The FDA summary describes the LifeVest as a prescription device: the prescribing physician sets the rate thresholds and pulse energy, and physicians review the ECG recordings sent through the manufacturer network. Product-code MVK describes the category as intended for use as prescribed and overseen by a physician. For anything about use, follow the manufacturer’s instructions for use and the prescriber’s guidance.
Common misconceptions
- It is not an AED. It monitors continuously and acts automatically on the wearer, whereas an AED is a portable device used on someone in cardiac arrest.
- It does not pace. Pacing is provided only by transvenous ICDs and CRT-Ds among the implanted devices, not by a WCD.
- An alarm does not always end in a shock. A conscious wearer can stop the sequence with the response buttons.
FAQ
Is a wearable defibrillator the same as an AED?
No. FDA classifies it under its own product code (MVK) as a wearable automated external defibrillator prescribed and overseen by a physician, and the NHLBI distinguishes it from the portable AED anyone can use on a person in cardiac arrest.
Does a wearable defibrillator pace the heart?
No. The NHLBI states that WCDs do not provide pacing therapy.
Can the wearer stop a shock?
Yes, before it is delivered. Holding the two response buttons stops the alarm sequence, which is designed to give a conscious wearer time to do so.
What is a wearable defibrillator made of?
A monitor, an electrode belt with four ECG and three therapy electrodes, a garment, a rechargeable battery pack and a holster, plus a charger/modem and a physician-facing data network.
How is it regulated?
As a Class 3 device through premarket approval; the original PMA (P010030) was approved on December 18, 2001.
Sources
- FDA, Summary of Safety and Effectiveness Data, PMA P010030/S056 (approval December 17, 2015). Checked November 1, 2026.
- openFDA, PMA database and device classification, product code MVK. Checked November 1, 2026.
- NLM, AccessGUDID record for the LifeVest Wearable Defibrillator. Checked November 1, 2026.
- NHLBI, How do defibrillators work? (updated June 6, 2023). Checked November 1, 2026.
