Every automated external defibrillator (AED) fixed to a wall in a London Tube station, GP surgery or community centre is, in effect, a small diagnostic computer. It does not simply deliver a shock on command — it reads the heart first and only acts if the rhythm it finds is one electricity can actually fix. Understanding how does a defibrillator work matters not just for the bystander who might one day use one, but for anyone thinking about how London’s health infrastructure is meant to function between the moment of collapse and the arrival of paramedics.
The London Health Infrastructure Review exists to trace exactly this kind of gap — between what "Better Health for London" recommended a decade ago and what is actually installed, findable and working on the ground across the capital’s boroughs today. Public-access defibrillation, and the national registry now built to support it, sits squarely inside that brief.
What an AED Actually Does When You Attach the Pads
Once the electrode pads are placed on a casualty’s chest, they pick up the heart’s electrical activity and pass it to the AED’s internal computer. That computer analyses the rhythm against a stored pattern library before it ever considers a shock. This is the step that makes the device automatic rather than a blunt electrical tool: the machine, not the bystander, decides whether a shock is appropriate, and it will refuse to deliver one if the rhythm does not call for it. That built-in refusal is the reason AEDs are considered safe for members of the public to use with only minimal training — the voice prompts guide the user, but the underlying decision is made by the rhythm analysis, not by guesswork.
Shockable Rhythms vs Non-Shockable Rhythms
Sudden cardiac arrest is not one single event electrically — the heart can stop effectively pumping in several different ways, and only some of them respond to a shock. AEDs are built around this distinction:
| Rhythm | Shockable? | What is happening in the heart |
|---|---|---|
| Ventricular fibrillation (VF) | Yes | Electrical signals become chaotic; the ventricles quiver instead of contracting, so no blood is pumped |
| Pulseless ventricular tachycardia (VT) | Yes | The ventricles beat far too fast to fill and pump properly, so there is no effective pulse |
| Asystole | No | No organised electrical activity at all — there is nothing for a shock to reorganise |
| Pulseless electrical activity (PEA) | No | Electrical signals look organised but the heart muscle is not responding to them |
In both shockable rhythms, a shock momentarily stops all electrical activity at once, giving the heart’s natural pacemaker cells a chance to restart in a coordinated rhythm. In the non-shockable rhythms, that reset has nothing to work with, which is why the AED will advise "no shock" and prompt the rescuer to continue chest compressions instead. This is also why CPR and defibrillation are described as complementary rather than interchangeable: compressions keep oxygenated blood moving to the brain and heart muscle, but only defibrillation can correct the underlying electrical fault in a shockable rhythm.
Why the Minutes Between Collapse and Shock Are the Whole Story
The clinical case for placing AEDs where the public can reach them, rather than relying solely on an ambulance crew arriving with one, comes down to timing. Survival chances from sudden cardiac arrest fall sharply for every minute that passes without defibrillation, and prompt defibrillation in the first few minutes after collapse offers by far the best chance of survival. An ambulance response, however well run, is rarely fast enough on its own to catch that window in a densely populated city — which is precisely the argument for a device being close by, charged, and locatable by the people who find the casualty.
The Circuit: Turning Scattered Devices Into a Findable Network
A defibrillator that nobody can find in time is functionally the same as no defibrillator at all, and that was the practical problem The Circuit was built to solve. Launched by the British Heart Foundation in October 2019, The Circuit is a national registry that records the location and status of public-access AEDs and shares that information directly with NHS ambulance services, so a 999 call handler can direct a bystander to the nearest working device rather than relying on local knowledge. It was developed in partnership with St John Ambulance, Resuscitation Council UK, NHS England and the Association of Ambulance Chief Executives, and by 2022 every ambulance service in the UK had been connected to the database. Government backing followed the same year, with the Department of Health and Social Care allocating a £1 million fund to help increase the number of registered devices in communities, on the basis that ongoing upkeep would fall to whoever owned or hosted each unit.
What The Circuit changes is not the physics inside the AED — that rhythm-analysis logic is unchanged — but the odds that a bystander is sent to a device that actually exists, is charged, and is unlocked in time to matter. An AED that is registered, maintained and known to the ambulance service is a meaningfully different asset from one that is simply present in a cabinet somewhere; the review’s interest in this space is in whether that registration and upkeep discipline is actually being followed through at borough level, not just whether cabinets have been installed.
Why This Belongs in a Health Infrastructure Review, Not Just a First-Aid Guide
The 2014 "Better Health for London" report treated urgent and emergency care as a system to be planned, not a set of isolated interventions, and public-access defibrillation is a clean example of what that planning looks like in practice: a piece of clinical equipment, a device standard that decides for itself when to act, and a data layer — The Circuit — that connects the two to the ambulance service that ultimately takes over. Understanding how the device itself works is what allows that infrastructure question to be asked properly: not "is there a defibrillator nearby", but "is there a registered, working one that the ambulance service already knows about, close enough to matter in the first few minutes". That is the standard the London Health Infrastructure Review measures against as it tracks how London’s boroughs have implemented the recommendations a decade on.