The defibrillator

Lesson 2 sent someone off to find a defibrillator. This lesson is what happens when they come back. Most people have walked past one on a wall, in a station or a gym, and never opened the box. The machine inside can do one thing a pair of hands can't: for some cardiac arrests, a shock is the treatment, and nothing else a bystander has will do instead. It is also built to be used by someone who has never seen one, and it will not shock a person who doesn't need it. What stops people is not knowing that, so this lesson takes the box off the wall.

Where the machine fits

The American Heart Association draws the whole response to a cardiac arrest as a chain. Its 2025 version has six links, in order: "Recognition and Emergency Activation", "High-Quality CPR", "Defibrillation", "Advanced Resuscitation", "Post-Cardiac Arrest Care" and "Recovery and Survivorship".⁴

Drawn by this course from the link names in the AHA's figure, which is copyrighted; the shading is this course's, not the AHA's.⁴

Look at the shaded boxes. The first three are yours. Lessons 2 to 4 were the first two links; this lesson is the third, and it's the last one a bystander can deliver before professionals arrive.

Two details from the AHA's text are worth knowing. The 2025 chain is a single one, for adults and children, in and out of hospital.⁴ And the AHA says what it left out: "The missing link from the 2025 Chain of Survival is prevention and preparedness."⁴ It plans to add that link later. Knowing where your nearest AED is, which is this lesson's first exercise, is a piece of preparedness.

RCUK's 2025 summary speaks of a chain of survival too, but this course didn't read a figure of its own, so the diagram is the AHA's. The order of actions is the same in both countries. RCUK's 2025 public steps put the AED last among the actions: "As soon as an AED is available, attach it and follow the AED instructions."¹

What a shock can fix, and what it can't

An AED, an automated external defibrillator, is a box with two sticky pads on a cable. The ERC's 2025 guideline describes it: a portable device with pads that attach "to a patient’s chest to detect the heart rhythm following suspected cardiac arrest."² So the first thing it does is not shock. It reads.

Then it decides. "If the rhythm is a shockable rhythm (ventricular fibrillation or pulseless ventricular tachycardia)," the ERC goes on, the machine prompts the rescuer "to deliver a direct current electric shock (defibrillation) to re-establish a coordinated heart rhythm."² Ventricular fibrillation, VF for short, and pulseless ventricular tachycardia are the two rhythms a shock can treat. Tachycardia means a fast rhythm; pulseless means it is producing no pulse. On this course's reading of the ERC's phrase "re-establish a coordinated heart rhythm", what the two have in common is that the heart's electrical activity has lost its coordination, so the heart is not pumping. The AHA adds that the left ventricle, the main pumping chamber you met in lesson 3, is "where VF often originates".³ The guidelines this course read say no more about what happens inside the heart during VF, and this lesson won't add a description of its own.

For every other rhythm, the answer is no. "For other heart rhythms (including asystole and a normal rhythm), no shock is advised."² Asystole is what people call the flat line; the guidelines this course read describe it no further.

That split cuts both ways. (The 18.4% is the first rhythm recorded, and the report does not say when in each arrest that was, so read it as a rough share, not a precise one.) For the fifth of people in a shockable rhythm, the AED is the treatment, and nothing else a bystander carries can do its job. For the other four fifths, the AED will say no shock at its first check, and what keeps them going until the crew arrives is your compressions. So the AED is never a reason to stop pushing, and "no shock advised" is never good news on its own.

Why it can't shock the wrong person

This is the fear that stops people opening the box, and the machine's design answers it. The ERC's 2025 guide for the public: "The rescuer cannot deliver a shock in error: the AED will not discharge if it does not detect appropriate conditions."⁶ The ERC's guideline chapter adds that "AEDs are accurate in their interpretation of the heart rhythm and are safe and effective when used by laypeople."²

There are two kinds of machine, and they differ at exactly this point. "Some AEDs (fully automatic AEDs) will deliver a shock automatically, while others (semi-automatic AEDs) will require the rescuer to press the shock button to deliver the shock" (ERC 2025; RCUK 2025 uses the same words).²¹ Either way, the decision to shock is the machine's. On a semi-automatic one, your job is to press the button when it tells you to, and to make sure nobody is touching the person when you do.

And the risk to you is low. RCUK 2025: "The risk of harm to rescuers from accidental shock during AED use is low."¹

Why the minutes matter

A shock works best early. The AHA's 2025 guideline reports a study from the ARREST registry in Amsterdam, of people who had a witnessed arrest outside hospital with VF as the first rhythm: the first shock succeeded 93% of the time when it came within 6 minutes, and 75% of the time when it was delayed beyond 16 minutes. "Every minute of delay to first shock was associated with a 6% decreased probability of survival to discharge."³

The ERC's 2025 chapter gives two figures for people in a shockable rhythm: "each minute delay to defibrillation is associated with 6 % higher probability of failure to terminate VF, and 3–6 % lower probability of survival to discharge."² Terminate VF means stop it. So the two guidelines agree on the direction and roughly on the size.

You may meet a bigger number. The ERC's own 2025 guide for the public says "Each minute of delay in getting to defibrillation reduces the probability of survival by 10%."⁶ The guideline chapters, which set out the studies behind their figures, give 6% (AHA) and 3 to 6% (ERC), and those are the figures taught here. The guide's point, that minutes count, is the same; its number is the larger one.

Two cautions on reading these. Each is "associated with", from observational data, so it describes what was seen across many arrests, not a law of nature. And neither guideline says how to turn "6% a minute" into a figure for ten minutes, so don't multiply it out. The two numbers the AHA reports make the point without arithmetic: 93% of first shocks worked early, and even beyond 16 minutes three in four still did, so late is worse without being hopeless.

In practice the wait is long. In CARES's 2025 US figures, the median EMS response time, EMS being emergency medical services, the US term for the ambulance service, was 7.6 minutes.⁷ The summary does not define exactly which interval that measures, but a median means about half of arrests waited longer. That is the gap a public AED is there to fill.

So why not shock at once, before any CPR?

Because, the AHA's supporting text says, the heart's reserves may run down: "when VF/pVT persists for a longer duration, the heart’s energy reserves may become depleted, reducing defibrillation effectiveness unless preceded by a period of CPR to restore these reserves before rhythm analysis."³ The reserves, the AHA says, are "of oxygen and other energy substrates", and "If replenished by a period of CPR before shock, defibrillation success improves significantly."³ That is the reason the AHA's text gives for CPR coming first, and for continuing it while the AED is fetched and set up; RCUK gives the instruction without a reason.

But no longer than that. The AHA recommends CPR "until a defibrillator or AED is applied" (AHA 2025, COR 1).³ The ERC reports an ILCOR review of five randomised trials comparing a shorter with a longer spell of CPR before the first shock, which found "No clear benefit from additional CPR before defibrillation"; its instruction is blunt: "Defibrillation should not be delayed to provide additional CPR."² The AHA's text reaches the same place from the other side: studies of about 30 seconds of CPR against up to 3 minutes before the first rhythm check "show no difference in defibrillation outcomes".³

Put together: push until the pads are on, then let the machine analyse and shock without waiting.

Using one, prompt by prompt

The steps are nearly word for word the same in RCUK's 2025 guideline and the ERC's, and the AHA's recommendations point the same way. RCUK's order follows; the AHA's graded recommendations, which point the same way, come after it.¹²³

1. Use it as soon as it arrives. "Use an AED as soon as it is available" (RCUK 2025).
2. Switch it on. "Open the AED case (if present). Some AEDs automatically turn on when opened. If not, identify the power button and turn it on."
3. Do what it says. "Follow the audio/visual prompts from the AED." The machine talks you through the rest, by what the ERC calls "an audible (and sometimes visual) prompt" (ERC 2025).
4. Pads on bare skin. "Attach the electrode pads to the person’s bare chest according to the position shown on the AED (or AED pads)." More on where, and on clothes, below.
5. Keep pushing while the pads go on, if there are two of you: "If more than one rescuer is present, continue CPR while the pads are being attached."
6. Hands off while it analyses. "Ensure that nobody touches the person whilst the AED is analysing the heart rhythm."
7. Hands off for the shock. "If a shock is indicated, ensure that nobody is touching the person." On a semi-automatic AED, you then press the button.
8. Straight back to compressions, shock or no shock. "After the shock has been delivered, immediately restart chest compressions. If no shock is indicated, immediately restart chest compressions."
9. Leave the pads on and keep listening. "Usually, the AED will instruct the rescuer to perform CPR, then, after a set time interval, the AED will instruct the rescuer to pause CPR to undertake rhythm analysis."

The American Red Cross's public steps add two practical points. "If necessary, wipe the chest dry" before the pads go on, and say "CLEAR!" loudly before the analysis and before the shock, so that everyone takes their hands off.¹⁴ Its page also tells you to plug the pad cable into the AED "if necessary", so on some machines it is already connected.

In the US the AHA's 2025 recommendations say the same things in their own form: CPR until the AED is applied (COR 1), pauses before and after a shock "as short as possible" (COR 1), and "it is reasonable to immediately resume chest compressions after shock administration rather than pause CPR to perform a postshock rhythm check" (COR 2a, LOE B-R).³ Lesson 4 explained the grades.

Why compressions restart straight after a shock

Step 8 feels wrong the first time, because pushing on a heart that a shock has just restarted seems like the last thing to do. The AHA answers in one sentence: "Defibrillation success refers to termination of VF/pVT, although this may not result in a perfusing rhythm."³ A perfusing rhythm is one that actually moves blood. So a shock can stop the VF and still leave a heart that is not pumping, and you can't tell which from the outside.

The evidence behind the instruction is the AHA's too. "Two RCTs enrolling more than 1000 patients did not find any increase in survival when pausing CPR to analyze rhythm after defibrillation," and "Observational studies showed decreased ROSC when chest compressions are not resumed immediately after shock."³ ROSC, from lesson 3, is the heart restarting effectively on its own. So you don't stop to check. The AED will check for you, on its own schedule.

That schedule also answers a question left from lesson 3. With two rescuers, the AED's analysis is a natural moment to swap compressors. The AHA says so: rotating "every 2 minutes is sensible because this approach maintains chest compression quality and takes advantage of when CPR would ordinarily be paused for rhythm analysis."³ The AHA wrote that sentence in its section on pauses generally, not only about lay rescuers, but the logic carries: when the machine says stand clear, the fresh pair of hands gets ready.

Where the pads go

Follow the pictures. The ERC: ILCOR "has issued a good practice statement that recommends following the manufacturer’s AED guidance and instructions for adult pad placement", and the ERC endorses it.² The AED or its pads show the position, and its "audio/visual prompts" are there to be followed (RCUK 2025).¹

For most machines that means the front-and-side position the ERC describes. "One defibrillation pad should be positioned below the patient’s right clavicle, just to the right of the upper sternal border. The other defibrillation pad should be centred on the patient’s left mid-axillary line, below the armpit."² In plain words: one pad just under the person's right collarbone, beside the top of the breastbone; the other on their left side, below the armpit, in line with the middle of the armpit. The AHA's text warns that the side pad should be "placed in the midaxilla and not too anteriorly",³ which means not creeping round onto the front of the chest. The ERC adds, "Pad placement should avoid breast tissue if possible."²

There is a second position, one pad on the front of the chest and one on the back. The ERC keeps it for when the first is not feasible, "if trained".² The AHA grades both positions as reasonable (COR 2a, LOE C-LD).³ For an adult, the front-and-side position is the one to learn, and the diagram on the pad is the one to follow. Children and small bodies are different, and lesson 6 covers them.

Clothes over the chest have to go, or at least move, because the pads need skin. Lesson 3 gave you the BHF's distinction: thin clothing can stay on for compressions, but not for the pads.¹⁷ If the chest is wet, wipe it dry (American Red Cross).¹⁴ None of the guidelines read for this lesson says anything about chest hair, jewellery or medication patches, so this lesson does not give rules for them; the machine's own prompts and the call handler are where that advice will come from.

Bras, and a gap in who gets the pads

Women who arrest in public are less likely than men to have an AED used on them. In CARES's 2025 figures, a bystander used an AED on 11.7% of women and 13.4% of men who arrested in a public place.⁷ The same kind of gap appears in who gets CPR, which lesson 4 set out.

The guidelines have a view on why. The AHA: "The need to apply pads or paddles directly to the bare chest may be a contributing factor to the observations that females experience significantly lower rates of public access defibrillation compared to males."³ The ERC says the gap "may stem from apprehension about exposing and touching a woman’s chest and fears of being accused of sexual assault."² Both are careful: "may be", "may stem". This is the guidelines' explanation for a measured gap, not a measured cause.

What they tell you to do is plainer. In the UK:

"Rescuers should prioritise correct pad placement and contact with bare skin. If this can be quickly achieved without removing the bra, then it is acceptable to keep the bra in place. If the bra interferes with correctly locating the pad position, then the bra should be removed." (RCUK 2025)¹

ILCOR, whose reviews all these guidelines draw on, says: "There is insufficient evidence to guide the routine removal of a bra, but it may not always be necessary to remove a bra for defibrillation. Pads must be placed on bare skin in the correct position, which may be possible by adjusting the bra's position rather than removing it (good practice statement)."⁵ The ERC's chapter says the same, with one difference in wording: where the bra is in the way, it "should be displaced"; the ERC's contrast throughout is with "unfastening or cutting" the bra.² The AHA's text, in the US, describes adjusting rather than removing as an option that "could mitigate factors such as discomfort with exposing a woman’s chest or fear of accusations of inappropriate touching or sexual assault."³

The difference between "removed" and "displaced" is small, and neither side is wrong: both want the pads on skin in the right place, as fast as possible. The underwire is the other worry. The AHA reports a study in pigs, published as a conference abstract, with "a 100% first-shock success rate even when self-adhering AED pads were applied directly over the metal underwire of a bra", and "No adverse events such as arcing, current redirection, or burning of the bra or the pig’s skin".³ The ERC says there is "no evidence that an underwire in a bra causes harm".² That's one animal study, so it's reassurance, not proof. It is enough that nobody should delay a shock to deal with wire.

And the ERC's closing line on the subject: rescuers "should prioritise life-saving interventions over concerns for modesty."²

Where the AEDs are

An AED you can't find in time does nothing, so the guidelines care as much about where they are as about how to use them.

Public access programmes. The AHA recommends that public access defibrillation programmes "be implemented in communities at high risk" of out-of-hospital arrest (AHA 2025, COR 1, LOE B-NR). The evidence it leads with is the PAD Trial, in which training lay volunteers in CPR and AED use "led to a doubling of OHCA survival compared with CPR training alone (30 survivors in 107 arrests versus 15 survivors in 128 arrests".⁴ It also says how far that has got: "Despite widespread implementation of AEDs in public spaces, use of public access defibrillators by lay rescuers remains low."⁴ In CARES's 2025 figures, a bystander applied an AED in 13.9% of arrests in public places.⁷ The ERC notes where lay rescuers have done best: "The highest survival rates following lay rescuer resuscitation have been reported after use of on-site AEDs such as at airports, casinos, sports facilities or train stations."²

Lesson 4's chart ended on its biggest bar. In CARES's 2025 figures, when a bystander used an AED, 32.9% of people survived to leave hospital, and that happened in only 2.5% of arrests (both figures leave out nursing homes and healthcare buildings).⁷ Read both with care. The 2.5% is the AHA's "remains low" in numbers. The 32.9% is not the AED's effect on its own: the report gives the raw figure with no adjustment, and since a bystander AED was used in 13.9% of arrests in public places against 2.5% overall, those arrests were weighted towards public places, where survival was 20.9% whatever happened.⁷ That reasoning is this course's, from the report's own numbers. What the registry shows is what went with a bystander using an AED, not how much the AED itself added. The ARREST figures above, from people who all had VF, are the closer look at what the timing of the shock does.

Signs. RCUK's 2025 guideline asks for clear signage, and says "Signage should state that AEDs can be used by anyone and that no training is needed."¹ The photograph below is what to look for: a sign with a heart and a lightning bolt, and a cabinet under it.

In the UK: The Circuit

The UK has a national register, The Circuit, led by the British Heart Foundation with RCUK and others. RCUK calls it "the national defibrillator network", and says that if an AED isn't registered, "ambulance service call handlers won't know it exists when that 999 call is made".⁸ "Over 110,000 defibrillators are registered on The Circuit," on RCUK's page in September 2026.⁸ RCUK's 2025 guideline: "The ambulance service should be able to direct callers to the nearest accessible AED during the 999 call."¹

Lesson 2 told you not to search a website in an emergency, and this is why. The public map, defibfinder.uk, is for finding AEDs in advance, and it says it "is not intended for use in an emergency."⁸ In an emergency the call handler has the same map and more.

In the US: no single register

The US has no national equivalent in the sources this course read. What it has is local systems. PulsePoint is an app that has to be connected to the local dispatch centre, so it exists community by community. People "trained in CPR and willing to assist" can be "notified if someone nearby is in cardiac arrest", and the app "directs these rescuers to the exact location of nearby AEDs".⁹ Members of the public who sign up are "only notified of nearby cardiac arrest events occurring in public places"; responders registered through an agency also get calls to homes.⁹ Whether your area has it depends on your dispatch centre. The 911 call-taker is still the first person to ask.

Volunteer apps

Both countries now use phone alerts to send trained volunteers to an arrest nearby, often with an AED. ILCOR's 2020 recommendation, kept unchanged in its 2025 review, is that willing citizens near a suspected arrest "should be notified" by app or text alert, a strong recommendation resting on very low certainty evidence.¹¹ In a trial by Ringh and colleagues, published in 2015, there was bystander CPR in 62% of arrests when trained volunteers were dispatched by phone, against 48% without.¹⁰ The ERC also reports a study in which survival from arrests at home rose from 26% to 39% after such a system was introduced.² But the AHA's 2025 review is candid: bystander CPR rose whenever an alert was accepted, with "inconsistent data on the incidence of ROSC and survival."⁴ The AHA grades these systems as reasonable (COR 2a, LOE B-NR).⁴ So they get more people doing CPR sooner, and whether that reliably turns into more survivors is not settled yet.

In the UK, RCUK's free Lifesaver course ends by offering to sign you up as a GoodSAM volunteer responder, one of these systems.¹⁶

Locked cabinets

Some public AEDs are kept in locked cabinets. The guidelines advise against it. ILCOR: "We advise against using locked cabinets for public access defibrillator storage (good practice statement)."⁵ RCUK 2025: "AED cabinets should be unlocked and readily accessible 24 hours a day, 7 days a week, 365 days per year."¹ The ERC gives the reasons: a low reported risk of theft, missing AEDs or vandalism, under 2%, and a finding among first responders that "half of all injuries sustained while accessing an AED were incurred when attempting to break glass".² If you meet a locked one, tell the call handler. Nothing read here says how cabinet codes are given out, but the ERC does say where the injuries came from, so don't put your fist through the glass.

The law, for AEDs

This is general information, not legal advice. The law differs by state and by country.

In the US, lesson 1 gave you the state Good Samaritan laws. On top of them sits a federal law, the Cardiac Arrest Survival Act of 2000. Its core sentence: "any person who uses or attempts to use an automated external defibrillator device on a victim of a perceived medical emergency is immune from civil liability for any harm resulting from the use or attempted use of such device".¹² The Act defines "perceived medical emergency" itself: circumstances in which someone's behaviour "leads a reasonable person to believe" that they have a life-threatening condition needing an immediate response "regarding the heart or other cardiopulmonary functioning".¹² So the test is what a reasonable person would have made of the scene at the time. The protection does not cover "willful or criminal misconduct, gross negligence, reckless misconduct", among other exceptions.¹² And it is a floor, not a ceiling: it applies "only to the extent that the State has no statute or regulations" giving that immunity.¹² Where your state has its own AED law, that law governs; where it has none, the federal one fills the gap. It requires no training of the person who uses the AED. The only training it mentions is a condition on the immunity of whoever acquired the device, who can lose it for failing to give "appropriate training in the use of the device to an employee or agent" who then used it; that is the owner's concern, not the rescuer's.¹²

In the UK, there's no AED statute in the sources read here. RCUK's booklet on the law, which lesson 1 used, covers AEDs alongside CPR, and gives as its example of acting reasonably that "it would be reasonable to carry out CPR and use an AED if no healthcare professionals were available."¹³ RCUK's 2025 guideline puts the practical point in one sentence: "Anyone can use an Automated External Defibrillator (AED)."¹

Reception to the fourth floor

Now the whole thing in one case, with two rescuers and a machine.

It's mid-morning in an office in Bristol. A man in his sixties stands up from his desk, sways and goes down. You and a colleague, Priya, reach him first. He does not respond when you shout and tap his shoulders. Priya calls 999 on speaker and puts the phone by his head; the call handler asks about his breathing, you describe slow, noisy gasps, and you're told to start compressions (RCUK 2025).¹ The call handler asks if there's a defibrillator. There is one in reception, four floors down, and a third colleague runs for it.

What happens next, step by step. Where a step opens with a spoken prompt, the words are this course's paraphrase of what a machine says, not any one model's wording.

1. Minute three: it arrives. You keep pushing. Priya opens the case; this model turns on as the lid opens and starts talking. (RCUK 2025: switch on, follow the prompts.)
2. Remove clothing from the chest. Priya unbuttons his shirt and pulls it open, and his vest (his undershirt) is pulled up and clear of his left side. You haven't stopped. (RCUK 2025: CPR continues while the pads go on.)
3. Attach pads as shown. She peels the first pad and puts it under his right collarbone, beside the breastbone, then the second on his left side, below the armpit, matching the drawing on each pad. (ERC 2025; the manufacturer's diagram.)
4. Analysing, don't touch the patient. You take your hands off and sit back. Priya says "Clear!" and checks nobody is touching him, including the colleague holding his hand. (RCUK 2025; American Red Cross.)
5. Shock advised, stand clear, press the button. It's semi-automatic. Priya looks along his body, says "Clear!" again, and presses. (RCUK 2025; ERC 2025.)
6. Start CPR. You're back on his chest within seconds, with no pause to look at his face or feel for anything. (RCUK 2025; AHA 2025, COR 2a.)
7. Analysing, a couple of minutes later. Hands off again. Priya, who has been resting, gets ready to take over as soon as it finishes. This time the machine advises no shock and tells you to start CPR. She starts. (RCUK 2025; the swap at the analysis is the AHA's reasoning, above.)
8. The paramedics arrive and take over. Until then you do what the machine says and keep swapping. (RCUK 2025: continue to follow the AED's instructions.)

Notice what nobody did. Nobody stopped CPR to fetch the machine, nobody stopped it for more than a few seconds, and nobody tried to work out his rhythm. The machine did the part that needs a machine.

The gym

This one is harder, because the hesitation is social, not technical.

It's early evening at a gym in Minneapolis. A woman in her thirties steps off a rowing machine, says she feels strange, and collapses. She does not respond. Her breathing is fast and shallow for a few seconds, then comes in slow gasps. A trainer calls 911 on speaker and starts compressions. A member brings the AED from the wall by the door and kneels beside her. She's wearing a close-fitting sports top with a sports bra under it. The member stops, the pads in her hand, and says, "I can't just strip her in front of everyone."

Now check it against the sources. The US guidance for the AED's use is the AHA's, and its text describes adjusting rather than removing a bra as a way to reduce "discomfort with exposing a woman’s chest",³ with ILCOR's good practice statement behind it: pads on bare skin in the correct position, "which may be possible by adjusting the bra's position rather than removing it".⁵ A sports bra is tight and wide, and the pad under the left armpit needs skin, so pushing the bra up or aside may not be enough. If it is not, it comes off, or is pulled out of the way however is fastest. The ERC's line applies here as much as anywhere: "prioritise life-saving interventions over concerns for modesty."²

The member's worry is the one the AHA and the ERC name, and the answer to it is the gap in the numbers: fewer women get a public AED, and both guidelines suggest this worry may be part of why.³²⁷ So the kindest thing for her dignity is to do it quickly and well. What this course suggests on top, none of it from a guideline and none of it worth a second's delay: the trainer asks the room to turn away, someone stands between her and the doorway, and when the crew takes over, a jacket goes over her.

The pads go on. The AED analyses, advises no shock and says to start CPR. The member looks at the trainer: does that mean she's all right?

It doesn't. She is still unresponsive and not breathing normally. The instruction is to restart compressions immediately and follow the AED (ERC 2025; RCUK 2025; American Red Cross),²¹¹⁴ and the AHA recommends keeping the pauses before and after a shock "as short as possible" (AHA 2025, COR 1).³ The pads stay on, because after a set interval the AED "will instruct the rescuer to pause CPR to undertake rhythm analysis" again (RCUK 2025).¹ The trainer and the member swap at each analysis until the paramedics arrive.

Children and drowning, briefly

Two situations change the order, and lesson 6 teaches both.

Children. Some AEDs have a child setting or child pads. The AHA says to use them under 8 years; RCUK says under 25 kg, which it puts at about 8 years, and lesson 6 sets the two side by side. Where there's nothing for children, both say to use the adult AED: RCUK, "If the AED does not have instructions for children, use it in standard adult mode" (RCUK 2025), and the AHA, that an adult AED "may be considered" (AHA 2025, COR 2b).¹⁸²⁰ Lesson 6 has the rest.

Drowning. The AHA says an arrest after drowning "is most often due to a hypoxic mechanism", lack of oxygen,²¹ and the ERC says a shockable rhythm is rare for the same kind of reason: "In drowning, an initial shockable rhythm is present in less than 10 % of cases because of the cardiac response to hypoxia" (ERC 2025).¹⁹ The AHA recommends CPR with breaths before the AED for a drowned person (AHA 2025, COR 1).²¹ Lesson 6 explains why.

What people get wrong

"An AED can restart a flat line." It advises no shock for asystole or a normal rhythm; it treats VF and pulseless VT (ERC 2025). A flat line needs CPR.²

"You need training to use one." "Anyone can use an Automated External Defibrillator (AED)" (RCUK 2025), and signs should say no training is needed.¹

"You could shock someone who doesn't need it." The machine decides, and "will not discharge if it does not detect appropriate conditions" (ERC 2025 guide for the public).⁶

"Stop CPR while someone fetches it." A helper fetches it; you keep pushing until the pads are on (AHA 2025, COR 1; RCUK 2025).³¹ Alone, the ERC says CPR "should not be delayed whilst locating and retrieving an AED"; follow the call handler.²

"Do a couple of minutes of CPR before the first shock." Once the pads are on, the shock shouldn't wait (ERC 2025).²

"No shock advised means they're fine." It means restart compressions (RCUK 2025).¹

"Cut or remove the bra every time." Pads on bare skin in the right place; move or remove the bra only if it's in the way (ILCOR 2025; RCUK 2025; ERC 2025).⁵¹²

"Each minute costs 10%, so after ten minutes there's nothing left." The link between delay and survival is real, and the guideline chapters put it at 6% a minute (AHA 2025) and 3 to 6% (ERC 2025).³² Neither multiplies it out, and in the registry study the AHA reports, three in four first shocks still worked beyond 16 minutes.³

"Look up the nearest one online." In the UK the call handler knows where registered AEDs are, and the public map isn't for emergencies (RCUK).⁸¹

Practice

For a feel of the real thing, the BHF's free RevivR course takes about fifteen minutes with a cushion and your phone, and includes using an AED; RCUK's Lifesaver is another way in.¹⁶ Neither replaces a class where you place training pads on a manikin, and the ERC says "CPR training should incorporate pad placement in people wearing bras."²

Connections

Lesson 2 sent a helper for the defibrillator and told you to ask the call handler, not a website, where it was. Lesson 3 gave you the compressions that keep the heart muscle supplied while it comes, and the two-minute swap that the AED's analysis now times for you. Lesson 4 gave you the grades in the parentheses and the CARES registry, whose figures this lesson used again for who gets an AED. Lesson 1 gave you state Good Samaritan law; this lesson added the federal floor for AEDs.

Lesson 6 takes children's pads, child modes and drowning, where the order changes. Lesson 16 comes back to how often to refresh all of this.

From earlier on the Core, Strength and Fitness told you to stop exercising when something felt wrong; the woman at the gym said she felt strange a moment before she fell, and this lesson is what a bystander does next. And Mental Fitness taught you to ask what the comparison group got. The trials behind "don't delay the shock for extra CPR" compared a short spell of CPR with a longer one, not with none, which is why they tell you nothing against pushing while the AED is on its way.

Go deeper

• RCUK, 2025 Adult basic life support guidelines, the sections "Using an Automated External Defibrillator (AED)" and "Where to place AEDs". Free, read here in full: the whole of this lesson's UK instruction in about twenty short sentences.
• ERC, European Resuscitation Council Guidelines 2025 Adult Basic Life Support, free on the publisher's site and read here in full. The sections on AEDs, pad position and bras set out the evidence behind each instruction, including how little of it there is.
• The Circuit, RCUK's page on the UK's national register, read here in full. If you look after an AED at work, a club or a village hall, this is where to register it so a 999 call handler can send people to it.
• RCUK and the American Red Cross each publish a short official AED film. This course has not checked either against the 2025 guidance, so neither is embedded here.

Sources

1. Resuscitation Council UK, 2025 Resuscitation Guidelines: Adult basic life support, 27 October 2025 . Read level: full, direct, re-read 2026-09-24. The AED steps, bras, signage, cabinets, rescuer safety, the child-mode line and "Anyone can use" an AED.
2. M. A. Smyth, S. van Goor, C. M. Hansen and colleagues, "European Resuscitation Council Guidelines 2025 Adult Basic Life Support", Resuscitation 215 (suppl 1), 2025, 110771. **Read level: full, direct, on the publisher's page**; every quotation here checked word for word on resuscitationjournal.com on 2026-09-24. What an AED does, the minutes, CPR before the shock, pad positions, bras, cabinets, volunteer responders.
3. M. E. Kleinman, J. E. Buick, N. Huber and colleagues, "Part 7: Adult Basic Life Support: 2025 American Heart Association Guidelines for CPR and Emergency Cardiovascular Care", Circulation 152 (suppl 2), 2025 . Read level: full, from the Internet Archive capture 20251026225858; sections 7.2, 9.1 and 9.2 re-read 2026-09-24, and the supportive-text quotations here checked on ahajournals.org the same day. The recommendation grades are from the capture's tables. Read at https://cpr.heart.org/en/resuscitation-science/cpr-and-ecc-guidelines/adult-basic-life-support through that capture.
4. C. Dezfulian, J. G. Cabañas, J. R. Buckley and colleagues, "Part 4: Systems of Care: 2025 AHA Guidelines", Circulation 152 (suppl 2), 2025 . Read level: partial, from the Internet Archive capture 20260208194135. Figure 2 (the six links) read on ahajournals.org; the chain, public access and mobile-technology supportive text checked there on 2026-09-24; the two recommendation grades from the cpr.heart.org copy, because the published tables are images.
5. J. E. Bray, M. A. Smyth, G. D. Perkins and colleagues for the ILCOR BLS Task Force, "Basic Life Support: 2025 International Consensus on CPR and ECC Science With Treatment Recommendations", Circulation, 2025 . Read level: full, from the accepted manuscript; the two quotations here checked word for word against the typeset text. Bras and locked cabinets.
6. European Resuscitation Council, The ERC Guidelines 2025 on Resuscitation for Everyone, 2025 . Read level: full, direct for the adult pages, the AED pages re-read 2026-09-24.
7. CARES, 2025 Non-Traumatic National Survival Report and 2025 Metrics Summary, April 2026. **Read level: full, direct.** First rhythm, survival by rhythm, public AED use by sex, median EMS response time, public bystander AED rate.
8. Resuscitation Council UK, "The Circuit". **Read level: full, direct.**
9. PulsePoint, "PulsePoint Respond" . **Read level: full, direct.** Coverage was not researched.
10. M. Ringh and colleagues, "Mobile-phone dispatch of laypersons for CPR in out-of-hospital cardiac arrest", New England Journal of Medicine 372, 2015, pp. 2316 to 2325 . Read level: abstract only.
11. R. Greif and colleagues, "Education, Implementation, and Teams: 2025 ILCOR CoSTR", Circulation 152 (suppl 1), 2025 . Read level: Internet Archive capture 20260430180745, recommendation sections. Notifying citizen responders.
12. Cardiac Arrest Survival Act, 42 U.S.C. §238q. **Read level: full, direct.**
13. Resuscitation Council UK, Cardiopulmonary resuscitation, automated defibrillators and the law, April 2018. **Read level: full, direct (pp. 1 to 13 closely).**
14. American Red Cross, "AED Steps", undated . Read level: full, direct, 2026-09-24.
15. Infrogmation, "AED - Automated external defibrillator in compartment with sign above", 2025, via Wikimedia Commons, CC BY-SA 4.0 . Licence checked and image viewed 2026-09-24.
16. British Heart Foundation, RevivR, and Resuscitation Council UK, Lifesaver. **Read level: both pages full, direct**; neither course was taken.
17. British Heart Foundation, How to do CPR, undated . Read level: full, direct.
18. Resuscitation Council UK, 2025 Resuscitation Guidelines: Paediatric basic life support, 27 October 2025 . Read level: full, direct for the AED section. The 25 kg cut-off, its "about 8 years", and the adult-mode line.
19. C. Lott, V. Karageorgos, C. Abelairas-Gomez and colleagues, "European Resuscitation Council Guidelines 2025 Special Circumstances in Resuscitation", Resuscitation 215 (suppl 1), 2025, 110753 . Read level: partial (drowning, opioid and hypothermia passages), direct, on the publisher's page. The drowning rhythm sentence, checked whole on the publisher's page on 2026-09-24.
20. B. L. Joyner, M. Dewan, A. Bavare and colleagues, "Part 6: Pediatric Basic Life Support: 2025 American Heart Association and American Academy of Pediatrics Guidelines", Circulation 152 (suppl 2), 2025 . Read level: full, direct, from the Internet Archive capture 20251026203140. Only the child AED recommendations are used here: the under-8 cut-off (COR 1) and the adult AED "may be considered" (COR 2b).
21. D. Cao, A. M. Arens, S. L. Chow and colleagues, "Part 10: Adult and Pediatric Special Circumstances of Resuscitation: 2025 American Heart Association Guidelines", Circulation 152 (suppl 2), 2025 . Read level: partial (the drowning sections among others), from the Internet Archive capture 20251027153841. The hypoxic-mechanism sentence and the CPR-with-breaths-before-AED recommendation (COR 1).