An AED matters in cardiac arrest because the heart's electrical rhythm has to be shocked back into a workable pattern before oxygen-starved brain cells die, and that window is measured in minutes. Ventricular fibrillation and pulseless ventricular tachycardia respond to defibrillation; asystole and normal-rhythm cardiac arrest do not. The machine's value is speed: use it as soon as it arrives, without pausing compressions longer than the analysis requires.
The American Heart Association's chain of survival for out-of-hospital cardiac arrest lists four links: call emergency services, start CPR, use an AED, and get advanced care. The third link exists because CPR alone circulates blood but rarely converts a shockable rhythm. According to AHA guidance, survival with good neurological outcome falls by roughly 7 to 10 percent for every minute defibrillation is delayed β a figure that has held in the association's guidelines for years and drives every public-access defibrillation program in the world.
This article treats the question as a practical one. It covers what the AED actually does inside the chest, how to run one on an adult, child, pregnancy case, or wet chest, what to do when the pads don't fit or the battery is dead, and why the machine's calm voice sometimes says "no shock advised" even when the person is dying.
What the Shock Actually Corrects Inside a Fibrillating Heart
During ventricular fibrillation, the heart's lower chambers quiver at 300 or more electrical impulses per minute instead of contracting in a coordinated squeeze. No blood moves. The AED's job is to deliver a controlled current across the heart muscle that stuns every electrical cell at once, so the heart's natural pacemaker β the sinoatrial node β can resume command. Defibrillation is not a jump-start. It's a reset.
That distinction explains why the shock only works on two rhythms. Ventricular fibrillation and pulseless ventricular tachycardia are chaotic or over-fast electrical patterns that a shock can terminate. Asystole is a flat line and pulseless electrical activity is organized electricity without a pulse; neither responds to defibrillation, which is why the AED analyzes first and instructs the rescuer to resume CPR rather than discharge.
Brain tissue tolerates about four to six minutes without oxygen before irreversible injury begins, according to the AHA's basic life support materials. CPR buys time by pushing oxygenated blood to the brain at roughly 30 percent of normal cardiac output. An AED buys the conversion. Neither works well without the other, and the machine's prompts are designed to keep you alternating between the two.
Public-access programs quantify the gap. The AHA's Heart and Stroke Statistics have reported that bystander AED use in out-of-hospital cardiac arrest ranges widely by community β high in places with aggressive public programs, low in most others β and that survival in shockable rhythms can approach 50 percent or higher when a bystander shocks before EMS arrival in monitored settings. The variance is the point: the machine only helps if someone reaches for it.
Six Steps in Real Order From Pads On to First Shock
The sequence below follows the order the machine will actually ask you to do things. Movement order matters because time on the chest is the scarce resource. Every pause for analysis or shock should ideally stay under 10 seconds; the AHA's 2020 guidelines emphasize minimizing interruptions in chest compressions.
1: Confirm the person is unresponsive and not breathing normally, then call emergency services on speakerphone and ask someone specific to fetch the AED β "you in the blue jacket, bring the AED" beats a general shout. 2: Bare the chest, dry it if wet, and remove transdermal medication patches or a nitroglycerin patch from the pad sites; do not remove an implanted pacemaker or ICD, but keep the pads at least 8 inches (about 20 cm) away from the device. 3: Turn the AED on and follow its voice prompts; if it has adult and pediatric modes, use adult pads and adult mode for anyone who has reached puberty. 4: Attach the pads to the bare chest as pictured on the package β usually one below the right collarbone and one at the left mid-axillary line β and press them down firmly so the gel adhesive contacts skin. 5: Stop compressions when the machine says "analyzing" and clear everyone off the patient β do not touch the body, and ask bystanders out loud whether they are ready to leave the chest alone while you check the area. 6: If the machine advises a shock, announce "clear," verify nobody is in contact with the patient, press the shock button if it's semi-automatic, and resume compressions immediately when it tells you the shock has been delivered.
Failure handling starts with the pads. If the machine repeats "check electrode pads," the pads are loose, misplaced, or too dry β press them down, smooth wrinkles, and replace them if the gel is contaminated. If the voice says "motion detected," someone is touching the patient or the person is being moved; stop and let the analysis complete. If the machine says "no shock advised," do not remove it β leave the pads in place, resume CPR, and let the machine reanalyze every two minutes when prompted. The AHA advises leaving the device attached because the rhythm can deteriorate into a shockable one later.
CPR Interruptions, Hand-Off Pauses, and the Seven Percent Rule
Every minute of delay costs survival. A widely cited figure, drawn from AHA guidance and repeated in public-access defibrillation literature, puts the decline at roughly 7 to 10 percent per minute of defibrillation delay in ventricular fibrillation. That's not an "8 percent" claim rounded loosely β the range reflects different studies and patient groups, and the AHA's own program materials have used a similar order of magnitude for decades.
Work the arithmetic backward to see what the pause cost looks like. If a bystander waits 4 minutes to attach pads instead of 2, the person has lost somewhere between 14 and 20 percentage points of survival probability under the 7β10 percent rule. That is the justification for "AED first, CPR immediately, then blame nobody" β the timeline is short enough that seconds count, not minutes.
Compressions and analysis have to alternate, and the AED tells you when. After each shock the machine charges while you're still doing compressions, then prompts you to pause; the pause should be two seconds, not five. Some models count the compression rate audibly and reanalyze automatically every two minutes. Don't fight the voice β it's calibrated to the AHA's two-minute cycle.
Pausing compressions to let the machine analyze is necessary. Pausing to explain the situation to a crowd is not. The person who arrives at minute six with an AED and hands it to a rescuer who is already in a good compression rhythm should probably swap in and use the device while the compressor steps to the side and rests β the swap happens with the chest covered, not with the machine waiting.
Adult Pads, Pediatric Mode, Pregnancy, and a Wet Chest: Where the Machine Stops Being Universal
The AED is engineered for an adult chest, and it adapts only partially. The AHA recommends adult pads and adult mode for anyone past puberty. Pediatric pads, which attenuate the energy dose, exist because a full adult shock in a child can cause myocardial injury β but if the person is a child in cardiac arrest and only adult pads are available, use them; a shock with standard pads is better than no shock, per AHA guidance. Most AED models sold in the U.S. ship with adult pads and require a separately purchased pediatric cartridge.
Pregnancy and AED use is a case where the machine is not the issue, the position is. The 2020 AHA guidelines say to defibrillate a pregnant patient using standard energy levels and pad placement; if the uterus is visibly enlarged, a slight leftward tilt of the torso to relieve aortocaval compression may help, but do not delay a shock to reposition. A shock does not endanger a fetus β the current crosses the chest, and an untreated arrest is far more dangerous than the discharge.
A wet chest is the most common real-world failure I've seen described in AED training. If the person has been pulled from a pool or is lying in rain, dry the chest with a towel or the person's own clothing before attaching pads; surface water can conduct current across the skin and cause burns and skipped shocks. If the patient is in standing water or on a metal surface, move the machine's energy from being shorted β place the patient on a dry surface and keep everyone clear, but don't leave them lying in a puddle.
Hairy chests trip up more rescuers than training scenarios admit. The pads won't stick, the machine can't analyze, and the voice keeps repeating the same fault. AHA guidance says to shave the area if a razor is available β the package on most public AED cabinets includes one, and the pads themselves are usually packaged with a razor inside their sealed bag. If no razor exists, press the pads down hard and accept a lower-quality signal rather than stopping CPR to hunt for a shave kit.
Implanted Defibrillators, Nitroglycerin Patches, and the Body That Has Its Own Machine
An implanted cardioverter-defibrillator is a small device under the skin of the chest that delivers its own shocks, and it is not a reason to withhold external defibrillation. The AHA's position is that external pads should be placed at least 8 inches away from the implanted device to avoid interference with its sensing and to avoid damaging the unit, and rescues proceed normally. The external shock and the internal one don't cancel β both work on the same electrical chaos.
Pain relief is a concern in the conscious-but-about-to-arrest patient, not in cardiac arrest. Once the person is unresponsive with no normal breathing, pain is not the operative variable; the machine will only advise a shock if the rhythm is shockable and the person is presumed pulseless. Rescuers do not need to worry about treating pain they can't assess. Do the steps.
Transdermal patches, especially nitroglycerin, fentanyl, and other medication patches, do belong in the checklist. The AHA and most AED manufacturers advise removing them from the pad placement sites in case they contain a conductive or flammable component β in practice the plan is simple: peel off anything stuck to the skin where the pads will go, wipe the area, then apply the pads.
The body's own device can save time even when an AED is en route. A patient with an implanted defibrillator may still need external shocks if the internal device doesn't fire or fails to convert the rhythm. When the external AED is called into action, the machine is not competing with the implant; it's supplementing it. The boundary: name the condition to advanced providers when they arrive so the implant is interrogated after the event.
What You Can and Cannot Ask of a Bystander
Bystanders are legally protected in every U.S. state by some form of Good Samaritan statute, and most state Good Samaritan laws cover AED use. The exact language varies; according to the National Conference of State Legislatures, AED provisions appear in statutes across all 50 states and the District of Columbia, but the standard β gross negligence versus ordinary care β differs. That's a lawyer's question in a specific case, not a rescuer's question at the scene.
You don't have to be certified to use a public AED. The AHA's design assumption is that the device can be operated by untrained laypeople, which is why it analyzes and prompts rather than requiring the operator to read a rhythm. Certification courses in CPR and AED use improve performance and reduce hesitation, but the machine is deliberately built to instruct first-time users from the moment it's opened.
You should use the AED even if you're uncertain the collapse is cardiac. The machine will not shock a heart that doesn't need it β analysis is the gate. The failure mode of a hesitant rescuer is not a bad shock; it's a machine sitting in its cabinet while the person's shockable rhythm burns irreversibly.
Where the chain most often breaks is elderly patients and crushed spaces. Care homes, hotel rooms, and small bathrooms mean narrow spaces where a breathing person is hard to move, and rescuers often lift the patient up or roll them to reach the pads. According to the AHA's resuscitation science materials, a person should be on a firm, flat surface for effective compressions β but if the space is cramped, drag a flat board or the AED case underneath, or accept a modest quality drop rather than wait for room. Consent, crowding, and bystander panic don't stop the machine; they only delay it.
Battery, Pads, Cabinet, and the Maintenance Bet That Decides the Outcome
The single most common reason a public AED doesn't work at the moment of need is an expired pad cartridge or a battery the building manager never noticed. According to the U.S. Food and Drug Administration, a substantial share of AED failures reported in its Manufacturer and User Facility Device Experience database trace to consumable components β batteries and pads β that were not charged or replaced on schedule. That's a maintenance problem, not a device flaw.
Batteries differ by maker: some are sealed units rated for four to five years under standby, others require weekly or monthly self-test checks. Pads are single-use and typically flagged with a "use by" date about two to four years out, depending on the manufacturer; the same prescription goes for all of them β inspect the expiration date on both the battery and the pad cartridge. If you're responsible for a device at a gym, school, church, or office, put the expiry dates in your calendar and re-order the way you would any safety consumable.
The actual failure-mode research gets thin when you ask how many machines sit broken in cabinets. The good news: every model mandates a visual indicator and an audible or visual self-test, and most public cabinets include a status light visible from the outside. That light is the first thing to check, and a green light with a dead battery is rare β a dead battery with a dead light confirms the stand-by system is doing its job.
There's a real self-interested answer here, too. A building is far more likely to have a call from a cardiac arrest than a fire, and the machine costs less than a mid-range laptop. The AHA's own programmatic guidance on public-access defibrillation describes the return as lives, not dollar ROI, but the decision framework is similar: coverage where people crowd, maintenance budget, and training replacement cycle. If your building has a machine and nobody knows who maintains it, that's the safety gap worth naming before the next board meeting.
Why Survival Data, Not Device Specs, Should Drive the Debate
Survival from out-of-hospital cardiac arrest with bystander AED use, in shockable rhythms, is the number that matters, and the AHA reports that people who are shocked by bystanders can have survival-to-discharge rates well above what EMS-only response produces in communities that have invested in public access. The variance across regions is large β a function of response times, bystander training rates, and the density of machines. Specific community figures change by year; the stable point is the direction of effect.
The same AHA statistics track bystander CPR rates rising over the last decade in the U.S., which is the lever that moves the AED's value. A machine used on a person who received no chest compressions in the previous four minutes has a worse problem than a machine used during good CPR. This is why public-access programs teach them together, and why a bystander who sees an AED but hasn't started compressions should hand the machine to someone else and stay on the chest.
A comparison row that public pages rarely build makes the case clearly: CPR alone in ventricular fibrillation β the approximate survival figure is single digits to about 10 percent without defibrillation; bystander CPR plus an AED before EMS in a shockable rhythm β survival in the tens of percent when the shock lands early, and up to roughly 50 percent or higher in ideal monitored or in-hospital settings. The machine doesn't double a survival number; it changes the order of magnitude in the right conditions. Those figures shift with the study and community, so treat them as ranges and check the AHA's current Heart and Stroke Statistics for year-specific figures.
What surprises rescuers most is the AED's calmness. In a real arrest, the machine's voice is slow, measured, and slightly loud, and it keeps saying "push hard, push fast" while the room around it is not calm. The practical takeaway is to let it set the cadence. You don't have to shout over it; you have to keep your hands on the chest between its prompts and move only when it says to move.
FAQ
Why does an AED say "no shock advised" when the person is clearly not breathing?
Because the machine is analyzing the electrical rhythm, not the breathing. Asystole and pulseless electrical activity are not shockable, and about half of out-of-hospital arrests outside monitored settings present in non-shockable rhythms. In that case, keep doing CPR and leave the pads attached β the AHA's two-minute reanalysis cycle matters because the rhythm can change.
Can I use an AED on someone with a pacemaker or a metal implant in their chest?
Yes. Place the pads at least 8 inches from the implanted device and avoid putting a pad directly over it. Metal implants like sternal wires or a hip prosthesis don't prevent AED use; the current pathway through the chest is the priority. Don't delay the shock to look for the device.
What if the AED fails or the pads won't stick?
Fall back to uninterrupted chest compressions and have someone call emergency services again with the update. Loose pads, a dead battery, or an expired cartridge are all maintenance failures rather than personal ones β the machine's indicator light and expiry dates are the preventable version of this gap. Don't stop CPR to troubleshoot.
Is an AED worth the cost for a small office or church?
The machine is cheap relative to a single cardiac arrest death and relatively cheap relative to most commercial safety equipment. Budget for the battery and pad replacement cycle, not just the device, and appoint a named person to check the status light monthly. The question that matters isn't the purchase price; it's whether the consumables are current when the cabinet opens.
Do you have to be certified to use an AED?
No. U.S. public-access AEDs are designed for lay use and most state Good Samaritan statutes protect untrained rescuers who act in good faith, though the exact legal standard varies by state. Certification reduces hesitation and improves CPR quality, which is why training programs pair the two.
How long should you pause compressions when the AED analyzes?
Aim for two to three seconds and never more than 10. The AHA's guidelines repeatedly emphasize minimizing interruptions; the machine's charge often happens while you're compressing, so the pause is only to avoid touching the patient during analysis. Resume immediately when the voice says to.
Can I use an AED on a child?
Yes. Use pediatric pads and pediatric mode if available. If only adult equipment is on hand, use it β the AHA's position is that a shock from adult pads is better than no shock. For infants under about 1 year, many devices require a pediatric attenuator or a different pad orientation; check the package.
What if the person is lying in water or on a wet floor?
Move them out of standing water or a puddle to a dry surface before defibrillating, and dry the chest with a towel or clothing. Water conducts electricity and can cause skin burns or a shorted shock. Don't delay a needed shock longer than the time it takes to move the person a step or two.
This article is general educational information about bystander response to cardiac arrest, not a diagnosis, treatment plan, or legal opinion. It does not promise outcomes; survival depends on the rhythm, response time, CPR quality, and advanced care available. For training, equipment selection, or legal questions, consult a qualified local CPR instructor, medical professional, or attorney, and for any person in immediate danger, call your local emergency number right away.