Using an AED: What the Machine Does and What You Do
An AED talks you through every step and decides for itself whether a shock is needed. What happens when you open one, and the situations people worry about.
Jarrett Chisholm, ACP · October 14, 2026
Where this is taught: CPR-A & AED, CPR-C & AED and Intermediate First Aid.
Automated external defibrillators hang on the walls of arenas, airports, offices, gyms and schools across Alberta, and most of them will never be opened. When one is needed, the most common problem is not that someone used it badly. It is that nobody brought it, or somebody brought it and nobody wanted to be the one to open the lid.
Image needed · hero
aed-pads-on-manikin-with-voice-prompt-screen.jpg
Top-down shot of an adult CPR manikin on a classroom floor. A training AED sits open beside it, cable connected, with one pad below the right collarbone and one on the left side below the armpit. A learner's hands hover just off the manikin, as if waiting for the analysis. Bright, even light.
Alt text ready: “A training AED open on the floor beside a CPR manikin, its two pads placed on the upper right chest and lower left side.”
The machine is designed for exactly that person: someone who has never used one, who is frightened, and who is kneeling on a floor next to a stranger. It talks. It tells you what to do next. It checks the heart rhythm on its own, and it decides by itself whether a shock is needed. Your job is to put it on the person quickly and follow the voice.
This post walks through what happens when you open one, what the machine is doing while you wait, and the handful of situations that make people hesitate.
What the AED is actually doing
In many adult cardiac arrests the heart is not still. Its electrical system has fallen into a chaotic quivering that moves no blood. A shock stops that chaos all at once, which gives the heart's own pacemaker a chance to take over again with a normal beat.
An AED reads the heart's rhythm through the two pads you stick on the chest. If it finds one of the rhythms a shock can treat, it charges and tells you to press the shock button, or on some models delivers the shock itself after a warning. If it does not find one, it tells you not to shock and to continue CPR. That decision is the machine's, not yours. You cannot talk it into shocking a rhythm it has not identified as shockable. a free lab showing the heart rhythms an AED is looking for.
It is also worth knowing what the AED cannot do. It does not restart a heart that has no electrical activity at all, and it does not replace CPR. After every shock, or every no-shock decision, the machine will tell you to go straight back to compressions. That is not a sign it failed. It is the plan.
Step by step
Keep CPR going while someone else opens the AED, if there are two of you. Stopping compressions to fiddle with the case costs blood flow. If you are alone, get the AED on as soon as it arrives.
The steps below are the general sequence. Every model is a little different in layout, which is why the voice prompts matter more than any memorised order. Listen to it and do what it says.
What using an AED looks like:
- Open it and turn it on. Many turn on when the lid opens.
- Expose the chest. Cut or pull clothing out of the way. Dry the skin if it is wet.
- Remove any medication patch that is in the way of a pad, wiping the skin after.
- Peel the pads and place them where the pictures on them show: usually one below the right collarbone and one on the left side below the armpit.
- Plug the pad cable in if it is not already connected.
- Stand clear while it analyses. Say it out loud so nobody touches the person.
- If it says to shock, make sure nobody is touching the person, then press the button.
- Go straight back to compressions when it tells you to, and keep following the prompts until paramedics take over.
Image needed · diagram
aed-pad-positions-adult-and-small-child.svg
Two simple torso outlines side by side in charcoal on white. Adult: one gold rectangle below the right collarbone, one on the left side below the armpit. Small child: one gold rectangle centre of chest on a front view, one centred between the shoulder blades on a back view. A small note that pads must not touch or overlap.
Alt text ready: “Two outline figures showing AED pad positions: on an adult, upper right chest and lower left side; on a small child, one pad on the front of the chest and one on the back.”
The situations that make people hesitate
Most hesitation around an AED comes from a handful of worries. Each one has a simple answer, and none of them is a reason to leave the machine in its case. the one-day child care course that covers child and baby pad placement.
| The worry | What to do |
|---|---|
| The chest is wet, from rain, snow or sweat | Wipe the chest dry before placing the pads. Move the person off standing water if you can. |
| Hairy chest | Press the pads down firmly. If they will not stick, use the razor in the kit, or pull off one set of pads to remove hair and use a second set. |
| A medication patch on the chest | Take it off and wipe the skin before placing a pad there. |
| A lump under the skin from a pacemaker or implanted defibrillator | Place the pad at least 2.5 cm (1 inch) away from it. |
| The person is wearing a bra | Guidelines now say it may be reasonable to adjust the bra rather than remove it, as long as the pads go on bare skin in the right places. |
| The person is a child or a baby | Use child pads if the kit has them. If the chest is too small for the pads to sit at least 2.5 cm apart, place one on the front and one on the back. |
| Jewellery on the chest | Move or remove anything that would sit under or between the pads. |
Almost all of these come down to the same idea: get the pads onto dry skin, in the right places, with nothing metal or medicated in between, and let the machine make the decision.
Stand clear means everyone
The one real safety rule is that nobody touches the person during analysis or a shock. Touching during analysis can confuse the reading. Touching during a shock can pass some of the current to you. Say stand clear in a loud voice, look up and down the body, and only then press the button.
The moment the shock is delivered, or the machine says no shock is advised, get your hands back on the chest and restart compressions. The pause around a shock should be as short as you can make it.
You will not be the first to use one untrained
Public AEDs are placed on the assumption that the person who opens one may never have seen one before. That is why they talk, why the pads have pictures on them, and why the shock decision is taken out of your hands. Training still helps a great deal. It turns a strange object into a familiar one and takes the fear out of the lid. But untrained use is far better than no use.
If you can, get your hands on a training unit once. The first time you peel a pad and hear the voice start talking should not be the day it matters. a workplace first aid course that includes hands-on AED practice.
Quick answers
- Can an AED shock someone who does not need it?
- An AED analyses the heart rhythm itself and only advises a shock when it finds a rhythm a shock can treat. The rescuer does not make that decision. Your job is to place the pads correctly, keep everyone clear during analysis and shocks, and follow the prompts.
- Can you use an AED on a wet person?
- Yes, but dry the chest first so the pads stick and the current goes through the chest rather than across the water. If the person is lying in water, move them to a drier surface if you safely can.
- Can you use an adult AED on a child?
- Use child pads if the kit has them. If only adult pads are available, use them rather than no AED at all, and if the chest is too small for the pads to sit at least 2.5 cm apart, place one on the front of the chest and one on the back.
- Do I need training to use an AED?
- Public AEDs are designed to be used by people without training, with voice prompts and pictures on the pads. Training still helps, mostly by taking away the hesitation, but untrained use is far better than leaving the AED in its case.
Sources
- Canadian Red Cross: First Aid and CPR manual: remove clothing, jewellery and medication patches that interfere with pads, dry a wet chest, place pads at least 2.5 cm from a pacemaker and at least 2.5 cm apart, and use front and back placement when the chest is too small. (checked 2026-09-23)
- Heart and Stroke Foundation of Canada: Canadian edition of the 2025 guideline highlights: it might be reasonable to adjust the position of a bra instead of removing it when placing defibrillation pads. (checked 2026-09-23)