CPR vs AED: What's the Difference and When to Use Each

What CPR does

When the heart stops pumping effectively, circulation stops. Chest compressions manually push blood — and the oxygen already in it — to the brain and heart muscle.

CPR does not usually restart a heart. What it does is buy time, keeping tissue viable until something can correct the underlying rhythm. Without it, the window closes fast: permanent brain damage begins within roughly four to six minutes.

What an AED does

Most sudden cardiac arrests start as a chaotic electrical rhythm — the heart quivers instead of pumping. An automated external defibrillator reads that rhythm and, if it is shockable, delivers a controlled shock to stop it, giving the heart's natural pacemaker a chance to resume a normal beat.

The critical detail: an AED will not shock a heart that does not need it. The device analyses first. You cannot shock someone by mistake — it simply will not deliver one.

Why the combination matters

CPR alone rarely converts cardiac arrest. Defibrillation alone, arriving late with no CPR in between, usually arrives at a heart that has already deteriorated past the point of being shockable. Together, they work — which is why every survival chain puts immediate CPR first and early defibrillation right behind it.

Using an AED: what actually happens

1. Turn it on. It talks you through everything from here. Some units power on when you open the lid.
2. Bare the chest. Cut or tear clothing. Dry the skin if wet. Shave heavy chest hair if a razor is in the kit — pads need skin contact.
3. Place the pads as pictured: upper right chest below the collarbone, lower left side below the armpit.
4. Stand clear while it analyses. Nobody touches the patient — movement corrupts the reading.
5. Deliver the shock if prompted, after confirming everyone is clear.
6. Resume compressions immediately. Do not pause to check for a pulse unless the device tells you to.

Situations people hesitate over

• Water — move the person off standing water and dry the chest. Rain alone is not a reason to delay.
• Pacemaker or implanted device — visible as a lump under the skin. Place the pad at least an inch away from it.
• Medication patch — remove it and wipe the skin before placing a pad over that spot.
• Children under 8 or under 55 lbs — use paediatric pads or the paediatric setting if available. If not, adult pads are appropriate. Do not withhold defibrillation.
• Metal surface — fine, as long as no rescuer is touching the patient during the shock.

Where this goes wrong

Almost never the device. AEDs are designed for untrained bystanders and they narrate every step. The failures are human: nobody starts compressions, nobody knows where the AED is, or everyone waits for the ambulance.

If your workplace has an AED, two questions are worth answering today: can every shift say where it is within ten seconds, and has anyone checked the pad expiry date this year? Pads and batteries expire, and an AED with dead pads is a wall ornament.

Bottom line

CPR sustains; an AED corrects. Start compressions immediately, get the AED on as fast as someone can bring it, and resume compressions between shocks. Both skills are covered in every course we run — Heartsaver CPR & AED for the general public, BLS for healthcare providers.