How Does CPR Work (and When Do You Do It)?

Someone goes down in front of you — unresponsive, not breathing normally — and the room freezes for a second. That frozen second is exactly what CPR is meant to fill. It isn’t a restart button for the heart, and it isn’t only for hospitals. It’s a way of doing the heart’s job from the outside, by hand, until help and equipment arrive. The 1-minute demonstration video at the top of this page shows adult CPR being performed, so you can see the rhythm and hand position rather than just reading about them.

Myth: CPR restarts the heart

Not quite. When the heart stops beating effectively — a cardiac arrest — blood stops carrying oxygen to the brain and the rest of the body. Pressing down on the centre of the chest squeezes the heart and moves some of that blood onward; releasing lets the chest come back up so the heart can refill. You’re buying time, not fixing the underlying problem. That distinction matters, because it explains why CPR works best as part of a chain: call emergency services straight away, start compressions, and use a defibrillator as soon as one is available. Our explainer on what happens during a heart attack, minute by minute covers one of the events that can lead to this point — though a heart attack and a cardiac arrest aren’t the same thing.

Myth: you need mouth-to-mouth, or you’ll do it wrong

A lot of people hold back because they picture complicated breaths and worry about getting a sequence wrong. For an adult who has collapsed, emergency guidance for bystanders generally centres on calling for help and giving firm, regular chest compressions — pushing hard and fast in the centre of the chest and letting it rise fully between pushes — with rescue breaths included if you’ve been trained to give them. If you’re unsure, a dispatcher can talk you through what to do in the moment. Doing compressions while you wait is far better than standing back because the full trained routine feels out of reach. The detail does differ for children and babies, which is why the companion ABC’s of Adult CPR video is worth watching alongside the demonstration above if you want the adult sequence talked through in parts.

Fact: the box on the wall helps, too

Many public buildings now keep an AED — an automated external defibrillator — in a marked cabinet. You don’t need to diagnose anything to use one. The device speaks its instructions aloud, tells you where to place its pads, and will only advise a shock if it detects a rhythm that might respond to one. CPR and an AED are partners, not alternatives: compressions keep blood moving while the AED is fetched and set up, and you resume compressions when it tells you to.

Honestly, the best time to learn this is before you ever need it. A short first-aid course lets you practise on a manikin, feel how firm compressions actually are, and hear an AED trainer talk — none of which a video alone can give you. But even reading and watching now helps. If the moment ever comes, you won’t be starting from zero.

MedicalVideos.com has published trusted medical videos since 2007. The site was founded by Dr. Mohamed Ibrahim, MD, PhD — physician, researcher, and former Duke faculty.

Educational only — this article is general information about CPR, not medical advice and not a substitute for first-aid training. In a real emergency, call your local emergency number immediately and follow the dispatcher’s instructions.

Frequently Asked Questions

When should CPR be started?
When someone is unresponsive and not breathing normally, after calling emergency services (or asking someone else to call). If you’re unsure, the emergency dispatcher can guide you.

Is a heart attack the same as a cardiac arrest?
No. A heart attack is a problem with blood flow to the heart muscle, and the person is often still conscious. A cardiac arrest is when the heart stops beating effectively and the person collapses and is unresponsive — that’s when CPR is used.

Can I hurt someone by doing CPR?
Compressions need to be firm to work, and tired arms are normal. In a genuine arrest, starting compressions promptly and following dispatcher or AED instructions matters far more than worrying about doing them perfectly.

Do I need to be certified to do CPR?
No. Anyone can call for help, start compressions, and follow an AED’s spoken instructions. Training is strongly worthwhile, though — a course lets you practise properly before it counts.