CPR does not restart a stopped heart. It buys time. Chest compressions keep oxygenated blood moving to the brain and heart muscle so that a defibrillator — when it arrives — still has something to work with. That is the whole purpose, and understanding it changes how urgently people act.
Survival from an out-of-hospital cardiac arrest falls sharply for every minute without CPR and defibrillation. The person who happens to be standing there matters more than anyone who arrives later.
This is an explainer, not training. Reading it will not make you competent to perform CPR. If you might ever need to, do a practical course — the section at the end explains why that is not a throwaway line.
What cardiac arrest actually is
A cardiac arrest is when the heart stops pumping effectively. The person collapses, becomes unresponsive, and stops breathing normally. Without intervention it is fatal within minutes.
It is not the same as a heart attack, though people use the terms interchangeably. A heart attack is a blockage in a coronary artery — the person is usually conscious and in pain. A heart attack can lead to cardiac arrest, but they are different events needing different responses.
The chain of survival
Four links, each dependent on the one before:
- Early recognition and call for help — spotting cardiac arrest and dialling 999
- Early CPR — keeping blood moving
- Early defibrillation — the shock that can restore an effective rhythm
- Post-resuscitation care — hospital treatment
Bystanders own the first three links. By the time professionals arrive, the outcome has often already been determined.
When to start CPR
Start if the person is unresponsive and not breathing normally. That is the whole test.
The critical trap: agonal breathing. In the first minutes of cardiac arrest, many people make occasional gasping, snoring or gurgling sounds. It looks like breathing. It is not. Agonal gasps are a sign of cardiac arrest, and mistaking them for normal breathing is one of the most common reasons bystanders do nothing.
If you are unsure whether breathing is normal, treat it as not normal and start. The 999 call handler will help you decide, and they will talk you through CPR — stay on the line and put the phone on speaker.
Adult CPR — the sequence
- Check for danger. Do not become a second casualty.
- Check for a response. Shout and gently shake the shoulders.
- Open the airway. Head tilt, chin lift.
- Check breathing for no more than 10 seconds. Look, listen, feel. Remember agonal gasps are not normal breathing.
- Call 999 — speaker on. Send someone for the nearest defibrillator; if you are alone, the call handler can tell you where one is.
- Start chest compressions.
- Use a defibrillator as soon as one arrives — switch it on and follow the spoken instructions.
Chest compressions — the numbers that matter
Per Resuscitation Council UK guidance for adults:
| Element | Target |
|---|---|
| Hand position | Centre of the chest — lower half of the breastbone |
| Depth | At least 5 cm, but no more than 6 cm |
| Rate | 100–120 compressions per minute |
| Recoil | Let the chest come all the way back up; do not lean on it between compressions |
| Interruptions | As few and as short as possible |
Full recoil is the element most often done badly. If you lean on the chest between compressions, the heart cannot refill, and compressions that do not let it refill move very little blood.
100–120 per minute is faster than most people expect — roughly two compressions per second.
Hands-only CPR, or 30:2?
If you are not trained in rescue breaths, or unwilling or unable to give them: do compression-only CPR. Continuous compressions, no pauses for breaths. This is explicitly endorsed, and it is vastly better than doing nothing.
If you are trained and willing: 30 compressions to 2 rescue breaths. Deliver each breath over about a second, using just enough air to make the chest start to rise. Avoid over-inflating — excessive ventilation does harm.
The message is deliberately permissive. Hesitation kills far more people than imperfect technique.
Defibrillators
An automated external defibrillator (AED) is designed to be used by an untrained bystander. It talks you through every step, and it will not deliver a shock unless it detects a rhythm that needs one — you cannot shock someone by mistake.
- Switch it on and follow the voice prompts
- Attach the pads to bare skin as shown on the diagrams
- Nobody touches the casualty while it analyses or shocks
- Resume compressions immediately after the shock unless told otherwise
Many AEDs are registered on the national defibrillator network, so a 999 call handler can direct you to the nearest one.
Children and infants are different
Paediatric resuscitation differs in important ways, because cardiac arrest in children is usually caused by a breathing problem rather than a heart rhythm problem.
- Trained rescuers give 5 initial rescue breaths before starting compressions
- The compression-to-ventilation ratio for trained paediatric rescuers is 15:2
- Compression depth is about one third of the chest depth — one or two hands for a child, two fingers or two thumbs for an infant
If you are untrained and faced with a child in cardiac arrest, use the adult sequence rather than doing nothing. Call 999 and follow the call handler.
Anyone responsible for children professionally should hold a paediatric first aid qualification — see our Paediatric First Aid course as awareness support alongside it.
When to stop
Continue until:
- Emergency services take over
- The person shows clear signs of life — waking, moving purposefully, breathing normally
- You are too exhausted to continue safely (swap with someone roughly every 2 minutes if you can)
- It becomes unsafe to continue
The three things that stop people helping
“I’ll break their ribs”
Rib fractures do happen, particularly in older people. They are a recognised and survivable consequence of effective compressions. Someone in cardiac arrest who receives no CPR will die; ribs heal.
“I’ll get sued”
In England and Wales, the Social Action, Responsibility and Heroism Act 2015 requires courts to consider whether a person was acting for the benefit of others or intervening in an emergency. It is not blanket immunity, and the UK has no “Good Samaritan law” granting one — but claims against bystanders who attempt to help in good faith are vanishingly rare in practice.
“I’ll do it wrong”
Imperfect CPR massively outperforms no CPR. The 999 call handler will coach you. Doing something is the decision that matters.
Why an explainer is not enough
Everything above is knowledge. Compressions at the right depth and rate, for many minutes, without leaning on the chest, on a real body, while frightened, is a physical skill. It is learned on a manikin with someone correcting you — which is precisely why FAW and EFAW qualifications require an in-person practical assessment and cannot be completed online.
If your role might ever require you to act, book a practical course. Reading this is a start, not a substitute. See what first aid training you actually need.
Further reading
- Resuscitation Council UK — Adult basic life support guidelines
- Resuscitation Council UK — Basic life support FAQs
- NHS — How to perform CPR
Awareness training
Our Basic Life Support course covers the chain of survival, recognising cardiac arrest, and the principles of resuscitation and defibrillator use. Also: Workplace First Aid · Anaphylaxis Awareness · Paediatric First Aid
Related reading: What first aid training do you actually need? · Workplace first aid kits: what the law requires
£9 per course. CPD accredited, 100 % online, verifiable e-certificate on completion. Pay for 2, get 3 — any 3 courses for £18.
Important: this page and our courses are general information and CPD-accredited awareness training — not medical advice, and not a substitute for practical resuscitation training. They are not Ofqual-regulated qualifications and do not make you a first aider. CPR is a physical skill that must be learned and assessed in person; FAW and EFAW qualifications require face-to-face practical assessment and cannot be obtained online. Clinical guidance is updated periodically — always follow current Resuscitation Council UK guidance and the instructions of the 999 call handler. In an emergency, call 999 immediately.