Cardiac Arrest and Heart Attack: How to Tell Them Apart

Both a heart attack and a cardiac arrest are medical emergencies, and both need a 999 call straight away. The difference is what is happening inside the body. In a heart attack, the blood supply to part of the heart muscle is blocked, but the heart is still pumping, so the person stays conscious and breathing¹. In a cardiac arrest, the heart has stopped pumping blood around the body, the person loses consciousness and stops breathing normally³.

That difference decides what you do. A heart attack needs the person sat down, kept still and kept calm while the ambulance comes⁵. A cardiac arrest needs chest compressions started immediately, and a defibrillator fetched if there is one nearby⁴. Every minute that passes without CPR and defibrillation, the chance of survival falls by about 10%⁶.

Most cardiac arrests happen at home, so the person who needs help is likely to be someone you know⁸. You do not need training to start. Calling 999, starting chest compressions if it is safe to do so, and sending someone for a defibrillator is the response the ambulance services ask of ordinary bystanders⁴.

A heart attack is a blocked blood supply to the heart

A heart attack happens when an artery becomes obstructed, restricting the flow of blood to the heart⁵. NHS inform describes it as a reduction in blood supply to part of the heart muscle¹⁴. When a coronary artery is blocked by a blood clot, the muscle beyond the blockage does not get oxygen or nutrients, and there is a risk the heart could stop beating¹⁴.

The person is usually conscious and breathing, because the heart is still sending blood around the body¹. That is the key fact for a bystander: a heart attack is a heart in trouble, not a heart that has stopped. It is still an emergency. The London Ambulance Service warns that left untreated it can lead to a cardiac arrest, which is when the heart stops beating⁹.

What to do, in order:

1. Call 999 or 112 and tell them you think someone is having a heart attack¹⁵.
2. Sit the person down, keep them at rest and keep them calm⁵.
3. Stay with them and watch for any change: if they become unresponsive and stop breathing normally, it has become a cardiac arrest and you start CPR¹.

St John Ambulance and the British Red Cross both stress calling for emergency help straight away rather than waiting to see whether symptoms settle¹⁵. The Scottish Ambulance Service lists heart attack among the conditions where you should call 999 immediately, and the South Western Ambulance Service says every second counts with these conditions¹⁷.

A cardiac arrest is when the heart stops pumping

A cardiac arrest is a medical emergency that happens when the heart suddenly stops pumping³. This prevents blood from flowing to the brain and other vital organs, depriving them of oxygen³. The North West Ambulance Service calls it the most extreme emergency, when the heart stops beating in a normal way, preventing blood from pumping around the body¹⁹.

The signs are unmistakable once you know them. If someone has a cardiac arrest, they will stop breathing or will not breathe normally, and they will lose consciousness³. They will not respond to stimulation²⁰. A heart attack is one cause, but not the only one: a cardiac arrest can also follow choking or trauma⁵, and the British Heart Foundation lists severe bleeding, a sudden drop in oxygen levels, electrocution and drug overdose among other causes²¹.

The response is a sequence, and it starts before the ambulance arrives:

1. Check for danger before approaching the person¹.
2. Call 999 immediately¹.
3. Start chest compressions, often called CPR, if it is safe to do so¹.
4. Send someone to get a defibrillator if one is nearby⁴.

Ambulance service call handlers can guide you through CPR and will direct you to the closest public access defibrillator⁵. You do not need formal training to begin; the London Ambulance Service describes this intervention as calling 999, starting chest compressions if it is safe to do so, and sending someone for a defibrillator, even without formal training⁴.

Heart attack or cardiac arrest: how to tell which one is happening

The single question that decides your next move is whether the person is conscious and breathing normally. In a heart attack, the heart is still sending blood around the body and the person remains conscious and is still breathing¹. In a cardiac arrest, they lose consciousness and stop breathing or stop breathing normally³.

If you are not sure, act as though it is a cardiac arrest. The cost of starting CPR on someone who did not need it is far lower than the cost of waiting. Check for danger before you approach the person⁵.

A stroke can be just as life-threatening as a heart attack and can have similar causes, so sudden weakness or facial droop is another reason to call 999 immediately²². The South Western Ambulance Service lists chest pain, breathing difficulties, heart attack and cardiac arrest among the reasons to call 999¹⁸.

Using a defibrillator: the public used one in only 9% of cardiac arrests

A defibrillator is a device used to give an electric shock to help restart a patient's heart when they are in cardiac arrest⁶. It is designed to be used by anyone: the machine talks you through each step, and you cannot shock someone who does not need it.

The problem is that they are not used often enough. In 2024, defibrillators were used by members of the public in only 9% of cardiac arrests¹⁰. The British Heart Foundation reports that less than 10% of out-of-hospital cardiac arrests see a defibrillator being used²³. In the East of England, defibrillators were available within 500 metres of 79% of cardiac arrest cases but were only used in 10% of incidents¹³. The equipment is often close by; the gap is people knowing to fetch it.

How to get one to the person:

• Call 999 first. The emergency medical dispatcher will guide you to the nearest available defibrillator and provide clear instructions on how to access it¹³.
• Send a second person to fetch it while you stay and do compressions⁴.
• If the cabinet is locked, the call handler can give you the code¹³.

Why speed matters: survival falls about 10% a minute

For every minute that passes once in cardiac arrest, a person loses a further 10% chance of survival⁶. The South Western Ambulance Service puts it as a 10% decrease per minute without bystander CPR⁷, and the Resuscitation Council UK gives the same figure for each minute without CPR and defibrillation²⁴. The Scottish Ambulance Service and the Northern Ireland Ambulance Service give the same rate¹.

The result is stark. In the UK, only around 1 in 10 people survive an out-of-hospital cardiac arrest¹². The Resuscitation Council UK gives the same figure¹¹. In 2015, 1 in 20 people survived a cardiac arrest to hospital discharge in Scotland¹, which shows how far survival has moved, and how far it still has to go.

This is why bystander action is the deciding factor. The ambulance service cannot reach everyone in the first few minutes, but a neighbour, a family member or a passer-by can. The British Heart Foundation's Heartsafe programme uses the message that 80% of cardiac arrests happen in the home, which is why it teaches CPR to ordinary households²⁵.

CPR: what it is and how to start

CPR stands for cardiopulmonary resuscitation. It combines chest compressions and rescue breaths to give a person the best chance of survival following a cardiac arrest²⁶. It is an emergency lifesaving procedure, performed when the heart stops beating, often called a cardiac arrest²⁷.

For an ordinary bystander, the simplest version is hands-only CPR: chest compressions without rescue breaths. The Scottish Ambulance Service describes the core action as starting chest compressions, often called CPR¹. The Yorkshire Ambulance Service advises calling 999 and starting CPR straight away if you witness a cardiac arrest²⁸.

If you have not been trained, the call handler will talk you through it⁵. If you have a defibrillator, use it as soon as it arrives and follow its spoken instructions⁶. If the person is female or has larger breasts, standard compressions on the centre of the chest are still correct; St John Ambulance has specific guidance on CPR on women and other people with breasts²⁶.

The Chain of Survival and how to be a link in it

The Chain of Survival is the sequence of actions that gives someone in cardiac arrest the best chance: recognising the emergency and calling 999, starting CPR straight away, using a defibrillator as soon as one is available, and the advanced care that follows when the ambulance crew arrives. Most cardiac arrests happen at home, so the person who needs help is likely to be someone you know and care about¹¹.

You can be a link in it in several ways:

• Learn CPR. The British Heart Foundation's free RevivR course teaches you to identify a cardiac arrest, make a simulated 999 call, perfect your CPR technique and learn how to use a defibrillator, in four steps²⁷.
• Attend a community session. Ambulance services run free community CPR and defibrillator familiarisation sessions covering how to recognise a cardiac arrest, how to perform effective CPR, how to safely use a public access defibrillator, and what happens when you call 999 and how you fit into the response¹².
• Register as a GoodSAM responder. GoodSAM volunteers can receive alerts on their mobile phone when someone nearby suffers a cardiac arrest, enabling them to provide potentially life-saving assistance before emergency services arrive²⁹. The app may alert you when you are in the immediate vicinity of a cardiac arrest, so that you can start CPR in the critical first minutes³⁰.
• Check your local defibrillator. The British Heart Foundation's National Defibrillator Network, known as The Circuit, maps public defibrillators so that ambulance services can direct callers to them²³.

In Northern Ireland, the ambulance service has worked in partnership with GoodSAM since July 2019 to improve survival from cardiac arrest by alerting nearby trained responders²⁹. GoodSAM volunteers can receive alerts on their mobile phone when someone nearby suffers a cardiac arrest, enabling them to provide potentially life-saving assistance before emergency services arrive²⁹.

Where to get help and training

Call 999 for either emergency. The Scottish Ambulance Service, the London Ambulance Service, the Welsh Ambulance Service and the South Western Ambulance Service all list heart attack and cardiac arrest among the conditions where you should call 999 immediately¹. In Scotland, NHS inform advises phoning 999 immediately and asking for an ambulance if someone stops breathing, loses consciousness or does not respond to stimulation²⁰.

For training, the options are free and local:

• Online: the British Heart Foundation's RevivR course covers CPR and defibrillator use²⁷. The Resuscitation Council UK's Lifesaver training is also free¹¹.
• In person: ambulance services run community CPR and defibrillator sessions¹². The South Western Ambulance Service offers CPR and defibrillator training¹².
• At home: the Heartsafe programme teaches households, because most cardiac arrests happen at home²⁵.

If you have a defibrillator at home, a club or a village hall, the Resuscitation Council UK publishes guidance on public access defibrillators for communities¹⁰. The Scottish Ambulance Service also publishes a public access defibrillator information pack³¹.

For the wider picture of what to do in a medical emergency at home, see First Aid at Home: A Household Guide. For the step-by-step sequence before CPR, see What to Do First: Danger, Response, Airway and Breathing. For hands-only CPR in detail, see How to Do CPR: Hands-Only Steps for Adults. For defibrillators, see Defibrillators (AEDs): What They Do and How to Use One and Public Access Defibrillators: Finding the Nearest One. For a suspected heart attack specifically, see What to Do if Someone Is Having a Heart Attack.