Sudden cardiac arrest

Dr Lynn Thomas, OStJ, BSc, MB.BS, MA, FRCPClinically reviewed: 18/09/2026

If someone has become unresponsive, they could be in cardiac arrest. Find out what to look for and what to do, including how to do CPR and use a defibrillator.

What is a cardiac arrest?

A cardiac arrest happens when someone’s heart stops.

If someone has become unresponsive, they could be in cardiac arrest and you need to act quickly. Call 999 or 112 for emergency help and start cardiopulmonary resuscitation (CPR), using a defibrillator (AED) if available.

Signs and symptoms

If someone has a cardiac arrest, they will usually:

be unresponsive
not be breathing normally
- about half will stop breathing altogether
- about half will be taking very abnormal breaths that are usually slow, gasping, and sometimes noisy
- occasionally athletes that experience sudden cardiac arrest initially continue almost normal breathing
have no signs of life (such as purposeful movement, blinking, swallowing, etc)

For more information on this topic visit Resuscitation Council UK.

What to do

1
If you find someone collapsed, you should first perform a primary survey (check for a response, shout for help, call 999 or 112 for an ambulance, open their airway, and look for normal breathing and signs of life for no more than 10 seconds).
If they are unresponsive and not breathing normally, start chest compressions and ask a helper to find a bring a defibrillator, if available.
If you're on your own, do not leave the casualty to look for a defibrillator yourself. The ambulance controller can tell your helper where the nearest defibrillator is.
2
Start chest compressions. Ideally, this should be on a firm surface. For example, if they are on a soft bed, and if it is safe for you to do so, move them to the floor.
kneel close to the casualty and put the heel of your hand on the middle of their chest. Put your other hand on top of your first hand and if you can, interlock your fingers
lean forwards so your shoulders are over your hands, keep your arms straight and press down hard, to a depth of about five to six centimetres before releasing the pressure and allowing the chest to come back up
push at a rate of 100 to 120 per minute
listen to instructions from the ambulance controller. They will tell you what to do and help you to push at the right speed
3
If you are trained to do so, after 30 compressions, provide two rescue breaths. If you are unable or unwilling to provide rescue breaths, give continuous chest compressions.
tilt the casualty's head backwards, lift their chin, and pinch the soft part of their nose closed
take a breath and put your mouth around the person's mouth to make a seal
blow into their mouth steadily and firmly for up to one second, until the chest rises
remove your mouth and watch the chest fall
this is a rescue breath, do this twice
It is important not to interrupt chest compressions for too long so each time you attempt rescue breaths, stick to two attempts to get the breaths in. If one or both don't work, return to chest compressions.
Keep alternating 30 compressions with two breaths (30:2) until:
emergency help arrives and takes over
the person starts showing signs of life and starts to breathe normally
you are too exhausted to continue. If there is a helper, you should swap every one to two minutes, but try to minimise interruptions to chest compressions
a defibrillator is connected to the casualty and tells you to stop CPR while it analyses the heart rhythm
4
When the helper returns with a defibrillator (AED), ask them to switch it on and take the pads out while you continue with CPR. The defibrillator will give audio or visual prompts on what to do.
They should remove or cut through clothing to get to the casualty's bare chest. This includes removing bras, bindings, or restrictive clothing the person is wearing. They may also need to wipe away any moisture or sweat.
The helper should attach the pads to the casualty’s bare chest by removing the backing paper and applying the pads as shown.
the first pad should be on the upper right side, below the collarbone
the second pad should be on the casualty’s left side just below and in line with the armpit
if the casualty has larger breasts, moving the left breast back towards the centre of the chest will allow correct placement of the AED pad
5
The defibrillator will analyse the heart's rhythm. Stop CPR and make sure no one is touching the casualty. It will then give a series of visual and verbal prompts that should be followed.
If the defibrillator tells you that a shock is needed, tell people to stand back. The defibrillator will tell you when to press the shock button. After the shock has been given, the defibrillator will tell you to continue CPR for two minutes before it re-analyses.
If the defibrillator tells you that no shock is needed, continue CPR for two minutes before the defibrillator reanalyses.
6
If the casualty shows signs of becoming responsive. like coughing, opening their eyes, speaking, or starting to breathe normally, put them in the recovery position.
Leave the defibrillator attached. Monitor the casualty's level of response and prepare to give CPR again if necessary.
7
They should attach the pads to the casualty’s chest by removing the backing paper. Applying the pads in the positions shown.
The first pad should be on the upper right side below the collarbone.
The second pad should be on the casualty’s left side below and in line with the armpit.

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