Updated 7/2/25

Causes and Effects Of Cardiac Arrest

Cardiac arrest is when the heart stops contracting effectively or stops completely. The most common cause of cardiac arrest is heart attack, but there are other causes, see later on.

Anatomy Of The Heart

Note – a heart attack is a cause of a cardiac arrest, but it is not the same thing. Heart attack is described in the Heart Attack and Angina section.

When a cardiac arrest occurs, all blood flow ceases in the body. The effect on the brain is loss of consciousness and cessation of all breathing*. Brain injury and brain death occur rapidly if the cardiac arrest is left untreated for 3-4 minutes because the brain cannot tolerate more than about 4 minutes without oxygen before becoming irreversibly damaged. If the casualty has severe hypothermia (e.g. a temperature of 25 degrees c) when they have their cardiac arrest, their brain will survive for longer.

*Agonal breathing may occur, this is abnormal looking gasps that sometimes occur in the first few seconds after a cardiac arrest. These rapidly cease. Agonal breathing must not be confused with normal breathing.

EARLY DEFIBRILLATION IS THE KEY TO SURVIVAL

Chain Of Survival

The best chance of survival is immediate treatment via defibrillation, if indicated by the AED. The effectiveness of defibrillation decreases the longer you wait to deliver a shock. Therefore, the AED must be applied as soon as a cardiac arrest is diagnosed (whilst someone else is doing chest compressions).  CPR (cardiopulmonary resuscitation) is used for circulatory support until a rhythm can be restored: in most circumstances CPR by itself will not restore a normal rhythm !!!

Only some cardiac rhythms will respond to defibrillation. These are called “shockable rhythms”. Those that will not respond are called “Non-shockable rhythms”. During resuscitation, the rhythm shown on the ECG (electrocardiogram) can change back and forth between these two.

For more details on cardiac arrest see ‘Casualty Care in Mountain Rescue’.

The following video shows the different contraction patterns of a normal heart and some of the most common types of cardiac arrest.

Cardiac Rhythms

Unfortunately the survival rate of individuals suffering out-of-hospital cardiac arrest is poor. In our situation where we are unlikely to be present at the time of the arrest, survival is very unlikely.

Symptoms and Signs

No breathing

Look, feel (the diaphragm) and listen for breathing for 10 seconds.

 

When To Attempt Resuscitation

 Resuscitation should be started immediately if:

  • Witnessed collapse (by MR)

  • Bystander CPR is taking place

For asystole in the presence of hypothermia or avalanche, see the relevant sections.

Resuscitation should not be started if:

  • Other injuries incompatible with life are present

  • It has been over 20 minutes since the collapse, no bystander CPR has taken place

  • Sustained CPR and timely evacuation would be impossible

Treatment

Remember: EARLY DEFIBRILLATION IS THE KEY TO SURVIVAL

1). Immediately start CPR at 30:2. i.e 30 chest compressions to two breaths. Aim for high quality continuous chest compressions with minimal interruptions. In children (0 - 18 yrs) and victims of drowning, it is essential to start with five rescue breaths. NB Chest compression-only CPR is now advocated if there is no medical airway equipment available (e.g. i-gel, pocket mask).

How to do effective chest compressions:

  • Kneel by the side of the victim. If on your own, kneel at the casualty’s head, as in the video below.

  • Place the heel of one hand in the centre of the victim’s chest (which is the lower half of the victim’s breastbone (sternum))

  • Place the heel of your other hand on top of the first hand Interlock the fingers of your hands and ensure that pressure is not applied over the victim's ribs

  • Keep your arms straight

  • Do not apply any pressure over the upper abdomen or the bottom end of the bony sternum (breastbone)

  • Position your shoulders vertically above the victim's chest and press down on the sternum to a depth of 5–6 cm

  • After each compression, release all the pressure on the chest without losing contact between your hands and the sternum;

  • Repeat at a rate of 100–120 minute

CPR & AED demo

2). Use the BVM (ideally 2 person technique) to achieve a total of two effective rescue breaths. Do not interrupt compressions by more than 10 seconds to deliver two breaths. Then return your hands without delay to the correct position on the sternum and give a further 30 chest compressions.

3). Cut away clothing using trauma shears.

4). Turn on the AED and follow voice prompts.

5). Check for, and remove where appropriate, check that there are no ‘5 P’s’ before placing the pads: Patches, Pendants, Piercings, Pacemakers and Perspiration and excessive body hair.

Correct Pad Placement

6). Place the pads in the positions marked on the packaging and then connect the lead to the machine.  Once connected the light will turn off.

7). Press the blue button to start analysing the cardiac rhythm.  Stop chest compressions and do not touch the casualty whilst the AED is analysing.

8a). If no shock is advised immediately restart chest compressions.

8b). If a shock is advised, wait whilst the AED charges.

9). Once charged, the person delivering chest compression MUST ENSURE that everyone is standing clear of the casualty before delivering the shock. If oxygen is being delivered by a mask, it must be removed prior to delivering the shock.  However if an igel is being used, there is no need to remove oxygen before delivering the shock.

Beware – The FRED AED can be fooled by movement, on one occasion an earlier model being used as a monitor (no longer team practice) advised team members to deliver a shock to a wet and shivering casualty who was sat up and talking.  Do not deliver shocks to fully conscious talking casualties!

10). After delivering the shock, immediately resume chest compressions. There must be no delay in getting back on the chest.  Do not wait hoping to see a response and do not check the pulse at this stage.

11). As soon as possible ventilate the casualty using an i-gel and bag. Continue using 30:2.

12). Give high flow oxygen by connecting the cylinder to the bag. The i-gel is a closed system. Oxygen does not need to be removed prior to delivering a shock.

Remember – As soon as practicable switch from delivering chest compressions manually to using the Autopulse (an automated machine to deliver effective chest compressions). The AutoPulse is calibrated to continue at 30:2.

Urgent evacuation is always warranted

Setting up the FRED AED and voice prompts:

Look for and treat reversible causes of Cardiac Arrest

Some causes of cardiac arrest are reversible. Whilst resuscitation is ongoing, as part of the management of cardiac arrest, you must identify and treat these if possible.

Look for and treat the 4 H’s and T’s. The ones in bold are the most relevant to MR and these are the ones you need to remember:

H's and T's

  • Hypothermia. Follow the MREW Hypothermia Protocol.

  • Hypoxia e.g. from drowning, anaphylaxis. Give oxygen.

  • Hypovolaemia e.g. from blood loss, anaphylaxis. Will need IV fluids.

  • Hyper/hypokalaemia – severely abnormal levels of potassium in the blood. This is included for information only. There is no intervention available to MR with cas care skills or the equipment we carry.

 

  • Toxic e.g. from drugs. Give oxygen. Support breathing. Monitor for cardiac arrest.

  • Thrombosis e.g. a blood clot lodged in the heart muscle (heart attack), a blood clot lodged in the lungs (pulmonary embolism).

  • Tension pneumothorax. Air collecting between the lung and the chest wall, resulting in lung collapse which then puts pressure on the heart and can result in cardiac arrest.

  • Tamponade – trapping of fluid (usually blood) under the the sack in which the heart is enclosed. This compresses the heart and prevents it beating effectively. NB this is included for information only.

 

When To Stop CPR

There are three reasons to stop CPR.

1). The casualty recovers:

  • Spontaneous breathing & a pulse returns

Remember – spontaneous breathing takes longer to return than the pulse so continued respiratory support may be necessary

  • The casualty regains consciousness

If the casualty recovers, monitor the ECG, (using the Viatom) pulse, BP, oxygen saturation and respiration, as it will provide early warning if they start to deteriorate again.

2). Practicalities:

  • You are too exhausted to continue

  • CPR cannot be continued during the evacuation (less of a problem if the AutoPulse is used)

  • The area becomes unsafe

3). The casualty has died

  • CPR for 20 minutes (except in the case of hypothermia with no contraindications to starting CPR such as injuries incompatible with life)

  • Injuries become incompatible with life, e.g. prolonged severe bleeding

Before stopping, always make sure there are no treatable reversible causes e.g. hypoxia, severe hypothermia.

See the Resuscitation Decision Tree protocol to aid your decision making. This is in the back of the Cas Care book and also in all the Cas Card folders in the medical sac for use on the hill. If you decide to end CPR or do not start the Recognition Of Life Extinct (ROLE) Protocol will need to be completed. Again this protocol is at the back of the Cas Care book and also in all the Cas Card folders in the medical sac for use on the hill.

Signs and symptoms that a cardiac arrest is about to occur

Whilst cardiac arrest can strike without warning, there are also circumstances when it’s more likely to occur.  Being prepared to react quickly in these situations can make the difference between life and death.

There is a higher risk of an arrest occurring:

  • after (or during) a myocardial infarction (Heart Attack)

  • when moving a severely hypothermic casualty

  • tension pneumothorax

  • drowning

  • when suffering from severe hypoglycemia

  • during prolonged fitting (due to lack of oxygen)

In these circumstance it is sensible to apply (but not connect) the defibrillator pads in case they become necessary. Monitor closely and ensure the AutoPulse is on scene and an i-gel and bag are to hand. Organise the Team into roles.

Watch for the most commonly observed symptoms preceding an arrest, shortness of breath with no other causes is the commonest symptom

  • chest pain

  • fainting (syncope)

  • cold sweat

  • casualty has a feeling that they are going to die

  • ECG Rhythms - for interest

Knowledge of ECG rhythms is useful, but not necessary for diagnosing and attempting to treat cardiac arrest.  The AED will determine whether a rhythm is responsive to defibrillation for you and prompt you accordingly.

Below are examples of:

1) normal (sinus) rhythm;

2) the two most common shockable rhythms (ventricular fibrillation (known as VF) and ventricular tachycardia (known as VT); and °

3) no rhythm, called asystole (commonly known as flat line).  Contrary to its common name, in individuals whose heart has recently stopped pumping, asystole will show as a flat, slightly undulating line, not a completely horizontal line as you see on TV.

An Example Of A Normal Cardiac Rhythm

An Example Of A Cardiac Arrest Rhythm

An Example Of A Cardiac Arrest Rhythm

An Example Of No Cardiac Rhythm

CPR and defibrillation being done for real.

Note the following about this video:

  • Immediate call for help

  • Early CPR instituted (straight on the chest)

  • AED on chest as soon as available

  • Shock delivered

  • Uninterrupted CPR

  • Minimal off-chest time (only pauses for rhythm checks & delivering shock)

  • Each person can only do 2 minutes good quality CPR

  • ROSC means Return of Spontaneous Circulation

  • The man delivering the breaths is using an i-gel or similar plus Bag-Valve