Ask the expert: what is cardiac arrest?

Cardiac arrest is a medical emergency where the heart suddenly stops pumping blood around the body. This prevents your organs from getting vital oxygen and nutrients. If it isn’t treated immediately, it is fatal. 

Most cardiac arrests happen at home with fewer than one in five happening in public spaces. When cardiac arrests occur in public spaces, in seven out of 10 cases, a member of the public who is trained in potentially life-saving cardiopulmonary resuscitation (CPR) is present to start CPR. 

Here, we will explore what causes cardiac arrests and how it differs from a heart attack, as well as symptoms and risk factors. 

What is the difference between cardiac arrest and a heart attack?

A heart attack happens when one of the blood vessels that supply the heart with oxygen-rich blood, ie one of the coronary arteries, becomes blocked. Without this blood supply, parts of the heart tissue start to die. 

Cardiac arrest occurs when the heart stops beating due to a disruption in the heart’s rhythm, that is, the electrical signals that regulate the beating of the heart. An uncontrolled irregular heartbeat is called an arrhythmia.

While a heart attack is one of the most common causes of cardiac arrest as it can trigger an arrhythmia, a cardiac arrest can be caused by other conditions too, such as genetic (inherited) conditions that affect the heart’s electrical circuits. 

What conditions can cause cardiac arrest?

As mentioned above, a heart attack can cause cardiac arrest. However, there are several other conditions — both heart-related and not related to the heart directly — that can lead to a cardiac arrest. 

Inherited heart conditions that increase your risk of cardiac arrest include Brugada syndrome and long QT syndrome, as well as those that cause thickening of the heart wall (hypertrophic cardiomyopathy).

Severe blood loss, for example, due to a road traffic accident or other trauma, can cause a sudden drop in blood pressure that the heart can’t cope with. This can, consequently, trigger cardiac arrest. 

Similarly, a sudden drop in oxygen levels, for example, if drowning or choking, can also trigger cardiac arrest. 

Other causes of cardiac arrest include a drop in body temperature below 35°C (hypothermia), very low blood sugar (severe hypoglycaemia), taking illicit drugs, a drug overdose, and abnormal levels of calcium and/or potassium in blood or other bodily fluids (electrolyte levels).

What are the symptoms of cardiac arrest?

Cardiac arrest occurs very quickly and results in a sudden loss of consciousness, with no pulse or breathing. 

If it isn’t treated immediately, it is always fatal. 

Who is most at risk of cardiac arrest? 

Factors that increase your risk of a heart attack (ie blockage of your coronary arteries with fatty deposits) increase your risk of cardiac arrest. This includes high blood pressure, high cholesterol, heart failure, smoking, diabetes and being overweight or obese.

Your risk of cardiac arrest that is not caused by a heart attack is largely dependent on whether you have a family history of inherited heart conditions that cause arrhythmias and/or hypertrophic cardiomyopathy, and whether you have a weakened heart. 

Who should get screened for heart conditions? 

If you have a family history of individuals prematurely dying as a consequence of cardiac arrest with no identified cause, it is important to get screened for heart conditions. 

The UK charity Cardiac Risk in the Young (CRY) has more information on screening for heart conditions in young adults.

Cardiac screening usually involves an electrocardiogram (ECG) and an echocardiogram (echo). Depending on the results of these tests, you may be referred to a doctor who specialises in treating heart conditions (a cardiologist).

What should you do if someone has a cardiac arrest? 

If someone collapses and becomes unresponsive in front of you, check if you can rouse them. If you can’t, call 999 and then check whether they are breathing. 

If they are not breathing and you have been trained in CPR, you can start chest compressions. If you have not been trained in CPR, you should wait for assistance from someone who is trained in CPR or for the paramedics to arrive. 

How is cardiac arrest treated in a hospital setting? 

If a cardiac arrest is caused by a heart attack, the blocked coronary artery will be opened via an angioplasty. 

If a cardiac arrest is caused by certain arrhythmias, namely ventricular tachycardia or ventricular fibrillation, a defibrillator can be used to restore a normal heart rhythm. If an underlying cause can then be identified, such as low oxygen, potassium or blood sugar levels, this will be treated.