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Electrocardiogram (ECG)


Expert reviewer, Dr Tim Cripps, Consultant Cardiologist
Next review due June 2023

An electrocardiogram (ECG) is a test to record the rhythm and electrical activity of your heart. It can help to find out if there are any problems with your heart.

In an ECG, sensors are attached with sticky pads to your chest, arms and legs. The sensors pick up electrical signals from your heart. These are sent to an ECG machine, which records the signals and prints out a reading on paper.

An electrocardiogram (ECG) reading

Why might I need an ECG?

You may be advised to have an ECG if you have symptoms of a heart condition. These might include chest pain, palpitations (when you can feel your heart thumping or throbbing), shortness of breath and dizziness.

Your doctor might recommend having an ECG to check your heart if you have certain other conditions or you’ve had an accident or injury. It might also be one of the general health tests you have before an operation.

An ECG can detect different heart problems including:

  • if you’re having a heart attack or your heart has been damaged by a previous heart attack
  • an enlarged heart that’s under strain
  • fast, slow or irregular heartbeats called arrhythmias

If you want to know more about why an ECG is recommended in your particular circumstances, ask your doctor.

For more information about what can cause an abnormal ECG, see our FAQs below.

Types of ECG

The type of ECG your doctor recommends will depend on your symptoms and what type of heart problem you might have. There are three main types of ECG.

  • A resting or standard ECG – this simple test is carried out while you’re lying down and relaxed. It usually only takes a few minutes to do.
  • An exercise ECG – this is done while you’re exercising, usually walking on a treadmill or cycling on an exercise bike. It shows how your heart copes under strain. You may have this test to help discover if you have coronary heart disease or to investigate irregular heartbeats that happen when you exercise.
  • An ambulatory (24-hour) ECG or Holter monitor – for this test you wear an electronic recorder for a day or two. It records the activity of your heart over this time and helps to show irregular heartbeats that may only happen occasionally.

If you get palpitations (abnormal rhythms) that happen from time to time but not every day, your doctor may recommend a device called a cardiac event recorder. This can record your heartbeat over a longer period of time. There are two main types.

  • Portable event recorders are devices which you carry or wear and activate yourself when you’re having symptoms.
  • Implantable loop recorders (ILR) – these are devices that are placed under the skin on your chest.

If your doctor recommends a particular type of ECG, they’ll give you information about the test and what it involves.

Preparing for an ECG

You don’t normally need any special preparation for a resting ECG. You may be able to have this test at your GP practice. Don’t use any oils or moisturisers on your skin when you go for the ECG because the sticky pads may not stick. Don’t wear too many layers or anything that’s difficult to remove because you’ll have to undress to the waist to have the ECG.

You usually need to go to hospital to have an exercise ECG or an ambulatory or event recorder. For these tests, your hospital will give you any information you need about how to prepare.

For an exercise ECG, it’s best to wear comfortable clothes and shoes. You may be asked to avoid having a heavy meal or having drinks with caffeine in for a few hours before the test. Your hospital may also ask you to stop taking some of your medicines a day or two before your exercise ECG. Only stop any medicines if you have been specifically told to.

What happens during an ECG?

The health professional performing your ECG (the practitioner) will talk to you beforehand to explain exactly what will happen and what you can expect. Be sure to ask any questions you have. It’s important that you understand what will happen before giving your consent to go ahead.

Here we give some general information about what may happen during these tests. Your experience may be different in some ways. Always follow the instructions your doctor or hospital gives you.

Resting ECG

It only takes a few minutes to have a resting ECG. It’s a simple, painless test.

Your ECG practitioner will ask you to undress above your waist and lie on a bed or couch. They’ll need to prepare the areas of skin where they will be sticking the electrodes. This may mean cleaning the areas of skin, exfoliating it with a paper towel or gauze swab or shaving any hair with a razor. Your practitioner will check with you before they do this. They will then place the electrodes in position on your arms, legs and chest, using sticky patches. Once you have the electrodes on, your practitioner will cover you with a gown if you wish.

The electrodes are connected by wires to a recording machine. When you’re ready, your ECG practitioner will begin a recording on the machine. The machine will print a record of your heartbeat onto a paper strip or store the data on a computer. Try to lie still and relax as much as possible during the recording – moving or tensing your muscles can affect the recording. Once your practitioner has got a good recording, they’ll remove the electrodes from your body.

Exercise ECG

An exercise ECG usually takes about 20 minutes. Your health professional carrying out the test will attach electrodes to you in the same way as described for a resting ECG. You’ll then be asked to start walking on a treadmill or cycle on an exercise bike at a gentle, slow pace. As the test goes on, the slope or speed of the treadmill will increase or the bike will become harder to pedal. This will make your heart work harder.

Your ECG will be monitored while you’re exercising, along with your blood pressure. The ECG practitioner won’t make you do more than you’re able to manage. But if you have any chest pains, shortness of breath or feel unwell in any way, let them know and they can stop the test. For more information about not being able to complete the exercise, see our FAQ: What happens if I can’t finish the exercise ECG? below.

24-hour ECG

For this test, you’ll have electrodes stuck to your chest and connected to a small, portable recorder. You wear this on a belt around your waist.

You can go about your normal activities while you’re wearing the 24-hour recorder but don’t have a bath or shower with the recorder on. You may be asked to keep a diary of everything that you do during the time period and note down if you have any symptoms. After the 24 hours are up, you’ll need to go back to hospital to return the recorder. You might need to wear it for longer if your doctor thinks it would be useful to track your heart’s activity over more time.

Cardiac event recorders

A portable cardiac event recorder is a small electrical device that you wear or have on you all the time for up to a month. You’ll usually either hold it, or wear it on your wrist – you don’t wear electrodes on your chest for these monitors. When you have symptoms such as chest pain, dizziness or palpitations, you press a button and the device will record your ECG. Sometimes, you’ll need to place the device on your chest, but your doctor will explain exactly how your event recorder works. You’ll also be shown how to send your ECG recordings to the hospital using your phone.

An implantable loop recorder (ILR) is a small device that your doctor will place just under the skin on the front of your chest. You’ll be given a local anaesthetic when the ILR is fitted, so you’ll stay awake but won’t feel any pain. An ILR will continuously monitor your heart and record any unusual heartbeats. You can also start a recording if you notice any symptoms. Your ILR will stay in place until your doctor has been able to diagnose any problems. It will then be removed in a procedure similar to when it was put in.

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What to expect afterwards

Your doctor will use your ECG test results to help assess if you have a heart condition. Sometimes, your doctor may discuss the results of the ECG with you straight away, but sometimes it needs to be sent to a specialist to check. The results will then be sent to the doctor who ordered your test.

An ECG can give doctors an idea of if there are problems with your heart, but it doesn’t always give the full picture. Heart problems don’t always show up on an ECG. If your resting ECG looks normal, you may need more tests – including other types of ECG – to look into what’s causing symptoms.

On the other hand, your ECG may look abnormal even if your heart is healthy. You may need to have other tests, such as an echocardiogram, to find out more.

What are the risks?

A resting ECG is a very simple procedure and is completely painless. The recording machine doesn’t send any electricity into your body – it just records your body’s own electrical activity.

When the electrode stickers are removed this might feel a little bit uncomfortable, a bit like having a sticking plaster removed. It’s possible you might have some redness and swelling on your skin afterwards because of a reaction to the adhesive used – but this is rare.

There’s a very small risk that the extra strain on your heart during an exercise ECG may trigger problems with your heart. This is very unlikely though, as you’ll be monitored at all times during the test. If your doctor, nurse or technician thinks there’s any risk of you becoming unwell, they’ll stop the test.

Frequently asked questions

  • An ECG records the electrical activity that spreads through your heart as it beats. The recording looks like a wavy line, with a series of bumps and spikes. These bumps and spikes relate to the different phases of your heartbeat. By looking carefully at this recording your doctor gets a lot of information about how well your heart is working.

    A normal ECG follows a similar pattern in everybody. If you have a problem with your heart, the pattern may look different. The waves may be too big or too small, too close together or too far apart, some of the bumps may be missing. How the waves look depends on what’s wrong with your heart. See the next FAQ to find out more about the medical conditions an ECG can help your doctor diagnose.

  • An ECG can show if you have any of a number of heart conditions, how severe these are and what kind of treatment you may need.

    An abnormal ECG can be caused by various heart conditions, including:


    But bear in mind that an abnormal ECG doesn’t always mean that there’s something wrong with your heart. Other things that can sometimes cause changes in your ECG include:

    • certain medicines, such as antidepressants and drugs for arrhythmia
    • having higher or lower than normal levels of certain minerals such as magnesium and calcium
    • other medical conditions such as pulmonary embolism and stroke

    Your ECG can also be affected by things like if you’re anxious or if you’ve had a hot or cold drink. Your doctor will take all these things into account when analysing your results.

  • If you feel really tired, dizzy or unwell, you can ask to stop. But you’ll be encouraged to do as much as you can so you get the most value from the test. If you’re unable to finish, you may be referred for other tests such as scans of your heart.

    An exercise ECG is designed to see how well your heart works when you’re being active and putting a bit more strain on your heart. As the exercise gradually increases during the test, it may make you feel uncomfortable and you may feel a little out of breath. But it shouldn’t be more than you can manage.

    You’ll be carefully monitored during the test. The nurse, technician or doctor doing the test will check your blood pressure, heart rate and ECG throughout. They’ll stop the test if they have any concerns for your health.

    You may find it helpful to know that most people find they can finish an exercise ECG test.



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  • Reviewed by Pippa Coulter, Freelance Health Editor, June 2020
    Expert reviewer, Dr Tim Cripps, Consultant Cardiologist
    Next review due June 2023

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