Electrocardiogram (ECG)

Expert reviewer, Dr Tim Cripps, Consultant Cardiologist
Next review due December 2020

An electrocardiogram (ECG) is a test to record the rhythm and electrical activity of your heart to see if it’s healthy.

Having an ECG doesn’t hurt. An ECG machine just records the electrical impulses your heart makes naturally – it doesn’t put any electricity into your body.

During the test, sensors are attached with sticky pads to your chest, arms and legs. These pick up electrical signals and send them back to the ECG machine. Your doctor will then carefully examine the readings to see if they show a problem.

An electrocardiogram (ECG) reading

Why might I need an ECG?

Your GP, or any other doctor looking after you, may recommend you have an ECG if you have symptoms of a heart condition. These might include chest pain, palpitations (abnormal heart rhythm), shortness of breath and dizziness. You may also have an ECG if you’ve been diagnosed with high blood pressure.

Sometimes an ECG is one of the general health tests you have before an operation.

An ECG can show 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 an ECG shows, and which medical conditions 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 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. It shows how your heart copes under stress. This test helps to find out if you have coronary heart disease, which is when the arteries to your heart become narrowed.
  • An ambulatory (24-hour) ECG – for this 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 cardiac event recorder. These 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) are devices 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.

See our section below on what happens during an ECG for more information about these.

Preparing for an ECG

You don’t normally need any special preparation for a resting ECG. You may have this test carried out at your GP practice. Don’t use any oils or moisturisers on the skin when you go for the ECG, or the sticky pads may not stick. Try and wear loose clothing and not too many layers as you’ll have to undress to the waist to have the ECG done.

With other types of ECG, your hospital will give you information 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.

If you’re having an ambulatory (24-hour) ECG, you may want to have a bath or shower before you go. This is because you won’t be able to have a bath or shower while the ECG recording equipment is attached.

What happens during an ECG?

Your doctor, nurse or technician will discuss with you what will happen before, during and after your procedure. This is a chance for you to ask questions. 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 about 5 minutes to have a resting ECG. It’s a simple, painless test.

Your nurse, technician or doctor will ask you to take off your clothing above your waist and lie on a bed or couch. They’ll stick some patches called electrodes onto your arms, legs and chest. If you have a hairy chest, they might need to shave some small patches to help the electrodes stick to your skin. Once you have the electrodes on, your doctor, nurse or technician will cover you with a gown if you wish.

Whoever is doing your ECG will then attach the electrodes with wires to a recording 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.

Exercise ECG

An exercise ECG usually takes about 20 minutes. Electrodes will be attached to you from the recording machine in the same way as in 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 and heart rate. The test will finish when whoever is doing your test has the readings they need. This is often when you reach a level of exercise that’s been decided in advance. The test is not to check the amount of exercise you can complete. It’s just to record your heart when you’re doing as much as you safely can. So don’t worry if you have to stop after a short time, as long as that’s your maximum possible effort.

They may also stop the test if your blood pressure changes, you have chest pains or your ECG tracing shows particular changes.

You can ask for the test to be stopped at any time if you feel unwell or cannot continue. See our FAQ below for more information about what happens if you can’t complete the exercise ECG.

24-hour ECG

For this test, you’ll wear a small, portable recorder that’s attached to a belt you wear around your waist. Some small patches (electrodes) will be stuck onto your chest and connected to the recorder with wires.

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 test 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 carry with you all the time for up to a month. You may be given a recorder to wear on your wrist. Or you may have electrode stickers placed on your chest and then connected by wires to a device at your waist. You’ll be told exactly what to do to make sure it records your heart’s electrical activity when you have symptoms. The results from the ECG recordings need to be sent to the hospital by telephone – you’ll be shown how to do this.

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 have a local anaesthetic so you’ll stay awake but won’t feel any pain while this is being done. 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 until your doctor is happy that enough recordings have been made. It’s then removed in a procedure similar to when it was put in.

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What to expect afterwards

Your doctor will look at the ECG recording to get an idea of whether you have a heart condition. Depending on where you have your test and who does it, you may get the results straight away or at your next appointment.

Heart problems don’t always show up on an ECG. On the other hand, your ECG may look abnormal even if your heart is healthy. You may need to have more tests, such as an echocardiogram, to find out more if your doctor thinks there’s a problem. Or, if your heart looks normal, to look into what’s causing symptoms.

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. Rather like having a sticking plaster removed.

There’s a very small risk of complications during an exercise ECG. The extra strain on your heart from exercising may cause:

  • abnormal heartbeats
  • chest pain (angina)
  • a heart attack

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 are waveforms that 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 varies little from person to person. Each beat produces the same pattern. 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, or 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 a number of heart conditions, how severe these are and what kind of treatment you may need. But bear in mind too that an abnormal ECG doesn’t always mean that there’s something wrong with your heart.

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

    • abnormal heart rhythms (arrhythmias), such as ectopic beats, atrial fibrillation, atrial flutter, ventricular tachycardia and heart block. See our topic on arrhythmias for more information about these.
    • a heart attack
    • coronary heart disease
    • heart failure
    • problems with your heart muscle (such as thickening) and heart muscle disease (cardiomyopathy)

    Some medicines can also cause an abnormal ECG.

  • 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.

    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.

    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. Tell them if you have any symptoms, such as chest pain, trouble breathing or if you feel dizzy. They’ll stop the test themselves 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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Related information

    • Clinical guidelines by consensus: Recording a standard 12-lead electrocardiogram. Society for Cardiological Science and Technology., published September 2017
    • ECG: A methodical approach. PatientPlus., last checked 19 May 2016
    • Cardiovascular medicine. Oxford handbook of clinical medicine (online). Oxford Medicine Online., published September 2017
    • Assessment of palpitations. BMJ Best Practice., last updated November 2017
    • Treadmill stress testing. Medscape., updated 26 December 2016
    • Exercise tolerance testing. PatientPlus., last checked 2 April 2015
    • Ambulatory ECG monitoring and related investigations. PatientPlus., last checked 25 January 2013
    • Cardiomyopathies. PatientPlus., last checked 20 June 2014
    • Electrocardiography. The MSD Manuals., last full review/revision September 2017
    • Stress testing. The MSD Manuals., last full review/revision September 2017
    • What is an electrocardiogram? British Heart Foundation., accessed 19 December 2017
    • Tests. British Heart Foundation., accessed 19 December 2017
    • What is a stress test? British Heart Foundation., accessed 19 December 2017
    • Cardiac event recorder. American Heart Association., updated 21 December 2016
    • Electrocardiogram (ECG or EKG). American Heart Association., updated 11 September 2015
  • Reviewed by Dr Kristina Routh, Freelance Health Editor, Bupa Health Content Team, December 2017
    Expert reviewer, Dr Tim Cripps, Consultant Cardiologist
    Next review due December 2020