ECG (electrocardiogram)

How the heart works

About ECG

An ECG is a simple test to record information about your heartbeat and the rhythm of your heart. An ECG measures the electrical signals that cause your heart to beat. During the test, a number of wires are connected to your arms, legs and chest and these pick up the electrical signals. These signals can be seen on a screen or are traced out on a piece of paper.

You may have an ECG done at your GP surgery or in hospital. An ECG is one of the tests you may have if you’re taken to Accident and Emergency because you have chest pains or an abnormal heart rate.

There are a number of reasons why you may need to have an ECG. You may have one to check for problems with your heart if you're having symptoms such as dizziness, chest pain or an abnormal heart rate. You may also have one as routine before an operation or part of a health check.

An ECG can show a number of different heart problems including:

  • a previous heart attack or a heart attack that is happening at the time of the test
  • an enlarged heart that is working under strain
  • fast, slow or irregular heartbeats called arrhythmias

There are a number of different types of ECG. These are listed below.

  • The standard ECG, which is sometimes called a resting ECG. This is taken while you're resting.
  • An exercise ECG, which is taken while you're exercising. This shows how your heart copes under stress. The test can help to find out if you have coronary heart disease (when the arteries to your heart become narrowed). If you have recently had heart surgery or a heart attack, an exercise ECG can help your doctor to decide how much exercise it’s safe for you to do. It’s sometimes called a stress test or treadmill test.
  • A 24-hour ECG is a test where you wear an electronic recorder for 24 hours. It’s sometimes called a Holter monitor or ambulatory ECG. It shows the activity of your heart over a day and night. It's useful for showing irregular heartbeats which may only happen occasionally. It can be used for more than 24 hours, if necessary.
  • Cardiac event recorders can record your heartbeat over a longer period of time. There are two main types of recorders. Portable event recorders are devices which you hold up to your chest and turn on when you’re having symptoms. Implantable loop recorders (ILR) are devices put in under the skin on your chest. These continuously monitor your heartbeat and can stay in place for a year or longer. An ILR is useful for recording symptoms that don’t happen very often, such as dizzy spells or fainting.

Preparing for an ECG

A resting ECG can often be done at your GP practice. No preparation is normally needed for this.

If you're having an exercise ECG, 24-hour ECG or cardiac event monitoring, you will need to go to hospital to have the test, or to have the equipment fitted. You will have your ECG done, or the equipment fitted, as an outpatient and should follow any instructions given to you in your appointment letter.

If you’re having an exercise ECG, you should wear comfortable clothes and shoes. Don't have a heavy meal just before the test. You may be asked to stop taking some of your medicines a few days before your exercise ECG. Your doctor will tell you in advance if you need to do this.

Your doctor, nurse or technician will discuss with you what will happen before, during and after your procedure. This is your opportunity to understand what will happen, and you can help yourself by preparing questions to ask about the risks, benefits and any alternatives to the procedure. This will help you to be informed, so you can give your consent for the procedure to go ahead, which you may be asked to do by signing a consent form.

What happens during an ECG

Resting ECG

The standard, resting ECG takes a few minutes. You will be asked to undress to the waist and lie down on your back on a bed or couch. A number of sticky patches called electrodes will be stuck onto your arms, legs and chest. If you have a lot of hair on your chest, some small patches may need to be shaved to help the electrodes make contact with your skin.

The electrodes are attached to a recording machine by wires. When your heart beats, it produces electrical signals which are picked up by the electrodes and transmitted to the recording machine. The machine then prints a record of your heartbeat onto a paper strip or straight onto a computer. You should lie still and be as relaxed as possible when the recording is being taken. If you move or if your muscles are tense, this can affect the recording.

                            Resting ECG

Exercise ECG

An exercise ECG usually takes about 15 minutes. During the test, electrodes from the recording machine are connected to you with wires in the same way as a standard ECG. You will be asked to exercise, either by walking on a treadmill or cycling on a stationary exercise bike. You will start exercising gently at a 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 causes your heart to work harder.

Your doctor or technician will monitor your ECG every few minutes while you're exercising, along with your blood pressure and heart rate. The test finishes when the doctor or technician has the readings he or she needs. The test may also be stopped if your blood pressure changes, if you have chest pains or if you become short of breath. You can ask for the test to be stopped if you feel unwell.

                                                           Exercise ECG

24-hour ECG

For this test, you will be asked to wear a small portable tape recorder, attached to a belt around your waist. Wires from the recorder are connected to three or four small sticky patches (electrodes) that are taped onto your chest.

While you’re wearing the 24-hour recorder, you can go about your normal activities for the day. However, you shouldn't have a bath or shower with the recorder on. During the test you may be asked to keep a diary of everything that you do and note when you have any symptoms. At the end of the 24 hours, you can remove the electrodes and recorder and return it to the hospital.

                                                               24-hour ECG

Cardiac event recorders

A portable cardiac event recorder is a small electrical device that you carry with you at all times. When you have symptoms, such as palpitations, you place the device on your chest and switch it on to record your ECG. You then contact the hospital and they will tell you what you need to do to get the readings to them. Your doctor or a technician at the hospital can then analyse your results and tell what to do next.

An ILR is a small, slim device that is inserted just under the skin on the front of your chest. This is done using a local anaesthetic. This completely blocks pain from the chest area and you will stay awake during the procedure. An ILR continuously monitors your heart and records any unusual heartbeats. You can also start a recording if you notice any symptoms.

What to expect afterwards

If your GP carries out your ECG, he or she may discuss the results with you immediately after the test. If you have the test in hospital, your results will be sent to the doctor who requested your test, and he or she will discuss them with you at your next appointment.

If your doctor thinks there is a problem, you may need to have further heart tests.

If your ECG is normal, your doctor may suggest other tests to find out what is causing your symptoms.

What are the risks?

A standard ECG is a very simple procedure and is completely painless. The recording machine can't give you an electric shock or affect your heart in any way.

There is a very small risk of complications during an exercise ECG. The extra demand on your heart from exercising may cause shortness of breath, abnormal heartbeats (arrhythmias), chest pain (angina) or a heart attack.

You will be monitored at all times during the test and told to stop if the technician or doctor thinks there is a risk of you becoming unwell. A medical team will always be on hand in case of an emergency.

How will the doctor know if there is something wrong with my heart after having an ECG?


The print-out that is produced at the end of your electrocardiogram (ECG) shows the rhythm and rate of your heart and may show evidence of abnormalities such as a previous heart attack or a thickened heart wall (left ventricular hypertrophy). Your doctor can read the ECG and identify any possible heart problems.


An ECG recording looks like a wavy line, with a series of bumps and spikes. These relate to the different phases of each heartbeat. Your doctor will look at each part of your ECG recording and can see if there are problems with certain areas of your heart, or with the way it's beating. In a normal heartbeat, the pattern of bumps and spikes is the same in each heart beat and similar in everyone.

However, if you have a problem with your heart, these bumps and spikes may look different. They may be too big or too small, too close together or too far apart, or some of the bumps may be missing. How the spikes and bumps look depends on what is wrong with your heart.

Looking at the ECG recording can give your doctor an idea of whether you have a heart condition. However, problems with your heart may not always show up on an ECG, or your ECG may look abnormal even if your heart is healthy. If your doctor thinks there is a problem, you may need to have further heart tests such as an echocardiogram.

What could be wrong with me if I have an abnormal ECG?


An ECG is an investigation to look for problems with your heart. It can show a number of heart conditions including a previous heart attack, thickening of the heart wall (left ventricular hypertrophy) and abnormal heart rhythms. However, an abnormal ECG doesn't always mean that there is something wrong with your heart, and a normal ECG may not mean that you don’t have heart disease.


An ECG can help your doctor to see whether you have any heart problems and if you do, how severe those problems are and what kind of treatment you may need.

There are a number of things that can cause an abnormal ECG, including various heart conditions, as well as other factors. These include:

  • abnormal heart rhythms (arrhythmias), such as ectopic beats, atrial fibrillation, atrial flutter, ventricular tachycardia and heart block
  • heart valve disease
  • acute or previous heart attack
  • coronary heart disease
  • abnormalities of the heart muscle (such as thickening) and heart muscle disease (cardiomyopathy)
  • certain medicines, including beta-blockers and digoxin

If your ECG is abnormal, your doctor will explain what will happen next. You may need to have further tests or you may need treatment. This could include medicines or surgery.

What happens if I'm unable to cope with the exercise in the exercise ECG?


If you become very tired, dizzy or feel unwell, you can ask for the test to be stopped. However, you should try to do as much as you can to get the most value out of the test.


An exercise ECG is designed to see how well your heart works when you’re active and your heart is under strain. As the exercise gradually gets harder during the test it may make you feel uncomfortable, for example, you may feel a little out of breath. The test isn’t like doing an intensive workout and most people find that they are able to do more than they thought and complete the test. However, it’s important to remember that it’s not a test for how fast you can complete the exercise.

You will be monitored all the time while the test is carried out. The doctor or technician carrying out the test will be checking your blood pressure, heart rate and ECG throughout the test. He or she will ask you to stop if they record a sudden change in your blood pressure or heart rate. The test will also be stopped if you have any symptoms, such as chest pain, trouble breathing or if you feel dizzy. You can ask for the test to be stopped at any time if you start to feel unwell.

If you can't do the test because you have a medical condition that prevents you from exercising, for example, severe arthritis, your doctor may arrange for you to have other tests instead. If you can walk easily, you’re fit enough to do the test.

If the technician or doctor carrying out the test has any concerns about symptoms you experience during the test, you will be examined by a doctor before you're allowed to go home.