What is an electrocardiogram (ECG)?

man lying on bed having an ECG

An electrocardiogram, or ECG, looks at your heart’s rate, rhythm and electrical activity. In the first of our new series, Professor Peter Macfarlane tells Senior Cardiac Nurse Emily Reeve what this simple test involves.

Why would I have this test?

An ECG is one of the most common heart tests. It is the only way of uncovering certain problems with the heart’s electrical impulses. There are a number of reasons why someone may have an ECG, including an irregular heartbeat, shortness of breath when they exert themselves, significantly high blood pressure, palpitations or a suspected heart valve problem. It can also be a useful way of ruling out problems. If your doctor recommends an ECG, think of it as a basic investigation.

What does the test involve?

You don’t need to do anything to prepare for an electrocardiogram

The ECG is a simple test, with 10 electrodes used to record 12 different views of your heart’s electrical activity. You don’t need to do anything to prepare for it. An electrode is attached to each ankle and wrist with sticky pads and six more are attached to the chest. The patient lies almost flat with the head and chest raised a little. Relaxing for a few minutes before the recording is made is important, as this allows the electrode connections to stabilise and means the ECG will be more reliable. During this time, your details can be recorded on the ECG machine.

Will it hurt?

No, it is completely painless.

Are there any after effects?

Very rarely someone may have a slight skin reaction to the electrodes, but normally there are no after effects.

What will it tell my doctor?

An abnormal ECG can tell your doctor if you have any of these issues:

  • irregular heartbeat (an arrhythmia)
  • problems with the spread of electrical activity within the heart
  • an enlarged heart
  • areas of the heart with reduced blood supply
  • a ‘silent’ heart attack (an interruption to blood flow in the coronary arteries without usual heart attack symptoms).

In an acute emergency, the ECG can help your doctor treat you. For example, the electrical source of a high heart rate may be located, which will determine appropriate treatment.

Will I need more tests afterwards?

Depending on the ECG results and the symptoms being experienced, you may need other tests. For example, if your ECG at rest is normal, but you experience chest pain when walking upstairs or uphill, you would probably be given an exercise ECG, which is recorded while you walk on a treadmill.

Depending on the ECG results and the symptoms being experienced, you may need other tests

In turn, this test might lead to additional investigations, such as a CT angiogram or a coronary angiogram to see for certain whether anything is obstructing blood flow to the hearts. 

If someone experiences palpitations (a noticeably rapid, strong or irregular heartbeat), then a longer ECG recording may be required to determine whether these are due to an abnormal heart rhythm. This recording can last for several days, using a small wearable monitor. This is sometimes called an ambulatory ECG or a Holter ECG after the person who invented the technique. There are different types of Holter recorders, but they all aim to detect ECG changes which may occur at random times.

What are the latest developments in this area?

There have been advances in miniaturising ECG equipment, particularly for long-term ECG recording. The term ‘wearables’ now describes, for example, a tightly fitting T-shirt with ECG electrodes built in so that it can be worn during sporting or other daily activities while an ECG is recorded. Other devices record only a single lead of an ECG using a mobile phone. They may be useful for a patient with an intermittent problem, so they can record the event easily when it occurs.

Can computers help doctors to interpret ECGs?

Yes – it is possible to build software into an ECG machine to give an interpretation of rhythm, conduction abnormalities and other forms of ECG abnormality such as enlargement of the heart. ECG results are dependent on age, sex and race. The computer can store normal limits based on these factors, and at the very least, an automated ECG interpretation gives your doctor a second opinion.

Can a computer also give inaccurate interpretations?

If an ECG has ‘noise’, perhaps due to poor contact between the electrodes and the skin, then additional waves may appear in the ECG and perhaps confuse the computer logic. The experienced eye can instantly recognise such problems, which might not always be recognised by a computer.

How is your work helping to improve the diagnosis of acute heart attack?

Our work in Glasgow has shown that there are differences in the normal ECG: (a) between men and women, and (b) depending on age. More recently, we have developed ECG databases in India and Nigeria, showing that different ethnic groups have different normal limits of ECG waves.

Thanks to our work, the internationally agreed definition of a heart attack, published in 2012, takes a patient’s age and sex into account to improve the ECG diagnosis of an acute heart attack. We have added race-based criteria locally in the interim. The patient’s treatment plan differs depending on the ECG appearances, so our work is helping to improve care for patients with heart attacks.

Professor Peter Macfarlane

Professor Peter Macfarlane 

Emeritus Professor, Honorary Senior Research Fellow, University of Glasgow

  • Vice President of the International Society of Computerized Electrocardiology.
  • Former president of the International Society of Electrocardiology and Board of Computing in Cardiology.
  • Author of more than 300 peer-reviewed papers and lead author of Comprehensive Electrocardiology.


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