An electrocardiogram (ECG) is a medical test that detects cardiac (heart) abnormalities by measuring the electrical activity generated by the heart as it contracts. The machine that records the patient’s ECG is called an electrocardiograph. The electrocardiograph records the electrical activity of the heart muscle and displays this data as a trace on a screen or on paper. This data is then interpreted by a medical practitioner. ECGs from healthy hearts have a characteristic shape. Any irregularity in the heart rhythm or damage to the heart muscle can change the electrical activity of the heart so that the shape of the ECG is changed. A doctor may recommend an ECG for people who may be at risk of heart disease because there is a family history of heart disease, or because they smoke, are overweight, or have diabetes, high cholesterol or high blood pressure. They may also recommend an ECG if a person is experiencing symptoms such as:
- chest pain
- shortness of breath
- fainting, or
- fast or irregular heartbeats (palpitations).
ECGs are often performed to monitor the health of people who have been diagnosed with heart problems, to help assess artificial cardiac pacemakers or to monitor the effects of certain medications on the heart.
The ECG procedure
There is no need to restrict food or drink before having an ECG test. Always let your doctor know what medications you are taking before you have an ECG, and if you have any allergies to adhesive tapes that may be used to attach electrodes.
When you go for an ECG test, you will need to remove your upper clothing so that electrodes can be attached to your chest and limbs. (For women, wearing a separate top with trousers or a skirt can allow easy access to the chest.) The selected sites are shaved, if necessary.
Electrodes (sensors) are attached to the chest, arms and legs with suction cups or sticky gel. These electrodes detect the electrical currents generated by the heart – these are measured and recorded by the electrocardiograph.
The three major types of ECG are:
- resting ECG – you lie down for this type of ECG. No movement is allowed during the test, as electrical impulses generated by other muscles may interfere with those generated by your heart. This type of ECG usually takes 5 to 10 minutes
- ambulatory ECG – if you have an ambulatory or Holter ECG you wear a portable recording device for at least 24 hours. You are free to move around normally while the monitor is attached. This type of ECG is used for people whose symptoms are intermittent (stop-start) and may not show up on a resting ECG, and for people recovering from heart attack to ensure that their heart is functioning properly. You record your symptoms in a diary, and note when they occur so that your own experience can be compared with the ECG
- cardiac stress test – this test is used to record your ECG while you ride on an exercise bike or walk on a treadmill. This type of ECG takes about 15 to 30 minutes to complete.