An electrocardiogram is commonly called an ECG or EKG and uses a machine to measure and record (on paper or a computer screen) the electrical activity in the heart. With each heartbeat, the heart emits a series of electrical discharge spikes that can be recorded using electrodes on the surface of the body.
What is the test used for?
The shape and pattern of the ECG spikes can assist in diagnosing a wide range of heart problems such as:
- muscle defect;
- enlargement of the heart;
- congenital defects;
- heart valve disease;
- arrhythmias (abnormal heart rhythms);
- tachycardia or bradycardia (heart rate too fast or too slow);
- ectopic heartbeat;
- coronary artery disease;
- inflammation of the heart (myocarditis);
- inflammation around the heart (pericarditis);
- changes in the amount of electrolytes (chemicals in the blood); and
- myocardial infarction (heart attack), past or present.
How the test is done
You will be asked to remove all clothes and jewellery to the waist (including bras in women) so the electrodes can be placed on specific sites on the chest wall, arms and legs. The electrodes are held in place by elastic straps, suction caps or sticky pads. Sometimes the skin may need to be shaved where the electrode recording patches are to be placed to make the electrical signal easier to detect and to reduce discomfort when the patches are removed. For a standard 12-lead ECG, the electrodes are placed on all 4 limbs and 6 positions on the chest wall. Additional leads are sometimes added for a 15-lead ECG.
You will be asked to remain still, breathing normally while the machine is started and a sample (usually 3 to 4 seconds) from each electrode site is recorded. The ECG is usually monitored continuously and you may be asked to hold your breath for short periods during the procedure (to stop the movement of your chest wall interfering with the signal). The machine will pick up electrical activity from the leads and then produce a graph consisting of a continuous up and down line that can look a little like a geographical map, but represents your heart's activity.
A normal healthy heart has a regular characteristic pattern, but any abnormality or damage to the heart will show up differently to the normal heart pattern. Letters of the alphabet (P, R, Q, S, T) identify the different spikes of the printout and these are then read by the doctor to indicate problem areas in the heart. The reading of an ECG is quite complex and may take time; doctors receive special training in this area. Some ECG machines have special software that helps with the interpretation of the test.
The ECG may be used continuously during some procedures and surgeries. People who are in hospital with certain conditions may also need continuous ECG monitoring. In an attempt to diagnose certain heart rhythm problems, portable ECG monitors may need to be worn for 24 hours or more to give an extended picture of the heart's rhythm.
Unfortunately, the ECG shows only the condition and function of the heart at the exact time that the reading is recorded — it cannot always predict what will happen to the heart in the future.
Specialised ECG recordings
- The ECG may be used continuously during some procedures and surgeries. People who are in hospital with certain conditions may also need continuous ECG monitoring.
- Ambulatory ECG: to help diagnose certain heart rhythm problems, portable ECG monitors may need to be worn for 24 hours or more to give an extended picture of the heart's rhythm, particularly when symptoms are occurring intermittently.
- Exercise ECG: An ECG recording is done as you exercise on a treadmill or exercise bike. This can help show the presence of narrowing of the coronary arteries.
Complications and risks
There are generally no risks associated with this test as the procedure only monitors the electrical impulses and does not emit electricity itself, so there is no danger of shock. Occasionally, the adhesive gel used at the electrode sites can cause some local skin irritation.
ECGs are not 100 per cent accurate because some heart conditions are not detectable. People with suspected heart disease or who have had a heart attack may need more than one ECG, but there is no need for regular testing in healthy people unless there is an inherited risk or it is needed for work or life insurance requirements.