An ECG is a recording of the electrical activity of the heart. The heart muscle contracts in response to certain electrical stimuli which are generated from a number of sources and which allow the rate and rhythm of the heart to be determined. When the heart muscle contracts, the electrical activity can be recorded.
Therefore an ECG gives a recording of how fast the heart beats and also assesses whether it is beating in a normal rhythm or an abnormal rhythm (arrhythmia). An example of a normal tracing from an ECG is shown in Figure 1.
If the heart is beating abnormally then this is easily detected and the cause can often be determined. When the heart is affected by certain diseases such as poor blood supply and a lack of oxygen, as occurs in angina then changes may appear on the ECG to confirm this.
Furthermore, when the heart is damaged as in a heart attack (myocardial infarction) then changes in the ECG are usually apparent. If the ECG is produced when someone is inactive this is called a resting ECG. As some abnormalities are only apparent when the heart is placed under stress then patients may be asked to exercise and this is called an exercise ECG.
Technique and preparation
No preparation is generally required. A standard ECG is a simple and short test. The patient is asked to lie on a couch. ECG leads (electrical wires that connect the patient to the ECG machine) are applied to various parts of the body and chest and a recording obtained.
If the resting ECG is normal an exercise ECG is obtained. The patient is monitored doing a standard set of exercises such as stepping up and down or walking on a treadmill until symptoms are reproduced. ECG recordings are taken at the end of the examination and at regular intervals following completion of the test.
Discomfort, risks and after affects
A resting ECG does not involve any discomfort. Obviously chest pain may be experienced at some point during an exercise ECG. There are few significant risks attached with the exercise ECG, although obviously patients should not be encouraged to exercise beyond certain levels and there is a small risk of precipitating a heart attack during a test.
