ECG (Electrocardiogram)
An electrocardiogram (ECG, sometimes EKG) is a recording of the electrical activity of the heart. It is the most commonly used cardiac test — quick, painless, and gives a great deal of information in a few seconds.
What does an ECG show?
The ECG records the small electrical signals generated by the heart with every beat. From the shape and timing of those signals, a cardiologist can assess:
- Rhythm — the rate, regularity and origin of each heartbeat. Atrial fibrillation, supraventricular tachycardia, ventricular ectopy and many other rhythm problems are diagnosed on the ECG.
- Conduction — how the electrical signal travels through the heart. Heart block, bundle branch block, and pre-excitation (Wolff-Parkinson-White) are visible.
- Ischaemia and previous heart attack — characteristic changes can suggest reduced blood flow to the heart muscle, or evidence of previous damage.
- Chamber enlargement — left or right ventricular hypertrophy, atrial enlargement.
- Electrolyte disturbances and drug effects — for example abnormal potassium levels.
A normal ECG does not exclude heart disease — many cardiac conditions can be present despite a normal-looking ECG — but an abnormal ECG often guides the next step.
What are the different types of ECG?
- Resting ECG — a single recording (around 10 seconds) taken with the patient lying still. The standard test in any cardiology consultation.
- Exercise (stress) ECG — the ECG is recorded while the patient walks on a treadmill or rides an exercise bike, to look for changes that only appear with exertion. Increasingly replaced by stress echocardiography or CT coronary angiography for assessment of coronary disease, but still useful in selected cases.
- Ambulatory ECG monitoring — a recording over hours, days or even years to capture intermittent symptoms. See the dedicated page on ambulatory ECG monitoring.
What can I expect?
For a resting ECG, you will be asked to remove your top and lie on an examination couch. Ten small sticky electrodes are placed on the chest and limbs, connected to the ECG machine by wires. The recording itself takes around 10 seconds. There is no preparation, no needles, and no radiation. You can eat and drink normally before and after, and drive yourself home.