What an ECG tells your doctor, and why it only takes minutes.

An ECG — an electrocardiogram — is one of the oldest tests in medicine and still one of the most useful. It is also one of the least understood by the people having it, largely because it takes so little time that it can feel like it cannot have told anyone much.

What it actually measures

Every heartbeat starts as an electrical signal that travels through the heart muscle in a set sequence and makes it contract. That electrical activity is detectable at the skin. An ECG places electrodes on the chest, arms and legs and records the signal from several angles at once, producing the familiar tracing.

Nothing is sent into you. The machine only listens.

What it can show

• Rhythm. Whether the heart is beating regularly, too fast, too slowly, or irregularly. Atrial fibrillation is often picked up this way, sometimes in someone who had no idea.
• Signs of strain or thickening of the heart muscle, which can reflect long-standing high blood pressure.
• Evidence of a past heart attack, sometimes one the person did not know they had had.
• Changes suggesting the heart muscle is not getting enough blood, which is why an ECG is done early when someone has chest pain.
• Conduction problems — delays or blocks in the electrical pathway.
• Effects of some medicines and electrolyte problems, which can alter the tracing in recognisable ways.

What it cannot show

This is the important half, and it is the part that gets glossed over.

A normal ECG does not mean your heart is fine. It records what was happening in those few seconds while you were lying still. Someone with significant narrowing of the coronary arteries can have a completely normal resting ECG, because the problem only declares itself under load. That is why an exercise stress test, a scan, or blood tests are sometimes the next step.

It also does not directly show the valves, or the pumping strength of the heart — an echocardiogram does that.

So an ECG is one piece of information, read alongside your symptoms, your history, your blood pressure and often blood tests. Your doctor is interpreting the whole picture, not the tracing on its own.

What to expect on the day

1. You will be asked to remove clothing from the upper body and lie down. A gown or sheet is provided, and you can ask for a chaperone at any point.
2. Ten sticky electrodes are placed — six across the chest, one on each arm and leg. Some chest hair may need trimming for them to stick.
3. You lie still and breathe normally for about ten seconds while the recording is taken. Talking, shivering or moving adds interference, so it is worth being still.
4. The electrodes come off, and that is it.

The whole thing usually takes a few minutes. It does not hurt. There is no radiation, no injection, no preparation and no fasting.

Because it is done here rather than referred out, your doctor reads it in the same visit and talks you through it while you are still in the room. If it needs to go to a cardiologist, it goes with the referral.

When one might be suggested

Common reasons include chest discomfort, palpitations or a sense that the heart is racing or skipping, breathlessness, dizziness or fainting, a heart murmur, monitoring of blood pressure or heart medicines, before some surgery, and as part of some workplace assessments.

The one thing that cannot wait

If you are having chest pain, tightness or pressure now — especially with sweating, nausea, breathlessness, or pain spreading to the arm, neck or jaw — call 000. Do not drive yourself and do not book an appointment for later in the week. Ambulance crews carry ECG machines and can begin treatment on the way.

General information only. This article is general health information for anyone who finds it useful. It is not advice about your own situation and it does not replace a consultation with a doctor who knows you. If you are worried about a symptom, book an appointment or call us on (02) 6530 7404. Last reviewed 14 July 2026.