An ECG takes five minutes and answers some questions completely while leaving others untouched. Here is what it can tell you, and what a normal result does not rule out.
An electrocardiogram records the electrical activity your heart is already producing. Ten stickers on the skin pick up those signals from twelve different angles, and the machine prints the result as a trace. Nothing is passed into the body, there is no radiation, and it takes about five minutes.
What it answers well
- Rhythm — whether the heart is beating regularly, too fast or too slowly, and where the rhythm is coming from.
- A heart attack happening now. This is the single most important thing an ECG does, and it does it within minutes.
- A heart attack that happened in the past, sometimes one the person never knew about.
- Thickening of the heart muscle, usually from years of high blood pressure.
- Conduction block, where the electrical signal is delayed or interrupted on its way through the heart.
- Effects of potassium and calcium disturbance, and of certain medicines.
What it cannot tell you
This is where most misunderstandings arise, and it matters more than the list above.
The ECG also records only the seconds it is running. A rhythm disturbance that comes and goes will be entirely invisible unless it happens during those seconds — which is why Holter monitoring is arranged for palpitations rather than repeating ECGs.
And it says nothing directly about the pump or the plumbing. It cannot measure how strongly the heart squeezes, cannot show the valves, and cannot display the coronary arteries. Those need an echocardiogram and, where indicated, angiography.
On being told your ECG is abnormal
ECGs are reported as abnormal for a great many reasons, and a substantial number of those are harmless variations, long-standing patterns of no consequence, or artefact from movement during the recording. Automated interpretations printed at the top of the trace are known to over-call abnormalities, and they are generated by software, not read by a doctor.
What makes an ECG meaningful is context: your symptoms, your examination, and comparison with a previous trace. An old ECG showing the same pattern turns an alarming finding into a stable one. This is the practical reason to keep your old reports and bring them.
The most valuable ECG is often the one taken a year ago, sitting in a folder at home.