What is an ECG

What it is

An ECG - electrocardiogram, also written EKG - records the heart's electrical activity through small sticky electrodes placed on the chest, arms, and legs. Every heartbeat is driven by an electrical signal, and the ECG draws that signal as a waveform.

The test is quick, painless, and involves no radiation and no electricity going into the body: the electrodes only listen. A standard recording takes about ten seconds once the electrodes are on.

What it shows

The ECG is the first-line test for heart rhythm problems: it shows whether the rhythm is regular, too fast, too slow, or chaotic, and it identifies specific arrhythmias such as atrial fibrillation.

It also shows signs of a heart attack - both one happening now and evidence of an old one - and clues about enlarged heart chambers, poor blood flow to the heart muscle, and some effects of medicines and electrolyte imbalances.

Because it is fast and available nearly everywhere, the ECG is usually the first heart test done in chest pain, palpitations, fainting, and shortness of breath.

Variants: longer recordings

A standard ECG captures only a few seconds, and problems that come and go often slip past it. For those, longer recordings exist: a Holter monitor records continuously for a day or two, event recorders are worn for weeks and activated during symptoms, and implantable loop recorders can watch for years.

An exercise ECG (stress test) records while the person walks or cycles, to see how the heart's electrical behavior changes under load.

Reading it carefully

ECG interpretation is a skill: the same squiggle means different things in different clinical contexts, and automated readings printed by the machine are only a draft that a clinician must review.

A normal ECG does not rule out heart disease - many people with significant coronary disease have normal resting tracings between events. And an abnormal ECG does not always mean disease: athletes, for example, commonly show patterns that would be concerning in others.

The ECG is a powerful first look, not a verdict. It points the next step: reassurance, monitoring, imaging, or urgent treatment.