What Is an ECG and What Does It Show?

Ten stickers, a tangle of wires, a machine that beeps — and, for a lot of people, one quiet worry: is this thing about to send electricity through me? It's the most common misunderstanding about the ECG, and it's exactly backwards. An electrocardiogram — ECG, or EKG in the older spelling — puts no electricity into your body at all. It does the opposite: it listens to electricity that's already there. The lecture-style video at the top of this page, "How to read ECG Part 1," is the clinician's side of that listening — it works through an introduction, how rate and axis are read, and then chamber hypertrophy, bundle branch block, and myocardial infarction. Here's the plain-English version of what all of that means.

Your heart runs on signals — the stickers just write them down

Every heartbeat begins as a small electrical signal that spreads through the heart muscle in an orderly wave, telling it when to squeeze. Electrodes — the stickers placed on your chest, arms, and legs — pick that activity up through the skin, and the machine draws it as a repeating wave pattern called a tracing. A standard ECG views the heart's activity from several angles at once, called leads, which is why there are so many stickers for what feels like a very short test. If you'd like to see where they actually go, this site's ECG leads placement video shows the positions being set in just over a minute. The whole recording itself is brief and painless — you're simply lying still while the machine does the writing.

What a clinician is looking for in the squiggle

That jagged line carries more information than you'd expect. Its spacing shows the heart's rate — how fast it's beating — and whether the rhythm is regular or uneven. Its shape and timing hint at how smoothly the electrical wave is travelling: a detour or delay in the heart's wiring shows up as a pattern clinicians call a bundle branch block, and an enlarged or overworked chamber can change the tracing's proportions, which is the chamber hypertrophy section of the video above. The pattern can also suggest that heart muscle has been injured. That's why an ECG is one of the first tests done when a heart attack is suspected — our minute-by-minute guide to what happens during a heart attack shows where the tracing fits into that race against time. Nothing on the tracing is read in isolation, though; it's always interpreted alongside your symptoms and history.

The honest limits of a very short snapshot

Here's the flip side, and it's the second myth worth clearing up: a normal ECG is reassuring, but it isn't a certificate that everything about the heart is fine. The tracing only records the few moments while you're lying on the couch. A rhythm problem that comes and goes may simply not happen during those moments, and some conditions barely touch the heart's electrical pattern at all. When clinicians need a longer look, they may turn to extended monitoring — a small recorder worn while you go about your day — or to other tests entirely. Think of the ECG the way your care team does: a fast, painless first window into how the heart is working electrically, valuable precisely because it takes minutes and asks nothing of you but to lie still.

MedicalVideos.com has published trusted medical videos since 2007. The site was founded by Dr. Mohamed Ibrahim, MD, PhD — physician, researcher, and former Duke faculty.

Frequently Asked Questions

Does an ECG hurt, or will I feel electricity?
No. The electrodes only record the heart's own electrical activity from the surface of your skin — nothing is sent into your body. The stickers go on, you lie still briefly, and they come off again.

How long does an ECG take?
The recording itself takes only a few moments; with placing and removing the electrodes, the whole test is a matter of minutes. There's no recovery time afterwards — you carry on with your day as normal.

If my ECG is normal, does that mean my heart is fine?
It's a good sign, but it's a snapshot of a few moments, not a complete check of the heart. Your clinician reads it together with your symptoms and history and may suggest further tests or longer monitoring if something still needs explaining.

What's the difference between an ECG and an EKG?
None — they're two abbreviations for the same test, electrocardiogram. EKG comes from the German spelling, Elektrokardiogramm, and stuck in everyday use; you'll see both used interchangeably.

Educational only — this article is general information about ECG testing, not medical advice. What an ECG shows in your case, and whether you need one or further tests, should be decided with a qualified clinician who knows your health history.