What’s an ECG?

You asked: What’s an ECG?

ECG Picture

The Basics

An ECG, or electrocardiogram – also known as EKG – is a picture of the heart’s electricity at any moment in time. Just like how your phone or camera creates an image from capturing light of different wavelengths that we see as colors, an ECG captures electrical activity from different parts of the heart and turns that into some squiggles on a piece of paper that represent the amount of electrical activity at any one moment in time traveling through the heart. As you’re probably well aware, we live in a 3D world so just how you can take a picture of the same object from different angles, an ECG can take pictures of the heart’s electricity from different angles to provide different information about where the electricity is going and when. With ECGs, we call those different angles leads of the ECG. A standard medical ECG contains 12 leads, or 12 different viewing angles.


If you’ve ever had an ECG done on you or wondered what the difference is between the one your doctor gets and the one you might get with an AliveCor device or the new Apple Watch 4, the answer is simple. The one you get with these consumer friendly products are generally a single lead. That’s good enough for telling certain information about your heart like how fast it’s beating and what the heart rhythm is, but a lot of information about your heart is left out. For example, a 12-lead ECG can give us information about your heart’s size, your heart rhythm, whether you’ve had damage to your heart such as heart attacks or blockages, and whether there are any abnormalities in your heart’s electricity (called the conduction system).

The Details

For those who are curious about what we look at and what it all means, here’s an example of an ECG and what it all means. For a little background, it might be helpful to check out these overviews of how the heart works and how the heart knows to keep beating first.

The normal way the heart beats starts at the top of the right atrium in a place called the sinus node. The electricity from the sinus node then travels through the right and left atria (the top chambers of the heart) and then down to the atrioventricular (AV) node, which is a pit stop for electricity before it goes down to the ventricles. The ventricles are where the bulk of the muscle is in the heart and are responsible for pumping blood out to either the lungs on the right side or the entire body on the left side. After the heart muscle has finished conducting electricity for that beat, called depolarization, it has to get ready to conduct electricity for the next beat, called repolarization.

Remember that the 12 different leads are just different “camera angles” of the same thing. Here’s what they represent:

P wave


The P wave is the part of the ECG that represents when electricity is traveling through the atria. This is normally the first part of the heart’s cycle.


QRS Complex


The QRS Complex is the part representing the electrical conduction through the ventricles. It’s taller than the P wave, which means there is more electricity. That’s because there is much more muscle present in the ventricles than the atria. The ventricles do all the hard work!


T Wave


The T Wave is the last part of the heart’s electrical activity that we capture with the ECG and represents what we call repolarization of the ventricles. This refers to all the cells moving ions like sodium, potassium, and calcium around so they can get ready for the next beat of your heart.


Now that you know the basics of ECGs, you can read your own next time you’re at Heartbeat!

Do you have questions for us? Ask us here.