How does an ECG work?

Chris gets his heart checked out with a standard ECG test.
13 June 2016

Interview with 

Dr James Rudd, Addenbrooke's Hospital


ECG stands for "electrocardiogram" and this is the heart trace corresponding toECG heart the electrical signals produced by the heart as it beats. To find out what it's like to undergo the procedure, Chris Smith turned himself into a medical guinea pig and went along to Addenbrooke's Hospital in Cambridge...

James - I'm James Rudd. I'm a consultant cardiologist at Addenbrooke's Hospital in Cambridge, and we are in the ECG recording department and we are going to record your ECG today. So, if you'd like to follow me, we will head through into the examination room...

This is Laura. She is a cardiac physiologist and she's going to be recording the ECG today for you.

Chris - Hi Laura.

Laura - Hi Chris.

Chris - So what do you want me to do?

Laura - What I need you to do is remove the clothes from your top half so I can get to your bare chest...

Chris - Ok, I'm down to my bare chest...

Laura - Ok. So if could lie down so that your heads on the pillow and just make yourself nice and comfortable.

Chris - I don't normally get to lie down at work in the hospital.

Laura - Are you comfortable now?

Chris - I'm very comfortable.

Laura - So, I just need to shave your chest so that I can pop the stickers on.

Chris - When you say stickers?

Laura - The electrodes for the ECG.

Chris - Right, OK. I didn't realise that I was going to get a shave when I came to work.

Laura - Yep. That's a hairy chest there. So are you ready for this one?

Chris - I think  so...

Laura - Just on the other side... There we go.

Chris - So you've just taken a little bit right in the middle, basically between the beast, isn't it?

Laura - Yes, just either side of your sternum there.  Are you allergic to alcohol wipes or disinfectants?

Chris - I'm not allergic to alcohol - I know that for a fact. Alcohol wipes - no fine.

Laura - No bar here, unfortunately. I'm just going to wipe the areas where I'm going to put the electrodes, just to make sure there's a good contact with your skin, so it might feel a bit cold now. OK?

Chris - You're also cleaning up my ankles?

Laura - Yes, so stickers go on your arms and legs as well as on your chest. Right, so lot's of stickers now...

Chris - They look like they've got a sort of jelly on the back - the sticky with the jelly?

Laura - Yes. There is a gel and that makes a contact with your skin to pick up the electrical activity.

Chris - So, there's one gone on my left arm at the top on my shoulder, one gone on the right side of my chest. That's two on the chest. Is there a particular place you put them?

Laura - Yes. Each sticker has a specific position so that each ECG we do is exactly the same for every person. So I'm just going to get all the wires now and I'm going to place these on all of the stickers.

Chris - They've got almost like little crocodile clips like you'd connect up to battery or something on the end of the wires. They're going on these tabs?

Laura - They clip onto the stickers. They just stick onto the electrodes. OK?

Beep, Beep...

Chris - OK. So now I have wires everywhere?

Laura - Yes, ten wires. So now I need you to lie back into the bed and just relax as much as you can so that it makes a nice, clear recording...

Beep, Beep, Beep...

Laura - And that's your ECG done.

Chris - Right, now it comes to the verdict! I'm going to talk to the cardiologist and find out what is shows...

James - So we've got your ECG in front of us now Chris, and what we can see is a piece of graph paper with some black lines on. In essence, we've got a plot of voltage against time. So this is a snapshot of your heartbeat over about five seconds and, as you may be aware, the heart is a large muscle, it's about the size of your fist. And within the heart are specialist cells which are called pacemaker cells and these cells generate very small voltages which we can pick up on the surface of the skin using the ECG test.

Chris - OK. Can you then take me through James, how what we're seeing on this piece of paper relates to what my heart is doing.

James - So, the very first part of the trace here. I'm just pointing to what looks like a humpedback bridge. This is called a P wave and this actually happens when the atria at the top of the heart are full of blood and begin to pump the blood into the ventricles, and the atria are the holding chambers, if you like, at the top of the heart and they receive blood from the rest of the body and also from the lungs.

The next thing we see is a very sharp up and then down stroke and this is when the main pumping chambers of the heart, the ventricles, start to contract. They expel the blood around the body and they also push blood to the lungs as well.

Finally, we have another humpback bridge. This is called the T wave of the ECG and this is when the electrical activity of the heart is resetting itself back to normal, and the ventricles and atria are relaxed and getting themselves ready for the next heartbeat.

Chris - How could a cardiologist like you then take that trace and see when a person has a problem?

James - So there are several elements we can look at. We can look at the heart rate, we can look at whether the heartbeats are happening regularly or irregularly, and we can also actually look at the waves themselves. Sometimes they have unusual shapes, which we recognise as being a signal of an abnormality in the heart.

Chris - If I had heart disease. Say a third up coronary artery - not enough blood is getting to my heart. Would you be able to tell that from an ECG?

James - In most cases we can, yes, and it's always the first test that we do.  As soon as you come into hospital, we would do an ECG, particularly if you had symptoms of a heart problems like chest pain or palpitations. An ECG is a really quick, inexpensive tool for giving us a clue as to exactly what's going on with the heart.


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