The electrocardiogram. We see it in hospitals all the time. But how do you read it? What do the parts of the graph mean?
I promise that by the end of this video, you'll get it. So let's do it! Leslie Samuel here from Interactive Biology, where we're making biology fun. And let's talk about the electrocardiogram, the EKG or ECG for short. An electrocardiogram is a test that records the electrical activity of the heart.
Now in a previous video, we looked at how the signals of the heartbeat are generated. And for a brief review, the signal for the heart to contract, it starts with the SA node in the right atrium. That signal then spreads throughout the atria causing them to contract. Then the signal reaches the AV node, which then goes via the bundle fibers all the way to the Purkinje fibers, sending that signal throughout the ventricles for the ventricles to contract.
Now, if that sounds confusing, don't worry. I'll link to my video about that in the description for you to check it out. For now, let's continue with the EKG.
We use the EKG to test for irregularities in how the heart is functioning. You've probably either seen this firsthand in a hospital or maybe even on TV, but you can look at the electrocardiogram and it will tell you if the electrical activity of the heart is working the way it should. Now, the way we do this is by placing skin electrodes on different parts of the body. And these electrodes are able to record the electrical activity of the heart. And the result is that it gives you a graph that looks like this.
There's normally a beep associated with it, so let's add that in. Yep, that's the beep. Now, what I want to do is look at each component of the electrocardiogram and break it down.
Cool? All right, let's do it. The first thing we see is this little bump on the graph.
This is called the P-wave. Then we have this structure here called the QRS complex. And lastly, we have another little bump called the T-wave.
Let's start by looking at the P-wave. As a reminder, once the SA node generates that signal, the signal then spreads to the muscle cells in the left and right atria. This is because of a unique characteristic of cardiac muscle cells in that they are all electrically connected to the muscle cells around them.
So if you get a signal in one, it'll spread to the rest. It's all or nothing. And once that signal spreads throughout the atria, that electrical activity is recorded on the EKG as the P wave.
That's the first structure we see, and it represents the spread of the electrical signal throughout the atria or the depolarization of the atria. Now, going back to what we know about how that signal spreads, it then goes to the AV node, and then that signal spreads throughout the rest of the fibers to the ventricles. And as that signal spreads through the ventricles, that's shown in the EKG as the QRS complex. So the QRS complex shows us the depolarization of the ventricles. Now you'll notice that the QRS complex is significantly larger than the P-Wave.
Why is this? Well, if you look at the structure of the heart, what you're gonna see is that the ventricles are significantly larger than the atria. There's a lot more muscle there.
Now this makes sense because while the atria, they only have to pump blood right next door to the ventricles, those ventricles are responsible for pumping the blood. all throughout the body. So they need more muscle. And as a result, the signal that we see is gonna be larger than the signal from the atria.
It makes sense. Lastly, we have the T wave. Now, after depolarization, we have repolarization. And in this case, when the ventricles repolarize, we see this structure right here, the T wave.
And as a reminder, first depolarization happens, which then causes contraction. Then repolarization happen, which causes. relaxation. Okay, so you're probably wondering why we don't see the repolarization of the atria on the EKG. Well, it does happen, but here's the thing.
The repolarization of the atria happens while the ventricles are depolarizing. So it gets overshadowed by the much larger QRS complex, and we don't see it on the graph. So we have the P wave representing atrial depolarization, then the QRS complex representing ventricular depolarization, and the T wave representing ventricular repolarization, and that's the electrocardiogram, the EKG.
But don't stop here. I mean, you gotta understand how those electrical signals results in actual contraction of the heart. And that's what I talk about in this next video. So click over there and check it out.