Normal 12 Lead ECG

Today we're going to take a look at a normal 12 lead ECG. I think it's really important to recognize what normal looks like and recognize it easily so that when you encounter abnormal, you'll recognize that right away. Having a good template of normal in your head makes it much easier to see abnormal. The ECG we're looking at today runs three channels. That means there are three channels running simultaneously and printing on the paper. The first channel starts, starts with lead I and then becomes AVR right about here, and then becomes v1 and then becomes v4. Simultaneously, the second channel is running, and it begins with lead 2, and then becomes AVL and so on. Then simultaneously, the third channel is running, and it begins with lead 3. The fact that these are simultaneous makes a lot of ECG interpretation easier because we know that each beat is represented in this case three times. In some ECGs you look at, you might see four channels, and then you see that beat four times. So the beats I'm circling are all actually depictions of one heartbeat or one moment in time. It's really great when you realize that some leads show things better than other leads. In this case, lead 3 doesn't show P waves very well. So if I had any concerns about whether or not there were P waves, I would just look straight up and then I would see that there are P waves up here in lead two. And so the patient has P waves. That's not a concern now.

So what is it that makes this particular ECG normal? Well, until you have a lot of experience and a lot of training, it's probably safe not to call an ECG normal or unremarkable. On your run report, there might be things that you don't know about and that another person, like a cardiologist, might say an abnormality. So it would be better for you not to say normal. But it's still a good idea to know what a normal ECG looks like and know the basic things to check to see if there are any abnormalities. First thing I notice here is that the rhythm is very regular. I don't have a rhythm strip at the bottom, but each of these channels really is a timeline and runs a straight rhythm strip all the way across from left to right. Just. Just have to be aware that at each lead change, the look of the QRs, the look of the Ps and Ts will change a little bit, but it's still a regular rhythm. I can easily see that the QRSS march out. The rate is 77. And also there are P waves in front of every qrs. So it's very easy to see here in lead one. But of course, they're also present in every lead. If they're present in one lead, they're present in all of them. So P waves are present. It's a regular rhythm, and the rate is 77, so we know we have normal sinus rhythm, and that is regular.

Another thing that I noticed that's normal about this ECG is the PR interval. In this case. I can look at the PR interval up here where the machine has read it, and it says 152. That's measured in milliseconds. If your machine measures in seconds, there will be a decimal point in front of that and the reading will be 0.152. A normal PR interval, you'll remember, is 0.12 to 0.20 seconds, which is three to five little blocks. So it's usually pretty easy to just look at the PR interval and see if it contains more than one big block from beginning to end. So there's the beginning of that P wave and there's the beginning of the QRS in AVR there. And I see that it doesn't go over one big block. So. So that makes it very easy to see that this PR interval is normal. The next interval to look at is the QRS duration, which is right here. The machine has measured it as 79 milliseconds, which is the same thing as 0.079, or you could say 0.08, which is a normal QRS duration. QRS duration is measured from the beginning of the QRS, and I mark it here in v1 to the end of the QRS, and it should be as skinny as possible. The more narrow a QRS is, the better it is, because that's telling us the health of the conduction system and specifically the bundle branches. If they're working top notch, they're going to be fast and conduct that impulse through the ventricles really quickly. And remember, time on the ECG is measured horizontally. So a narrow QRS is ideal and 0.08 is just fine. It's perfect. In another video, we'll get into QT intervals and QT corrected, and we'll talk about long QT syndrome. We'll save that for another. Another day. And also axis. This axis is represented in numerical, and so we'll save that because that's another whole discussion. But all of these are normal, so we'll leave it at that for now. As far as axis goes, the Quick and easy way to tell if the heart is depolarizing in the right direction. In other words, the heart is the right size, it's positioned correctly, and all of it is working. The conduction system is working properly. Simply look at leads one, two, and three, the first three in the column here. And lead two should be in the normal person. If everything is normal, lead two should be the tallest upright QRS of those three. There's a lot of variation between people, and it's okay if lead two and lead one are pretty similar. But a normal axis or normal direction of depolarization would result in a tall upright lead 2. So we like to see that, and we also like to see an upright P wave in lead ii, because that means the same thing is going on in the atria. The P wave is traveling from the SA node toward the AV node, which is down and to the left. And that should produce an upright P wave in lead two to be normal. And speaking of P waves, there should be a P in front of every qrs, even if they're a little hard to see, like lead three. We know lead three has a P wave because Lead II and Lead I do, but there should be a P wave in front of every qrs. And there is in this case, another very important screening for the field that needs to be done quickly but also accurately is screening for ST elevation. And I usually recommend that we look at the heart as a map, which we'll also do in another video. But if we look at the heart as a map, we look at the walls separately. So, for instance, leads 2, 3, and AVF represent a view or an electronic view of the inferior wall. So I'm going to circle those. This is what I mean by making a map.

And when I scan those three related leads for ST elevation, what. What I'm looking at is the J point. And the J point is right where the QRS ends and the ST segment begins. That can be a little hard to see in some leads, but in these three leads, we're not having any trouble at all. So that's the J point. The J point should be in line with the TP segment, which is right here. Nice straight line here. And that's when nothing is going on in the heart. So that should be your true baseline. If you have a tachycardia and you can't see the TP segment, it's too short, then your next best choice for baseline is to look at the PR segment, which is in front of the qrs. So what we want to see in a normal ECG is that the J point is in line with the baseline or the isoelectric line, not elevated and not depressed. And we're seeing that here. Another thing we want to see in a normal ECG is that the ST segments have a generally concave upward appearance. They might be a little flat in some leads. I wouldn't worry too much about it if there's just a slight change. But overall in general, throughout the whole ecg, we want to see a concave upward appearance, kind of like a little smile. Flat isn't good and convex upward isn't good. Would look like a frown. But once in a while, especially on the right side like V1 and lead three, which represent right sided leads. A little flattening isn't anything to worry about, but I would definitely look at the entire ECG for a general feeling of convex upward and J points not elevated. I would do that screening on all the walls. So we just did the inferior wall and you'd want to screw screen the high lateral wall, which is this one.

And you would want to screen AVR and V1 together because they represent a view of the high part of the anterior wall or the very proximal portion of the left coronary artery, which is a very bad place to have an mi. So you want to screen those. AVR looks a little bit frowning there, but not abnormally. And the J point isn't elevated, so I wouldn't be concerned about that. The anterior wall is represented by all the leads over on the right of the paper. And you know, it's such a large area that we break it down into segments. So V1 and V2 show us the septal area of the anterior wall or the anterior septum. And I want to screen them for us to changes. I then want to look at V3 and V4 together and screen them for ST changes. So put them in a little box together and then at the end we want to look at V5 and V6 together and check for ST changes. And of course, if we find ST elevation, then we would possibly be looking at a diagnosis of anterior wall mi. So when we've completed screening each area of the ECG for ST elevation, we also want to look for other abnormalities like T wave inversions and pathological Q waves, which we don't have in this ecg. If you do encounter some isolated T wave inversions or flattened T waves in lead III or in lead V1, you kind of see flat T waves in lead three here and kind of flat in V1, don't be concerned. That's fairly normal on the right side of the heart. And remember, Lead 3 and Lead V1 are both on the right, so I wouldn't be concerned about those. But if you have T wave inversions in any of the other leads, it could be a sign of ischemia or some other problem. So that would take us out of the realm of normal, and we have to investigate further.

There are no pathological Q waves in this ecg. In fact, this particular ECG has no normal Q waves either. Occasionally, we'll see a small downward deflection in lead two or sometimes in lead one, but it's fairly rare. And a little tiny Q wave is not a sign of any abnormality. It's just a sign of we're seeing the septum depolarize a little bit ahead of the rest of the heart. But if we see a downward deflection at the beginning of a qrs, and that downward deflection is at least one quarter of the entire height of the qrs, then we'd be concerned about an area of necrosis, and we don't have that here either.

We may not have covered everything that's normal about this ecg, but we've made a pretty good start. And I have one more thing to mention, and that is called R wave progression. When you look over at the chest leads, which are the V1 through V6 leads over here on the right side of the paper, v1, if you'll remember, is the electrode that you place to the right of the sternum in the fourth intercostal space. Since our heart primarily depolarizes from right to left, the left ventricle depolarizes from right to left. V1 is in a position to see that depolarization wave move away from it. And so v1 in a normal ECG is a negative lead or downward.

There should be a very small little R wave at the beginning of V1. That's an upward deflection, and that represents the septum of the heart. It depolarizes in the opposite direction from the rest of the left ventricle. So v1 would see the septum depolarizing toward its electrode and the rest of the heart away from. So V1 should be a little R wave followed by a deep S wave. So it would have a pattern of RS. Now, if you move over to V6 on the chest, you've moved to the underarm area in the mid axillary line. Now, the heart hasn't Changed just because we moved our electrode. The heart's still doing the same thing. And now we're standing in front of that depolarization wave. So it's the. The heart's depolarizing toward us, and that gives V6 a primarily positive or almost 100% positive deflection. In this case, it is 100%. That's normal. What else is normal is that we should see a progression from negative to positive as we go across the V leaves. Remember, you place them pretty symmetrically across the chest. They're evenly spaced in most cases. And so we should see the R wave gradually getting more prominent and the S wave gradually getting less prominent. By V3 or V4, we usually see about a half and half distribution of R to S. So you see V4 here is a little bit more R, a little bit less S. By V5, we've almost lost our S wave, and by V6 we've lost it. That is called R wave progression, and that's normal. If R wave progression isn't normal on your ecg, there's either a problem with the way the electrodes were placed on the patient, or there may be an abnormality in the heart. And so you should always double check your electrode placement before you accept that as your final ecg. Another thing worth mentioning about the V leads is that you might think that if a QRS is all upright, that it should be taller than all the other qrss. And it seems kind of strange that an all upright QRs like V6 would be shorter overall than V3, which is dividing its energy between up and down or negative and positive.

The reason for this is the heart is actually closer to the chest wall in the areas where you've placed V2, V3 and V4. So you're going to pick up more voltage than you would from V1 or V6. That's in a normal heart. The heart should be very close to the chest wall in the midclavicular line. So around V2 to V4, you should have very tall QRS compared to the others. You also may see taller T waves, and that doesn't mean there's something wrong with them. It just means that you're close to the heart, your electrodes picking up more of the energy. So don't be surprised at the size change. It's the progression from negative to positive that's really the important thing that we want to see. Thanks for watching. I hope this has helped and I hope you'll more easily be able to recognize normal features of the ecg so that then abnormal will be easier for you too.