12 Lead ECG Explained [Micro Lecture]

In today’s micro-lecture, Australian Paramedical College Hon. Snr. Lecturer Sam Willis talks about 12-Lead ECG Acquisition and Interpretation which in paramedic practice, helps trying to rule out whether the patient is presenting with signs and symptoms of a cardiac condition or if it is related to something else.

In today’s lecture, we’re going to talk about 12-Lead ECG Acquisition and Interpretation. My name’s Sam Willis and I’m a senior lecturer for the Australian Paramedical College. In your career as a paramedic, at some point in the assessment cycle, you are going to need to try and rule out whether the patient is presenting with signs and symptoms of a cardiac condition or whether those signs or symptoms are related to some other medical condition. Now of course one of the ways that we do this is through the use of a 12-lead ECG. Now of course the 12-lead is part of the assessment process. It does replace a physical examination and we would always teach you guys to treat the patient and not the machine.

In this session, we’re going to look at the importance of a 12-lead ECG machine and monitoring in helping you guys differentiate between cardiac and non-cardiac conditions as well as talking about when to acquire the ECG because we wouldn’t necessarily do it on everybody. We are going to talk you through the steps and we’re also going to talk you through the steps of interpreting the ECG as well.

Now I like to use a case study to highlight the need for doing an ECG. This is a typical case study that you guys will be called to once you’re state paramedics. You are called to a 65 year old male at his home address. At home, is complaining of left sided chest tightness. When you guys arrive, he’s holding his chest. He appears pale and sweaty and he’s telling you that he’s going to die. When you undertake a 12-lead ECG, you know it is the ST-segment elevation in leads II, III, and aVF. Now of course that’s a shortened version of it because these patients who are clearly having a STEMI, an ST Elevation Myocardial Infarction, they will usually be having nausea and vomiting. They will be quite anxious. They’ll have blue lips. They will be telling you that they’re going to die quite emphatically with emphasis and they’ll usually be quite aggressive as well. It’s your job to try and calm the entire situation down whilst being able to at the same time reassure the patient and give them the pharmacology that they need.

Let’s put this into context then. The 12-lead ECG really is a standard part of primary practice. Historically, paramedics used to only be able to access the lead II and we’ll talk about the difference between a 12-lead and a lead II later on. 12-lead ECG acquisition and interpretation has become an important part of primary care and therefore what that means is as paramedics, you need to be able to undergo a systematic approach to be able to apply and interpret the findings of the 12-lead ECG.

Now let’s try and remind you guys what a 12-lead is. Imagine this image here is a picture, lots of different pictures of the heart. Imagine if you take a camera and you take a picture this way and you press the button. Then you move it and you look this way and you press the button, and you move it this way and you press the button. That’s exactly what a 12-lead ECG is. It’s lots of different views of the heart. Now when I say the heart, I actually mean the electrical conduction system within the heart and not the heart itself. Actually, what you’re seeing here is the view of the movement of the electricity through the different regions of the heart. Now just in this image here, you have different leads. These are your 12 leads. Now ironically, it’s called a 12-lead ECG because you’re all seeing 12 different views of the heart, but there’s actually only 10 dots, 10 electrodes that you place on the chest. That’s what I find interesting but never mind, that’s avoiding the situation.

Lead I is here. Lead II is here. Lead is here. aVR is the fourth lead. aVL, aVF. Lead V1, V2, V3, V4, V5, V6. Now you will notice in this ECG that they look slightly different. I’m going to show you why. Now for those of you who have got any type of understanding of ECGs, you’ll see that this is a pretty bad ECG. This patient’s probably quite unwell. But the purposes of me showing you this one is for those of you who have never seen a 12-lead ECG before. This is what an ECG looks like. First and foremost, you have this little square symbol here. This tells you that the paper speed is 25 millimeters per second and that the machine is being calibrated. Then you have all these different views. Remember what we said about looking in the different angles. That’s what this is showing you.

Now we can’t actually have a 12-lead ECG session without reminding you guys about the conduction system within the heart. Now we’re not going to spend a prolonged period of time going back over the basics, but instead just to remind you guys that it all starts here at the sinoatrial node or the pacemaker. It all starts here. The electricity then moves down these conduction fibers, these bundles, down to the atrioventricular node. Then down to the bundle of His and into the left and right bundle branches. Then finally into the Purkinje fibers. What you should probably notice by now is that the electricity is moving in different directions. Here it’s moving diagonally downwards whereas here it’s moving left and right. Then it’s moving downwards again and a little bit to the left and the right. Then it’s moving back up towards the sinoatrial node. Now that’s exactly what this next slide shows.

The direction of the electricity is referred to as the cardiac axis. Now the cardiac axis tells you and shows you the direction of the electricity. Now to try and make this as simple as possible, I want you to try and think back to your earliest studies of ECGs. Now if this is the first session you’re having, I’m going to try and do my best to explain as much as I can. Now the cardiac axis, as I’ve said, really just refers to the direction of the electricity. Don’t make it any more complex than that. Now this triangle here on the screen, it’s called Einthoven’s triangle. I’m glad my brain kicked in there.

Imagine that you’ve just put three sticky dots, three electrodes onto your patient. One on the patient’s right arm, one of the left arm, one on the left leg. Now what you’ll find is that the electrical energy coming from the sinoatrial node and from the image that I just showed you, from the electrical conduction system, it will be picked up in this triangle. The electrodes really do pick up the electrical signal and they read the electrical signal and it’s presented on a piece of paper as you can see here. Now go back to the ECG slide that I showed you a moment. Look here. Lead I, lead II, lead III, aVF, aVL, and aVR. What we can see form this image is that we’re looking in lead I at the energy moving across like that. If you remember back to this image here, it’s really this area here. Lead I, again, that’s lead I there, is showing the movement of electricity going across the heart and so on and so forth down to lead II, down to lead III, and so on and so forth.

This is just another view of what I’ve just told you. Here you can see the electrical movement, the movement of the electrical activity. These relate to the different leads within the body, within the ECG. Cardiac axis. Whenever somebody says to you cardiac axis, all you need to understand is that it’s the movement of the electrical signal moving through the different parts of the conduction pathway.

Imagine this. You’ve managed to get yourself a 12-lead ECG. We’re going to talk a little bit in a moment about how we do that and the process and the steps. Then what you need to do is to be able to interpret it. Now in a moment, we’re going to talk through the different steps of interpreting a 12-lead, but once you’ve got that, you need to relate it to this. These really are the locations. Lead II, III, and aVF, these are referring to the inferior part of the heart. Likewise, the green parts are the lateral parts. Anterior, septal. So this is the septum of the heart. Then aVR really is just an inverted lead V2. If we go back up here. aVR is an upside down version, it’s a mirror image of lead V2. Really the only time you tend to see a positive and when we say positive, we mean these guys are all positive. Anything about the isoelectric line, it’s a positive deflection. The only times we tend to see a positive aVR is really only dextrocardia and a number of other very, very rare medical conditions.

Let’s talk about undertaking an ECG then. What you need is a set of electrodes or a sticky dot, whatever you want to call them. Now you can use the word sticky dot to your patient by all means, but medically speaking, they are electrodes. You need the leads and you need the machine to be able to undertake it. There are other things that you need as well if you’re going to go through the process from start to finish, such as handy contamination gel. You’re going to need your gloves. The gloves are not used to keep you or your patient clean unless of course they’re indicated for it, but some people like to use the gloves because it helps to make a very intricate and delicate and careful procedure seem a lot more professional, but that’s really a personal thing because again, if your patient’s not indicated for gloves, then you don’t need to. But imagine doing a 12-lead on a female. Of course, it makes complete sense that you’d put the gloves on to help the female patient feel much more relaxed.

Now the role of the electrode. On the backside of the electrode is a sticky gel. The role of the electrode is to collect the electrical signal which will then be sent down the wires and interpreted using a complex set of algorithms into the ECG. Now when you’re undertaking ECG interpretation, never try and interpret an ECG from the screen itself. Always print it off because it can look completely different, not to mention the fact that it’s difficult to list an entire 12-lead on the screen. These leads are different colors for a reason. However, just be mindful that different manufacturers do create different colors, so where possible, always go with the lettering that’s on the ECG leads. For example, if it says LL, it means left leg, left leg or left leg.

Here we have typical electrode placement. Now the first thing you’ll do is to put the … These are called the [inaudible 00:11:57] leads. If you weren’t going to put the chest leads on or the anterior leads on, then all you need to do just to do a basic interpretation is to put these four leads on. Now in workshop one, we’ll be demonstrating this among your peers so you’ll be recognizing the indications for undertaking an ECG on a patient as well as being able to actually practice. Now you can either put these electrodes on the main torso to prevent movement or you can put them on the limbs themselves.

When you get down to doing the 12-lead, V1 lead goes into the fourth intercostal space. So you feel down one, two, three, four, and place the first dot there. V2 is in the fourth intercostal space but on the opposite side of the body. Then we miss V3. The reason being is because we then go down to the fifth intercostal space, midclavicular line, and put V4 there. Then we go back and put the V3 in the middle, so leave yourself enough space. Notice how it’s actually on the rib. Then we miss V5 and V6 is midaxilla in a direct line of the same line as V4. Then we go back and we put V5 there. That’s as simple as it gets. Again, as I’ve said, you’ll get a chance to practice this in the workshop.

Interpretation then. When you’re looking at lead II, let’s go back a few slides and remind you what that looks like. Now most of these 12-lead ECGs, they will print off these, all the different leads, and they will have an additional long lead II because most of this interpretation that I’m going to talk you through actually is done in lead II first.

First and foremost, look at the paper speed. It has to be 25 millimeters per second. If it’s too fast or too slow, it changes the imagine. Then you look at the heart rate. On most occasions, it tells you what the heart rate is at the top of the ECG. Then you look at the P wave. Is there a P wave before every QRS? Is it nice and round? What’s the PR interval? The PR interval should only be three to five small squares. Anything more than five small squares and you’ve got what’s called a first degree heart block. Then look at the QRS complex. Are the QRS complexes after every P wave? Is the QRS duration less than two and a half small squares? If not, then there’s a block there. Also, are the QRS lines nice and straight? Then look for the T wave. Is there a T wave after every QRS? Because then you have to remember, the P wave represents atrial depolarization or contraction. QRS complexes represent ventricular contraction or depolarization, and the T wave represents ventricular polarizations.

Then you would go down the rest of the leads. Is aVR negative? It should be. Is the ST-segments in each of the ECG leads elevated or depressed? It shouldn’t be. They should all be isoelectric. In other words, the line itself should be nice and straight and it shouldn’t be depressed or elevated. Or are there any other abnormalities? This really comes down to your reading and your interpretation from other texts.

Okay. 12-lead acquisition then. We’ve talked a little bit about the electrodes. We’ve talked about the different types of leads. Now identifying the requirement for 12-lead ECG, most patients who present with chest pains are going to get a 12-lead ECG. Now I would go as far to say every patient with chest pain gets it even if the signs and symptoms look explicitly to do with a chest infection. In other words, sharp stabbing pain rather than chest tightness. Always place it on everybody. Always consider anybody who’s presenting to you with very vague symptoms that they should always get it. “I’m not sure.” “I don’t feel very well, but I can’t put my finger on it.” Anybody with a chest pain. Anybody who is giving you very vague symptoms would always be indicated for an ECG.

I would also go as far to say is always have your crew mate with you when you do the 12-lead and if it’s a female patient and you’ve got a female crew mate with you, it’s a bit of a no brainer. Get her to do the ECG, but that’s purely for patient comfort. No other reason. Likewise, when you’re placing the dots onto a female, make sure if you do need to lift the breast tissue, then you use the back of your hand. Now placing the electrode on the breast tissue is not going to cause you a huge problem, but the adipose tissue underneath the breast itself is going to interfere with the signal. Always seek consent, explain the procedure. They may have never had this done before. Explain what you want to do. “I need to expose you and I need to place these dots in certain places. Is that okay with you?”

Relax the patient in a semirecumbent position on the bed. Now you can also do it in a flat position. There are techs that say that’s actually the better position, but most paramedics tend to do it in this semirecumbent upright position because it’s more comfortable for the patient as long as they stay nice and still. Clear the chest of any hair by using the razor. Connect the electrodes to the leads first. In other words, put the actual dots on the leads first rather than putting the dots in the places that we showed you and then attaching the leads because you’re making it very uncomfortable for the patient.

Once you’ve done that, ask the patient to stay nice and still and quiet for a few seconds, for approximately 10 seconds, no coughing or talking, anything like that. Record the 12-lead and do several records of it, so press the button that you need to press and print off several copies of it. Therefore, you can monitor that on the way to hospital. Then you can interpret it using the process that we talked about in this lecture.

In today’s session, we talked about the importance of the 12-lead ECG, when to do it, demonstrate how to do it, and use a stepwise approach to interpret the ECG. Now the ECG session itself is something that you are going to become better and better at the more practice you get. Of course, some students love it, some students hate it, but regardless, as long as you’re able to understand and interpret the basics of an ECG, that’s what’s important. You also have to understand that the more reading you do, the better you will become. There are some great websites out there that can help you with this.

I hope you’ve enjoyed this session on 12-lead ECG.

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