 What's up YouTube? This is Ali Hader here, also known as Your Heart Doc. Thank you for checking in my channel. Today we're going to be talking about EKGs. So by far the biggest request I get on my Instagram account is people who want to learn more about EKGs. I think the key is not to get bogged down by trying to memorize criteria, but to get a good hold on the basics and the fundamentals and the mechanism. So today I want to go over the basics about what is going on during the EKG tracing during a single cardiac cycle. So where do those waves and intervals actually originate from within the conduction system of the heart as we go through an entire cardiac cycle and heartbeat? If we can get a hold on that, it's really going to help us out. So we're going to be using my little iPad here, so let's get to it. So here we go. All right. We're going to dive right into it. This is a diagram that I've created of the heart and its conduction system. Just to orient ourselves, up here is the right atrium. Here is the left atrium. Down here is the right ventricle. This is the left ventricle. Right here is a tricuspid valve. This is the mitral valve. And here is the intervertricular septum. These are the important landmarks you've got to understand when we're going to go through this. So the first thing you're going to notice on a surface EKG is the isoelectric line. This is the period of non-conduction. So there's no depolarization going on here. Nothing's firing. You're going to see in between heartbeats. And the first thing that starts out the cardiac cycle is the firing of the sinoatrial node right here in number one. And that's what's going to start our cardiac cycle in the EKG. When this fires, that's not the P wave. When this fires, this sends a signal throughout the atrium. It's the signal of depolarization throughout the atrium that's going to give us our broad-based P wave. So the P wave is actually formed, number two, by the depolarization of the atrium. So when your P wave is enlarged or abnormal, this can be indicative of right atrium or left atrium enlargement. Now once those signals travel through the atrium, they're going to enter this kind of final common pathway or this gatekeeper, if you will, the AV node. And that's because the only way a signal can get from the atrium to the ventricle in a normal heart is through the AV node. Now there are other circumstances that can exist, such as an accessory pathway, like you see in WPW, where actually conduction can go down through the pathway into the ventricle in an abnormal situation. But aside from that, everything's going through the AV node. Now there's a delay when you get things through the AV node. Looking a little closer, signals that hit the AV node from the atrium, there's going to be a slowing of conduction before it hits the ventricular side, okay? This slowing of conduction is our PR interval, okay? And this can be influenced by the autonomic nervous system. High vagal tone will increase your PR interval. Similarly, certain medications can also impact AV nodal conduction, such as digioxin, for example, a common one that can increase your PR interval because it's reducing AV nodal conduction by way of increasing vagal tone. So getting back to our diagram here, this here, hence, is the PR interval. That is the delay through the AV node, and that is, again, number three over here. Once conduction leaves the AV node, it's going to enter a super important structure, the bundle of his. This little bundle of his right over here, this is a specialized and important conduction tissue that is basically the final common pathway down to the ventricles. It's sort of the left main of the conduction system. This is sort of represented here near the end of the PR interval, although you can't really see the his conduction on a surface EKG, but this is a very common bundle that we evaluate when we're doing an EP study, or an electrophysiology study to figure out if someone has advanced conduction disease. Block in the level of the his, so if you have block in this conduction tissue, this is high-grade block. So MOBITS 2, third degree, okay? These are the blocks that often will require pacemakers when they're occurring at the his, whereas blocks to the level of the AV node are often a first degree block or a wankybock or a MOBITS 1, okay? These are often due to high vagal tone and not as serious. So once conduction leaves the his, it's going to enter down into the rapidly conducting bundle branches through the ventricular septum. And this is the left bundle and the right bundle. Now the left bundle is divided into the posterior fascicle, left posterior fascicle, and the left anterior fascicle. And these are all kind of fast conducting highways and signals are going to be going down rapidly into these little prokinji fibers, which are also kind of specialized rapidly conducting tissue. The same time conduction is going to be doing the right bundle, okay? Depolarizing the right ventricle. The idea is to get a uniform simultaneous depolarization of the heart in unison. And that's what's going to be giving us our QRS complex. And this is the highest of voltage you see on the EKG. And that's because the ventricle, the left ventricle, is the most important and thickest structure. What you're seeing on the QRS is actually depolarization of the myocardial cells. This is not conduction down the pathways. This is myocardial cells depolarizing, okay? That's why when you have left ventricular hypertrophy, you see this voltage is increased because you have an increased thickness and more voltage being manifested. Normal duration of the QRS is less than 120 milliseconds. And again, the PR interval less than 200 milliseconds is normal. Now once the conduction gets down these pathways and depolarizes all the ventricles, you're going to then get repolarization. And that's the T-wave. This represents repolarization, okay? This is when the whole heart resets and is ready for the next signal to come down. So in a nutshell, that's kind of what's happening when we have one single cardiac cycle on the EKG. One important thing to remember, everything's about vectors, right? So if you look at the sum vector here in terms of electrical signal, you're going to see an electrical signal kind of going in this direction, okay? And that's influenced mainly by the myocardial cells because they're providing most of the voltage, okay? But this vector, this concept of the vector is important when you're thinking about what different leads, what location they are, and what they're seeing, which we're going to talk about later. So hopefully this is going to give you a better sense of what's actually going on in the surface EKG. And this is going to help you apply your knowledge and your analysis of EKGs as we move on to more advanced stuff.