Welcome back to the EZMed channel where medical and science topics are made easy. In this video you're going to learn some simple tricks that will help you remember the different types of heart blocks. This will be a general overview and then we'll go in much more detail about each heart block and other EZMed videos.
But I wanted to have one quick concise video you could come back to as a reference and find these tricks that will help you study for exams and be a good resource throughout your career. Let's first define what an AV or atrioventricular heart block is. And in order to do that, we're going to take a look at the conduction system.
Now there was a previous EasyMed video where we walked through the conduction system of the heart step by step, and I'll link that down below in the description. So we're just going to do a brief review here. In a normal functioning heart, the SA node spontaneously produces action potentials or electrical impulses, and this is what becomes the heart rate. The action potential produced by the SA node will then travel through the atria using the internodal pathway and Bachmann's bundle.
As the action potential travels through the atria, which are the top two chambers, it depolarizes the cardiac myocytes or muscle cells and this leads to atrial contraction. Well now that electrical impulse needs to be transmitted from the atria, the top two chambers, to the ventricles, which are the bottom two chambers, and it does so by passing through the AV node, followed by the bundle of His, then the right and left bundle branches, and finally the Purkinje fibers. As the action potential travels through this part of the conduction system, It leads to depolarization of the ventricles and ventricular contraction occurs. An AV or atrioventricular heart block is when the electrical transmission from the atria to the ventricles is interrupted. Either it's delayed or it's completely blocked.
So what this means is the action potentials produced by the SA node have difficulty traveling from the atria down to the ventricles. And this is usually due to a problem either in the AV node or a little bit lower in the bundle of his or the right and left bundle branches. There are three main types of AV heart blocks.
First degree, second degree, and third degree. There are two different types of second degree heart blocks and we're going to be walking through each of those here shortly. Now in a previous EasyMed video we used the heart block poem to remember the different types of AV heart blocks.
And the reason I'm recreating this video is while this poem is helpful it is a lot of words to memorize. So if it works for you then great, keep using it. But the next trick I'm going to show you simplifies these four main types of heart blocks into just a few words. So first let's go over this heart block poem as a quick refresher. If the R is far from the P, then it must be a first degree.
If PR gets longer, then a QRS drop, then it must be a type 1 WinkyBock. If PR stays normal and QRS quits, then it must be a type 2 Mobitz. If P and QRS beat independently, then it must be a complete third degree.
We're going to clarify what that type 1 WinkyBock versus type 2 Mobitz actually means here shortly, but you can see how this poem uses a lot of words and it's tough to memorize. So the next trick I'm going to show you simplifies these four types of heart blocks into just a few words and it's much easier to memorize. So if you can remember this it will help you on a lot of exams.
First degree far away p, winky bach longer than drop, second degree drop randomly, third degree beat independently. Now winky bach is a type of second degree heart block and we're going to talk more about that here shortly, but once you understand that you'll see this trick is very easy to use. Let's now walk through these different types of heart blocks starting with a first degree. For a first degree you'll want to remember the phrase far away p and if you're using the poem it's if the r is far from the p then it must be a first degree. What this refers to is a prolonged pr interval.
Now there's a previous EasyMed video where we talk about the different parts of an EKG so if you need a quick refresher on the different waves and intervals I encourage you to go check that out. It'll be linked down below in the description. So in a first degree heart block, the PR interval is prolonged to greater than 200 milliseconds.
Remember the P wave represents atrial depolarization and ultimately atrial contraction, and the QRS complex represents ventricular depolarization and ultimately ventricular contraction. So that PR interval represents the time it takes for the electrical impulse to travel from the atria to the ventricles. In a first degree heart block, the PR interval is prolonged. And what this means is the electrical transmission from the atria to the ventricles is delayed. Usually this occurs in the AV node.
Now despite that delay there's still a QRS complex that follows each P wave in a first degree heart block. And what this means is each electrical impulse is still being transmitted down to the ventricles, it's just taking longer to do so. And that's one of the differentiators between a first degree heart block and a second and third degree heart block. In those heart blocks we don't necessarily have a QRS complex that follows each P-weight and we're going to be talking about those here shortly. But let's first talk about the two different types of second degree heart blocks.
Second degree heart blocks can be divided into Mobitz type 1 and Mobitz type 2. Mobitz type 1 is also referred to as Wienkebach and the easy way to remember Wienkebach as a Mobitz type 1 is to think of the word wink and we wink with one eye. So WinkyBach type 1. Let's now take a closer look at each one of these starting with WinkyBach. So for WinkyBach we said to remember the phrase longer than drop and if you're using the POM it's if the PR gets longer than a QRS drop then it must be a type 1 WinkyBach. What this refers to as a second degree Mobitz type 1 or WinkyBach is when the PR interval progressively gets longer and then all of a sudden the QRS complex drops. So if we take a look at the image we can see the PR interval is slowly getting longer and longer.
and eventually we don't have a QRS complex that follows the P wave. This is different than the first degree heart block where we had a prolonged PR interval but we still had a QRS complex that followed each P wave. So what's happening in a second degree Mobitz type 1 is the conduction from the atria to the ventricles is slowly being more delayed with each beat and that's why we see that PR interval progressively getting longer with each beat.
Eventually the conduction from the atria to the ventricles is completely blocked. and that's when we get a dropped QRS complex. This is known as the Wienkebach phenomenon. After we get that drop beat, conduction to the ventricles resumes and we repeat the cycle again.
Moving on to a second degree Mobitz type 2, we said to remember the phrase second degree drop randomly, and if you're using the poem it's if PR stays normal and QRS quits, then it must be a type 2 Mobitz. What this refers to as a second degree Mobitz type 2 is when the PR interval remains constant. and the QRS complex randomly drops.
This is different than the Wickebach where we saw the PR interval getting longer and longer and then the QRS complex dropped. Now if we take a look at the image we can see the PR interval stays the same and then all of a sudden we get a P wave without a QRS complex that follows it. So what this means is the electrical impulse travels at a constant rate from the atria to the ventricles and that's why we see a constant PR interval.
Then all of a sudden that electrical impulse is completely blocked and it can't travel from the atria to the ventricles and that's when we get that dropped QRS complex. This usually occurs in the bundle of his or bundle branches and sometimes in the AV node. This is a second degree Mobitz type 2. The final type of heart block is a third degree, also referred to as a complete heart block. And the phrase to remember here is beat independently.
And if you're using the poem it's if P and QRS beat independently then it must be a complete third degree. So what this means is the electrical signal between the atria and ventricles is completely blocked. As a result, there's no electrical communication between the top two chambers and the bottom two chambers. This forces the atria and ventricles to each generate their own action potentials, and we can see that on the EKG rhythm strip shown.
The blue arrows point to the P waves, which represent atrial depolarization, and the red arrows point to the QS complexes, which represent ventricular depolarization. And we can see there's no relationship between the P waves and QRS complexes. They're beating at their own rhythm.
And it's these junctional escape rhythms in the ventricles that allow for ventricular depolarization and contraction to occur without needing to receive an impulse from the atria. And this is what maintains cardiac function in a third degree heart block. Hopefully this was a good review of the different types of heart blocks. If you're new to the channel make sure to subscribe so you don't miss out on future videos that make medical and science topics easy.
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