EKG Interpretation, part 9: Atrioventricular Blocks (AV Blocks)

by Cathy Parkes July 03, 2020

Full Transcript

In this video, we are going to talk about atrioventricular blocks, or AV blocks. So there are four kinds that you'll need to know. We have a 1st degree AV block, and then we have 2nd degree type 1, which is also called Mobitz I or Wenckebach. Then we have a 2nd degree AV block type 2, which is also called Mobitz II, and then we have a 3rd degree AV block as well.

Okay, here we are looking at a 1st degree AV block. So a 1st degree AV block causes a prolonged impulse conduction time from the atria to the ventricles, due to a delay in the AV node. So typically, treatment of a 1st degree AV block is not necessary. However, we will want to monitor the patient's rhythm to make sure it doesn't progress into a more severe block. So when we analyze this strip, we can see that it is regular. We have the same distance between our R waves. We have the same distance between our P waves. So both the atrial and ventricular heart rhythms are regular. Our heart rate is approximately 60 beats per minute, because there are 25 small boxes between these R waves. So if we take 1500 divided by 25, that equals about 60. In terms of the the P wave. The P wave is nice and upright, and it's the right duration and amplitude. However, we can see that the PR interval is prolonged. It is over 5 small boxes in duration or over 0.2 seconds in duration. So that is not expected, and that is a key finding with a first-degree AV block. So the way that you can remember this, there is a couple of different ways. There is a cute poem, which some of you may have seen. So for first-degree AV block it goes, if the R is far from P, then you have first degree. So you can see that our R wave is far from the P wave. We have that prolonged PR interval. So that is one way to remember. The other analogy which my MedSurg professor taught us and which I will explain to you here and with the other blocks, is that you have a dysfunctional couple that lives together. The woman is the P wave, the guy is the QRS complex. And with a first-degree AV block, he comes home late every night. So he always comes home, but he comes home late. And then you will see how this little analogy plays out as we talk about the other blocks. Okay, here we have a second-degree AV block type 1, which causes a progressive increase in the impulse conduction times between the atria and the ventricles until one impulse fails to conduct. So usually second-degree type 1 AV blocks are temporary and do not require treatment, however, if cardiac output is insufficient, then atropine can be used for patients with this type of block. So if we take a look at this strip, we can see that our atrial heart rhythm is regular. So if we look from one P wave to the next, the distance between those P waves is consistent. However, our ventricular heart rhythm is irregular because we are missing some QRS complexes. So we have an equal distance to this one, to this one, but then we have this missing QRS complex. So we have this big distance between R wave to R wave. So ventricular heart rhythm is irregular. If we look at the P waves, those are nice and upright and of the right duration and amplitude. However, when we look at the PR intervals, you will see that they progressively get bigger and bigger, right? Longer and longer. So we've got a nice short PR interval, then a little longer, then a little longer here until our whole QRS complex is dropped, and this is second-degree AV block type 1. So in terms of how you remember this, to add onto our cute little poem, the hint is longer, longer, longer, drop. Then you have Wenckebach. Okay? And then to carry out our analogy of the dysfunctional couple, again, the woman is the P wave, the guy is the QRS complex. He comes home later and later every night until he just doesn't show up one night. So again, we'll carry this analogy through to the next block as well.

Here we have a 2nd degree AV block type 2, so this type of block causes a sudden failure of impulse conduction from the atria to the ventricles, without a progressive increase in conduction time, like we saw with type 1. So patients who have this type of AV block, it's usually permanent, and they usually require a pacemaker. So if we look at this strip, you will see that our atrial heart rhythm is regular, so there's an equal distance between these P waves. However, our ventricular heart rhythm will be irregular, because we have these missing QRS complexes. So the distance between this R wave and this one is very different than between these two R waves. And then if we look at the P waves, those are nice and consistent, they're upright, they are of proper duration and amplitude. In terms of the PR intervals on this type of strip, the PR intervals will be consistent. So they may be of normal duration, like under 0.2 seconds, or they may be prolonged. But the key thing with a second-degree AV block type 2 is that they will be consistent across the strip, not getting longer, and longer, and longer like we saw with type 1. And then, the big thing you'll notice here are these absent QRS complexes. They're just missing in parts of the strip. So in terms of how we remember this, if you want to follow along with the little, cute poem, the next line of that poem is, "If some Ps don't get through, then you have Mobitz 2." And then, if you want to remember this with the dysfunctional couple analogy, remember the woman is the P wave and the guy is the QRS complex. So in this scenario, he comes home at the same time every night. It may be late or it may be on time, but he comes home at the same time, but every once in a while he just doesn't show up just randomly. So with the second-degree AV block type 1, she kind of had some warning, right? He would come home later, and later, and later until he didn't show up. But here, she really doesn't have warning. He comes home consistently and then just doesn't show up randomly. So hopefully that's helpful.

Here we have a third-degree AV block. So this type of block causes a complete failure in all impulse conduction from the atria to the ventricles. So a patient with a third-degree AV block will require a pacemaker. So if we look at this strip here, we can see that the ventricular heart rhythm is regular, so there's equal distance between the R waves. And our atrial heart rhythm is also regular because there's equal distance between these P waves. But you will notice that P waves are not associated with QRS complexes. So normally we have a nice little P wave followed by a QRS complex and then the T wave. But here, the P waves are kind of doing their own thing, and the QRS complexes are doing their own thing because there is no impulse conduction between the atria and the ventricles.

So when you look at a third-degree AV block, there will be a lot more P waves than there are QRS complexes, and there's no association between the two. So in terms of remembering this with the poem, this line of the poem goes, "If Ps and Qs don't agree, then you have third degree." And then with my dysfunctional couple analogy, again, the woman is the P wave, and the guy is the QRS complex. They're living in the same home, but they are not dealing with each other. So she comes and goes as she pleases; he comes and goes as he pleases. There is no communication or association between the man and the woman. They're just living in the same house but really have no relationship at all. So that wraps up our blocks, and hopefully, those explanations were helpful for you. We will go into other EKG abnormalities in my next video. Thanks for watching!


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