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

by Cathy Parkes July 03, 2020 Updated: July 29, 2021

In this article, we'll explain everything you need to know about atrioventricular (AV) blocks, including a 1st degree AV block, 2nd degree AV block type 1 (Mobitz I or Wenckebach), 2nd degree AV block type 2 (Mobitz II), and the 3rd degree AV block.

As we explained in our article on the steps in the heart conduction, normally the atrium initiates an electrical impulse, which eventually reaches the ventricles. Atrioventricular blocks are a type of heart block that occurs when that impulse is impaired (blocked) in some way.

The EKG interpretation video series follows along with our EKG interpretation flashcards, which are intended to help RN and PN nursing students study for nursing school exams, including the ATI, HESI, and NCLEX.

1st degree AV block on EKG

1st degree AV block

A first degree AV block causes a prolonged impulse conduction time from the atria to the ventricles, due to a delay in the AV node.

Treatment

Treatment of a 1st degree AV block is usually not necessary, however, it's important to monitor the patient's rhythm to make sure it doesn't progress into a more severe block.

EKG Analysis

On the strip above, you can see that the rhythm is regular. There is an equal distance between R waves, which mean the ventricular rhythm is regular, and an equal distance between P waves, which means the atrial rhythm is regular.

Heart rate

In the EKG strip shown above, there are approximately 25 small boxes between the R waves. 1500 divided by 25 is 60, so the heart rate is about 60 beats per minute

Components

In the EKG strip shown above, the P wave is upright as expected, and has the normal duration of .12-.20 seconds long and amplitude of 2.5mm high. However, we can see that the PR interval is prolonged. It is approximately 8 small boxes in duration, or approximately .32 seconds, which is longer than the expected duration of .20 seconds. The prolonged PR interval is the key indicator of a 1st degree AV block.

Tip to remember

There is a cute poem that might help you remember that a prolonged PR interval is the key indicator of a 1st degree AV block. The poem is: "If the R is far from P, then you have a first degree."

Another story that might help you remember is to think of a dysfunctional couple living together. The sensible partner is the P wave, the wayward partner is the QRS complex (R). With a first degree AV block, R comes home late every night. R always comes home, but they come home late.

2nd degree AV block type 1 (Mobitz I, Wenckebach) on EKG

2nd degree AV block type 1 (Mobitz I, Wenckebach)

A second degree AV block type 1, which is also known as Mobitz I or Wenckebach, causes a progressive increase in the impulse conduction times between the atria and ventricles until one impulse fails to conduct.

Treatment

Usually, second degree AV blocks type 1 are temporary and do not require treatment. However, if the patient's cardiac output is insufficient, then the antiarrhythmic atropine can be used for patients with this type of block.

Atropine is a key cardiac med you will need to know in your pharmacology studies and is one of the important meds covered in our Pharmacology flashcards for nursing students.

EKG Components

In the strip shown above, the distance between P waves is consistent, which means that the atrial heart rhythm is regular. However, because there are some missing QRS complexes, there is not a consistent amount of space between R waves, which means that the ventricular heart rhythm is irregular.

The P waves are upright and the expected duration and amplitude. However, the PR intervals in the strip shown above get progressively longer and longer.

Tip to remember

To remember 2nd degree AV block type 1 (also known as Mobitz I or Wenckebach) you can remember a cute poem or the story of the dysfunctional couple.

The cute poem is, "Longer, longer, longer, drop. Then you have Wenckebach."

In the story of the dysfunctional couple, remember that the P wave is the sensible spouse while the QRS complex (R) is the wayward one. In a 2nd degree AV block type I, The P wave stays at home, while the QRS complex is staying out later, and later, and later, each night. Until one night, the QRS complex does not come home at all.

2nd degree AV block type 2 (Mobitz II) on EKG

2nd degree AV block type 2 (Mobitz II)

A 2nd degree AV block type 2 is also known as Mobitz II, and this type of block causes a sudden failure of impulse conduction from the atria to the ventricles, without a progressive increase in conduction time. This is the difference between a 2nd degree AV block type 1 and 2nd degree AV block type 2; type 1 has a progressive increase in conduction time, while type 2 does not.

Treatment

A 2nd degree AV block type 2 is usually permanent and a patient will require treatment with an artificial pacemaker.

EKG Components

In the 2nd degree AV block type 2 EKG strip shown above, you can see there is an equal distance between P waves, so we can confirm the atrial rhythm is regular. However, there are missing QRS complexes, which means there is not an equal distance between R waves, and so the ventricular heart rhythm is irregular.

The P waves in 2nd degree AV block type 2 are upright, and the expected duration and amplitude. The PR intervals are also consistent; though they may be normal duration or they may be long, the key indicator that this is a 2nd degree AV block type 2 and not a type is that the PR intervals are consistent.

Another key indicator is that the QRS complexes are missing.

Tips to remember

To remember 2nd degree AV block type 2 (also known as Mobitz II) you can remember a cute poem or the story of the dysfunctional couple.

The cute poem is, "if some Ps don't get through, then you have a Mobitz II."

In the story of the dysfunctional couple, remember that the P wave is the sensible spouse while the QRS complex (R) is the wayward one. In a 2nd degree AV block type II, the P wave stays at home, while the QRS complex comes home at consistently the same time—except one day QRS randomly does not come home!

3rd degree AV block on EKG

3rd degree AV block

A 3rd degree AV block causes a complete failure in all impulse conduction from the atria to the ventricles.

Treatment

A patient with a 3rd degree AV block will require an artificial pacemaker.

EKG components

In the 3rd degree AV block EKG strip shown above, there are equal distances between the R waves, so we know that the ventricular heart rhythm is regular. The P waves are also consistent distances apart, so we know that the atrial heart rhythm is also regular.

The key indicator that this is a 3rd degree AV block EKG strip is that the P waves are not associated with the QRS complexes. If you recall from our article on EKG Basics, a normal EKG is supposed to show a P wave, followed by a QRS complex, followed by a T wave.

But in the case of a 3rd degree AV block, the P waves are completely independent of the QRS complex. There is no impulse conduction between the atria and the ventricles, so these components are not following their expected, interconnected pattern. You can also discern that they are not connected because there are more P waves than QRS complexes.

Tip for remembering

To remember 3rd degree AV block, you can remember a cute poem or the story of the dysfunctional couple.

The poem is: "If R and P don't agree, then you have a third degree!"

The story of the dysfunctional couple is as follows. The P wave and QRS complex live in the same home, but they are not connected. The P wave comes and goes as they please, and the QRS complex comes and goes as they please. They do not communicate, and have basically no relationship!


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 Med-Surg 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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