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Heart Blocks Overview · Acedra
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Heart Blocks Overview
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0:02
well welcome to another MedCram lecture we're going to talk about the second half of rhythm in our rates rhythm axis and we're specifically going to talk about blocks so blocks prevents electrical conduction so what I've got drawn here is an overview of the electrical conduction of the heart which we've talked about before and specifically we're looking at the SA node the AV node the right bundle branch and the left bundle branch area and
0:29
specifically we're going to talk about blocks in those areas now you may notice here that there is an anterior and there is a posterior branch of that left bundle and so we're also going to talk about hemi blocks but this is kind of a generalized overview of all of the different areas where you can get blocks so you've got to know what those look like we're going to expand significantly on this AV node which is right here because
0:53
there's different levels of block okay there's something called a first degree a second degree and finally a third degree or complete heart block so let's take a look at those so you will know and see examples of the different types of blocks okay so let's start with the SA node so with a block in the SA node there is some feeling that the SA node actually generates electrical activity but it just can't exit the SA node and so what happens is with the lack of any
1:22
type of escape rhythm there is a lack of AP wave because we know that the P wave comes from the SA node okay so here's an example of a SA node block you can see clearly here that we've got P waves and then nothing basically no activity whatsoever and the approximate time period is about 800 in terms of milliseconds and here we've got about 2,500 so that's about three beats that were missed and then it resumes back up and comes back down to about 800
1:54
and so you can see here what's missing is an entire P wave and this goes on for a couple of seconds and there is no escape rhythm sometimes you can have an escape rhythm sometimes you might not have an escape rhythm and the escape rhythm could be ventricular it could be atrial in this situation so that is an example of a sinus node block so the thing that I think you ought to know for the SA node is specifically that it's the same
2:23
timing and the p-waves looks the same they're the same distance from the QRS complex pretty straightforward okay let's talk about AV node blocks let's talk about the primary AV block which is written as a one with a circle so that's a first-degree block a first-degree block basically just lays the communication between the atrium and the ventricle so let's go back and take a look at the QRS and the P wave to show you what we're going to see that so when
2:57
looking at the PQRS and T wave complex what we're looking at specifically here is we're looking at the PR interval and that is from the very beginning of the P wave until the very beginning of the QRS complex and so it is this distance here specifically that we are looking at for there to be a first-degree AV block there has to be an elongated PR interval now the limit for this is 0.2 seconds or one big box remember the one big box has five little boxes inside of it and one
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big box is equal to 0.2 seconds so if for some reason the PR interval is longer than 0.2 seconds by definition or one box you are going to have at least a first-degree AV block okay so let's take a look at an example here of an EKG let's take a look here at lead 2 which is probably the best lead to see P waves and you can see clearly there's a P wave right there and here there is a QRS complex and if we measure the distance from the beginning of the P wave which
4:06
begins right there to the Q wave the R wave I should say the QRS complex which is right there clearly we can see that that distance is bigger than one large box therefore this is going to be a first-degree AV block okay so let's review a first-degree AV block is going to have a point two zero second PR interval or longer than a point two second PR interval and that's going to define our first degree AV block a second degree AV block has two
4:43
types and this is what gets a little confusing there is two names for the second degree AV block one of them is known as a wanker Bock and that's otherwise known as a mobitz type one or there's a mobitz type two so a mobitz type one a mobitz type 2a mobitz type one is also known as a Weinke Bock so just be aware of that confusion for some reason that's what they've done now a second degree AV block has two types as we just mentioned the wanker Bach or
5:19
mobitz type one actually occurs in the AV node and so as a result of that it is susceptible to parasympathetic innervation however a second degree AV block that is mobitz type two is actually below the AV node and has no input from parasympathetic fibers the second degree is worse and the first degree is not as bad usually the second degree AV block that is a mobitz type one or a Weinke bock usually is transient and can go away however an AV
5:52
block of the second degree that is a mobitz type two is usually more permanent and usually has to be treated with some sort of pacemaker and we'll talk about how to differentiate those two very very shortly but I want to make sure you are aware that there are two types of second-degree AV blocks when kabak or mobitz type one and the more dangerous one is mobitz type 2 so let's go ahead and take a look at what they look like okay so here's a good example
6:19
of what they look like here we have mobitz type one as we mentioned otherwise known as wanker Bock and we have mobitz type two we'll talk about two to one block in just a second remember what we said mobitz type one is a second-degree AV node block that occurs actually in the AV node okay is affected by parasympathetics okay whereas the mobitz type 2 is a second-degree AV block and even though it's known as an AV block the actual
6:53
block itself is below the AV node and as a result of that there is no parasympathetic innervation which can be helpful we'll talk about that so for a mobitz type 1 because the block is more or less in the AV node what do you think you're going to see you're gonna see PR intervals that get bigger and bigger as the block gets worse and worse why because the PR interval is made in the AV node secondly because the block is fairly high up your QRS complexes are
7:29
going to be relatively narrow so let's write it here PR interval is good to be increased and the QRS is actually going to be normal as opposed to the mobitz type to second-degree AV block which is below the AV node it's relatively lower down and so what you're typically to see here is the PR interval is going to be okay but the QRS is typically good to be increased now we don't see that here in this example but that's just something you want to think about when you're
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looking at other examples if you want to differentiate
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but the primary differentiation for a second degree AV block between a mobitz type 1 and a mobitz type 2 is this and this is very important that's probably the most important thing to know is looking at the PR interval and noticing that in a mobitz type 1 or a when kabak as it's known as it gets longer and longer and longer and you can see that here very clearly the PR PR PR is getting longer longer longer and then finally what happens is you drop a QRS
8:46
complex in other words it's infinitely long if you wish then it starts over again PR interval starts over at the same length as it did at the beginning of the cycle so we have a way of naming this and this has to do with cycles and series so we look at this ratio of cycles in series so what is a cycle and what is a series so the cycle is how many P waves are there and in this case there's 1 2 3 4 so how many cycles would there be in this there would be 4 when
9:19
we put a little line and how many series are there how many QRS series are there in this case there are 3 so the series is always going to be a number that's one less than the cycle so this would be a mobitz type 1 second degree AV block because the PR interval is getting increasingly longer with each cycle or series until finally there is a missing QRS complex because there are four cycles of P waves we put a 4 and because there were three series that got longer
9:49
and longer until finally one dropped we put a 3 so this is a 4 to 3 ratio of a second degree AV block mobitz type 1 or when kabak ok now what would happen if we did a vagal maneuver if we did a vagal maneuvers in this situation that would increase the parasympathetic nervous systems drive to the AV node so what would that do that would make in other words this worse the block would be worse it would block it at the AV node because that's where this block is
10:22
it's at the AV node so as a result of that what you would actually see is you could have this four to three go to a five to four it would increase the number in other words instead of having three qrs's and missing one you could have four or five or six and then have one that's missing now that actually seems better but in fact the qrs's are becoming increasingly more longer in length and you would miss a cycle so that's going to be important
10:55
differentiator when we talk about mobitz type two so let's review a mobitz type one mobitz type one or when kabak is a second-degree AV node block it is at the AV node therefore it is susceptible to parasympathetic activity the PR interval becomes longer and longer with each successive cycle until finally the series ends and you have a missing QRS complex because it is innervated by the parasympathetic nervous system if you stimulate that how could you do that by
11:27
doing a valsalva maneuver or a vagal maneuver that would increase the parasympathetic Drive to the AV node and that would cause a increased blocking of this PR interval okay so that's mobitz type one now let's move on to mobitz type two as we talked about with mobitz type two this is technically at the AV node but actually it's really below it and as a result it does not have any parasympathetic nervous system activity because it's not a block that is high up
11:54
in the AV node the PR interval is usually okay but what you might see is an increased QRS complex maybe maybe not you might so let's look here these PR intervals are the same they are not changing until finally there's a dropped beat there's a dropped QRS if you will so this is far more serious because this means that the block is further down below the AV node there's no escape mechanism from the AV node you'd have to have a an escape mechanism from further
12:27
down in the bundle branch specifically and in this way we still get a missed beat it's still a second-degree AV block but it's below the AV node now instead of naming it the same way we do a pair which is cycle series we do it in a different way here the way we measure it here is by looking at the block so what we look at is how many P waves are there 1 2 3 4 and so the cycles is still the same so in this case it's 4 but then instead of looking at how many series of
13:02
PR intervals get longer we actually go the other direction and we ask how many missing QRS complexes are there so in this case this would be a 4 to 1 AV block which would be a mobitz type 2 and as we mentioned no parasympathetic nervous system activity there because it is below the AV node if we did a parasympathetic nervous system stimulation in this situation a mobitz type 2 which is a non Weinke Bach second-degree AV block what would happen
13:35
is we would block the AV node which is not where the problem is but actually interestingly what would happen is is that the AV node itself would be slowed down it would be partially blocked and it would actually be more in line with the block that is below it that is causing the mobitz type 2 so as a result of that instead of making this worse it could actually turn it from a four-to-one block to actually one-to-one conduction we you actually don't have a
14:03
block so that's an interesting distinction because what we could have if time would allow here we could have a 2 to 1 block now that's going to be very difficult because remember what we have here in mobitz type 1 in mobitz type 1 we're looking for increasingly long PR intervals well what happens if the block is such that you get 1 conduction and then you don't get any conduction and then you get 1 conduction and then you don't get any conduction in that
14:31
situation there are not enough series to see whether or not the PR interval is getting longer and so when you have something called the two-two-one block where you have the conduction of a PR and then no conduction of a PR and then the conduction of a PR again you're stuck you don't know if this is a mobitz type 1 or a mobitz type 2 whether it's when you bought or not Winky Bock and so again you've got to look for those
14:59
things that we talked about if this were a mobitz type one you would expect to see a large PR interval and you would expect to see a narrow QRS complex and if you look very carefully here you can clearly see that this has a relatively large PR interval and so just looking at this you would say that this is probably most likely a mobitz type one that's causing this AV block similarly you would get a narrow qrs which is exactly what you're seeing here
15:28
however if it was a mobitz type to which the block is below the AV node you would not expect to have an enlarged PR interval you would expect to have a normal PR interval but you might see a widened QRS complex which you're not really seeing here either so this is a situation where you're suspicious that this could be a mobitz type one but you're not sure so what's one way you can differentiate this even more and we've talked about it it's doing a
15:56
parasympathetic maneuver remember what we said again in a parasympathetic maneuver it would turn a 4 to 3 into a 5 to 4 so this could turn in this case it would be a 2 to 1 it could make it a 3 to 2 if it were a mobitz type 1 or if it were mobitz type 2 and we did a vagal maneuver it should not affect it at all so it would stay exactly the same or as we mentioned it could turn it into a one-to-one conduction okay so why is that the key here is that the
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parasympathetic nervous system activation is only going to affect a mobitz type 1 because a mobitz type 1 block is specifically dead set right into the AV node whereas a mobitz type 2 is below the AV node that is the key ok let's look at this example here this is uh this is an interesting example that will test our abilities again the best place to look at P waves is and lead to here we see a P wave right there here we see a P wave right there here we see a P
17:00
wave right here but no QRS afterwards so what we have how many cycles are there there are three cycles okay and how many Q ress complexes there are two but how many blocks are there so if we thought it was a wanker Bock we would use this terminology but if we think it's a mobitz type - we would use a three to one block so let's see which one it is remember what we said that if it were a wanker Bock it would be at the AV node
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and we would expect an elongated PR interval so let's take a look at the PR interval first and see here we see the P wave starting right about there and here we see the QRS complex starting right about there that to me looks like it's at or below one box if we look over here you can see here's a QRS complex the same one and P wave there that's less than a box so to me and what I'm looking at all of these and look pretty much about the same in this situation here -
17:59
that we have a PR interval that's not getting any longer and it's less than point two so that to me makes me think that this is actually a mobitz type - or a non wanker box second degree AV block what was the other characteristic well you know that the non wanker Bach or mobitz type two is a block that is below the AV node okay even though it's classified as an AV node block and as a result of that your QRS complex is typically good to be widened and
18:28
certainly we can see here that that QRS complex and that one there in fact all of these QRS complexes are a little bit wider than we would expect so again here we have two things going for us that make us think that this is a second-degree mobitz type two or non-winged kabak AV node block now interesting if you look out here what do we have now this is a good example where you go from a three to one mobitz type two block to a two to one mobitz type
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two block in the same patient we've got a p-wave conducting a qrs then we have a p-wave with no qrs then we have a p-wave with a qrs and then a p-wave without a qrs and this is basically identical to what we showed you before except here we actually have the widened QRS this is definitely a mobitz type two with a two to one AV very sure of that in this case okay let's quickly review before we go on to the third degree AV block so we're
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looking at second degree AV block and we're over here the two types so specifically we've got mobitz type 1 or Weinke Bock remember it's at the AV node there is parasympathetic you're gonna have PR intervals that are longer and longer and longer okay and your QRS is good to be normal typically these can be treated without pacemakers mobitz type 2 or non wanker Bach remember it's below the AV node just below it because of that the PR
20:10
interval is actually going to be okay but the QRS is going to be longer perhaps and here because it's below the AV node there is no parasympathetic activity associated with it okay well thanks for joining us join us on the next lecture where we're going to get into straight-out third-degree our block
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