Heart Blocks on EKG / ECG Explained Clearly (MedCram EKG / ECG Course)
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Heart Blocks on EKG / ECG Explained Clearly (MedCram EKG / ECG Course)

November 5, 2019


Welcome to another MedCram lecture, we’re gonna talk about the second half of rhythm in our rate, rhythm, axis (discussion). And, we’re specifically gonna talk about blocks so blocks prevent
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 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 hemiblocks. 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 there’s different levels of block. 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 type of escape rhythm there is a lack of a P wave
because we know that the P wave comes from the SA node.
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 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 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 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 big box is equal to 0.2 seconds so if
for some reason the PR interval is longer than 0.2 seconds, or
one box, by definition, 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 begins right there to the R
wave, 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. Let’s review: a first-degree AV block is going
to have a .20 second PR interval or longer than a
.20 second PR interval and that’s going to define our first degree AV
block a second degree AV block has two types and this is what gets a little
confusing there are two names for the second degree AV block one of them is
known as a Wenckebach and that’s otherwise known as a mobitz type one. Or
there’s a mobitz type two. So a mobitz type one is also known as a Wenckebach 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 Wenckebach or 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 Wenckebach usually is
transient and can go away however an AV 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 Wenckebach 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 of what they look like here we have
mobitz type one as we mentioned otherwise known as Wenckebach 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, and is affected by parasympathetics.
Whereas a mobitz type 2 is a second-degree AV block and even though
it’s known as an AV block the actual 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 going to be relatively narrow so let’s
write it here PR interval is going 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 OK, 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 looking at other examples if you want to
differentiate, 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 Wenckebach 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 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 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 4 cycles of P waves, we put a 4, and because there
were 3 series that got longer 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 Wenckebach. OK, now what would happen if we did a vagal maneuver? If we did a vagal
maneuver in this situation that would increase the parasympathetic nervous
systems drive to the AV node so what would that do? That would make this worse, the block would be worse… it would block it at the AV node,
because that’s where this block is it’s at the AV node. So as a result of that
what you would actually see is you could have this 4 to 3, go to a 5 to 4… it would
increase the number, in other words instead of having 3 QRS’s and missing 1
you could have 4 or 5 or 6 and then have one that’s missing now that actually
seems better, but in fact the QRS’s is are becoming increasingly longer in
length, and you would miss a cycle so that’s gonna be an important differentiator
when we talk about mobitz type 2… so let’s review a mobitz type 1: mobitz type
1 or Wenckebach 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 doing a valsalva maneuver or vagal
maneuver that would increase the parasympathetic drive to the AV node and
that would cause a increase blocking of this PR interval. 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 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 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 up here
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?
One two three four and so the cycles is still the same, so in this case it’s four
but then instead of looking at how many series of 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 four to one, AV block, which would be a
mobitz type two, 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 Wenckebach second-degree AV block, what would happen
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 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, where you actually don’t have a block so
that’s an interesting distinction because what we could have, if time would
allow here, we could have a two to one 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 one conduction and then you don’t get any conduction, and
then you get one conduction and then you don’t get any conduction, in that
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-to-one block
where you have the conduction of 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
one or a mobitz type two, whether it’s Wenckebach or not Wenckebach.
Again, you’ve got to look for those 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. 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. However if it was a mobitz type 2, 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 parasympathetic maneuver.
Remember what we said: in a parasympathetic maneuver it would turn a
4 to 3 into a 5 to 4, so 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, so why is that? The key here is that the 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 an interesting example that will test our abilities.
Again the best place to look at P waves is and lead 2. Here we see a P wave
right there, here we see a P wave right there, here we see a P wave right here,
but no QRS afterwards, so how many cycles are there? There are
three cycles. And how many QRS complexes? There are two. But how many
blocks are there? So if we thought it was a Wenckebach, we would use this
terminology, but if we think it’s a mobitz type two, 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
Wenckebach, it would be at the AV node 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
they look pretty much about the same in this situation here too, that we have a
PR interval that’s not getting any longer and it’s less than .20, so
that to me makes me think that this is actually a mobitz type 2 or a non Wenckebach, second degree AV block.
What was the other characteristic? Well you know that
the non Wenckebach or mobitz type two is a block that is below the AV node,
even though it’s classified as an AV node block, and as a result of that
your QRS complex is typically gonna be widened and 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 2 or
non Wenckebach 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 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 block, very sure of that in
this case. OK let’s quickly review before we go on to the third-degree AV
block, so we’re looking at second degree AV block, and over here the two
types, so specifically, we’ve got mobitz type one or Wenckebach, remember it’s at
the AV node, there is parasympathetic, you’re gonna have PR intervals that are
longer, and longer, and longer, and your QRS is gonna be normal. Typically
these can be treated without pacemakers. Mobitz type two or non Wenckebach,
remember it’s below the AV node, just below it,
because of that, the PR 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. OK, well
thanks for joining us join us, on the next lecture where we’re going to get
into third-degree heart block

Only registered users can comment.

  1. As always MED CRAM is on point! Thank you for this exceptional, comprehensive review of Heart Blocks on ECG. V/r DJK

  2. Thanks for watching! Just so you know, our entire video collection is at MedCram.com

    Our videos include:
    – Enough relevant anatomy/physiology to allow key concepts to stick and be remembered (not memorized)
    – Simple illustrations (rather than overpacking slides with info)
    – Topics that are relevant to your medical education and career.

    Hope to see you soon at www.MedCram.com

  3. why are sa blocks not researched well? whenever i search i only get info on av and no info on SA blocks. what could be the reason?

  4. Wow, these are specific concepts that I've NEVER seen ANYONE teach apart from you! I always learn new things even if the video is about basic concepts that I have read/watch loads of time. Kudos and thanks! Keep up the good work!!

  5. This video is fantastic until you find out you have to pay over a $100 for the rest of the videos 🙁

  6. i signed up. I was worried about the money and my time. But, well done, good speaker Dr Seheult. Good content. I signed up for EKG and Cardiology and soon i will add Hematology = i did see one on Lymphoma. Dr Seheult is internal medicine background
    good..not too fast nor too slow, good diagrams etc. Will re-share now

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