Heart Blocks Explained – First, Second, Third Degree and Bundle Branch on ECG

By Adem Lewis / in , , , , , , , , , , , , , , , , , , , , , , , , , , /

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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 four cycles of P waves we put a 4 and because
there were three 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
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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 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 – 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 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 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 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 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

7 thoughts on “Heart Blocks Explained – First, Second, Third Degree and Bundle Branch on ECG

  1. Thanks for watching and subscribing. See the rest of the ECG video series at https://www.medcram.com/courses/ekg-ecg-interpretation-explained-clearly

  2. i am consfused. you said because the PR interval is getting longer as the block in AV gets worse the QRS gets narrowers but then on the ECG exaple you said QRS is normal. please can you help me on this

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