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
