Let's talk through
how you get a signal from the SA node,
all the way down to these cardiac myocytes.
There are some specific pathways,
by which you’re going to go through.
So, you start at the SA node,
you go to the AV node,
you go down the Purkinje fibers
and that is how this process works.
But I'll tell you something.
Some of the places along
this conduction pathway
are faster than others.
So, let's start with the fastest stuff, right?
The fastest thing we have are Purkinje fibers.
Purkinje fibers are really quick conductors.
So, as so soon as they get electricity,
they move it right on.
Right behind that is the
right and left bundle branches.
So, those are the areas right along the septum.
They also conduct electricity very rapidly.
The ones that conduct it a little bit are in
the atrial muscle and ventricular muscle.
This is kind of a moderate level of electrical conduction
in terms of speed.
What’s the slowest –
what is the slowest process
in this whole thing?
The weakest link.
It is the slow portion
and that is the AV node.
So, the AV node slows things down.
It is a slow conductor,
while the other things like
Purkinje fibers move really quick.
So, it just depends upon
which area you are in the heart,
of which of these particular conduction velocities
will allow for the process to happen.
If you think about it,
it makes sense that these will have different rates.
Well, first, you need to signal
the top portion of the heart to contract,
but you need to wait
until the ventricles fill up
before you contract them.
And that's one of the primary
purposes of the slow AV node
that allows it to slow down,
so you have enough time to fill the ventricles.
Once the ventricles are full,
then you can contract them optimally.
But you can't have that process,
you couldn’t depolarize the
whole heart at the same speed
or you wouldn't get this pumping ability.
Once you have information sent down
the bundle branches to Purkinje fibers,
that is important to happen very quickly
because you want to make sure
that they contract as a unit.
So, these conduction velocity speeds
make a lot of physiological sense
and that is why we go through them in this way,
so you have a good feel for
how the conduction process
works with the mechanics
and how this process works to be able
to pump blood throughout the body.
So, there are a couple of factors
that affect the conduction velocity.
The first is sympathetic stimulation.
So, this fight or flight response
or this fight or aggressive conflict mediation response
allows for there to be faster
conduction of current.
We call this positive dromotropy.
So, dromotropy is speed of conduction.
What slows down the speed of conduction?
The parasympathetic nervous system.
And these are done through acetylcholine.
This decreases the conduction
through places like the AV node,
and we call that negative dromotropy.
So, we have chronotropy,
is heart rate.
Dromotropy is speed of conduction.
Now, anytime we talk about pathology,
there are places where this whole
process could go awry, right?
It seems like that anytime we
talk about something how neat it is,
how fast it is,
how cool it is,
okay there's a place when –
there’s a time when it is going to go wrong.
And there, of course, is in the
conduction pathway as well.
The AV node is one area of pathology
a lot of problems happen with the AV node,
in that maybe it slows conduction too much
and doesn't let it move through very rapidly.
There also can be blocks
in places like the Bundle of His.
There can be also blocks in places like the bundle branches,
the right or the left.
And each one will look a little
bit different on electrocardiogram.
There also can be some
other blocks downstream,
but they’re a little less likely,
and so what we do is we focus on the bundle branches
and the AVs
because they are the most important.
Because we cover the most important stuff here.
So, let's walk through a couple of the AV blocks.
So, what do we mean by AV block?
It doesn’t always mean a block.
It doesn't mean that it
doesn't travel through at all.
What we’re talking about
here is that it slows it down,
maybe it delays it,
or maybe it doesn't let it go through.
But it doesn't mean that it's fully blocking it
until you get to a type III block.
So, what is a type I block?
It delays the conduction
through the AV node.
So we just have here –
think of it like a gate.
You know, you've been on the interstate
and you've been on a large highway,
in which all at once,
you have a tollgate out
and it slows everybody down.
And it’s these kind of gates that
don't stop movement of traffic,
but they slow that movement of traffic down.
So, the interesting thing about type I AV blocks
is that the rate is lower,
but you still have a normal sinus rhythm.
What do we mean by a sinus rhythm?
It has a P wave, a QRS complex,
and a T wave.
If we move to a more serious block,
like a type II block,
here is where some action
potentials don't make it through.
Many of them are slowed down,
but some don't make it through.
There are a couple of different
types of AV blocks that are type II.
There's a type I and a type II.
They usually refer to this as
Mobitz type I and Mobitz type II.
But these can be different ways
in which you slow rates through the AV node
and this results in a bradycardia.
Heart rate below 60.
So, this is a time when a slow heart rate
is not necessarily in an endurance athlete.
It's in pathology in someone who
can't have their heart beat fast enough
because they have one of these AV blocks.
The most serious problem here are
AV blocks that are AV type 3.
These do not allow for the propagation of an action
potential from the SA node to the Bundle of His.
So, this is even a more problematic condition.
You will no longer have a sinus rhythm.
And you'll have a severe ventricular bradycardia.
In fact, the ECG will look much different
because there will be some P waves,
which is atrial depolarization,
with no captured QRS complexes.
We’ll review this subsequently.
We have some time denoted
just to look at what an ECG looks like
because we know it's hard for you,
so we want to make sure you got it.
So, we spend some extra time
on that in a future time.