- [Voiceover] Let's look at a single
heartbeat in EKG.
So here on the right side we have
a single beat on EKG.
Now, we're looking at lead II.
Lead II has a negative electrode
on the right hand and a positive
electrode on the left foot.
In a healthy heart the wave
of depolarization overall runs
from the top right to the bottom left.
So you can see that lead II measures
voltage created in this direction.
Therefore, lead II is commonly singled out
for more simple information such as
heart rate and rhythm because it
gives you a pretty good view of the waves
particularly the P wave.
Often people say that lead II is the best
lead to see the P wave.
An EKG gives us information on
voltage over time, or the change
in voltage over time.
So voltage is on the Y axis,
time is on the X axis.
Every small box running in the X axis
or time direction represents 0.04 seconds.
And there are one, two, three, four, five
small boxes per every big box.
So you can do the math, and you
can calculate that each big box
represents 0.2 seconds.
Conduction starts with the heart's
dominant pacemaker the SA node.
The SA node initates waves of
depolarization that run through the atria
and go to the AV node.
And on EKG this depolarization through
the atria is seen as this purple wave
here called the P wave.
The P wave is usually about two
small boxes or 0.08 seconds.
Again, the P wave represents atrial
depolarization.
And what happens after the atria
depolarizes? Well you get subsequent contraction.
After the signal reaches the AV node
conduction slows down.
This is for a couple of reasons.
One is that AV nodal cells are smaller
in diameter than the other cells
and as a rule the smaller the diameter
of the cell the slower the conduction.
In fact, AV nodal cells are among
the smallest diameter cells in the body.
Also, conduction runs primarily through
calcium channels in the AV node.
Calcium channels have inherently slow
kinetics and this is in contrast to fast
sodium channels that we see in the
ventricular conduction system.
This delay in conduction is really important
because it allows time for the atria
to contract and relax, which optimizes
the time for the ventricles to fill.
So, again we have the signal leaving
the SA node going to the AV node where
conduction slows down.
This all happens before ventricular
depolarization.
On the EKG the time it takes from the beginning
of atrial depolarization all the way up
until ventricular depolarization is seen
as the PR interval.
The PR interval is typically 0.12 seconds,
or three boxes.
It shouldn't ever be longer than 0.2 seconds
or something's wrong.
The signal travels from the AV node
to the His bundle.
Once we hit the His bundle this is where
conduction starts moving rather quickly.
The His bundle bifurcates into the left
and right bundle branches.
And these branches branch into terminal
Purkinje fibers.
The depolarization from the His bundle
down the bundle branches and to the
Purkinje fibers leads to mass depolarization
of ventricular cells, and is seen as the
QRS complex on EKG.
Again, the QRS complex represents
ventricular depolarization.
Like we said before, this is very rapid
because we're using fast acting sodium
channels now.
So typically the QRS complex is about 0.12
seconds.
It shouldn't be any longer than that.
Early on depolarization the septum
is depolarizing and the septum depolarizes
from the left septum to the right septum.
So you have the wave of depolarization
going from the left septum to the right septum.
You'll notice that this wave of depolarization
moves in a different direction than lead II,
which is why you get this first initial
negative deflecting Q wave on EKG.
However, quickly after the wave
depolarization spreads throughout the ventricles,
which is why you have the positive high
amplitude R wave shortly after.
Something else to note is that
during ventricular depolarization you
also have atrial repolarization.
So, at the same time as the ventricles are
depolarizing the atria are repolarizing.
However, the EKG signal that you would
see from atrial repolarization is actually
lost because the QRS complex dominates
since there are far more ventricular cells
than there are atrial cells.
So you don't see the atrial repolarization
on EKG because it's buried in the QRS complex.
So just after ventricular depolarization
and in the initial phase of ventricular
repolarization there's a flat segment
on the EKG known as the ST segment.
And the ST segment represents a period where
there is no net current.
That's to say there's no large electrical vectors
in any direction.
However, this doesn't mean there's not a lot
going on.
During the ST segment is when the ventricles
are contracting and are pumping blood
to the aorta and to the pulmonary artery.
Finally, the ventricular cells repolarize
and this is seen as the T wave on EKG.
The T wave is flatter and longer than the QRS
complex because repolarization is a slower
process than depolarization.
Again, this represents ventricular repolarization.
The T wave typically takes up about
four boxes or 0.16 seconds.
Now something you might be asking yourself
is why is the PR interval called the PR
interval when it goes from the P to the Q wave?
Well, remember we said that the PR interval
is from the beginning of atrial depolarization
up until the start of ventricular depolarization.
Sometimes there's no Q wave on the EKG.
It's just not found.
So ventricular depolarization, or the start of it,
is actually the start of the R wave,
which is why it's called the PR interval.
If there is a Q wave you can call it the
PQ interval, however, to generalize we call
it the PR interval for the times that the Q wave
is lost.
Something else you might be wondering
is why is the T wave positive as well
as the R wave.
Why are the waves representing ventricular
depolarization and ventricular repolarization
in the same direction?
And to answer this you need to remember
two things.
One is that the EKG gives you information
on the difference in charge.
So it gives you information on the changing
imbalance of potential.
The other thing to note is that the epicardial
cells repolarize before the endocardial cells.
What do I mean by that?
Well, remember how we said that the
wave of depolarization goes down the bundle
of His, the right and left bundle branches
and to the Purkinje fibers.
So you can imagine that the cells on the inside
or the endocardium depolarize first and the
wave of depolarization spreads to the
outside because we have the Purkinje fibers
spreading the depolarization to the outer cells
or the cells of the epicardium.
Let's cut out this part of the heart so I can
show you in more detail.
Just remember the EKG sees cells at rest
as being positive on the outside.
The wave of depolarization comes down
and depolarizes the cells in the endocardium first.
So, those will be seen as negative
while they're still positive on the outside
because the depolarization hasn't gone through yet.
Eventually, the endocardium and the epicardium
will be depolarized and while typically cells that
depolarize first repolarize first it's different
in the heart ventricles.
Epicardial cells actually repolarize before
endocardial cells.
So the EKG sees them as positive
before endocardial cells are positive.
You'll notice that during depolarization the
vector created and during repolarization
the vector created are both in the same
direction, which is why the peak of the QRS
complex and the T wave are in the same direction.