This is the classic cardiac cycle.
we’re going to start off
in isovolumic contraction.
So, I know these terms are a little bit difficult,
but you must invest
the time in to learn them
because this is how
everyone is going to refer
to the various stages of the cardiac cycle
and when blood is being pumped out.
What does isovolumic mean?
Iso meaning the same,
volumetric is the volume
is the same during a contraction.
whatever is being filled,
this is the left ventricular end-diastolic volume.
What we have is then a
depolarization of the left ventricle.
And remember, that's the
QRS complex from your ECG.
At this point,
the left ventricular pressure increases,
which is the pressure within the left ventricle.
The mitral valve closes,
and this left ventricular volume then
is maintained throughout this
particular portion of the cycle.
The big thing that looks like that's changing here
is that left ventricular pressure is increasing.
So, the volume doesn't change,
just the pressure.
Now, we enter force of ejection.
There's going to be a rapid ejection,
which is number three.
There's going be reduced ejection,
which is number four.
When does ejection happen?
As soon as the aortic valve opens.
So, you build up pressure within this left ventricle,
you’re squeezing in on this fluid,
but the fluid is not traveling anywhere
and then, BOOM,
pops through the aortic valve.
At that point,
blood is now ejected into the aorta
and then out through the systemic vasculature.
What happens here in terms of the ECG?
This is at the time
when the T wave occurs.
And remember from the ECG,
the T wave is during ventricular repolarization.
The last item to kind of consider
is that left ventricular end-systolic volume
will be at the lowest point
at the end of the reduced ejection
or phase 4.
So, where do you get left ventricular end-diastolic volume?
That is the spot that you started
with for isovolumic contraction.
Where do you get
left ventricular end-systolic volume?
That’s the last point of the ejection phase.
Isovolumic relaxation is the next phase,
and this is when the aortic valve closes.
So, once again,
you're at a place
where both valves are closed,
both aortic and the mitral valve,
and they will be closed until the pressure drops
to a low enough level
to open up that mitral valve.
Notice here that the volume
again doesn't change,
but the pressure is rapidly dropping.
The next phases are the ventricular filling.
Ventricular filling is so important.
We almost always overlook it.
Because you need to fill up the left ventricle
before you have anything to contract.
that's the bolus of blood
that has returned to the left ventricle.
We break it up into a
couple of different components.
One is a rapid filling
and the other is a reduced filling.
That is, six is rapid,
seven is reduced.
You notice here that it's a longer phase.
the heart spends more time in diastole
than it does in systole.
So, if you want to think about it
in terms of the speed,
think of this.
The filling phase always accounts for
more time than the contraction phase.
you can appreciate through this type of graph
that there is a large amount of volume
going into that left ventricle.
if you haven't thought about this before,
if you think about the filling of the left ventricle,
most of the time,
when you first learn this,
you always feel that,
okay, it fills a little bit
and then the top part of the heart contracts
and pushes in the rest of the blood, right?
Well, not so much.
The ventricular filling component
encompasses most of the diastolic filling,
probably 90% of it is done passively.
You only add about 10% through atrial contraction.
And that is shown here, the atrial contraction.
You see a very small blip in pressure
and that increases about
the last 10% of ventricular filling.
When we’re trying to quantify
how much blood leaves the heart,
it's probably best to do that
in something called an ejection fraction.
An ejection fraction,
is just a concept of how this process works.
It's usually measured with echocardiography.
So, you can put someone
and measure their left ventricular end-diastolic volume,
the left ventricular end-systolic volume,
you divide those –
that quantity by end-diastolic volume again.
And that gives you a fraction.
And that fraction should be,
in the average person,
somewhere between 55 and 60.
However, if you have heart failure,
that is a person that has a low ejection fraction,
and this can be very dramatic
or it can just be a partial
decrease in ejection fractions.
So, ejection fraction is an important clinical monitor
for looking at the volume
that leaves the left ventricle.