Now, we're going to connect two big points here.
We've talked about the cardiac cycle
and we talk about the cardiac cycle over time.
Because there is a
time component to the ECG.
We went through all of these different items,
such as there’s an R-R interval,
there's a P-R interval,
there's a Q-T interval.
They'll have specific, specific times.
That helps us understand
how the heart works in sequence.
But to really understand the function of the heart,
we’re going to have to plot the graph
in a little different manner.
To do this,
we are going to flip two of the axes.
We’re going to remove time out of the equation
and we’re going to put this left ventricular volume
from being on its own in the cardiac cycle,
we’re going to slide it over to the x-axis.
And on the y-axis,
all we’re going to have is left ventricular pressure,
which is seen here in the solid red line above.
So, let's now talk through what things happen
in a pressure-volume curve.
Make sure you follow
along this very carefully,
so that you can link everything
back to the cardiac cycle
because I've taught this for many years
and I know that it's very easy
to understand the cardiac cycle
with all of its different components.
You know you have a contraction.
You have relaxation.
You have filling.
You have all these things
that come in sequence.
Some people don't see the sequence
on a pressure-volume curve as quickly,
but as soon as you do,
you will now have insight
and ways that you can now express
changes in the cardiovascular system
that you don't have the ability
to express with the cardiac cycle.
So, if we plot left ventricular volume on the X,
left ventricular pressure on the Y,
we're going to start at the end systolic volume
and start with filling.
And that's denoted as A on your figure.
So, the filling phase is along the bottom,
then you have isovolumic contraction,
then you have this rapid and reduced ejection
and then isovolumic relaxation.
So, you have all of the same components there,
but now they're just located in
a loop rather than over time.
So, what happens at the various spots.
Well, the mitral valve opens at A,
so now you can start to fill the left ventricle.
You can have rapid filling,
and the atrial kick.
Then the mitral valve closes
and that’s denoted as B.
this A and B are important points.
This corresponds to your
left ventricular end-systolic volume at A
and left ventricular end-diastolic volume at B.
You subtract those two
and you get stroke volume.
Then you have an isovolumetric change,
and that is seen by that vertical line up.
So, now, you're going from B to C.
And as soon as you hit C,
you've now eclipsed the pressure of the aorta.
So, now, you can open up the aortic valve.
Once aortic valve is open,
you have a rapid ejection
followed by a reduced ejection,
then the aortic valve closes,
and that’s denoted here by D.
Between D and A is simply isovolumic relaxation.
So, you see how you can
express the entire cardiac cycle
on this one little curve.