pressure-volume loop generation

RICHARD E. KLABUNDE:
This lecture

describes the generation of
ventricular pressure-volume

loops.

Pressure-volume loops
are a way of depicting

ventricular function
during the cardiac cycle.

Specifically, it's a way--

these loops are a
way of depicting

the changes in ventricular
pressure and volume

that occur as the heart
undergoes contraction

and relaxation.

Before we actually look
at pressure-volume loops

and how they are generated,
it's first important for you

to make sure that you already
understand the functional

anatomy of the heart and also
the cardiac cycle, both of which

are found on cvphysiology.com.

Let's now take a look at
the pressure-volume loops

and their generation.

The pressure-volume loop
is shown on the right.

And this loop on the
right is generated

from the pressure tracings
of the left ventricle

and the volume
tracings that occur

during a single cardiac cycle.

So let's just take this through
step by step first of all

to review how the cardiac
cycle generates the pressure

and volume changes.

And then we'll see how those
are translated into the drawing

of the pressure-volume loop.

The beginning of this gray
phase, this phase b here,

which is called
isovolumetric contraction,

this is initiated by the
appearance of the QRS complex

in the electrocardiogram.

In other words, it's initiated
by ventricular depolarization.

Once the ventricles
depolarize, the muscle fibers

then begin to contract.

When they first
begin to contract

and the pressure within the
ventricle begins to increase,

the mitral valve
suddenly closes shut.

The pressure continues to
rise, but during this phase

of isovolumetric
contraction, no blood

is ejected from the ventricle
because the pressure has not yet

exceeded the aorta.

And therefore, when we look at
the volume tracing, it is flat.

There is no change in volume.

And that's why we call this
the isovolumetric phase

of contraction.

At the point in which
the aortic valve opens,

we will continue
to have generation

of ventricular pressure up
to a peak systolic pressure.

And then the
ventricle will begin

to relax because of the
appearance of the T wave

in the electrocardiogram.

And it will continue to
relax, and the pressure

will begin to fall in
the left ventricle.

Well, during this phase,
this phase c here,

which represents ejection,
we see a rapid decline

in left ventricular
blood volume.

Now, at the point that the
ventricular pressure falls

significantly below
the aortic pressure,

then we have the
aortic valve closing,

which corresponds to this
point on the pressure

diagram and this point
here on the volume diagram.

The ventricle then undergoes
isovolumetric relaxation

because all the heart valves
are closed during this phase.

And so the pressure falls, and
there is no change in volume.

Finally, the pressure
reaches a level

in which it falls below
the left atrial pressure.

And once the left
ventricular pressure

falls below the left
atrial pressure,

then blood can move
from the left atrium

into the ventricle
across the mitral valve

as the mitral valve opens.

And the ventricle will now
begin to fill with blood,

and its pressure after
an initial decrease

will then begin to increase.

But as you can see here,
once that mitral valve opens,

there's a rapid and
then a reduced phase

of ventricular filling.

Now, to generate the
pressure-volume loop,

all we are doing to generate
this loop is plotting

left ventricular pressure
against left ventricular volume

at all points in the
cardiac cycle diagram.

So let's begin
with a point here,

which is the point of
mitral valve closing, which

is the beginning of the phase
of isovolumetric contraction.

Well, at this point here, you'd
have a certain pressure, perhaps

10 millimeters of
mercury in the ventricle.

And you would have a
end diastolic volume

in the ventricle that might
be 125 millimeters of blood.

Well, this pressure
and this volume

would be represented
by this point

here on the
pressure-volume loop.

As the ventricle undergoes
isovolumetric contraction,

we saw that the
pressure here increases,

but there is no
change in volume.

Well, on a
pressure-volume diagram,

that would be represented
as a vertical line.

The pressure increases, but
there is no change in volume

because no blood is being
ejected from the ventricle.

Once the ventricular
pressure exceeds

the aortic pressure and
the aortic valve opens,

now we will have a decrease
in ventricular volume,

while at the same time we
will have an increase and then

a decrease in
ventricular pressure.

And if you were to plot
each pressure and volume

at different points in time
during this phase of ejection,

you would generate a curve
that would look like this.

So this portion of the curve,
c, represents this region

of systolic ejection
by the ventricle.

When we reach the point
where the aortic valve closes

that represents this point here.

The ventricle then undergoes
isovolumetric relaxation,

so there is a fall in pressure
but no change in volume.

And that's depicted by
another vertical line

as the pressure falls, but
there is no change in volume.

At point 4, that represents
mitral valve opening.

And as the mitral valve opens,
then the ventricular volume

will start to increase again as
the ventricle begins to fill.

And at first, the
pressure will still

continue to fall as the
ventricle fills, as shown here.

But then we will see the
ventricular pressure as well as

the volume increases until
we complete the loop.

There's an important
feature on this loop

that I want you to note.

And that is the stroke
volume of the ventricle--

which we have defined previously
as the end diastolic volume

minus the end systolic volume--

is represented by the width
of the pressure-volume loop.

Because on the right edge
of the pressure-volume loop,

this vertical line of
isovolumetric contraction,

that volume of blood represents
the end diastolic volume

of the ventricle.

Over here on this
side of the loop,

this vertical line represents
the end systolic volume.

Therefore, the difference
between these two

represents the stroke
volume of the ventricle.

This slide is a
animation showing

how the pressure in the volume
changes during a cardiac cycle.

And as we look at
the top here, this

is the tracing of the
electrocardiogram showing

the QRS complex
right here, which

will then be followed by
isovolumetric contraction

in the ventricle.

And all we're doing
here is generating

the same pressure
and volume changes

seen in the previous slide.

And I will stop talking
here in just a moment.

And I just want you to look
at these loops which are also

found on cvphysiology.com.

And you can see how
different portions

of the pressure-volume
changes over time

are represented by
different regions

of the pressure-volume loop.

In a subsequent
lecture, we will look

at how changes in
cardiac preload

and afterload and inotropy
or contractility affect

the pressure-volume loops.