Hello, my name is Bridget Mazzuco coming to you from Gulfcoast Ultrasound Institute with
today’s hot tip on how to calculate the aortic valve area using the continuity equation.
First, let’s go over what the continuity equation actually is.
The continuity equation for the aortic valve area equals the cross sectional area of the
left ventricular outflow tract times the velocity time integral (VTI) of the left ventricular
outflow tract over the VTI of the aortic valve. This is the actual continuity equation. We
use the continuity equation to calculate the aortic valve area. A lot of people get confused
because there are a lot of different ways this formula can be shown all meaning pretty
much the same thing. In this formula, also showing you how to do
the continuity equation to figure out an aortic valve area, the formula seems a little bit
different. Sometimes we’ll see the continuity equation like this. This formula is just representing
a simplified version of the previous full formula.
When we break down how to calculate the cross sectional area, we need to multiply .785 times
the diameter squared to obtain that number. So this version is just showing you specifically
how to calculate the cross sectional area of the left ventricular outflow tract.
This is another simplified version of the continuity equation, in which the formula
changes slightly to substitute velocities for VTI’s. I always recommend using a VTI
instead of the peak velocity because the peak velocity will be less accurate if the shape
of the velocity curve is a-typical. The reason why we always want to trace the
Doppler waveform when evaluating the aortic valve, and not just use the caliper to calculate
peak velocity, is because tracing the envelope will also give you a mean pressure gradient
which is a measurement that is essential to severe aortic stenosis. It will also give
you all of your other values such as a stroke volume of the LVOT, mean and max velocities,
and a maximum pressure gradient. So what exactly does this all mean and how to I obtain these
measurements in my study? Let’s go over to the machine and take a look.
The first thing we need to do to calculate an aortic valve area is to obtain a parasternal
long axis view, and sineloop to get to early to mid systolly when the aortic valve is at
its maximum opening. We then need to do a diameter of the LVOT measurement.
We do this measurement right before the opening of the cusps, and we need to make sure that
we enter it into our system so later on when we go to do our velocities in apical 5-chamber,
the number is there. The next thing we want to do is get a velocity
or VTI of the left ventricular outflow tract. So we put on our color, and we put on our
pulse wave Doppler,
and we want to make sure we trace our waveform
starting from the baseline,
and this is to make sure we’re getting our VTI.
And as you’ll notice, when I traced
it, I’m getting a VTI, a max velocity, a mean velocity, a max and mean pressure gradient
and I’m also getting the stroke volume of the LVOT, which some doctors are going to
find really important.
The last thing we’re going to do to get the aortic valve area
is do continuous wave Doppler right through the aortic valve.
We’re going to freeze the image and syne loop through it, and then once again we want to
trace to envelope so we can get our VTI’s. When I’m done tracing this envelope the
aortic valve area’s going to pop up because I did all of my 3 steps.
So once again, I have an aortic valve area VTI and then I actually have an aortic valve
area that was used calculating using the maximum velocities, and that’s how you calculate
an aortic valve area using the continuity equation, three simple easy steps.
Now you give it a try.