How To Calculate Aortic Valve Area Using The Continuity Equation

Don’t be threatened by using the continuity equation to calculate aortic valve area.  Even though most of our echo machines will calculate it for us these days, it’s still important to know how it’s used.  Interested in knowing more about calculating aortic valve area with echocardiography?  Keep reading.

What Is Normal Aortic Valve Area?

A normal aortic valve area is greater than or equal to 2.0 cm2. In people with normal aortic valves, the normal aortic valve area range is 2.0 cm2 and greater.  As the severity of aortic stenosis increases, increases in valve gradients aren’t really present until the aortic valve area narrows down to less than half of its normal size.

The pressure gradient across a stenotic aortic valve is directly related to the valve orifice area and the forward flow that crosses the aortic valve. So in the case of decreased cardiac output, relatively low pressure gradients can be seen in patients with severe aortic stenosis.

On the other hand, during exercise or other high flow states, systolic gradients can be measured in patients with minimally stenotic or even normal valves.

How To Calculate Aortic Valve Area

In order to accurately calculate aortic valve area, complete assessment of the following must be performed.

• Measurement of the aortic transvalvular flow
• Determination of the transvalvular pressure gradient
• Calculation of the aortic valve area

Aortic Valve Area Calculations

There are several different methods of determining the aortic valve area. In echocardiography however, there is one method of calculating the aortic valve area that is used more often than any other method.  That is the aortic valve area continuity equation.

The Aortic Valve Area Continuity Equation

Calculating the aortic valve area by using the continuity equation is a relatively simple method to use. And these days, most modern echocardiography machines will calculate this for you.  All you need to do as a sonographer is input the required data.

What Is The Continuity Equation For Aortic Valve Area?

The aortic valve area continuity equation is this:

A1 x V1 = A2 x V2

OR:

AVA(cm2) = CSA of lvot(cm2) x lvot VTI(cm) / AoV VTI(cm)

Continuity Equation Steps:

1. From the parasternal long axis view, determine the cross sectional area (CSA) of the left ventricular outflow tract (LVOT). To do this, measure the LVOT diameter during early to mid systole at the insertion points of the aortic valve cusps.  Be sure to zoom in to make this measurement!

CSA of lvot = .785 x LVOT diameter2

2.  From the apical 5 chamber view, measure the LVOT VTI by placing the pulsed wave (PW) sample approximately 3 mm from the aortic valve inside the LVOT. If the sample volume is too close to the aortic valve, you may get increased flow from the aortic valve.  Take care to measure only the flow in the LVOT.
3. From the same apical 5 chamber view, use the continuous wave (CW) Doppler to measure the maximum aortic velocity and measure the aortic VTI.
4. In an effort to make sure you have the maximum aortic velocity, measure the flow from multiple windows. The most common windows to measure aortic velocities are the apical, right parasternal, suprasternal notch and even the subcostal window.

Severity Scale Of Aortic Valve Stenosis

Below is a range of severity of aortic valve stenosis.

A normal aortic valve area is greater than or equal to 2.0 cm2.

• Mild aortic stenosis: Valve area is between 1.5-2.0 cm2 with a pressure gradient of less than 25 mmHg
• Moderate aortic stenosis: Valve area is 1.0-1.5 cm2 with a gradient of 25-40 mmHg
• Severe aortic stenosis:  Valve area is less than 1.0 cm2 and with a pressure gradient of greater than 40 mmHg
• Extremely severe: Valve area is less than 0.6 cm2 and the pressure gradient is more than 60 mmHg.