The evaluation and screening of aortic stenosis is a routine calculation performed on all complete echocardiograms. The detailed evaluation involves multiple key parameters that make up the equation that determines the aortic valve area (AVA). With echo being the gold-standard to evaluating and diagnosing the severity of aortic stenosis, it is crucial we understand the methodology of the continuity equation and how the fluid dynamics play a key role in the calculation.
The continuity equation is based on the conservation of mass… since the volume of blood cannot be ‘lost’ – this theory supports the concept that what flows in, must flow out! In other terms: the volume proximal to valve is the same as the volume downstream from the valve.
It evaluates the highest gradients across the valve to determine the effective orifice area (EOA). Due to fluid dynamics, higher gradients of a stenotic valve will occur post-valve (downstream) at the narrowest portion of jet flow, this is the location of the EOA. The calculated EOA is smaller than the anatomical orifice area (AOA), which is the planimetry traced area. The EOA (continuity equation) is the primary predictor of clinical outcomes.
The continuity equation involves 4 key elements:
The stroke volume through the LVOT is equal to the stroke volume through the aortic valve. We can obtain 3 of the 4 components by calculating the stroke volume of the LVOT and measuring the peak AV velocity (V2). This allows us to solve for the 4th component, which is the AVA!
By filling in echo-obtained calculations into the conservation of mass theory, we can calculate the continuity equation for the aortic valve area (AVA)!
To calculate the AVA using the continuity equation, we need 3 parameters:
Let’s break down the equation to better understand the methodology behind it!
First, let’s discuss how we calculate Doppler stroke volumes. Doppler stroke volume (SV) is the volume of blood that passes through a specific location during each beat [mL]. Since the continuity equation states there is equal SV of blood going in and coming out of an orifice, we need to calculate the SV of one of the locations.
We can calculate the SV of the LVOT by measuring the:
To determine the cross-sectional area (cm²), we need to measure the diameter (d) of the LVOT.
The second part to calculating the stroke volume is to obtain a pulsed-wave (PW) Doppler of the LVOT.
Now that we have the 2 components, we can calculate the Doppler SV of the LVOT!
We have now obtained 2 of the 3 components needed for the continuity equation, we need the last part:
The 3rd component needed to calculate the AVA is the peak AV VTI! This antegrade systolic flow should be evaluated in multiple acoustic windows to determine the highest velocity.
Review our past blogs that cover tips and techniques for measuring and obtaining the aortic valve velocity!
Finally we can calculate the AVA using the continuity equation after obtaining the 3 parameters: LVOT diameter, LVOT VTI & AV Peak VTI.
The chart listed below are the cut-off ranges for determining the severity of stenosis using the continuity equation:
There are limitations to the continuity equation due to the multiple variables involved in the equation. The following are key limitations that should be taken into consideration when determining the AVA:
There are other theoretical concerns regarding the continuity equation as well:
On a positive note- there are advantages to the continuity equation method too!
The continuity equation is an important concept to understand and apply to our patients. Being able to comprehend the methodology and flow dynamics is essential to ensuring proper and correct methods when calculating the aortic valve area.
Andrea Fields MHA, RDCS
Lang, R. M., MD, Badano, L. P., MD, & Mor-Avi, V., PhD. (2015). Recommendations for Cardiac Chamber Quantification by Echocardiography in Adults: An Update from the American Society of Echocardiography and the European Association of Cardiovascular Imaging. JASE, 28(1), 1-53. Retrieved March 1, 2017, from http://asecho.org/wordpress/wp-content/uploads/2015/01/ChamberQuantification2015.pdf