The continuity equation calculates aortic valve area (AVA) by equating stroke volume through the left ventricular outflow tract (LVOT) to stroke volume through the aortic valve. The equation is:
AVA = (Cross-sectional area of LVOT) × (LVOT VTI) / (Aortic valve VTI)
The cross-sectional area of the LVOT is derived from the LVOT diameter, using the formula π × (diameter/2)^2. Because the diameter is squared in this calculation, even a small error in measurement leads to a significant error in the calculated valve area.
This makes the LVOT diameter measurement the greatest source of error when calculating AVA by the continuity equation. Errors in Doppler velocity measurements (LVOT velocity or aortic jet velocity) are also important but less impactful compared to diameter measurement error.
Aortic valve planimetry is a direct measurement method, not part of the continuity equation. LVOT velocity recorded with pulsed Doppler and aortic jet velocity by continuous wave Doppler are important but not the greatest error source.
This is a well-established concept described in the "Textbook of Clinical Echocardiography, 6e", Chapter on Valvular Stenosis and Continuity Equation Methodology【20:370-375†Textbook of Clinical Echocardiography】.
