What is the continuity equation for aortic valve area?

The continuity equation states that blood flow is conserved across the left ventricular outflow tract (LVOT) and the aortic valve. AVA = (LVOT area x LVOT VTI) / AV VTI. LVOT area is calculated from the LVOT diameter as pi x (diameter/2)^2. VTI (velocity-time integral) is measured by pulsed Doppler at the LVOT and continuous-wave Doppler at the aortic valve. This is the recommended method in current clinical guidelines for assessing aortic stenosis severity.

What AVA indicates severe aortic stenosis?

According to ACC/AHA and ESC guidelines, severe aortic stenosis is defined as an AVA of less than 1.0 cm^2 (or indexed AVA less than 0.6 cm^2/m^2 of body surface area). Moderate stenosis is 1.0 to 1.5 cm^2, and mild stenosis is greater than 1.5 cm^2. Very severe stenosis is sometimes defined as AVA less than 0.6 cm^2. The AVA is used alongside mean gradient (severe at over 40 mmHg) and peak velocity (severe at over 4 m/s) to classify stenosis. Discordant findings between these parameters require additional evaluation.

What is the difference between AVA and indexed AVA?

Aortic valve area (AVA) is the absolute opening area of the valve in cm^2. Indexed AVA (AVAi) adjusts for body size by dividing AVA by the body surface area (BSA) in m^2. AVAi is particularly useful in patients with small or large body habitus because a small absolute AVA may represent less severe stenosis in a very small patient, and a larger AVA may still represent significant stenosis in a very large patient. The threshold for severe stenosis by AVAi is less than 0.6 cm^2/m^2.

Aortic Valve Area Calculator

This calculator works out aortic valve area (AVA) from echocardiography readings using the Doppler continuity equation, the method recommended in current cardiology guidelines for grading aortic stenosis severity. You enter the left ventricular outflow tract (LVOT) diameter, the LVOT velocity-time integral, and the aortic valve velocity-time integral, plus optional mean gradient and peak velocity figures from the echo report. Add height and weight and pick a body surface area formula (Du Bois or Mosteller) to also get an indexed AVA, which adjusts for body size, and enter a stroke volume index if you want the tool to flag possible low-flow, low-gradient disease. The calculator returns the AVA in cm², the indexed AVA in cm²/m², the LVOT area, and a severity rating of mild, moderate, severe or very severe, alongside a full breakdown of the continuity equation working and a written interpretation of the result. Because LVOT diameter is squared in the formula, small measurement errors are magnified considerably, so use the most accurate diameter reading available and check the AVA against the mean gradient and peak velocity for consistency. This tool is for educational and reference purposes only. It does not replace a full echocardiographic assessment, and any decision about ongoing monitoring or aortic valve replacement should be made by a qualified cardiologist reviewing the complete clinical picture.

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Updated June 2026 Based on ACC/AHA 2014 Valvular Heart Disease guidelines and ASE/EACVI echocardiography standards.

1. Echocardiography Measurements

LVOT diameter (cm)

LVOT VTI (velocity-time integral, cm)

Aortic valve VTI (cm)

Mean gradient (optional, mmHg)

mmHg

Peak velocity (optional, m/s)

m/s

2. Patient Details (optional)

Height (for indexed AVA)

Weight (for indexed AVA)

BSA method

Stroke volume index (for low-flow, optional, mL/m²)

mL/m²

Aortic Valve Area Results

AVA
-
cm² (continuity equation)

Indexed AVA

cm²/m²

LVOT Area

cm²

Severity

Based on AVA

Continuity Equation Breakdown

LVOT diameter-

LVOT area (pi x r²)-

LVOT VTI-

LVOT stroke volume-

Aortic valve VTI-

AVA = LVOT SV / AV VTI-

Severity Classification

AVA-

Indexed AVA (AVAi)-

Mean gradient-

Peak velocity-

BSA-

Overall classification-

Interpretation: Enter measurements above to see the result.

Aortic Stenosis Severity Classification

Classification	AVA (cm²)	Indexed AVA (cm²/m²)	Mean Gradient (mmHg)	Peak Velocity (m/s)
Very severe	< 0.6	< 0.4	> 60	> 5.0
Severe	< 1.0	< 0.6	> 40	> 4.0
Moderate	1.0 to 1.5	0.6 to 0.85	20 to 40	3.0 to 4.0
Mild	> 1.5	> 0.85	< 20	< 3.0

How Aortic Valve Area is Calculated

The aortic valve area (AVA) is the effective orifice area of the aortic valve during systole. In clinical echocardiography, it is most commonly measured using the continuity equation, which is based on the conservation of flow: the volume of blood passing through the left ventricular outflow tract (LVOT) must equal the volume passing through the aortic valve orifice.

The Continuity Equation

The formula is:

AVA = (LVOT area x LVOT VTI) / AV VTI

where:

LVOT area = pi x (LVOT diameter / 2)² measured from the parasternal long-axis view just below the aortic valve
LVOT VTI = velocity-time integral at the LVOT from pulsed-wave Doppler in the apical 5-chamber or 3-chamber view
AV VTI = velocity-time integral at the aortic valve from continuous-wave Doppler

The product LVOT area x LVOT VTI equals the LVOT stroke volume (in mL). Dividing by AV VTI gives the effective aortic valve orifice area in cm².

Worked Example

Using the default values:

LVOT diameter = 2.0 cm, so LVOT area = pi x 1.0² = 3.14 cm²
LVOT VTI = 24 cm, so LVOT stroke volume = 3.14 x 24 = 75.4 mL
AV VTI = 80 cm
AVA = 75.4 / 80 = 0.94 cm²

An AVA of 0.94 cm² with a mean gradient of 45 mmHg and peak velocity of 4.2 m/s is consistent with severe aortic stenosis.

Indexed AVA

The indexed aortic valve area (AVAi) accounts for patient body size by dividing the AVA by the body surface area (BSA). It is particularly useful for patients with unusually small or large body habitus. The Du Bois formula for BSA is: BSA = 0.007184 x height(cm)^0.725 x weight(kg)^0.425. The Mosteller formula is: BSA = sqrt(height x weight / 3600). Severe stenosis by AVAi is defined as less than 0.6 cm²/m².

Low-Flow, Low-Gradient Aortic Stenosis

In patients with reduced left ventricular ejection fraction (LVEF below 50%) or reduced stroke volume index (SVI below 35 mL/m²), severe stenosis may produce low gradients despite a small valve area. This is termed "classical low-flow, low-gradient" severe aortic stenosis and requires dobutamine stress echocardiography or CT aortic valve calcium scoring for further evaluation. A stroke volume index below 35 mL/m² with AVA below 1.0 cm² and mean gradient below 40 mmHg should prompt additional investigation.

Other Assessment Methods

The Gorlin formula, which uses catheter-derived data, was the historical gold standard but has largely been replaced by the non-invasive continuity equation. CT measurement of aortic valve calcium score is increasingly used as a complementary method, particularly in discordant cases. A calcium score above 2,000 AU in men or above 1,200 AU in women supports severe stenosis even with low gradients.

Clinical Context

AVA is one of several parameters used to grade aortic stenosis. Guidelines recommend integrating AVA, mean gradient, peak velocity, and stroke volume index with clinical findings (symptoms, LV function, valve morphology) before making management decisions. Aortic valve replacement (surgical or transcatheter TAVI/TAVR) is indicated for severe symptomatic aortic stenosis. This calculator is an educational tool; it does not replace clinical assessment by a qualified cardiologist.

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Sources and method: Otto CM et al. 2020 ACC/AHA Guideline for the Management of Patients With Valvular Heart Disease. Nishimura RA et al. 2014 AHA/ACC Guideline. Baumgartner H et al. 2017 ESC/EACTS Guidelines on Valvular Heart Disease. Gorlin R, Gorlin SG. Hydraulic formula for calculation of the area of the stenotic mitral valve, other cardiac valves, and central circulatory shunts. Am Heart J. 1951. Continuity equation: AVA = (pi x (LVOT d/2)² x LVOT VTI) / AV VTI.

This calculator is for educational and reference purposes only. It does not constitute medical advice and must not be used as the sole basis for clinical decisions. All echocardiographic measurements should be reviewed by a qualified cardiologist in the context of the full clinical assessment. Measurement errors in LVOT diameter are amplified because diameter is squared in the calculation; a 5% error in LVOT diameter results in approximately a 10% error in AVA.