Echo Reference — Valve Stenosis
Aortic Stenosis Severity Grading on Echocardiography and AVA Calculator
Adult severity thresholds for peak velocity, mean gradient, aortic valve area, indexed AVA and dimensionless index — with a continuity-equation calculator, a stepwise approach to discordant and low-flow low-gradient AS, dobutamine stress echo and CT calcium score criteria, and the measurement pitfalls that most often mislead.
Severity Grading
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| Parameter | Aortic sclerosis | Mild | Moderate | Severe |
|---|---|---|---|---|
| Peak velocity, Vmax (m/s) | ≤ 2.5 | 2.6 – 2.9 | 3.0 – 3.9 | ≥ 4.0 |
| Mean gradient (mmHg) | — | < 20 | 20 – 39 | ≥ 40 |
| AVA, continuity (cm²) | — | > 1.5 | 1.0 – 1.5 | < 1.0 |
| Indexed AVA (cm²/m²) | — | > 0.85 | 0.60 – 0.85 | < 0.60 |
| Dimensionless index, DI (VTI ratio) | — | > 0.50 | 0.25 – 0.50 | < 0.25 |
Very severe AS: Vmax ≥ 5.0 m/s or mean gradient ≥ 60 mmHg. Low flow: stroke volume index ≤ 35 mL/m². High-gradient AS (Vmax ≥ 4.0 m/s or mean gradient ≥ 40 mmHg) is severe regardless of the calculated AVA unless a high-flow state or a measurement error explains the gradient. Aortic sclerosis is thickened or calcified leaflets with Vmax ≤ 2.5 m/s — a low velocity alone does not diagnose sclerosis; valve morphology is required, and mild AS likewise requires a thickened or calcified valve, not just a velocity of 2.6–2.9 m/s.
Conventions. ESC/EACTS 2025 writes its low-flow definitions as AVA ≤ 1.0 cm² and SVi ≤ 35 mL/m² where ASE/EACVI use < 1.0 and < 35; the boundaries are the same to one decimal place, and a value a few hundredths either side of a cut-off does not change the diagnosis on its own.
Key Equations
Continuity equation
AVA = (CSALVOT × VTILVOT) ÷ VTIAV
Flow in = flow out; result in cm²
LVOT area
CSALVOT = π × (D ÷ 2)² = 0.785 × D²
Diameter is squared — its error dominates
Dimensionless index
DI = VTILVOT ÷ VTIAV
Peak-velocity ratio is an accepted alternative
Stroke volume
SV = CSALVOT × VTILVOT
mL per beat
Stroke volume index
SVi = SV ÷ BSA
Low flow ≤ 35 mL/m²
Gradients (Bernoulli)
ΔP = 4v²; mean gradient ≈ 2.4 × Vmax²
Use the approximation as an internal consistency check
AVA Calculator — Continuity Equation
Calculates aortic valve area, dimensionless index, stroke volume and their indexed values, grades each parameter separately, and gives an integrated interpretation that flags discordance in both directions. Peak velocity, mean gradient and LVEF are optional but are needed for the integrated read. BSA is calculated by the Du Bois formula from height and weight, or can be entered directly.
Aortic Stenosis — Continuity Equation
LVOT diameter, LVOT VTI and AV VTI are required. Add Vmax, mean gradient, LVEF and BSA (or height and weight) for flow state, indexed values and the integrated interpretation.
Worked Example
LVOT diameter 2.0 cm gives a cross-sectional area of 0.785 × 2.0² = 3.14 cm². With an LVOT VTI of 22 cm the stroke volume is 3.14 × 22 = 69 mL. With an AV VTI of 95 cm, AVA = 3.14 × 22 ÷ 95 = 0.73 cm² and DI = 22 ÷ 95 = 0.23. At a BSA of 1.85 m², indexed AVA is 0.39 cm²/m² and SVi is 37 mL/m² (normal flow). With Vmax 4.3 m/s and mean gradient 45 mmHg this is concordant high-gradient severe AS. If instead the CW trace showed Vmax 3.4 m/s and mean gradient 28 mmHg, the same AVA would be normal-flow low-gradient — and the first step is the LVOT diameter: re-measuring it at 2.1 cm rather than 2.0 cm raises the AVA to 0.80 cm² without any change in the Doppler traces.
How the Measurements Are Acquired
The continuity equation is only as good as its three inputs, and the LVOT diameter and PW sample position must be treated as a pair — the diameter is measured where the velocity is sampled. Record blood pressure at the time of the study: hypertension changes loading and flow and can push gradients and calculated AVA in either direction, so a borderline or discordant study should be reassessed once BP is controlled.
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| Measurement | View / modality | Technique | Most common error |
|---|---|---|---|
| LVOT diameter | Zoomed parasternal long-axis, mid-systole | Inner edge to inner edge, parallel to the valve plane, at the annulus or within 5–10 mm below it — at the same position as the PW sample volume, and at the same position on every serial study. 3D or CT LVOT area can be used to investigate a clearly elliptical outflow tract, but the resulting hybrid AVA is larger and is not read against the 2D cut-offs (see pitfalls). | Oblique or non-zoomed measurement; measuring in diastole; including annular calcium in the diameter. A 1 mm error changes AVA by roughly 8–10 %. |
| LVOT VTI | Apical 5- or 3-chamber, PW Doppler | Sample volume at the diameter position, just proximal to the flow-acceleration zone. Accept a narrow, laminar envelope with a clear closing click; trace the modal velocity. If the envelope broadens, move 1–2 mm apically. | Sampling too deep in the LVOT underestimates VTI, lowers the calculated AVA and overestimates severity; sampling inside the acceleration zone does the reverse. |
| AV Vmax and VTI | CW Doppler — apical, right parasternal, suprasternal, subcostal; non-imaging (Pedoff) probe | Interrogate every window and report the highest velocity obtained. Trace the outer edge of the dense envelope, excluding faint fringes and transit-time artefact. Average 3 beats in sinus rhythm. | Angle error from a single apical window underestimates velocity and gradient; contamination by an MR jet overestimates both. |
| Mean gradient | Same CW trace | Software integrates the instantaneous gradients across ejection. Check the trace against Vmax: mean gradient should approximate 2.4 × Vmax². | Tracing the mean velocity and squaring it, or including the closing click, distorts the mean gradient. |
| Rhythm | All Doppler measurements | In atrial fibrillation, average 5–10 consecutive beats or use cycles of matched R–R interval for LVOT and AV traces. Avoid post-ectopic beats. | Mismatched cycle lengths between the LVOT and AV traces make AVA and DI meaningless. |
| Blood pressure and BSA | Cuff pressure at the time of echo; height and weight | Document BP; reassess borderline or discordant studies once hypertension is controlled. Index AVA and stroke volume to BSA, stating the formula. | Indexing exaggerates severity in obesity (BSA rises with weight while the annulus does not); in small adults it helps avoid overcalling severe AS when the absolute AVA is small but the indexed AVA is ≥ 0.6 cm²/m². |
Discordant AS — Stepwise Assessment
SVi > 35 mL/m² — normal flow
SVi ≤ 35 mL/m² — low flow
Step 1 — Confirm the Measurements
Measurement error produces exactly the same pattern as true low-flow AS, and the stroke volume index is calculated from the same LVOT diameter and VTI as the AVA — an underestimated diameter lowers both, so the study looks "low flow" for the same reason it looks "severe". Re-measure the LVOT diameter in a zoomed mid-systolic frame, confirm the PW sample position matches it, re-interrogate the jet from every window, and check the R–R intervals used for the LVOT and AV traces. Use the indexed AVA in small or large adults, and consider 3D or CT LVOT area if the outflow tract is elliptical — reporting the method, since hybrid AVA is not interchangeable with 2D continuity AVA. Only once the numbers survive this step should a low-flow paradigm be invoked.
Step 2 and 3 — Flow State and Classification
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| Category | Definition | Usual mechanism | Confirmation |
|---|---|---|---|
| Normal-flow low-gradient | AVA < 1.0 cm², MG < 40 mmHg, SVi > 35 mL/m², LVEF ≥ 50 % | Usually moderate AS; small annulus in a small patient; residual measurement error | Indexed AVA and DI; CT calcium scoring if concern persists |
| Low-gradient, normal flow, reduced LVEF | AVA < 1.0 cm², MG < 40 mmHg, SVi > 35 mL/m², LVEF < 50 % | Not a defined guideline category — dilated LV with preserved stroke volume; measurement error must be excluded | Integrated reassessment; CT calcium scoring |
| Classical low-flow low-gradient | AVA ≤ 1.0 cm², MG < 40 mmHg, SVi ≤ 35 mL/m², LVEF < 50 % | Reduced LV systolic function lowers stroke volume; true severe AS and pseudo-severe AS look identical at rest | Low-dose dobutamine stress echo (flow reserve, projected AVA); CT calcium scoring if no flow reserve |
| Paradoxical low-flow low-gradient | AVA ≤ 1.0 cm², MG < 40 mmHg, SVi ≤ 35 mL/m², LVEF ≥ 50 % | Small, hypertrophied, restrictive LV; AF; significant MR; older women predominate | Exclude measurement error, then CT calcium scoring |
Transaortic flow rate (stroke volume ÷ ejection time) below about 200 mL/s is an alternative marker of low flow that is less dependent on body size and is worth reporting where SVi sits near the cut-off.
The Other Direction — High Gradient With AVA Above 1.0 cm²
Vmax ≥ 4.0 m/s or mean gradient ≥ 40 mmHg with a calculated AVA > 1.0 cm² is also discordant, and is not moderate AS by default. Look first for an overestimated LVOT diameter (a 1 mm overestimate inflates the AVA by about 10 %), then for a high-flow state — significant aortic regurgitation, anaemia, thyrotoxicosis, fever or sepsis, an arteriovenous fistula, pregnancy — and check the indexed AVA in large patients. If none of these explains the gradient, high gradients indicate severe AS, and the guidelines grade the valve as severe on the gradient.
Dobutamine Stress Echo and CT Calcium Scoring
Low-dose dobutamine (5 to a maximum of 20 µg/kg/min in 3–5-minute stages, stopped at the top dose, once flow augmentation is adequate, or for symptoms, arrhythmia or hypotension) is used in classical low-flow low-gradient AS with LVEF < 50 % to separate true severe from pseudo-severe stenosis and to document flow (contractile) reserve. In paradoxical LFLG-AS with preserved LVEF, CT calcium scoring is generally preferred; DSE is not routinely used, although selected cases may warrant specialist assessment. ESC/EACTS 2025 places CT aortic valve calcium scoring alongside dobutamine stress echo as a confirmatory test, and CT is the route of choice when there is no flow reserve or when DSE is contraindicated or indeterminate.
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| DSE response | Interpretation | Next step |
|---|---|---|
| Flow reserve (SV rises ≥ 20 %); AVA stays ≤ 1.0 cm² and mean gradient rises to ≥ 40 mmHg (Vmax ≥ 4 m/s) | True severe AS | Severe AS confirmed — proceed on symptoms and LV function |
| Flow reserve; AVA rises above 1.0 cm² (typically > 1.2 cm²) with mean gradient still < 40 mmHg | Pseudo-severe AS | Moderate stenosis — the low resting AVA reflected reduced opening force. Treat the cardiomyopathy; re-image at interval |
| Flow reserve but an intermediate response (AVA around 1.0–1.2 cm², mean gradient 30–40 mmHg) | Indeterminate | Projected AVA at a normal flow rate; CT calcium scoring |
| No flow reserve (SV rises < 20 %) | Conventional criteria may remain indeterminate | Projected AVA may still help if the transvalvular flow rate rises by ≥ 15 %; otherwise CT calcium scoring. Absent flow reserve marks higher operative risk but does not exclude benefit from valve replacement — once severe AS is confirmed, SAVR or TAVI is recommended regardless of flow reserve (ESC/EACTS 2025) |
CT Aortic Valve Calcium Score
| Aortic valve calcium (Agatston units) | Men | Women |
|---|---|---|
| Severe AS unlikely | < 1,600 AU | < 800 AU |
| Indeterminate | 1,600 – 2,000 AU | 800 – 1,200 AU |
| Severe AS likely | ≥ 2,000 AU | ≥ 1,200 AU |
| Severe AS very likely | ≥ 3,000 AU | ≥ 1,600 AU |
| Calcium density — severe AS likely (AU per cm² of annulus area) | ≥ 500 AU/cm² | ≥ 300 AU/cm² |
Measurement Pitfalls and Special Situations
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| Situation | Effect on grading | Approach |
|---|---|---|
| LVOT diameter error | The largest source of error because the diameter is squared, and the effect is proportional rather than fixed. At a true diameter of 2.0 cm, measuring 1.9 cm lowers the AVA by 9.75 % (0.73 → 0.66 cm² in the worked example) and measuring 2.1 cm raises it by 10.25 % (→ 0.80 cm²); at 2.5 cm a 1 mm error is about 8 %. | Zoomed PLAX, mid-systole, inner edge to inner edge, at the PW sample position. 3D or CT-derived LVOT area can be used when the outflow tract is elliptical, but state the method when reporting hybrid AVA: CT-derived LVOT area gives a larger AVA that is not interchangeable with 2D continuity AVA, with an equivalent severe cut-off nearer 1.2 cm² (Clavel 2015). |
| LVOT PW sample position | Too deep in the LVOT: VTI underestimated → AVA too small → severity overestimated. Inside the flow-acceleration zone: VTI overestimated → AVA too large → severity underestimated. | Accept a narrow, laminar envelope with a closing click; adjust the position by 1–2 mm rather than insisting on a fixed distance from the valve. |
| CW alignment and window | Any angle between beam and jet underestimates velocity (cos θ) — 20° costs about 6 % of velocity and 12 % of gradient. Eccentric jets, common in bicuspid valves, are missed from the apex alone. | Apical, right parasternal, suprasternal and subcostal windows with a non-imaging probe; report the highest velocity obtained. |
| MR jet mistaken for the AS jet | MR begins at mitral closure and spans both isovolumic periods, so it is longer than the AS signal, usually faster (5–6 m/s) and denser, and it overestimates both Vmax and mean gradient. | Time the signal against the ECG and the aortic closing click, compare duration with the LVOT trace, and confirm on the colour map. |
| Pressure recovery | In a small ascending aorta (sinotubular junction ≤ 30 mm) the Doppler gradient overestimates the net pressure loss and AS severity. | Energy loss index: ELI = [AVA × AAo ÷ (AAo − AVA)] ÷ BSA, using the aortic area at the sinotubular junction; ELI ≤ 0.6 cm²/m² supports severe AS. |
| Hypertension at the time of the study | Changes in loading and flow can push gradients and calculated AVA in either direction, so severity may be over- or underestimated and the flow state misread. | Record cuff BP with the study; reassess after BP control when results are borderline or discordant. |
| Atrial fibrillation | Beat-to-beat variation in stroke volume; a short preceding R–R interval underestimates the gradient, a long one overestimates it. Mismatched cycles between LVOT and AV traces invalidate AVA and DI. | Average 5–10 consecutive beats, or use cycles of matched R–R interval for both traces; avoid post-ectopic beats; state the method in the report. |
| Coexisting aortic regurgitation | AR increases forward stroke volume across the valve, so gradients are higher for a given AVA. The continuity AVA (and DI) remain valid, but the gradients are not artefact: in mixed aortic valve disease they reflect the total haemodynamic load and remain prognostically important. | Grade both lesions and report the combined burden. Do not dismiss a high gradient as "just the AR". |
| Coexisting mitral regurgitation | Significant MR reduces forward aortic flow: low gradients and a low SVi despite normal LVEF, mimicking paradoxical LFLG-AS. Also the commonest source of CW jet contamination. | Continuity AVA and DI; CT calcium scoring where the flow state is confounded. |
| High-output states | Anaemia, thyrotoxicosis, fever or sepsis, arteriovenous fistula and pregnancy inflate gradients out of proportion to the orifice. | Rely on AVA and DI; re-image once the high-output state has resolved. |
| Body size | Indexing exaggerates severity in obesity because BSA rises with weight while the annulus does not. In very small adults an AVA < 1.0 cm² with indexed AVA ≥ 0.6 cm²/m² may represent moderate AS. | Report both absolute and indexed AVA and interpret with the gradients and calcium burden. |
| Bicuspid aortic valve | Presents one to two decades earlier than calcific trileaflet AS; jets are eccentric and calcium is lighter for the same severity, so CT calcium thresholds may underperform. Aortopathy is common. | Multiple CW windows; report the aortic root and ascending aorta on every study (see Aortic Dimensions). |
| Subvalvular or supravalvular obstruction; HCM | A high, late-peaking (dagger-shaped) LVOT velocity means the continuity equation and the total CW gradient no longer describe the valve alone. | Localise the level of obstruction with colour and PW mapping; report valvular and subvalvular components separately. |
Progression and Surveillance
Haemodynamic progression averages roughly 0.3 m/s per year in Vmax, 7 mmHg per year in mean gradient and 0.1 cm² per year in AVA across cohorts, but the range is wide and individual prediction is poor: progression is slower in mild, lightly calcified disease and faster with heavy calcification, older age, chronic kidney disease and bicuspid morphology. A Vmax increase of ≥ 0.3 m/s per year with moderate-to-severe calcification identifies rapid progressors at high event risk and, in asymptomatic severe AS, is an intervention criterion in ESC/EACTS 2025 (Class IIa in patients at low procedural risk, alongside Vmax > 5 m/s or mean gradient ≥ 60 mmHg, and BNP more than three times the age- and sex-adjusted normal) and in ACC/AHA 2020.
| Severity | Surveillance echo — asymptomatic, normal LV function (ACC/AHA 2020) |
|---|---|
| Mild | Every 3–5 years |
| Moderate | Every 1–2 years |
| Severe | Every 6–12 months |
These intervals apply to asymptomatic patients with normal LV function. ESC/EACTS guidance is similar — six-monthly for severe AS, yearly for calcified mild or moderate AS, and every two to three years for younger patients with mild non-calcified disease. New symptoms, a fall in LVEF, exercise-induced symptoms or a change in examination findings prompt reassessment irrespective of the interval.
Frequently Asked Questions
What defines severe aortic stenosis on echocardiography?
A peak velocity ≥ 4.0 m/s or mean gradient ≥ 40 mmHg with an aortic valve area ≤ 1.0 cm² (indexed ≤ 0.6 cm²/m²). When gradients and valve area agree, grading is straightforward. When the AVA is in the severe range but the gradients are not, the study is discordant, and the flow state, LVEF and — where the echo cannot settle it — dobutamine stress echo or CT calcium scoring determine whether the stenosis is truly severe.
What is low-flow low-gradient aortic stenosis?
An AVA ≤ 1.0 cm² with a mean gradient below 40 mmHg and a stroke volume index ≤ 35 mL/m². Classical low-flow low-gradient AS occurs with a reduced LVEF (below 50 %) and is assessed with low-dose dobutamine stress echo. Paradoxical low-flow low-gradient AS occurs with a preserved LVEF, usually in a small hypertrophied ventricle, and is usually confirmed with CT aortic valve calcium scoring. Before either label is applied, the LVOT diameter and Doppler sampling must be verified, because measurement error produces the same pattern.
Why do the valve area and the gradient disagree?
The gradient depends on flow as well as on orifice size, whereas the AVA is calculated from three measurements, one of which is squared. Discordance therefore arises from low flow (reduced LVEF, a small hypertrophied ventricle, atrial fibrillation, mitral regurgitation), from measurement error (most often the LVOT diameter or the PW sample position), from body size, or from a high-flow state that inflates the gradient. Around a third of patients assessed for AS show some form of discordance.
How is the aortic valve area calculated?
By the continuity equation: the volume of blood crossing the outflow tract equals the volume crossing the valve, so AVA = (LVOT area × LVOT VTI) ÷ AV VTI. The LVOT area is derived from its diameter (π × radius²), which is why a 1 mm error in the diameter changes the AVA by roughly 10 %. The dimensionless index (LVOT VTI ÷ AV VTI) avoids the diameter altogether but is still subject to Doppler sampling errors; a value below 0.25 supports severe AS.
When is CT calcium scoring used rather than dobutamine stress echo?
CT aortic valve calcium scoring is independent of flow, so it is generally preferred in paradoxical low-flow low-gradient AS with preserved LVEF, and is the confirmatory test in classical low-flow low-gradient AS when there is no flow reserve or dobutamine echo is contraindicated or indeterminate, and in normal-flow low-gradient AS when doubt persists. Sex-specific thresholds apply: severe AS is likely above about 2,000 Agatston units in men and 1,200 in women, and very likely above 3,000 and 1,600. It scores the valve, not the coronary arteries, and can underestimate severity in fibrotic, bicuspid or rheumatic valves.
How often should aortic stenosis be re-scanned?
For asymptomatic patients with normal LV function, the ACC/AHA intervals are every 3–5 years for mild AS, every 1–2 years for moderate AS and every 6–12 months for severe AS. New symptoms, a fall in LVEF, exercise-induced symptoms or a change in examination findings warrant an earlier study regardless of the schedule.
Related Echocardiography References
References
- Baumgartner H, Hung J, Bermejo J, et al. Recommendations on the echocardiographic assessment of aortic valve stenosis: a focused update from the European Association of Cardiovascular Imaging and the American Society of Echocardiography. J Am Soc Echocardiogr. 2017;30(4):372–392. doi:10.1016/j.echo.2017.02.009
- Praz F, Beyersdorf F, Haugaa K, Prendergast B, et al. 2025 ESC/EACTS Guidelines for the management of valvular heart disease. Eur Heart J. 2025; ehaf194. doi:10.1093/eurheartj/ehaf194
- Otto CM, Nishimura RA, Bonow RO, et al. 2020 ACC/AHA Guideline for the Management of Patients With Valvular Heart Disease. Circulation. 2021;143(5):e72–e227. doi:10.1161/CIR.0000000000000923
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