Aortic Stenosis: Echocardiographic Assessment

Aortic stenosis (AS) is narrowing of the aortic valve orifice that obstructs left ventricular outflow during systole. It is the most common primary valve lesion requiring intervention in high-income countries, and echocardiography is the first-line and usually definitive test for diagnosis and severity grading. This tutorial covers etiology, clinical presentation, and the 2D and Doppler assessment used to grade severity, with thresholds drawn from the 2020 ACC/AHA and 2025 ESC/EACTS valvular heart disease guidelines.

Etiology

• Degenerative (senile, fibrocalcific) — progressive calcification of a trileaflet valve, most commonly seen in patients presenting in their late 60s to 70s. This is the leading cause of AS in high-income countries.
• Congenital — most often a bicuspid aortic valve (BAV), which calcifies and becomes stenotic roughly a decade earlier than trileaflet degenerative disease. A bicuspid valve is also one of the recognized cardiac anomalies associated with congenital absence of the pericardium, and carries its own distinct ascending aortic dilation pattern — see Diseases of the Aorta for the specific aneurysm surveillance thresholds that apply to BAV.
• Rheumatic — commissural fusion following rheumatic heart disease, less common in regions with access to childhood rheumatic fever treatment but still relevant globally, and almost always accompanied by rheumatic mitral disease.

Clinical Presentation

Symptoms — the classic triad, typically appearing only once stenosis is severe:

• Angina, from increased myocardial oxygen demand in a hypertrophied ventricle facing a fixed obstruction
• Syncope, especially with exertion, due to a fixed cardiac output that cannot rise to meet peripheral vasodilation
• Heart failure, a late manifestation and a marker of a poor prognosis if intervention is delayed

Auscultation:

• A harsh, crescendo-decrescendo systolic ejection murmur, best heard at the right upper sternal border and radiating to the carotids
• An S4 gallop, reflecting atrial contraction against a stiff, hypertrophied ventricle
• Pulsus parvus et tardus — a slow-rising, late-peaking carotid pulse — on physical exam

Complications of longstanding pressure overload include LV hypertrophy, elevated LV end-diastolic and left atrial pressures, late LV systolic dysfunction, and (rarely) pulmonary hypertension, along with an increased risk of infective endocarditis and sudden cardiac death.

Diagnostic Testing Beyond Echocardiography

• ECG — LV hypertrophy, left atrial enlargement, and atrial fibrillation may all be seen, though none are specific enough to grade severity on their own.
• Chest X-ray — LV hypertrophy, left atrial enlargement, and post-stenotic dilation of the ascending aorta.
• Cardiac catheterization — directly measures the peak-to-peak and mean transvalvular pressure gradients, and can calculate AVA using the Gorlin formula. Largely reserved today for cases where noninvasive imaging is inconclusive or discordant with the clinical picture, since it is invasive and gradients are flow-dependent here too.

2D and M-Mode Findings

Bicuspid Aortic Valve

• Thickened leaflets with an eccentric closure line on M-mode. An eccentricity index (EI) ≥1.5 supports a bicuspid valve, where EI = 0.5 × (A/a) — A being the internal aortic root diameter at the onset of diastole, and a being the distance from cusp coaptation to the nearest aortic wall at the onset of diastole.
• On 2D imaging: thickened leaflets with systolic doming, and a “football-shaped” systolic orifice best seen in the parasternal short-axis view.
• A raphe — an underdeveloped, fused cusp — can give the false appearance of three leaflets in diastole; the parasternal short-axis view in systole is the more reliable view to correctly identify a bicuspid valve.
• BAV is the most common congenital cause of AS and often becomes symptomatic between the ages of 20 and 50 — distinctly earlier than calcific degenerative AS, which is a useful clue to etiology on its own.

Degenerative or Rheumatic Aortic Stenosis

• Increased leaflet thickness with reduced excursion (typically < 15 mm) on M-mode and 2D imaging
• Thickened, calcified valve leaflets on 2D short- and long-axis views
• Post-stenotic dilation of the ascending aorta
• LV hypertrophy with increased mass; decreased LV systolic function is a late finding. This hypertrophy is secondary and typically concentric — worth distinguishing from the asymmetric, often more severe hypertrophy of hypertrophic cardiomyopathy, particularly when the two coexist or when the aortic valve gradient seems disproportionate to the degree of stenosis seen on 2D imaging.

Doppler Assessment of Severity

Four Doppler-derived parameters are used, together, to grade severity:

1. Peak aortic jet velocity (Vmax) — obtained with continuous-wave Doppler, interrogating the valve from multiple acoustic windows (apical, right parasternal, suprasternal, supraclavicular, subcostal, left parasternal) to ensure the beam is as parallel to flow as possible and the true peak is captured.
2. Mean transvalvular gradient — derived from the traced CW Doppler velocity curve using the simplified Bernoulli equation.
3. Aortic valve area (AVA) by the continuity equation — calculated from the LVOT diameter, LVOT velocity-time integral (VTI), and aortic valve VTI:

AVA (cm²) = (LVOT area × LVOT VTI) ÷ AV VTI, where LVOT area is calculated from the measured LVOT diameter assuming a circular cross-section. Because diameter is squared in the area calculation, small measurement errors in LVOT diameter produce disproportionately large errors in the final AVA — this is the single most common source of continuity-equation inaccuracy in practice.

4. Velocity ratio (dimensionless index) — the ratio of LVOT VTI to AV VTI (or peak LVOT velocity to peak AV velocity). Because it is a ratio, it doesn’t require the LVOT diameter measurement at all, which makes it a useful cross-check when that measurement is unreliable. A velocity ratio ≤0.25 suggests severe AS is highly likely, consistent between the ACC/AHA and ESC/EACTS guidelines.

Echocardiographic assessment of aortic stenosis — video walkthrough

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[00:00] Welcome to this overview of the echocardiographic assessment of aortic stenosis. [00:12] We'll cover the three key Doppler parameters: peak velocity, mean gradient, and aortic valve area by the continuity equation. [00:35] We'll also discuss why flow matters, and how low-flow, low-gradient disease can make grading severity more difficult. [01:05] This is placeholder transcript text for the video embed component and should be replaced with a real transcript before publication.

Doppler Technical Pearls

• Evaluate LV systolic function alongside velocity — flow state has a direct effect on peak aortic velocity and gradient (see Flow-Gradient Patterns below).
• Significant concomitant aortic regurgitation (3+ or 4+) increases forward stroke volume across the aortic valve, which increases velocity and gradient independent of true stenosis severity — AVA by the continuity equation becomes the more reliable parameter in this situation, since it accounts for the actual flow.
• Don’t confuse the AS jet with a mitral regurgitation jet on CW Doppler from the apical window — the MR jet typically has a longer duration and a higher velocity, reflecting the larger pressure gradient between LV and left atrium compared to LV and aorta.
• Jet contour can offer a qualitative clue to severity: an asymmetric, triangular contour that peaks early in systole is more typical of mild AS, while a symmetric, rounded contour that peaks late (jet peak occurring beyond 50% of ejection time) is more typical of severe AS. This is supportive, not a primary grading criterion.
• When findings are borderline or inconsistent with the clinical picture, catheterization remains a reasonable way to confirm the mean gradient and AVA.

Grading Severity

ACC/AHA Table 13 (Stages of AS) gives the following peak velocity and mean gradient bands. Note that AVA is a supportive, not required, criterion for severe AS once the velocity/gradient threshold is met:

Severity	Peak velocity (Vmax)	Mean gradient	AVA
Mild	2.0–2.9 m/s	< 20 mmHg	—
Moderate	3.0–3.9 m/s	20–39 mmHg	—
Severe	≥ 4.0 m/s	≥ 40 mmHg	typically ≤ 1.0 cm² (or AVAi ≤ 0.6 cm²/m²), but not required to define severity
Very severe	≥ 5.0 m/s	≥ 60 mmHg	—

A velocity ratio ≤0.25 supports severe AS as well. Some older echocardiography-specific classifications reserve 2.0–2.5 m/s for “aortic sclerosis” (valve thickening without significant obstruction) and begin “mild AS” at 2.6 m/s — if you see that convention elsewhere, it hasn’t been superseded so much as it predates the current guideline table above, which the site follows as the primary reference.

Flow-Gradient Patterns

Because gradient is flow-dependent, AS is further categorized by flow state whenever echocardiographic parameters are discordant (e.g., a small AVA with a gradient below the severe threshold). Stroke volume index (SVi) is used to define low versus normal flow, conventionally at a threshold of 35 mL/m²:

• Concordant severe (high-gradient) AS — mean gradient ≥40 mmHg, Vmax ≥4.0 m/s, AVA ≤1 cm² (or ≤0.6 cm²/m²). Considered severe regardless of flow state.
• Classic low-flow, low-gradient AS — mean gradient < 40 mmHg, AVA ≤1 cm², SVi ≤35 mL/m², LVEF < 50%.
• Paradoxical low-flow, low-gradient AS — mean gradient < 40 mmHg, AVA ≤1 cm², SVi ≤35 mL/m², LVEF ≥50% (a small, hypertrophied, poorly compliant ventricle producing reduced stroke volume despite preserved EF). This is also the pattern cardiac amyloidosis frequently produces when it coexists with severe AS — worth actively considering, since roughly 1 in 8 patients referred for TAVI with severe AS has coexisting amyloid.
• Normal-flow, low-gradient AS — mean gradient < 40 mmHg, AVA ≤1 cm², SVi >35 mL/m², LVEF ≥50%. Usually represents only moderate stenosis.
• Discordant high-gradient AS — mean gradient ≥40 mmHg but AVA >1 cm². Considered severe unless caused by a reversible high-flow state.

In the two low-flow, low-gradient patterns, further testing is used to distinguish truly severe from pseudo-severe disease:

• Dobutamine stress echocardiography (DSE) — an increase in stroke volume of ≥20% with dobutamine defines the presence of flow (contractile) reserve, which helps interpret whether the gradient response reflects true valve severity. Note that more recent evidence has questioned flow reserve’s independent prognostic value specifically — see Stress Echocardiography for how this finding has evolved and why projected AVA is now often preferred for classifying severity in this population. The same stress-testing principle — that resting hemodynamics can be misleadingly normal — applies to prosthetic aortic valves as well.
• Aortic valve calcium scoring by CT — in men, a score >2000 Agatston units (AU) indicates severe AS with high sensitivity and specificity (~85%), while a score < 1600 AU makes severe AS unlikely. In women, the corresponding thresholds are >1200 AU (severe likely) and < 800 AU (severe unlikely) — women generally calcify less for a given severity of stenosis, hence the lower cutoffs.

Common Pitfalls

• Missing the true peak velocity by relying on a single Doppler window — always interrogate from multiple acoustic windows.
• Mismeasuring the LVOT diameter, which is squared in the continuity equation and is a major source of error; the velocity ratio avoids this specific pitfall.
• Ignoring flow state when a gradient looks only moderately elevated in a ventricle with reduced stroke volume.
• Overlooking concomitant significant AR, which inflates velocity and gradient independent of true AS severity.
• Confusing pseudo-severe and truly severe AS in low-flow, low-gradient physiology without dobutamine stress testing or calcium scoring when the distinction changes management.

When to Refer

Guideline recommendations for intervention depend on symptom status, LVEF, severity, and exercise response, and are outside the scope of this tutorial — see the current ACC/AHA and ESC/EACTS valvular heart disease guidelines cited below for intervention thresholds and timing.