What are the key transthoracic echocardiography parameters (peak aortic jet velocity, mean trans‑valvular gradient, aortic valve area, dimensionless index) that define severe versus moderate aortic stenosis, and how should low‑flow, low‑gradient disease be evaluated?

Aortic Stenosis Echocardiographic Parameters

Defining Severe vs. Moderate Aortic Stenosis

Severe aortic stenosis is defined by any one of three primary criteria: peak aortic jet velocity ≥4.0 m/s, mean transvalvular gradient ≥40 mmHg, or aortic valve area (AVA) <1.0 cm², with velocity and gradient measurements being the most robust parameters when properly obtained. 1, 2

Standard Severity Classification

The key transthoracic echocardiography parameters stratify AS severity as follows 3, 2:

Parameter	Mild	Moderate	Severe
Peak velocity	2.6-2.9 m/s	3.0-4.0 m/s	≥4.0 m/s
Mean gradient	<20 mmHg	20-40 mmHg	≥40 mmHg
AVA	>1.5 cm²	1.0-1.5 cm²	<1.0 cm²
Indexed AVA	>0.85 cm²/m²	0.60-0.85 cm²/m²	<0.6 cm²/m²
Dimensionless index (velocity ratio)	>0.50	0.25-0.50	<0.25

Velocity and gradient measurements represent the most reliable parameters for severity assessment in clinical practice, as long as they are properly performed with meticulous Doppler technique. 1

Algorithmic Approach to AS Severity Assessment

Step 1: Obtain Peak Velocity and Mean Gradient

• Use continuous-wave Doppler from multiple acoustic windows (apical, right parasternal, suprasternal) with a dedicated small dual-crystal transducer 3, 2
• Record the highest velocity signal obtained from any window 1
• Ensure proper Doppler alignment (parallel to flow) to avoid underestimation 2
• Calculate mean gradient by averaging instantaneous gradients over the entire ejection period, not from mean velocity 1

Step 2: Classify as High-Gradient or Low-Gradient AS

High-gradient AS (velocity ≥4 m/s AND mean gradient ≥40 mmHg):

• This confirms severe AS and requires no further testing to establish severity 1
• Only exception: abnormally high flow states (stroke volume index >58 mL/m²) from anemia, hyperthyroidism, arteriovenous shunts, or significant aortic regurgitation may cause high gradients with only moderate stenosis 1

Low-gradient AS (velocity <4 m/s OR mean gradient <40 mmHg):

• Proceed to Step 3 1

Step 3: Calculate AVA and Assess Flow Status

• Calculate AVA by continuity equation using LVOT diameter measured at the annulus or within 2 mm below it 4
• Critical pitfall: LVOT diameter measured too distally systematically underestimates stroke volume and overestimates stenosis severity 2, 4
• Calculate stroke volume index (SVi) to define flow status 1:
  • Normal flow: SVi ≥35 mL/m²
  • Low flow: SVi <35 mL/m²

Step 4: Apply Flow-Based Diagnostic Algorithm

If AVA ≥1.0 cm²: Moderate AS (not severe) 1

If AVA <1.0 cm² with normal flow (SVi ≥35 mL/m²):

• Severe AS is very unlikely despite calculated AVA <1.0 cm² 1, 4
• This constellation almost always represents moderate AS with measurement error 4
• Re-measure LVOT diameter and check for large LVOT causing AVA overestimation 1, 4

If AVA <1.0 cm² with low flow (SVi <35 mL/m²):

• Proceed to Step 5 based on ejection fraction 1

Step 5: Evaluate Low-Flow, Low-Gradient AS

Low-Flow, Low-Gradient AS with Reduced LVEF (<50%)

Perform low-dose dobutamine stress echocardiography (DSE): 1, 4

• Start at 5 mcg/kg/min, increase by 5 mcg/kg/min every 3-5 minutes to maximum 20 mcg/kg/min 4
• Measure AS velocity, mean gradient, stroke volume, and AVA at each stage 1
• Use the same LVOT diameter throughout all stages 1

Interpretation of DSE results: 1

• True severe AS: AVA remains ≤1.0 cm² AND velocity ≥4.0 m/s or mean gradient ≥30-40 mmHg at any flow rate
• Pseudo-severe AS: AVA increases to >1.0 cm² with dobutamine (stenosis not truly severe)
• Contractile reserve present: Stroke volume increases by ≥20% from baseline (predicts better surgical outcomes)
• Contractile reserve absent: Stroke volume fails to increase by ≥20% (predicts high surgical mortality, though valve replacement may still improve outcomes)

Low-Flow, Low-Gradient AS with Preserved LVEF (≥50%)

This "paradoxical" low-flow state occurs in elderly patients with hypertrophied, small ventricles and restrictive physiology 1, 4

First, rigorously exclude measurement errors: 1, 4

• Re-verify LVOT diameter measurement
• Confirm low flow (SVi <35 mL/m²) using alternative techniques: 3D TEE for LVOT measurement, cardiac CT, or CMR 1

Diagnostic criteria requiring multiple concordant findings: 1

Clinical criteria:

• Physical examination consistent with severe AS
• Typical symptoms without alternative explanation
• Age >70 years

Qualitative imaging:

• Left ventricular hypertrophy (consider coexisting hypertension)
• Reduced LV longitudinal function (global longitudinal strain <16%) without other explanation 4
• Small LV cavity (end-diastolic volume index <55 mL/m²) with relative wall thickness >0.5 4

Quantitative imaging:

• Mean gradient 30-40 mmHg (measured when normotensive) 1
• AVA ≤0.8 cm² 1
• Confirmed low flow by non-Doppler techniques

Obtain CT aortic valve calcium score (primary confirmatory test): 1, 4

Calcium Score (Agatston units)	Interpretation
Men ≥3000, Women ≥1600	Severe AS very likely
Men ≥2000, Women ≥1200	Severe AS likely
Men <1600, Women <800	Severe AS unlikely

Dobutamine stress echo should be avoided or used with extreme caution in paradoxical low-flow AS due to marked hypertrophy and small cavities 4

Critical Pitfalls to Avoid

Measurement Errors

• LVOT diameter underestimation is the most common error, leading to flow underestimation and overestimation of stenosis severity 2, 4
• Inadequate Doppler alignment (non-parallel to flow) underestimates velocity and falsely lowers severity 2
• Confusing mitral regurgitation jet with AS jet: MR extends from mitral valve closure to opening (longer duration), whereas AS jet is shorter 4

Physiologic Factors

• Record blood pressure at every examination: Hypertension alters peak velocity and mean gradient 2
• Measure when normotensive in paradoxical low-flow AS to avoid underestimating gradients 1

Inconsistencies Between Parameters

• Research demonstrates that AVA = 1.0 cm² corresponds to mean gradient = 32 mmHg and velocity = 3.7 m/s, creating discrepancies in 18-19% of cases 5
• When parameters are discordant, prioritize velocity and gradient over AVA if measurements are technically sound 1
• Discrepancy rates are substantially higher (40%) in low-flow states 5

Role of Dimensionless Index

The dimensionless index (velocity ratio = LVOT velocity/AS velocity) provides an alternate measure that is less flow-dependent and can corroborate AVA findings 1, 6:

• Severe AS: <0.25
• Particularly useful during dobutamine stress echo to track changes 1
• Remains reliable during intraoperative TEE when gradients may decrease 6