How is the severity of aortic stenosis graded by echocardiography (echo) in patients with suspected aortic stenosis?

Aortic Stenosis Severity Grading by Echocardiography

Aortic stenosis severity is graded by integrating three primary echocardiographic parameters: peak aortic jet velocity, mean transvalvular gradient, and aortic valve area calculated by the continuity equation, with severe AS defined as velocity ≥4 m/s, mean gradient ≥40 mmHg, and valve area ≤1.0 cm². 1

Primary Severity Parameters

The classification system uses the following thresholds 1:

Severe AS:

• Peak aortic jet velocity ≥4.0 m/s
• Mean pressure gradient ≥40 mmHg
• Aortic valve area (AVA) ≤1.0 cm²
• Indexed AVA ≤0.6 cm²/m²

Moderate AS:

• Peak velocity 3.0-3.9 m/s
• Mean gradient 20-39 mmHg
• AVA 1.0-1.5 cm²

Mild AS:

• Peak velocity 2.5-2.9 m/s
• Mean gradient <20 mmHg
• AVA >1.5 cm²

Critical Measurement Technique

LVOT diameter measurement is the single largest source of error because it is squared in the continuity equation, magnifying small measurement errors 2. The LVOT should be measured at the base of the aortic valve cusps or 1-5 mm below 1.

Peak velocity must be obtained from multiple acoustic windows (apical, right parasternal, suprasternal, subcostal) to capture the highest velocity signal and avoid underestimation from beam misalignment 2, 1.

Algorithmic Approach Based on Gradient

High Gradient AS (Mean Gradient ≥40 mmHg)

This represents the straightforward scenario 1:

• High gradient generally confirms severe AS regardless of flow state 1
• No additional testing needed to confirm severity 1
• The only exception is abnormally high flow states (stroke volume index >58 mL/m²) from conditions like anemia, hyperthyroidism, or arteriovenous shunts, which should be corrected before reassessment 1

Low Gradient AS (Mean Gradient <40 mmHg)

This is the "difficult track" requiring extensive evaluation 1:

Step 1: Calculate AVA

• If AVA >1.0 cm², this indicates moderate AS 1
• If AVA <1.0 cm², proceed to flow assessment 1

Step 2: Determine Flow Status

• Normal flow: Stroke volume index ≥35 mL/m² 1
• Low flow: Stroke volume index <35 mL/m² 1

Step 3: Flow-Based Classification

Normal Flow, Low Gradient (AVA <1.0 cm²):

• Severe AS is very unlikely despite calculated AVA <1.0 cm² 1
• Most likely represents moderate AS with measurement error 1
• Carefully recheck LVOT measurement for underestimation 1, 2

Low Flow, Low Gradient with Reduced EF (<50%):

• Perform dobutamine stress echocardiography (≤20 μg/kg/min) 1
• True severe AS: Mean gradient ≥40 mmHg AND AVA ≤1.0 cm² at any dobutamine stage 1
• Pseudosevere AS: AVA increases >0.3 cm² or to >1.2 cm² with gradient remaining <40 mmHg 1
• If no contractile reserve (stroke volume increase <20%), use **CT calcium scoring**: severe AS confirmed by >2000 Agatston units (men) or >1300 units (women) 1

Low Flow, Low Gradient with Preserved EF (≥50%)—"Paradoxical Low Flow":

• This represents a particularly challenging subset 1
• Consider 3D echocardiography to verify stroke volume and AVA measurements 1
• CT calcium scoring is highly valuable: >1200 Agatston units (women) or >2000 units (men) confirms severe AS 1
• Dobutamine stress echo may help distinguish true from pseudosevere AS 1

Common Pitfalls and How to Avoid Them

LVOT Area Underestimation:

• The LVOT is elliptical, not circular 2
• Using only the sagittal diameter systematically underestimates LVOT area, leading to underestimation of stroke volume and AVA 1
• Consider 3D transesophageal echo or CT for direct LVOT planimetry 1, 2

Hypertension Effects:

• Elevated blood pressure can alter velocity and gradient measurements 2
• Record blood pressure during examination 1
• If blood pressure and valvulo-arterial impedance (Zva) are elevated (>4.5-5 mmHg/m²), optimize antihypertensive therapy and repeat echo after normalization 1

Discordant Parameters:

• When AVA <1.0 cm² but velocity <4 m/s and gradient <40 mmHg, this is the most challenging scenario 2
• Never rely on a single parameter in isolation 2
• Systematically exclude measurement errors for all components 1, 2

Integration with Additional Parameters

Beyond the primary grading parameters, assess 1:

• Valve morphology: Tricuspid vs bicuspid, degree of calcification
• LV function and geometry: Ejection fraction, wall thickness, chamber dimensions
• Hemodynamic consequences: Left atrial size, pulmonary pressures
• Symptom status: Critical for clinical decision-making

Clinically significant progression is defined as mean gradient increase >10 mmHg or AVA decrease >25% from baseline 2.