How is the severity of aortic stenosis (AS) classified by valve area?

Classification of Aortic Stenosis Severity by Valve Area

Aortic stenosis is classified as severe when the aortic valve area (AVA) is less than 1.0 cm², which is associated with poor prognosis and increased mortality if left untreated. 1

Comprehensive Classification System

Aortic stenosis severity is classified based on valve area measurements as follows:

Classification	Aortic Valve Area (cm²)	Indexed AVA (cm²/m²)	Velocity Ratio
Mild	> 1.5	> 0.85	> 0.50
Moderate	1.0-1.5	0.60-0.85	0.25-0.50
Severe	< 1.0	< 0.6	< 0.25

Measurement Methods and Considerations

Continuity Equation Method

The primary method for determining AVA is the continuity equation:

• AVA = (LVOT area × LVOT VTI) / Aortic valve VTI
• Where LVOT = left ventricular outflow tract; VTI = velocity time integral

This can be simplified to:

• AVA = LVOT area × (LVOT velocity / Aortic jet velocity) 1

Important Clinical Considerations

1. Body Size Adjustment:

   • AVA should be indexed to body surface area (BSA) in small adults, children, and adolescents
   • Severe AS is defined as indexed AVA < 0.6 cm²/m² 1
   • Indexing is controversial in obese patients as valve area doesn't increase with excess body weight

2. Velocity Ratio (Dimensionless Index):

   • Calculated as LVOT velocity / Aortic valve velocity
   • Severe AS is indicated by a ratio < 0.25 1
   • Advantage: Eliminates potential errors in LVOT area measurement

3. Planimetry:

   • Direct measurement of anatomic valve area
   • Less reliable with TTE due to shadowing from calcification
   • More accurate with TEE but still challenging in heavily calcified valves 1

Special Considerations in Discordant Measurements

When AVA and gradient measurements are discordant, several subtypes of severe AS should be considered:

1. Low-Flow, Low-Gradient AS with Reduced EF (Stage D2):

   • AVA < 1.0 cm²
   • Mean gradient < 40 mmHg
   • EF < 50%
   • Stroke volume index < 35 mL/m² 1
   • Requires dobutamine stress echocardiography to distinguish true severe AS from pseudosevere AS

2. Paradoxical Low-Flow, Low-Gradient AS with Normal EF (Stage D3):

   • AVA < 1.0 cm²
   • Mean gradient < 40 mmHg
   • EF ≥ 50%
   • Stroke volume index < 35 mL/m² 1
   • Often associated with small, hypertrophied LV with concentric remodeling

3. Normal-Flow, Low-Gradient AS:

   • AVA < 1.0 cm²
   • Mean gradient < 40 mmHg
   • Stroke volume index ≥ 35 mL/m²
   • May represent measurement errors or early severe AS 1

Common Pitfalls in AVA Measurement

• LVOT Diameter Measurement Errors: Underestimation of LVOT area is the most common source of error, leading to falsely small AVA calculations 1
• Angle-Related Errors: Improper alignment of Doppler beam can lead to velocity underestimation
• Hypertension: Can alter peak velocity/mean gradient measurements and should be recorded during examination 1
• Concomitant Aortic Regurgitation: Can increase stroke volume and affect gradient measurements

Prognostic Implications

• Patients with AVA < 0.8 cm² have worse outcomes compared to those with AVA between 0.8-0.99 cm² 2
• Among patients with AVA 0.8-0.99 cm², those with normal-flow, low-gradient pattern have better prognosis than those with high-gradient or low-flow patterns 2
• The current AVA threshold of 1.0 cm² for severe AS has higher sensitivity (91%) for predicting adverse outcomes compared to a threshold of 0.8 cm² (61%) 2

Algorithmic Approach to Discordant Measurements

1. First, rule out measurement errors (especially LVOT diameter)
2. Assess flow status (normal vs. low) and gradient (high vs. low)
3. In low-flow states, consider dobutamine stress echocardiography
4. Evaluate for other factors affecting measurements (hypertension, anemia)
5. Consider additional imaging (CT calcium scoring) in cases of persistent uncertainty

By carefully integrating valve area measurements with flow status and gradient information, clinicians can accurately classify AS severity and make appropriate management decisions to improve patient outcomes.