How is severe aortic stenosis (AS) classified?

Classification of Severe Aortic Stenosis

Severe aortic stenosis is classified by any one of three primary hemodynamic criteria: aortic valve area (AVA) <1.0 cm², peak aortic jet velocity ≥4.0 m/s, or mean pressure gradient ≥40 mmHg, with further subclassification based on flow status and left ventricular ejection fraction. 1

Primary Hemodynamic Criteria for Severe AS

The diagnosis of severe AS requires meeting any one of the following thresholds 1:

• Peak aortic jet velocity ≥4.0 m/s 1
• Mean transvalvular pressure gradient ≥40 mmHg 1
• Aortic valve area <1.0 cm² 1
• Indexed AVA <0.6 cm²/m² (particularly important in small adults, children, and adolescents) 1
• Velocity ratio <0.25 1

Ideally, all three criteria should be concordant in the severe range, but discordance requires integration with additional imaging and clinical data 1.

Subclassification of Severe AS

High-Gradient Severe AS (The "Easy Track")

This represents the most straightforward diagnosis where velocity ≥4 m/s and mean gradient ≥40 mmHg clearly indicate severe AS 1. High-gradient AS generally confirms severity without requiring additional evaluation to establish the diagnosis 1.

Further categorization by flow status 1:

• Normal-flow, high-gradient AS: Stroke volume index (SVi) ≥35 mL/m² 1
• Low-flow, high-gradient AS: SVi <35 mL/m² 1

Both have prognostic implications, but severity is confirmed by the high gradient alone 1.

Low-Gradient Severe AS (The "Difficult Track")

This challenging category requires extensive evaluation and is subdivided by ejection fraction 1:

Low-Flow, Low-Gradient AS with Reduced LVEF

Defined by 1:

• AVA <1.0 cm² 1
• Mean gradient <40 mmHg 1
• LVEF <50% 1
• SVi <35 mL/m² 1

Dobutamine stress echocardiography is required to distinguish true-severe from pseudo-severe AS by assessing contractile reserve (≥20% increase in stroke volume) 1. In patients without contractile reserve, valve calcification assessment by CT becomes the primary determinant of severity 1.

Low-Flow, Low-Gradient AS with Preserved LVEF (Paradoxical Low-Flow AS)

This represents the most challenging subgroup where 1:

• AVA <1.0 cm² 1
• Mean gradient <40 mmHg 1
• LVEF ≥50% 1
• SVi <35 mL/m² 1

This typically occurs in patients with small, hypertrophied ventricles and requires meticulous exclusion of measurement errors, particularly underestimation of LVOT area 1.

Staging System for AS

The ACC/AHA guidelines provide a comprehensive staging system 1:

Stage C: Asymptomatic Severe AS 1

• C1: Severe AS with normal LVEF, mild LV hypertrophy, and LV diastolic dysfunction 1
• C2: Severe AS with LVEF <50% 1

Stage D: Symptomatic Severe AS 1

• D1: High-gradient severe AS (velocity ≥4 m/s, mean gradient ≥40 mmHg) 1
• D2: Low-flow, low-gradient severe AS with reduced LVEF 1
• D3: Low-flow, low-gradient severe AS with normal LVEF 1

Critical Diagnostic Pitfalls

Measurement Errors to Avoid

• Underestimation of LVOT area is the most common source of error, leading to underestimation of flow and overestimation of stenosis severity 1
• Inadequate Doppler alignment can underestimate velocity and falsely lower severity assessment 1
• Hypertension can alter peak velocity and mean gradient; blood pressure should be recorded at every examination 1

Discordant Measurements

When AVA suggests severe AS but gradients are low with normal flow (SVi ≥35 mL/m²), severe AS is very unlikely and the patient likely has moderate AS with miscalculated AVA 1. The inconsistency reflects that an AVA of 1.0 cm² correlates to a mean gradient of only 21 mmHg and velocity of 3.3 m/s at normal flow rates 2.

Conversely, when high gradients (≥40 mmHg) occur with AVA >1.0 cm², this typically indicates high transvalvular flow from conditions like significant aortic regurgitation, anemia, or hyperthyroidism 1. Reversible causes must be excluded, but the high gradient still indicates severe LV pressure overload requiring intervention 1.

Special Considerations

• Very severe AS is defined as velocity ≥5.0 m/s or mean gradient ≥60 mmHg, indicating extremely rapid disease progression 1
• Body size indexing is controversial in obese patients but essential in children, adolescents, and small adults 1
• Research demonstrates that outcomes vary significantly within the AVA 0.8-0.99 cm² range, supporting the current 1.0 cm² cutoff for severe AS 3