What is the classification of aortic stenosis?

Classification of Aortic Stenosis

Aortic stenosis (AS) is classified primarily by hemodynamic parameters including peak velocity, mean gradient, and valve area, with severity categorized as mild, moderate, or severe based on specific cutoff values.

Standard Classification by Severity

Hemodynamic Parameters

• Aortic Sclerosis

  • Peak velocity: ≤2.5 m/s
  • No significant hemodynamic obstruction

• Mild AS

  • Peak velocity: 2.6–2.9 m/s
  • Mean gradient: <20 mmHg
  • Aortic valve area (AVA): >1.5 cm²
  • Indexed AVA: >0.85 cm²/m²
  • Velocity ratio: >0.50

• Moderate AS

  • Peak velocity: 3.0–4.0 m/s
  • Mean gradient: 20–40 mmHg
  • AVA: 1.0–1.5 cm²
  • Indexed AVA: 0.60–0.85 cm²/m²
  • Velocity ratio: 0.25–0.50

• Severe AS

  • Peak velocity: ≥4.0 m/s
  • Mean gradient: ≥40 mmHg
  • AVA: <1.0 cm²
  • Indexed AVA: <0.6 cm²/m²
  • Velocity ratio: <0.25 1

Flow-Gradient Classification

When there is discordance between valve area and gradient measurements, AS can be further classified based on flow status and gradient:

Normal Flow, High Gradient (NF-HG)

• AVA <1.0 cm²
• Mean gradient ≥40 mmHg
• Stroke volume index (SVi) ≥35 mL/m²
• Typically represents "classic" severe AS 1

Low Flow, High Gradient (LF-HG)

• AVA <1.0 cm²
• Mean gradient ≥40 mmHg
• SVi <35 mL/m²
• Associated with worse prognosis than NF-HG 1

Low Flow, Low Gradient (LF-LG) with Reduced EF

• AVA <1.0 cm²
• Mean gradient <40 mmHg
• LVEF <50%
• SVi <35 mL/m²
• Also called "classical" LF-LG AS 1
• Requires dobutamine stress echocardiography to differentiate true severe from pseudo-severe AS

Low Flow, Low Gradient (LF-LG) with Preserved EF

• AVA <1.0 cm²
• Mean gradient <40 mmHg
• LVEF ≥50%
• SVi <35 mL/m²
• Also called "paradoxical" LF-LG AS 1, 2
• Often associated with small, hypertrophied LV cavity with concentric remodeling

Normal Flow, Low Gradient (NF-LG)

• AVA <1.0 cm²
• Mean gradient <40 mmHg
• SVi ≥35 mL/m²
• LVEF ≥50%
• May represent measurement errors or inherent discrepancy between AVA and gradient cutoffs 1, 2

Clinical Staging System

The American College of Cardiology/American Heart Association guidelines propose a staging system:

• Stage A: At risk for AS (bicuspid valve, aortic sclerosis)
• Stage B: Progressive AS (mild to moderate)
• Stage C: Asymptomatic severe AS
  • C1: Normal LVEF
  • C2: Reduced LVEF
• Stage D: Symptomatic severe AS
  • D1: High-gradient AS
  • D2: Low-flow/low-gradient AS with reduced LVEF
  • D3: Low-flow/low-gradient AS with preserved LVEF 1, 2

Special Considerations

1. Measurement accuracy: Ensure proper alignment of Doppler beam and accurate LVOT diameter measurement to avoid errors in AVA calculation 2

2. Body size adjustment: Indexing AVA to body surface area (AVA/BSA) is important in small adults, children, and adolescents, with severe AS defined as indexed AVA <0.6 cm²/m² 1

3. Discordant measurements: When AVA and gradient measurements are discordant, additional testing may be needed:

   • Dobutamine stress echocardiography for LF-LG with reduced EF
   • CT calcium scoring for LF-LG with preserved EF (severe AS likely if ≥2000 Agatston units in men or ≥1200 in women) 2

4. Hypertension: Can alter peak velocity/mean gradient and should be recorded for every examination 1

The classification of AS requires integration of multiple parameters rather than relying on a single measurement, with careful attention to flow status and ventricular function to ensure accurate diagnosis and appropriate management decisions.