AS severity criteria: the four parameters and how they integrate
ACC/AHA VHD 2020 evaluates AS severity using AVA, Vmax, mean gradient, and DVI. Understanding what each threshold means — and why the parameters can disagree — is the foundation for correct severity classification.
The AS Severity Tool classifies a hemodynamic phenotype from your numbers. This page explains what each threshold means, why discordance occurs, and how low-flow states change the interpretation.
Key takeaway
Severe AS is usually identified by AVA < 1.0 cm², Vmax ≥ 4.0 m/s, and mean gradient ≥ 40 mmHg. But Vmax and mean gradient are highly flow-dependent. In low-flow states, truly severe AS may present with a gradient below 40 mmHg. DVI is useful as a cross-check because it avoids LVOT diameter measurement, but it should be interpreted together with the full echo profile.
Key points
- AVA < 1.0 cm² suggests severe AS, but AVA is calculated by the continuity equation and is sensitive to LVOT diameter measurement error.
- Vmax ≥ 4.0 m/s and mean gradient ≥ 40 mmHg strongly support severe AS when flow is normal — both fall when stroke volume is low.
- In low-flow states, Vmax and mean gradient can underestimate stenosis severity even when AS is truly severe.
- DVI < 0.25 supports severe AS and is useful when AVA and gradient are discordant or LVOT diameter measurement is uncertain.
- Low flow is defined by stroke volume index < 35 mL/m² — at this level, gradient may be below 40 mmHg even in truly severe AS.
- When AVA and gradient disagree, identify the flow state first — not which number to trust.
When to read this
Read this when an echo report includes AS parameters and you want to understand what AVA, Vmax, mean gradient, DVI, and SVI mean — before using the AS Severity Tool.
The four core parameters for AS severity
| Parameter | Mild | Moderate | Severe | Flow dependence |
|---|---|---|---|---|
| Aortic valve area (AVA) | > 1.5 cm² | 1.0–1.5 cm² | < 1.0 cm² | Moderate — calculated by the continuity equation; sensitive to LVOT diameter measurement error |
| Peak aortic velocity (Vmax) | < 3.0 m/s | 3.0–3.9 m/s | ≥ 4.0 m/s | High — scales with flow; low SV reduces Vmax even in severe AS |
| Mean pressure gradient | < 20 mmHg | 20–39 mmHg | ≥ 40 mmHg | High — proportional to velocity squared; falls significantly with low flow |
| Dimensionless velocity index (DVI) | > 0.35 | 0.25–0.35 | < 0.25 | Relatively low — LVOT VTI divided by aortic valve VTI; not affected by LVOT diameter error. Interpret together with other parameters. |
Low-flow patterns: when standard thresholds are harder to interpret
| Pattern | SVI | LVEF | Key finding | Next step |
|---|---|---|---|---|
| Normal-flow high-gradient | ≥ 35 mL/m² | Any | AVA < 1.0 cm² AND mean gradient ≥ 40 mmHg — concordant | Severe AS confirmed — proceed to intervention assessment |
| Classical LFLG (low-flow low-gradient, reduced EF) | < 35 mL/m² | < 50% | AVA < 1.0 cm², gradient < 40 mmHg — reduced LVEF with low flow; distinguish true severe AS from pseudo-severe AS | Confirm severity: low-dose dobutamine stress echocardiography and/or CT aortic valve calcium scoring |
| Paradoxical LFLG (low-flow low-gradient, preserved EF) | < 35 mL/m² | ≥ 50% | Small hypertrophied LV — low SVI despite preserved EF | Confirm severity: CT aortic valve calcium scoring is useful when echo findings are discordant |
| Discordant AS (normal flow, AVA severe, gradient non-severe) | ≥ 35 mL/m² | Any | AVA < 1.0 cm² but gradient < 40 mmHg — AVA and gradient disagree despite preserved flow | Check LVOT diameter for measurement error; assess DVI; consider additional imaging when needed |
The DVI as a cross-check
The dimensionless velocity index divides LVOT VTI by aortic valve VTI. Because it does not require LVOT diameter, it is less affected by one of the most common sources of AVA calculation error.
A DVI below 0.25 supports severe AS, especially when AVA and gradient are discordant or when LVOT diameter measurement is uncertain. However, DVI should not be used as a standalone decision-maker — it should be interpreted together with AVA, Vmax, mean gradient, SVI, LVEF, and valve calcification.
Perioperative relevance: what these thresholds mean at the bedside
- High-gradient severe AS — Vmax ≥ 4.0 m/s or mean gradient ≥ 40 mmHg — indicates fixed obstruction to LV outflow. Induction and intraoperative management should prioritize maintaining preload, afterload, sinus rhythm, and coronary perfusion pressure.
- Suspected LFLG AS before non-cardiac surgery should prompt cardiology review when feasible. These patients are at high risk of hemodynamic instability under anesthesia — severity and intervention eligibility should be clarified before surgery.
- A preoperative report showing AVA 0.9 cm² with mean gradient 28 mmHg should not be labeled moderate AS until flow state has been assessed (SVI, LVEF).
- For perioperative planning, the AS phenotype matters more than a severity grade alone — the AS Severity Tool returns a phenotype such as high-gradient AS, classical LFLG, paradoxical LFLG, or discordant AS.
The most common perioperative misread
A mean gradient of 28–35 mmHg is often interpreted as moderate AS. In a patient with SVI < 35 mL/m², however, a truly severe valve may generate only a modest gradient. If AVA is in the severe range but gradient is not, check SVI and LVEF before downgrading severity.