Aortic Size Index (ASI)
Normalises aortic diameter to body surface area to stratify the risk of rupture or dissection, complementing absolute diameter thresholds.
The aortic size index indexes maximal aortic diameter to body surface area, recognising that a given diameter carries greater risk in a smaller patient. Davies and colleagues showed that ASI stratifies the yearly risk of rupture, dissection or death in thoracic aortic aneurysm better than absolute diameter alone.
📐 Formula
ASI = aortic diameter (cm) / body surface area (m²)
🎯 When to Use
- Thoracic aortic aneurysm surveillance, particularly in patients of small or large body habitus.
- Risk stratification when absolute diameter is borderline for intervention.
- Serial CT/MR/echo follow-up of ascending or descending aortic dilatation.
⚙️ How It Works
- Measure the maximal aortic diameter perpendicular to the vessel axis.
- Calculate body surface area (e.g. Mosteller or Du Bois formula).
- ASI = diameter (cm) / BSA (m²).
- Interpret with absolute diameter thresholds.
📊 Interpreting the Result
- ASI < 2.75 cm/m²: low yearly risk (≈4%).
- ASI 2.75–4.25 cm/m²: moderate yearly risk (≈8%).
- ASI > 4.25 cm/m²: high yearly risk of rupture/dissection (≈20% and above).
- Absolute thresholds still apply: ascending aorta ≥ 5.5 cm (or ≥ 5.0 cm in connective-tissue disease/bicuspid valve) generally warrants surgical consideration.
⚠️ Limitations & Pitfalls
- Derived largely from ascending thoracic aneurysm cohorts; less validated for abdominal or descending segments.
- Growth rate and aetiology (bicuspid valve, Marfan/Loeys-Dietz, family history) modify risk independently of ASI.
- BSA formula choice and measurement plane affect the result.
📚 References
- Davies RR, Gallo A, Coady MA, et al. Novel measurement of relative aortic size predicts rupture of thoracic aortic aneurysms. Ann Thorac Surg. 2006;81(1):169-177.
- Hiratzka LF, Bakris GL, Beckman JA, et al. 2010 ACCF/AHA guidelines for the diagnosis and management of thoracic aortic disease. Circulation. 2010;121(13):e266-e369.
⚕️ For clinical decision support only. Always verify calculations and apply clinical judgment.