Indexed Aortic Root Size: Normal Values and Clinical Thresholds
The indexed aortic root size at the sinuses of Valsalva should be ≤2.1 cm/m² in adults, with normal values averaging 1.7 ± 0.2 cm/m² in men and 1.8 ± 0.2 cm/m² in women, and aortic root dilatation is definitively diagnosed when the z-score exceeds 2.0. 1, 2
Calculation Method
Body surface area indexing is calculated by dividing the measured aortic root diameter (in cm) by the patient's BSA (in m²). 1
For more precise assessment, particularly in children and individuals with extreme body sizes, z-scores are preferred over simple BSA indexing. 1, 2 The z-score formula is:
- Expected aortic root size = 2.423 + (age in years × 0.009) + (BSA in m² × 0.461) - (sex [1=male, 2=female] × 0.267) 1
- z-score = (observed diameter - expected diameter) / 0.261 1
Normal Indexed Values by Population
Adults
Absolute normal values at the sinuses of Valsalva: 1
- Men: 3.4 ± 0.3 cm (indexed: 1.7 ± 0.2 cm/m²)
- Women: 3.0 ± 0.3 cm (indexed: 1.8 ± 0.2 cm/m²)
Age-stratified equations for predicting normal diameter based on BSA: 1
- Children and adolescents (<20 years): y = 1.02 + 0.98x (where x = BSA)
- Adults 20-39 years: y = 0.97 + 1.12x
- Adults ≥40 years: y = 1.92 + 0.74x
Children
In pediatric populations, height is the best predictor of aortic dimensions (r = 0.93-0.95), superior to BSA. 3 However, BSA-based nomograms remain widely used and show strong correlation (r = 0.93). 4
Pathologic Thresholds
Z-Score Based Criteria (Preferred Method)
Aortic root dilatation is definitively recognized when z-score >2.0, corresponding to the 98th percentile. 1, 5
Severity grading by z-score: 1, 5
- Mild dilatation: z-score 2.0-3.0
- Moderate dilatation: z-score 3.01-4.0
- Severe dilatation: z-score >4.0
A z-score of 3.0 corresponds to the 99.9th percentile. 1
Indexed Diameter Thresholds
Upper limit of normal indexed diameter: 2.1 cm/m² provides 98% specificity for detecting aortic dilatation in both men and women. 4 This threshold is particularly useful when z-score calculations are not readily available.
Clinical threshold for males: Indexed diameter >2.2 cm/m² (or absolute diameter >40 mm) warrants evaluation for underlying aortopathy. 6, 2, 5
Absolute Diameter Thresholds
While indexing is preferred, absolute thresholds remain clinically relevant: 2, 5
- >40 mm in adult males: Unequivocal aortic root enlargement requiring evaluation
- >34 mm in adult females: 99th percentile, triggers further assessment
Critical Measurement Technique
All measurements must be performed correctly to avoid false diagnoses:
- Timing: Measure at end-diastole (except annulus, which is measured at mid-systole) 1
- Technique: Use leading-edge to leading-edge convention for echocardiography 1, 2
- Orientation: Measurements must be strictly perpendicular to the aortic long axis 1, 2
- Location: Measure maximal diameter at the sinuses of Valsalva 1
Common Pitfall to Avoid
Never apply M-mode nomograms to 2D echocardiographic measurements. This error falsely diagnoses aortic dilatation in 40% of normal children and 19% of normal adults because 2D measurements at the sinuses are systematically larger than M-mode values. 4, 2
Imaging modality matters: Echocardiographic measurements (sinus-to-sinus) yield values 2-3 mm larger than CT/MRI measurements (sinus-to-commissure). Serial imaging should use the same modality. 1, 2
Follow-Up and Referral Recommendations
When Indexed Diameter is Normal (<2.1 cm/m² or z-score <2.0)
No specific aortic surveillance is required in asymptomatic patients without risk factors. 1
When Indexed Diameter is Borderline (2.0-2.1 cm/m² or z-score 1.5-2.0)
Annual echocardiographic surveillance is reasonable, particularly in patients with bicuspid aortic valve, family history of aortic disease, or connective tissue disorder. 1
When Aortic Root is Dilated (>2.1 cm/m² or z-score >2.0)
Immediate evaluation for underlying aortopathy is mandatory, including: 1, 5
- Detailed family history of aortic disease, sudden death, or connective tissue disorders
- Physical examination for Marfan syndrome features
- Genetic evaluation if syndromic features present
- Assessment for bicuspid aortic valve
Referral to specialized center with expertise in genetic aortic disease is recommended when: 1
- z-score >3.0
- Rapid progression (>0.5 cm/year)
- Family history of aortic dissection
- Clinical features suggesting genetic syndrome
Surveillance Intervals Based on Severity
Mild dilatation (z-score 2.0-3.0): Annual echocardiography 1, 5
Moderate dilatation (z-score 3.01-4.0): Every 6 months 1, 5
Severe dilatation (z-score >4.0): Every 3-6 months and cardiothoracic surgery consultation 1, 5
Aortic root >50 mm (or >45 mm with bicuspid valve or Marfan syndrome): Surgical evaluation regardless of symptoms or z-score. 1, 5
Special Considerations
Women require particular attention: Late results after surgery for aortic regurgitation are less satisfactory in women than men because intervention occurs later when absolute diameter thresholds (validated primarily in men) are reached. By that time, the degree of dilatation normalized for BSA is much higher in women. 1 Therefore, indexed values or z-scores are especially critical in women.
Hypertension effect: Blood pressure has minimal impact on aortic root diameter at the sinuses of Valsalva but does affect more distal aortic segments. 1
Risk despite "normal" dimensions: 42% of type A aortic dissections occur with diameters <50 mm, and 12% of women dissect at <40 mm. 1 Therefore, clinical context and risk factors must always be considered alongside measurements. 5