Aortic Height Index Calculation
For a patient with height 182 cm (1.82 m) and ascending aorta diameter 46 mm (4.6 cm), the Aortic Height Index (AHI) is 2.53 cm/m, which falls below the threshold associated with increased risk of adverse aortic events.
Calculation Method
The AHI is calculated by dividing the maximum aortic diameter (in cm) by the patient's height (in meters) 1, 2:
- Aortic diameter: 46 mm = 4.6 cm
- Height: 182 cm = 1.82 m
- AHI = 4.6 cm ÷ 1.82 m = 2.53 cm/m
Risk Stratification Based on AHI
The 2022 ACC/AHA guidelines and supporting research establish clear risk thresholds 1, 2:
- AHI <2.43 cm/m: ~4% average yearly risk of complications
- AHI 2.44-3.17 cm/m: ~7% average yearly risk of complications
- AHI 3.21-4.06 cm/m: ~12% average yearly risk of complications
- AHI ≥4.1 cm/m: ~18% average yearly risk of complications
This patient's AHI of 2.53 cm/m places them in the second risk category with approximately 7% yearly risk of aortic adverse events (dissection, rupture, or death). 2
Clinical Implications for This Patient
Current Surgical Thresholds
The ACC/AHA guidelines state that surgery may be reasonable when AHI ≥3.21 cm/m in asymptomatic patients when performed by experienced surgeons in a Multidisciplinary Aortic Team 1. This patient does not meet the AHI threshold for surgical consideration based on indexed measurements alone. 1
However, the absolute diameter of 4.6 cm also requires consideration 1, 3:
- Standard surgical threshold is ≥5.5 cm for ascending aortic aneurysms 1, 3
- Earlier intervention at ≥5.0 cm is reasonable at experienced centers 1, 3
- For concomitant cardiac surgery, replacement is reasonable at ≥4.5 cm 1, 3
Why Height-Based Indexing Matters
The 2024 ESC guidelines emphasize that body surface area (BSA) indexing can underestimate risk in overweight patients, making height-based indexing (AHI) increasingly preferred 1. Height is particularly valuable because it reflects genetic contribution to body size and remains stable, unlike weight 1, 2.
The AHI is especially useful for patients at extremes of height distribution (>1 standard deviation above or below mean), where absolute diameter thresholds may be misleading 1. For this patient with height 182 cm (approximately average for males), both absolute diameter and indexed measurements provide complementary information.
Important Caveats
Limitations of Diameter Alone
Approximately 60% of acute type A aortic dissections occur at diameters <5.5 cm, demonstrating that absolute diameter is an imperfect predictor 1. This underscores why indexed measurements like AHI improve risk stratification, particularly for identifying high-risk patients who fall below traditional size thresholds. 1, 2
Additional Risk Factors to Consider
The following factors may warrant earlier intervention even when AHI and absolute diameter are below surgical thresholds 1, 3:
- Growth rate ≥0.3 cm/year over 2 consecutive years or ≥0.5 cm in 1 year 1, 3
- Family history of aortic dissection 3
- Genetic conditions (Marfan, Loeys-Dietz, bicuspid aortic valve) 3
- Symptoms attributable to the aneurysm 1, 3
- Concomitant aortic valve disease requiring surgery 1, 3
Measurement Standardization
The 2024 ESC guidelines mandate that aortic diameters be measured perpendicular to the longitudinal axis using the double-oblique technique on CT or MRI, and that serial imaging use the same modality and measurement method 1. Oblique measurements can overestimate true diameter and lead to inappropriate clinical decisions. 1, 4