Nakata Index: Definition, Calculation, and Fontan Suitability Criteria
What is the Nakata Index?
The Nakata index (also called PA-index) is an angiographic measurement that quantifies the total cross-sectional area of the pulmonary arteries normalized to body surface area 1. It serves as a critical metric for assessing pulmonary artery size adequacy before Fontan operation and other congenital heart surgeries.
Calculation Method
The Nakata index is calculated as: (Right PA cross-sectional area + Left PA cross-sectional area) / Body Surface Area, expressed in mm²/m² 1, 2.
The measurement is obtained by:
- Measuring the cross-sectional area of the right pulmonary artery (RPA)
- Measuring the cross-sectional area of the left pulmonary artery (LPA)
- Adding these areas together
- Dividing by the patient's body surface area (BSA)
Normal Values
The normal Nakata index is 330 ± 30 mm²/m² and remains consistent across a wide range of body surface areas from infancy through adolescence 1.
Adequate Values for Fontan Operation
For Fontan candidacy, a Nakata index >250 mm²/m² is considered adequate, with values >250 mm²/m² associated with excellent survival (12 of 13 patients survived in the original validation study) 1. In contrast, patients with a Nakata index <250 mm²/m² had significantly higher mortality from severe heart failure (p<0.01) 1.
Historical Context and Modern Perspective
The original 1984 study by Nakata established these thresholds:
- >250 mm²/m²: Good candidates for Fontan procedure
- <250 mm²/m²: Higher risk, consider alternative approaches first 1
However, more recent evidence suggests that small pulmonary arteries may not be as prohibitive as historically believed 3. A 2014 study of 146 Fontan patients found that those with a Nakata index ≤150 mm²/m² (modified ≤100 mm²/m²) were not at higher risk for longer mechanical ventilation, prolonged ICU stay, extended hospital stay, or effusions 3. This challenges the traditional contraindication of small pulmonary arteries.
Important Clinical Considerations
The decision to proceed with Fontan should not be postponed solely due to smaller pulmonary arteries 3. The risks of palliative procedures performed to increase PA size—including volume overload of the single ventricle and complications from prolonged cyanosis—must be carefully weighed against potential benefits 3.
Pulmonary artery size shows an inverse correlation with aortopulmonary collateral (APC) flow (r=-0.50, p=0.002), suggesting that smaller pulmonary arteries may stimulate APC development 2, 4. This relationship is clinically relevant as APC flow affects ventricular dimensions but does not necessarily impair ventricular function 2.
Measurement Modalities
While the Nakata index was originally developed using cardiac catheterization angiography 1, cardiac MRI (CMR) is now indicated as a Class I recommendation for routine pre-Fontan assessment when there is no primary indication for intervention 5. CMR provides accurate pulmonary artery measurements comparable to catheterization while avoiding procedural risks 5.
For patients weighing >10 kg, CT angiography-derived Nakata index measurements are reliable and comparable to catheterization 6, offering another non-invasive alternative.
Dynamic Changes Over Time
The Nakata index decreases significantly after Glenn procedure and Fontan completion 7. In one study, the index decreased from 351.9 mm²/m² pre-Glenn to 226.4 mm²/m² post-Glenn (p<0.003), and in one-stage Fontan patients, it decreased from 318.7 mm²/m² to 120 mm²/m² at mean 7.4-year follow-up (p<0.0001) 7. This relative decrease suggests that pulmonary artery size should not be an absolute limiting factor in treatment decisions 7.