What a single “1.3 mm minimum diameter” can and cannot tell you

Among the Pediatric Heart Network's published echocardiographic Z-score regressions, the predicted mean for the proximal right coronary artery is RCA(prox) = 0.012047694 + 2.301336377 × BSA^0.45, in millimetres. It comes from the PHN normal echocardiogram database and describes coronary internal diameter in normal children in a standard view.

For a body surface area of 1.12 m² — roughly a school-age child of about 143 cm and 32 kg — the predicted value is about 2.43 mm. Treated as a circular lumen, that is a cross-sectional area of about 4.65 mm². These two figures exist only to set the scale.

The caveats have to follow immediately. The equation is built on standard measurements in normal children, whereas “1.3 mm at its narrowest” in an AAORCA report is most likely the minor axis of an anomalous ostium or intramural segment. Dividing one by the other, or deriving a Z score, looks precise while putting two different measurands into one formula.

The same phrase can come from the minor axis of the anomalous ostium, the smallest minor axis within the intramural segment, the narrowest point of the interarterial segment, or a visually narrowest width on one longitudinal reformat. The first three are different anatomic locations; the last does not even fix the measuring plane.

Only a true cross-section perpendicular to the vessel yields minor axis, major axis, and area together. That is why comparability requires the plane to be stated, not just a number. Recent work proposing standardized nomenclature for AAOCA addresses exactly this kind of same-word-different-meaning problem.

Take the cross-sectional area of the normal RCA beyond the anomalous segment as the reference and the minimum area within the intramural segment as the lesion, and divide. This adjusts inherently for age, body surface area, the territory differences that follow from coronary dominance, and individual vessel calibre, so it is more robust than comparison against population norms.

The value shows in an example: a distal reference area of 4.0 mm² and a narrowest intramural area of 2.0 mm² gives an area stenosis of about 50%. That statement can be checked by another reader, repeated over time, and compared before and after surgery. “About 1.3 mm at its narrowest” supports none of those.

Coronary dominance also finds its proper place here. Left dominance implies a smaller distal right-coronary territory, so that vessel's normal calibre may legitimately be narrower. That is a reason not to treat a population mean as an individual's expected value; it is not a reason to forgo surgery, since no pediatric AAORCA risk data are stratified by dominance.

A study of 81 adults with R-AAOCA collected CCTA geometry and invasive FFR together. For predicting abnormal flow under exercise-like (dobutamine) stress, geometric quantities such as the ostial minor axis were more informative than the binary presence of an intramural course, with hydraulic diameter and resistance-type composite indices adding further information.

In the same study, adenosine FFR was abnormal in about 6.2% and dobutamine FFR in about 19.8%. Fixed and dynamic geometry are therefore not the same thing: one cross-section on a static scan cannot capture the further deformation that comes with aortic distension and a rising heart rate during exercise.

These results come from adults with a mean age of about 52 and represent an emerging line of work; they provide no threshold usable for pediatric stratification. The one conclusion they support is that geometry, not an isolated diameter, is what is worth measuring.

In the 2026 Guangdong cohort of 151 children with AAORCA (mean age at diagnosis 7.40 ± 4.87 years), among the 129 with CTA data an interarterial course appeared in 48.8% and ostial stenosis in 39.5%. The study grouped a slit-like ostium, ostial narrowing, and reduced luminal diameter together as “ostial stenosis.”

Ostial stenosis carried the strongest univariable association with symptoms: OR 4.30 (95% CI 2.14–8.65, P<0.001). After multivariable adjustment the estimate became OR 0.40 (95% CI 0.15–1.01, P=0.052), with the direction reversing. This cannot be read as “ostial stenosis multiplies risk 4.3-fold”; it reads as an apparent association that adjustment for age and other factors largely explains away.

The surgical side points the same way. Forty-six patients (30.5%) had surgery, 44 of them unroofing; among the 45 with complete intraoperative data, 44/45 (97.8%) had an intramural course and 40/45 (88.9%) ostial stenosis, while only 13/45 (28.9%) had an interarterial course. In practice, the patients reaching the operating room were those with ostial and intramural problems, not those defined by running between the great arteries.

Placing this next to the Texas Children's cohort of 220 completes the picture: there, intramural length was identical in patients with and without ischemia (median 5 mm, IQR 4–7, P=0.65). Together the two datasets show the centre of gravity moving from course classification toward ostial and proximal geometry — without any single measure yet being strong enough to decide surgery alone.

The first question: what are the minor axis, major axis, and area at the narrowest point, and what percentage of the patient's own distal reference area is lost. This requires no new study, only re-quantification of the original images by a team familiar with coronary anomalies.

The second: under stress that approaches real exertion, does the right coronary territory become ischemic. Geometry can indicate probability; it cannot substitute for that answer.

Until both are answered, an argument about one isolated millimetre figure can be neither confirmed nor refuted, and therefore cannot move the decision forward.