Z-scores of the fetal aortic isthmus and duct: an aid to assessing arch hypoplasia

• OBJECTIVE: Prenatal diagnosis of isolated coarctation of the aorta suffers from high false positive and false negative rates. The aim of our study was to develop Z-scores for the aortic isthmus in normal fetuses as a reference for fetuses with suspected coarctation. METHODS: The aortic isthmal diameter, immediately proximal to the insertion of the arterial duct, was measured prospectively in the transverse (three vessel and trachea) and sagittal views in 221 normal fetuses at 18 to 37 weeks' gestation. The ductal diameter was measured immediately before it entered the descending aorta in the same view. All measurements were repeated three times by a single investigator and averaged. A second investigator re-measured the images of 50 cases to assess interobserver variability. Z-scores were created relating isthmal and ductal diameters to femur length and gestational age. The ratio between the isthmal and ductal diameters was calculated. RESULTS: The formula used to calculate Z-scores for the three diameters was: [ln(measured isthmal diameter) - (m ln(femur length or gestational age) + c)]/root MSE, where c is the intercept, m is a multiplier and MSE is the mean squared error. The ratio between isthmal and ductal diameters was close to a constant value of 1 (95% CI 0.97-1.01), regardless of the value of femur length or gestational age. CONCLUSION: We have defined Z-scores for the fetal aortic isthmus and arterial duct measured in the three vessels and trachea view and for the isthmus in the sagittal plane. In suspected coarctation, these Z-scores and the isthmal to ductal ratio may help in longitudinal assessment of the aortic arch and aid in the prenatal diagnosis of coarctation.

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