Increased Pulmonary-Aortic Interspace in Fetal Right Aortic Arch: A Matched Case-Control Study.

Introduction: The prenatal detection rate of a right aortic arch (RAA) has increased with the implementation of the three-vessel view (3VV) to the second-trimester anomaly scan formed by the pulmonary artery (PA), aorta (Ao), and superior vena cava (SVC). We examined the value of measuring the distance between PA and Ao in the 3VV in cases with an RAA. Methods: We conducted a case-control study in which fetuses with an isolated RAA were matched to 3 healthy controls. Using 3VV images, the distances between PA, Ao, and SVC were measured and the ratio between PA to Ao (PAAo) distance and Ao to SVC (AoSVC) distance was calculated. Results: Fifty-four RAA cases and 162 matched controls were included. The mean absolute distance PAAo was 3.1 mm in cases and 1.8 mm in controls (p < 0.001), and the mean PAAo/AoSVC ratio was 2.9 and 1.4, respectively (p < 0.001). The ROC curve of PAAo/AoSVC ratio showed a cut-off point of 1.9 with sensitivity and specificity over 87% for the diagnosis of RAA. Conclusions: The pulmonary-aortic interspace and the PAAo/AoSVC ratio were significantly larger for RAA cases as compared to controls. If an increased pulmonary-aortic interspace is observed, a PAAo/AoSVC of >= 1.9 can be helpful in the diagnosis of an RAA.