Isthmus Aortae-Its Definition and Significance

In order to reach a clear definition of the Isthmus aortae (BNA, INA and PNA), the outer and inner diameters of six portions of the aortic arch, as shown in Fig.1, were measured in 24 human adults and 20 fetuses. The narrowing of the vessel was actually located between the origin of the left subclavian artery and the insertion of the ductus arteriosus in fetuses, but was found to be unremarkable in adults. The term Isthmus aortae thus belongs to the embryological nomenclature. Summarizing the previously reported cases of anomalies of the aortic arch, it can be said that the ductus arteriosus usually remains in loco, in spite of the transformation of the aortic arch, connecting the pulmonary trunk with the left aortic arch or the beginning of the descending aorta, crossing over the left bronchus. When this situation is considered from the stand point of embryology together with the upward migration of the developing left subclavian artery, the presence of a developmental weak point (Locus minoris resistentiae) can be suspected in the developing left aortic arch. This portion corresponds to the Isthmus aortae in normal cases, and most anomalies of the aortic arch are genetically based on maldevelopment of this portion.