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Changes after birth in the tunica media and in the internal diameter of the aortic isthmus in normal newborns.

In 104 hearts of infants and children, who died from non-vascular diseases, and whose ages ranged up to 10 years after birth, narrowing of the aortic isthmus, i.e. an internal diameter of 80% or less of that of the descending aorta, was found in 26 cases (24%). The remaining cases showed a lesser degree or no narrowing of the aortic isthmus. Among the 38 cases who died within 2 days after birth (gestational age range 21 to 42 weeks) narrowing of the aortic isthmus, as defined above, was present in only 14 cases (37%). This is much less frequent than is suggested in the literature. No correlation was found between presence, and degree, or absence of narrowing of the aortic isthmus and the relative sizes of the aortic and pulmonary narrowing of the aortic isthmus and the relative sizes of the aortic and pulmonary ostia. Possibly, to define the limits of the normal condition of the aortic isthmus, not only its diameter, but also the characteristics of its tunica media may be of importance. Therefore, the thickness and the packing density of the elastic fibres of both the aortic isthmus and the descending aorta were measured in 69 cases, equally distributed over the material. It is reasoned that in the 3 out of a total of 26 cases, in which not only a marked narrowing of the aortic isthmus was observed but also a relative underdevelopment of its tunica media, a persistent narrowing might have developed, whereas development of such a persistent narrowing seems less likely with a tunica media of normal volume and structure.