'adult Pattern' of Pulmonary Vessels in Newborn Infants.

The study of pulmonary vessels has assumed great importance in the last decade since the advent of corrective surgery for congenital cardiac defects. Several papers have been written on the structure and state of these vessels, in utero, in neonatal life and during later years (Edwards, 1957; Civin and Edwards, 1951; Dammann and Ferencz, 1956a). These papers stress the basic differences in the pulmonary vessels during these different periods. The vessels showing the most striking differences between intrauterine and other periods of life are the small muscular arteries and arterioles. These vessels in the foetus have thick walls with narrow lumina (Figs. 1 and 2), thus producing high pulmonary vascular resistance (Civin and Edwards, 1951; Edwards, 1957): the lumen-to-wall ratio of these vessels in the foetus is 1 : 1. The narrow lumen is thought to be due partly to vasoconstriction and partly to the presence of thick muscular media in these vessels (Edwards, 1957). This arrangement is most advantageous during intrauterine life, when most of the blood from the right ventricle is to be diverted to the systemic circulation through the ductus arteriosus, since the lungs have no aerating function (Barclay, Franklin and Prichard, 1944; Lind and Wegelius, 1949). After birth, however, all the right ventricular blood must be shunted through the lungs. To enable this to happen the pulmonary vessels dilate, their walls become thin and the lumen becomes wide. The media of the small muscular artery becomes thin, and the media of the arteriole almost completely disappears, and the lumina of these vessels become much larger than the wall, the lumen-to-wall ratio finally reaching the adult