Aspects of applied physiology of neonatal circulation.

In talking to clinical students and junior resident staff I have frequently heard the remark that "I have never understood the foetal circulation". As an understanding of this circulation and its conver-sion to the adult form is fundamental to an apprecia-tion of the circulatory and respiratory physiology and pathology of the newborn infant, I must begin this lecture by mentioning the most important points. FOETAL CIRCULATION Fig. 1 is a diagram of the adult circulation as Harvey saw it; Fig. 2 is based on this same diagram, but has had added to it the complexities of the foetal state. The conventions in this diagram are as follows: the volume of flow in the various chan-nels is represented by the diameter of the vessels, and the colours indicate the degree of oxygenation of the blood. One sees that the flow through the pulmonary artery is very small (Harvey thought there was no flow through the lung in the foetal state), and circulation is only possible by the pro-vision of two foetal shunts around the lungs: (1) the foramen ovale leading relatively oxygenated blood from the umbilical vein to the left atrium, and (2) the ductus arteriosus carrying blood from the pulmonary trunk to the aorta. The pulmonary trunk conducts blood of poor oxygenation (at 52 per cent saturation as compared with 80 per cent in the umbilical vein in the lamb (Born et al.,1954)). In the left atrium there is a mixture of the de-oxygenated blood passing through the pulmonary arteries, with more oxygenated blood arriving from the foramen ovale, and hence the blood in the first part of the aorta supplying the heart, the right arm, and the head and neck is slightly more oxygenated