THE FETAL AND NEONATAL CIRCULATION 



' 6 45 



pulmonary ventilation and perfusion to provide full 

 oxygenation of the blood in order to maintain the 

 necessary oxygen supply to the body tissues; it is, 

 therefore, closely linked with lung development 

 which occurs relatively late in intrauterine life. 

 Most of the quantitative data relating this develop- 

 ment to length of gestation are, again, supplied by 

 Dawes and his colleagues in the lamb fetus. At 90 

 days of age fluid starts to collect in the alveolar 

 spaces of the lungs (93). Shortly afterward the dis- 

 tensibility of the lungs begins to increase so that, at 

 a given inflation pressure, older lambs obtain more 

 tidal air per kg body weight (fig. 24); ventilation 

 also starts to cause a decrease in pulmonary vascular 

 resistance and there is a larger blood flow for the same 

 perfusion pressure (fig. 25). By 110 days gestational 

 age, about 28 to 30 weeks on the human scale, arti- 

 ficial ventilation can raise the arterial oxygen satura- 

 tion to 95 per cent and independent existence is 

 possible; in the nonviable premature this cannot occur 

 and death is due to asphyxia. 



The development of many other physiological 

 mechanisms must also influence the successful opera- 

 tion of ventilation and perfusion of the lung tissues. 

 Among these will be the level of the arterial blood 

 pressure, closure of the foramen ovale and ductus 

 arteriosus, and the presence of sufficient surface 

 active substance to prevent collapse of the expanded 

 alveoli (145); Avery & Mead (18) have found the 

 surface activity of lung extracts from premature 

 infants to be only one-third of that from normal 

 full-time lungs. 



Congenital Heart Disease 



The transition from the fetal to the adult course of 

 the circulation may not take place because of in- 



herent congenital abnormality or be protracted on 

 account of a difficult labor and the ensuing asphyxia. 

 Rowe (164) gives a concise account of the physical 

 signs and the physiology of both, together with the 

 possibilities of their treatment (122). The physio- 

 logical disturbances accompanying congenital mal- 

 formations may be divided into three groups: /) a 

 left-to-right shunt through a patent ductus arteriosus, 

 2) the retention of a fetal type of flow through both 

 ductus arteriosus and foramen ovale, and 3) simple 

 intracardiac arteriovenous shunts. Only the first 

 group have normal arterial oxygen saturations. 



The physical signs of congenital heart disease are 

 frequently difficult to distinguish from the transient 

 abnormalities due to respiratory disturbances at 

 birth; Rowe also divides these infants into three main 

 groups. In the first are those who do not breathe 

 readily at birth and who have a murmur due to 

 patency of the ductus arteriosus: Burnard (51) has 

 observed a midsystolic murmur in 70 per cent of 

 such infants, and considers it due to swift turbulent 

 flow through a ductus only partially constricted on 

 account of asphyxia; the direction of this flow will 

 depend upon the relative pressures in the pulmonary 

 and aortic trunks. Lind & Wegelius (129) have 

 angiocardiographic evidence for delayed closure of 

 the ductus arteriosus following asphyxia neonatorum. 

 In the second group, apnea may develop suddenly 

 following normal respiration of a few hours to 4 weeks 

 duration; a loud continuous murmur, due to a left- 

 to-right shunt through the opened ductus arteriosus 

 is heard. The third group of premature infants, with 

 classical respiratory distress syndrome have, on ac- 

 count of the high lung resistance, a pulmonary ejec- 

 tion click to the second heart sound; during the 

 recovery phase a midsystolic sound is also heard as 

 the ductus arteriosus narrows. 



REFERENCES 



1. Acheson, G. H., G. S. Dawes, and J. G. Mott. Oxygen 

 consumption and the arterial oxygen saturation in new- 

 born lambs. J. Physiol., London 135: 623, 1957. 



2. Adams, F. H., N. Assali, M. Cushman, and A. Westen- 

 sten. Interrelationships of maternal and foetal circulations. 

 I. Flow-pressure responses to vasoactive drugs in sheep. 

 Pediatrics 27 : 627, 1 961. 



3. Adams, F. H., and J. Lind. Physiologic studies on the 

 cardiovascular status of normal infants (with special 

 reference to the ductus arteriosus). Pediatrics 19: 431, 1957. 



4. Aitken, E. H., R. V. Eton, B. Eton, J. R. K. Preedv, 

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foetal and maternal plasma at parturition. Lancet 2 : 

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5. Alexander, P. P. and D. A. Nixon. The foetal kidney. 

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6. Althoff, H., and H. Werner. Vorkommen und Bedeu- 

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