THE FETAL AND NEONATAL CIRCULATION 



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VENTILATION 

 BEGUN 



M I NUTE. S 



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fig. 18. Ventilation of the lungs of a mature fetal lamb 

 caused the mean left atrial pressure to rise above the pressure 

 in the inferior vena cava (IVC) ; occlusion of the umbilical 

 cord caused the IVC pressure to fall. Both, therefore, contribute 

 to the rapid reversal of the pressure gradient across the foramen 

 ovale, resulting in its closure after birth. (Modified from G. S. 

 Dawes. Changes in the circulation at birth. Brit. Med. Bull. 

 17: 152, 1961.) 



The mechanism for the functional closure of the 

 ductus venosus is unknown but it is important that 

 this should take place early in neonatal life. The 

 formation of an Eck fistula, with the portal blood 

 short-circuiting the liver, passing through the portal 

 sinus and straight into the inferior vena cava, might 

 explain the hypoglycemia and icterus which some- 

 times occurs in the neonatal period, especially in pre- 

 mature infants. Cardiac catheterization through the 

 umbilical vein depends upon anatomical patency of 

 the ductus venosus; there is evidence that it is either 

 closed or absent in about 30 per cent of newborn 

 infants (167). However, when the ductus venosus is 

 patent it may be visualized by radiopaque sub- 

 stances up to 12 days after birth (146). 



Fetal Channels in the Thorax 



The first breath initiates the changes in course of 

 the blood streams in the heart: expansion of the lungs 

 decreases the resistance in the small vessels and the 

 resulting increase in pulmonary blood flow raises 

 the left atrial pressure above that in the inferior vena 

 cava, closing the foramen ovale functionally (fig. 18); 

 this closure is assisted by the fall in the inferior vena 

 caval pressure due to the temporarily reduced venous 

 return to the heart, following occlusion of the um- 

 bilical vessels. The whole volume of inferior caval 

 blood now joins the superior caval blood in the right 



atrium to maintain the high pulmonary blood flow. 

 As a result of the reduced pulmonary vascular re- 

 sistance the pulmonary arterial pressure falls below 

 the systemic level and blood flow through the ductus 

 arteriosus is diminished. 



The radiological studies in the sheep and the hu- 

 man infant (26, 129), at first suggested that when 

 respiration is off to a flying start the functional clos- 

 ure of both the foramen ovale and the ductus ar- 

 teriosus is immediate. However, anatomical closure is 

 not complete for some weeks and there is evidence 

 that blood may flow through both these channels, 

 probably intermittently, for about a fortnight after 

 birth; this is demonstrated in the angiocardiographic 

 studies and in dye dilution curves which, in normal 

 babies, are characteristic of pathological states with a 

 patent ductus (149). More direct evidence for a 

 patent ductus with a left-to-right shunt has been 

 obtained in mongol (117) and in normal infants (3); 

 during cardiac catheterization it was found that blood 

 obtained from the pulmonary artery contained more 

 oxygen than that collected from the right auricle 

 and, in addition, the pulmonary arterial pressures 

 were higher than expected. Dawes and his colleagues 

 have measured this flow in the lamb and find it con- 

 siderable (76). Blood flowing through the wide open 

 ductus arteriosus creates no murmurs, but as the 

 vessel constricts there is turbulence of the swiftly 

 flowing stream and murmurs attributed to this can 

 be heard in both the sheep (76) and the human in- 

 fant (50). The direction of this shunt may be from 

 left to right or right to left according to the relative 

 pressures in the pulmonary and aortic trunks. Follow- 

 ing expansion of the lungs, the pulmonary arterial 

 pressure falls relative to the systemic pressure and 

 there is the possibility of a left-to-right shunt; if this 

 occurs, the work of the left heart will be increased 

 but, during recirculation of the blood through the 

 lungs, there is a further opportunity for oxygen up- 

 take which is advantageous when the ventilation is 

 poor. During asphyxia or crying the pulmonary ar- 

 terial pressure rises and may exceed the systemic 

 pressure causing the possibility of a right-to-left 

 flow again, when the lower half of the body will 

 probably receive blood of a lower oxygen content 

 than the upper half (88). 



The wall of the ductus arteriosus has a sphincter- 

 like structure and the musculature a poor nerve 

 supply. In the lamb fetus the lumen is nearly as 

 large as the pulmonary artery and descending aorta 

 and the blood flow through it approximately one- 

 third of the combined output of the two ventricles (76) ; 



