THE FETAL AND NEONATAL CIRCULATION 



i * >-> 7 



genetic and environmental factors as well as the 

 pressure within them, the fact that the great vessels 

 do enlarge and the pulmonary vascular bed does 

 increase must mean that there is an increasing vol- 

 ume of flow during intrauterine life; however, it is 

 not known whether the proportion of this blood flow 

 to the total cardiac output changes or remains con- 

 stant during development. Dawes et al. (76) have 

 estimated that, in the near term sheep fetus, about 10 

 per cent of the combined ventricular output perfuses 

 the lungs. 



Hepatic Blood Supply and the Ductus Venosus 



The liver is probably supplied by the most oxy- 

 genated blood in the body; the umbilical vein carries 

 well-oxygenated portal blood from the placenta 

 and a hepatic branch leaves, before the ductus veno- 

 sus, to supply the left lobe of the liver, nearly two- 

 thirds of the whole organ. The volume of this flow 

 is large, representing over 50 per cent of the cardiac 

 output (since the umbilical blood flow is about 57% 

 of the cardiac output in the lamb and probably only 

 a small proportion passes through the ductus veno- 

 sus), and the oxygen tension is unlikely to be greatly 

 reduced by mixture with hepatic arterial blood. 

 Emery (89) found more hemopoietic foci in the right 

 side of the liver than in the left and degenerative 

 changes are observed more frequently on the right 

 side of the liver at autopsy in stillborn infants and 

 following neonatal deaths (101). It is possible that the 

 reduction in oxygen supply to the liver, following 

 birth, is a factor in the development of physiological 

 icterus (178). 



The presence of a ductus venosus is not universal 

 but it is patent in the lamb and monkey and in the 

 human infant at term (27, 71, 129); at the junction 

 with the umbilical vein, the vessel possesses a muscular 

 sphincter, which is innervated by postganglionic 

 branches of the vagus nerve. Cineangiography sug- 

 gested to Barclay et al. (26) that only a small propor- 

 tion of the umbilical venous blood flow passed through 

 the ductus venosus in the lamb but no direct meas- 

 urements have yet been made. It has been suggested 

 that the sphincter closes in response to a rise in um- 

 bilical venous return to the heart (155); conversely 

 it may regulate hepatic blood flow itself or the placen- 

 tal blood flow since the main resistance to the um- 

 bilical blood flow resides in the liver. A large flow 

 through the ductus venosus would ensure a good 

 supply of arterialized blood to the head but the fact 

 that a ductus venosus is not always present suggests 



that no special mechanism exists for supplying the 

 brain with the most arterialized blood. Experimental 

 occlusion of the ductus venosus in the mature lamb, 

 caused no significant change in arterial blood pres- 

 sure, heart rate, or carotid arterial 0> saturation 

 (12). Rostral to the ductus venosus the umbilical 

 vein continues as the portal sinus and joins the portal 

 vein where arterialized and venous blood meet in 

 unknown quantities. 



FETAL HEART 



The development of activity in the mammalian 

 heart has been observed in hanging drop cultures of 

 whole embryonic rat vesicles (96) : the earliest con- 

 tractions occurred in the left ventricle and were fol- 

 lowed by a slower rhythm in the right ventricular 

 tube; when the two ventricles joined the left became 

 the pacemaker. The auricles beat a little later and the 

 sinus venosus last, finally bringing the ventricular 

 rhythm under their control at an early stage in de- 

 velopment. 



Recording of the electrical activity of the heart 

 in utero has not been frequently attempted in experi- 

 mental animals but would provide both fundamental 

 knowledge of the development of the propagated 

 impulse and enable the fetal heart rate to be counted 

 with minimal disturbance during growth. The im- 

 pulse is large enough to record in rabbit and guinea 

 pig fetuses, 15 g in weight (32, 133). Recording of the 

 ECG of the human fetal heart in utero, using leads 

 placed in the mother's vagina or rectum, or on her 

 abdomen has been employed for many years to 

 monitor the heart rate, particularly during difficult 

 labors (127); the method is not, however, widely 

 used and it is possible that the electrophonocardio- 

 graph will be simpler and less subject to interference 

 from the maternal heart (175). Electrocardiograms 

 obtained from human infants at Cesarean section 

 show all the deflections characteristic of the adult as 

 early as the second month; in the full-term infant 

 there is a small right ventricular preponderance 

 corresponding to the slightly greater relative weight 

 of this ventricle at birth. The left ventricle starts to 

 exceed the right 3 months after birth and by 6 months 

 of age the deflections are usually identical with those 

 of the adult; this is due both to growth of the left 

 ventricle and to involution of the right ventricle 

 (121). The T wave is frequently of low amplitude at 

 birth and becomes negative shortly afterward (178); 

 the sign may be reversed again by the administration 



