THE FETAL AND NEONATAL CIRCULATION 



l6 35 



steady rise in the blood hemoglobin in fetuses of all 

 species during gestation. Most are born with levels 

 which are higher than that of the mother (29) and 

 erythropoietic concentration is known to be high in 

 the cord blood of many species (6, 41). Third, the 

 average blood flow through the fetal tissues is high. 

 This has not been compared with the adult values 

 for each individual tissue but estimates of the fetal 

 cardiac output in the lamb are high, as already de- 

 scribed and amount to an average tissue flow of 

 about 120 ml per kg per min, which is at least twice 

 the flow in the adult sheep. These cardiac output 

 measurements have been calculated indirectly from 

 umbilical blood flow measurements and the dis- 

 tribution of blood within the fetus with an open chest 

 and are, therefore, probably an underestimate. 

 Approximate calculations for the human fetus also 

 suggest that the average body blood flow is high. 



INFLUENCE OF HYPOXIA AND ASPHYXIA 

 ON THE FETUS 



The effects of a prolonged reduction in oxygen 

 supply have been observed in fetuses born to mothers 

 at high altitude and the possibility of hypoxia as a 

 cause of congenital malformation has already been 

 discussed : experimentally the influence of acute hy- 

 poxia, produced by maternal breathing of low- 

 oxygen gas mixtures has been studied the most 

 frequently. The results of true asphyxia may be ob- 

 served during marked impairment of the maternal 

 placental circulation or the mechanical obstruction 

 of the umbilical vessels. 



Hemoglobin 



The possibility of a rise in blood hemoglobin 

 concentration, in response to a reduced oxygen 

 supply, first attracted the attention of Joseph Bar- 

 croft (25) who correlated the fetal hemoglobin level 

 with the percentage saturation of the umbilical vein 

 blood with oxygen, in the lamb at term. This idea 

 has proved most controversial clinically (118, 182), 

 particularly because subsequent investigators did not 

 heed Barcroft's warning concerning the difficulties of 

 collecting a good specimen of umbilical vein blood, 

 and his awareness of the variety of conditions which 

 might bring about rapid changes in the oxygen satu- 

 ration of cord blood. Neither the oxygen saturation 

 of the blood in the umbilical vessels at birth, nor 

 the total level of hemoglobin in the blood and the 

 relative proportion of fetal hemoglobin, contributing 



to this, have proved to assist in the interpretation of 

 either the extent or duration of any impairment of 

 the intrauterine environment. However, it is now 

 certain that fetal hemopoietic tissues can respond 

 when oxygen availability is reduced for the young 

 born to llamas at 15,000 ft have higher blood hemo- 

 globin concentrations than those born at sea level 

 (150). It is interesting to speculate on this response 

 at altitude in the fetus: as described, erythropoietic 

 concentration is high in cord blood at sea level and 

 may represent the fetal response to low arterial oxy- 

 gen tensions despite the adequate availability of 

 oxygen to the majority of fetal tissues; adult hemo- 

 poietic tissue, however, will also respond at 25,700 ft 

 (7,830 m) when the arterial oxygen tension is reduced 

 to 33 mm Hg (151), a value which is normal for the 

 fetus. Born et al. have shown that fetal hemoglobin 

 concentrations increase during acute hypoxia in the 

 lamb, which suggests that red cells may have been re- 

 leased from the spleen or there may only have been a 

 loss of plasma to the extracellular space (39). 



Blood Flow 



It is doubtful if the possibility of increasing the 

 tissue and placental blood flow in response to a re- 

 duction in oxygen supply is significant in the fetus: 

 in the lamb, younger than 60 days of gestational age, 

 both umbilical and tissue blood flows will probably 

 fall since there are no reflex mechanisms to elevate 

 the blood pressure and the depleted oxygen supply 

 will cause bradycardia. Later, when the cardio- 

 vascular reflexes begin to be developed, a rise in 

 arterial pressure and carotid and umbilical blood 

 flow is observed in response to asphyxia or low oxy- 

 gen tensions; these responses occur when the fetal 

 arterial oxygen saturation is reduced to 50 to 35 

 per cent, following the administration of 7.5 to 5.0 per 

 cent oxygen in nitrogen to the mother (39). The 

 increase in carotid and umbilical blood flows prob- 

 ably occurs at the expense of the blood supply to the 

 major portion of the body, for it is unlikely that the 

 cardiac output increases: there is no experimental 

 evidence to support this statement, but it is known that 

 hypoxia does not increase the cardiac output in the 

 newborn lamb (57); this lack of response is possibly 

 related to the very high cardiac output at this time, 

 for the fetal cardiac output at term is at least twice 

 that of the adult per kg body weight. The decrease 

 in oxygen consumption of the hind quarters in the 

 lamb during hypoxia may be evidence for peripheral 

 vasoconstriction. 



