THE FETAL AND NEONATAL CIRCULATION 



I 63 I 



fig. 9. Response of the fetal heart rate and arterial pressure 

 in the lamb during hypoxia. The rise in blood pressure is accom- 

 panied by a) tachycardia during the administration of 10' ", < > 

 to the mother, b) bradycardia during ventilation with 6% Oj. 

 [From Born et at. (39)-] 



choline also causes bradycardia and hypotension 

 early in gestation. Dawes and his colleagues consider 

 that the range of effectiveness, per kg of body weight, 

 of those autonomic drugs does not differ from 60 to 

 160 days in the lamb, and is about the same as in 

 the adult for both the lamb and the fetal rabbit (70, 

 74). In the lamb at term, with ventilation established, 

 adrenaline and noradrenaline cause a greater rise in 

 blood pressure following occlusion of the umbilical 

 cord, when the low resistance circuit of the placenta 

 is absent. Equal doses of these drugs are also more 

 effective when injected into the femoral vein and pass 

 straight to the left side of the heart and to the coro- 

 nary circulation, than after injection into the jugular 

 vein when the drug has first to traverse the lungs. 

 Since suprarenal venous blood enters the inferior 

 vena cava, there will be the possibility of a rise in 

 the fetal blood pressure and an increase in the placen- 

 tal blood flow during stress; the effectiveness of sym- 

 pathomimetic amines liberated during the stress of 

 asphyxia in the fetus may be limited by the reduced 

 responsiveness of the cardiovascular system during 

 asphyxia (195). The adrenal glands and accessory 

 organs contain a pressor substance early in develop- 

 ment in the sheep (54) and in the human infant 

 (186). It is also known that sympathomimetic amines 

 are released into the adrenal veins during asphyxia 

 by about 90 days gestation in the sheep; this libera- 

 tion is due to the direct action of asphyxia on the 



adrenal medulla. The splanchnic nerves do not 

 take part in the release until shortly before term. 

 It is perhaps significant that noradrenaline predomi- 

 nates for its pressor activity is the greater and the 

 stimulating action on metabolism apparently more 

 effective than that of adrenaline in the young animal 

 (136). 



The catecholamine concentration of human fetal 

 heart, kidney, and lung during the first trimester 

 was found to be roughly similar to that in adult 

 organs though the brain contained much smaller 

 concentrations than in the adult (99); again, norepi- 

 nephrine predominated suggesting its early appear- 

 ance at sympathetic nerve endings, but no dopamine 

 was found in any tissue studied. 5-Hydroxytrypta- 

 mine is found in the blood platelets of the fetal guinea 

 pig two-thirds of the way through gestation and is 

 still lower than the adult at term (174); the brain 

 levels, however, approximate to those of the adult 

 at term (120). The high estrogen and progesterone 

 content of fetal blood (4) may influence both cardio- 

 vascular development and the responses of the vessels. 

 The action of many other pharmacological substances 

 on the fetus has been reviewed recently (20). 



The importance of cardiovascular regulating 

 mechanisms to the fetus in utero is questionable; 

 asphyxia and hemorrhage are probably the only 

 stresses which the fetus encounters. The responses to 

 hemorrhage have not been frequently studied and 

 Mott suggests that they might possibly be a better 

 indication of the homeostatic capacity of the fetal 

 circulation than the response to asphyxia since the 

 fetus is more resistant to hypoxia than the adult 

 (140). 



FETAL PLACENTAL BLOOD FLOW 



Effective Perfusion Pressure. Resistant e 0] 

 the Placental Circulation 



The effective perfusion pressure across the fetal 

 placental circulation increases as the arterial pressure 

 rises with gestational age. Figure 10 shows some com- 

 parative values for umbilical arterial and venous 

 pressure measurements in the lamb (25). Reynolds & 

 Paul (160) have observed umbilical venous pressures 

 as high as 35 mm Hg in the lamb at term; the reason 

 for this rise in umbilical venous pressure may be re- 

 lated to an increased resistance to flow in the fetal 

 liver, through which most of the umbilical blood 

 passes during development. The sphincter of the 



