1642 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



Cord clamped after 3 mm _^* 



J< 



Cord clamped immediately 



\y^ 



Cord clamped during delivery 



AGE (hr ) 



I 2 3 4 24 



fig. 21. Three representative records of the changes in the 

 systolic pressure of normal babies during the first 24 hours of 

 life. [From Ashworth & Neligan (14).] 



factor which must influence all but the cardiometer 

 results in animals, which also have their inherent 

 disadvantages. The average cardiac output of the 

 human infant, 540 ml per min, corresponds to a 

 value of 180 ml per kg per min and is about double 

 the value in the adult per kg body weight; the cardiac 

 index is 2.5 liters per min per m 2 . Assuming a new- 

 born heart rate of 140 per min, the stroke volume 

 will be approximately 4 ml. It may be noted that if 

 the estimates of cardiac output in utero are correct, 

 200 ml per kg per min (17), the value does not change 

 in the neonatal period and no increase in oxygen 

 consumption is observed (58). On the other hand, 

 there is evidence for an increase in cardiac output 

 following birth in the lamb: Cross et al. (57) made 

 calculations using the Fick principle and obtained 

 values of 325 ± 30 ml per kg per min, which com- 

 pared with the near term intrauterine estimate of 

 235 ml per kg per min for both ventricles; a single 

 ventricle has therefore increased its output three- 

 fold. This increase may be the response to the raised 

 oxygen consumption which occurs in the lamb at 

 birth or it may be the expression of the better measure- 

 ments which are possible after birth. 



Systemic Pressure, Cardiovascular Reflexes 

 and Peripheral Resistance 



When the changes in systemic arterial pressure at 

 delivery are measured, a discrepancy exists between 

 the lamb and the human baby; namely, a small 

 transient rise of pressure is observed following the 



initiation of ventilation or occlusion of the cord in 

 the lamb (66) while, remarkably, in the human 

 infant no change of pressure is seen (183, 196). There 

 are many possible explanations for this difference: 

 first, the different types of maternal placental circu- 

 lation; second, the influence of contraction of the 

 uterine muscle on this circulation; and third, the 

 alteration of distribution of blood between the 

 placenta and fetus before the arterial measurements 

 are made. The arterial pressure measurements in 

 lambs have all been made on fetuses delivered by 

 Cesarean section and, as the sheep uterus is not very 

 reactive to surgery, the maternal and consequently 

 the fetal placental circulations are probably not 

 greatly impaired. When the lamb is delivered vagin- 

 ally the maternal placental blood flow is not reduced 

 during labor and does not decrease until separation 

 of the placenta some hours after delivery of the fetus 

 (15). The temporary rise in pressure observed is 

 therefore probably due to the removal of the low 

 resistance circuit of the placenta, and a small rise in 

 arterial pressure following cord occlusion has also 

 been observed in the rhesus monkey delivered by 

 Cesarean section (71). In the adult animal the reduc- 

 tion of a circulating bed even of the same resistance 

 as the total vascular bed, raises the arterial blood 

 pressure (23). In the human, contraction of the uterus 

 during labor probably reduces both the maternal and 

 fetal placental blood flow and therefore much of the 

 low resistance circuit of the placenta is gradually 

 removed before the arterial pressure measurements 

 are made as the cord is tied. Other hemodynamic 

 factors, such as the relative distribution of blood 

 between the fetus and the placenta and the relative 

 proportions of the cardiac output which traverse 

 the placenta, might also influence any change in 

 systemic pressure at birth. In the lamb, at term, only 

 1 5 per cent of the total blood volume is to be found 

 in the placenta and 60 per cent of the cardiac output 

 traverses this vascular bed. The human placenta 

 contains 30 per cent of the total circulating blood 

 volume at term (173), but the portion of the cardiac 

 output perfusing it is not known. 



Ashworth & Neligan (14) have used the con- 

 ventional inflatable cuff and manometer and a 

 sensitive pulse indicator to measure the arterial 

 pressure in the newborn infant's arm, and report 

 marked changes in systolic pressure within the first 

 24 hours of life. The initial pressures, within 2 min 

 of delivery, ranged from 1 16 to 52 mm Hg and there 

 was subsequently a fall of up to 54 mm Hg (fig. 21); 

 delay in clamping the cord postponed this fall, but 



