THE FETAL AND NEONATAL CIRCULATION 



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AGE IN DAYS 



5 15 25 35 



fig. 20. Right ventricular systolic pressure in 15 puppies 

 during the first 5 weeks of neonatal life. [From Rudolph el al. 

 (166).] 



pulmonary blood flow at the onset of pulmonary 

 ventilation in the lamb; similar changes in blood 

 chemistry, and vasodilator drugs, also increase blood 

 flow in the unexpanded fetal lung (73) which would 

 suggest that a decrease in vascular resistance is not 

 necessarily due to uncoiling of vessels as suggested by 

 Reynolds (156). Once started, the increase in blood 

 flow itself together with the raised left atrial blood 

 pressure may help to maintain a low pulmonary 

 vascular resistance as it does in the adult lung (42). 

 Pulmonary vascular resistance has been calculated in 

 the human infant at birth and during the first 3 weeks 

 of life from measurements of the pulmonary artery 

 pressure and cardiac output, determined by the Fick 

 principle (164). Within a few hours of delivery the 

 pulmonary vascular resistance is about 550 dynes per 

 sec per cm -s in comparison with an assumed fetal 

 value of 8,000 dynes sec cm -5 . This neonatal value 

 is still considerably higher than that found in the 

 older infant and the adult, but is already much less 

 than the systemic vascular resistance; it declines to 

 the adult level by 6 months of age by which time 

 the walls of the pulmonary arterioles are reduced in 

 thickness (62). The lung blood volume does not 

 change immediately following this large drop in re- 

 sistance (66) but a considerable increase has been 

 demonstrated within the first 24 hours of life in the 

 guinea pig (92). 



The pulmonary arterial pressure is reduced by 

 about a half to approximately 35 mm Hg during 

 the immediate postnatal period in both the lamb 

 and the human infant, and in the puppy (165, 166). 

 The final reduction in pressure occurs gradually over 



the following weeks (fig. 20). The thickness of the 

 walls of the two ventricles is nearly equal in the 

 fetus with a slight preponderance of the right 

 chamber: while the pulmonary vascular resistance 

 and arterial pressure are falling and the systemic 

 vascular resistance and pressure are rising in the 

 newborn period, the right ventricular wall decreases 

 in thickness and the left ventricular wall increases in 

 thickness; in the human infant these changes are 

 nearly completed within the first month of life (121). 



The Heart 



The immediate changes in heart rate in the human 

 infant following a normal birth are variable and 

 transient (173). In the lamb delivered by Cesarean 

 section the heart rate slows when the cord is clamped 

 (66): this may be reflex in origin for the arterial 

 pressure is raised, but may also be due to the direct 

 effect of asphyxia on the pacemaker; the bradycardia 

 is followed by tachycardia once respiration and 

 oxygenation of the blood are established. During the 

 first 2 days after birth the heart rate of the human 

 infant is usually lower than in utero, about 120 

 beats per min, and rises during the first week of 

 life (14, 21, 198). The temporary bradycardia is 

 possibly the combined effect of the low body tempera- 

 ture during this period (52) and the residual effect 

 of perinatal asphyxia. In the newborn kitten and 

 puppy the heart rate varies widely, ranging from 

 180 to 260 per min during the first 15 weeks of life 

 (114). The heart rate of the newborn monkey is 

 205 ± 20 (sd) beats per min (1 16). 



The heart volume has been measured radiologically 

 in the human infant (124) and found to have an 

 average value of 48 ml in 55 infants on the day of 

 birth; during the first hour of life there was an in- 

 crease in volume with a return to the immediate 

 postnatal level within 3 hours. During the subsequent 

 4 days of life the volume diminished by 25 per cent 

 and thereafter increased; the decrease in heart size 

 was more pronounced in premature babies. These 

 early changes in heart volume and the enlargement 

 of the heart which occurs following birth asphyxia 

 (51) need to be made simultaneously with other 

 circulatory measurements, for a better understanding 

 of the events taking place. 



Cardiac output measurements in newborn human 

 infants by the dye dilution technique (149) and using 

 the Fick principle (3) provide a very wide range of 

 values 180 to 850 ml per min, which is probably ex- 

 plained by the patency of the fetal channels; this is a 



