THE FETAL AND NEONATAL CIRCULATION 



'643 



40 SO I20 160 



fig. 22. Arterial blood pressures before ( ) and after), ) 



birth, showing the continuous course of the rise with increasing 

 age, in the rabbit, cat, sheep, and monkey. (Modified from 

 G. S. Dawes. Changes in the circulation at birth. Brit. Med. 

 Bull. 17: 150, 1 96 1.) 



did not influence its magnitude. It is tempting to 

 suggest that the wide range of initial pressures is due 

 to varying degrees of asphyxia during birth, but no 

 proof exists for this explanation. The pressures rise 

 gradually during the second day of life and during 

 the subsequent weeks. 



Once the temporary interruptions of parturition 

 are over, the mechanisms which have been responsible 

 for the gradual rise in arterial pressure throughout 

 gestation will, probably, be extended into the neo- 

 natal period: these mechanisms are, however, likely 

 to be modified by the different internal environment 

 of the young free animal, as compared with the 

 fetus, and by many other factors which will vary 

 with the species; orthostatic factors and the mode of 

 life will be among these. The rate of rise in arterial 

 pressure is rapid in small animals and the mean pres- 

 sure is about doubled during the first 6 weeks of life, 

 approaching the adult level; in the sheep and monkey 

 the rise is slower (fig. 22). In the human infant, who 

 has been a repeated subject for blood pressure meas- 

 urements, the rise is slow during the first 9 months 

 of life (fig. 23) and continues well into adolescence 

 and throughout adult life (194). [But see also (193). 

 Ed.] 



The newborn is a more satisfactory experimental 

 subject than the fetus for, under experimental condi- 

 tions, an established respiration provides a more 

 constant internal environment than the placental 

 circulation. The differences between the cardio- 

 vascular responses of newborn and adult animals are 

 of a quantitative rather than a qualitative nature: 



£lOO 



S so 



a. 



8 so 

 o 



-I 

 3 



y 



_j 

 o 



H 



to 



z 

 < 



40 



20 



ft* * 



54 BABIES 



< 

 □ 



9 

 DAYS 



6 

 WEEKS 



3 6 



MONTHS 



fig. 23. Mean arterial blood pressures at birth and during 

 the first few months of life in normal infants. [From Holland & 

 Young, Brit. Med. J. 2: 1331, 1956.] 



in the newborn monkey there is evidence for func- 

 tional baroreceptor and chemoreceptor activity, 

 yet bradycardia and hypotension still follow acute 

 hypoxia (71). In the young growing rabbit (70, 83), 

 kitten, and puppy ( 1 1 4) a gradual increase in vasocon- 

 strictor tone in the systemic circulation can be 

 demonstrated by the responses to asphyxia and to the 

 injection of hexamethonium. Downing's (83) observa- 

 tions show that the threshold for baroreceptor stimu- 

 lation in young rabbits is about 40 mm Hg. 

 Hutchinson el al. (11 4) have also demonstrated in the 

 newborn kitten and puppy that the carotid sinus- 

 cardiac center mechanism will respond to a rise in 

 pressure but not to a fall. These findings may be ex- 

 plained by Landgren's (126) observations that 40 

 mm Hg is just within the recording range of the 

 baroreceptors; any stimulus which raises the pressure 

 will elicit a response, especially if the pulse pressure 

 is also increased (85), but a further fall will be in- 

 effective. As the resting arterial pressure rises and 

 approaches the maximum sensitivity range of the 

 baroreceptors, 85 to 100 mm Hg, the reflexes 

 become more active; for instance, following the injec- 

 tion of adrenaline the percentage decrease in heart 

 rate increases in relation to the percentage rise in 

 blood pressure in the growing rabbit and kitten. The 

 direct action of adrenaline on the heart could only 

 be demonstrated in the youngest animals following 

 doses so small that the blood pressure did not rise 

 sufficiently to elicit a reflex bradycardia. In contrast, 

 the young kitten heart was found to be more sensitive 

 to acetylcholine than the peripheral vessels; with 

 small doses a marked bradycardia accompanied the 

 fall in blood pressure in the kitten, while reflex 



