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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



12 - 



100 



fig. 15. Oxygen consumption per kilogram body weight at 

 different arterial oxygen saturations. (•) Fetal lambs; (©) 

 lambs less than 1 day old; (o) lambs 1-10 days old after the 

 rise in minimal oxygen consumption (95 r o confidence limits 

 are shown); horizontal lines indicate the range of observations. 

 [From Cross el al. (57).] 



Oxygen Consumption 



The fetus has a third mechanism of defense at 

 low oxygen tensions, that of lowering its oxygen con- 

 sumption. Cross et al. have shown that this occurs 

 when the umbilical arterial oxygen falls to 50 per cent 

 saturation in the lamb, and the effect increases as the 

 arterial oxygen tension is reduced still further (57) 

 (fig. 15). The fall in oxygen utilization may be due 

 primarily to the decrease in blood flow to the majority 

 of tissues, as discussed above, for it is accompanied 

 by an accumulation of lactic acid and depletion of 

 tissue glycogen stores (171). It would be interesting 

 to know if the tissue temperature falls as it does in the 

 adult when the blood flow and oxygen supply are re- 

 duced to muscle (169). A reduction in oxygen con- 

 sumption with low arterial oxygen tensions is not 

 readily demonstrated in the adult animal, for the 

 cardiac output increases and the heart is liable to 

 sudden failure before low oxygen tensions are reached 

 (1). The inability of the fetal cardiac output to in- 

 crease, and the capacity of the heart to continue to 

 beat during asphyxia, must be important for survival 

 during birth. 



The oxygen consumption of newborn animals, at 

 their neutral temperature, increases after birth at 

 intervals which vary with the species (67, 72). In 

 the lamb, the minimal oxygen utilization is trebled 



within 24 hours of delivery to correspond with the 

 metabolic requirements of its surface area, and usu- 

 ally no shivering occurs (72): this recently acquired 

 increase in oxygen consumption is not well main- 

 tained when the arterial oxygen saturation is lowered. 

 Hill has also observed that the increase in oxygen 

 consumption without shivering, in response to a 

 low environmental temperature, is particularly sus- 

 ceptible to hypoxia (107). 



Heart Rate During Reduction in Maternal 

 Placental Blood Flow 



The influence of asphyxia on the fetal heart rate 

 in utero and its relationship to the degree of reduc- 

 tion in maternal placental blood flow, or the short- 

 term placental reserve, has important practical 

 applications. The physiology of the response of the 

 fetal cardiovascular system to asphyxia has already 

 been discussed. It is generally agreed that tachy- 

 cardia is the first indication of intrauterine asphyxia 

 at term in the human infant (108), and in the lamb. 

 Born et al. observed that tachycardia did not occur in 

 the lamb until the umbilical arterial oxygen satura- 

 tion was reduced to 50 to 35 per cent, during the 

 administration of 7.0 to 5.5 per cent oxygen to the 

 maternal sheep; bradycardia, most usually associated 

 with intrauterine asphyxia, was not observed until 

 the oxygen saturation reached 20 per cent (39). 

 The influence of the accumulation of carbon dioxide, 

 occurring in asphyxia, is not known. The time 

 course of both the cardiac acceleration and slowing 

 observed experimentally and clinically is very vari- 

 able, depending upon the rate of onset and degree 

 of asphyxia induced, the existing oxygen environ- 

 ment and the previous asphyxial history. For in- 

 stance, a sustained acceleration is readily observed 

 as the tension of oxygen administered to the maternal 

 animal is gradually lowered, but it is only transient 

 when nitrogen is inspired by the mother or when 

 the cord is tied; frequently acceleration does not 

 precede the bradycardia in the latter circumstances. 

 Hon (108) has described two time courses for fetal 

 bradycardia in the human infant during labor: the 

 one, which he describes as physiological, occurs fol- 

 lowing a uterine contraction and most usually in 

 vertex presentations; the heart slows briefly and 

 recovers swiftly within 15 sec. The second, which 

 Hon calls pathological, has a longer time course and 

 is considered to be possible evidence of previous 

 asphyxia or a permanent reduction in uterine blood 

 flow. An example of the influence of limiting the 



