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HANDBOOK OF PHYSIOLOGY 



CIRCULATION II 



of adrenaline and the observers attributed this to a 

 rise of pulmonary arterial pressure. 



Heart Rate. Regulating Mechanisms 



In the smaller animals systematic correlation of 

 the fetal heart rate with age has not been frequently 

 made and when the uterus is opened the data may be 

 questionable on account of cooling and hypoxia 

 (189). In the guinea pig and the rabbit the heart 

 rate has been counted from ECG records taken with 

 the uterine wall intact and the maternal abdomen 

 opened under saline at 37 C (133); the heart rates in 

 both increased from 160 to 320 per min during 20 to 

 67 days in guinea pig and 25 to 31 days in the rabbit 

 fetuses, but the range was wide, possibly on account 

 of intrauterine hypoxia. The maternal heart rates 

 also varied widely and within the same range as the 

 full-term fetus. In the guinea pig fetus a slight slow- 

 ing occurred when the uterine wall was incised and 

 allowed to contract around the large vessels supply- 

 ing the placenta. In the rabbit fetus postmaturity 

 did not influence the heart rate (134). The heart 

 rate of the monkey fetus at Cesarean section is 140 

 to 1 70 beats per min (71 ). 



Barcroft and colleagues (25) counted the lamb 

 heart rate in utero with a stethoscope and found 

 that it increased during the first two-thirds of preg- 

 nancy to 150 beats per min and thereafter fell slowly 

 to 128 beats per min at term; in a larger series in 

 which the heart rates were obtained from blood 

 pressure tracings, the rate rose throughout gestation 

 to about 200 beats per min at term (39). The ewe 

 heart rate is normally 100 to 120 per min. In two 

 studies in humans the fetal heart rate was also ob- 

 served to be faster in midfetal life, 156 per min, than 

 just before birth, when the average was 142 per min. 

 These differences are small, however, and probably 

 not significant; in one study the counts were made 

 with a stethoscope (1 76) and in the second from ECG 

 recordings (192). 



The pattern of changes in fetal heart rate in utero 

 will be determined by the rate of development of the 

 pacemaker rhythm and the onset of subsequent vagal 

 restraint. The anatomical pathways of the parasym- 

 pathetic system are laid down early, and vagal fibers 

 may be observed in the A-V bundle in a 6-weeks- 

 old human fetus (184), before the inhibitory response 

 of the isolated cardiac muscle to acetylcholine is ob- 

 served (194). Vagal tone is not apparent in utero or 

 subsequently in the guinea pig, rabbit, or cat: the 

 full-term fetal heart rates are the same as in the adult, 



and in the cat the heart rate is uninfluenced, in both 

 the newborn and the adult, by section of the vagus 

 nerves (114); the latter, however, is not particularly 

 good evidence. Vagal tone was considered by Bar- 

 croft and colleagues (25) to be present in the sheep 

 fetus toward term, for they found that bilateral sec- 

 tion of these nerves increased the heart rate; this, 

 however, was not confirmed by Born et al. (39). 

 Stimulation of the peripheral cut end of the vagus 

 nerve will cause bradycardia in the sheep fetus half- 

 way through gestation, though the heart will re- 

 spond to intravenous acetylcholine earlier (74). 

 The isolated fetal heart is very sensitive to acetyl- 

 choline but it is not possible to correlate this with the 

 age of the fetus (19). The influence of atropine on 

 fetal heart rates and a comparison with its action in 

 the adult of the same species is practically unknown; 

 late in intrauterine life atropine in the fetal circula- 

 tion causes an acceleration of the fetal guinea pig 

 heart (97). In the pregnant woman atropine in the 

 maternal circulation ( 1 1 o) abolishes asphyxial fetal 

 bradveardia, but there is no evidence for its influence 

 on the normal heart rate nor independent evidence 

 for its placental transfer. 



The sympathetic pathways are known to be laid 

 down early in development in the kitten and the hu- 

 man fetus (44, 103); the lamb heart is able to acceler- 

 ate in response to intravenous adrenaline two-thirds 

 of the way through the gestation period and at term 

 the sensitivity of the fetal heart is little different from 

 that of the adult in both the sheep (74) and the rabbit 

 (70) ; earlier observers frequently found a decreased 

 sensitivity and this was possibly due to anoxia (194). 

 Again, the isolated heart is sensitive to adrenaline 

 and noradrenaline and there is no correlation with 

 the age of the fetus (19). 



Cardiac Output 



Fetal cardiac output was measured in the goat 

 both cardiometrically and using the Fick principle 

 by Barcroft and his colleagues (25); they estimated 

 that it increased from 1 1 3 ml per kg body weight per 

 min at 89 days of age to 193 ml per kg body weight 

 per min at 150 days, full term. Dawes et al. (75) cal- 

 culated that the cardiac output of both ventricles in 

 the lamb at term was 235 ml per kg per min, know- 

 ing the umbilical blood flow and estimating that it 

 formed about 57 per cent of the combined cardiac 

 output. Assali et al. (17) made similar calculations 

 in human fetuses of 9 to 28 weeks gestation and found 

 the cardiac output to be 200 ml per kg per min; 



