UTERINE BLOOD FLOW 



1607 



physiological burden on the circulation led Burwell 

 (47-49) to regard it as an arteriovenous shunt. The 

 blood -volume increase of the human uterus that 

 occurs is reported by Caton et al. (52). The effect on 

 the maternal heart rate, cardiac output, and blood 

 volume are comparable to the effect of a major arterio- 

 venous shunt in the cardiovascular system. In the 

 latter part of pregnancy increases are seen in resting 

 heart rate, cardiac output, circulation rate and blood 

 volume [Hamilton (91), Palmer & Walker (169)]. 

 In the last few weeks of pregnancy there begins to be a 



z 4 



" i!60 h 

 1180 



-: x / \ 



• — • 



^ UTERINE CONTRACTION 



MINUTES __ 



^__ 1 // ET^a 



10 



20 



60 



90 



fig. 20. Average uterine blood flow at term in uterus of ewe. 

 The time scale does not refer to the duration of uterine con- 

 tractions or to the duration of the decrease in flow. Although 

 recordings were taken continuously, for the reason of space 

 economy only one single contraction, with two blood flow 

 readings, is given for each rate of oxytocin infusion. In early 

 labor, uterine contractions occurred every 8-10 min and lasted 

 for about 20-25 sec. Each contraction was accompanied by a 

 decrease in flow. When the rate of oxytocin was doubled, con- 

 tractions occurred every 4-5 min and lasted for 35-40 sec, and 

 the flow was more reduced. With a further increase in the 

 infusion rate, the contractions lasted for 45-60 sec and the 

 reduction in flow was more marked. Note the rebound in flow 

 during uterine relaxation. [Assali et al. (10).] 



decline in each of these as maternal oxygen consump- 

 tion increases. The cause of these declines is not 

 known, but one is reminded of the profound uterine 

 ischemia seen in rabbits (vide supra) toward the end 

 of pregnancy. The concept of the gravid uterus actin? 

 as an A-V shunt was first set forth in 1938 [Burwell 

 (49)] and supported by the later studies cited above. 

 The most recent summary is by Burwell (47). The 

 circulatory load as a pathophysiological mechanism 

 upon the circulation after birth when the A-V shunt 

 is removed has drawn the attention of Schwarz (217) 

 and the ischemia of the parturient uterus noted by 

 Thoma (234). 



Assali et al. (7, 8) were the first investigators to 

 catheterize the uterine vein in women for the purpose 

 of withdrawing blood samples that could be employed, 

 when combined with simultaneous arterial samples, 

 to use the Fick principle (nitrous oxide) in estimating 

 uterine blood flow. Studying women in normal preg- 

 nancy, Assali et al. (9) found that the flow was 15 ml 

 per 100 g of tissue per min. This value, approximating 

 150 ml per min, is surely low (see above). However, 

 with the same method, they observed a decrease to 

 9 ml per 1 00 g of tissue per min in the first 24-hour 

 postpartum period. Before commencing these studies, 

 Assali (7) reviewed and criticized methods used 

 previously. 



A new departure in measurement of uterine blood 

 flow was reported by Assali et al. (10) who monitored 

 blood flow in a uterine artery of the pregnant ewe 

 with an electromagnetic flowmeter during spontane- 

 ous and induced labor. Uterine contractions, spon- 

 taneous or induced, were accompanied by a significant 

 decrease in blood flow which was more or less pro- 

 portional to the strength of the contraction. During 



Pr«- 1 



pactum^ 

 delivery 



6 7 8 



DAY POST-PARTUM 



10 



-/>- 



14 



fig. 21. Average uterine blood How 

 obtained during the puerperium in the 

 ewe. Delivery of the placenta occurred 

 between day 1 and day 2. Note the 

 precipitous fall in flow which occurred 

 after the expulsion of the placenta. The 

 progressive decrease during the post- 

 partum period coincided with uterine 

 involution. [Assali et al. (10).] 



