Fetal Placental Blood Flow Research Articles

Catecholamine physiology in the ovine fetus: III. Maternal and fetal response to acute maternal exercise

A study was made of the effects of maternal exercise on fetal plasma concentrations of catecholamines in nine ewes with chronically catheterized singleton fetuses at 125 to 137 days' gestation. The ewes were subjected to acute treadmill exercise of 2.5 mph for 45 minutes with continuous recording of maternal and fetal blood pressures. Samples of arterial blood were obtained for measurement of catecholamines, glucose, and blood gases. Changes in blood flow in fetal organs in response to maternal exercise were assessed by injection of radioactive microspheres. The maternal plasma catecholamine responses were related to the severity of the exercise stress as indicated by the index of cardiac effort. The fetal responses did not correlate with maternal cardiac effort. A significant decrease in fetal Po2 with a moderate alkalosis occurred, accompanied by a significant elevation in circulating levels of norepinephrine. At the peak of exercise, there was an increase in fetal renal, adrenal, and placental blood flows, as compared to the control period.

Placental Blood Flow and Transfer of Nutrient Analogs in Large, Average, and Small Guinea Pig Littermates

Placental blood flow in the maternal side and transfer of [14C]-alpha isobutyric acid (AIB) and [3H]methylglucose (MG) were measured in 22 pregnant guinea pigs at various gestational ages. The fetuses were divided in three groups according to their body weights: small, average, and large. Body weight was 85.25% of average values in the small fetuses and 114.12% in the large fetuses. Placental weight was 121.73% of average in the large fetuses and 84.42% in the small fetuses. Placental blood flow was 134.48% of average in large fetuses and 73.18% in small fetuses. AIB and MG transfer were significantly lower in the small fetuses (80.33% and 86.06%, respectively, of average values). In contrast, in large fetuses, AIB transfer was 123.43%, and MG transfer was 113.96% of average. Significant correlations were found between fetal and placental weight and placental blood flow and transfer of AIB and MG in the various groups. Placental transfer of AIB and placental blood flow were significantly correlated in the small (r = 0.59) and average weight fetuses (r = 0.37). In addition, the slope of the regression curve for AIB was significantly steeper in the small fetuses when compared with the slope of average and large fetuses. Placental blood flow and transfer of MG were significantly correlated only in the large fetuses (r = 0.48). In the small fetuses, the rate of AIB transfer was proportionally more reduced than that of MG transfer as the rate of placental blood flow decreased. The results demonstrate that spontaneous fetal growth retardation in the guinea pig is associated with a smaller placenta, a reduced placental blood flow, and a reduced transfer of AIB and MG. Inasmuch as in the small fetuses AIB transfer was proportionally more reduced than MG transfer, it is suggested that in addition to the reduced blood flow limited availability of certain essential amino acids may be a cause of fetal growth retardation.

Responses of fetal sheep to simulated no-decompression dives.

The effect of simulated standard no-decompression dives to 60 and 100 ft of seawater was tested in 12 near term sheep carrying 16 fetuses. In the immediate postdive period there were no significant changes in fetal blood pressure or fetal placental or renal blood flow, but the maternal blood pressure was elevated and the maternal placental blood flow was depressed. Six surgically prepared fetuses were dived to 100 ft. Five died within 20 min of ascent and the sixth suffered severe cardiac arrhythmia and hypotension. At autopsy all fetuses were observed to have massive bubbling in the arterial system and heart. Five fetuses were dived to 100 ft without surgery. Two were alive 3 h later and no bubbles were present at autopsy, and three were born alive at term. With the 60-ft dives, three fetuses were subjected to surgery and all suffered massive bubbling. Two fetuses were dived to 60 ft without surgery; one was alive after 3 h and the other was born alive at term. We conclude that surgery and monitoring result in the formation of postdive gas bubbles that would not otherwise appear.

Placental blood flows and oxygen transfer during uterine contractions: A mathematical model

Theoretical calculations have been performed on the effects of changes in intrauterine pressure on maternal and fetal placental blood flows and pressures and transplacental O 2 exchange. Initial conditions and contraction profiles may be “assumed normal” or abnormal. The model predicts that the fetal O 2 deficit is most sensitive to the maximum intensity of the contraction and maternal mean arterial blood pressure, less sensitive to the duration of the contraction and far less sensitive to the value of maternal arterial PO 2. The umbilical venous PO 2 falls to a minimum at the peak of a contraction. Its level is most sensitive to contraction intensity and maternal blood pressure; while the value of maternal arterial PO 2 has less effect and the duration of contraction has essentially no effect. There is interaction between factors so that the effect of simultaneous changes is not necessarily in proportion to the effects of individual changes.

Circulatory Changes in the Reproductive Tissues of Ewes during Pregnancy

The blood flows to reproductive organs were measured by means of radionuclide-labeled microspheres in 24 pregnant ewes with gestational ages ranging from 38 to 141 days. The microspheres were injected in the left ventricle of the non-anesthetized animal 4–7 days after surgery and while uterine blood flow was recorded continously by means of electromagnetic probes on both uterine arteries. The blood flow to ovaries with corpus luteum was maximum in early pregnancy and greater than in ovaries without corpus luteum. Placental blood flow increased from ∼30 to ∼300 ml/min during the stage of placental growth (40–90 days). In the last 50 days of pregnancy placental weight declined, whereas placental flow continued to rise and attained ∼1,500 ml/min near term. The growth curves of fetal weight and placental blood flow followed a similar pattern, but the early increase in placental flow was more rapid than the increase in fetal weight. Hence placental flow per gram of fetus was larger in early pregnancy (∼9 ml/min·g) than near term (∼0.25 ml/min·g). In twin pregnancies the placenta of each fetus received a smaller blood flow than the placenta of a singleton of comparable age. Mammary blood flow demonstrated no change until the last 60 days, when it increased from ∼10 to ∼200 ml/min. This phenomenon coincided with an increase in mammary weight and the final progressive increase of placental blood flow.

Fetal blood volume and fetal placental blood flow in lambs.

SummaryIn the conscious, nonmedicated sheep fetus, blood volume strongly influences placental blood flow, a change of 1% in volume being associated with a change of 1.7 ± 0.1% in flow. This relationship is valid between blood volumes of 80 to 110% of normal, is independent of time over a period of several days and independent of concomitant changes in fetal hemoglobin concentration.We are indebted to Mr. Frederick Arfmann, Dr. J. Metcalfe and Mrs. Alice Fitzgerald for help with this study.

Relationships between fetal weight, placental weight and maternal placental circulation in the rabbit at different stages of gestation.

Summary. Fetal weights, placental weights, myometrial, vaginal and maternal placental blood flows (estimated with radioactive micro-spheres) were measured in forty-one rabbits at 16, 20, 24 or 28 days of gestation; term occurs around Day 31. Multiple linear regression analysis indicated that fetal weight and placental weight were positively related at all stages of gestation examined. Fetal weight and placental blood flow were negatively related at Day 16, but positively related at Days 20, 24 and 28. The conceptus adjacent to the ovary had greater placental weight and flow values than the means from all conceptuses in the horn by Day 20 but fetal weight was only greater by Day 28. The relevance of these findings to the determination of fetal weight is discussed.

Uneven distribution of maternal and fetal placental blood flow, as demonstrated using macroaggregates, and its response to hypoxia.

A technique is described for studying the distribution of blood flow to the maternal and fetal placental vessels in sheep and dogs with radioactive labeled macroaggregates of albumin. When the maternal animal breathed room air the distribution of maternal placental blood flow was uneven among the cotyledons as well as within a given cotyledon. Fetal blood flow was also distributed nonuniformly among and within the cotyledons. The relation of maternal to fetal placental blood flow was also markedly uneven (coefficient of correlation, tau = 0.066). After the animal was made hypoxic by breathing 10-12% O(2) the distribution of maternal, fetal, and maternal/fetal placental flows became more uniform. The coefficient of correlation of maternal to fetal flow was high (tau = 0.53, P < 0.01). While the maternal animal breathed room air, after ligation of a major branch of the umbilical artery the distribution of maternal, fetal, and maternal/fetal flows in the remaining two-thirds to three-fourths of the placenta became more uniform. The correlation coefficient for maternal to fetal flow was high (tau = 0.35, P < 0.01).It appears that under normal circumstances with uneven distribution of blood flows there is a considerable portion of the placenta that does not receive blood flow in optimum quantities to promote efficient O(2) exchange. Failure to consider the influence of nonuniform maternal flow/fetal flow will result in overestimation of mean maternal-fetal oxygen tension gradients, and thus underestimation of the placental diffusing capacity for oxygen. In response to maternal hypoxia or compromise of the fetal placental circulation the distribution of maternal, fetal, and maternal/fetal flows becomes more uniform, thereby increasing the efficiency of placental O(2) exchange.

Respiratory Function of the Placenta as Determined with Carbon Monoxide in Sheep and Dogs*

A technique is described for studying the respiratory function of the placenta using carbon monoxide, a gas whose exchange across the placenta between the maternal and fetal circulations is limited by diffusion rather than blood flow. During the steady state before the introduction of CO, the normal concentration of carboxyhemoglobin in the ewe, [COHb](M), is approximately 0.90%, and that in the fetus is 2.9%, the ratio [COHb](F)/[COHb](M) being 3.2. In dogs the corresponding values are 1.9%, 4.8%, and 2.4%. After the introduction of CO into the mother animal, CO diffused across the placenta slowly with an equilibration half-time of approximately 2 hours. The average carbon monoxide diffusing capacity (D(Pco)) of the placenta during maternal to fetal exchange was 0.54 ml per (minute x mm Hg x kg fetal weight) (SD +/- 0.13) in sheep and 0.57 ml per (minute x mm Hg x kg) (SD +/- 0.18) in dogs. The fetal to maternal placental diffusing capacity in two sheep was 0.54 ml per (minute x mm Hg x kg). Calculations considering the relative rates of reaction of O(2) and CO with red cell hemoglobin and the relative rates of diffusion of the two gases suggest that the true D(Po2) should be about 1.2 to 2 times greater than the D(Pco) or 0.65 to 1.1 per (minute x mm Hg x kg). This is about 5 times greater than the reported value of D(Po2) calculated from measurements of P(O2) in the mixed uterine and umbilical venous blood. With a diffusing capacity of this magnitude the maternal and fetal placental end capillary P(O2) would approach equilibrium, becoming too small to measure, and the calculation of D(Po2) would be unreliable. We suggest that the apparent end capillary P(o2) gradients of 15 to 20 mm Hg, obtained from sampling uterine and umbilical venous blood, result from a combination of uneven distribution of maternal and fetal placental blood flow and from placental oxygen consumption.