Cardiorespiratory responses to graded reductions of uterine blood flow in the sheep fetus

Pregnant sheep were chronically instrumented with fetal and maternal catheters and an inflatable occluder and electromagnetic flow transducer were placed on the uterine artery. Uterine blood flow was reduced for approximately 15 minutes to 25 percent, 50 percent, or 75 percent of control uterine blood flow. Fetal blood gases, arterial blood pressure, heart rate and regional distribution of blood flow (by radioactive microspheres) were measured. With progressive reduction of uterine blood flow there was an increasing degree of fetal asphyxia, as measured by blood gases and acid base state. At moderate degrees of asphyxia the fetus responded by redistribution of blood flow to certain organs, namely heart, brain, and adrenal gland, thus preserving oxygenation of these organs. During the most severe degree of asphyxia induced by reduction of uterine blood flow to 25 percent of control there is a reduction of fetal blood flow due to generalized vasoconstriction of essentially all organs. We hypothesize that this is due to the inability of the vasodilator mechanisms to sufficiently oppose the vasoconstrictor mechanisms. Also, because the oxygen consumption of the "vital" organs would be decreased this can be described as the stage of decompensation.     

The relationship of umbilical glucose uptake to uterine blood flow

In 30 experiments performed on 5 pregnant sheep, the rate of glucose transfer from the placenta to fetus via the umbilical circulation was measured while varying uterine blood flow by means of a cuff-type occluder and while maintaining a constant maternal glucose concentration by means of a 'glucose clamp'. Over the range of uterine blood flows obtained, there was no significant effect on the simultaneously measured umbilical blood flow. Fetal glucose uptake and arterial glucose concentration remained normal as the uterine blood flow rate decreased from 600 to 300 ml per min per kg of fetus. At blood flow rates less than 300 ml.min-1.kg-1, the fetal glucose uptake decreased and became negative in one instance while the arterial glucose concentration became variable and markedly increased in 2 animals. This increase in fetal glucose concentration was associated with a decrease in the uterine oxygen delivery rate, a decrease in fetal oxygen content and a decrease in fetal oxygen uptake. These observations support the concept that fetal glucose metabolism is altered by severe hypoxia and demonstrate that there is little effect of uterine blood flow on fetal glucose uptake in the normal physiological range.     

Foetal circulatory responses to arrest of uterine blood flow in sheep: effects of chemical sympathectomy.

Acute foetal asphyxia, caused by arrest of uterine blood flow, increases both sympathetic activity and peripheral vascular resistance and decreases blood flow to peripheral organs (Jensen et al., J. Dev. Physiol., 9, 543-559). The rapidity and uniformity of this peripheral vasoconstriction suggest that the sympatho-neuronal system may reflexly cause these initial blood flow changes during acute asphyxia. To test this hypothesis, we studied 5 intact and 6 chemically sympathectomized (6-hydroxy-dopamine, 46.1 +/- 6 mg/kg foetal weight) chronically prepared normoxaemic foetal sheep in utero at 0.9 of gestation. Organ blood flows (microsphere method), plasma concentrations of catecholamines, vasopressin, and angiotensin II, acid-base balance and blood gases were measured before, during and after arrest of uterine blood flow for 2 min, i.e., at 0, 1, 2, 3, 4 & 30 min. In intact foetuses there was a progressive increase in arterial blood pressure and a rapid circulatory centralization in favour of the brain stem and heart and at the expense of most of the peripheral organs. The changes in peripheral blood flow during and after asphyxia were well reflected by those in the skin and scalp. In chemically sympathectomized foetuses, arterial blood pressure fell transiently at 1 min of asphyxia and cardiac output was redistributed towards the carcass and intestinal organs at the expense of the heart, spinal medulla, and placenta. We conclude that in foetal sheep at 0.9 of gestation, the short-term adaptation to arrest of uterine blood flow is a rapid and profound peripheral vasoconstriction to effect an increase in arterial blood pressure. This initial response during circulatory centralization, which is necessary to increase or maintain blood flow to the heart, brain stem, and placenta, is blunted by sympathectomy. Thus, the foetal sympatho-neuronal system is important for short-term adaptation to and intact survival of asphyxia.     

Effects of severe reduction in maternal placental blood flow on blood flow distribution in the sheep fetus

To test the hypothesis that fetal lambs are able to maintain oxygen delivery to myocardial, brain and adrenal tissues during reduction in uterine blood flow to 25% of control, we performed experiments on five ewes and their fetuses. A snare occluder was placed around the maternal common hypogastric artery and catheters were placed for measurement of blood pressures, flows, blood gas tensions, pH and oxygen content. After a five day recovery period, control measurements were made. The snare occluder was then closed until the artery was fully occluded. The arterial occlusion caused uteroplacental blood flow to fall to 32 +/- 4% and maternal placental blood flow to fall to 25 +/- 3% of control values. This level of asphyxia was maintained for 19 +/- 3 minutes, when maternal and fetal blood flows were measured again. In response to occlusion, fetal ascending aortic PO2 fell from 21 +/- 2 (SEM) to 13 +/- 2 mmHg (P less than or equal to 0.01), oxygen content from 4.3 +/- 0.3 to 1.4 +/- 0.2 mM (P less than or equal to 0.01) and pH from 7.37 +/- 0.01 to 7.21 +/- 0.05 (P less than or equal to 0.01). PCO2 rose from 48 +/- 1 to 62 +/- 3 mmHg (P less than or equal to 0.01). Fetal arterial blood pressure increased from 51 +/- 3 to 61 +/- 3 mmHg (P less than or equal to 0.001) and heart rate decreased from 172 +/- 10 to 104 +/- 4 beats.min-1 (P less than or equal to 0.01). The heart, brain and adrenals showed vasodilation in response to the asphyxic stimulus.(ABSTRACT TRUNCATED AT 250 WORDS)     

Studies on experimental growth retardation in sheep. The effects of a small placenta in restricting transport to and growth of the fetus

Fetal and placental growth rate in sheep has been manipulated by removal of endometrial caruncles prior to conception. This produced two groups of fetuses, one in which prenatal growth rate was similar to normal and a second group in which the fetuses were about half of the normal size. The mortality in the latter group was high, particularly after catheterisation, and there was evidence of early intra-uterine death and fetal reabsorption. Prior to 125 days the relationship between fetal and placental size was poor, but after 126 days a close correlation between the two was apparent. The small fetuses had comparably small placentas and in all cases there was a close relationship between fetal and placental weight. The experimental growth retardation was associated with hypoglycaemia, hypoxia and hypoinsulinaemia. Plasma T3, T4 and particularly prolactin were very low in the small fetuses whilst levels of cortisol and alanine were high. In contrast to the controls these fetuses showed little evidence of net glucose, alanine or lactate consumption. Infusion of 50% glucose into the pregnant ewe, sufficient to elevate maternal plasma glucose concentrations 2 to 3 fold, caused a comparable increase in the plasma concentrations of normal fetuses but only a 50% rise in the concentration in small fetuses. In contrast administration of 50% O2 to the ewes sufficient to cause a 2 to 3-fold increase in maternal PO2 caused only a small increase of arterial PO2 of normal fetuses but doubled that to normal levels in small fetuses. The results are discussed in relation to the effect of reduced placental size causing a fall in placental and transport and transport capacity and significance of this to the associated fetal growth retardation.     

Relationship between fetal electrocorticographic changes and umbilical blood flow in the near-term sheep fetus

Local blood flow was measured with radioactive microspheres in 9 near-term ewes 2 min into successive high and low voltage electrocortical activity states. In an additional 8 animals the umbilical blood flow was measured using an electromagnetic flow-probe on the common umbilical vein. The microsphere data indicated that the blood flow during low and high voltage electrocortical activity was 185 +/- 22 ml/min per kg of fetus (SEM) and 165 +/- 22 ml/min per kg of fetus (P less than 0.01) respectively. Using the electromagnetic flowprobe the average flow during low and high voltage electrocortical activity was 203 +/- 14 ml/min per kg of fetus and 196 +/- 13 ml/min per kg of fetus (P less than 0.05) respectively. We observed that the decrease in the umbilical blood flow preceded the change from low to high voltage electrocortical activity by approximately 1 min. In that time the flow is significantly lower than it was during the preceding measurements taken during the low voltage electrocortical activity periods. This depression was still significantly lower at 3 min into the high voltage electrocortical activity whereas at 5 min into the high voltage state it was elevated to near average values. We conclude that the umbilical blood flow, on the average, is lower in high voltage states than it is in low voltage states and that this change precedes the switch from low to high voltage electrocortical activity.     

Exercise responses in pregnant sheep: oxygen consumption, uterine blood flow, and blood volume

In an attempt to explore the acute maternal responses to exercise we measured oxygen consumption, uterine blood flow, and blood volume in 13 chronically catheterized pregnant sheep at rest and while exercising on a treadmill. With maximal exercise O2 consumption increased 5.6 times, from a resting value of 5.8 +/- 0.3 (SE) to 32.1 +/- 2.8 ml X min -1 X kg -1, cardiac output increased 2.7 times, from 149 +/- 8 to 404 +/- 32 ml X min -1 X kg -1, and arteriovenous oxygen content difference increased 2.1 times, from 3.9 +/- 0.2 to 8.0 +/- 0.4 ml X dl -1. Total uterine blood flow decreased from a mean resting value of 292 +/- 6 to 222 +/- 19 ml X min -1 X kg fetus -1 near exhaustion during prolonged (40 min) exercise at 70% maximal oxygen consumption. Maternal blood volume decreased 14% (P less than 0.01) from 67.5 +/- 3.7 to 57.8 +/- 3.6 ml X kg -1 during this exercise period, with a 20% decrease in plasma volume without a change in red cell volume. We conclude that uterine blood flow decreases during maternal exercise. However, hemoconcentration helps to maintain a relatively constant oxygen delivery to the uterus.     

Uterine blood flow, oxygen and glucose uptakes at mid-gestation in the sheep

In early ovine fetal development, the placenta grows more rapidly than the fetus so that at mid-gestation the aggregate weight of placental cotyledons exceeds fetal weight. The purpose of this study was to compare two separate methods of measuring uterine blood flow and glucose and oxygen uptakes in seven mid-gestation ewes, each carrying a single fetus. Uterine blood flow to both uterine horns was measured by microsphere and by tritiated water steady-state diffusion methodology. Calculations of tritiated water blood flows and oxygen and glucose uptakes were based on measurements of arteriovenous concentration differences across each uterine horn. The distribution of blood flow and oxygen uptake between the two uterine horns was strongly correlated with placental mass distribution. The two methods gave comparable results for uterine blood flow (457 +/- 35 vs 476 +/- 35 ml/min), oxygen uptake (457 +/- 35 vs 476 +/- 35 mumol/min), and glucose uptake (63 +/- 8 vs 64 +/- 6 mumol/min). Uterine blood flow was approximately 38% of the late gestation value and 56.1 +/- 1 times higher than umbilical blood flow. Uteroplacental oxygen consumption was about 58% of late gestation measurements and 3.9 +/- 0.5 times higher than fetal oxygen uptake. We confirm that the large placental mass of mid-gestation is associated with high levels of maternal placental blood flow and placental oxidative metabolism.     

Effects of chronic reduction in uterine blood flow on fetal and placental growth in the sheep

Pregnancy is associated with a significant increase in uteroplacental blood flow (UBF), which is responsible for delivering adequate nutrients and oxygen for fetal and placental growth. The present study was designed to determine the effects of vascular insufficiency on fetal and placental growth. Thirty-nine late-term pregnant ewes were instrumented to investigate the effects of chronic UBF reduction. Animals were split into three groups based on uterine blood flow, and all animals were killed on gestational day 138. UBF, which began at 851 +/- 74 ml/min (n = 39), increased in controls (C) to 1,409 +/- 98 ml/min (day 138 of gestation) and in the moderately restricted (R(M)) group to 986 +/- 69 ml/min. In the severely restricted (R(S)) group, UBF was only 779 +/- 79 ml/min on gestational day 138. This reduction in UBF significantly affected fetal body weight with R(M) fetuses weighing 3,685 +/- 178 g and R(S) fetuses weighing 2,920 +/- 164 g compared with C fetal weights of 4,318 +/- 208 g. Fetal brain weight was not affected, whereas ponderal index was significantly reduced in R(M) (2.94 +/- 0.09) and R(S) fetuses (2.49 +/- 0.08) compared with the value of the C fetuses (3.31 +/- 0.08). Placental weight was also significantly reduced in the R(M) group, being 302 +/- 24 g, whereas the R(S) group placenta weighed 274 +/- 61 g compared with the C values of 414 +/- 57 g. Fetal heart, liver, lung, and thymus were all significantly smaller in the R(S) group. Thus the present study shows a clear relationship between the level of UBF and both fetal and placental size. Furthermore, the observation that fetal brain weight was not affected, whereas fetal body weight was significantly reduced suggests that this experimental preparation may provide a useful model in which to study asymmetric fetal growth restriction.     

Matching of maternal and fetal flow ratios in the sheep placenta

Local interaction of maternal and fetal placental blood flows was studied in two groups of unanaesthetized near-term sheep. Five sheep were exposed to a simulated dive to 100 feet of seawater (4.03 atmospheres) for 25 min. Six fetuses received an infusion of noradrenaline (6.8 micrograms/[kg x min]). Radioactive microspheres were administered simultaneously to mother and fetus before (control) and after (test) the experimental manipulation. Maternal and fetal relative activities, defined as % of total placental radioactivity divided by % of total placental weight, were calculated for 1-g pieces of cotyledonary tissue under control and test conditions. Pieces of cotyledons were defined as matched if the direction of change in relative activity from control to test was the same for mother and fetus. In the absence of an interaction between the maternal and fetal placental circulations, the probability of a piece of cotyledon being matched is 0.5. In each series of experiments the proportion of all cotyledon pieces having maternal and fetal relative activities that changed in the same direction was significantly greater than 0.5. Thus, the majority of the placental mass responds to a physical or chemical perturbation of the fetus in such a way that changes in relative perfusion are qualitatively matched in the adjacent maternal and fetal placental circulations.     

The effect of elevation of maternal plasma catecholamines on the fetus and placenta of the pregnant sheep

To study the effects of reduced uterine blood flow on fetal and placental metabolism, adrenaline has been infused at physiological doses (0.5 microgram/min per kg) into the circulation of the pregnant sheep. This gives a reduction of about one third of uterine blood flow at days 120-143 of pregnancy, but causes no significant change in umbilical blood flow. In contrast to the effects of constricting the uterine artery to reduce blood flow to a similar degree, placental oxygen consumption was reduced and that, together with a large increase in lactate production, indicated the placenta became hypoxic. The fetal blood gas status and hence oxygen consumption was not affected significantly. A consistent arterio-venous difference for glucose across the umbilical or uterine circulations was not detected unless the uterine blood flow was comparatively high. Glucose balance across the uterus showed a close linear relationship with uterine blood flow and more particularly with the supply of glucose to the uterus. There was clear evidence for glucose uptake by the placenta and fetus and also glucose output by both. The latter was more common when uterine blood flow was comparatively low or reduced by adrenaline infusion. The results are consistent with the concept that glucose supply has to be maintained to the placenta even at the expense of fetal stores, although lactate can substitute if there is enhanced output because of fetal hypoxia. They indicate that placental mobilisation of glycogen can lead to a net output of glucose to the mother. The manner of communicating to the fetus changes in placental state that occur during maternal adrenaline infusion is not clear. However towards the end of the 60 min infusion, elevation of fetal plasma adrenaline, probably resulting from a breakdown of the placental permeability barrier, may be an important signal.     

The placenta releases branched-chain keto acids into the umbilical and uterine circulations in the pregnant sheep

There was net uptake of branched-chain keto acids by the fetus from the umbilical circulation. Mean fetal uptake of the 3 keto acids 2-keto isovalerate, 2-keto isocaproate and 2-keto methylvalerate was 1.8 mumol/min per kg of fetus. The concentrations in the umbilical vein for these keto acids were 10.9 +/- 3.8 microM (mean +/- SD: 2-keto isovalerate), 19.7 +/- 6.1 microM (2-keto isocaproate) and 14.8 +/- 5.3 microM (2-keto methylvalerate) respectively. The coefficients of extraction for the same keto acids were 17.2%, 16.8% and 11.9% respectively. Fetal uptakes (both mumol/min and mumol/min per kg fetus) were positively correlated with umbilical supply. There were concentration gradients across the placenta, with fetal concentration: maternal concentration ratios of 3.3 +/- 1.5 for 2-keto isovalerate, 2.1 +/- 0.8 for 2-keto isocaproate and 1.3 +/- 0.6 for 2-keto methylvalerate. The net release of 2-keto acids into the umbilical circulation may conserve the carbon skeleton of branched-chain amino acids for fetal metabolism and growth. In the uterine circulation there was not a consistent pattern of release from or uptake by the uteroplacental tissues. It is suggested that branched-chain keto acids may contribute to fetal growth or energy metabolism.     

Effects of reducing uterine blood flow on fetal blood flow distribution and oxygen delivery.

We examined the effect of graded reduction in uterine blood flow on distribution of cardiac output and oxygen delivery to fetal organs and venous blood flow patterns in 9 fetal sheep using the radionuclide-labeled microsphere technique. We reduced uterine blood flow in two steps, decreasing fetal oxygen delivery to 70% and 50% of normal, and compared the results with those from a similar study from our laboratory on graded umbilical cord compression. With 50% reduction in fetal oxygen delivery, blood flow and the fraction of the cardiac output distributed to the brain, heart, and adrenal gland increased and that to the lungs, carcass, skin, and scalp decreased. Oxygen delivery to the brain and myocardium was maintained, while that to the adrenal doubled, and that to the brain stem increased transiently. The decrease in oxygen delivery to both carcass and lower body segment correlated linearly with oxygen consumption (P less than 0.001). The proportion of umbilical venous blood passing through the ductus venosus increased from 44.6% to 53% (P less than 0.05). The preferential distribution of ductus venosus blood flow through the foramen ovale to the heart and brain increased, but that to the upper carcass decreased so that ductus venosus-derived blood flow to the upper body did not change. Hence, the oxygen delivered to the brain from the ductus venosus was maintained, and that to the heart increased 54% even though ductus venosus-derived oxygen delivery to the upper body fell 34%. Abdominal inferior vena caval blood flow and its contribution to cardiac output decreased, but the proportion of the abdominal inferior vena caval blood distributed through the foramen ovale also increased from 23.0 to 30.9%. However, the actual amount of inferior vena caval blood passing through the foramen ovale did not change. There was a 70% fall in oxygen delivery to the upper body segment from the inferior vena cava. A greater portion of superior vena caval blood was also shunted through the foramen ovale to the upper body, but the actual amounts of blood and oxygen delivered to the upper body from this source were small. Thus, graded reduction of uterine blood flow causes a redistribution of fetal oxygen delivery and of venous flow patterns, which is clearly different from that observed previously during graded umbilical cord occlusion.