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.     

Effects of intravascular infusions of plasma on placental and systemic blood flow in fetal sheep

Six singleton fetal sheep of 118-122 days gestational age were instrumented with flow sensors on the brachiocephalic artery, the postductal aorta, and the common umbilical artery and with arterial and venous intravascular catheters. At 125-131 days of gestation, we started week-long continuous recordings of flows and pressures. After control measures had been obtained, the fetuses were given continuous intravenous infusions of adult sheep plasma at an initial rate of 229 ml/day. After 1 wk of infusion, fetal plasma protein concentrations had increased from 34 to 78 g/l, arterial and venous pressures had increased from 42 to 64 and from 2.7 to 3.7 mmHg, and systemic resistance (exclusive of the coronary bed) had increased from 0.047 to 0.075 mmHg.min(-1).ml(-1), whereas placental resistance had increased from 0.065 to 0.111 mmHg.min(-1).ml(-1). Fetal plasma renin activities fell as early as 1 day after the start of infusion and remained below control (all changes P < 0.05). All flows decreased slightly although these decreases were not statistically significant. Thus the increase in arterial pressure was entirely due to an increase in systemic and placental resistances.     

Umbilical artery flow velocity waveforms and placental vascular resistance during maternal placental outflow obstruction in sheep

This study was designed to test the hypothesis that the pulsatility index (PI) of the umbilical artery flow velocity waveform varies as a function of placental vascular resistance. Placental vascular resistance was raised by a one-minute occlusion of the maternal inferior vena cava. Occlusion of the maternal inferior vena cava resulted in a decrease in fetal heart rate from 183 +/- 7.8 beats/min to 142 +/- 8.6 beats/min at the end of occlusion (P less than 0.05). Placental vascular resistance increased from 0.113 +/- 0.021 mmHg.ml-1.min during control to 0.151 +/- 0.033 mmHg.ml-1.min (P less than 0.05) during occlusion. The pulsatility index increased from 1.05 +/- 0.05 to 1.85 +/- 0.4 (P less than 0.05) during occlusion. After parasympathetic blockade with atropine fetal heart rate did not change during occlusion. Placental vascular resistance increased from 0.091 +/- 0.014 before to 0.121 +/- 0.021 mmHg.ml-1.min during occlusion (P less than 0.05). The pulsatility index increased from 0.98 +/- 0.1 before to 1.12 +/- 0.12 during occlusion (P less than 0.05). These results support the hypothesis that, in the fetal sheep, placental vascular resistance is one of the determinants of the pulsatility index of the umbilical artery.     

Effects of prolonged reduction in blood flow on submandibular secretory function in anesthetized sheep.

Submandibular vascular and secretory responses to parasympathetic chorda-lingual (C-L) stimulation were investigated in anesthetized sheep before, during, and after an intracarotid (ic) infusion of endothelin-1 (ET-1). Stimulation of the peripheral end of the C-L nerve at 4 and 8 Hz produced a frequency-dependent reduction in submandibular vascular resistance (SVR) associated with a frequency-dependent increase in submandibular blood flow, salivary flow, and Na+, K+, and protein output from the gland. During stimulation at 4 Hz, ic ET-1 significantly increased SVR (P < 0.01), without significantly affecting either the aortic blood pressure or heart rate. Submandibular blood flow (SBF) was reduced by 48 +/- 4% and the flow of saliva by 50 +/- 1%. The effect on blood and salivary flow persisted for at least 30 min after the infusion of ET-1. The reduction in SBF was associated with a diminution in the output of Na+,K+, and protein in the saliva (P < 0.01). These effects persisted for 30 min after the infusion of ET-1 had been discontinued and were linearly related to the flow of plasma throughout.     

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.     

Regional blood flow distribution in fetal sheep with intrauterine growth retardation produced by decreased umbilical placental perfusion

To determine the capacity of the fetus to adapt to chronic O2 deficiency produced by decreased placental perfusion in the early development of growth retardation, we embolized the umbilical placental vascular bed of fetal sheep for a period of 9 days. Fetal umbilical placental embolization decreased arterial O2 content by 39%, decreased total placental blood flow by 33%, and produced a 20% reduction in mean fetal body weight. Neither the combined ventricular output nor the regional blood flow distribution was significantly different between the 8 growth-retarded and 7 normally grown fetuses despite the 39% decrease in fetal arterial O2 content. Thus a 33% reduction in total placental blood flow restricts normal fetal growth, but does not exceed the placental circulatory reserve capacity necessary to maintain normal basal metabolic oxygenation. Because the proportion of combined ventricular output to the placenta at rest is decreased in late IUGR fetuses but not in early IUGR fetuses, despite chronic oxygen deficiency, we conclude that the growth retarded fetus maintains a normal regional blood flow distribution until the placental circulatory reserve capacity is depleted.     

Restriction of placental function alters heart development in the sheep fetus

Placental insufficiency, resulting in restriction of fetal substrate supply, is a major cause of intrauterine growth restriction (IUGR) and increased neonatal morbidity. Fetal adaptations to placental restriction maintain the growth of key organs, including the heart, but the impact of these adaptations on individual cardiomyocytes is unknown. Placental and hence fetal growth restriction was induced in fetal sheep by removing the majority of caruncles in the ewe before mating (placental restriction, PR). Vascular surgery was performed on 13 control and 11 PR fetuses at 110-125 days of gestation (term: 150 +/- 3 days). PR fetuses with a mean gestational Po(2) < 17 mmHg were defined as hypoxic. At postmortem (<135 or >135 days), fetal hearts were collected, and cardiomyocytes were isolated and fixed. Proliferating cardiomyocytes were counted by immunohistochemistry of Ki67 protein. Cardiomyocytes were stained with methylene blue to visualize the nuclei, and the proportion of mononucleated cells and length and width of cardiomyocytes were measured. PR resulted in chronic fetal hypoxia, IUGR, and elevated plasma cortisol concentrations. Although there was no difference in relative heart weights between control and PR fetuses, there was an increase in the proportion of mononucleated cardiomyocytes in PR fetuses. Whereas mononucleated and binucleated cardiomyocytes were smaller, the relative size of cardiomyocytes when expressed relative to heart weight was larger in PR compared with control fetuses. The increase in the relative proportion of mononucleated cardiomyocytes and the relative sparing of the growth of individual cardiomyocytes in the growth-restricted fetus are adaptations that may have long-term consequences for heart development in postnatal life.     

Effects of maternal blood loss on embryonic and placental development in the rat.

The effects of acute loss of maternal blood on embryonic and placental development was examined in 50 rats on Days 8 or 9 of gestation. Blood was withdrawn from conscious, cannulated rats over a 1-min period at 1-0 or 2-0 ml/100 g body weight. These degrees of blood loss were expected to produce a mild (about 50%) and severe (about 80%) reduction in uterine blood flow, respectively, for at least 15 min. There was no evidence that loss of blood affected either fetal survival and malformation rates or fetal weights and sex ratios. The anaemia resulting from haemorrhage lasted no longer than 6 days. Placental weights were 11% higher in rats losing 2-0 ml blood/100 g than in controls (P less than 0-05). It appears that the 8- or 9- day rat embryo is highly resistant to the partial reduction in uterine blood flow, maternal anaemia and other possible challenges induced by maternal loss of blood at levels sufficient to affect the mothers.     

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.     

Effects of maternal nutritional status on fetal and placental growth and on fetal urea synthesis in sheep

Fetal and placental growth, and fetal and maternal urea synthesis in late gestation, were studied in 2-year-old Corriedale ewes on a maintenance ration (M) except when subjected to moderate dietary restriction from day 50 to day 100 (RM), day 100 to day 135 (MR) or day 50 to day 135 (RR). In comparison with fetuses of ewes maintained throughout the experiment (MM), RR fetuses were smaller and RM fetuses were larger whereas MR fetuses were unaffected; all restrictions were associated with increased placental size. Fetal urea synthesis at day 133 in the well-nourished ewes (MM) was 21.5 mg N h-1 kg-1 increasing to, respectively, 25.7, 27.3 and 38.8 mg N h-1 kg-1 in groups MR, RM and RR; these values were 1.6, 3.9, 2.2 and 3.8 times the maternal rates of synthesis. On the basis of the observed urea synthesis rates, amino acid oxidation could have accounted for up to, respectively, 32, 38, 40 and 57% of fetal oxygen consumption in groups MM, MR, RM and RR. Amino acids, in addition to their role in tissue accretion, may be key energy substrates for the fetus.     

Effect of cooling and heating on the regional distribution of blood flow in fetal sheep

This is a study on the effect of cooling and heating amniotic fluid on blood flow to fetal tissues and organs. In 8 unanaesthetized, chronically-catheterised fetal sheep (129-137 days gestation) cold or warm water was passed through tubing encircling the fetus in utero and blood flow was measured using the radionuclide-labelled 15 mu spheres. Following cooling for 30 min, amniotic fluid temperature fell 9.6 degrees C to 29.9 +/- 2.1 degrees C (SEM) fetal arterial temperature fell 2.37 degrees C to 37.30 +/- 0.36, and maternal arterial temperature fell 0.53 degrees C to 38.58 +/- 0.16. Blood flow through the fetal skin fell 60% (P less than 0.01) to 13.6 ml/min per 100 g tissue. Blood flow to the brown fat increased 186% (P less than 0.05) to 99.6 ml/min per 100 g. Following warming for 20 min, fetal temperature rose to 40.43 +/- 0.19 degrees C, and skin blood flow did not change significantly relative to initial control period but rose 200% above that during cooling (P less than 0.01). During both cooling and heating, blood flow to the adrenals rose significantly (P less than 0.05) whereas flow to the carcass, brain, kidneys, and placenta was not altered detectably. Continuous sampling of blood from the inferior vena cava during microsphere injection failed to detect any evidence of arterio-venous shunting through the skin at any temperature studied. Overall, the blood flow responses are consistent with a thermoregulatory role for the skin and brown fat in the near-term fetal sheep.     

Effects of exercise and heat stress on regional blood flow in pregnant sheep

Radioactive microspheres were used to measure cardiac output and blood flow to most major tissues, including those in the pregnant uterus, in late-pregnant ewes at rest and during treadmill exercise (approximately 3-fold increase in metabolic rate for 30 min) in thermoneutral (TN) (dry bulb temperature (Tdb) = 13 degrees C, wet bulb temperature (Twb) = 10 degrees C) and mildly hot (MH) (Tdb = 40 degrees C, Twb = 27 degrees C) environments. Exercise caused major increases in blood flow to respiratory muscles, nonrespiratory limb muscles, and adipose tissue, and flow was decreased to some gastrointestinal tissues, spleen, pancreas, and to placental and nonplacental tissues in the pregnant uterus. Heat exposure had relatively little effect on these exercise-induced changes, except that flow was further increased in the respiratory muscles. Results are compared with those of a similar study on nonpregnant sheep in which changes in muscle, skin, and visceral flows during exercise were attenuated by heat exposure. It is suggested that redistribution of blood flow from the pregnant uterus, which in resting ewes took 22% of cardiac output, is a significant buffer against the potentially deleterious effects of combined exercise and heat stress on blood flow to exercising muscles and thermoregulatory tissues.     

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.