Alterations in fetal and placental deoxyribonucleic acid synthesis rates after chronic fetal placental embolization

Nutrition for Multiples

In 2012 there were 135,943 infants of multiple pregnancies born in the United States, nearly a 2-fold increase since 1980, with twins accounting for 96% of all multiple births. To date, most perinatal morbidities associated with multiple births have proven resistant to technological or pharmaceutical interventions. Maternal nutrition can have a profound effect on the course and outcome of multiple pregnancy, with the goal of achieving optimal intrauterine growth and birthweights, and minimizing prenatal and perinatal complications for the mother and her children.

Natriuretic peptide levels in intrauterine growth-restricted fetuses with absent and reversed end-diastolic flow of the umbilical artery in relation to ductus venosus flow velocities

OBJECTIVE

To determine if changes in natriuretic peptide levels [propeptide of atrial natriuretic peptide (proANP) and N-terminal propeptide of brain natriuretic peptide (NT-proBNP)] can be demonstrated in fetuses with serious growth restriction and Doppler ultrasonographic findings of hemodynamic compromise.

METHOD

Natriuretic peptides of the umbilical vein (proANP and NT-proBNP), arterial and venous blood gases, as well as lactate levels of the umbilical vein were determined in 38 appropriate for gestational age (AGA) fetuses, and 24 intrauterine growth-restricted (IUGR) fetuses with absent (zero-flow) and 17 IUGR fetuses with reversed end-diastolic flow (reverse-flow) of the umbilical artery.

RESULTS

A continuous increase in natriuretic peptides in the presence of progressive deterioration was shown. ANP concentrations were significantly lower in the AGA group (mean 818 fmol/mL; range 508-991 fmol/mL) than the zero-flow group (mean 19,680 fmol/mL; range 13,680-25,080 fmol/mL) and the reverse-flow group (mean 33,880 fmol/mL; range 24,365-41,786 fmol/mL). Significant differences were further demonstrated in BNP concentrations of the AGA group (mean 32 fmol/L; range 25-38 fmol/L), zero-flow group (mean 1191 fmol/L; range 908-1655 fmol/L), and reverse-flow group (mean 3063 fmol/L; range 1281-4968 fmol/L). Additionally, significant differences in lactate concentrations were observed for the three groups: AGA, 2.6 mmol/L (range 2-3 mmol/L); zero-flow group, 4.1 mmol/L (range 3.3-5 mmol/L); and reverse-flow group, 7.7 mmol/L (range 5.9-13.7 mmol/L). Increased ANP levels correlated with the arterial base excess and lactate concentrations, as well as with the flow velocities in the ductus venosus during atrial contraction (a-wave), and both ductus venosus indices (S-a)/D and (S-a)/V(mean). No correlations were established for BNP.

CONCLUSIONS

As components of a compensatory mechanism, natriuretic peptides exert an influence on the cardiovascular function of the fetus.

Aetiology and pathogenesis of IUGR

Intrauterine growth restriction (IUGR) is a major cause of perinatal mortality and morbidity. A complex and dynamic interaction of maternal, placental and fetal environment is involved in ensuring normal fetal growth. An imbalance or lack of coordination in this complex system may lead to IUGR. Animal studies have given us an insight into some aspects of the basic pathophysiology of IUGR, and recent technologies such as Doppler studies of maternal and fetal vessels have added further information. The aetiologies of IUGR are diverse, involving multiple complex mechanisms, which make understanding of the pathophysiology difficult. However, particular focus is placed on the mechanisms involved in uteroplacental insufficiency as a cause of IUGR, as (1) it is common, (2) outcome can be good if timing of delivery is optimal and (3) it may be amenable to therapy in the future. While the research into the pathophysiology of IUGR continues, there have been interesting discoveries related to the genetic contribution to IUGR and the intrauterine programming of adult-onset diseases attributed to IUGR.

Placental insufficiency decreases cell cycle activity and terminal maturation in fetal sheep cardiomyocytes

Umbilicoplacental embolization (UPE) in sheep has been used to investigate the effects of placental insufficiency on fetal development. However, its specific effects on the heart have been little studied. The aim of this study was to determine the effects of placental insufficiency, induced by UPE, on cardiomyocyte size, maturation and proliferation. Instrumented fetal sheep underwent UPE for either 10 or 20 days. Hearts were collected at 125 +/- 1 days (10 day group) or 136 +/- 1 days (20 day group) of gestation (term approximately 145 days). Cell size, maturational state (as measured by the proportion of binucleated myocytes) and cell cycle activity (as measured by positive staining of cells for Ki-67) were determined in dissociated cardiomyocytes. UPE fetuses were hypoxaemic, but mean arterial pressures were not different from controls. UPE fetuses were lighter than control fetuses (10 days: -21%, P < 0.05; 20 days: -27%, P < 0.01) and had smaller hearts, but heart weight was appropriate for body weight. Neither lengths nor widths were different between control and UPE cardiomyocytes at either age. Ten days of UPE did not significantly alter the proportion of binucleated myocytes or cell cycle activity in either ventricle. However, 20 days of UPE reduced cell cycle activity in both ventricles by approximately 70% (P < 0.05); the proportion of binucleated myocytes was also lower in UPE fetuses at this age (left ventricle: 31.1 +/- 12.0 versus 46.0 +/- 6.6%, P < 0.05; right ventricle: 29.4 +/- 12.3 versus 46.3 +/- 5.3%, P < 0.05). It is concluded that in the absence of fetal arterial hypertension, placental insufficiency is associated with substantially depressed growth of the heart through suppressed proliferation and maturation of cardiomyocytes.

Nutrition in multiple gestations

Multiple pregnancies represent a state of magnified nutritional requirements, resulting in a greater nutrient drain on maternal resources and an accelerated depletion of nutritional reserves. Maternal weight gain to 20 weeks and between 20 and 28 weeks has the greatest effect on birthweight in twin and triplet pregnancies, particularly among underweight women. Parity, which most likely represents a higher proportion of body fat, has a positive effect on pregnancy outcome, with an average 7 to 10 days longer gestation for multiparous versus nulliparous women. In addition to being the nutrients most often lacking in a woman's diet, calcium, magnesium, and zinc have been identified as having the most potential for reducing pregnancy complications and improving outcomes.

Morphologic alterations in ovine placenta and fetal liver following induced severe placental insufficiency

OBJECTIVES

Umbilical-placental embolization with microspheres has been used as a model of placental insufficiency and intrauterine growth restriction (IUGR). However, the effects of embolization on placental structure and organ morphology of the resulting IUGR fetus are relatively unexplored. In this study using ovine fetuses, we determined the location and distribution of microspheres within the placenta and explored the extent of placental and fetal organ morphologic changes induced by placental embolization. We hypothesized that microspheres administered into the umbilical circulation over 4 days would cause placental damage without significant morphologic alterations in fetal kidney or liver.

METHODS

Eleven pregnant sheep at 118 +/- 1 (SE) days' gestation were studied. In six fetuses, embolization was induced by injections of 15-microm diameter microspheres on 4 successive days into the fetal descending aorta proximal to the umbilical arteries. Five fetuses served as time controls.

RESULTS

In embolized fetuses, microspheres were detected in the placenta embedded in the fetal cytotrophoblastic layer or maternal parenchyma adjacent to villous cytotrophoblasts. Fetal cytotrophoblasts appeared normal except for loss of distinct separation between fetal and maternal cell layers. Microspheres were also detected in the fetal membranes within capillaries. The body weights of embolized fetuses were lower than controls, as were the body weight-normalized liver but not kidney weights. In the liver of the embolized fetuses, the number of hematopoietic cell clusters was markedly reduced, whereas the fetal kidneys appeared normal.

CONCLUSIONS

We conclude that after 4 days of umbilical-placental embolization, microspheres were concentrated at the fetal villi proximal to the apical maternal-fetal interface and in the fetal membranes. There were noticeable morphologic changes in the embolized placentas, with no apparent gross damage to the placenta. The reduction in fetal liver weight and liver extramedullary hematopoietic cell abundance associated with embolization may predispose the fetus to alterations in liver function that could persist after birth.

Placental insufficiency and its consequences

Placental insufficiency is a process leading to progressive deterioration in placental function and a decrease in transplacental transfer of oxygen and nutrients to the fetus. The resulting fetal hypoxemia is the major stimulus involved in the reduction in fetal growth as an attempt to reduce metabolic demands by the growing fetus. Fetal growth restriction (FGR) is the second cause of perinatal death after prematurity and can complicate up to 6% of all pregnancies. It is becoming apparent that its occurrence has major impacts on the fetus and placenta with consequences on the cardiovascular, metabolic and neurological development up to adulthood. We are just starting to unveil some of the basic mechanisms involved in this complex adaptation that may lead to reprogramming of fetal organs development mostly the heart, pancreas, lungs and brain. It is becoming clear that future research is needed to develop strategies to improve antenatal detection of FGR, in addition to reduce the risk of abnormal neurodevelopment during childhood, and onset of common diseases in adulthood following pregnancies complicated with placental insufficiency.

Liver blood perfusion as a possible instrument for fetal growth regulation

Placental and fetal liver blood perfusions are reduced in intrauterine growth-restricted human fetuses. We hypothesized that changes in fetal liver blood supply can alter fetal growth. In nine ewes with twin pregnancies at a gestational age of 119+/-2 days, a stent (4 mm) was placed into the ductus venosus of one twin (DV(stent) group). Alternatively, in 17 near term sheep with twin (n=11) or singleton (n=6) pregnancies, a DV was blocked with an embolization coil (DV(coil) group) for about one week. The cell proliferation rate (pKi-67) was determined in the liver, heart, skeletal muscle, kidneys and placenta. The dilatation or occlusion of the DV did not change placental perfusion on the first day or later after surgery. The liver blood supply was decreased in the DV(stent) group by more than half from 499+/-371 to 278+/-219 ml min(-1) (mean+/-s.d., n=4), and increased two-fold in the DV(coil) group (P< 0.05). The percentage of liver/body weight was decreased from 3.9+/-0.6 per cent in control twin to 3.0+/-0.2 per cent (n=3) in the DV(stent) group. Occlusion of the DV lead to an increase in the percentage of liver/body weight from 3.4+/-0.8 per cent to 4.3+/-0.8 per cent (n=11, P< 0.05). Reduced liver blood supply in the DV(stent) group was associated with a decrease of cell proliferation in the liver from 12.43+/-2.31 to 6.5+/-0.62 (nuclei microm(2) 10(-4), n=3, P=0.058), in heart from 1.14+/-0.03 to 0.93+/-0.02 (nuclei microm(2) 10(-4), P< 0.05), and in skeletal muscle from 0.82+/-0.05 to 0.54+/-0.01 (nuclei microm(2) 10(-4), P< 0.05). The increased liver blood perfusion following occlusion of the DV increased cell proliferation sixfold in the liver, (n=9, P< 0.005) and twofold in heart muscle, skeletal muscle and the kidneys (P< 0.05), whereas no significant difference was seen in the placenta. The expression of mRNA for IGF-I and IGF-II in the liver was increased in the DV(coil) group. In conclusion, these results suggest that liver blood perfusion can regulate fetal growth.

Obstruction of ductus venosus stimulates cell proliferation in organs of fetal sheep

In growth restricted fetuses, hepatic blood flow is reduced. This suggests the hypothesis that liver blood flow controls fetal growth. In 11 near term sheep the ductus venosus was blocked with an embolization coil in one fetus (experimental) and left patent in the twin (control). Arterial catheters were placed in both fetuses. After termination [mean (s.d.) 5 days (2) after surgery] the fetal body and organs were weighed. The cell proliferation rate (pKi-67) was determined in tissue samples of the liver, heart, skeletal muscle, kidneys and placenta (n=6). Blood flow through the umbilical vein measured by Doppler ultrasound did not differ in control and experimental fetuses [experimental: 600 (101) ml/min; control: 626 (89) ml/min]. In experimental fetuses, blood flow through the ductus venosus was negligible (colour Doppler), and thus hepatic blood flow was increased. Absolute and relative (percentage of body weight) liver weights were increased in experimental fetuses [liver weight: 119 (34) g versus 84 (17) g; relative liver weight: 4.3 (0.8) per cent versus 3.4 (0.8) per cent;P=0.002, n=11]. The cell proliferation rate was increased significantly (twofold) in heart muscle, skeletal muscle and kidneys, and sixfold in liver. It is concluded that increases of hepatic blood flow stimulate cell proliferation in major organs of the ovine fetus.

Kynurenic acid in brain and cerebrospinal fluid of fetal, newborn, and adult sheep and effects of placental embolization

Concentrations of the endogenous glutamate receptor antagonist kynurenic acid (KA) were measured in various brain regions and in cisternal cerebrospinal fluid of fetal, newborn, and adult sheep. KA concentrations were significantly higher in the fetal brain and cerebrospinal fluid at 90 and 140 d gestation compared with postnatal ages. In fetuses of 132-139 d gestation, KA concentrations in cerebrospinal fluid collected by drainage from an indwelling cisternal catheter increased significantly after infusion of the organic acid transport inhibitor probenecid (100 or 200 mg/kg, i.v.) indicating active transport of KA out of the fetal brain. In fetuses in which the umbilical circulation had been chronically restricted from 120 to 140 d gestation by partial embolization of the placenta, plasma concentrations of the KA precursor kynurenine were significantly lower than in control fetuses, and KA concentrations in the hypothalamus and hippocampus were significantly reduced; other brain regions were not affected. These results indicate that the production of KA is higher in the fetal brain compared with the newborn and adult brain. Because KA diminishes the risk of excitotoxic neuronal damage under hypoxic-ischemic conditions, the high levels of KA in the brain before birth may have a neuroprotective function. The decrease of KA concentrations in the hypothalamus and hippocampus after umbilical embolization suggests that, after chronic hypoxia in utero, these regions of the brain may become more vulnerable to subsequent episodes of acute hypoxia or ischemia encountered in late gestation or during parturition.

The effect of repeated acute hypoxaemia on fetal cardiovascular development in the sheep

1. Hypoxaemia during intrauterine life may be important in the development of cardiovascular diseases in later life. Thus it was the aim of this study to investigate the effect of repeated acute hypoxia on the cardiovascular development and growth of the fetus. 2. Fourteen fetal sheep (105-109 days gestational age) were instrumented with amniotic and vascular catheters, an electrocardiogram (ECG) electrode and a flow probe around the femoral artery. Seven animals were given repeated acute isocapnic hypoxaemia (Pa,O2 reduced to ca. 13 mmHg) for 1 h every day for 14 days and they were compared to seven animals which remained normoxic throughout with respect to fetal mean arterial blood pressure (MAP), fetal heart rate (FHR), and fetal baro- and chemoreflexes. 3. No differences were found between the two groups of fetuses in FHR, MAP, baro- or chemoreflexes, femoral blood flow, femoral vascular resistance or fetal growth. 4. Repeated acute hypoxaemia of a moderate degree over a period of 2 weeks in late gestation does not affect cardiovascular development or growth in the fetal sheep.

Fetal sheep endocrine responses to sustained hypoxemic stress after chronic fetal placental embolization

The purpose of this study was to determine the endocrine and circulatory responses of the ovine fetus, near term, to sustained hypoxemic stress superimposed on chronic hypoxemia. Fetal sheep were chronically embolized (n = 7) for 10 days between 0.84 and 0.91 of gestation via the descending aorta until arterial oxygen content was decreased by approximately 30%. Control animals (n = 8) received saline only. On experimental day 10, both groups were embolized over a 6-h period until fetal arterial pH decreased to approximately 7.00. Regional distribution of lower body blood flows was measured on day 10, before and at the end of acute embolization. On day 10, the chronically embolized group had lower arterial oxygen content (P < 0.05), Po2 (P < 0.01), and placental blood flow (P < 0.05) than controls and higher prostaglandin E2 (PGE2) and norepinephrine plasma concentrations (both P < 0.05). In response to a superimposed sustained hypoxemic stress, there was a twofold greater increase in PGE2 in the chronically embolized group than in the control group (P < 0.05). However, the increase in fetal plasma cortisol in response to superimposed hypoxemic stress was similar in both groups, despite significantly lower adrenocorticotropic hormone and adrenal cortex blood flow responses in the chronically hypoxemic group (both P < 0.05). We conclude that PGE2 response to a sustained superimposed reduction in placental blood flow, leading to metabolic acidosis, is enhanced under conditions of chronic hypoxemia and may play an important role for the maintenance of the fetal cortisol response to an episode of superimposed acute stress.

Pulse oximetry--physiological considerations

Fetal well-being depends on the level of oxygenation in vital organs such as the heart and brain. In this review, we discuss the physiological parameters which underlie the use of pulse oximetry to evaluate fetal conditions intrapartum. Whilst the measurement of haemoglobin oxygen saturation (SaO2) depends on partial pressure of oxygen (PO2), the relation is alinear, is relatively insensitive to changes in PO2 at the upper physiological range, and it is affected by the Bohr shift. Oxygen content of the blood cannot be determined without measurement of haemoglobin (Hb) content and this can change quite quickly. In hypoxia for example, oxygen delivery to an organ, e.g. the brain, cannot be assessed without simultaneous measurement of blood flow, which again changes with fetal condition. Lastly, it is not possible to gauge fetal tissue unless some measure of, for example, cytochrome aa3 oxidation is used: tissue oxidation in relation to oxygen delivery can change due to local vascular readjustments and changes in metabolism. We conclude that use of SaO2 to assess fetal well-being is fraught with difficulties, and that much more research is needed before its routine clinical use can be considered.