Non-invasive evaluation of placental blood flow: lessons from animal models

Velocity-selective arterial spin labeling perfusion measurements in 2nd trimester human placenta with varying BMI

INTRODUCTION

Proper placental development is crucial to fetal health but is challenging to functionally assess non-invasively and is thus poorly characterized in populations. Body mass index (BMI) has been linked with adverse outcomes, but the causative mechanism is uncertain. Velocity-selective arterial spin labeling (VS-ASL) MRI provides a method to non-invasively measure placental perfusion with robustness to confounding transit time delays. In this study, we report on the measurement of perfusion in the human placenta in early pregnancy using velocity-selective arterial spin labeling (VS-ASL) MRI, comparing non-obese and obese participants.

METHODS

Participants (N = 97) undergoing routine prenatal care were recruited and imaged with structural and VS-ASL perfusion MRI at 15 and 21 weeks gestation. Resulting perfusion images were analyzed with respect to obesity based on BMI, gestational age, and the presence of adverse outcomes.

RESULTS

At 15 weeks gestation BMI was not associated with placental perfusion or perfusion heterogeneity. However, at 21 weeks gestation BMI was associated with higher placental perfusion (p < 0.01) and a decrease in perfusion heterogeneity (p < 0.05). In alignment with past studies, perfusion values were also higher at 21 weeks compared to 15 weeks gestation. In a small cohort of participants with adverse outcomes, at 21 weeks lower perfusion was observed compared to participants with uncomplicated pregnancies.

DISCUSSION

These results suggest low placental perfusion in the early second trimester may not be the culpable factor driving associations of obesity with adverse outcomes.

A review of feto-placental vasculature flow modelling

The placenta provides the vital nutrients and removal of waste products required for fetal growth and development. Understanding and quantifying the differences in structure and function between a normally functioning placenta compared to an abnormal placenta is vital to provide insights into the aetiology and treatment options for fetal growth restriction and other placental disorders. Computational modelling of blood flow in the placenta allows a new understanding of the placental circulation to be obtained. This structured review discusses multiple recent methods for placental vascular model development including analysis of the appearance of the placental vasculature and how placental haemodynamics may be simulated at multiple length scales.

Relationship between placental hemodynamics and placental histological analysis in third trimester

AIM

To investigate whether vascularization index (VI), flow index (FI), and vascularization flow index (VFI) correlate with the pathological structure of the placenta and whether there were any differences in VI, FI, VFI, and placental pathological structure between the normal and preeclampsia (PE) groups.

METHODS

Fifty-five pregnant women (normal group, n = 27; PE group, n = 28) underwent VI, FI, and VFI at four locations in the placenta during the second and third trimesters. Two hematoxylin and eosin (HE)-stained specimens of the postpartum placenta were prepared. We randomly selected two of these locations and used ImageJ, an open-source image package, to quantify intervillous blood vessels (IBV), intervillous spaces (IS), and intervillous blood vessels + intervillous spaces (IBV + IS) per unit placenta and analyzed their correlation with VI, FI, and VFI.

RESULTS

There was no positive correlation between VI, FI, VFI, IBV, IS, and IBV + IS. There were no significant differences in VI, FI, and VFI between the normal and PE groups; however, there were significant differences in IBV, IS, and IBV + IS in the PE group compared to those in the normal group.

CONCLUSIONS

Placental hemodynamics measured by VI, FI, and VFI were not positively correlated with placental morphology in the third trimester. There were no differences in the VI, FI, and VFI in the third trimester between the normal and PE groups, suggesting that these may reflect placental circulatory insufficiency.

Umbilical Vein Pulse Wave Spectral Analysis: A Possible Method for Placental Assessment Through Evaluation of Maternal and Fetal Flow Components

OBJECTIVES

Placental blood flow analysis is complicated by having both maternal and fetal flow components. Using the Fast Fourier Transform (FFT) of the umbilical venous pulse wave spectra (PW) envelope, we could simultaneously assess maternal/fetal blood flow in the placenta and investigate if normal and intrauterine growth restriction (IUGR)/pre-eclamptic pregnancies could be distinguished.

METHODS

This retrospective study included normal gestations (N = 11) and gestations with IUGR, pre-eclampsia, or both (N = 13). Umbilical vein PW were acquired and spectral envelopes were identified as a function of time and analyzed by FFT. Base-10 logarithms of the ratios of the maternal/fetal spectral peaks (LRSP) were compared in normal and IUGR/pre-eclamptic populations (two-tailed t-test). Body mass index (BMI), gestational age at scan time, placental position, and weight-normalized umbilical vein blood volume flow (two-tailed t-test, analysis of variance [ANOVA] analysis) were tested. P < .05 was considered significant.

RESULTS

The LRSP for normal and IUGR/pre-eclamptic pregnancies were 0.141 ± 0.180 and -0.072 ± 0.262 (mean ± standard deviation), respectively (P = .033). We detected differences between normal gestations and combinations of LRSP and weight-normalized umbilical venous blood flows. Placental effects based on LRSPs and blood flow may act synergistically in cases with both pre-eclampsia and IUGR (P = .014). No other significant associations were seen.

CONCLUSIONS

In this preliminary study, we showed that umbilical venous flow contains markers related to placental maternal/fetal blood flow, which can be used to assess IUGR and pre-eclampsia. When coupled with umbilical cord blood flow, this new marker may potentially identify the primary causes of the two conditions.

An MRI approach to assess placental function in healthy humans and sheep

KEY POINTS

Human placental function is evaluated using non-invasive Doppler ultrasound of umbilical and uterine artery pulsatility indices as measures of resistance in placental vascular beds, while measurement of placental oxygen consumption ( V O 2 ) is only possible during Caesarean delivery. This study shows the feasibility of using magnetic resonance imaging (MRI) in utero to measure blood flow and oxygen content in uterine and umbilical vessels to calculate oxygen delivery to and V O 2 by the gravid uterus, uteroplacenta and fetus. Normal late gestational human uteroplacental V O 2 by MRI was ∼4 ml min^-1  kg^-1 fetal weight, which was similar to our MRI measurements in sheep and to those previously measured using invasive techniques. Our MRI approach can quantify uteroplacental V O 2 , which involves the quantification of maternal- and fetal-placental blood flows, fetal oxygen delivery and V O 2 , and the oxygen gradient between uterine- and umbilical-venous blood, providing a comprehensive assessment of placental function with clinical potential.

ABSTRACT

It has not been feasible to perform routine clinical measurement of human placental oxygen consumption ( V O 2 ) and in vitro studies do not reflect true metabolism in utero. Here we propose an MRI method to non-invasively quantify in utero placental and fetal oxygen delivery ( D O 2 ) and V O 2 in healthy humans and sheep. Women (n = 20) and Merino sheep (n = 10; 23 sets of measurements) with singleton pregnancies underwent an MRI in late gestation (36 ± 2 weeks and 128 ± 9 days, respectively; mean ± SD). Blood flow (phase-contrast) and oxygen content (T1 and T2 relaxometry) were measured in the major uterine- and umbilical-placental vessels, allowing calculation of uteroplacental and fetal D O 2 and V O 2 . Maternal D O 2 (ml min^-1  kg^-1 fetus) to the gravid uterus was similar in humans and sheep (human = 54 ± 15, sheep = 53 ± 21, P = 0.854), while fetal D O 2 (human = 25 ± 4, sheep = 22 ± 5, P = 0.049) was slightly lower in sheep. Uteroplacental and fetal V O 2 (ml min^-1  kg^-1 fetus; uteroplacental: human = 4.1 ± 1.5, sheep = 3.5 ± 1.9, P = 0.281; fetus: human = 6.8 ± 1.3, sheep = 7.2 ± 1.7, P = 0.426) were similar between species. Late gestational uteroplacental:fetal V O 2 ratio did not change with age (human, P = 0.256; sheep, P = 0.121). Human umbilical blood flow (ml min^-1  kg^-1 fetus) decreased with advancing age (P = 0.008), while fetal V O 2 was preserved through an increase in oxygen extraction (P = 0.046). By contrast, sheep fetal V O 2 was preserved through stable umbilical flow (ml min^-1  kg^-1 ; P = 0.443) and oxygen extraction (P = 0.582). MRI derived measurements of uteroplacental and fetal V O 2 between humans and sheep were similar and in keeping with prior data obtained using invasive techniques. Taken together, these data confirm the reliability of our approach, which offers a novel clinical 'placental function test'.

Pregnancy-specific physiologically-based toxicokinetic models for bisphenol A and bisphenol S

Predictions from physiologically based toxicokinetic (PBTK) models can help inform human health risk assessment for potentially toxic chemicals in the environment. Bisphenol S (BPS) is the second most abundant bisphenol detected in humans in the United States, after bisphenol A (BPA). We have recently demonstrated that BPS, much like BPA, can cross the placental barrier and disrupt placental function. Differences in physicochemical properties, toxicokinetics, and exposure outcomes between BPA and other bisphenols prevent direct extrapolation of existing BPA PBTK models to BPS. The current study aimed to develop pregnancy-specific PBTK (p-PBTK) models for BPA and BPS, using a common p-PBTK model structure. Novel paired maternal and fetal pregnancy data sets for total, unconjugated, and conjugated BPA and BPS plasma concentrations from three independent studies in pregnant sheep were used for model calibration. The nine-compartment (maternal blood, liver, kidney, fat, placenta and rest of body, and fetal liver, blood and rest of body) models simulated maternal and fetal experimental data for both BPA and BPS within one standard deviation for the majority of the experimental data points, highlighting the robustness of both models. Simulations were run to examine fetal exposure following daily maternal exposure to BPA or BPS at their tolerable daily intake dose over a two-week period. These predictive simulations show fetal accumulation of both bisphenols over time. Interestingly, the steady-state approximation following this dosing strategy achieved a fetal concentration of unconjugated BPA to levels observed in cord blood from human biomonitoring studies. These models advance our understanding of bisphenolic compound toxicokinetics during pregnancy and may be used as a quantitative comparison tool in future p-PBTK models for related chemicals.

Spiral artery blood flow during pregnancy: a systematic review and meta-analysis

Background

Downstream remodeling of the spiral arteries (SpA) decreases utero-placental resistance drastically, allowing sustained and increased blood flow to the placenta under all circumstances. We systematically evaluated available reports to visualize adaptation of spiral arteries throughout pregnancy by ultra-sonographic measurements and evaluated when this process is completed.

Methods

A systematic review and meta-analysis of spiral artery flow (pulsatility index (PI), resistance index (RI) and peak systolic velocity (PSV)) was performed. English written articles were obtained from Pubmed, EMBASE and Cochrane Library and included articles were assessed on quality and risk of bias. Weighted means of Doppler indices were calculated using a random-effects model.

Results

In healthy pregnancies, PI and RI decreased from 0.80 (95% CI: 0.70–0.89) and 0.50 (95% CI: 0.47–0.54) in the first trimester to 0.50 (95% CI: 0.45–0.55, p < 0.001) and 0.39 (95% CI: 0.37–0.42, p < 0.001) in the second trimester and to 0.49 (95% CI: 0.44–0.53, p = 0.752) and 0.36 (95% CI: 0.35–0.38, p = 0.037) in the third trimester, respectively. In parallel, PSV altered from 0.22 m/s (95% CI: 0.13–0.30 m/s) to 0.28 m/s (95% CI: 0.17–0.40 m/s, p = 0.377) and to 0.25 m/s (95% CI: 0.20–0.30 m/s, p = 0.560) in the three trimesters. In absence of second and third trimester Doppler data in complicated gestation, only a difference in PI was observed between complicated and healthy pregnancies during the first trimester (1.49 vs 0.80, p < 0.001). Although individual studies have identified differences in PI between SpA located in the central part of the placental bed versus those located at its periphery, this meta-analysis could not confirm this (p = 0.349).

Conclusions

This review and meta-analysis concludes that an observed decrease of SpA PI and RI from the first towards the second trimester parallels the physiological trophoblast invasion converting SpA during early gestation, a process completed in the midst of the second trimester. Higher PI was found in SpA of complicated pregnancies compared to healthy pregnancies, possibly reflecting suboptimal utero-placental circulation. Longitudinal studies examining comprehensively the predictive value of spiral artery Doppler for complicated pregnancies are yet to be carried out.

Toward a better understanding of the effects of endocrine disrupting compounds on health: Human-relevant case studies from sheep models

There are many challenges to overcome in order to properly understand both the exposure to, and effects of, endocrine disruptors (EDs). This is particularly true with respect to fetal life where ED exposures are a major issue requiring toxicokinetic studies of materno-fetal exchange and identification of pathophysiological consequences. The sheep, a large, monotocous, species, is very suitable for in utero fetal catheterization allowing a modelling approach predictive of human fetal exposure. Predicting adverse effects of EDs on human health is frequently impeded by the wide interspecies differences in the regulation of endocrine functions and their effects on biological processes. Because of its similarity to humans as regards gestational and thyroid physiologies and brain ontogeny, the sheep constitutes a highly appropriate model to move one step further on thyroid disruptor hazard assessment. As a grazing animal, the sheep has also proven to be useful in the evaluation of the consequences of chronic environmental exposure to "real-life" complex mixtures at different stages of the reproductive life cycle.

Systematic evaluation of velocity-selective arterial spin labeling settings for placental perfusion measurement

Purpose

Placental function is key for successful human pregnancies. Perfusion may be a sensitive marker for the in vivo assessment of placental function. Arterial spin labeling (ASL) MRI enables noninvasive measurement of tissue perfusion and it was recently suggested that ASL with velocity‐selective (VS) labeling could be advantageous in the placenta. We systematically evaluated essential VS‐ASL sequence parameters to determine optimal settings for efficient placental perfusion measurements.

Methods

Eleven pregnant women were scanned at 3T using VS‐ASL with 2D multislice echo planar imaging (EPI)‐readout. One reference VS‐ASL scan was acquired in all subjects; within subgroups the following parameters were systematically varied: cutoff velocity, velocity encoding direction, and inflow time. Visual evaluation and region of interest analyses were performed to compare perfusion signal differences between acquisitions.

Results

In all subjects, a perfusion pattern with clear hyperintense focal regions was observed. Perfusion signal decreased with inflow time and cutoff velocity. Subject‐specific dependence on velocity encoding direction was observed. High temporal signal‐to‐noise ratios with high contrast on the perfusion images between the hyperintense regions and placental tissue were seen at ~1.6 cm/s cutoff velocity and ~1000 ms inflow time. Evaluation of measurements at multiple inflow times revealed differences in blood flow dynamics between placental regions.

Conclusion

Placental perfusion measurements are feasible at 3T using VS‐ASL with 2D multislice EPI‐readout. A clear dependence of perfusion signal on VS labeling parameters and inflow time was demonstrated. Whereas multiple parameter combinations may advance the interpretation of placental circulation dynamics, this study provides a basis to select an effective set of parameters for the observation of placenta perfusion natural history and its potential pathological changes.

Toxicokinetics of bisphenol A, bisphenol S, and bisphenol F in a pregnancy sheep model

Bisphenol A (BPA), S (BPS), and F (BPF) are among the most abundant bisphenols detected in humans, yet pregnancy toxicokinetics for BPS or BPF remain unknown. Because gestational BPS can disrupt placental function and result in reproductive and metabolic disorders in the progeny, the aim of the study was to investigate BPS and BPF toxicokinetics during pregnancy using an in vivo approach. Fetal catheterizations were conducted in pregnant sheep (n = 6) at mid-pregnancy and injected with either a single dose of BPS (n = 3, 0.5 mg/kg, s.c.), or a combination of BPS, BPF, and BPA (n = 3, 0.5 mg/kg for each chemical, s.c.). Maternal and fetal blood and urine and amniotic fluid were collected over 72 h and analyzed for bisphenols by HPLC-MS/MS. We observed significant differences in half-life, maximum concentration, and total body clearance in maternal circulation among bisphenols. Longer half-lives were observed in fetal vs. maternal circulation for all bisphenols. Fetal toxicokinetics differed among bisphenols with BPS having the longest fetal half-life. All bisphenols reached basal levels at 48 h in maternal plasma, but were still detectable in amniotic fluid, fetal urine, and fetal plasma at 72 h. In this first pregnancy toxicokinetic study of BPS and BPF we have demonstrated maternal and fetal toxicokinetic differences among all three bisphenols. Higher BPS persistence in the fetal compartment warrants studies into progeny adverse outcomes following gestational exposure. Additionally, toxicokinetic differences among bisphenols call for a more careful approach when extrapolating kinetic information from one bisphenol chemical to another.

Contrast-enhanced ultrasonography of maternal and fetal blood flows in pregnant bitches

We evaluated the potential usefulness of CEUS to assess fetal-maternal circulation during pregnancy in dogs. Nine bitches were examined at 23, 30, and 45 days of gestation using an ultrasound machine (LOGIQ E9) and SonoVue^® contrast media as echo-signal enhancer. Qualitative and quantitative evaluation of contrast enhancement patterns of uterine artery and utero/placental vessels were performed on recorded images. Independently of the gestational periods, the qualitative evaluation showed the initial wash-in phase from the first appearance of the uterine artery to the rapid distribution in embryonic vesicles or placenta to the progressive washout, whilst there was no enhancement of either embryos or fetuses in any bitch. Independent of gestational age, parameters derived from quantitative analysis of time intensity-curves of contrast enhancement (peak intensity, time to peak, rise time, washout) did not vary between proximal placenta, distal placenta, and uterine artery. With the progression of gestation, AUC values did not change in both proximal and distal placenta, but in the uterine artery it was lower (P ≤ 0.05) at day 30 than at day 23 (464.8 ± 16.1 vs.596.4 ± 28.1, respectively). In conclusion, CEUS appears to safely permit evaluation of the maternal and fetal vessels in the first two third of gestation, without any clinically relevant adverse effects. Further studies in a larger number of bitches in different stages of pregnancy are needed to establish standard parameters for normal pregnancies that can be used to detect abnormalities of pregnancy.

Gestational bisphenol S impairs placental endocrine function and the fusogenic trophoblast signaling pathway

Exposure to bisphenolic chemicals during pregnancy occurs in > 90% of pregnancies. Bisphenolic compounds can cross the placental barrier reaching fetal circulation. However, the effects of emerging bisphenolic compounds, such as bisphenol S (BPS), on placental function remain untested. The aim was to determine if bisphenol A (BPA) or BPS, at an environmentally relevant dose, impairs placental function. Pregnant sheep were randomly distributed into three treatment groups (n = 7-8/group): control, BPA, and BPS. All animals received daily injections of corn oil (control), BPA, or BPS (0.5 mg/kg; s.c.; internal fetal doses were ~ 2.6 ng/mL unconjugated BPA and ~ 7.7 ng/mL of BPS) from gestational day 30-100. After a 20-day washout period, placentas were weighed and placentomes collected. Placental endocrine function was assessed on biweekly maternal blood samples. Gestational exposure to BPS, but not BPA, reduced maternal circulating pregnancy-associated glycoproteins without change in placental weight or placental stereology. BPS-exposed placentas had 50% lower e-cadherin protein expression, ~ 20% fewer binucleate cells, and ~ threefold higher glial cell missing-1 protein expression. BPA placentas were not affected highlighting the intrinsic differences among bisphenolic chemicals. This is the first study to demonstrate that gestational BPS can result in placental endocrine dysfunction and points to a dysregulation in the fusogenic trophoblast signaling pathway.