Longitudinal doppler sonographic measurements of vascular impedance in the central and peripheral spiral arteries throughout pregnancy

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.

Feasibility of three dimensional power Doppler ultrasonography methods to assess placental perfusion

OBJECTIVES

Given the crucial role of the placenta in establishing a healthy pregnancy, reliable non-invasive methods to measure placental perfusion are desirable. The aim of this study is to determine the reproducibility and potential bias in different three-dimensional power Doppler (3DPD) methods assessing placenta perfusion.

METHODS

Ten singleton pregnancies around 16 weeks of gestation, with an anteriorly located placenta and centrally inserted umbilical cord were included in this study. Eight different combinations of a specific placental sweep and sonobiopsy method were used to evaluate placental perfusion. Vascularization index (VI), flow index (FI) and vascularization-flow index (VFI) were determined offline using the 4D-view program. Reproducibility and repeatability of the methods, expressed as correlation coefficients and Bland-Altman mean differences, were calculated. Differences between sampling methods were analyzed using t-test or Mann-Whitney U test.

RESULTS

Intra- and inter-class correlation coefficient (CC) was highest when using a spherical centrally placed sonobiopsy of 2 cm³ in a whole placenta sweep (method 1; IntraCC VI 0.985, FI 0.769, VFI 0.993, InterCC VI 0.986, FI 0.784, VFI 0.987). Overall, intraCCs were higher compared to interCCs. Lowest mean differences in VI and FI were found comparing spherical to manual sonobiopsies, whereas the mean differences in VFI were lowest when comparing central versus peripheral located sonobiopsies. Comparing the three vascular indices, best median intra- and interCC and lowest mean differences were found for VFI.

CONCLUSIONS

Three dimensional placental vascularization analysis showed best reproducibility using whole placental sweep volume and centrally located, spherical sonobiopsy of 2 cm³.

Human Placenta Blood Flow During Early Gestation With Pseudocontinuous Arterial Spin Labeling MRI

BACKGROUND

Noninvasive measurement of placental blood flow is the major technical challenge for predicting ischemic placenta (IPD). Pseudocontinuous arterial spin labeling (pCASL) MRI was recently shown to be promising, but the potential value in predicting the subsequence development of IPD is not known.

PURPOSE

To derive global and regional placental blood flow parameters from longitudinal measurements of pCASL MRI and to assess the associations between perfusion-related parameters and IPD.

STUDY TYPE

Prospective.

POPULATION

Eighty-four women completed two pCASL MRI scans (first; 14-18 weeks and second; 19-24 weeks) from prospectively recruited 118 subjects. A total of 69 subjects were included for the analysis, of which 15 subjects developed IPD.

FIELD STRENGTH/SEQUENCE

3T/T₂ -weighted half-Fourier single-shot turbo spin-echo (HASTE) and pCASL.

ASSESSMENT

Four perfusion-related parameters in the placenta were derived: placenta volume, placental blood flow (PBF), high PBF (hPBF), and relative hPBF. The longitudinal changes of the parameters and their association with IPD were tested after being normalizing to the 16th and 20th weeks of gestation.

STATISTICAL TESTS

Comparisons between two gestational ages within subjects were performed using the paired Wilcoxon tests, and comparisons between normal and IPD groups were performed using the unpaired Wilcoxon tests.

RESULTS

The difference between the first and second MRI scans was statistically significant for volume (156.6 cm³ vs. 269.7 cm³ , P < 0.001) and PBF (104.9 ml/100g/min vs. 111.3 ml/100g/min, P = 0.02) for normal subjects, indicating an increase in pregnancy with advancing gestation. Of the parameters tested, the difference between the normal and IPD subjects was most pronounced in hPBF (278.1 ml/100g/min vs. 180.7 ml/100g/min, P < 0.001) and relative hPBF (259.1% vs. 183.2%, P < 0.001) at 16 weeks.

DATA CONCLUSION

The high perfusion-related image parameters for IPD were significantly decreased from normal pregnancy at 14-18 weeks of gestation.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY STAGE: 1 J. Magn. Reson. Imaging 2020;51:1247-1257.

Volume and vascularity: Using ultrasound to unlock the secrets of the first trimester placenta

Fetal growth restriction (FGR) is a major cause of perinatal morbidity and mortality. Identifying which pregnancies are at risk of FGR facilitates enhanced surveillance and early delivery before fetal demise can ensue. However, existing risk stratification strategies yield an unacceptably low detection rate. A robust and reliable first trimester screening test for FGR would not only enable high-risk women to be appropriately monitored but would facilitate future trials for possible interventions to enhance fetal growth. Both the volume and vascularity of the first trimester placenta has been demonstrated to be linked to adverse pregnancy outcomes including FGR and pre-eclampsia. The investigation of novel ultrasound markers for FGR are discussed along with the development of methods for fully automatic placental volume estimation which has the potential for use as part of a multi-variable population-based screening test.

Measurement of spiral artery jets: general principles and differences observed in small-for-gestational-age pregnancies

OBJECTIVE

To investigate whether the jets of blood from the mouths of the spiral arteries could be measured reliably, as well as their relationship with the uterine artery (UtA) and any differences in small-for-gestational-age (SGA) pregnancies.

METHODS

Participants underwent serial ultrasound scans, from 11 weeks' gestation. Pulsatility index (PI) and resistance index (RI) of jets into the intervillous space (IVS) and UtA were recorded at every visit. Intra- and interobserver variability studies were performed. Customized birth weight centiles were calculated and SGA was defined as < 10(th) centile. Linear mixed model analysis was used to allow for the longitudinal nature of the data.

RESULTS

Sixty-six women were recruited; 58 remained normotensive and delivered at term. Of these, six women delivered SGA newborns and 52 delivered appropriate-for-gestational-age newborns. All had pulsatile jets until 20 weeks' gestation. The PI and RI of the jets decreased with advancing gestation, following a trend similar to that of the UtAs. There was no correlation between the jets and UtA waveforms when gestational age was controlled for. For intraobserver variability the intraclass correlation coefficient was 0.9. The interobserver study showed no significant difference between the observers. Mixed model analysis demonstrated that PI and RI of jets were different in SGA pregnancies (P < 0.06). This difference was not seen for the UtAs (P = 0.8).

CONCLUSION

This technique enables examination of characteristics of the jets of blood flowing from spiral arteries into the IVS. It is both precise and reproducible, with biologically plausible results. Further work is required to assess differences in pregnancies with adverse outcomes.

Developmental changes in spiral artery blood flow in the human placenta observed with colour Doppler ultrasonography

OBJECTIVES

Our current knowledge of the physiological dilatation of spiral arteries in pregnancy, is based on histology. Real-time ultrasound visualisation of these changes may aid understanding of abnormal placentation. This study aimed to investigate if changes in the spiral artery blood flow can be followed 'in vivo' and explore the novel phenomenon of the larger 'mega-jets'.

METHODS

Colour Doppler ultrasonography was used to identify the most prominent jets at blood from the spiral artery into the intervillous space. Their velocity, width and length were recorded seven set time points during pregnancy.

RESULTS

Fifty two uncomplicated, term normotensive pregnancies were studied. Width and length of the jets' Doppler signals increased with gestation, the velocity decreased. The length of the jets shows a bi-modal frequency distribution. The width of the signals of longer ('mega') jets was significantly greater (p = 0.001) than that of the jets (mean 4.3 mm (3.1-5.9) versus 3.8 mm (1.8-5.8) respectively) at 34 weeks. However, there was no significant difference in the peak systolic velocity (p = 0.2).

CONCLUSION

This study confirms that ultrasound can be used to study the gestation dependent changes in the haemodynamics of the placental basal plate predicted, but not proven, by histologic data. The bi-modal distribution of jet lengths suggests that mega-jets are a separate entity to 'normal' jets. That they are significantly wider than 'normal' jets and yet maintain the same velocity of blood flow suggests that they have a greater volume of blood flow. The mechanism for this is hypothesised and their apparent relationship with simple placental lakes discussed.

Rheological and physiological consequences of conversion of the maternal spiral arteries for uteroplacental blood flow during human pregnancy

Physiological conversion of the maternal spiral arteries is key to a successful human pregnancy. It involves loss of smooth muscle and the elastic lamina from the vessel wall as far as the inner third of the myometrium, and is associated with a 5–10-fold dilation at the vessel mouth. Failure of conversion accompanies common complications of pregnancy, such as early-onset preeclampsia and fetal growth restriction. Here, we model the effects of terminal dilation on inflow of blood into the placental intervillous space at term, using dimensions in the literature derived from three-dimensional reconstructions. We observe that dilation slows the rate of flow from 2 to 3 m/s in the non-dilated part of an artery of 0.4–0.5 mm diameter to approximately 10 cm/s at the 2.5 mm diameter mouth, depending on the exact radius and viscosity. This rate predicts a transit time through the intervillous space of approximately 25 s, which matches observed times closely. The model shows that in the absence of conversion blood will enter the intervillous space as a turbulent jet at rates of 1–2 m/s. We speculate that the high momentum will damage villous architecture, rupturing anchoring villi and creating echogenic cystic lesions as evidenced by ultrasound. The retention of smooth muscle will also increase the risk of spontaneous vasoconstriction and ischaemia–reperfusion injury, generating oxidative stress. Dilation has a surprisingly modest impact on total blood flow, and so we suggest the placental pathology associated with deficient conversion is dominated by rheological consequences rather than chronic hypoxia.

Non-invasive method of evaluation of trophoblast invasion of spiral arteries in monochorionic twins with discordant birthweight

To test the hypothesis that variation in birthweight between twin pairs is due to discordant placental development, we determined spiral arterial blood flow by colour pulsed Doppler ultrasound scan. We prospectively studied 24 twin pregnancies in the late second trimester with (n=12) and without (n=12) inter-pair difference in estimated birthweight of > or = 20 per cent. In the discordant growth group, there were seven cases with chronic twin-twin transfusion syndrome (TTTS) and five without. The blood flow in spiral artery of each twin's portion of the placenta was assessed by resistance index (RI) by colour flow pulsed Doppler within a 5 cm radius of cord insertion. In twins with discordant weight, RI was increased in the growth restricted (FGR) twin than the appropriate for gestational age (AGA) co-twin (0.46 +/- 0.02 vs 0.3 +/- 0.01; P< 0.001) and the control group (P< 0.001). However, delta RI was comparable between twins with and without TTTS (0.13 +/- 0.01 vs 0.19 +/- 0.02; P=NS). No such differences were found between concordant twin pairs (0.28 +/- 0.01 vs 0.29 +/- 0.1; P=NS) and AGA twins of the discordant growth group. This study indicates that growth restricted twins have increased resistance to blood flow in the spiral arteries than the AGA co-twins. This observation, therefore, suggests non-physiological remodelling of the maternal spiral arteries in response to migrating trophoblast in placental bed of FGR MC twins.