Does the umbilical artery systolic/diastolic ratio reflect flow or acidosis? An umbilical artery Doppler study of fetal sheep

The role of umbilical artery velocities in the clinical assessment of twin-twin transfusion syndrome at stage

OBJECTIVE

To analyze the discordances of the umbilical artery velocities between pregnancies with twin-twin transfusion syndrome (TTTS) at stage I and those with normal monochorionic-diamniotic (MCDA) twins, and investigate the value of their discordances in predicting TTTS at stage I.

METHODS

We recruited 58 twin pregnancies with TTTS at stage I and 60 normal MCDA twin pregnancies in a tertiary referral center retrospectively. The umbilical artery velocities and their discordances were compared between the normal and TTTS twins.

RESULTS

The discordances of umbilical artery mean diastolic velocity (UA-MDV), umbilical artery time-averaged maximum velocity (UA-TAmax), umbilical artery peak systolic velocity (UA-PSV), and umbilical artery end-diastolic velocity (UA-EDV) were higher in the TTTS group than in the normal group. In TTTS co-twins, the UA-MDV, UA-TAmax, UA-PSV, and UA-EDV in recipients were higher than those in donors. The discordances of UA-TAmax and UA-PSV were found to be independent predicting factors for TTTS at stage I.

CONCLUSION

Co-twin umbilical artery velocity discordances were significantly associated with stage I TTTS. The results suggest that UA-TAmax and UA-PSV might be new parameters for predicting TTTS at stage I.

Elevated umbilical artery systolic/diastolic ratio in the absence of fetal growth restriction

PURPOSE

To evaluate whether patients with isolated elevation of umbilical artery (UA) systolic/diastolic (S/D) ratio are at increased risk for adverse perinatal outcome.

METHODS

This is a retrospective cohort study of 330 patients who underwent routine evaluation at our maternal fetal medicine center. We regularly perform UA S/D ratio analysis with every third trimester sonogram. All identified patients were included and divided into four groups based on estimated fetal weight (EFW) and UA S/D ratio. Perinatal outcome was compared between the groups.

RESULTS

Regardless of the EFW, fetuses with persistent elevated UA S/D ratio showed significantly more preterm deliveries (p < .001), neonatal intensive care unit (NICU) admissions (p < .001), longer stay in the NICU (p < .001) and lower birth weight (p < .001) relative to controls. Stepwise logistic regression analysis demonstrated that being a member in any study group significantly and independently predicted birth weight less than the 10th percentile and preterm delivery. Patients with persistently elevated S/D ratio were significantly and independently from other factors, more likely to have a newborn admitted to the NICU.

CONCLUSION

Our results indicate a suboptimal perinatal outcome in all pregnancies with an elevated UA S/D ratio. These fetuses may benefit from intensive monitoring.

Diffusion-weighted MR imaging of the placenta in fetuses with placental insufficiency

PURPOSE

To evaluate diffusion-weighted magnetic resonance (MR) imaging of the human placenta in fetuses with and fetuses without intrauterine growth restriction (IUGR) who were suspected of having placental insufficiency.

MATERIALS AND METHODS

The study was approved by the local ethics committee, and written informed consent was obtained. The authors retrospectively evaluated 1.5-T fetal MR images from 102 singleton pregnancies (mean gestation ± standard deviation, 29 weeks ± 5; range, 21-41 weeks). Morphologic and diffusion-weighted MR imaging were performed. A region of interest analysis of the apparent diffusion coefficient (ADC) of the placenta was independently performed by two observers who were blinded to clinical data and outcome. Placental insufficiency was diagnosed if flattening of the growth curve was detected at obstetric ultrasonography (US), if the birth weight was in the 10th percentile or less, or if fetal weight estimated with US was below the 10th percentile. Abnormal findings at Doppler US of the umbilical artery and histopathologic examination of specimens from the placenta were recorded. The ADCs in fetuses with placental insufficiency were compared with those in fetuses of the same gestational age without placental insufficiency and tested for normal distribution. The t tests and Pearson correlation coefficients were used to compare these results at 5% levels of significance.

RESULTS

Thirty-three of the 102 pregnancies were ultimately categorized as having an insufficient placenta. MR imaging depicted morphologic changes (eg, infarction or bleeding) in 27 fetuses. Placental dysfunction was suspected in 33 fetuses at diffusion-weighted imaging (mean ADC, 146.4 sec/mm(2) ± 10.63 for fetuses with placental insufficiency vs 177.1 sec/mm(2) ± 18.90 for fetuses without placental insufficiency; P < .01, with one false-positive case). The use of diffusion-weighted imaging in addition to US increased sensitivity for the detection of placental insufficiency from 73% to 100%, increased accuracy from 91% to 99%, and preserved specificity at 99%.

CONCLUSION

Placental dysfunction associated with growth restriction is associated with restricted diffusion and reduced ADC. A decreased ADC used as an early marker of placental damage might be indicative of pregnancy complications such as IUGR.

SUPPLEMENTAL MATERIAL

http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.10092283/-/DC1.

Control of the fetoplacental circulation

This year marks the 250th anniversary of the discovery by William Hunter of the existence of two distinct circulations within the human placenta. Until relatively recently the placenta has been viewed with “respect” – a passive structure which occasionally elicited fear and anxiety if implanted either too low or too deep. More recently our understanding of perinatal physiology, blood flow regulation and epidemiological data linking prenatal events with adult disease has created renewed interest in the placenta. This review will focus on the regulation of fetal blood flow in the placenta, the possible mechanisms whereby it may be deranged and why this might be relevant to adult disease.

Ultrasonic transit-time measurement of blood flow in the umbilical arteries and veins in the bovine fetus during stage II of labor

The vitality of the bovine fetus during parturition depends on an intact umbilical circulation to supply adequate amounts of oxygen and nutrients to the fetus. The goal of the present study was to measure the blood flow in the umbilical vessels during stage II of labor and to determine when blood flow ceases in the umbilical cord. In 20 cows, ultrasonographic transducers were placed on one umbilical vein and one umbilical artery after rupture of the allantochorionic sac, and the blood flow volume per unit time was measured. At the same time, a pressure transducer was placed into the uterus to measure uterine pressure. Parturition was spontaneous in all 20 cows. In 20 live calves born, pH, base excess and lactate concentration were measured in the blood immediately after birth. During the last 90 min before birth the mean total umbilical blood flow (artery and vein combined) was 1.186+/-0.028 L/min. Calves with a blood pH> or =7.2 (n=13) had a higher mean total blood flow than calves with a pH<7.2 (n=7; 1.243+/-0.038 versus 1.095+/-0.038 L/min). In calves with a blood pH<7.2, the mean total blood flow decreased from 1.178+/-0.134 at 20 min before birth to 0.959+/-0.126 L/min at the end of stage II of labor. During this time period, the arterial blood flow did not differ between calves with a blood pH> or =7.2 and<7.2, but venous blood flow decreased significantly in calves with a blood pH<7.2. During uterine contractions, the total umbilical blood flow decreased significantly by 0.22 L/min. The blood flow in the umbilical artery and vein ceased before the calves were completely born.

Betamethasone effects on ovine uterine and umbilical placental perfusion at the dose used to enhance fetal lung maturation

OBJECTIVE

The purpose of this study was to examine glucocorticoid effects on umbilical placental perfusion.

STUDY DESIGN

Pregnant sheep instrumented with uterine and umbilical ultrasound transit-time flow probes received 2 doses of 12 mg betamethasone (n = 6) or saline (n = 5) intramuscularly 24 hours apart.

RESULTS

Maternal blood pressure and uterine flow did not change during glucocorticoid exposure. Fetal blood pressure increased, and umbilical resistance showed a transient increase after each injection (P < .05), followed by an increase of umbilical flow (P < .05) that was closely correlated to an increase in fetal heart rate (r = 0.85, P < .001), which determines cardiac output of the developing heart. Umbilical waveform indices were decreased over the entire treatment period, indicating a decrease of resistance in the fetoplacental microcirculation (P < .05).

CONCLUSION

Fetoplacental perfusion during glucocorticoid exposure is not limiting for nutrition exchange. Clinical interpretation of Doppler waveform indices needs to be interpreted with caution because they do not reflect dynamics of umbilical placental perfusion.

Doppler-derived umbilical artery absolute velocities and their relationship to fetoplacental volume blood flow: a longitudinal study

OBJECTIVES

To construct reference ranges for serial measurements of the umbilical artery (UA) absolute blood flow velocities in the second half of pregnancy and to test the hypothesis that significant associations exist between UA velocities and placental volume blood flow assessed from umbilical vein blood flow velocities and diameter.

METHODS

This was a prospective longitudinal study of the umbilical circulation. UA absolute velocities and umbilical vein blood flow were measured at 4-weekly intervals between 19 and 42 weeks' gestation in 130 low-risk singleton pregnancies. A total of 511 observations were used to construct the reference ranges and assess the association between UA absolute velocities and placental volume flow using multilevel modeling.

RESULTS

Both UA absolute velocities and placental volume blood flow showed a steady increase throughout the second half of pregnancy. However, the gestational age-related increase in the UA end-diastolic velocity (EDV) was greater than the corresponding increase in the peak systolic velocity (PSV). The time-averaged intensity-weighted mean velocity (TAWMV)/time-averaged maximum velocity (TAMXV) was 0.6 indicating probably not a completely parabolic velocity profile. There was a significant positive association (P < 0.00001) between UA absolute velocities and placental volume blood flow, but this association was modified by the gestational age. The intraobserver coefficients of variation for the UA PSV, EDV, TAMXV and TAWMV and placental volume blood flow were 10.17%, 16.29%, 11.46%, 18.18% and 8.61%, respectively.

CONCLUSION

We have established new reference ranges for the UA absolute velocities based on longitudinal data. They show a significant association with fetoplacental volume blood flow and may have a clinical value in the assessment of the umbilical circulation.

Idiopathic fetal growth restriction: a pathophysiologic approach

Known causes of fetal growth restriction (FGR) can be traced in up to 40 percent of the cases. In the remainder of cases, FGR is idiopathic in origin. Evidence from experimental studies, antenatal findings at Doppler velocimetry of the uterine and umbilical arteries, and placental pathology studies suggests that idiopathic FGR can be divided into three groups: 1) primary abnormality in the uteroplacental perfusion; 2) primary abnormality in the fetoplacental perfusion; and 3) abnormal villous structure at the interface between fetal and maternal circulation. An improved understanding of the pathophysiology of idiopathic FGR is crucial to gain insight into its pathogenesis.

Umbilical artery Doppler velocimetry: clinical utility in high-risk pregnancies

An abnormal result of an umbilical artery Doppler study reflects the presence of placental vascular pathologic mechanisms and identifies pregnancies at increased risk for perinatal mortality. Recent reviews of the clinical utility of umbilical artery Doppler study have concluded that it should not be routinely used as a screening modality for the general obstetric population and have suggested that further research is required. However, metaanalysis of published peer-reviewed and randomized controlled trials indicates that its use in high-risk pregnancies is associated with a significant decrease in perinatal mortality without an increase in the rate of inappropriate obstetric intervention. This clinical opinion serves to underscore the relevance of umbilical artery Doppler velocimetry to clinical practice and to suggest that an abnormal result of an umbilical artery Doppler study should be added to the current list of indications for intensive fetal surveillance.

Atrial natriuretic peptide: a vasodilator of the fetoplacental circulation?

Paired maternal and fetal atrial natriuretic peptide concentrations were measured in 62 percutaneous umbilical blood samplings performed principally for the assessment and treatment of rhesus isoimmunization. Pretransfusion fetal atrial natriuretic peptide levels were significantly higher than maternal atrial natriuretic peptide levels (median 117 pg/ml vs median 32 pg/ml; p less than 0.001); paired pretransfusion fetal and maternal atrial natriuretic peptide samples showed a weak correlation with each other (R2 = 17%; p = 0.002). Fetal atrial natriuretic peptide levels correlated inversely with hematocrit (R2 = 14%; p = 0.003), but not with albumin or gestational age. Paired pretransfusion and posttransfusion (median = 134 pg/ml) fetal atrial natriuretic peptide levels (n = 38) showed a significant rise after transfusion (p less than 0.001); this rise was related to the percentage of fetoplacental blood volume transfused (R2 = 33%; p = 0.035). In a subgroup of 26 procedures, change in fetal atrial natriuretic peptide levels was weakly correlated with transient reductions in the Doppler systolic/diastolic ratio of the umbilical artery (R2 = 14%; p = 0.07). These data support work in animals that indicate a role for atrial natriuretic peptide in the human fetus, but these data do not confirm that atrial natriuretic peptide modulates fetoplacental vascular impedance in the human fetus.