Brian J. Trudinger

Fetal umbilical artery flow velocity waveforms and placental resistance: pathological correlation

Summary. Placental microvascular anatomy was correlated with antenatal assessment of the umbilical circulation in 106 patients to further validate the measurement of the A/B ratio (the ratio of peak systolic to least diastolic flow velocity) of the umbilical artery flow velocity time wave‐forms as an index of blood flow resistance. Three groups of patients were studied: a normal group of 38 uncomplicated pregnancies, a control group of 33 potentially ‘at risk’ pregnancies with a normal A/B ratio matched by risk factors and gestation with the third group of 35 pregnancies with a high A/B ratio. Placental arterial resistance was quantitated by counting the number of small muscular arteries (<90 μm diameter) in the tertiary stem villi in a standard microscopic field (mean 18·5 fields/placenta). The modal small arterial vessel count was shown to be significantly less in the group with a high fetal risk and a high A/B ratio (1–2 arteries/field) than in both the normal and control groups (7–8 arteries/field). The tertiary villus count did not vary between groups. Antenatal studies of umbilical artery flow velocity waveforms with Doppler ultrasound identify a specific microvascular lesion in the placenta characterized by obliteration of small muscular arteries in the tertiary stem villi.

Fetal umbilical artery flow velocity waveforms and placental resistance: clinical significance

Summary. Since the umbilical arteries carry fetal blood to the placenta we studied flow velocity waveforms in these vessels with a simple continuous wave Doppler system to assess placental blood flow. The ratio of peak systolic to least diastolic (A/B) flow velocity was measured as an index of placental flow resistance. In 15 normal pregnancies there was a small but significant decrease in this ratio through the last trimester. The A/B ratio was measured on 436 occasions in 168 high‐risk pregnancies. In 32 of 43 fetuses subsequently shown to be small for gestational age there was an increase in placental flow resistance with reduced, absent or even reversed flow in diastole. This finding was also present in the one fetus which died in utero. Serial studies in patients with fetal compromise indicated increasing flow resistance, a reverse of the normal trend. These results were not available to the clinician yet of 24 fetuses born before 32 weeks 13 had a high A/B ratio, and all of them were born electively. Maternal hypertension was associated with an increase in fetal placental flow resistance. The umbilical artery A/B ratio provides a new and non‐invasive measure of fetoplacental blood flow resistance.

Uteroplacental blood flow velocity-time waveforms in normal and complicated pregnancy.

Summary. A simple continuous wave Doppler ultrasound system for recording arterial flow velocity waveforms in branches of the uterine artery in the placental bed is described. Twelve normal pregnancies were studied serially from 20 weeks to delivery. The diastolic flow velocity expressed as a percentage of the systolic provides an index of downstream vascular bed resistance and perfusion. This always exceeded 50% in normal pregnancy and there was a small increase with gestational age indicative of a decreasing flow resistance. Of the 91 complicated pregnancies, studied because of potential uteroplacental insufficiency and fetal risk, 25 resulted in the birth of an infant small‐for‐gestational‐age. In 15 the uterine artery flow velocity waveform revealed a pattern of low diastolic flow velocity. It is postulated that these represent a subgroup of growth‐retarded fetuses in whom there is reduced uterine artery perfusion. Reduced uterine artery diastolic flow velocity in these patients was associated with reduced umbilical artery diastolic flow velocity on the fetal side of the placenta. In contrast the 10 small‐for‐gestation infants associated with normal uterine artery waveforms suggest a primary fetal cause. Twelve patients with severe hypertensive disease of pregnancy were studied. Nine were associated with reduced uterine artery diastolic flow velocity (reduced uterine artery perfusion) consistent with vasospasm in the branches of the uterine artery in the placental bed.

A comparison of fetal heart rate monitoring and umbilical artery waveforms in the recognition of fetal compromise

Summary. Antenatal fetal heart rate monitoring was compared with the study of umbilical artery flow velocity waveforms for the recognition of fetal compromise in 170 patients considered at high fetal risk. In 53 patients the infant had a 5‐min Apgar score of <7 and/or a birthweight < the 10th centile of weight for gestation. Fetal heart rate traces were classified as reactive or non‐reactive and also assessed with a modified Fischer score. The systolic/diastolic A/B ratio was measured in the umbilical artery waveform. Fetal compromise was more efficiently recognized by study of the umbilical artery waveforms. The sensitivity of assessment by umbilical artery waveforms was 60% compared with 17% and 36% respectively, for the two methods of scoring fetal heart rate traces. This was not associated with an increase in false‐positive results as the predictive value of both positive (64% compared with 69 and 58%) and negative (83% compared with 72 and 75%) results was similar when umbilical artery waveform analysis was compared with the two methods of scoring fetal heart rate traces. Specificity was also similar (85% compared with 97 and 88%).

Fetal umbilical artery flow velocity‐time waveforms in twin pregnancies

Summary. The umbilical artery flow velocity‐time waveforms were studied in 76 twin pregnancies. The ratio of peak systolic (A) to least diastolic (El) velocity was calculated for each fetus as an index of umbilical placental flow resistance. Seventy‐one sets of twins were studied within 14 days before delivery. In 65 cases both twins were alive at the time of study. In 32 pregnancies both fetuses were of birthweight appropriate for gestational age (AGA) and had A/B ratios within the normal singleton range. In 33 pregnancies one or both of the liveborn infants were small for gestational age (SGA) and in 78% of these at least one fetus had an elevated A/B ratio. Discordancy in birthweight and A/B ratio was associated with growth retardation. Clinically manifest twin‐to‐twin transfusions occurred in five of the ten pregnancies resulting in an SGA infant (eight with discordant weight) associated with a normal and concordant A/B ratio. Two twin‐to‐twin transfusions were associated with perinatal death. The placentas were examined in 61 patients. In 43 dichorionic pairs the A/B ratio was elevated in 12 of the 18 where there was at least one SGA infant. There was a greater incidence of growth retardation in the monochorionic pairs (12 of 18). Only seven of these were identified by an elevated A/B ratio. In 10 of these 18 pairs vascular anastomoses were demonstrated on placental inspection and in a further two there was evidence of twin‐to‐twin transfusion by haemoglobin discrepancy. Twin pregnancy may result in the birth of a small‐for‐dates infant because of intrauterine growth retardation or twin‐to‐twin transfusion. An abnormally elevated A/B ratio identifies growth retardation. In twin‐to‐twin transfusion the A/B ratio of the two fetuses is similar (crossed circulation) yet difference in size may be seen on ultrasound measurement.

Umbilical and uterine artery flow velocity waveforms in pregnancy associated with major fetal abnormality.

Summary. Flow velocity waveforms from the umbilical artery and branches of the uterine artery in the placental bed were recorded using continuous wave Doppler ultrasound. The records of 26 patients with pregnancy complicated by major fetal abnormality were reviewed to determine the changes of a primary fetal disturbance. The systolic/diastolic (A/B) ratio was used as an index of blood flow resistance. In 13 of the 26 patients the umbilical artery waveform systolic/diastolic ratio was high. It is postulated that in these patients there is a process of obliteration of small arteries in the placenta that is triggered by the abnormal fetus. In all patients the uterine artery waveform was normal. In seven of these 13 patients the infant had a birthweight > 10th centile. Placental weights were examined. A small placenta expressed as either low weight for gestational age or low placental/feta1 weight ratio was associated with a normal umbilical artery waveform. It is suggested that fetuses in these groups have a low growth potential. In contrast a high placental/fetal weight ratio was associated with an abnormal umbilical artery waveform pattern.

HLA‐G deletion polymorphism and pre‐eclampsia/eclampsia

Objective To investigate a HLA‐G deletion polymorphism in pre‐eclamptic pedigrees and the general population.

The effect of epidural anaesthesia for caesarean section on maternal uterine and fetal umbilical artery blood flow velocity waveforms

Summary. The blood flow resistance in the maternal uteroplacental and fetal umbilical artery circulation was studied in eight otherwise normal patients undergoing elective lower segment caesarean section. The systolic/diastolic A/B ratio (the ratio of peak systolic to least diastolic flow velocity) for the uteroplacental and fetal umbilical circulation was determined from the artery blood flow velocity‐time waveform and used as an index of blood flow resistance. Each patient received a 1 litre intravenous crystalloid infusion before an epidural bupivicaine injection. Both crystalloid infusion and epidural anaesthesia resulted in a significant decrease in the maternal uteroplacental systolic/diastolic (A/B) ratios, associated with a decrease in fetal umbilical artery A/B ratio. This study suggests a beneficial fetal effect from the improved maternal uterine perfusion after epidural anaesthesia.

Fetal volume blood flow and umbilical artery flow velocity waveform analysis: a comparison

Summary. The estimation of fetal umbilical vein and aortic volume blood flow was compared with umbilical artery flow velocity‐time waveform analysis in 42 pregnancies. The volume blood flow measurements were made with a combined linear array B‐mode and pulsed Doppler ultrasound system. The umbilical artery flow velocity‐time waveform analysis was made with either pulsed or continuous wave 2 MHzDoppler ultrasound and the systolic/diastolic (A/B) ratio was used as an index of flow resistance. There was no difference when continuous wave was compared to pulsed Doppler ultrasound as a method of recording flow velocity waveforms. Analysis of the umbilical artery flow velocity‐time waveform was more sensitive (100%‐50%) than the measurement of umbilical vein volume blood flow (ml/kg/min) in the detection of the small‐for‐gestational age (SGA) fetus. The predictive value of a positive test was higher (71%‐45%) while specificity was similar (88%‐81%). Descending aortic volume flow was reduced in only one SGA fetus. The increase in umbilical placental downstream resistance (high systolic/ diastolic ratio) was associated with a reduction in the percentage of fetal aortic blood flow directed to the umbilical placental circulation.

Treatment of Preterm Cervical Dilatation with Glyceryl Trinitrate, a Nitric Oxide Donor

EDITORIAL COMMENT: This paper reports very impressive results of treatment of cervical dilatation with glyceryl trinitrate in what seems to be a very high‐risk group of 10 women. The details provided of Case 1, table 1, suggest that glyceryl trinitrate patch therapy may be considered as an alternative to cervical ligation, at least in selected patients where insertion of a cervical stitch has contraindications (haemorrhage, uterine contractions, possibility of infection). It is early days yet, but the editorial committee wish to endorse the authors conclusion that ‘the data presented here supports the need to perform randomized placebo‐controlled trials so that the role of glyceryl trinitrate, if any, and safety of its use can be established’. Should such trails include prophylactic use of this regimen in women without cervical dilatation when there is a past history of recurrent mid‐trimester fetal loss from premature labour ± premature rupture of the membranes? These women are significant contributors to the 35–40% of all perinatal deaths that occur associated with delivery between 20 and 28 weeks gestation.