Umbilical Flow Velocity Research Articles

Hyrtl's anastomosis, the only connection between the two umbilical arteries. A study in full term placentas from AGA infants with normal umbilical artery blood flow.

The aim of this study was to describe the variable anatomy in the anastomosis between the umbilical arteries for better understanding of the physical characteristics of the umbilical flow velocity waveform (FVW). The arterial system of 67 placentas from pregnancies with normal umbilical FVW and resulting in a full-term AGA infant was visualized by angiography. The method allowed study of the anatomy of the anastomosis between the umbilical arteries and calculation of the relative placental area supplied by each umbilical artery. In 60 cases there was one anastomosis between the umbilical arteries, in one case there were two, in four the anastomosis was absent, and another two cases had a single umbilical artery. The anastomosis was represented by a vessel, a fenestration or coalescence of the umbilical arteries. In case the anastomosis diameter was of at least that of the umbilical arteries, they supplied in mean 26% and 74% (+/- 8.2%) of the placental area respectively. When the anastomosis diameter was smaller than that of the umbilical arteries their supply areas were in mean 41% and 59% (+/- 6.0%) respectively. In placentas lacking anastomosis the two umbilical arteries supplied 45% and 55% (+/- 2.6%) respectively, indicating a higher degree of symmetry. By using angiography we were able to demonstrate the variable anatomy of the anastomosis of Hyrtl. We found the occurrence and width of the anastomosis was correlated to the symmetry in size between the supply areas of each umbilical artery.

Changes in umbilical venous velocities with physiologic perturbations

Objective: The purpose of this study was to determine the direction of transmission of umbilical venous Doppler flow velocity changes in human fetuses. Study Design: Strip chart recordings of simultaneously measured umbilical arterial and venous velocities were examined at two sites in the umbilical cord, one near the fetus (proximal) and one near the placenta (distal). Fetuses with venous pulsations or breathing episodes were included. At both locations time from venous pulsation to arterial systole was measured in fetuses with venous pulsations and duration of phase delay between arterial diastolic velocity minimum and venous velocity minimum was measured in fetuses with breathing episodes. Results: In 21 fetuses with venous pulsations the pulsations occurred earlier in the cardiac cycle at proximal sites (0.12 ± 0.04 second before systole) and later at distal sites (0.02 ± 0.04 second before systole; P < .001). Phase delays in venous velocities in the 5 fetuses with breathing episodes were also longer at distal sites than at proximal sites ( P < .011). Conclusion: Changes in umbilical venous velocities occurred earlier at sites that were closer to the fetus. These findings suggest that changes in umbilical venous velocities originate in the fetal venous system and are transmitted to, rather than from, the placenta. (Am J Obstet Gynecol 2000;182:835-40.)

Necrotizing enterocolitis in infants born to women with severe early preeclampsia and absent end-diastolic umbilical artery doppler flow velocity waveforms.

The aim of this study was to determine the prevalence of necrotizing enterocolitis (NEC) in infants born to a homogeneous group of women with severe preeclampsia before 34 weeks' gestation and who had absent end-diastolic umbilical artery Doppler flow (AEDF) or normal umbilical Doppler flow velocities (NUFV). A total of 242 infants were entered into the study. The mean birth weight was 1260.5 g (SD = 339) and the mean gestational age 30.5 weeks (SD = 2.0). Sixty-eight (28%) infants had AEDF, 43 (18%) had umbilical artery Doppler flow velocities between the 95th and 99th percentile, and 131 (54%) had NUFV. Forty-one (18%) infants developed NEC, of whom 20 (8%) developed definite and advanced NEC (grade 2 and 3). Of these, 16(80%) had grade 2 and 4(20%) had grade 3. Twenty-one (8%) infants developed suspected NEC (grade 1). The mean onset of grade 1 NEC (7.2 days) occurred significantly earlier than in those with grades 2 and 3 NEC (18.7 and 23.3 days, respectively). Of the 21 infants with grade 1 NEC, 10 (48%) had AEDF and 9 (43%) had NUFV. None of the infants with grades 2 or 3 NEC had AEDF. We conclude that although chronically hypoxemic fetuses born to women with severe early onset preeclampsia and AEDF respond by redistributing blood flow to vital organs and away from the gut; the intestinal compromise is of insufficient magnitude to induce intestinal necrosis or NEC. Enteral feeding, however, should be introduced cautiously in infants with AEDF, as so-called suspected NEC developed significantly more often in these infants.

Association between a low umbilical artery pulsatility index and fetal distress in labor in very prolonged pregnancies

Objective: To investigate the association between fetal, umbilical and uterine circulatory changes and adverse perinatal findings in very prolonged pregnancies. Study design: 44 women proceeding to 43 completed weeks of gestation with the intention of a trial of vaginal delivery were studied prospectively with ultrasound Doppler velocimetry. An intensified fetal surveillance was routinely commenced at 42 weeks and only uncomplicated pregnancies were allowed to proceed. The endpoint perinatal measures were oligohydramnios, fetal meconium release, fetal distress in labor and birth asphyxia. Flow variables in different groups were compared with the Mann-Whitney U test, Student's unpaired t-test, Wilcoxon signed-rank matched-pairs test. Fisher's exact test and contingency table analysis, and a two-tailed P value < 0.05 was considered statistically significant. Results: The umbilical artery pulsatility index was significantly lower in cases of fetal meconium release ( n = 12) and fetal distress ( n = 7). The umbilical venous flow velocity was significantly lower in cases of meconium, and the fetal aortic volume flow significantly higher in cases of fetal distress. No significant flow changes were found in connection with oligohydramnios ( n = 5) and birth asphyxia ( n = 2). Uterine flow was not significantly affected in any group. Conclusions: In very prolonged pregnancies, fetal distress in labor was not associated with an increased placental vascular resistance. In contrast to previous reports, the umbilical artery pulsatility index was low in cases of fetal distress and meconium release. The etiology is unknown, but a subclinical fetal hypoxia might have triggered a vasodilation of placental vessels. Vasodilation at an unchanged volume flow could also explain the decrease of umbilical venous flow velocity. The increased aortic volume flow indicates an increase of cardiac output in fetuses later developing distress in labor.

Effects of therapeutic amniocentesis on uterine and umbilical artery velocimetry in cases of severe symptomatic polyhydramnios

The objective of this study was to determine the effects of removal of amniotic fluid in cases of symptomatic severe polyhydramnios on Doppler waveform indices of the uterine and umbilical arteries and flow velocities of the uterine arteries. Nine women underwent therapeutic amniocentesis during ten pregnancies for symptomatic polyhydramnios due to Beckwith-Wiedemann Syndrome (n = 1), esophageal atresia (n = 2), chorioangioma (n = 1), twin-twin transfusion syndrome (n = 3), a presumed autosomal recessive syndrome (n = 2), and an unbalanced double translocation (n = 1; partial dup 3q and partial del 9p syndrome). An average of 2.78 ± 0.9 (range 1–4) 1 of fluid were removed at each procedure between the gestational ages of 18 and 34 weeks (mean of 28 weeks). The systolic/diastolic (S/D) ratio, pulsitility index (PI), and resistance index (RI) of the uterine and umbilical arteries were obtained before and after the procedure using color and pulsed Doppler. After angle correction, the peak systolic velocity (PSV) and mean velocity (MV) in centimeters/second (cm/s) of the uterine arteries were also determined. The presence or absence of a uterine artery waveform notch was determined. Dominant uterine arteries were defined as those with lower impedance indices or higher flow velocities. Statistical analysis was performed with the Wilcoxon signed-rank test. Significance was set at P < 0.05. There was a significant increase in the median value of the uterine artery MV (43.8 vs. 81.1 cm/s, P = 0.005) and PSV (74.2 vs. 125.5 cm/s, P = 0.007) after amniocentesis. The uterine S/D (3.0 vs. 1.84, P = 0.007), PI (1.12 vs. 0.68, P = 0.008), and RI (0.60 vs. 0.45, P = 0.005) impedance indices significantly decreased following amniocentesis. When uterine arteries were categorized as dominant vs. nondominant, there were greater improvements in impedance indices and flow velocities in the nondominant uterine arteries. There were three cases of unilateral and one case of bilateral early diastolic notches of the uterine artery waveforms which either resolved (n = 4) or improved (n = 1). There was no effect on the umbilical artery impedance indices. Therapeutic amniocentesis significantly improved uterine artery impedance indices and resulted in improved flow velocities, while there was no effect on umbilical artery waveform indices. The procedure resulted in the disappearance or improvement of the uterine waveform notch. Our findings suggest that in cases of severe polyhydramnios abnormal uterine artery velocimetry may not be due to lack of trophoblastic invasion of the spiral arteries but to increased intrauterine pressure secondary to polyhydramnios. © 1996 Wiley-Liss, Inc.

Effects of therapeutic amniocentesis on uterine and umbilical artery velocimetry in cases of severe symptomatic polyhydramnios.

The objective of this study was to determine the effects of removal of amniotic fluid in cases of symptomatic severe polyhydramnios on Doppler waveform indices of the uterine and umbilical arteries and flow velocities of the uterine arteries. Nine women underwent therapeutic amniocentesis during ten pregnancies for symptomatic polyhydramnios due to Beckwith-Wiedemann Syndrome (n = 1), esophageal atresia (n = 2), chorioangioma (n = 1), twin-twin transfusion syndrome (n = 3), a presumed autosomal recessive syndrome (n = 2), and an unbalanced double translocation (n = 1; partial dup 3q and partial del 9p syndrome). An average of 2.78 +/- 0.9 (range 1-4) 1 of fluid were removed at each procedure between the gestational ages of 18 and 34 weeks (mean of 28 weeks). The systolic/diastolic (S/D) ratio, pulsitility index (PI), and resistance index (RI) of the uterine and umbilical arteries were obtained before and after the procedure using color and pulsed Doppler. After angle correction, the peak systolic velocity (PSV) and mean velocity (MV) in centimeters/second (cm/s) of the uterine arteries were also determined. The presence or absence of a uterine artery waveform notch was determined. Dominant uterine arteries were defined as those with lower impedance indices or higher flow velocities. Statistical analysis was performed with the Wilcoxon signed-rank test. Significance was set at P < 0.05. There was a significant increase in the median value of the uterine artery MV (43.8 vs. 81.1 cm/s, P = 0.005) and PSV (74.2 vs. 125.5 cm/s, P = 0.007) after amniocentesis. The uterine S/D (3.0 vs. 1.84, P = 0.007), PI (1.12 vs. 0.68, P = 0.008), and RI (0.60 vs. 0.45, P = 0.005) impedance indices significantly decreased following amniocentesis. When uterine arteries were categorized as dominant vs. nondominant, there were greater improvements in impedance indices and flow velocities in the nondominant uterine arteries. There were three cases of unilateral and one case of bilateral early diastolic notches of the uterine artery waveforms which either resolved (n = 4) or improved (n = 1). There was no effect on the umbilical artery impedance indices. Therapeutic amniocentesis significantly improved uterine artery impedance indices and resulted in improved flow velocities, while there was no effect on umbilical artery waveform indices. The procedure resulted in the disappearance or improvement of the uterine waveform notch. Our findings suggest that in cases of severe polyhydramnios abnormal uterine artery velocimetry may not be due to lack of trophoblastic invasion of the spiral arteries but to increased intrauterine pressure secondary to polyhydramnios.

Evaluation of umbilical venous blood flow by Doppler color flow mapping and conventional ultrasonographic methods.

Ultrasonographic measurement of umbilical venous blood flow as a reflection of placental blood flow has been used for assessment of fetal well-being. Because fetal and maternal factors may prevent satisfactory measurement of umbilical venous diameter and flow velocity, an alternative method would be helpful. We found that, in 35 human fetuses between 18 and 35 weeks of gestation, that both variables for umbilical venous flow calculation can be measured from a single Doppler color frame with close measurement agreement to conventional combined cross-sectional pulsed Doppler ultrasonographic measurements. Thus, this method provides an alternative to the conventional method or may be used for a cross-check and has the potential for automated flow calculation.

Reduced contractile effect of endothelin-1 and noradrenalin in human umbilical artery from pregnancies with abnormal umbilical artery flow velocity waveforms.

This study on the human umbilical artery was undertaken in order to elucidate possible correlations between changes in response to vasoactive substances in vitro and abnormal umbilical artery flow velocity waveforms in vivo associated with preeclampsia and intrauterine growth retardation. The vascular reactivity to endothelin-1, noradrenalin, serotonin, the thromboxane A2 analogue U46619, substance P and prostacyclin was determined in umbilical artery segments from 13 normal pregnancies and 29 pregnancies complicated with preeclampsia and/or intrauterine growth retardation with normal or abnormal umbilical flow velocity waveforms. The contractile effect in vitro of endothelin-1 and noradrenalin was reduced in segments from pregnancies complicated by abnormal umbilical flow velocity waveforms in vivo. No differences were detected in the contractile effect of serotonin and U46619, or in the relaxatory effect of substance P and prostacyclin. In conclusion, endothelin-1- and noradrenalin-related mechanisms could be involved in the abnormal umbilical flow velocity waveforms associated with preeclampsia and intrauterine growth retardation.

Lack of association between Doppler velocimetry and synthesis of prostacyclin and thromboxane in umbilical cord vessels from growth retarded fetuses

The aim of this study was to investigate the possible association between Doppler velocimetry and synthesis of prostacyclin (PGI2) and thromboxane A2 (TxA2) in umbilical cord vessels. The hypothesis was that an altered balance between PGI2 and TxA2 production is associated with a change of artery flow resistance. 17 cases with a suspicion of intrauterine growth retardation and 21 normal pregnancies were studied. The umbilical artery pulsatility index (PI) and venous mean velocity were recorded in vivo by Doppler velocimetry. Cord vessel prostanoid synthesis was determined in vitro. The Mann-Whitney U test and simple linear regression were used for statistical analyses. The umbilical vessel synthesis of both PGI2 and TxA2 was in general lower in small-for-gestational age (SGA) cases (n = 10) compared to appropriate-for-gestational age (AGA) (n = 28). In the vein, the PGI2/TxA2 ratio was significantly lower in SGA cases. No certain correlations were found between umbilical artery PI and venous velocity, respectively, and PGI2 or TxA2, or their ratio. The prostanoid synthesis was in general lower in SGA cases, resulting in a significantly lower PGI2/TxA2 ratio in the umbilical vein. This indicates that fetal growth retardation might be associated with a disturbed endothelial function in this vessel. The synthesis of PGI2 and TxA2 in the juxtaplacental umbilical cord vessels was not correlated to the umbilical artery PI or venous flow velocity. It is possible that an altered release of prostanoids in the placental vasculature and tissue accounts for the rise of umbilical artery flow resistance instead.

Umbilical cord venous pulsation in normal fetuses and its incidence after 13 weeks of gestation

A total of 1175 umbilical venous Doppler flow velocity waveforms were acquired from 449 normal pregnant women whose fetuses were delivered at term without neonatal asphyxia or anomalies. Arterial and venous flow velocity waveforms were acquired from a free loop of the umbilical cord. The incidence of umbilical venous pulsation decreased from 30.4% at 13 ∼ 14 weeks of gestation to 14.8% at 21–22 weeks of gestation. After 23 weeks, this incidence was settled within 5.0∼7.8%. There was no difference between the umbilical arterial resistance index of the fetuses with venous pulsation and without pulsation before 22 weeks. But the resistance index was lower in the pulsatile fetuses than in the nonpulsatile fetuses after 23 weeks of gestation. The origin of venous pulsations before early second trimester was suspected to be different from that after late second trimester.

Presence of pulsations and reproducibility of waveform recording in the umbilical and left portal vein in normal pregnancies.

Reproducibility and inter-observer variability of intra- and extra-abdominal umbilical venous flow velocity and left portal venous flow velocity as well as heart-synchronous waveform pulsations in these vessels were studied in 23 women at 34-38 weeks of normal pregnancy.Limited reproducibility, expressed by large intra-patient coefficients and limits of agreement between two observers, was established for all standardized recording sites. Pulsations, defined as negative venous deflections of at least 10% of the mean velocity, were demonstrated at all locations ranging from 19.6% of the measurements at the free-floating loop of the umbilical vein to 78.4% of the measurements at the left portal vein. The present study shows that the limited reproducibility of venous flow velocity waveforms should be taken into consideration, and that the presence of pulsations can be demonstrated in normal late pregnancy.

Fetal echogenic gut: a marker of intrauterine gut ischaemia?

The pathophysiological significance of fetal echogenic gut (FEG) is unknown. Our aim was prospectively to evaluate FEG in infants with intrauterine growth retardation (IUGR) and absent umbilical artery end diastolic flow velocities. Over a 15 month period, nine infants with FEG met these criteria. Nine infants who, on antenatal assessment, had demonstrated IURG and absent umbilical artery end diastolic flow velocities, but no evidence of FEG, were selected as case-controls. Gastrointestinal function was then prospectively evaluated in both groups after delivery. All liveborn infants received nasogastric feeds of breast milk by 8 days of age. All in the FEG group developed marked abdominal distension, large, bile stained, nasogastric aspirates, and constipation requiring rectal washouts. This led to a discontinuation of enteral feeds on one or more occasions. Two patients in the FEG group required water soluble contrast enemas in order to relieve intestinal obstruction. In the control group, 3/9 patients had abdominal distension, but no rectal washouts were given and enteral feeds were not interrupted. The median (range) time to tolerate full enteral feeds was 15 (7-32) days in the FEG group, compared with 4 (1-8) days in the control group. In the FEG group 5/6 patients required parenteral nutrition for 5-27 days. In the control group one patient required parenteral nutrition over a period of four days only. No child had necrotising enterocolitis or cystic fibrosis. When FEG is observed in the fetus with IUGR, problems with enteral feeding should be anticipated.

Low target birth weight or growth retardation? Umbilical Doppler flow velocity waveforms and histometric analysis of fetoplacental vascular tree

The placental vascular architecture of small-for-gestational-age fetuses seems to have an impact on the flow patterns in the umbilical arteries. Blood flow velocity waveforms of the umbilical arteries were measured by Doppler ultrasonography in nine small-for-gestational-age fetuses with elevated systolic/diastolic ratios of the umbilical arteries, seven small-for-gestational-age fetuses with normal flow patterns, and 14 appropriate-for-gestational-age fetuses with normal flow patterns. After delivery histomorphometric placental investigations were performed. Reduced end-diastolic flow velocities were significantly associated with both a reduction of vascularization within the terminal villi and adverse diffusion conditions, indicating insufficient functional maturity. The perfusion and diffusion capacity of small-for-gestational-age placentas with normal umbilical artery flow velocity waveforms was similar or even slightly better compared with the appropriate-for-gestational-age control values. These data suggest that Doppler flow velocimetry in the umbilical arteries is predictive of a vascular lesion within the placentas of small-for-gestational-age fetuses.

Antenatal tests of fetal welfare and development at age 2 years

Our objective was to examine the outcomes at 2 years of age of fetuses delivered electively before 34 weeks, studied antenatally with two tests of fetal well-being. Forty-two fetuses from high-risk pregnancies delivered electively by cesarean section before 34 weeks were stratified into normal versus abnormal subgroups with umbilical Doppler flow velocity waveform and fetal heart rate results. Developmental outcome was assessed at 2 years. Two comparison groups were also selected: 40 matched premature controls delivered spontaneously before 34 weeks and 67 normal babies delivered spontaneously at term. Frequency outcome data were tested with chi 2 analyses and the remainder with analyses of variance. Within the electively delivered study group poor cognitive progress at 2 years was more strongly associated with an abnormal fetal heart rate result than an abnormal Doppler result. Compared with the premature control and normal term groups, electively delivered fetuses were significantly delayed in growth, cognition, and motor development (p < 0.005). Adverse fetal welfare in a high-risk obstetric sample was associated with poorer outcome at 2 years. However, the whole of the group of fetuses from such high-risk pregnancies showed significant developmental delay compared with normal term children and, more importantly, matched premature infants delivered spontaneously from otherwise uncomplicated pregnancies.

Uterine and umbilical flow velocity waveforms in normotensive and hypertensive subjects during the angiotensin II sensitivity test

The systemic and uterine circulatory effects of angiotensin II infusion in five normotensive and 10 hypertensive subjects at the end of the third trimester of gestation were studied. Color Doppler ultrasonography was used for investigating the systolic/diastolic ratios on flow velocity waveforms in the uterine and umbilical arteries. A significant increase was observed in the systolic/diastolic ratio in the uterine artery in both groups. In normotensive subjects a differential effect of angiotensin II on the systemic diastolic blood pressure and on the systolic/diastolic ratio was observed; the uterine circulation had a slower response and a faster recovery than the maternal diastolic blood pressure. In hypertensive subjects this relative refractoriness to angiotensin II was less marked; the increase in uterine vascular resistance was faster and the recovery slower than in the normotensive group. No changes in the systolic/diastolic ratio in the umbilical artery were observed in either group. These results indicate that the uterine circulation of normotensive subjects is remarkably more refractory to angiotensin II than the circulation of hypertensive subjects.

EFFECT OF ADRENALINE ON EXTRADURAL ANAESTHESIA, PLASMA LIGNOCAINE CONCENTRATIONS AND THE FETO-PLACENTAL UNIT DURING ELECTIVE CAESAREAN SECTION

Extradural anaesthesia was induced with either 2% lignocaine or 2% lignocaine with adrenaline 1:200,000 in 20 patients undergoing elective Caesarean section. With the adrenaline-containing solution, a smaller dose of lignocaine was required to produce an adequate block and the lignocaine concentrations in both mother and neonate were significantly smaller compared with the plain solution. Arterial pressures were less in the adrenaline group, but there was no difference in umbilical flow velocity waveform, fetal heart rate or fetal outcome. Neither feto-placental circulation nor fetal outcome were affected adversely by episodes of hypotension or the ephedrine used for treatment.

Maternal angiotensin sensitivity and fetal Doppler umbilical artery flow waveforms

The angiotensin pressor response was investigated in normotensive pregnancies which had umbilical Doppler flow velocity waveforms suggestive of placental vascular disease. Of the 36 pregnancies studied at between 24 and 38 weeks gestation, 18 had a positive response to the angiotensin pressor test, these women were delivered earlier (35.3 vs 38.5 weeks, P = 0.015), had a lower mean birthweight centile (14 vs 36, P less than 0.001) and higher frequency of fetal distress in labour (40% vs 7%, P = 0.06) when compared with the 18 women who had a negative response. The Doppler umbilical systolic-diastolic (S-D) ratio decreased with gestation in the negative group, suggesting continuing placental growth and vascular expansion, whereas the S-D ratio increased in the positive group (P less than 0.001), indicative of vascular obliteration. We suggest that the positive angiotensin pressor response is primarily associated with the placental vascular pathology. Angiotensin infusion had no acute effect on maternal uteroplacental or fetal umbilical artery flow velocity waveforms.

Absent or reversed end diastolic flow velocity in the umbilical artery and necrotising enterocolitis.

Absent or reversed end diastolic flow (AREDF) velocities in the umbilical artery were identified in 25 high risk pregnancies. In six pregnancies the fetus was abnormal and all but one of these ended in perinatal death. Of the 19 morphologically normal fetuses, three died in utero and there were four neonatal or infant deaths. The mortality rate was 48% for all pregnancies and 37% for those with morphologically normal fetuses. There was a highly significant increased risk for the development of necrotising enterocolitis in these morphologically normal fetuses with AREDF (53%) compared with controls (6%) who did have umbilical artery end diastolic flow velocities in fetal life. There were no significant differences between the matched pairs for parameters of neonatal outcome chosen to reflect neonatal morbidity. These findings demonstrate the close association between AREDF and necrotising enterocolitis that appears to be independent of other variables such as degree of growth retardation, prematurity, and perinatal asphyxia.

Umbilical artery velocimetry and non-stress test in monitoring high-risk pregnancies. A comparative longitudinal study.

In a longitudinal comparative study, umbilical artery velocimetry and the non-stress test were used in parallel for surveillance of 153 hospitalized high-risk pregnancies. The occurrence of abnormal flow velocity waveforms of the umbilical artery, with increased pulsatility index but maintained diastolic flow velocity, was an inconsistent finding and often related to pregnancy complications other than intrauterine growth retardation. Among patients with small-for-gestational age fetuses or pre-eclampsia with a small-for-gestational age fetus, there was a significantly higher frequency of abnormal umbilical flow velocity waveforms. This was a consistent finding and was often associated with an absence of diastolic flow velocity and operative delivery for fetal distress. An abnormal non-stress test had a significant relationship with intrauterine growth retardation, but a lower predictive capacity for this condition than the abnormal umbilical artery flow velocity waveform. In patients with small-for-gestational age fetuses and no other complication of pregnancy, umbilical artery velocimetry can be used as a reliable tool for identifying those fetuses that need intensified surveillance. In cases with additional complications, a combined use of blood velocimetry and the non-stress test is advocated.

Doppler velocimetry: critical analysis of umbilical, cerebral and aortic reference values

Cross-sectional reference standards of the umbilical flow velocity waveforms were investigated from 18 to 38 weeks of gestation on 482 normal pregnancies. A significant decrease in the pulsatility index (PI) and systolic/diastolic ( SD) ratio was observed during gestation. This phenomenon was confirmed in a longitudinal study on 150 patients of the same population. The two slopes of cross-sectional and longitudinal data were not significantly different. A bimodal distribution of PI and S D ratio was observed. Six per cent of data fell above the vast majority of cases. The dimension of the population studied allowed us to consider these findings not as outliers but as an interesting transient phenomenon of very low end-diastolic velocities which can occur in normal fetuses, without concomitant variations on the thoracic aorta and middle cerebral artery. Cross-sectional reference ranges were studied for these vessels on the same population from 26 to 38 weeks of gestation. A significant decrease in the PI of the middle cerebral artery was observed versus a significant increase in the PI of the thoracic aorta. These latter indices could be used to obtain indirect information on the normal redistribution of blood flow in the human fetus in the third trimester of gestation. However, the great variability observed should warn against the risks of a simplistic clinical use of these observations.