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 MH_zDoppler 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.     

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 MH_zDoppler 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.     

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

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 greater than 10th centile. Placental weights were examined. A small placenta expressed as either low weight for gestational age or low placental/fetal 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.     

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.     

Fetal blood flow in pregnancies complicated by intrauterine growth retardation

A noninvasive pulsed Doppler ultrasound technique was used to characterize blood flow in the descending thoracic aorta and the intra-abdominal part of the umbilical vein in 159 fetuses suspected of intrauterine growth retardation (IUGR) on the basis of ultrasound fetometry. From this group, 74 infants with IUGR (defined as gestational age-related birth weight of 2 standard deviations [SD] or more below the population mean) were born. The blood flow results were not available to the clinicians managing the pregnancies. Blood flow mean velocity in the fetal aorta was lower, pulsatility index and rising slope higher, and umbilical volume flow and umbilical flow per 100 g placental tissue were lower in the pregnancies with IUGR than in 21 normal pregnancies. The waveform of the maximum aortic velocity envelope was related to operative delivery for fetal distress, Apgar score, and umbilical cord blood pH. The pulsatility index and the configurational assessment of the diastolic part of the waveform were combined to form a new concept, the blood flow class. The blood flow class was abnormal in 57% of the fetuses classified as having IUGR at birth and in 93% of those growth-retarded fetuses who subsequently developed signs of fetal distress requiring operative delivery. Waveform analysis, in terms of blood flow class, seems to be a useful tool in the surveillance of fetuses when IUGR is suspected. Abnormal blood flow class is a marker of fetal distress and probably gives an earlier indication than antenatal nonstressed cardiotocography. The results of this study point to a strong association between IUGR and impaired fetal blood flow. The aortic volume blood flow, unlike waveform analysis, does not seem to be a variable sensitive enough to predict fetal outcome in the individual pregnancy.     

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

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.     

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.     

Umbilical and uterine flow velocity waveforms in pregnancies complicated by major fetal anomalies

A continuous wave Doppler unit was used to obtain umbilical and uterine artery flow velocity waveforms in pregnancies complicated by a major fetal abnormality. A total of 139 examinations were performed on 32 women between 26 to 41 weeks' gestation, and the records were reviewed to determine the changes associated with fetal malformation. The systolic/diastolic (A/B) ratio was used as an index of blood flow resistance in the umbilical artery and the systolic minus diastolic divided by systolic (A-B)/A for the branches of the uterine artery. Seventeen out of 32 patients showed high systolic/diastolic ratio in waveforms taken from the umbilical artery. In 30 out of 32 patients the uterine artery waveform was normal (in two patients the results were equivocal). It appears that a fetal mechanism may determine the changes in the umbilical placental circulation resulting in an umbilical artery pattern of high flow resistance in more than half of the patients with congenital anomalies.     

Direct determination of the ovine fetal umbilical artery blood flow waveform

Ultrasonographic umbilical artery blood flow velocity waveform analysis has been proposed as a means of noninvasive assessment of fetal well-being. We computed waveform indices from directly measured umbilical artery blood flow in chronically instrumented ovine fetuses from 109 to 138 days of gestation (term, 145 days). The three waveform indices (systolic/diastolic ratio, pulsatility index, and resistance index) correlated significantly with each other (r = 0.90 to 0.98). These indices progressively decreased with gestation and were significantly correlated with calculated umbilical vascular resistance (r = 0.68 to 0.70, p less than 0.01) and with umbilical blood flow (r = -0.71, p less than 0.01). During the final week of pregnancy, systolic/diastolic ratio could be predicted by the combination of placental size (total cotyledonary mass), fetal size (ponderal index), and either umbilical blood flow or umbilical vascular resistance (multiple linear regression, r2 = 0.94). Fetal heart rate declined from day 109 of gestation to 138 days. Fetal heart rate was significantly correlated with waveform indices only when values exceeded 170 beats/min (r = -0.37 to -0.51). Ovine fetal umbilical artery waveform indices changed at approximately the same rate as those reported for human fetuses in late gestation on the basis of external Doppler ultrasonographic velocity measurements. These results suggest that the sheep is a suitable model for investigations of umbilical artery waveform analysis.     

Ultrasound measurement of fetal blood flow in predicting fetal outcome

Summary. The efficacy of fetal blood flow assessment in predicting fetal outcome was evaluated in 159 pregnancies suspected of intrauterine growth retardation (IUGR). Blood flow in the fetal aorta and umbilical vein was measured with imaging and pulsed Doppler ultrasound. Volume blood flow values and variables describing the waveform of the maximum aortic blood velocity were checked for relations to subsequent fetal outcome. A new semi-quantitative velocity waveform variable, blood flow class (BFC), was designed and tested. The occurrence of IUGR, imminent fetal distress, a low Apgar score at f and 5 min, and a low pH in the umbilical artery and vein were adopted to characterize fetal outcome. Receiver operating characteristic curves were used to demonstrate the sensitivity and false positive rate, and the Cohen's Kappa index was used to compare the predictive capacity of the various blood flow variables. BFC, describing the blood velocity waveform with emphasis on its end-diastolic part, was found to be the most powerful marker of imminent fetal asphyxia (Kappa = 0.66) and of intrauterine growth retardation (Kappa—0.48).     

Ultrasound measurement of fetal blood flow in predicting fetal outcome

The efficacy of fetal blood flow assessment in predicting fetal outcome was evaluated in 159 pregnancies suspected of intrauterine growth retardation (IUGR). Blood flow in the fetal aorta and umbilical vein was measured with imaging and pulsed Doppler ultrasound. Volume blood flow values and variables describing the waveform of the maximum aortic blood velocity were checked for relations to subsequent fetal outcome. A new semi-quantitative velocity waveform variable, blood flow class (BFC), was designed and tested. The occurrence of IUGR, imminent fetal distress, a low Apgar score at 1 and 5 min, and a low pH in the umbilical artery and vein were adopted to characterize fetal outcome. Receiver operating characteristic curves were used to demonstrate the sensitivity and false positive rate, and the Cohen's Kappa index was used to compare the predictive capacity of the various blood flow variables. BFC, describing the blood velocity waveform with emphasis on its end-diastolic part, was found to be the most powerful marker of imminent fetal asphyxia (Kappa = 0.66) and of intrauterine growth retardation (Kappa = 0.48).     

Placental morphology in relation to umbilical artery blood velocity waveforms

The association between umbilical artery flow velocity waveforms, placental morphology and arterial vascular pattern was investigated in 30 pregnant women at risk for intra-uterine growth retardation. The blood velocity waveform was assessed in the umbilical arteries with pulsed Doppler ultrasound.

Placentas from fetuses with an end-diastolic zero flow were small and thick with an extrachorial configuration, marginal cord insertion, magistral or mixed allantochorial vessel pattern and few cotyledons. The incidence and the extension of gross lesions were slightly increased in these placentas compared to placentas from fetuses with a normal S/D ratio (peak systolic velocity/minimum diastolic velocity).

Placentas from fetuses with an increased S/D ratio (> +2SD) were large and thin with a high maximum diameter/maximum thickness ratio. Heavily smoking mothers were overrepresented in the group, with an increased S/D ratio and corresponding SGA infants.

End-diastolic zero flow in the umbilical artery was strongly correlated with placental developmental abnormalities.     

Fetal blood flow in diabetic pregnancy

Forty pregnant diabetic women were examined with combined Doppler and real-time ultrasound. The volume blood flow in the fetal aorta and umbilical vein was higher and the pulsatility index (PI) in the aorta lower in the early third trimester, as compared with a reference group. Near term, the umbilical artery PI was higher in diabetic than in non-diabetic pregnancies, indicating a higher placental vascular resistance in the former group. The high umbilical artery PI occurred in fetuses who later developed distress in labor. Therefore, a high umbilical artery PI cannot be considered characteristic of diabetic pregnancy, although fetal distress might be more common in diabetic pregnancy. In addition, a high aortic volume flow was found in those fetuses who later develop distress in labor. This might be an expression of an early compensatory mechanism for increased placental vascular resistance. No specific flow variation was observed for any White class, or in association with hypertension or non-optimally regulated diabetes. The blood flow variables in growth-accelerated fetuses were comparable to those found in fetuses with normal weight. Since fetal distress might be more common in diabetic pregnancy, ultrasonic fetal blood flow measurements are recommended for antenatal fetal surveillance.     

Doppler study on pulmonary venous flow in the human fetus

OBJECTIVES: The fetal pulmonary circulation is different from the postnatal circulation: the flow is small. The arterial pressure is almost systemic, and the vascular resistance is high. Moreover, lungs are collapsed in the fetus. However, the pulmonary venous flow in the fetus has not been studied. The aims of this study were to look at flow patterns of pulmonary venous system in the fetus and to see whether there are any changes in Doppler parameters with fetal growth. METHODS: The pulmonary venous flow velocity was analyzed in 56 normal human fetuses. The fetal gestational age ranged from 17 to 37 weeks. Pulsed-Doppler echocardiography was used to investigate the right upper pulmonary vein in four-chamber view. After the pulmonary venous signal was obtained, by enlarging the sample size, the pulmonary arterial signal was superimposed for timing. Heart rate velocities at systolic peak (S), at diastolic peak (D), at nadir between S and D (O), and at nadir between D and the next S (X) and velocity-time integral were measured. RESULTS: The pulmonary venous signal showed basically continuous flow from pulmonary vein toward left atrium through all the cardiac cycles, and it had biphasic peaks: one during the systolic and the other during the diastolic phase. The systolic peak of pulmonary vein occurred slightly earlier or later than the systolic upstroke of pulmonary arterial waveforms. The systolic peak of pulmonary vein was higher than the diastolic peak in 45 out of 56 cases. Velocities of systolic and diastolic peaks and velocity-time integral of the pulmonary vein have significantly increased with fetal growth. CONCLUSIONS: In the fetus, the flow pattern of the pulmonary vein was similar to that of the postnatal pattern with the exception of absence of atrial reversal. Lower velocities may reflect a decreased pulmonary flow volume.     

Doppler assessment of umbilical artery blood flow for the prediction of outcome in fetal cardiac abnormality

In a series of 244 pregnancies referred for fetal echocardiography, the umbilical artery waveform was also studied by pulsed Doppler ultrasound. In 152 normal pregnancies diastolic flow in the umbilical artery was always detectable after 20 weeks gestation. In 4 of 58 normal patients examined before 20 weeks, diastolic flow was absent in some part of the recording. In 34 fetuses with congenital heart disease detected at between 18 and 37 weeks gestation, 10 were found consistently to have associated absent diastolic flow. Five of these fetuses died in utero between 5 and 21 days after the recording; three were aborted and the remaining two died in the neonatal period at 4 and 7 days after the examination. Fetal congenital heart disease with normal umbilical blood flow also had a poor prognosis in general, but the adverse outcome was much less immediate than in fetuses with absent diastolic flow. No correlation was found between the type of congenital heart disease and the characteristics of the umbilical artery waveform. Absent diastolic flow in the umbilical artery indicates a poor short-term prognosis for fetuses with congenital heart disease, particularly after 20 weeks, when fetal death is predictable.     

Fetal hypertension induced by norepinephrine infusion and umbilical artery flow velocity waveforms in fetal sheep

This study was designed to examine the effects of fetal hypertension on the umbilical artery pulsatility index. Fetal arterial blood pressure and umbilical venous pressure were measured in eight sheep, 3 to 5 days after surgery. Umbilical blood flow was measured with an electromagnetic flowmeter around the common umbilical vein. Umbilical artery flow velocity waveforms were obtained either by an indwelling 5 MHz pulsed Doppler device (n = 4) or transcutaneously by a 4 MHz continuous-wave Doppler device (n = 4). Fetal blood pressure was raised by intravenous infusion of norepinephrine 10 micrograms/min during 5 minutes. Norepinephrine infusion resulted in elevated arterial and umbilical venous pressures, accompanied by a bradycardia during the first 3 minutes. Umbilical blood flow, calculated placental vascular resistance, and umbilical artery pulsatility index did not change. After atropine administration, the norepinephrine-induced elevated arterial and umbilical venous pressures were accompanied by tachycardia, increased umbilical blood flow, and no change in placental vascular resistance and umbilical artery pulsatility index. It is concluded that fetal arterial hypertension provoked by norepinephrine infusion has no effect on placental vascular resistance, umbilical blood flow, and umbilical artery pulsatility index.     

The use of an indwelling Doppler probe to study acute changes in umbilical artery waveforms in the fetal sheep

A chronic sheep model for Doppler umbilical vascular analysis was developed, in which indwelling Doppler probes were used. These were designed with a fixed angle of insonation and implanted directly on the umbilical cord to register umbilical artery velocity waveforms. The fetuses in eight pregnant ewes underwent maternal aortic and umbilical cord constrictions producing serial blood flow reductions. Occlusion of the umbilical cord and maternal aorta caused distinctly different waveforms. Cord occlusion produced an immediate response with an elevated systolic/diastolic ratio and disappearance of diastolic velocity. Maternal aortic occlusion produced a delayed response with drops in both systolic and diastolic velocity; diastolic velocity never reached zero. Although systolic/diastolic ratios are believed to reflect placental resistance, the maintenance of the systolic/diastolic ratio with diminution of systolic velocity suggests declining fetal cardiac output as an additional factor. It is possible to differentiate uteroplacental from umbilicoplacental insufficiency by Doppler methods. With technologic improvements Doppler ultrasonography may allow better analysis of acute stressful conditions during human labor.     

Doppler assessment of umbilical artery blood flow for the prediction of outcome in fetal cardiac abnormality

Summary. In a series of 244 pregnancies referred for fetal echocar-diography, the umbilical artery waveform was also studied by pulsed Doppler ultrasound. In 152 normal pregnancies diastolic flow in the umbilical artery was always detectable after 20 weeks gestation. In 4 of 58 normal patients examined before 20 weeks, diastolic flow was absent in some part of the recording. In 34 fetuses with congenital heart disease detected at between 18 and 37 weeks gestation, 10 were found consistently to have associated absent diastolic flow. Five of these fetuses died in utero between 5 and 21 days after the recording; three were aborted and the remaining two died in the neonatal period at 4 and 7 days after the examination. Fetal congenital heart disease with normal umbilical blood flow also had a poor prognosis in general, but the adverse outcome was much less immediate than in fetuses with absent diastolic flow. No correlation was found between the type of congenital heart disease and the characteristics of the umbilical artery waveform. Absent diastolic flow in the umbilical artery indicates a poor short-term prognosis for fetuses with congenital heart disease, particularly after 20 weeks, when fetal death is predictable.     

Fetal umbilical artery flow velocity waveforms: prediction of small for gestational age infants and late decelerations in labour

The relationship between fetal umbilical artery waveform patterns and (a) small for gestational age infants or (b) late fetal heart rate (FHR) decelerations in labour was examined in 205 clinically high-risk pregnancies. All pregnancies were studied using continuous-wave Doppler Ultrasound within 7 days of delivery. Waveforms were analysed by calculating a ratio of peak systolic frequency to end diastolic frequency (A/B ratio). An abnormal Doppler result was defined as being above the 97th centile for gestation from our previously derived normal values. Abnormal Doppler results occurred in only 38% of small for gestational age (SGA) infants and in 70% of those developing late decelerations in labour. Doppler ultrasound is a poor predictor of SGA infants, but may identify a high proportion of cases at risk of developing late FHR decelerations in labour.     

Umbilical artery blood flow velocity waveforms during variable deceleration of the fetal heart rate

Blood flow velocities of the umbilical arteries were measured by Doppler ultrasonography during variable decelerations of the fetal heart rate. The flow velocity waveforms, being normal between uterine contractions, showed either an unchanged flow velocity waveform with an exclusive fetal heart rate effect on end-diastolic velocities or a rapid change to absent and reverse diastolic flow during the decelerations, indicating an abrupt increase in placental resistance with a halt in placental perfusion. Computer-aided reconstruction of the fetal heart rate curve revealed the exact temporal relationship between the reduction of umbilical artery perfusion and deceleration of fetal heart rate. We showed that variable decelerations of fetal heart rate can be observed during only slightly changed umbilical perfusion or can be caused by a halt in placental perfusion, which does not necessarily mean an absence of any movement of the fetal blood column, but is a result of a systolic forward and diastolic reverse flow to the same extent.