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.     

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.     

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.     

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

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 MHz Doppler 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

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.     

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.     

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.     

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.     

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.     

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

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 feto-placental blood flow resistance.     

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

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.     

Fetal blood velocity waveforms in normal pregnancies. A longitudinal study

Blood velocity in the umbilical artery, fetal descending aorta and fetal middle cerebral artery was recorded by means of 3.5 MHz pulsed Doppler ultrasound in 22 normal pregnancies. The measurements were performed longitudinally between 25 and 42 weeks of gestation. The maximum blood velocity curves were analysed for pulsatility index (PI), systolic/diastolic velocity ratio (S/D ratio) and in the middle cerebral artery also for the resistance index according to Pourcelot (RI). In the umbilical artery, all variables decreased significantly with increasing gestational age as an expression of the decreasing placental vascular resistance. PI in the descending aorta remained fairly constant during the examined period of gestation. In the middle cerebral artery, all three variables (PI, S/D ratio, RI) decreased significantly as gestation advanced. With the exception of the umbilical artery at 28-32 weeks, PI and S/D ratio were not influenced by the heart rate in any of the vessels examined. Diagrams of the gestational age-related reference values (means +/- 2 SD) for the variables of the blood velocity waveforms were established. Additionally, a ratio of the PI and S/D was calculated for the umbilical artery and middle cerebral artery.     

Fetal blood velocity waveforms in uncomplicated pregnancy

Umbilical artery velocity waves were measured in fetuses from 94 normal pregnant women. In all, 183 determinations were carried out from the 14th to the 40th week of gestation. A combination of pulsed echo and real-time scanning was used to obtain blood waveforms from the umbilical arteries. The umbilical artery velocity wave can be readily differentiated from other fetal signals by its pattern. The systolic peak of the velocity wave was divided by the end diastolic value, thereby giving an S/D ratio. The S/D ratio in normal pregnancy declined from 7.6 to 2.0 from 14 to 40 weeks. Analysis of these waveforms indicated that the placenta is an organ of low vascular resistance and that placental resistance to blood flow declines with advancing gestational age in normal pregnancy. The umbilical artery S/D ratio provides a new and non-invasive marker of fetoplacental blood flow resistance.     

Doppler umbilical and uterine flow waveforms in severe pregnancy hypertension

Summary. Doppler studies of the umbilical and uteroplacental bed arterial flow velocity waveforms were performed in a series of women suffering from severe proteinuric pregnancy hypertension. Ninety-five women were studied and the mean interval between the last study and delivery was 1–4 days. An abnormally elevated umbilical artery systolic:diastolic (AB) ratio was present in 61 (64%) of the women. All 10 perinatal deaths were associated with pregnancies with an abnormal umbilical artery AB ratio. An elevated umbilical artery AB ratio was significantly associated with small-for-gestation and neonatal morbidity. Infants associated with extremely high umbilical artery AB ratios (≥ 99th centile) spent twice as long in the neonatal nursery as those with a lower value. There was no relation between the duration of hypertension and the umbilical study result. The uteroplacental bed artery flow velocity waveform did not correlate with fetal or neonatal mortality or morbidity. Patients with abnormal uteroplacental AB ratios also had abnormal umbilical artery AB ratios. We conclude that an abnormal fetal umbilical artery Doppler flow velocity waveform correlates with adverse fetal outcome in severe proteinuric pregnancy hypertension and suggest that the associated placental lesion may precede the maternal hypertension.     

Doppler umbilical and uterine flow waveforms in severe pregnancy hypertension

Doppler studies of the umbilical and uteroplacental bed arterial flow velocity waveforms were performed in a series of women suffering from severe proteinuric pregnancy hypertension. Ninety-five women were studied and the mean interval between the last study and delivery was 1.4 days. An abnormally elevated umbilical artery systolic:diastolic (AB) ratio was present in 61 (64%) of the women. All 10 perinatal deaths were associated with pregnancies with an abnormal umbilical artery AB ratio. An elevated umbilical artery AB ratio was significantly associated with small-for-gestation and neonatal morbidity. Infants associated with extremely high umbilical artery AB ratios (greater than or equal to 99th centile) spent twice as long in the neonatal nursery as those with a lower value. There was no relation between the duration of hypertension and the umbilical study result. The uteroplacental bed artery flow velocity waveform did not correlate with fetal or neonatal mortality or morbidity. Patients with abnormal uteroplacental AB ratios also had abnormal umbilical artery AB ratios. We conclude that an abnormal fetal umbilical artery Doppler flow velocity waveform correlates with adverse fetal outcome in severe proteinuric pregnancy hypertension and suggest that the associated placental lesion may precede the maternal hypertension.     

The acute response of the umbilical artery pulsatility index to changes in blood volume in fetal sheep.

This experimental study was designed to test the effects of acute changes in fetal circulating blood volume on the umbilical artery pulsatility index (PI). Six fetal sheep were provided with an electromagnetic flow meter for measurement of umbilical venous blood flow, with catheters for determination of arterial blood pressure and umbilical venous pressure, and with a 5 MHz Doppler transducer around one umbilical artery for flow velocity waveform analysis. A catheter in the inferior vena cava was used to infuse 50 ml of maternal blood (hypervolemia) into the fetal circulation or to withdraw 50 ml of fetal blood (hypovolemia) after volume correction. Hypervolemia resulted in a rise in arterial pressure and umbilical venous pressure, without an effect on PI, umbilical blood flow or placental vascular resistance. Hypovolemia resulted in a decrease in fetal heart rate, arterial pressure, umbilical venous pressure and umbilical blood flow. Calculated placental vascular resistance was not changed, whereas the PI increased by 42%. We conclude that volume loading with 10-15% of fetal circulating volume does not affect the umbilical artery PI, whereas acute reduction of fetal blood volume with the same amount is associated with an increase in the umbilical artery PI, without changes in calculated placental vascular resistance.     

Umbilical artery and uteroplacental blood flow velocity waveforms in normal pregnancy--a cross-sectional study

Umbilical and arcuate artery blood flow velocity waveforms (FVW) were recorded in 125 normal singleton pregnancies from 20 to 42 weeks of gestation. The FVW were analysed for pulsatility index (PI), peak systolic velocity/minimum diastolic velocity ratio (S/D ratio), rising slope (RS) and descending slope (DS). Both in the umbilical and arcuate arteries, values for all variables declined with advancing gestation, indicating decreasing placental vascular resistance. The umbilical artery PI was unaffected by the fetal heart rate, but the arcuate artery PI was negatively correlated to the maternal heart rate (r = -0.40). The arcuate artery PI decreased by 0.00394 with each beat per minute increase in maternal heart rate. Normal limits (mean +/- 2 SD) were established for umbilical artery PI corrected for gestational age, and arcuate artery PI corrected for gestational age and maternal heart rate.     

Umbilical artery velocimetry in diabetes and pregnancy

Doppler studies of umbilical artery velocity waveforms were performed during the third trimester of pregnancy in 43 diabetic women (18 class A and 25 insulin dependent). A mean serum glucose value of 120 mg% or more was selected to indicate poor glucose control. Systolic to diastolic ratios of umbilical artery velocity waveforms were calculated to determine the degree of placental vascular resistance. A systolic to diastolic ratio of 3 or more was selected as the cut-off value for determining placental vascular disease. A significant positive correlation between systolic to diastolic ratios and serum glucose level (r = 0.52, P less than .001) was found. Elevated systolic to diastolic ratio was associated with increased number of stillbirths and neonatal morbidity. This study suggests that the risk of adverse outcome in diabetic pregnant women correlates with umbilical artery velocity waveforms.     

Color-Doppler for predicting vascular resistance to feto-placental blood flow and fetal well-being

Aloka Color-Doppler, model SSD-860, was used to observe the blood velocity waveforms in the umbilical artery and in the fetal descending thoracic aorta in 139 normal pregnant women from 16th to 40th week and 30 patients suffering from pregnancy induced hypertension (PIH syndrome). The ratio of systolic to diastolic peak flow (A/B) reflected the fetoplacental vascular resistance and peripheral resistance. The results demonstrated that the placental resistance in PIH syndrome and IUGR were much higher than that in normal pregnancy. The placental resistance decreased when patients condition improved after treatment and vice versa. This suggests that fetal blood velocity waveforms (FVW) is of great value in the intensive monitoring of IUGR fetuses. The blood velocity FVW was also of great value in evaluating the curative effects on PIH patients.