Blood viscosity and uterine artery flow velocity waveforms in pregnancy: a longitudinal study.

In the course of pregnancy, whole blood viscosity (WBV) decreases, plasma viscosity (PV) increases and uteroplacental resistance to blood flow falls. According to the Hagen-Poiseuille law, viscosity influences resistance to blood flow. To investigate the contribution of these haemorheological changes to the decrease in resistance to blood flow in the uteroplacental circulation, a serial study was performed. Using blood viscosimetry and pulsed Doppler ultrasound recordings of the uterine arteries, 21 women with normal pregnancies were studied every 8 weeks from 12 weeks onwards. Statistical analysis revealed no significant relationship between WBV, PV and pulsatility index of the uterine arteries. It is concluded that the changes in blood viscosities are of minor influence on resistance to blood flow in the uteroplacental circulation throughout normal pregnancy. The vascular changes e.g. the considerable increase of the diameter of the spiral arteries have a much greater impact on resistance to blood flow.

Uterine artery Doppler flow and uteroplacental vascular pathology in normal pregnancies and pregnancies complicated by pre-eclampsia and small for gestational age fetuses.

This study was conducted to investigate the association between uterine artery Doppler flow patterns and uteroplacental vascular pathology in normal and complicated pregnancies in view of the recently described concept of heterogeneous causes of hypertensive pregnancy complications. Forty-three women whose pregnancies were complicated by pre-eclampsia, the HELLP (Haemolysis, Elevated Liver enzymes, Low Platelets) syndrome and/or small for gestational age (SGA) fetuses and 27 women with normal pregnancies undergoing elective caesarean section were included. We obtained uterine artery Doppler waveforms at a mean of 4 days before delivery. Placental bed biopsies were obtained at caesarean section and analysed for physiological changes and pathological changes. We found that abnormal uterine artery Doppler flow was strongly associated with pregnancy complications. Absence of physiological changes was seen in 58 per cent of complicated pregnancies and 40 per cent of normal pregnancies. Pathological changes were seen in 58 per cent of complicated pregnancies and 53 per cent of normal pregnancies; they occurred in spiral arteries with and without physiological changes, and there was no significant correlation to Doppler results. In conclusion, absence of physiological changes is associated with abnormal uterine artery Doppler flow and pregnancy complications. However, there is a gradient in the severity of uteroplacental vascular pathology and the correlation with pregnancy complications is not as strong as previously thought. There is also a significant degree of uteroplacental vascular pathology in normal pregnancies with normal uterine artery Doppler flow. This variation may be partly due to sampling error, as a typical biopsy contains only one or two spiral arteries. We hypothesize that additional factors might be necessary to induce the clinical syndrome of pre-eclampsia.

Fetal Doppler velocimetry in the internal carotid and umbilical artery during Braxton Hicks' contractions.

Using Doppler ultrasound, previous studies revealed a considerable increase in vascular resistance in the uteroplacental circulation during Braxton Hicks' contractions. Consequently, uteroplacental blood flow is reduced and this affects placental oxygen transfer to the fetus, causing a fall in fetal arterial PO2. In view of the important role of arterial PO2 in the regulation of cerebral blood flow in the fetus, we hypothesised that Braxton Hicks' contractions cause a decrease in cerebral vascular resistance. A study was undertaken in 16 healthy near term pregnancies, using pulsed-wave Doppler ultrasound to evaluate the influence of Braxton Hicks' contractions on cerebral vascular resistance of the fetus. Flow velocity waveforms (FVWs) were recorded of the fetal internal carotid and umbilical artery and the Pulsatility Index (PI) was calculated. During Braxton Hicks' contractions the PI in the recorded vessels did not change. Fetal heart rate showed also no changes during Braxton Hicks' contractions. These findings indicate that resistance to blood flow downstream of these arteries, is not significantly altered, suggesting that Braxton Hicks' contractions have little or no effect on fetal haemodynamics and on fetal oxygenation in the healthy near term fetus.

Behavioural states and Doppler velocimetry of the renal artery in the near term human fetus.

Transition of fetal behavioural state 1F to 2F coincides with cardiovascular changes measured by Doppler velocimetry e.g. a decrease in pulsatility index (PI) in the internal carotid arteries and in the descending aorta, indicating redistribution of blood flow. Recently, we reported a considerable reduction in fetal urine production rate of 47% during fetal behavioural state 2F as compared to 1F. It was suggested that this reduction is caused by an increase in renal vascular resistance during 2F. Using Doppler ultrasound, flow velocity waveforms (FVW) of fetal renal arteries were recorded during behavioural states 1F and 2F. Fetal behavioural states 1F and 2F were assessed by recording fetal heart rate pattern, eye movements and body movements. The PI of the renal artery FVWs were calculated as an index of renal vascular resistance to blood flow. Fifteen healthy pregnant women between 36 and 40 weeks gestational age were studied and the relationship between fetal renal vascular resistance to blood flow and behavioural states was determined. We found that PI values in the renal arteries did not change relative to behavioural state 1F and 2F. These findings suggest that renal vascular resistance to blood flow is not appreciably different in 1F and 2F. This is in contrast with urine production rate which is almost reduced by half during 2F in the near term fetus.

Doppler ultrasound screening predicts recurrence of poor pregnancy outcome in subsequent pregnancies, but not the recurrence of PIH or preeclampsia.

OBJECTIVE: To assess the role of Doppler uterine artery screening in the prediction of recurring hypertensive disorders in a high-risk population. METHODS: Ninety-four women with a history of hypertensive disorders in previous pregnancies underwent ultrasound color Doppler to analyze blood flow in the uterine arteries at 21-22 weeks of gestation. We evaluated the performance of the Pulsatility Index (PI) as well as the diastolic notch to predict recurring hypertensive disorders. Outcome measures were the recurrence of hypertensive disorders, and poor pregnancy outcome due to intrauterine death growth retardation, intrauterine death, placental abruption, hemolysis, elevated liver enzymes, and low platelets (HELLP) syndrome, eclampsia, or premature birth. Onset of symptoms was before 35 weeks in all cases of poor pregnancy outcome. RESULTS: Doppler flow recordings were obtained from a well-defined location in both uterine arteries. The predictive value of the uterine artery PI for recurring hypertensive disease was poor and not significant; interestingly, however, the predictive values for poor pregnancy outcome were good (sensitivity 83%, specificity 71%, p < 0.001). The PI also provides a good test for intrauterine growth retardation (sensitivity 80%, specificity 69%, p < 0.01). The "diastolic notch" did not perform as well as the PI. CONCLUSIONS: Uterine artery screening did significantly predict the recurrence of poor pregnancy outcome due to hypertensive complications in this high-risk group. In contrast, gestational hypertension and preeclampsia with normal pregnancy outcome were not significantly predicted by uterine artery screening.

Ultrasound pulsed Doppler studies of the uteroplacental circulation: the influence of sampling site and placenta implantation.

The results of Doppler ultrasound studies on the uteroplacental circulation show considerable variations which might be well explained by differences in methods. Using pulsed wave Doppler ultrasound, we studied the uteroplacental circulation in 45 normal pregnancies. Flow velocity waveforms were obtained at three different sampling sites: in the uterine artery near its origin, near the crossing with the external iliac artery and in arcuate arteries near the placenta bed. Significantly lower pulsatility index (PI) values were found in the arcuate arteries near the placenta bed compared to the uterine artery at the point where it crosses the external iliac artery. Between the origin and the crossing site of the uterine artery, no significant difference was found. The influence of placental site upon the PI in uterine arteries near the crossing was investigated longitudinally in 18 normal pregnancies. Lower PI values were found in uterine arteries near the placental site only at 16 and 20 weeks of gestation.

Uteroplacental Doppler velocimetry during Braxton Hicks' contractions.

Pulsed-wave Doppler ultrasound was used to evaluate the influence of Braxton Hicks' contractions on flow velocity waveforms in the uterine arteries. Flow velocity waveforms were obtained from a standardized site, at the crossing of the uterine artery with the external iliac artery near the uterine wall. Doppler signals were recorded in 16 healthy near-term nulliparous pregnant women. During Braxton Hicks' contractions, a considerable increase in pulsatility index (PI) was found in the uterine arteries increasing from 0.71 +/- 0.21 (mean +/- SD) to 1.14 +/- 0.67 (p < 0.005). Maternal heart rate decreased slightly but significantly during contractions. Despite the high PI values that were found during Braxton Hicks' contractions, a diastolic notch in the flow velocity waveform was never noticed. The findings indicate that during Braxton Hicks' contractions, resistance to blood flow in the uteroplacental circulation is considerably increased.

Enhancement of hepatic artery resistance to blood flow in preeclampsia in presence or absence of HELLP syndrome (hemolysis, elevated liver enzymes, and low platelets)

OBJECTIVE: Our purpose was to test the hypothesis that the HELLP (hemolysis, elevated liver enzymes, and low platelets) syndrome is the result of excessive vasoconstriction of the hepatic arterial circulation. STUDY DESIGN: Doppler ultrasonography was used to measure the pulsatility index of the common hepatic artery in 14 women with preeclampsia, 15 with preeclampsia complicated by HELLP syndrome, and 8 with HELLP syndrome but without proteinuria. Gestational age ranged from 24 to 38 weeks. The study group was compared with a reference group (n = 42). RESULTS: Both in preeclampsia and in the HELLP syndrome the hepatic artery pulsatility index values were significantly increased compared with the reference group. However, no significant differences were found between the preeclamptic group, the HELLP group with proteinuria, and those with HELLP without proteinuria. CONCLUSIONS: These findings indicate that hepatic artery resistance to blood flow is increased in preeclampsia in the presence or absence of the HELLP syndrome. The results also demonstrate that vasoconstriction of the hepatic arteries is not more pronounced in the HELLP syndrome than in other manifestations of preeclampsia. Therefore factors other than vasoconstriction are likely to be responsible for the development of the HELLP syndrome.

Acute maternal rehydration increases the urine production rate in the near-term human fetus.

OBJECTIVE: We sought to investigate the effect of a decrease of maternal plasma osmolality produced by hypotonic rehydration on the fetal urine production rate in normal near-term human fetuses. STUDY DESIGN: Twenty-one healthy pregnant women attending the clinic for antenatal care were studied between 37 and 40 weeks' gestation. The fetal urine production rate was assessed by serial measurements of 3 diameters of the fetal bladder. The hourly fetal urine production rate was determined by linear regression analysis of the calculated bladder volumes versus time and was initially determined after a period of 4 hours of fluid deprivation. Thereafter, the women were asked to drink 1 L of water, and the hourly fetal urine production rate was assessed again. The hourly fetal urine production rate was only studied during behavioral state 1F because it is dependent on the behavioral state. The fetal behavioral state was determined by assessment of fetal heart rate, fetal eye movements, and fetal body movements. RESULTS: Successful recordings were obtained in 10 of the 21 women. The hourly fetal urine production rate increased significantly after hypotonic rehydration (P <.02). Compared with the initial hourly fetal urine production rate after 4 hours of fluid deprivation, the hourly fetal urine production rate showed an increase of 63.2% after hypotonic rehydration, from 38.2 +/- 16.3 mL/h to 62.4 +/- 34.6 mL/h (mean +/- SD). After rehydration, the baseline fetal heart rate fell significantly, from 141 +/- 6 to 132 +/- 8 beats/min (mean +/- SD; P =.005). CONCLUSION: The fetal urine production rate is augmented after acute maternal oral hypotonic rehydration after 4 hours of fluid deprivation. The current findings demonstrate that the near-term human fetus can handle such acute changes in fluid osmolality by increasing the urine production rate to maintain its fluid homeostasis. This mechanism implies that changes in maternal plasma osmolality and volume probably play an important role in determining amniotic fluid volume. Therefore the application of maternal hydration for the treatment of oligohydramnios should be further investigated.

Death of one twin followed by extremely variable flow velocity waveforms in the surviving fetus.

Intrauterine death of one fetus after the second trimester in a twin pregnancy, with continuation of the pregnancy is a rare complication. The risks of morbidity and mortality for the surviving fetus are high. A 32-year-old woman was admitted to the antenatal ward at 27 weeks gestation because of intrauterine death of one twin. During the first 24 h after the death of one twin, Doppler ultrasound assessment showed a remarkable variability in flow velocity waveforms in the umbilical artery of the surviving fetus. Changes from reversed to normal end-diastolic flow velocities were recorded within 6 min. These findings are explained by twin-to-twin transfusion due to intravascular blood pressure changes, or by release of vasoactive substances by the dead fetus.

Urine production rate is related to behavioural states in the near term human fetus.

OBJECTIVE: To investigate the relation between hourly fetal urine production rate (HFUPR) and behavioural states 1F and 2F (corresponding to quiet and active sleep, respectively) in normal near term fetuses. DESIGN: An observational study. SETTING: A clinic for antenatal care at a university hospital. SUBJECTS: Nineteen healthy pregnant women examined at 37 to 40 weeks of gestation. MAIN OUTCOME MEASURES: Fetal behavioural states (1F and 2F) were assessed by means of fetal heart rate patterns (FHR A and FHR B). Using real time ultrasonography, HFUPR (ml/h) was estimated during behavioural states 1F and 2F. RESULTS: During behavioural state 1F, HFUPR was significantly higher than during state 2F (P < 0.01). HFUPR falls by 47% from 50.8 +/- 24.4 ml/h in state 1F to 25.7 +/- 15.0 ml/h in state 2F. CONCLUSIONS: During active sleep (state 2F) hourly fetal urine production rate is considerably reduced as compared to quiet sleep (state 1F).

Pathophysiology and treatment of fetal anemia due to placental chorioangioma.

Placental chorioangiomas occur in 1% of pregnancies. Large chorioangiomas may cause serious complications such as fetal anemia, hydrops and fetal death. In this case report, a pregnancy complicated by a large placental chorioangioma is described. Severe fetal anemia without the occurrence of hydrops fetalis was suspected using ultrasound and Doppler examinations. Successful intrauterine blood transfusion was performed, with an unusually large amount of blood needed to obtain an adequate rise in fetal hematocrit. Two weeks later, at 32 weeks, the infant was born in good condition. In pregnancies with large chorioangiomas, we advise regular ultrasound and Doppler examinations, with the aim of detecting fetal anemia before hydrops develops. When anemia is suspected, fetal blood sampling is indicated and intrauterine transfusion therapy may be beneficial to preserve fetal health until maturity is reached.

Quantification of the diastolic notch in Doppler ultrasound screening of uterine arteries.

OBJECTIVE: To investigate a new method of quantification of the diastolic notch of the flow velocity waveforms of uterine arteries in the prediction of hypertensive disorders of pregnancy. METHODS: Pulsed-wave Doppler was used to obtain flow velocity waveforms (FVWs) from the uterine arteries at 21-22 weeks of gestation from 531 nulliparous women and 94 multiparous women at high risk. From the FVWs, both the pulsatility index (PI) and the notch index (NI) were calculated and the predictive values for both indices were compared using logistic regression analysis for mild and severe early onset hypertensive pregnancy complications. RESULTS: Both the PI and the NI were poor predictors for mild gestational hypertension and pre-eclampsia; predictive values for severe early onset disease, however, were much better. Logistic regression analysis showed the NI has no additional value compared with the PI in the prediction of either mild or severe disease. CONCLUSIONS: The NI offers the possibility to quantify the diastolic notch in uterine artery analysis. Compared to the PI, this does not lead to better predictive values for hypertensive disorders of pregnancy.

Second trimester Doppler ultrasound screening of the uterine arteries differentiates between subsequent normal and poor outcomes of hypertensive pregnancy: two different pathophysiological entities?

The 'classical' concept that pregnancy-induced hypertension (PIH) and pre-eclampsia (PE) primarily originate from defective placentation in early pregnancy has been challenged recently. There is growing evidence that other factors, including maternal predisposing conditions, also play a significant role in the pathophysiology of PIH and PE. The aim of the present study was to test the hypothesis that PIH and PE with an early onset and poor pregnancy outcome is associated with defective placentation, e.g. inadequate spiral artery dilatation and subsequent reduced uteroplacental perfusion, whereas PIH and PE with normal pregnancy outcome is not. Using Doppler ultrasound, we measured the uterine artery pulsatility index (PI) in a population of 531 nulliparous women in the 22nd week of gestation. Uterine artery PI was used as an index of resistance to blood flow in the uteroplacental circulation. Outcome measures were PIH/PE with or without poor pregnancy outcome, preterm birth and intra-uterine growth restriction (IUGR). The results revealed a striking difference between PI values for PIH/PE with and without poor pregnancy outcome. Uterine artery PI in the 22nd week was increased significantly in pregnancies which developed early-onset (before 35 weeks) PIH/PE with a poor pregnancy outcome. In contrast, uterine artery PI values were normal in women who developed PIH/PE, but had a good pregnancy outcome. There was a significant correlation between 22nd week uterine artery PI and subsequent preterm birth or IUGR. Our results indicate that only PIH/PE with poor pregnancy outcome is associated with defective placentation, whereas PIH/PE with good outcome is not. These findings support the concept of heterogeneous causes of hypertensive disorders of pregnancy.