Precision and accuracy of Doppler flow measurements. In vitro and in vivo study of the applicability of the method in human fetuses

The precision of the Doppler method for quantitative blood flow measurement in the fetal descending thoracic aorta and in the umbilical vein and for estimation of the Pulsatility Index from the velocity curve from fetal aorta was tested in vivo by examination of six pregnant women eight times. Two investigators examined each patient twice in random order upon two successive days. The diameter of the vessel was measured using planimetry on a magnified time-motion image of the diameter variations during the heart cycle, while the angle between the ultrasound Doppler beam and the vessel of interest was measured on the hard copy image. No systematic variation was found between observers, days, repeated observations or repeated readings of curves and images. The mean coefficient of variations was 5.6% for the quantitative flow per kilogram estimated fetal weight measured in the fetal descending aorta, 6.8% for the quantitative flow per kilogram estimated fetal weight measured in the umbilical vein and 9.8% for the Pulsatility Index. When the diameter of aorta was calculated as the mean of the maximal and the minimal diameter measured on the hard copy image, the mean coefficient of variation for the flow increased to 9.4%. In vitro tests of the Doppler instrument and the real-time scanner revealed a systematic overestimation of Doppler measured flow of only 4.4% compared with the true flow, and a real-time scanner underestimation of vessel diameter of only 1.1%.     

Blood flow in the fetal descending aorta; intrinsic factors affecting fetal blood flow, i.e. fetal breathing movements and cardiac arrhythmia

An ultrasonic method combining real-time ultrasonography and pulsed Doppler technique was used for the examination of blood flow in the fetal descending aorta. The mean aortic blood flow velocity in the last trimester of normal pregnancies was 29.0 cm/s; the peak maximum velocity 97.3 cm/s and the mean blood flow 238.4 ml/min/kg. The blood flow velocity did not change significantly with gestational age, the aorta diameter showed a linear growth. During labour, the aortic blood flow in undistressed fetuses was not different from the flow recorded during late pregnancy. Fetal breathing movements modulate the flow velocity signals in the descending aorta, the inferior vena cava and the umbilical vein of the fetus; therefore, when quantifying fetal blood flow, only periods without fetal breathing movements should be considered. A group of fetuses with various types of cardiac arrhythmias was examined. Postextrasystolic potentiation was found to be present already during intrauterine life. The present method enables quantitative evaluation of the hemodynamic effects of cardiac arrhythmias.     

Fetal arterial Doppler studies in twin-twin transfusion syndrome.

This cross-sectional study investigates the circulatory profile of the donor and recipient fetuses in pregnancies with twin-twin transfusion syndrome manifested by acute polyhydramnios during the second trimester of pregnancy. Doppler investigations of the umbilical arteries and of the fetal descending thoracic aortas and middle cerebral arteries were performed in both fetuses of 27 pregnancies with twin-twin transfusion syndrome at 18 to 25 (mean, 21.7) weeks' gestation. Significant differences from normal values were increased umbilical artery pulsatility index and decreased aortic mean velocity in both donor and recipient fetuses, decreased middle cerebral artery pulsatility index in recipients and decreased middle cerebral artery mean velocity in donors. Increased umbilical artery pulsatility index in some donor and recipient fetuses may be the consequence of abnormal placental development and polyhydramnios-related compression, respectively. Doppler findings in the fetal circulation are compatible with hypovolemia in the donor and hypervolemia with congestive heart failure in the recipient.     

Doppler assessment of the normal early fetal circulation.

Combined transvaginal and transabdominal Doppler ultrasound allows recording of fetal intra- and extracardiac flow velocity waveforms in late first- and early second- trimester pregnancies. At 10-12 weeks, end-diastolic flow velocities were always absent in the fetal descending aorta and umbilical artery, but were present in over half of the intracerebral artery waveforms. The pulsatility index in the three vessels decreased significantly with advancing gestational age, suggesting a reduction in fetal and umbilical placental vascular resistance. Peak velocities during atrial contraction (A-wave) were nearly twice as high as those during early diastolic filling (E-wave), reflecting low ventricular compliance. Continuous forward flow in the umbilical vein was associated with a pulsatile systolic and diastolic forward flow in the ductus venosus. Retrograde flow was only present in the inferior vena cava.     

Doppler ultrasound in the fetus: a review of current applications

With the recent introduction of Doppler pulsed and continuous wave as well as color-flow mapping, assessment of blood velocity in the human fetus has added a new dimension to fetal assessment. Although investigators initially examined blood flow in the descending aorta and umbilical vein, there was a wide variation in normal values because of the difficulty of accurately measuring the area of the vessels, a requirement for computation of blood volume. Using duplex sector scanners, velocity and blood flow have been obtained from the right and left ventricular chambers and aortic and pulmonic outflow tracts. At the present time the clinical application of the latter measurements is still under investigation. Doppler color-flow mapping appears to be promising for elucidation of abnormal flow in fetuses suspected of having structural and/or functional cardiovascular disease. Because of the difficulty in computing volume flow due to the above mentioned factors, more recently investigators have examined angle independent parameters of blood velocity from the aorta, carotid and umbilical arteries in an attempt to quantify peripheral resistance. This latter technique appears to be promising for elucidation of placental pathology as is found in a number of fetal and placental diseases.     

Antenatal fetal blood flow in the descending aorta and in the umbilical vein and their ratio in normal pregnancy

One hundred and sixty sequential measurements of umbilical vein blood flow (UVBF) and descending aorta blood flow (DABF) in normal fetuses were performed in utero by the duplex real-time ultrasound and pulsed Doppler technique. Throughout pregnancy from 26 to 41 weeks the blood flow velocity of the umbilical vein (UV) and the descending aorta (DA) remained relatively constant, while the diameter of UV and DA, UVBF and DABF increased with advancing gestational age. Moreover, the UVBF-to-DABF ratio was nearly constant, with a mean value of 64% from 26 weeks onwards. The DABF-to-abdominal area (DABF-to-AA) ratio was constant throughout pregnancy, with a mean value of 7.0 ml/min/cm2. We call these two constants the umbilical-aortic index and descending-aortic index. These relationships may be of help in the diagnosis of intrauterine growth retardation and other fetal disorders.     

Fetal and uteroplacental blood flow in prolonged pregnancies. A clinical study.

In a prospective study during 1 year, 102 women with prolonged pregnancies (more than 294 completed gestational days) were followed with serial pulsed Doppler blood flow examinations every 2nd day, either to spontaneous onset of labor (n = 82) or to induction of labor due to subsequently occurring complications (n = 20). Mean aortic blood velocity did not change significantly with gestational age beyond 294 days either in fetuses with normal outcome or in fetuses that developed asphyxia at birth. The flow velocity waveforms in the descending aorta, the umbilical artery, the common carotid artery and the uterine artery did not change significantly compared to the values at term. Abnormal flow velocity waveforms in the fetal descending aorta, umbilical artery or uterine artery had no significant relationship to fetal asphyxia. Absence of diastolic flow velocities was not found in any of the vessels examined, indicating that the fetuses did not suffer from chronic hypoxia in utero and that aging of the placenta did not alter fetal and uteroplacental blood flow. Notching of the aortic flow velocity waveform was a common finding among prolonged pregnancies. The hemodynamic implications and consequences of this phenomenon are discussed.     

Time domain measurement of blood flow in the human fetal aorta during normal pregnancy.

OBJECTIVE: To evaluate blood-volume flow-rate measurement in the fetal descending thoracic aorta using a non-invasive, non-Doppler, ultrasound technique. METHODS: This was a cross-sectional, observational study. Volume flow measurements were obtained from the descending thoracic aorta in 59 human fetuses between 20 and 40 weeks' gestation. These were uncomplicated pregnancies that resulted in the live births of appropriately grown infants. The measurements were obtained using a time domain processing technique: color velocity imaging quantification (CVI-Q). RESULTS: The blood-volume flow rate increased consistently from the second trimester until term. The mean values ranged between 100 mL/min at 20 weeks' gestation and approximately 350 mL/min at term. The normalized (weight-adjusted) volume flow rates decreased with increasing gestation, from a maximum of 626 mL/min/kg at 23 weeks to a minimum of 45.6 mL/min/kg at 37 weeks. CONCLUSIONS: The regular measurement of blood-volume flow in the descending fetal thoracic aorta is feasible using CVI-Q. Although there is a considerable learning curve, with adequate training there are potential clinical applications for this non-Doppler technique. However, limitations exist with the currently available technology for clinical use in fetal vascular studies.     

Fetal Doppler velocimetry at high altitude.

OBJECTIVES: To determine impedance and velocity characteristics of the fetal circulation using Doppler ultrasound, at extremely high altitude (4300 m) in the Peruvian Andes compared to an ethnically similar population at sea level. METHODS: This was a cross-sectional study of 196 women resident at high altitude (Cerro de Pasco, 4300 m above sea level) and 196 women resident at sea level (Lima) with normal singleton pregnancies. Pulsatility index (PI), maximum velocity (Vmax) and minimum velocity (Vmin) in the umbilical artery, the descending aorta, the middle cerebral artery and the ductus venosus were compared between the two populations using fractional polynomial regression analysis. RESULTS: The PI was higher at high altitude than at sea level in the umbilical artery (regression coefficient = 0.112, P < 0.001), and not significantly different in the descending aorta, middle cerebral artery and ductus venosus. Vmax was lower at high altitude than at sea level in all three arterial vessels assessed; Vmin was lower in two: the umbilical artery and the descending aorta. The high-altitude/sea-level ratios for umbilical artery Vmax and Vmin were 0.93 and 0.82, respectively (P < 0.001 for each), the ratios for descending aorta Vmax and Vmin were 0.93 and 0.89, respectively (P = 0.003 and P < 0.001, respectively), and the regression coefficient for the middle cerebral artery Vmax was -2.844 (P = 0.003). There was no significant difference in the middle cerebral artery Vmin or in the ductus venosus Vmax and Vmin. CONCLUSIONS: Despite the lower ambient oxygen at high altitude and an increase in umbilical artery PI, the fetal circulation does not exhibit a 'brain sparing effect'. This and the overall decrease in blood flow velocities in the fetal circulation at high altitude may be due to the increased fetal hematocrit, which will result in increased blood viscosity.     

Changes in fetal hemodynamics with ritodrine tocolysis.

OBJECTIVE: To evaluate the effect of ritodrine on the fetal cardiovascular system. METHODS: Cardiac and extracardiac Doppler waveforms were recorded in 12 fetuses prior to and during ritodrine therapy used for preterm labor. Maternal and fetal heart rates, the Doppler pulsatility indices of the umbilical artery, middle cerebral artery, descending thoracic aorta and renal artery, and time velocity integrals of the atrioventricular valves and the ductus arteriosus, were measured. RESULTS: Ritodrine infusion caused an increase in maternal and fetal heart rates, the left cardiac output as measured by the product of time velocity integral and heart rate, and the pulsatility index of the middle cerebral artery, and a decrease in the pulsatility index of the umbilical artery. CONCLUSIONS: Ritodrine infusion may alter placental and cerebral blood flow and may have a selective effect on the left side of the heart.     

Retrograde net blood flow in the aortic isthmus in relation to human fetal arterial and venous circulations.

OBJECTIVES: To characterize changes in the human fetal arterial and venous circulations associated with retrograde aortic isthmus net blood flow. METHODS: Study groups consisted of fetuses with placental insufficiency and/or fetal growth restriction and either antegrade (Group 1; n = 18) or retrograde (Group 2; n = 11) net blood flow in the aortic isthmus. The control group comprised 31 fetuses in uncomplicated pregnancies. Pulsatility indices of the umbilical, middle cerebral and proximal pulmonary arteries and the descending aorta, and pulsatility indices for veins of the ductus venosus and inferior vena cava were calculated. Right and left ventricular fractional shortenings were ascertained. The coronary artery blood flow was visualized and the presence of tricuspid regurgitation was noted. RESULTS: In the study groups, the umbilical artery and descending aorta pulsatility indices were significantly higher (P < 0.05), and those of the middle cerebral artery lower (P < 0.001), than in the control group, with no difference between the two study groups. The proximal pulmonary artery pulsatility index was significantly higher in Group 2 (P < 0.001) than in Group 1 and the control group. In Group 2, the right ventricular fractional shortening was significantly lower (P < 0.01) than in Group 1. Coronary artery blood flow was visualized significantly more often (P < 0.03) and tricuspid regurgitation was present more frequently (P < 0.003) in Group 2 than in Group 1. In Group 2, the ductus venosus pulsatility index for veins was significantly higher than in Group 1 (P < 0.01) and the control group (P < 0.01), with no difference in the inferior vena cava pulsatility index for veins. CONCLUSIONS: Fetuses with retrograde aortic isthmus net blood flow demonstrate a rise in right ventricular afterload and increased pulsatility in ductus venosus blood velocity waveforms.     

Doppler evaluation of fetal hemodynamics

Human umbilical hemodynamics was investigated in 27 pregnant patients with gestational ages from 29 to 43 weeks using a pulsed Doppler flowmeter. The signals were analyzed by Fast Fourier Transform using a minicomputer. Coherent averaging of the signals, along with the computation of maximum velocity, mean velocity and the first moment of Fourier spectra about the zero frequency axis were performed. Hemodynamic indices were measured utilizing a comprehensive feature characterization technique of the blood flow velocity waveform. It was observed that coherent averaging reduced spectral variance thus relatively simplifying the analyses. The Pulsatility Index based on the first moment of the Fourier spectra indicated a higher umbilical flow change during the fetal cardiac cycle, than that predicted by the peak velocity Pulsatility Index. We also observed a high Pulsatility Index in association with three cases of oligohydramnios indicating the possibility of an elevated placental circulatory impedance. Variation in the peak velocity up to 20% was also noted with fetal breathing movements. Further studies are necessary to determine the clinical relevance of these findings.     

多普勒超声对高原地区胎儿脐动脉血流的研究

目的运用多普勒超声测量高原地区正常妊娠胎儿脐动脉血流,观察高原环境对胎儿脐带胎盘循环的影响。方法随机选取在外院(海拔2300 m)进行产前检查的正常妊娠孕妇103例为高原观察组,随机选取同期在我院(海拔44 m)进行产前检查的正常妊娠孕妇216例为平原对照组。高原组与平原组均按照孕周分为五组:20~23+6周、24~27+6周、28~31+6周、32~35+6周、36~40周,运用多普勒超声对两组胎儿脐动脉血流搏动指数(PI)、阻力指数(RI)、收缩期峰值流速与舒张末期流速比值(S/D)进行测定。结果孕中晚期五组中,高原组与平原组正常妊娠胎儿PI、RI和S/D差异均无统计学意义(P>0.05)。结论海拔2300 m高原地区正常妊娠胎儿脐带胎盘循环阻力与平原地区无明显差异。     

Ductus venosus blood velocity and the umbilical circulation in the seriously growth-retarded fetus.

Based on the assumption that the ductus venosus is regulator of oxygenated blood in the fetus, the present study investigated the blood flow velocity of the ductus venosus in relation to the umbilical circulation in the that seriously growth-retarded fetus. The study group of 38 fetuses (gestational week 17-39) had no chromosomal aberrations or structural malformations and had an ultrasonographic biometry of < 2.5th centile and birth weight of 相似文献   

Reverse flow in the human fetal descending aorta as a sign of severe fetal asphyxia preceding intrauterine death

Flow velocity waveforms (FVW) in the descending part of the human fetal aorta were monitored in 35 cases of severe intrauterine growth retardation (IUGR) showing pathological intrauterine circulation (end-diastolic block). In 5 of the 35 fetuses, reversed aortic flow was observable during diastole and the simultaneously recorded CTG findings were pathological. All 5 fetuses having this circulatory sign died in utero within the next day. Controls were 260 healthy fetuses with normal growth curves; no end-diastolic block or reverse flow was found in this group. Diastolic reverse flow in the fetal descending aorta appears to reflect severely altered circulation preceding imminent fetal death.     

Nonuniformity of fetal umbilical systolic/diastolic ratios as determined with duplex Doppler sonography

Fetal umbilical arterial systolic/diastolic (S/D) ratios are said to be uniform throughout the umbilical cord. We evaluated this phenomenon by performing duplex Dopper on 73 fetuses at three sites in the umbilical cord: near the placental origin, in the free-floating midcord, and near the fetal abdominal wall insertion. Twenty-one of 73 fetuses examined (29%) demonstrated pronounced nonuniformity of the S/D ratios with a discrepancy of 1 or greater between any two S/D measurements. Studies demonstrated the highest S/D ratios at the fetal abdominal wall and lowest S/D ratios at the placental origin. These findings suggest that umbilical arterial flow velocity waveforms (FVWs) and S/D ratios are not uniform throughout the cord in all patients. The significance of this finding is uncertain; however, this variability in umbilical cord flow velocity waveforms stresses the importance of identifying the portion of the umbilical cord from which S/D ratios are obtained.     

Venous Doppler velocimetry in the surveillance of severely compromised fetuses.

OBJECTIVE: To investigate whether venous Doppler velocimetric signs of cardiac decompensation might predict fetal demise in severely compromised fetuses. MATERIAL AND METHODS: This was a prospective study involving 154 growth-restricted fetuses, 37 of which were found to have reversed flow in the umbilical artery (BFC III). Doppler velocimetry of the right hepatic vein and ductus venosus were investigated serially and the presence of umbilical venous pulsations also registered. Only the final examination prior to birth or fetal demise was accepted for analysis and related to obstetric outcome defined as gestational age at birth, birth weight and perinatal mortality. In cases of BFC III the venous velocimetry of 15 nonsurviving fetuses was compared to that of the 22 survivors. RESULTS: There was a significant correlation between venous blood velocity and placental vascular resistance. In the right hepatic vein there was a significant decrease in peak systolic and end-systolic velocities and an increase of maximum velocity during atrial contraction and pulsatility (P < 0.05). A decrease of all velocities and increase of pulsatility were noted in the ductus venosus (P < 0.05). A reversed flow in the ductus venosus was found in 9/37 fetuses and double umbilical venous pulsations in 16/37 fetuses. However, the hepatic vein seemed to be a better predictor of impending mortality than the ductus venosus. Changes in diastolic venous blood velocity and a double pulsation in the umbilical vein were closely related to perinatal mortality, although these parameters did not provide a useful threshold to optimize the timing of delivery. CONCLUSION: Diastolic venous velocimetry changes significantly in severely compromised fetuses. These changes might be of great clinical value in deciding on the timing of delivery to minimize damage to the fetus and newborn.     

Ventriculovascular physiology of the growth-restricted fetus.

OBJECTIVES: To examine the mechanisms by which intrauterine growth restriction may influence later cardiovascular risk by comparing the ventriculovascular physiology of gestational age- and weight-matched growth-restricted and normal fetuses. DESIGN: A prospective longitudinal observational study of 20 normal fetuses studied from 20 weeks to term at monthly intervals was compared with a growth-restricted cohort examined in the interval between diagnosis and delivery. The last values before delivery of the growth-restricted cohort were compared with the normal cohort in two analyses matched for weight and for gestation. Arterial and venous vessel wall physiology and aortic pulse wave velocity were examined longitudinally in the thoracic descending aorta and inferior vena cava using an ultrasonic phase-locked echo-tracking system. Serial echocardiographic examinations were performed assessing structure, ventricular dimensions, function and Doppler flows. RESULTS: There was a linear increase in cardiac preload and relative pulse amplitude in the inferior vena cava with gestation. In normal fetuses, the aortic pulse wave velocity, maximum incremental and late decremental velocities increased with gestation whilst the relative pulse amplitude decreased reflecting falling distal impedance. In both age- and weight-matched analyses, the growth-restricted fetuses showed significantly reduced values reflecting the chronic fetal ventriculovascular responses to increased placental impedance. Pulse wave velocity increased with gestation and was significantly less in the growth-restricted cohort. CONCLUSIONS: Growth restriction is associated with abnormal ventriculovascular physiology that represents a successful adaptive response to raised placental impedance and reduction in wall stress as evidenced by the lower fetal pulse wave velocity in growth-restricted fetuses. However, whilst fetal adaptive mechanisms may aid survival they may result in cerebral and vascular abnormalities that prejudice later cardiovascular health.