Agenesis of the ductus venosus and its correlation to hydrops fetalis

Absence of the ductus venosus is a rare vascular anomaly. We report a late onset of a hydrops fetalis seen in a fetus at 34 completed weeks of gestation. A persistence of the cranial parts of the left and right umbilical veins and of the paired cranial vitelline veins with an absent ductus venosus led to a bilateral hydrothorax, ascites and skin-edema. Postnatally the hydrops resolved within 7 days most probably due to the change from the fetal to the adult circulation. The abnormal venous system was confirmed by angiography. Agenesis of the ductus venosus can manifest in two different morphologic patterns: The umbilical vein drains exclusively into the left branch of the intrahepatic portal vein or the umbilical vein drains into the inferior vena cava or directly into the right atrium by-passing the liver completely. In both patterns, the preferential direction of the flow towards the foramen ovale is not present. While the first pattern leads to hyperperfusion of the liver parenchyma, the latter would result in reduced perfusion and oxygenation. Our findings suggest that agenesis of ductus venosus might induce hydrops fetalis. We conclude, that in every case of hydrops fetalis the venous system should be evaluated by ultrasonography prenatally and/or immediately postnatally.     

Three-dimensional color power imaging of the fetal hepatic circulation

OBJECTIVE: The purpose of this study was to describe the use of three-dimensional power Doppler ultrasonography to identify vascular congenital anomalies of fetal portosystemic and umbilical venous systems. STUDY DESIGN: In a prospective study, the hepatic and umbilical venous systems were examined in 390 fetuses with two-dimensional ultrasonography, color, and spectral Doppler imaging. Fetuses suspected to have anomalies of the portal system and ductus venosus were additionally examined with three-dimensional power Doppler ultrasonography. RESULTS: Vascular anomalies were identified in 8 fetuses (absent ductus venosus, n=4; direct connection between the umbilical vein and the right atrium, n=2; and direct connection between the umbilical vein and the inferior vena cava, n=2) out of the 310 in which the venous system could be adequately imaged (prevalence=2.6%). Three-dimensional power Doppler imaging showed the course of the umbilical vein, its relationship to the portosystemic circulation, and whether a ductus venosus was present. CONCLUSION: Three-dimensional power Doppler ultrasonography can be used to image normal fetal hepatic and portal circulation, as well as identify anomalies of the fetal portosystemic and umbilical venous systems.     

Agenesis of the ductus venosus that is associated with extrahepatic umbilical vein drainage: prenatal features and clinical outcome

OBJECTIVE: We sought to study the clinical and ultrasonographic findings and outcomes of fetuses with ductus venosus agenesis as the result of extrahepatic umbilical vein drainage. STUDY DESIGN: This was an observational study of 12 fetuses who were diagnosed with these anomalies between 1995 and 2001. RESULTS: The umbilical vein drained either directly into the right atrium (n = 7 fetuses), the inferior vena cava (n = 3 fetuses), or the iliac vein (n = 2 fetuses). Combined cardiac output (n = 8 fetuses) was nearly 2-fold increased to 891 +/- 210 mL/kg per minute. Other relevant findings were (1) considerable umbilical vein enlargement (12/12 cases; range, 8-13 mm) with an accelerated (peak, 0.53 +/- 0.1 m/s), (2) pulsatile Doppler flow pattern within its intra-abdominal course, (3) cardiomegaly (12/12 fetuses), (4) secundum atrial septal defects (5/12 fetuses), (5) extracardiac anomalies (5/12 fetuses), (6) polyhydramnios (4/12 fetuses), and (7) progressive heart failure (3/12 fetuses). Two fetuses died: 1 death was unexplained at 29 weeks of gestation; the other death from hydrops occurred after delivery at 32 weeks of gestation. So far, none of the survivors have had other long-term sequelae that were related to ductus venosus agenesis. CONCLUSION: Careful assessment of the umbilical venous return and the ductus venosus should be part of the routine evaluation of every fetus with heart failure and polyhydramnios.     

Reference values of fetal ductus venosus,inferior vena cava and hepatic vein blood flow velocities and waveform indices during the second and third trimester of pregnancy

Objective Our objective was to establish reference values for ductus venosus, inferior vena cava and hepatic vein flow velocities during ventricular systole (S-wave) and diastole (D-wave), the lowest forward velocity during atrial contraction (a-wave), the intensity-weighted mean flow velocity (Vmean) and different calculated indices.Methods Venous flow velocity waveforms were obtained from 329 singleton pregnancies at 20–42 weeks of gestation by pulsed-wave color Doppler. Reference values were constructed by means of a quadratic regression model after logarithmic transformation of original data.Results With advancing gestational age the peak velocity index for the vein (PVIV) and pulsatility index for the vein (PIV) decreased whereas blood flow velocities increased. Blood flow velocities were highest in the ductus venosus and lowest in the right hepatic vein. Values for PVIV and PIV were highest in the hepatic vein and lowest in the ductus venosus. During atrial contraction there was a blood flow towards the fetal heart in the ductus venosus, whereas in the inferior vena cava and in the hepatic vein blood flow was either in the opposite from the fetal heart (reverse flow), or there was absent flow (zero flow) or flow was towards the fetal heart (positive flow).Conclusions The reference ranges and calculated velocities established in this study may be utilized in studies dealing with the role of ductus venosus and inferior vena cava blood flow in fetuses with chromosomal abnormalities or congenital heart disease as well as hypoxic conditions. We speculate, that the reduction in PVIV and PIV with advancing gestational age may reflect a decrease in cardiac afterload as a result of maturation of diastolic ventricular function.     

Doppler assessment of the fetal venous system

This article describes the achievements in Doppler measurements of the fetal venous circulation with emphasis on the clinical impact of these techniques. In rhesus isoimmunization, fetal venous flow assessment gives useful information on the fetal haematologic condition and on the impact of blood transfusion. In first trimester fetuses, Doppler evaluation of the ductus venosus and umbilical vein could attribute to the detection of cardiac defects and/or chromosomal abnormalities. The inferior vena cava flow velocity waveform could be studied in cases of fetal arrhythmias to be able to diagnose the type of fetal heart rhythm disturbances. One of the more important applications of venous Doppler assessment is its use in the evaluation of the intrauterine growth retarded fetus, who is suffering from placental insufficiency. Decrease of the late diastolic flow component in the ductus venosus waveform and the presence of umbilical venous pulsations are distinct alterations, which have been detected before cardiotocogram deterioration occurs. The clinical possibilities of venous Doppler measurements are limited, and the use of the techniques requires intensive training.     

Phasic blood flow patterns in the common umbilical vein of fetal sheep during umbilical cord occlusion and the influence of autonomic nervous system blockade

The blood flow pattern in the common umbilical vein is under normal conditions nonpulsatile in contrast to the flow in the fetal inferior vena cava. We observed pulsatile flow patterns in the common umbilical vein of fetal lambs during changes in the fetal hemodynamic equilibrium. These pulsations may influence the mixing of oxygen-rich ductus venosus blood and oxygen-poor inferior vena cava blood. This study deals with the phasic changes in umbilical venous blood flow during cord occlusion. The experiments were performed in eight chronically instrumented fetal lambs between 114 and 133 days gestation (term 146 days). Umbilical venous blood flow was measured with an electromagnetic flow transducer around the intraabdominal common part of both umbilical veins. The fetuses were provided with catheter in the fetal abdominal aorta and with electrodes for monitoring arterial blood pressure and heart rate. Occlusion of the umbilical cord was performed by means of an inflatable balloon occluder around the total cord (occlusion time 20 to 90 seconds). Occlusions were performed in fetuses with an intact autonomic nervous system and after blockade of the alpha-adrenergic, beta-adrenergic or cholinergic part of the autonomic nervous system.     

Hemodynamics of the ductus venosus.

Although the ductus venosus has a similar function in human as in animal pregnancies (to regulate the shunting of oxygenated blood from the umbilical vein towards the left atrium), the amount of blood shunted in the human fetus seems to be less (25-40%) than in the animal (50%). The degree of shunting depends both on the resistance of the portal vasculature in the liver as well as the resistance of the ductus venosus itself. Neural and endocrine regulation plays a role in this distribution, as do fluid mechanical forces; blood viscosity and umbilical venous pressure are powerful determinants. There is a high degree of shunting at reduced umbilical venous pressure, and by increasing hematocrit, and viscosity, the distribution shifts from the liver to the ductus venosus. Additionally, the ductus venosus acts as transmission line in the opposite direction for the atrial pressure waves. Shape, viscosity, compliance, and particularly the diameter of the inlet are suggested to influence the pulsatility of the blood velocity at the ductus venosus inlet--and determine the degree of wave transmission into the umbilical vein. Occurrence of umbilical venous pulsation, an important diagnostic sign, is also dependent upon the size and compliance of the umbilical vein.     

Prenatally diagnosed portocaval shunt and postnatal outcome: a case report

Congenital absence of the portal vein (CAPV) is a rare anomaly in the form of a portocaval shunt, whereby the intestinal and splenic venous drainage bypasses the liver and drains directly into the systemic circulation. We report a case of CAPV diagnosed prenatally after the recognition of a dilated umbilical vein draining directly into a large inferior vena cava (IVC). The IVC then drained into the right atrium of a dilated, hyperdynamic heart. The ductus venosus could not be identified. Repeated postnatal scans showed a gradual disappearance of venous lakes in the region of the porta hepatis and a clear drainage of the splenic vein to the left renal vein and the superior mesenteric vein to the IVC. From birth up to twelve months follow-up there was no evidence of liver dysfunction, encephalopathy or liver lesions.     

Neonatal nucleated red blood cell counts in growth-restricted fetuses: relationship to arterial and venous Doppler studies.

OBJECTIVE: Elevated nucleated red blood cell count in neonatal blood and Doppler-detected circulatory decompensation in fetuses with intrauterine growth restriction are associated with hypoxemia. We sought to determine the relationship between the nucleated red blood cell count at birth and the circulatory status of fetuses with intrauterine growth restriction. STUDY DESIGN: Eighty-four fetuses with elevated umbilical artery pulsatility index values >2 SD above the gestational age mean and a subsequent birth weight <10th percentile were examined serially. Umbilical and middle cerebral artery pulsatility index, inferior vena cava and ductus venosus peak velocity index, and flow pattern in the umbilical vein (umbilical vein constant vs pulsatile) were recorded. Fetuses were grouped as follows, on the basis of the last examination before delivery: 1, elevated umbilical artery pulsatility index only; 2, middle cerebral artery pulsatility index >2 SD below the gestational age mean in addition to abnormal umbilical artery pulsatility index; 3, either peak velocity index >2 SD above the gestational age mean in the inferior vena cava and ductus venosus or pulsatile flow in the umbilical vein, or both. Nucleated red blood cells per 100 white blood cells were ascertained in a peripheral blood sample obtained within 1 hour of delivery with daily follow-up samples until the nucleated red blood cell count was <5/100 white blood cells. RESULTS: Groups 2 (median 38.5, range 1-273) and 3 (median 145, range 2-3180) had higher nucleated red blood cell counts than group 1 (median 8.5, range 1-270) (P <.05 and P <.005, respectively). The persistence of the nucleated red blood cell count elevation was also longer in groups 3 (median 4 days, range 1-19 days) and 2 (median 2. 5 days, range 1-7 days) than in group 1 (median 1 day, range 1-8 days). Neonates in group 3 also had lower platelet count, hemoglobin value, hematocrit value, and white blood cell count. The umbilical cord artery bicarbonate level was the strongest independent determinant of the peak nucleated red blood cell count and persistence of nucleated red blood cell elevation (r (2) = 0.27, P <. 001 and r (2) = 0.47, P <.0001). CONCLUSION: Increasing abnormality of arterial and venous flows in fetuses with intrauterine growth restriction is associated with increasing nucleated red blood cell count at birth. Metabolic acidemia rather than altered PO (2 ) associated with this circulatory state appears to be the main determinant of the rise in nucleated red blood cells.     

Doppler sonography of the fetal venous circulation

Venous Doppler sonography has been used for assessment of the fetal hemodynamics in the last 15 years. The velocimetries of the central fetal veins — umbilical vein (UV), ductus venosus (DV), hepatic veins (HV) and inferior vena cava (IVC) — reflect the cardiac function and its impairment due to changes in cardiac preload or afterload, and due to changes in the cardiac rhythm. It is possible to see the severity of the disturbance in cardiac function and venous Doppler is broadly used in the surveillance of pregnancies, which are disturbed by severe placental insufficiency, cardiac defects, arrhythmias, anemia, hydrops fetalis and hyper- or hypovolemia as in twin-to-twin transfusion syndrome. Herein we summarize the past, present and eventual future developments of venous Doppler sonography, reflecting the physiology and pathophysiology of the fetal venous circulation and describing the state of the art of its clinical application.     

Internal jugular vein blood flow in normal and growth-restricted fetuses

OBJECTIVE: To assess internal jugular vein blood flow patterns during the second half of pregnancy in normal and growth-restricted fetuses. METHODS: We did Doppler ultrasound studies of internal jugular veins and the inferior vena cavas longitudinally on 21 normal singleton fetuses from 20 weeks to term, and on eight growth-restricted fetuses with absent end-diastolic flow at the umbilical artery (UA). The three components of the venous flow velocity waveforms were used to calculate peak velocity ratio: Peak systolic velocity (S wave) minus reverse peak velocity (R wave) divided by peak velocity during early diastole (D wave) and velocity time integral ratio: systolic velocity time integral minus reverse velocity time integral divided by velocity time integral during early diastole. Statistical analysis of longitudinal measurements used K-related samples Friedman test; groups were compared with Mann-Whitney U test and chi(2) test. RESULTS: In normal fetuses we found significant increases in peak velocity ratio and velocity time integral ratio of internal jugular veins and the inferior vena cavas throughout gestation. The mean +/- standard deviation (SD) of the internal jugular veins peak velocity ratio (1.12 +/- 0.4 versus 1.46 +/- 0.15, P <.05) and velocity time integral ratio (1.1 +/- 0.2 versus 1.55 +/- 0.17, P <.05) were significantly lower in growth-restricted fetuses compared with normal fetuses at 28-32 weeks' gestation but inferior vena cava indices were not. None of the eight growth-restricted fetuses had umbilical venous pulsations or changes in inferior vena cava or ductus venosus blood flow patterns. All had arterial pH above 7.15 at birth. CONCLUSION: Growth-restricted fetuses with absent end-diastolic velocity in the UA have changes in internal jugular vein blood flow patterns that probably indicate increased cerebral blood flow, more evidence of redistribution of blood flow in growth-restricted fetuses that can be used to maintain them.     

Pulsations of the umbilical vein: pathophysiologic aspects and fetal outcome

Continuous forward flow to the fetal heart in the umbilical vein is a normal Doppler finding. Altered fetal hemodynamics can cause a pulsatile flow pattern in the umbilical vein. Pulsations in the umbilical vein were diagnosed in 14 pregnancies complicated by fetal hydrops, cardiac malformations, arrhythmia or severe intrauterine growth retardation. To document the fetal outcome, the results were analysed retrospectively. Compared with a normal Doppler group (N = 56), a significantly higher rate of perinatal death (Alpha < 1%), (64% vs 1.75%) was diagnosed, when pulsations in the umbilical vein were present. The Apgar-score was significantly lower (4.5 vs 8.4) (p < 0.0002) in the group with pulsations in the umbilical vein. There was no significant difference of pH between the newborns of the two groups. Eight fetuses developed hydrops. Thirteen fetuses had increased reverse flow in the inferior vena cava. The vena cava of the acardiac fetus could not be identified. The knowledge of the poor outcome and the pathophysiologic relationships of fetal hemodynamics may be useful in clinical management. Therefore Doppler examination of the umbilical vein should be performed in high-risk pregnancies.     

The assessment of fetal well-being by venous Doppler velocimetry

The assessment of fetal well-being by venous Doppler velocimetry, especially in cases of intrauterine growth restriction, has been growing in importance as a number of researchers have been improving their studies^1 - 5 as well as a consequence of better technologies in ultrasound equipment. Since different authors have studied the fetal blood circulation with color Doppler, initially focusing their attention on the arterial system^6 - 12, many questions and doubts have remained without a reasonable answer, especially concerning the optimal time at which to deliver these fetuses. Among these authors, some have expressed the opinion that all biophysical methods, including the cardiotocogram, should be used before the decision of fetus delivery should be made^13,14. It is possible, nowadays, to study several venous vessels, such as the ductus venosus, inferior vena cava, umbilical vein, portal vein, hepatic veins¹⁵, and, more recently^1,2, the cerebral transverse sinus. The purpose of this review is to describe the fetal circulation, the technique and rationale behind measuring venous Dopplers and to assess the clinical applications of venous Doppler velocimetry.     

Doppler measurements of blood flow velocity and waveforms in fetal venous circulation

Ultrasonic velocimetry of the fetal circulation became a very useful method in assessment of fetal well-being, especially in high risk pregnancies. There are many papers concerning on distribution and regulation of blood flow in umbilical artery, middle cerebral artery, and much less dealing with flow velocity waveforms in inferior vena cava and ductus venosus and their clinical significance. Fetal compromise is associated with significant alterations in the fetal arterial and venous circulation. Changes in venous Doppler waveforms develop due to increased afterload and perhaps myocardial failure in deterioration after arterial redistribution is established. Doppler investigation of the fetal venous circulation may play an important role in monitoring the redistribution and may help to determine the optimal time for delivery.     

Evaluation of ductus venosus and inferior vena cava by using multiple Doppler ultrasound parameters in healthy fetuses

Purpose  

The objective of this study was to describe multiple Doppler ultrasound parameters of ductus venosus and inferior vena cava in fetuses at gestational ages ranging from 22 to 38 weeks.     

Agenesis of the ductus venosus associated with direct umbilical venous return into the heart--case series and review of literature

OBJECTIVES: The purpose of our study was to review our own experience and the available literature on the prenatal diagnosis of absent ductus venosus associated with direct insertion of the umbilical vein into the heart. METHODS: A retrospective review of the database of a tertiary fetal cardiology centre. The literature was searched for cases with a prenatal diagnosis of such an umbilical venous anomaly. RESULTS: Between January 2000 and June 2003, nine fetuses were diagnosed as having absence of the ductus venosus with the umbilical vein directly draining into the heart. This represents the largest reported clinical experience of this anomaly. We also report, to our knowledge, the first prenatal diagnosis of insertion of the umbilical vein into the left atrium. Combining our series with those found in the literature, a total of 35 cases were identified. Cardiomegaly was reported in 25 of the 35 cases (71%). Structural cardiac abnormalities and other extra-cardiac anomalies were found in 13 of the 35 cases (37%). Hydrops was present or developed in 6 cases (17%). The outcome data were available for 33. The overall survival rate was 67% (22 of 33). CONCLUSIONS: Careful assessment of the ductus venosus and the umbilical vein should be a part of the evaluation of every fetus with unexplained cardiomegaly. All fetuses with abnormal connection of the umbilical vein should undergo a clinical and ultrasonographic assessment both in utero and after birth to exclude any cardiac and extra-cardiac abnormalities. During the prenatal period, serial ultrasound examinations are indicated and delivery is considered when there is evidence of progressive cardiovascular compromise.     

Umbilical vein pulsations: a physiologic finding in early gestation.

OBJECTIVES: The purpose of this study was to establish the incidence of umbilical vein pulsations in normal early pregnancies and to evaluate whether these pulsations are related to Doppler-measured vascular resistances in umbilical artery or central venous flow patterns. STUDY DESIGN: We performed a cross-sectional study on 257 uneventful pregnancies at 7 to 16 weeks of gestation by means of transvaginal color and pulsed Doppler ultrasonography. Blood flow velocity waveforms were recorded from the umbilical artery, umbilical vein and, in 48 cases, also from fetal inferior vena cava. RESULTS: Pulsations in the umbilical vein were evidenced in all cases until 8 weeks. From this gestational age on pulsations progressively disappeared, becoming completely absent at greater than or equal to 13 weeks. The incidence of pulsations in the umbilical vein was unrelated to umbilical artery pulsatility index values, the incidence of absent end-diastolic flow in umbilical artery, and fetal heart rate. In the inferior vena cava, reverse flow during atrial contraction was significantly higher (p less than or equal to 0.002) in cases with umbilical vein pulsations than in those cases without pulsations at a similar gestational age. CONCLUSIONS: Pulsations in the umbilical vein are physiologically present during the first trimester of pregnancy and seem to be related to inferior vena cava flow patterns.     

The ductus venosus

Until recently, our information on the ductus venosus was based on postmortem and experimental studies. The present review relates to the modern concept of this vein predominantly founded on clinical studies. Recent publications show that the blood distribution through the ductus venosus is particularly sensitive to changes in umbilical venous pressure, blood viscosity, and an active regulation of diameter of the entire ductus venosus. The mean fraction of umbilical blood shunted through the ductus is reduced from 30% to 20% during the second half of the human pregnancy, indicating that, during this period, the fetal liver has a higher priority than the shunting through the ductus venosus, apart from the compensatory redistribution needed during extreme challenges of placental compromize and hypoxemia. Additionally, the ductus venosus acts as a transmission line to the umbilical vein for pulse waves generated in the heart. These waves, reflecting cardiac function, are substantially influenced by the local variation of impedance and compliance.     

Venous Doppler in the sonographic surveillance of fetuses with supraventricular tachycardia.

OBJECTIVE: In animals and adult humans sustained supraventricular tachycardia leads to myocardial remodelling and dysfunction, persisting even after drug-induced cardioversion to sinus rhythm. This study was undertaken, to evaluate cardiac function in the human fetus by noninvasive determination of the degree of AV valve incompetence and venous blood flow, in order to enhance understanding of the pathophysiology of fetal supraventricular tachycardia. Furthermore, we wanted to determine the usefulness of these methods in the surveillance of these fetuses before and after drug-induced cardioversion. STUDY DESIGN: Eleven fetuses with supraventricular tachycardia between 24 and 35 weeks of gestation were studied. AV valve regurgitation and venous Doppler waveforms of the inferior vena cava and ductus venosus were evaluated before and after conversion to sinus rhythm. RESULTS: Three different groups of fetuses could be distinguished. The first group consisted of four fetuses with neither signs of hydrops nor AV valve incompetence. Venous indices normalized within one to four days (median 2.5 days) after conversion to sinus rhythm. The second group contained two fetuses with hydrops, but without AV valve incompetence. Their venous indices normalized at the day of conversion and 3 days later, respectively (median 1.5 days). The last group of five fetuses consisted of four fetuses with hydrops and AV valve regurgitation during supraventricular tachycardia. In one fetus with hydrops and supraventricular tachycardia the fetal heart rate was continuously decreased to a level of 160-190 beats/min under drug treatment, but no conversion to sinus rhythm occurred. The venous indices of these fetuses normalized within 12-42 days (median 27 days) after conversion. CONCLUSION: Our data suggest that in sustained fetal supraventricular tachycardia alterations of myocardial function similar to tachycardia-induced 'cardiomyopathy' occur. The severity of tachycardia-induced changes of cardiac function is reflected by the degree and persistence of AV valve incompetence, as well as by alterations of the venous blood flow pattern. Under clinical conditions, the latter can readily and well reproducibly be demonstrated by calculating the venous blood flow indices of the inferior vena cava and ductus venosus.     

Venous Doppler velocimetry in relationship to central venous pressure and heart rate during hypoxia in the ovine fetus.

OBJECTIVE: Characteristic changes in ductus venosus (DV) blood velocity and pulsations in the umbilical vein (UV) have been described during imminent fetal asphyxia. The aim of this study was to examine fetal venous blood velocity in relationship to pressure gradient across the DV during hypoxia in a fetal lamb preparation. METHODS: In general anesthesia, a cesarean section was performed on seven pregnant ewes, the fetus was exteriorized and put into a heated waterbath with uninterrupted umbilical circulation. Pressure measurements in the UV and inferior vena cava (IVC) were performed with the catheter tips on both sides of the DV. Fetal hypoxemia was induced by giving the ewe 12% oxygen in inhaling air. Pressure across the DV and Doppler velocimetry were repeatedly measured during hypoxemia. Blood velocity was recorded in the DV and UV by Doppler ultrasound. RESULTS: Before hypoxia the median pressure gradient across the DV was in systole 1 mmHg and 0.31 mmHg in end-diastole and during hypoxemia 1.5 mmHg and zero, respectively. The pressure difference across the DV was constant during hypoxemia irrespective of the presence of umbilical venous pulsations or heart rate. IVC-pressure was greatly influenced by fetal heart rate (FHR). A small but linear fall in systolic IVC pressure was seen with increasing FHR. In end-diastole the IVC pressure changed in a parabolic fashion, with increasing pressure during brady- and tachycardia. Pulsations in the UV also showed a parabolic relationship to FHR and central venous pressure. DV end-systolic and end-diastolic blood velocity changed during hypoxemia in direct relationship to FHR and central venous pressure, but without direct relationship to fetal blood gases. CONCLUSION: The pressure gradient across the DV is constant during hypoxemia. Changes in central and umbilical venous pressure are directly related to FHR. Umbilical venous and DV blood velocity changed in direct relationship to FHR and central venous pressure.