Doppler velocity assessment of venous return in the human fetus

Studies in the human fetus are limited by the methods available for investigation. Pressure and volume flow measurements in the fetal cardiovascular system require invasive techniques that are not performed at present. However, information on fetal circulatory performance may be helpful in the evaluation of pathologic conditions. With the introduction of Doppler ultrasound non-invasive examination of the fetal vessels became possible. In the last decade cardiovascular research in the human fetus has focused on the study of arterial, cardiac and umbilical blood flows. Four factors mainly determine cardiac performance: (i) afterload (ii) cardiac contraction force (iii) heart rate {iv) preload. Examination of the factor afterload in the fetal circulation has been characterized by Doppler studies of the fetal descending aorta and umbilical artery in the second half of pregnancy (Marsal et al. 1984; Trudinger et al. 1985;Tonge 1987). The second factor, cardiac contraction force, is even more difficult to study in the fetus. Efforts have been made by a number of investigators (Maulik et al. 1985; Kenny et al. 1986; Reed et al. 1986; Allan et al. 1987), who all tried to quantify cardiac stroke volume and force by means of Doppler velocimetry at the level of the atrioventricular valves and in the outflow tracts. However, it was pointed out that the reproducibility of these data is disappointingly low and large within and between variation was documented (Beeby et al. 1991). The third factor is the fetal heart rate, which is relatively easy to obtain. Studies have shown that as a result of the Frank-Starling mechanism fetal heart rate changes within the normal heart rate range do not seem to considerably influence fetal cardiac output (Kenny et al. 1987; van der Mooren et al. 1991). Finally, little is known about the factor preload and the hemodynamics of the fetal venous vasculature, although it has become clear from animal experimental work (Rudolph and Heymann 1967; Rudolph 1983) that venous return is an important factor in cardiac functioning. With the presence of three shunts (foramen ovale, ductus arteriosus and ductus venosus) and the placenta as a third circulation venous blood flow and pressures in the normally developing fetus are significantly different from the physiologic situation in adults. In this thesis data are presented o~ Doppler venous cardiac inflow, in particular from the umbilical vein, ductus venosus and inferior vena cava in (i) the late first and early second trimester fetus and (ii) the late second and third trimester fetus. The inclusion of early pregnancy flow studies was based on the significant changes occurring at placental level around 13 to 14 weeks of gestation with emphasis on the process of trophoblast invasion, resulting in low resistance placental vascular dynamics (Pijnenburg et al. 1980; Jauniaux et al. 1991). The transvaginal approach now allows fetal flow velocity waveform recording as early as 9 to 10 weeks. Late pregnancy studies were mainly performed to establish the effect of fetal variables such as breathing movements, behavioural states and cardiac arrhythmias on fetal venous cardiac inflow. Obviously, proper interpretation of venous inflow velocity parameters is only feasible when related to other parameters of cardiovascular performance, such as afterload and heart rate

Ductus Arteriosus Doppler in der ersten Schwangerschaftshälfte

Zusammenfassend stellen wir fest, daß es möglich ist, Flußgeschwindigkeiten durch den ductus arteriosus ab der 11. SSW zu messen. Enddiastolischer Flow tritt an der 13. SSW auf, vermutlich aufgrund abnehmender Afterload. Sowohl eine Abnahme der peripheren Afterload (Plazenta) als auch eine Zunahme der rechtsventrikulären Kontraktilität und das Auswurfvolumen sind für den Anstieg aller Blutflußgeschwindigkeiten im ductus arteriosus verantwortlich

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 atria1 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. Copyrigh

Doppler flow velocity waveforms in late first‐ and early second‐trimester fetuses: reproducibility of waveform recordings

The objective of the study was to assess the reproducibility of Doppler flow measurements of the fetal umbilical vein and artery, descending and ascending aorta, mitral and tricuspid valve, pulmonary artery, inferior vena cava and ductus venosus in early pregnancy. In a cross‐sectional study, Doppler measurements were obtained in a total of 54 women at 11–16 weeks of gestation, at 2–5 different vascular sites, on 3–5 different occasions, at 5‐min time intervals. The total variance in the various flow velocity parameters was partitioned in a between‐subject and within‐subject component by analysis of variance, and, from these calculations, coefficients of variation in waveform recording were calculated. Flow velocity waveform recording was characterized by coefficients of variation in the range 2.2–5.7% except for the acceleration time (18.6–24.5%) and percentage reverse flow (8.8%). In conclusion, our data suggest that fetal flow velocity waveforms in early pregnancy demonstrate good reproducibility in the individual subject, while all parameters depict larger variabilities for between‐subject values. Acceleration time turned out to be poorly reproducible. If a single flow velocity waveform measurement is used for the future evaluation of clinical conditions in early pregnancy, one has to consider that normal values will display a rather wide range. Copyrigh

Presence of pulsations and reproducibility of waveform recording in the umbilical and left portal vein in normal pregnancies

Reproducibility and inter‐observer variability of intra‐ and extra‐abdominal umbilical venous flow velocity and left portal venous flow velocity as well as heart‐synchronous waveform pulsations in these vessels were studied in 23 women at 34–38 weeks of normal pregnancy. Limited reproducibility, expressed by large intra‐patient coefficients and limits of agreement between two observers, was established for all standardized recording sites. Pulsations, defined as negative venous deflections of at least 10% of the mean velocity, were demonstrated at all locations ranging from 19.6% of the measurements at the free‐floating loop of the umbilical vein to 78.4% of the measurements at the left portal vein. The present study shows that the limited reproducibility of venous flow velocity waveforms should be taken into consideration, and that the presence of pulsations can be demonstrated in normal late pregnancy. Copyrigh

Changes in inferior vena cava blood flow velocity and diameter during breathing movements in the human fetus

Breathing movements in the human fetus cause distinct changes in Doppler flow velocity measurements at arterial, venous and cardiac levels. In adults, breathing movements result in a momentary inspiratory collapse of the inferior vena cava vessel wall. The study objective was to quantify the inferior vena cava flow velocity modulation during fetal breathing movements and to evaluate possible inferior vena cava vessel diameter changes in normal third‐trimester pregnancies. We studied 57 women after oral administration of dextrose (50 g). In 40 fetuses (n = 19, 27–32 weeks and n = 21, 36–39 weeks), fetal inferior vena cava waveforms were obtained during apnea and fetal breathing activity. In 30 fetuses (27–39 weeks) inferior vena cava vessel diameter changes were studied using the M‐mode during apnea and breathing movements. Peak and time‐averaged velocities of inferior vena cava flow velocity waveforms showed a gestational age‐independent increase of 60–160% during breathing activity. A temporary inferior vena cava vessel wall collapse (range, 50–83%) was recorded, which was significantly different from vessel diameter changes during apnea (range, 11–19%). The marked increase of inferior vena cava flow velocities is due to a raised thoraco‐abdominal pressure gradient, which may cause a reduction in vessel size and additional volume flow into the right atrium. The significance of the caval index for recognition of elevated right atrial pressure in abnormal human fetal development needs further investigation. Copyrigh