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.

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