Similar findings were made in color Doppler studies of human fetuses 42. This streaming effect appears to be supported by the crestlike caudal border of the foramen ovale 9. The considerably slower blood flow in the distal inferior vena cava forms a stream in the right ventral portion of the inferior vena cava that is directed into the right atrium and then into the right ventricle ( Fig. This streaming mechanism ensures an optimum oxygen supply to the brain and myocardium. The narrow ductal lumen causes marked acceleration of the blood flow, creating a stream in the left dorsal portion of the inferior vena cava with the preferential direction of flow through the foramen ovale into the left atrium and left ventricle. Studies in fetal lambs showed that approximately 50% of oxygenated umbilical venous blood flows through the ductus venosus. Studies in experimental animals have demonstrated the existence of two different directions of blood flow (streaming effect) in the thoracic portion of the inferior vena cava 9. The ductus venosus represents the first of three specific shunts in the fetal circulation and appears to be an important regulator in the distribution of oxygen-enriched blood. Besides the umbilical vein, inferior vena cava, and hepatic veins, Doppler examination of the ductus venosus has become a particular object of current scientific interest. With technical advances in ultrasound instrumentation and especially the advent of color Doppler sonography, it became possible to make a detailed evaluation of the fetal venous system. Although Doppler studies of volume flow in the intrahepatic umbilical vein were already being done in the early 1980 s, this method has not become widely established because of its poor reproducibility 11, 12, 18, 19, 20, 35, 36, 41. In recent years, there have been increasing numbers of Doppler ultrasound studies of the physiological and pathophysiological changes in the fetal heart and circulatory system, with special emphasis on the cardiac and venous vascular systems. In the past, scientific and clinical attention was focused primarily on examination of the uteroplacental vascular system and on the arterial system of the fetus. Since its initial description by Fitzgerald in 1977 13, the Doppler ultrasound assessment of the fetomaternal vascular system has become widely utilized in the surveillance of high-risk pregnancies.
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