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This chapter focuses on the Doppler interrogation of the fetal circulation. Other chapters in this book focus on Doppler velocimetry of the uterine arteries and fetal echocardiography. Randomized clinical trials have demonstrated that Doppler examination of the umbilical arteries can reduce perinatal mortality, and therefore this technique is no longer considered investigational. Moreover, the detection of an abnormal Doppler velocimetry of the umbilical artery allows classification of small for gestational age (SGA) fetuses into those who have "placental disease" (defined as an abnormal umbilical artery Doppler waveform) and those who have other conditions that may be responsible for their small size. A fraction of SGA fetuses are diagnosed at term and often do not have any abnormalities of the umbilical artery Doppler, but may have alterations of the cerebral circulation (anterior cerebral artery or middle cerebral artery). Therefore, practitioners need to be familiar with the sampling, interpretation, and clinical significance of these vessels. Similarly, Doppler examination of the venous system (ductus venosus, inferior vena cava (IVC), and umbilical vein) are informative of cardiac function and identify a fetus at risk for impending in utero death. Finally, examination of the peak systolic velocity of the middle cerebral artery has become important in the assessment of fetal anemia. Therefore, this chapter reviews in detail the anatomy, physiology, Doppler waveform morphology, normal ranges, and the interpretation of an abnormal waveform in clinical practice.
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Umbilical artery Doppler velocimetry (UADV) has become an important clinical tool in obstetrical practice. UADV examines the impedance to blood flow in the placenta, and therefore is a test of the placental stages. The umbilical arteries are easy to sample because they are long, nonbranching, and surrounded by amniotic fluid.
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Anatomy of the Umbilical Arteries
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The umbilical arteries are branches of the internal iliac artery, which have a pelvic segment around the fetal bladder (Figure 11-1). Thereafter, the umbilical arteries have an upward trajectory inside the fetal abdomen as they travel to the umbilicus to become part of the umbilical cord. The vessels travel in the umbilical cord until the insertion of the placenta. An anastomosis of the umbilical arteries located approximately 3 cm from the placental insertion, acting as a pressure-equalizing system between umbilical arteries, can be demonstrated with ultrasound and is called "Hyrtl anastomosis."1,2
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The umbilical cord contains 2 arteries and 1 vein. The arteries carry deoxygenated blood from the fetus to the placenta. Such blood is enriched with oxygen in the villous tree, and then returns to the fetus through the umbilical vein. Therefore, the umbilical arteries contain blood with a lower Po2 and pH than that in the umbilical vein.
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Physiology of the Umbilical Arteries
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