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KEY POINTS
The peak systolic velocity (PSV) and the pulsatility index (PI) are the commonly used clinical parameters for the evaluation of fetal cerebral blood perfusion.
Ultrasonic measurement of the middle cerebral artery peak systolic velocity (MCA PSV) can predict the existence of moderate-to-severe fetal anemia.
Under hypoxic conditions, the fetus redistributes its flow toward the brain, a phenomenon called brain sparing.56 This brain sparing effect is reflected in the MCA flow that shows decreased pulsatility at Doppler evaluation. Decreased pulsatility, which reflects low cerebral resistance to blood flow, constitutes a late manifestation, with acceptable specificity but low sensitivity for clinical applicability.
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The brain is not only the most complex and vulnerable fetal organ but is also the most difficult organ to study. The pioneering work of Rudolph et al was performed on a sheep model and set the basis for fetal brain hemodynamics in the 1970s.1 They found that the fetal brain may suffer from hypoxic damage in utero and that the hemodynamic reaction to hypoxemia is gestational age–dependent and exhibits very different patterns of reactions from those observed in extrauterine life. However, these important observations could not be validated in the human fetus because of the study limitations. Hence, no clinical implications were available until the development of the Doppler ultrasound technique for transcutaneous measurement of human fetal blood flow in the 1980s.2
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The introduction of Doppler ultrasound measurements enabled the assimilation of hemodynamic research into clinical practice. Doppler ultrasonography became an invaluable obstetric tool and has been in clinical use for over 35 years. It is used to assess both the fetal and placental circulation. Doppler enables clinicians to facilitate the diagnosis and monitoring of important conditions such as fetal growth restriction (FGR), fetal anemia, and complications of monochorionic twins.
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Commonly used Doppler assessments in obstetrics include but are not limited to the umbilical artery, middle cerebral artery, ductus venosus, and uterine arteries. This chapter addresses the recent use of Doppler evaluation of the fetal brain as a diagnostic, prognostic, and management tool in the developing fetus.
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DEVELOPMENT OF BRAIN VASCULARIZATION
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The development of the circulatory system of the fetal brain begins with the formation of six pairs of primitive branchial arch arteries at the 1.3 mm embryonic stage. Subsequently, vasculogenesis (differentiation of hemangioblasts into angioblasts) and angiogenesis (formation of new vessels) occurs. At 8 to 9 weeks gestational age, these vessels develop into the adult-like structure of the circle of Willis with its anterior, middle, and posterior arteries.3
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Ultrasonographic demonstration of brain blood flow in the first trimester is possible (Figure 22–1) and has been studied extensively.4 Some words of caution should be added regarding the safety aspects of such examinations. Most of the available epidemiological evidence on ultrasound safety is derived ...