Uterine blood low is estimated to rise from 50 mLlmin early in gestation to 500 to 750 mLlmin by term. he uterine artery Doppler waveform is characterized by high diastolic low velocities and markedly turbulent flow. Greater resistance to flow and development of a diastolic notch are associated with later development of gestational hypertension, preeclampsia, and fetal-growth restriction. Zeeman and coworkers (2003) also found that women with chronic hypertension who had elevated uterine artery impedance at 16 to 20 weeks' gestation were at greater risk to develop superimposed preeclampsia. However, the technique, best testing interval, and defining criteria for this indication have not been standardized. As the predictive value of uterine artery Doppler testing is considered to be low, its use for screening or for clinical decision-making is not recommended in either high-risk or low-risk pregnancies (Sciscione, 2009).
Doppler interrogation of the middle cerebral artery (MCA) has been investigated and applied clinically for fetal anemia detection and fetal-growth restriction evaluation. Anatomically, the path of the MCA is such that flow often approaches the transducer "head-on," allowing for accurate determination of low velocity (Fig. 10-52). he MCA is imaged in an axial view of the head at the base of the skull, ideally within 2 mm of the internal carotid artery origin. Velocity measurement is optimal when the insonating angle is close to zero, and no more than 30 degrees of angle correction should be used. In general, velocity assessment is not performed in other fetal vessels, because a larger insonating angle is needed and confers significant measurement error.
When fetal anemia is present, the peak systolic velociy is enhanced due to greater cardiac output and decreased blood viscosity (Segata, 2004). This has permitted the reliable, noninvasive detection of fetal anemia in cases of blood-group alloimmunization. Mari and colleagues (2000) demonstrated that an MCA peak systolic velocity threshold of 1.50 MoM could reliably identiy fetuses with moderate or severe anemia. As discussed in Chapter 15 (p. 303), MCA peak systolic velocity has replaced
FIGURE 10-52 Middle cerebral artery (MCA) Doppler. A. Color Doppler of the circle of Willis, demonstrating the correct location to sample the MCA. B. The waveform demonstrates a peak systolic velocity exceeding 70 cm/sec in a 32-week fetus with severe fetal anemia secondary to Rh alloimmunization.
invasive testing with amniocentesis as the preferred test for fetal anemia detection (Society for Maternal-Fetal Medicine, 2015).
MCA Doppler has also been studied as an adjunct in evaluation of fetal-growth restriction. Fetal hypoxemia is believed to result in increased blood flow to the brain, heart, and adrenal glands, leading to greater end-diastolic low in the MCA. his phenomenon, "brain-sparing," is actually a misnomer, as it is not protective for the fetus but rather is associated with perinatal morbidity and mortality (Bahado-Singh, 1999; CruzMartinez, 2011). The utility ofMCA Doppler to aid the timing of delivery is uncertain. It has not been evaluated in randomized trials or adopted as standard practice in the management of growth restriction (American College of Obstetricians and Gynecologists, 2015; Berkley, 2012).