Middle Cerebral Artery Doppler Velocimetry. Serial measurement of the peak systolic velocity of the fetal MCA is the recommended test for detection of fetal anemia (Society for Maternal-Fetal Medicine, 20 15a). he anemic fetus shunts blood preferentially to the brain to maintain adequate oxygenation. he velocity rises because of increased cardiac output and decreased blood viscosity. The technique is discussed
Fetus without anemia or with mild anemiac� nE 120 • Fetus with severe anemia .-u .�::100 u ..o� )� . ) 40n� �u
FIGURE 15-1 Doppler measurements ofthe peak systolic velocity in the middle cerebral artery (MeA) in 165 fetuses at risk for severe anemia. The blue line indicates the median peak systolic velocity in normal pregnancies, and the red line shows 1.5 multiples ofthe median. (Reproduced with permission from Oepkes 0, Seaward PG, Vandenbussche et al: Doppler ultrasonography versus amniocentesis to predict fetal anemia, N Engl J Med. 2006 Jul 13;355(2):156-164.) in Chapter 10 (p. 214) and requires training and experience (American College of Obstetricians and Gynecologists, 2016).
In a landmark study, Mari and coworkers (2000) measured the MCA peak systolic velocity serially in 111 fetuses at risk for anemia and in 265 normal control fetuses. he threshold value of 1.5 multiples ofthe median (MoM) for gestational age correctly identiied all fetuses with moderate or severe anemia. his provided a sensitivity of 100 percent, with a false-positive rate of 12 percent.
he MCA peak systolic velocity is followed serially, and values are plotted on a curve like the one shown in Figure 15-1. If the velocity is between 1.0 and 1.5 MoM and the slope is rising-such that the value is approaching 1.5 Movf-surveillance is generally increased to weekly Doppler interrogation. If the MCA peak systolic velocity exceeds 1.5 \10M and the gestational age is younger than 34 or 35 weeks, fetal blood sampling should be considered and followed by fetal transfusion if needed (Society for Maternal-Fetal Medicine, 2015a). he false-positive rate of MCA peak systolic velocity increases signiicantly beyond 34 weeks, due to the normal augmentation in cardiac output that develops at this gestational age (Moise, 2008; Zimmerman, 2002).
Amnionic Fluid Spectral Analysis. This test is included for historical interest. More than 50 years ago, Liley (1961) demonstrated the utility ofamnionic fluid spectral analysis to measure bilirubin concentration and to thereby estimate hemolysis severity. Amnionic luid bilirubin concentration was measured by a spectrophotometer and was represented as the change in optical density absorbance at 450 nm-10D450. The likelihood of fetal anemia was determined by plotting the 10D45o value on a graph that was divided into zones. hese zones roughly correlated with fetal hemoglobin concentration, and thus with anemia severity. he original Liley graph was valid from 27 to 42 weeks' gestation and was subsequently modiied by Queenan (1993) to include gestational ages as early as 14 weeks. However, the amnionic luid bilirubin level is normally high in midpregnancy, limiting the reliability ofthis technique.
Middle cerebral artery velocimetry is more accurate than 10D45o assessment and does not confer risks for increased alloimmunization associated with amniocentesis. It has replaced 10D45o assessment for this purpose.