Читать книгу Protocols for High-Risk Pregnancies - Группа авторов - Страница 76

As previously mentioned, fetal anemia resulting from maternal alloimmunization (except anti‐Kell) was historically detected by amniocentesis and assessment of the ΔOD450 and then confirmed by fetal blood sampling. Assessment of the MCA PSV has replaced the ΔOD450 as the gold standard test for fetal anemia. The MCA PSV has greater sensitivity for moderate to severe anemia than does the ΔOD450. Moreover, the ΔOD450 is not useful in Kell sensitization because fetal anemia in this condition is caused by bone marrow suppression rather than hemolysis and thus does not lead to RBC breakdown products in the amniotic fluid normally detected by the ΔOD450.

One of the resultant pathophysiological features present in Rh disease is a reduction in the viscosity of the fetal blood, which is secondary to a lower hematocrit. This results in an increase in the velocity of blood flow, which can be detected by pulsed‐wave Doppler velocimetry. One of the vessels branching off the circle of Willis is the MCA, which lends itself to interrogation by Doppler because of its location and because the paired middle cerebral arteries carry 80% of the cerebral blood flow. The MCA is first identified with the use of color flow Doppler. With an angle of insonation of 0° (or less than 30° with angle correction), pulse‐wave Doppler velocimetry is used to obtain the flow velocity waveform. From the flow velocity waveform, one can determine the PSV across serial peaks of the systolic component of the flow velocity waveform profile and obtain a mean systolic peak velocity of the MCA flow waveform. Nomograms are available for PSV in the MCA across gestation. Web‐based MCA PSV multiples of the median (MoM) calculators for a given gestational age are also available at perinatology.com.

When the MCA PSV surpasses the threshold of 1.55 MoM, there is a high risk of moderate to severe anemia in the fetus. The original report on this technique for assessing fetal anemia showed that a cut‐off 1.55 MoM has a sensitivity of 100% and a negative predictive value of 100% for moderate to severe anemia. Subsequent studies have shown similar favorable performance characteristics of this screening test. Once this value is surpassed, the fetus can undergo blood sampling to determine the actual fetal hematocrit and determine if a transfusion is necessary. Using Doppler to assess the MCA PSV and identifying anemia in the fetus in this fashion avoids the standard invasive procedural risks of amniocentesis (needed for the ΔOD450) including exacerbation of the Rh sensitization. The MCA PSV measurement has also been used to guide management in cases of anemia due to infections such as parvovirus, loss of a co‐twin in monochorionic pregnancies, and in TAPS.