Читать книгу Protocols for High-Risk Pregnancies - Группа авторов - Страница 79

The management of fetal anemia requires careful consideration of the etiological factors. The majority of cases will be due to RBC alloimmunization of the mother or parvovirus B19 infection.

Rh sensitization begins with identification of an isoimmunized patient from routine blood type and Rh, and antibody screening tests. When the antibody screen shows the presence of an antibody that places the fetus(es) at risk for fetal anemia, the patient needs to undergo serial screening with antibody titers. An alternative to serial antibody titers that is now more commercially available is the use of cell‐free (cf) DNA to see if the fetus carries the RBC antigen in question. If the antigen is absent in the fetus, no further testing is needed. If the antigen is present, the patient should be followed with antibody titers. Once a critical threshold has been reached by a specific titer of a given RBC antigen antibody, the patient must undergo evaluation for fetal anemia. Most hospital laboratories use either a 1:16 or 1:32 threshold cut‐off and it is essential that each practitioner knows the threshold for their particular hospital or laboratory. Once the critical threshold has been reached, the patient needs to undergo either (i) an amniocentesis for assessment of ΔOD450 in the amniotic fluid or (ii) MCA PSV assessment using pulsed‐wave Doppler velocimetry. If the latter is available, it should receive priority over the amniocentesis simply because it avoids the risk associated with the amniocentesis and has very good performance characteristics as a screening test for moderate to severe anemia. If an amniocentesis is performed and the ΔOD450 is in the high zone 2 or zone 3 of the Liley or Queenan curve, then that fetus must undergo fetal blood sampling for documentation of the anemia and transfusion (see Protocol 42). Alternatively, if the MCA PSV is used to assess fetal anemia, a 1.55 multiple of the median (MoM) value should be used as a threshold above which fetal blood sampling and transfusion are needed.

After one blood transfusion, the MCA PSV loses some accuracy and a different threshold for subsequent transfusion should be used (1.32 MoM has been suggested). The MCA PSV becomes increasingly less reliable for timing of subsequent transfusions and empiric intervals between transfusions are usually used: 7–10 days after the first transfusion, then two weeks until fetal bone marrow suppression is confirmed by Kleihauer–Betke stain and then three weeks thereafter. Administration of phenobarbital (30 mg PO TID) to enhance hepatic maturation can be considered at 34 weeks’ gestation or one week prior to delivery. Delivery of the anemic fetus receiving blood transfusion can generally be accomplished at between 36 and 37 weeks. If fetal blood sampling will be performed at a very preterm gestation, administration of betamethasone should be considered prior to the procedure.