In management of Rh isoimmunization, the middle cerebral artery peak systolic velocity (MCA-PSV) is used to monitor for fetal anemia. An MCA-PSV above which multiple of the median (MoM) indicates the need for cordocentesis and possible intrauterine transfusion?
- A >1.29 MoM
- B >1.75 MoM
- C >1.5 MoM ✓
- D >2.0 MoM
Explanation
MCA-PSV > 1.5 MoM (multiples of the median for gestational age) is the established threshold (Mari et al., 2000, NEJM) for diagnosing moderate-to-severe fetal anemia requiring consideration of intrauterine transfusion. At this threshold, the sensitivity for detecting moderate-severe anemia (hemoglobin deficit > 0.65 MoM) is approximately 100% with a false positive rate of 12%. The principle is that anemic fetuses develop a hyperdynamic circulation with increased cardiac output, which elevates MCA blood flow velocity detectable by Doppler. This has replaced invasive amniocentesis for ΔOD450 as the primary monitoring tool.
Reference: Williams Obstetrics, 26th ed.
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