In the management of Rh isoimmunization, the Middle Cerebral Artery (MCA) Peak Systolic Velocity (PSV) is used to predict fetal anemia. What is the pathophysiological basis for increased MCA-PSV in fetal anemia, and at what MCA-PSV threshold is fetal blood sampling/intrauterine transfusion indicated?

• A Fetal anemia reduces blood viscosity and increases cardiac output, increasing cerebral blood flow velocity; MCA-PSV >1.29 MoM indicates significant anemia requiring intrauterine transfusion
• B Fetal polycythemia from transfusion increases viscosity and reduces MCA-PSV below 0.80 MoM
• C Fetal anemia causes cerebral vasospasm; MCA-PSV >1.50 MoM (multiples of median for gestational age) indicates moderate-severe anemia warranting fetal blood sampling ✓
• D MCA-PSV reflects intracranial pressure; values >1.80 MoM indicate hydrops fetalis requiring immediate delivery

Explanation

In fetal anemia (from Rh isoimmunization or other causes), compensatory mechanisms include: increased cardiac output, redistribution of blood flow to vital organs (brain, heart, adrenals—the 'brain-sparing' effect), and reduced blood viscosity (fewer RBCs). The combination of increased cardiac output, reduced viscosity, and preferential cerebral blood flow increases the peak systolic velocity in the fetal MCA. The Mari et al. (1995) study validated that MCA-PSV >1.50 MoM (multiples of median for gestational age, using the established Mari nomogram) has sensitivity ~100% and specificity ~88% for predicting moderate-to-severe fetal anemia (hematocrit <30%). This threshold is the standard indication for cordocentesis (fetal blood sampling) followed by intrauterine transfusion if fetal anemia is confirmed.

Reference: Williams Obstetrics, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.