Middle cerebral artery peak systolic velocity (MCA-PSV) Doppler is used to monitor the fetus of an Rh-sensitised mother. An MCA-PSV >1.5 MoM indicates moderate-to-severe fetal anaemia. Why does fetal anaemia increase the MCA-PSV?

• A Anaemia causes cerebral oedema that compresses cerebral vessels, increasing flow velocity
• B Elevated bilirubin in fetal haemolytic disease directly dilates cerebral arteries
• C Anaemia causes reduced blood viscosity and cardiac output redistribution — higher flow velocity in the cerebral circulation compensates for reduced oxygen delivery ✓
• D Fetal tachycardia in anaemia increases cardiac output proportionately, raising all arterial velocities equally

Explanation

Fetal anaemia reduces blood viscosity (fewer red cells) and triggers a compensatory increase in cardiac output (increased stroke volume and heart rate). The brain is a high-priority organ; cerebral blood flow is maintained and preferentially increased. The MCA-PSV rises because reduced blood viscosity allows faster flow velocity for the same cardiac output, and increased cardiac output further elevates flow velocity in cerebral arteries. This haemodynamic change precedes the development of hydrops. Mari et al. established MCA-PSV >1.5 multiples of the median for gestational age as the threshold for considering intrauterine transfusion.

Reference: Williams Obstetrics, 26th ed.

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Written and medically reviewed by the StethoPrep medical team.