Determinants of the middle cerebral artery peak systolic velocity in the human fetus

Objective: The purpose of this study was to identify physiologic determinants of the peak systolic blood flow velocity (PSV) of the middle cerebral artery (MCA) in the human fetus. Study Design: MCA PSV was measured with pulsed wave Doppler ultrasound in human fetuses who underwent cordocentesis. Hemoglobin, hematocrit, and blood gas values were analyzed from umbilical venous blood, and the data were normalized for gestational age. Total oxygen content of fetal venous blood was calculated from oxygen saturation, hemoglobin value, and pO₂. Correlation and logistic regression analyses were performed to identify primary physiologic determinants of MCA PSV. Results: In 136 fetuses who underwent cordocentesis (predominantly for alloimmune disease), hematocrit, hemoglobin, and blood oxygen content correlated significantly with the MCA PSV (P < .01). Logistic regression modeling demonstrated that fetal hemoglobin content (odds ratio, 7.1; 95% CI, 3.71-13.7) and pCO₂, but not pO₂ or fetal blood oxygen content, accounted for increases in MCA PSV. Conclusion: Under physiologic circumstances, fetal hemoglobin, and not fetal oxygenation, primarily determines the middle cerebral artery peak systolic velocity.