Fetal Aortic-cerebral Doppler Resistance Index Ratio: An Indicator of Physiologic Blood Flow Distribution Donald School Journal of Ultrasound in Obstetrics and Gynecology, July-September 2009;3(3):91-95 91 Fetal Aortic-cerebral Doppler Resistance Index Ra

Identical hemodynamic impedance and constant ratio of the fetal descending aorta and middle cerebral artery of uncomplicated pregnancies at term. Fetal aortic-cerebral Doppler resistance index ratio: An indicator of physiologic blood flow distribution. Objective: To interpret the physiologic fetal arterial blood flow distribution by relating the vascular impedance of the fetal descending aorta (DA) and middle cerebral artery (MCA) and to establish the reference ranges for the aortic-cerebral Doppler resistance index ratio (ACRI). Study design: Ninety-six patients with uncomplicated pregnancies were recruited for the cross-sectional assessment of the Doppler resistance index (RI) in the fetal DA and MCA between the 38rd and 40th weeks of gestation. The normal ranges of the ACRI were calculated. A cut-off value was designed to facilitate the clinical application of the ACRI. Results: Between the 38th and 40th weeks of gestation in normal pregnancies the ACRI of healthy fetuses is constant, the overall mean is: 1.062 (+/ – 0.087). A single cut-off value of 1.2 is recommended to assist separating normal and pathologic arterial blood flow patterns. Conclusion: The normal ACRI reflects the identical vascular resistance of the descending aorta and the cerebral vessels, which maintains the physiologic fetal central arterial blood flow. Additional clinical studies are necessary to assess the diagnostic efficacy of the abnormal ACRI (>1.2) as a potentially useful marker of the centralized arterial circulation indicating the early stage of fetal hypoxemi