Only 48% of patients with HLHS have a measured CPR greater than 1, whereas 95% of normal fetuses have a measured CPR greater than 1-suggesting lower cerebral vascular resistance in fetuses with HLHS. Normal CPR is less than 1, meaning that cerebrovascular resistance is greater than placental vascular resistance. The pulsatility index is the difference between systolic flow velocity and diastolic flow velocity, divided by the time-averaged flow velocity ( ), and is related to downstream vascular resistance (higher pulsatility index is associated with higher downstream vascular resistance). CPR is the ratio of the pulsatility index measured in the middle cerebral artery to the pulsatility index measured in the umbilical artery. reported this low resistance by measuring the cerebroplacental Doppler ratio (CPR) in a study of fetuses with CHD.
24,72,73įetal ultrasonography and Doppler ultrasonography of the circle of Willis have demonstrated abnormal responses of the cerebral vasculature in fetuses with systemic outflow obstruction, showing lower than normal cerebral vascular resistance. Relative cerebral hypoxia in utero may contribute to the neurodevelopmental delays seen among these patients. 70,71 Studies of patients with TGA, for instance, show that fetal blood with the lowest saturation is directed to the brain and blood with the highest saturation is directed inferiorly to the abdominal organs. In many complex structural heart defects, especially those with interrupted left-sided outflow, this beneficial streaming is not present, and oxygenated blood flow is directed toward the ductus arteriosus. In the normal fetal circulation the eustachian valve and the foramen ovale direct the most highly oxygenated blood to the developing fetal brain via the left ventricular outflow tract.
Ken Brady MD, in Critical Heart Disease in Infants and Children (Third Edition), 2019 Neurologic Injury in Utero
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