Andersons pediatric cardiology 164

well-being, and arterial and venous Doppler waveforms have become incorporated as standard measurements in surveillance of the high-risk pregnancy,60,61 with the latter shown to provide a more specific predictor of fetal compromise (Fig 6.9).62 Of the Doppler measurements used in evaluation, absence or reversal of end-diastolic flow in the descending aorta or umbilical artery of the fetus is seen first and may be tolerated for a period of weeks in the compromised growth-restricted fetus, but once changes are seen in the systemic veins, emergent delivery is often the most appropriate management.63 FIG 6.9 Series of umbilical artery (UA) and umbilical vein (UV) Doppler recordings illustrating worsening placental function (A) Doppler of normal flow in the UA showing prograde flow in diastole (B) Normal flow in the UV showing absence of venous pulsation (C) Absence of end-diastolic flow in the umbilical artery (D) Abnormal venous pulsations associated with fetal hypoxemia (E) Reversed end-diastolic flow in the umbilical artery signifying increasing placental resistance (F) Triphasic flow pattern in the umbilical vein When placental function is severely reduced, as in fetuses with restricted growth, increased placental resistance leads to a reduction in total delivery of arterial oxygen to the fetus because of the reduction in mean placental return, even though the content of oxygen of the umbilical venous blood is often near normal It is vital to examine other vascular beds in the human fetus as the fetal brain, heart, and adrenal glands respond to this pathologic state by drawing increased flow, thus requiring an increase in combined ventricular output to provide it In the human fetus, the brain is the largest organ, and the healthy responsive fetus is able to reduce cerebral resistance by arteriolar dilation This has led to the adoption of the ratio of the cerebral to placental resistances as both an indicator of poor fetal growth and an independent predictor of intrapartum fetal compromise.64 Flow in the Venous Duct The venous duct occupies a unique physiologic position as a regulator of oxygen in the fetoplacental circulation Animal studies first demonstrated streaming of oxygenated blood from the umbilical vein through the oval foramen into the left