Chapter 187 Doppler Flow Studies
THE CHALLENGE
Scope of the Problem: Of pregnancies, 3% to 12% are at risk because of gestations that extend beyond term and more may be compromised by maternal disease states that affect fetal health or placental function (e.g., hypertension, diabetes), resulting in abnormalities of fetal growth or amniotic fluid volume and other problems. Current applications of Doppler flow studies are generally limited to cases of fetal growth restriction.
TACTICS
Relevant Pathophysiology: The Doppler principle states that when energy is reflected from a moving boundary, the frequency of the reflected energy varies in relation to the velocity of the moving boundary. In clinical practice, this principle is used to determine the velocity of blood flow in vessels because the frequency of sound reflected from moving blood cells is slightly altered in proportion to the velocity of the blood flow (and the cosine of the angle of incidence).
During the cardiac cycle, blood flow within the fetal circulation is pulsatile, with the difference in flow during systole and diastole gradually declining with gestational age and other factors. In the umbilical artery, this systolic to diastolic (S/D) ratio decreases from about four at 20 weeks, to less than three at 30 weeks, and finally to around two near term. Much of this change is mediated by the health and function of the placenta, and when this is compromised, diastolic flow diminishes. In extreme cases of fetal–placental compromise, diastolic flow may be absent or even show reversal of flow direction. Absent end-diastolic flow is associated with significant fetal compromise. Babies with abnormal umbilical artery Doppler blood flow results have a significantly higher rate of cesarean delivery for fetal distress, longer stays in the neonatal intensive care unit, and increased neonatal morbidity regardless of whether they were of normal size or growth restricted.
When there is fetal anemia, the associated increased cardiac output and relatively lower blood viscosity result in increased blood flow in the middle cerebral artery. This flow can be measured and used to evaluate fetuses with alloimmunization. (Fetuses with blood flow greater than 1.5 times the median [multiples of median, or MoM] are correctly identified with anemia with only a 12% false-positive rate.) Increased middle cerebral artery blood flow has also been proposed as a marker for altered blood flow before other indicators of hypoxemia may be present.
Uterine artery blood flow increases from about 50 mL/min in early gestation to 500 to 750 mL/min by term. Doppler flow studies of the uterine artery have been used in an effort to predict the development of pre-eclampsia and other complications. Unfortunately, uterine artery Doppler flow velocity appears to have limited diagnostic accuracy in predicting pre-eclampsia, intrauterine growth restriction (IUGR), and perinatal death.
Strategies: Doppler flow studies may be used to assess blood flow in the umbilical blood vein and arteries, fetal brain, and fetal heart. A Doppler flow study is often used when a fetus has intrauterine growth restriction or abnormalities of amniotic fluid volume.
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