compared with the umbilical artery is greater than 1 With hypoxia or inadequate cardiac output, there may be “cephalization” of flow, characterized by a ratio of resistance in the middle cerebral artery compared with the umbilical artery of less than 1 Arterial Duct The arterial duct plays a key role in the fetal circulation, allowing most of the deoxygenated blood to bypass the lungs, which are under high resistance in utero At 20 weeks’ gestation, about 13% of the combined cardiac output reaches the lungs, although this percentage increases to nearly 25% by 30 weeks’ gestation.73 The normal Doppler flow pattern for the arterial duct demonstrates the majority of flow in systole, with only a small proportion occurring in diastole The arterial duct should be large and unrestrictive in utero Indeed, constriction of the arterial duct leads to increased afterload on the right ventricle, which may be characterized by tricuspid regurgitation, pulmonary insufficiency, and right ventricular dysfunction.74 Multiple medications—including corticosteroids, high-dose aspirin, and prostaglandin synthetase inhibitors—have been shown to cause constriction of the arterial duct.75 In cases of severe constriction of the arterial duct, prompt recognition of this finding, discontinuation of causative agents, and early delivery may be critical to avoid intrauterine fetal demise Fig 8.6 demonstrates both the normal and a constricted arterial duct Doppler flow pattern FIG 8.6 Doppler spectral display of the arterial duct (A) Normal Doppler flow pattern with only a small amount of flow in diastole (B) Abnormal Doppler flow pattern with significantly increased diastolic flow in a fetus with constriction of the arterial duct See Videos 8.1 through 8.4 Umbilical Vein In the umbilical vein there is normally continuous forward flow at low velocity Reference values for the umbilical venous Doppler have been published previously.76–78 As in the case of the umbilical artery, tracings should be obtained during fetal apnea, since respiratory effort in the second and third trimesters may influence the umbilical venous Doppler flow pattern As central venous pressure increases, notching is seen at the end of diastole in the umbilical venous flow In severe cardiovascular compromise, absence of flow at enddiastole and venous pulsations may be noted.37 Fig 8.7 illustrates both the normal umbilical venous Doppler flow pattern and an abnormal pattern with venous pulsations in a fetus with heart failure FIG 8.7 Doppler spectral display of the umbilical vein (A) Normal low velocity flow (B) Venous pulsations in a fetus with heart failure Venous Duct The venous duct is a key site of shunting within the fetal cardiovascular system, enabling highly oxygenated blood within the umbilical vein to bypass most of the hepatic circulation and return to the heart via the inferior caval vein Absence of the venous duct may be associated with aneuploidy and congenital anomalies79 and can also lead to the development of hydrops fetalis if the umbilical vein bypasses the liver and inserts directly into the inferior caval vein.80 Resistance within the venous duct gradually declines over the course of gestation The normal Doppler flow pattern—comprising s, d, and a waves—is shown in Fig 8.8 The s wave corresponds to ventricular systole, the d wave to ventricular diastole, and the a wave to the nadir during atrial contraction Reference ranges for the venous duct over the course of gestation have been published.81–83 After 18 weeks’ gestation, flow in the venous duct should be all antegrade with atrial contraction However, as central venous pressure increases, first there is decreased flow, next there is diminished flow, and finally there is reversal of flow with atrial contraction in the venous duct (see Fig 8.8) Reversal of flow with atrial contraction may be seen in a variety of pathologic conditions, including the Ebstein anomaly, pulmonary atresia with intact atrial septum, complete heart block, and intrauterine growth restriction In these cases, central venous pressure is elevated FIG 8.8 Doppler spectral display of the venous duct (A) Normal Doppler flow pattern in the venous duct Note that blood flow is phasic but all antegrade below the baseline (B) Abnormal flow pattern with reversal of flow with atrial contraction in a fetus with hydrops fetalis and heart failure Atrioventricular Valves The Doppler flow pattern across the mitral and tricuspid valves reflects important information regarding diastolic function and ventricular compliance In contrast to mature myocardium, fetal myocardium comprises a greater amount of noncontractile elements,84 causing impaired relaxation Consequently the normal Doppler flow consists of a two-peak pattern with a smaller E wave, representing passive filling during early diastole, and a larger a wave, representing active atrial contraction In severe cases of diastolic dysfunction— such as in the recipient twin with twin-twin transfusion syndrome, or in ventricles compromised by endocardial fibroelastosis—the E and a waves may merge into a single peak Fig 8.9 illustrates the two-peak Doppler flow pattern