Fetal Doppler: Umbilical Artery, Middle Cerebral Artery, and Venous System
Section snippets
Umbilical Artery
Umbilical artery angle independent indices (pulsatility index or systolic/diastolic (S/D) ratio) decrease with advancing gestation because of a decreased placental vascular resistance, which physiologically occurs with advancing gestation.1, 2 In pathologic conditions, such as in intrauterine-growth-restricted (IUGR) fetuses, the umbilical artery waveforms change and the angle-independent indices become abnormal (values above their reference ranges). These changes reflect an increased placental
Middle Cerebral Artery
Angle-independent indices differ among the different cerebral arteries.5 The middle cerebral artery is the most studied cerebral artery because (a) it is easy to sample; (b) it provides information on the cerebral blood flow in normal and IUGR fetuses; and (c) it can be sampled at an angle of 0° between the ultrasound beam and the direction of the blood flow.6 Therefore, for the middle cerebral artery we are able to determine angle-independent indices (the most used is the pulsatility index)
Umbilical Vein
After 15 weeks gestation, the umbilical vein normally has a continuous blood flow but becomes pulsatile in pathological cases, such as in IUGR and hydropic fetuses.8 For the umbilical vein we use a qualitative assessment: continuous versus pulsatile blood flow.
Atrioventricular Valves
The atrioventricular valves (mitral and tricuspid) are characterized by two peaks—the “E” wave that corresponds to the rapid filling of the ventricles and the “A” wave that corresponds to the atrial contraction. The “A” wave is taller than the “E” wave,9, 10 which may suggest that atrial contraction is important in the fetus, and is associated with stiffness of fetal cardiac chambers. With advancing gestation, the E/A ratio increases. By contrast, after birth, and also in the adult, the “E”
Ductus Venosus
Ductus venosus waveforms are characterized by two peaks, the S and D, followed by a nadir, the a wave (Fig. 1).12 Hemodynamically, these phases reflect the rapid chronologic change in pressure gradients between the umbilical vein and the right atrium. In appropriate-for-gestational-age fetuses, there is forward flow at the ductus venosus, and the pulsatility index for veins (S-D/a) decreases with advancing gestation. However, in IUGR fetuses, the pulsatility index increases, and in the most
Diagnosis of Fetal Anemia
The middle cerebral artery (MCA) can be insonated at an angle of 0° between the ultrasound beam and the direction of the blood flow and, consequently, the real velocity of the blood flow can be determined.6 The lowest intra- and interobserver variability is obtained when the MCA proximal to the transducer is sampled soon after its origin from the internal carotid artery, without the use of an angle corrector, by using a 1- to 2-mm sample volume.13 A peak systolic velocity (PSV) above 1.50 MoM (
What Is New in IUGR Research?
We and others have reported a temporal sequence in the cardiovascular system of IUGR fetuses.26, 27, 28, 29 We have also reported that the MCA PSV is increased in IUGR fetuses and that this increase predicts perinatal mortality more accurately than the MCA pulsatility index.30 In addition, there is a correlation between the MCA PSV and a low pO2 and high pCO2 in IUGR fetuses.31
Recently, there has been much interest in finding a method to decide when to deliver the IUGR fetus. It has been
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