Table 20.1 Assessment of Maternal and Fetal Doppler Waveforms Using Different Assessment Methods
Figure 20.1 This figure depicts the calculation of the semiquantitative Doppler indices on an arterial waveform. The gray-filled area defines time-averaged maximum velocity (Tmax). D, end diastole; S, peak systole.
trophoblastic invasion and remodeling of the maternal spiral arteries. The UtAs arise from the anterior division of internal iliac arteries and divide into the arcuate, radial, and spiral arteries. Impedance to blood flow in the UtAs decreases as gestational age advances with the development of a low-resistance vascular bed (Figure 20.3). The initial fall in vascular resistance during the first and second trimesters is attributed to trophoblastic invasion of spiral arteries. The continued decrease in resistance in the UtAs into the third trimester may be explained by a hormonal effect on the elasticity of the arterial walls.7 These physiologic changes in spiral arteries result in an increase in the end-diastolic flow of the uterine arteries.11 Placental ischemic lesions and abnormal development of the spiral arteries may be depicted in the uterine arteries by increased resistance and/or presence of a diastolic notch, which is associated with hypertensive disorders of pregnancy (Chapter 27) and FGR.11 Color Doppler should be used to locate the UtAs as they cross medial to the external iliac arteries (Figure 20.4).12 The region of interest revealed by Doppler should be magnified, and a color box should be placed. The Doppler gate is placed within the straight portion of the UtA before it enters the myometrium.
UA generally do not start to increase until approximately 60% to 70% of the placental vascular tree is not functioning.2 This highlights the presence of extensive disease before Doppler detection is possible and emphasizes the reserve capacity of the placenta.4 The abnormal UA Doppler waveform is characterized by a pattern of present, absent, or reversed diastolic flow velocities relative to the PSV (Figure 20.6).
should be <30°; optimally as close to 0° as possible. Angle correction can be used to measure PSV in the fetal MCA.13,14 The region of interest should be magnified so that the MCA occupies more than 50% of the screen, and the full length of the MCA should be visualized. Fetal breathing and excessive pressure on the fetal head by the transducer may change the MCA waveform. The highest PSV value should be measured, and these steps should be repeated at least three times to improve the accuracy and reproducibility of the measured PSV.15
be identified. This area is identified by looking for an aliasing effect where the blood flow velocity accelerates due to the narrow lumen of the DV. A pulsed-wave Doppler gate of 1 mm in width should be placed over this area, and the waveform should be obtained with the smallest possible angle of insonation (Figure 20.9). When five constant waveforms with a good signal-to-noise ratios are obtained, the frozen image can be traced, outlining the waveform from the beginning of ventricular systole to the end of atrial systole.18 The DV waveform is influenced by several intrinsic fetal factors including fetal breathing movements, behavioral states, and cardiac arrhythmias. The DV should be sampled during fetal rest and in the absence of fetal breathing.
reduced myocardial compliance and fluid accumulation; impaired neural crest cell migration to the neck and conotruncus secondary to a hypoxic insult; and abnormal innervation or endothelial thickening of the DV.25,26,27,28 In fetuses with normal nuchal translucency measurements, abnormalities in the DV waveform are associated with adverse pregnancy outcomes, including cardiovascular defects, FGR, renal anomalies, and perinatal death.24 Identification of abnormalities in the DV waveform in the first trimester should be a clue to the clinician that such pregnancies should be monitored closely, and additional surveillance may include fetal echocardiography and third-trimester growth ultrasound in addition to the routine detailed anatomic review at 18 to 20 weeks’ gestation.