another continuously captures signals. In M-mode imaging, continuous-wave Doppler is used to evaluate motion through time, however, it cannot image individual vessels.
Pulsed-wave Doppler uses only one crystal, which transmits the signal and then waits until the returning signal is received before transmitting another one. It allows precise targeting and visualization of the vessel of interest. Pulsed-wave Doppler can be conigured to allow color-low mapping-such that blood flowing toward the transducer is displayed in red and that flowing away from the transducer appears in blue. Various combinations of pulsed-wave Doppler, color-flow Doppler, and real-time sonography are commercially available.
Umbilical artery Doppler has been subjected to more rigorous assessment than any previous test of fetal health. he umbilical artery difers from other vessels in that it normally has forward low throughout the cardiac cycle. Moreover, the amount of low during diastole increases as gestation advances, and this reflects decreasing placental impedance. As a result, the SID ratio normaly declines rom approximatey 4.0 at 20 weeks' gestation, to generaly less than 3.0 ter 30 weeks' and inaly to 2.0 at term. Because of downstream impedance to low, more end-diastolic low is observed at the placental cord insertion than at the fetal ventral wall. hus, abnormalities such as absent or reversed enddiastolic low will appear first at the fetal cord insertion site. The International Society of Ultrasound in Obstetrics and Gynecology recommends that umbilical artery Doppler measurements be made in a free loop of cord (Bhide, 2013). However, assessment close to the ventral wall insertion may optimize measurement reproducibility when low is diminished (Berkley, 2012).
The waveform is considered abnormal if the SID ratio is >95th percentile for gestational age. In extreme cases of growth restriction, end-diastolic flow can become absent or even reversed (Fig. 44-8, p. 854). Such reversal of end-diastolic flow has been associated with greater than 70-percent obliteration of the small muscular arteries in placental tertiary stem villi (Kingdom, 1997; Morrow, 1989).
As described in Chapter 44 (p. 854), umbilical artery Doppler aids management of fetal-growth restriction and has been associated with improved outcome in these cases (American College of Obstetricians and Gynecologists, 2015). It is not recommended for complications other than growth restriction. Similarly, its use for growth-restriction screening is not advised (Berkley, 2012). Abnormal umbilical artery Doppler findings should prompt a complete fetal evaluation, if not already done, because abnormal measurements are associated with major fetal anomalies and aneuploidy (Wenstrom, 1991).
Doppler evaluation of the ductus arteriosus has been used primarily to monitor fetuses exposed to indomethacin and other nonsteroidal antiinlammatory agents (NSAIDs). Indomethacin, which is used by some for tocolysis, may cause ductal constriction or closure, particularly when used in the third trimester (Huhta, 1987). The resulting increased pulmonary flow can cause reactive hypertrophy of the pulmonary arterioles and eventual development of pulmonary hypertension. In a review of 12 randomized controlled trials involving more than 200 exposed pregnancies, Koren and coworkers (2006) reported that NSAIDs raised the odds of ductal constriction 15-fold. When these agents are indicated, their duration is typically limited to less than 72 hours. And, women taking NSAIDs are closely monitored so that these can be discontinued if ductal constriction is identified. Fortunately, ductal constriction is often reversible after NSAID discontinuation.