Wave reflections in the umbilical artery measured by Doppler ultrasound as a novel predictor of placental pathology

Abstract

Background: The umbilical artery (UA) Doppler pulsatility index is used clinically to detect elevated feto-placental vascular resistance. However, this metric is confounded by variation in fetal cardiac function and is only moderately predictive of placental pathology. Our group developed a novel ultrasound methodology that measures wave reflections in the UA, thereby isolating a component of the Doppler signal that is specific to the placenta. The present study examined whether wave reflections in the UA are predictive of placental vascular pathology. Methods: Standard clinical Doppler ultrasound of the UAs was performed in 241 pregnant women. Of these, 40 women met narrowly defined preset criteria for the control group, 36 had maternal vascular malperfusion (MVM) and 16 had fetal vascular malperfusion (FVM). Using a computational procedure, the Doppler waveforms were decomposed into a pair of forward and backward propagating waves. Findings: Compared to controls, wave reflections were significantly elevated in women with either MVM (p<0.0001) or FVM pathology (p = 0.02). In contrast, the umbilical and uterine artery pulsatility indices were only elevated in the MVM group (p<0.0001) and there were no differences between women with FVM and the controls. Interpretation: The measurement of wave reflections in the UA, combined with standard clinical ultrasound parameters, has the potential to improve the diagnostic performance of UA Doppler to detect placental vascular pathology. Identifying women with FVM pathology is particularly challenging prenatally and future investigations will determine if women at risk of this specific placental disease could benefit from this novel diagnostic technique.

Reflections illuminate antenatal detection of placental pathology

“…Several findings in the study of Cahill et al [3] are noteworthy. First, the average umbilical artery reflection coefficient of the MVM and FVM groups was higher than in control patients, with the increased reflection coefficient in MVM presumably secondary to the fetal consequences of vascular pathology affecting the maternal side of the placenta.…”

“…Current antenatal detection of placental pathology with routine ultrasound screening is only moderately successful for MVM, and difficult to attain for FVM. Through a newlydeveloped analytic technique which quantifies the degree of wave reflection within arteries of the umbilical cord using Doppler velocity waveforms obtained during antenatal ultrasound screening, Cahill et al [3] provide a potential means for not only improving antenatal detection of MVM, but also permitting antenatal recognition of FVM.…”

“…Although the umbilical artery pulsatility index may be increased in the presence of placental pathology, antenatal prediction of MVM in patients using routine ultrasound screening is only moderately successful, even when combined with a clinical risk score and maternal serum biomarkers [4]. Cahill et al [3] have therefore sought to bolster diagnostic capability in this area via a more detailed analysis of the umbilical artery blood velocity waveform in patients classified as having MVM or FVM following examination of the placenta after birth.…”

“…The approach utilized by Cahill et al [3] is grounded in well-established physiological principles [5]. Thus, contraction of the heart generates flow (and pressure) waves which travel down the arterial tree.…”

“…The conventional ultrasound approach for obtaining a non-invasive reflection coefficient in the arterial circulation of children and adults requires near-simultaneous measurement of a Doppler blood velocity waveform and a blood pressure waveform (or a surrogate such as the arterial diameter waveform) at a single site, followed by deconvolution of these waveforms into forward and backward components using established wave separation techniques [5]. However, for the less-accessible fetal circulation, Cahill et al [3] employed a recently-developed approach which uses just the blood velocity waveform, but from two spatially separated measurement sites. In essence, this approach utilizes the change in morphology of measured Doppler blood velocity waveforms between the fetal and placental ends of the umbilical arteries (typically a distance of~50 cm in humans) to derive the forward and backward wave components consistent with the observed change in morphology, thus permitting calculation of an umbilical arterial reflection coefficient [8].…”