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Despite the umbilical artery (UA) Doppler pulsatility index being used to detect elevated fetoplacental vascular resistance, the metric is confounded by variation in fetal cardiac function and is only moderately predictive of placental pathology.
To improve accuracy, an international group of investigators have developed a novel ultrasound methodology that measures wave reflections in the UA, thus isolating a component of the Doppler signal that is specific to the placenta.
A prospective study by the investigators in the journal EBioMedicine, published by The Lancet, assessed whether wave reflections in the UA are predictive of placental vascular pathology.
A total of 427 women, aged 18 to 45, with a singleton pregnancy and a body mass index (BMI) of less than 45 kg/m2 were recruited for fetal ultrasound examination from Mount Sinai Hospital in Toronto, Ontario, Canada, and Johns Hopkins Medicine in Baltimore, Maryland.
Standard clinical Doppler ultrasound of the UAs were conducted on 241 of the women between 26 and 32 weeks of gestation by certified sonographers.
Among the 241 women, 40 met narrowly defined preset criteria for the control group, 36 had maternal vascular malperfusion (MVM) and 16 had fetal vascular malperfusion (FVM).
A computational procedure allowed the Doppler waveforms to decompose into a pair of forward and backward propagating waves.
Compared to controls, wave reflections were significantly elevated in women with either MVM (P< .0001) or FVM pathology (P = .02).
But the umbilical and uterine artery pulsatility indices were only elevated in the MVM group (P< .0001). There were no differences in women with FVM or in the control group.
“The first harmonic reflection coefficient acts as a measure of hemodynamic impedance that is specific to the placenta because the formulation explicitly controls for upstream factors such as cardiac output that may bias other metrics,” wrote the authors.
Because the placentas in the control group had low reflection coefficients, the impedance of the placental vascular system is likely well matched to the umbilical cord and has a branching vessel pattern that is structured to transmit rather than reflect hemodynamic pulses.
The authors said this hypothesis is consistent with placental vascular development. The placentas with MVM and FVM pathologies had elevated reflection coefficients that indicate one or more distal sites of high hemodynamic impedance and/or a large impedance mismatch.
While it is unknown where the identified reflection arises within the placenta, a likely source are the capillaries within the terminal villi, because in both MVM and FVM pathology, the structure of the villi is abnormal.
The investigators recently used computational modeling to elucidate the forward and reflected waves via microcomputed tomography images of the fetoplacental vasculature in mice. They discovered that the terminal load impedance (capillary and/or vein) played a significant role in creating the observed reflected waveform.
The authors said measuring 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. This is particularly promising for FVM where there is currently no reliable method of detection.”
Because the data used in the study were collected via standard clinical ultrasound equipment and automated computational analysis, the data can be easily translated to use at other centers, according to the authors.
“Future investigations will determine if women at risk of this specific placental disease could benefit from this novel diagnostic technique,” they wrote.
Cahill LS, Stortz G, Chandran AR, et al. Wave reflections in the umbilical artery measured by Doppler ultrasound as a novel predictor of placental pathology. EBioMedicine. Published online May 5, 2021. doi:org/10.1016/j.ebiom.2021.103326