Umbilical cord blood flow is among the most highly desired parameters for monitoring fetal well-being. This is because cord blood flow directly reflects placental volume flow, which is considered to be as important in the fetus as cardiac output and lung perfusion are in adults.^{1} Yet, presently employed noninvasive methods, such as umbilical artery Doppler waveform analyses, use surrogate flow evaluation parameters, such as systolic/diastolic ratios, which do not directly reflect placental-fetal blood flow.^{2,3} Volume flow estimation overcomes this by measuring true flow, and it has been shown that volume flow changes in the umbilical vein occur before umbilical artery flow indices become abnormal.^{4}

Yet, the present volume flow measurement method has severe problems limiting its utility. These include technical difficulties in flow measurement in umbilical cords and faulty assumptions employed in the measurement. The present method using spectral Doppler is

*Q* = *V* × *A* (1),

where *Q* is volume flow, V is the mean velocity through the Doppler sample volume, and *A* is the cross-sectional area of the vessel of interest. This formula assumes that the 2D flow profile is cylindrically symmetric with a circular cross-section, and the line of the Doppler sampling cuts perfectly through the center of the sampled vessel. The velocity estimates require angle correction, and if the vessel is tortuous, as in umbilical cords, the sampling position placement and angle correction are hard to perform. Multiple investigators have warned that small errors in volume flow components can result in large errors in the calculation of volume flow.^{5-7}

A new, easy-to-perform volume flow method overcomes almost all of the limitations of the standard technique. The new method is angle independent, flow profile independent, and vessel geometry independent. It works as follows:

Volume flow is defined as the total flux across any surface, S, intersecting the vessel. This is written as

where Q is again volume flow, V is the local velocity through each area element dA, and “**∙**” is the dot product which projects the local velocity V onto the normal vector for each area element. This is known as Gauss’s theorem. The intersecting surface, known as the “C” surface, is very simple to obtain using 3D ultrasound (Figure^{8}). In order to validate this method, we obtained an AIUM EER-funded research grant.

We had 2 specific aims: 1) Test the reproducibility of the volume flow measurement, and 2) evaluate the relationship of volume flow to clinical outcome in a high-risk patient population.

In the first aim, we performed studies on 35 subjects between the gestational ages of 22–37 weeks, 26 high risk and 9 normal.^{9} We attempted to measure umbilical cord blood flow at 3 sites in the cord in each subject, and we averaged 28.3 ± 3.3 (mean ± standard deviation) samples per site. We used a GE LOGIQ E9 ultrasound system with a 2.0–8.0 MHz bandwidth convex array transducer to acquire multiple volume 3D color and power mode data sets. Since we were measuring mean blood flow, we assessed variability using relative standard error (standard error /mean) (RSE). The average RSE for blood flow at each cord position was ±5.6% while the average RSE among the measurements in each subject was ±12.1%.

For the second aim, we compared the volume flow measurements in 5 subjects that developed preeclampsia with the 9 normal subjects. Even with these small numbers, we detected a significant difference between the mean depth-corrected, weight-normalized umbilical vein blood volume flows in the two groups (*P* = .035). Further, blood flow abnormalities were detected either at the same time or preceded the hypertensive disorder in 4 of the 5 subjects. This is consistent with our prior publication where blood flow changes preceded the onset of pre-eclamptic symptoms in a study subject.^{8}

With the introduction of 2D array transducers, umbilical cord volume flow estimates can be performed in seconds and given the valuable information provided by this method, umbilical cord volume flow will hopefully become a standard component of fetal examinations.

References:

- Tchirikov M, Rybadowski C, Huneke B, Schoder V, Schroder HJ. Umbilical vein blood volume flow rate and umbilical artery pulsatility as ‘venous-arterial index’ in the prediction of neonatal compromise.
*Ultrasound Obstet Gynecol*. 2002;20:580-5. - Newnham JP, Patterson LL, James IR, Diepeveen DA, Reid SE. An evaluation of the efficacy of Doppler flow velocity waveform analysis as a screening test in pregnancy.
*Am J Obstet Gynecol*. 1990;162:403-10. - Acharya G, Wilsgaard T, Bernsten GKR, Maltau JM, Kiserud T. Doppler-derived umbilical artery absolute velocities and their relationship to fetoplacental volume blood flow: a longitudinal study.
*Ultrasound Obstet Gynecol*. 2005;25:444-53. - Rigano S, Bozzo M, Ferrazzi E, Bellotti M, Battaglia FC, Galan HL. Early and persistent reduction in umbilical vein blood flow in the growth-restricted fetus: a longitudinal study.
*Am J Obstet Gynecol*. 2001;185:834-8. - Evans DH. On the measurement of the mean velocity of blood flow over the cardiac cycle using Doppler ultrasound.
*Ultrasound Med Biol*. 1985;11(5):735-41. - Gill R. Measurement of blood flow by ultrasound: accuracy and sources of error.
*Ultrasound Med Biol*. 1985;11:625-41. - Lees C, Albaiges G, Deane C, Parra M, Nicolaides KH. Assessment of umbilical arterial and venous flow using color Doppler.
*Ultrasound Obstet Gynecol*. 1999;14:250-5. - Pinter SZ, Rubin JM, Kripfgans OD, Treadwell MC, Romero VC, Richards MS, Zhang M, Hall AL, Fowlkes JB. Three-dimensional sonographic measurement of blood volume flow in the umbilical cord.
*J Ultrasound Med*. 2012;31(12):1927-34. - Pinter SZ, Kripfgans OD, Treadwell MC, Kneitel AW, Fowlkes JB, Rubin JM. Evaluation of umbilical vein blood volume flow in preeclampsia by angle-independent 3D sonography [published online ahead of print December 15, 2017].
*J Ultrasound Med*. doi:10.1002/jum.14507.

*How do you determine umbilical cord blood flow? What problems have you encountered using the traditional method?* *Comment below or let us know on Twitter: *@AIUM_Ultrasound.

Jonathan Rubin, MD, PhD, FAIUM, is Professor Emeritus of Radiology at University of Michigan.