Umbilical Artery Wall Shear Stress and Control of the Feto-placental Circulation.

The aim of this paper is to review the area of wall shear stress (WSS) in umbilical arteries and to present a new theory for the short-term (seconds) control of placental resistance, which could be mediated by WSS. The endothelium senses changes in WSS, and a series of biological changes ensues with timescales of seconds to weeks. Wall shear stress mediates a control mechanism in which the arterial diameter changes in order to maintain WSS within a narrow range. Umbilical artery WSS has been estimated using a combination of ultrasound measurement of diameter and blood velocity (from maximum Doppler frequency shift), and computational fluid dynamics. The measurement of maximum blood velocity using commercial ultrasound systems is overestimated by typically 20%-40% in clinical ultrasound and 40%-60% in pre-clinical ultrasound. Measurements of WSS that use an estimate of maximum velocity from maximum Doppler frequency will also be overestimated by similar amounts. The overestimation of maximum velocity is due to geometric spectral broadening, which can be corrected at the time of data collection using measurements made from a string or similar phantom. A new hypothesis is described, which is that placental resistance is controlled on a timescale of a few seconds in order to maintain the umbilical artery flow rate constant. This hypothesis originates from observations made in a 1989 paper that investigated the relationship between the umbilical artery heart rate and resistance index. The key observation was that changes in heart rate were followed a few seconds later by changes in resistance index. It is proposed that the basis for the control could be endothelial detection of changes in WSS. Modern ultrasound systems have the technical capability to further investigate this hypothesis.