Fetal superior mesenteric artery: Longitudinal reference ranges and evidence of regulatory link to portal liver circulation

ObjectiveTo establish longitudinal reference ranges for the fetal superior mesenteric artery (SMA) flow velocity and pulsatility index (PISMA). Also to examine the hemodynamic relationship to venous liver perfusion and umbilical flow distribution in the liver, to other splanchnic arteries, and more generally to the middle cerebral and umbilical artery.MethodsProspective longitudinal study of 161 low-risk pregnancies using Doppler recordings including the SMA, repeated on 3–5 occasions at 3–5 weekly intervals. Umbilical venous flow was estimated, blood velocity in the shunt ductus venosus represented umbilico-caval (i.e. porto-caval) pressure gradient, and left portal vein blood velocity represented umbilical distribution within the liver. The correlation between PISMA and the splenic and hepatic artery PI were analysed (PISA and PIHA), and the association to middle cerebral and umbilical artery PI (PIMCA and PIUA) assessed.ResultsReference ranges for the SMA for gestational weeks 21–39 were based on 589 observations. Low impedance in the SMA (i.e. low PISMA) was associated with low umbilical flow and porto-caval pressure gradient (i.e. < 10th centile), and high distribution of umbilical flow to the right lobe (i.e. left portal vein blood velocity > 90th centile). PISMA correlated weakly with PISA and PIHA (r = 0.30, 95%CI 0.22–0.37, and r = 0.39, 95%CI 0.27–0.51, respectively). PISMA was positively associated with PIMCA and PIUA.ConclusionWe have provided longitudinal reference ranges for fetal SMA flow velocity and PI, and shown that the SMA, which perfuses the fetal gut, is also involved in the regulation of the liver perfusion.