The left portal vein is the watershed of the fetal venous system

Abstract

Critical fetal organs are preferentially supplied with oxygenated blood from the umbilical vein (UV) by way of the ductus venosus (DV). Under normal conditions a significant part of UV-blood flows steadily forward through the left portal vein (LPV). Blood flow through the LPV could reverse, however, in cases of absent or reversed endodiastolic flow in the umbilical arteries. We tested when fetal blood flow reversal occurs by studying 28 cases with pathological flow in the umbilical artery. In the LPV we observed normal nonpulsatile forward flow in 9 cases, pulsatile forward flow in 10 cases, and reversed flow in 9 cases. Reverse flow in the LPV correlated significantly with an elevated resistance index of the umbilical arteries. This reversal could have major physiological implications: Deoxygenated blood may be added via the LPV to the blood shifted through the DV and ultimately reach critical fetal organs. In extremis there could be a waterhose effect, whereby more blood flows through the DV than the UV that supplies it. The LPV is thus the watershed of the venous circulation of the fetus.

Ductus venosus shunting in growth‐restricted fetuses and the effect of umbilical circulatory compromise

“…In the recent study by Bellotti et al , as many as 12/30 fetuses were calculated to shunt ≥ 100% of the umbilical blood through the DV, 6/30 to shunt ≥ 150%, and 2/30 to shunt ≥ 200%16. The authors explain this with a possible additional contribution of portal blood through the left portal branch, a phenomenon that has been suggested as a compensatory mechanism in compromised cases25, 26, and which the authors could demonstrate in one of their fetuses. However, in the present study of more severe cases (fetal weight ≤ 2.5 th percentile), we had only 3/60 observations of shunting above 100% (102%, 104%, and 110%).…”

Fetal cardiac output, distribution to the placenta and impact of placental compromise

“…This is in keeping with other mechanisms seen in such fetuses: reduced size of and shunting through the foramen ovale 7,20 , increased resistance in the pulmonary circuit 21 , with correspondingly less venous return to the left heart, and retrograde blood flow at the aortic isthmus 7,22 to further supply the aortic arch and carotid arteries with right ventricular blood via the ductus arteriosus. A shift to the left of the watershed area between portal and umbilical venous supply to the liver 23,24 and an augmented blood velocity in the hepatic artery 25 will change the fetal circulation in the same direction. These are mechanisms of redistribution but also of increased recirculation of umbilical blood in the fetal body, which correspond to more extensive oxygen extraction.…”

“…• in 56 recordings and the median was 16 (IQR, [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] • in the remaining 141. The fetoplacental blood flow was calculated as π · (D/2) 2 · V wmean , and its fraction of the CCO was calculated as a percentage.…”

Aortic Isthmus and Cardiac Monitoring of the Growth-Restricted Fetus

“…The aortic isthmus represents the arterial watershed between the brachiocephalic (including brain) and subdiaphragmatic (including placenta) circulations, 1 whereas the left portal vein represents the venous watershed between the umbilical (placental) and systemic (splanchnic) circulations. 36 It has been argued that a true shunt, by definition, diverts blood from one circuit to another and that the aortic isthmus is the only true shunt in the fetal circulation. 4 However, the aortic isthmus does not behave like a classical shunt because physiologic fetal shunts normally close after birth but the watersheds remain patent.…”