Fetal hepatic blood flow has been measured using radiolabeled microspheres injected into the umbilical vein, where mixing may be incomplete. We introduce a technique using a steady state infusion of Indocyanine Green (ICG), a substance taken up solely by the liver postnatally. Methods: In 10 pregnant ewes, sampling catheters were placed in the fetal umbilical vein, left hepatic vein, and hindlimb artery. After recovery, ICG was infused at a constant rate into a fetal brachial vein, and allowed to reach steady state(≅ 60 minutes). Blood samples were taken for ICG concentration and oxygen content. By application of the Fick principle, left hepatic blood flow was calculated using liver ICG uptake rate and the concentration difference across the left hepatic circulation. At autopsy, catheter placement was verified and the fetal liver transected into left and right hepatic lobes and weighed.Results: (see Table). Oxygen content was 2.96mM (COV=0.14) in the fetal artery, 4.50mM (COV=0.10) in the umbilical vein, and 3.95mM (COV=0.14) in the left hepatic vein. ICG concentration in the umbilical vein and fetal artery were not different by paired ttest (0.221± 0.060 vs 0.221 ± 0.60abs, NS) indicating no placental ICG uptake. Conclusion: Using microspheres, mean estimates for fetal hepatic oxygen uptake in three seperate earlier studies were 1.96, 1.79, & 1.10 umol/min/gm. COV for the latter two studies were 0.65 & 0.48, respectively. Our results indicate that normal fetal hepatic oxygen uptake is less variable than previously shown, and that ICG may be a more precise tool for exploring the regulation of fetal hepatic metabolism.
