Fetal hepatic oxygen consumption under normal conditions in the fetal lamb

Previous measurements of fetal hepatic blood flow have relied on microsphere methodology. Estimates of fetal hepatic oxygen consumption (VO2), based on these measurements and the oxygen content difference across the fetal hepatic circulation, have been quite variable. To estimate hepatic VO2 in the fetal lamb by a different methodology, we applied the Fick principle using the steady-state uptake of indocyanine green (ICG) by the fetal liver to measure left hepatic blood flow in 10 pregnant ewes. Sampling catheters were inserted into the fetal external iliac artery, left hepatic vein, and umbilical vein. ICG was infused to steady state (for approximately 60 min) through a fetal brachial vein. Four sets of ICG concentration differences across the circulation of the left hepatic lobe were determined for each animal, and left hepatic lobe blood flow calculated. The oxygen concentration difference was measured simultaneously and VO2 of the left hepatic lobe calculated. In addition, we measured fetal VO2 and calculated the ratio of hepatic to fetal VO2. Left hepatic lobe blood flow was 382.30 ml/min/100 g tissue (COV = 0.32), a result statistically no different than in 4 animals with an independent measurement of hepatic blood flow using an ethanol equilibration method. Hepatic VO2 was 1.74 micromol/min/g tissue (COV = 0.13), and hepatic to fetal VO2 ratio was 18.23% (COV = 0.19). Our results indicate that normal fetal hepatic oxygen uptake per gram of tissue is less variable than previously suggested, and that ICG can be applied in the fetus for the purpose of hepatic blood flow measurement.