Clinical and physiologic implications of increased fetal oxygen consumption

In an effect to determine if placental diffusion reserves exceed fetal oxygen requirements we artificially increased fetal O2 consumption by infusing norepinephrine (NE) or triiodothyronine (T3) into 24 chronically catheterized fetal lambs. After 50 minutes of NE infusion, fetal O2 consumption rose 25%; after 5 days of T3 infusion, it increased 28%. In both instances there was a redistribution of organ blood flows, but placental diffusing capacity and the fetal venous and arterial blood gas values remained essentially unchanged. We concluded that both NE and T3 infusion can result in an increased rate of fetal metabolism with increased oxygen consumption and that the placental reserve for O2 diffusion exceeds normal oxygen requirements.