Redistribution of cardiac output and oxygen delivery in the hypoxemic fetal lamb

In hypoxia, fetal cardiac output and the product arterial O2 content × blood flow to the fetal heart and central nervous system (CNS) tend to remain constant. As a consequence the percentage of cardiac output directed to the heart and CNS increases hyperbolically in inverse relation to the oxygen content of the fetal ascending aorta, [O2]as. The fetal lamb maintains [O2]as approximately 0.45 mM (0.45 ± 0.02 SEM) higher than the O2 content in the abdominal aorta, [O2]ab, over a wide range of oxygenation. When [O2]as decreases below the 2 mM level, the [O2]as-[O2]ab difference (ΔO2) decreases also. A mathematical model of the fetal circulation shows that ΔO2 is a function of the ratio oxygen consumption of fetal upper body/abdominal aorta blood flow (V̇U/FA). The behavior of ΔO2 in hypoxia can be explained by assuming that the V̇U/FA ratio is maintained in moderate hypoxia and decreases in severe hypoxia.