Blood flow and oxygen delivery to fetal organs and tissues during sustained hypoxemia

To examine the fetal cardiovascular responses to a sustained reduction in O2 delivery (DO2), studies were conducted on 13 chronically instrumented fetal lambs (128-138 days gestation) made hypoxemic for 7.9 +/- 0.5 h by lowering maternal inspired O2 concentration to 9-10%. Fetal descending aortic PO2 fell initially from 18.0 +/- 1.0 to 10.7 +/- 0.6 mmHg, whereas pH decreased progressively from 7.326 +/- 0.006 to 6.843 +/- 0.023. Blood flow to the cerebral hemispheres, myocardium, and adrenal glands rose maximally by 110.2 +/- 22.5, 253.7 +/- 41.1, and 338.7 +/- 55.0%. Cerebral hemispheric DO2 fell progressively, whereas DO2 to the myocardium and adrenal was maintained until 7.9 h, when it fell significantly. There was also a rise in blood flow to brown adipose tissue. Blood flow to the gut and skeletal muscle was maintained, whereas flow to the spleen and kidney fell. DO2 to all these tissues fell markedly because of the progressive decline in blood O2 content. It is concluded that fetal cardiovascular function was well maintained in the face of severe hypoxemia and marked acidemia.