Fetal sheep endocrine responses to sustained hypoxemic stress after chronic fetal placental embolization

The purpose of this study was to determine the endocrine and circulatory responses of the ovine fetus, near term, to sustained hyperemic stress superimposed on chronic hypoxemia. Fetal sheep were chronically embolized (n = 7) for 10 days between 0.84 and 0.91 of gestation via the descending aorta until arterial oxygen content was decreased by similar to 30%. Control animals (n = 8) received saline only. On experimental day 10, both groups were embolized over a 6-h period until fetal arterial pH decreased to similar to 7.00. Regional distribution of lower body blood flows was measured on day 10, before and at the end of acute embolization. On day 10, the chronically embolized group had lower arterial oxygen content (P < 0.05), Po-2 (P < 0.01), and placental blood flow (P < 0.05) than controls and higher prostaglandin E-2 (PGE(2)) and norepinephrine plasma concentrations (both P < 0.05). In response to a superimposed sustained hypoxemic stress, there was a twofold greater increase in PGE(2) in the chronically embolized group than in the control group (P < 0.05). However, the increase in fetal plasma cortisol in response to superimposed hypoxemic stress was similar in both groups, despite significantly lower adrenocorticotropic hormone and adrenal cortex blood flow responses in the chronically hypoxemic group (both P < 0.05). We conclude that PGE(2) response to a sustained superimposed reduction in placental blood flow, leading to metabolic acidosis, is enhanced under conditions of chronic hypoxemia and may play an important role for the maintenance of the fetal cortisol response to an episode of superimposed acute stress.