REGIONAL BLOOD-FLOW AND THE ENDOCRINE RESPONSE TO SUSTAINED HYPOXEMIAIN THE PRETERM OVINE FETUS

To determine the circulatory response of the preterm fetus to a sustai ned hypoxic insult, regional blood how was measured (microsphere techn ique) in 12 unanesthetized fetal sheep (0.75 gestation) during a normo xic control period, after 1 h and 8 h of sustained hypoxemia, and afte r a l-h recovery period. Associated endocrine changes which might rela te to organ-specific changes in blood flow were also assessed. Myocard ial and cerebral blood flow were increased by 240 and 90%, respectivel y, such that oxygen delivery to the heart was well maintained througho ut the study, whereas that to the brain was significantly decreased by 8 h of hypoxic study. Regional blood flows for all structures within the brain showed similar percent increases, except that for the pituit ary gland, where the increase was much smaller, and that for the choro id plexus, where blood how actually fell. Whereas blood flow to upper body muscle showed no significant change throughout the study, that to the thyroid was increased by 70% by 1 h of hypoxic study but fell the reafter. Adrenal cortical blood flow relative to that of the medulla w as increased 3-fold by 8 h of hypoxic study, indicating a differential effect of sustained hypoxia on these vascular beds. Although pituitar y and thyroid blood flows showed no relationship to respective trophic and/or secretory hormones measured, values for adrenal cortical flow relative to medullary flow were well correlated with plasma concentrat ions of ACTH. It is concluded that the ''centralization'' of blood flo w to vital organs in response to a sustained hypoxic insult is qualita tively similar for both the preterm and near term ovine fetus and that hypoxic regulatory mechanisms may be better protective of the heart. Additionally, a role for the functional activation of the adrenal glan d in its blood flow response to sustained hypoxemia is suggested.