High-dose steroids prevent placental dysfunction after fetal cardiac bypass

Surgical treatment of certain congenital heart lesions in utero may have a therapeutic advantage over postnatal repair or palliation. For fetal heart surgery to be possible, a method to support the fetal circulation is necessary. Early experimental attempts at fetal cardiac bypass were unsuccessful because of increased placental vascular resistance during and after fetal cardiac bypass, which led to decreased placental flow, fetal asphyxia, and death. Our laboratory has demonstrated that the administration of indomethacin (a cyclooxygenase inhibitor) during fetal cardiac bypass prevents this increase in placental vascular resistance during and after fetal cardiac bypass. The specific mechanism by which indomethacin achieves this effect is likely to be either by inhibiting the production of a placental vasoconstrictive prostaglandin or by diverting substrate from the cyclooxygenase pathway to the lipoxygenase pathway, thereby potentially increasing the production of a placental vasodilating leukotriene. To examine these potential mechanisms in more detail, we inhibited both prostaglandin and leukotriene synthesis at the phospholipase stage with high-dose steroids. Fourteen fetal lambs were used in the study. Six animals received indomethacin (3 mg/kg), four received high-dose steroids (Solu-Medrol 50 mg/kg), and four animals were used as controls. Observations were made during a 1-hour prebypass period, a 30-minute bypass period, and a 2-hour postbypass period. Placental blood flow and placental vascular resistance were calculated at four times during the experiments: before sternotomy; after sternotomy; during bypass at 30 minutes; and 30 minutes after cessation of bypass. Similar to indomethacin, high-dose steroid administration during fetal cardiac bypass prevents the rise in placental vascular resistance and preserves placental blood flow during and after fetal cardiac bypass. This study suggests that the production of a placental vasoconstrictive prostaglandin is responsible for the increase in placental vascular resistance and decrease in placental blood flow observed after fetal cardiac bypass.