Fetal cardiac bypass alters regional blood flows, arterial blood gases, and hemodynamics in sheep

Successful fetal cardiac bypass might allow prenatal correction of some congenital heart defects. However, previous studies have shown that fetal cardiac bypass may result in impaired fetal gas exchange after bypass. To investigate the etiology of this impairment, we determined whether fetal cardiac bypass causes a redistribution of fetal regional blood flows and, if so, whether a vasodilator (sodium nitroprusside) can prevent this redistribution. We also determined the effects of fetal cardiac bypass with and without nitroprusside on fetal arterial blood gases and hemodynamics. Eighteen fetal sheep were studied in utero under general anesthesia. Seven fetuses underwent bypass without nitroprusside, six underwent bypass with nitroprusside, and five were no-bypass controls. Blood flows were determined using radionuclide-labeled microspheres. After bypass without nitroprusside, placental blood flow decreased by 25-60%, whereas cardiac output increased by 15-25%. Flow to all other fetal organs increased or remained unchanged. Decreased placental blood flow after bypass was accompanied by a fall in PO2 and a rise in PCO2. Nitroprusside improved placental blood flow, cardiac output, and arterial blood gases after bypass. Thus fetal cardiac bypass causes a redistribution of regional blood flow away from the placenta and toward the other fetal organs. Nitroprusside partially prevents this redistribution. Methods of improving placental blood flow in the postbypass period may prove critical to the success of fetal cardiac bypass.