FETAL CARDIAC BYPASS - IMPROVED PLACENTAL FUNCTION WITH MODERATELY HIGH-FLOW RATES

Prenatal correction of certain cardiac lesions with a poor prognosis m ay have advantages over postnatal repair. For this to be done, safe an d effective support of the fetal circulation must be devised. Studies involving fetal cardiac bypass have demonstrated progressive fetal hyp oxemia, hypercapnia, and acidosis, indicating placental dysfunction. W e performed fetal cardiac bypass in 18 fetal lambs (126 to 140 days' g estation) to assess the effect of now rate on fetal oxygenation and me tabolism and function of the placenta as an in vivo oxygenator. Fetal cardiac bypass was done for a 30-minute study period at normothermia i n all fetuses. During the study period the fetal aorta was cross-clamp ed and cold cardioplegia was administered to the heart so there was no fetal cardiac contribution to systemic output. Nine fetuses underwent studies at low flow rates (109 +/- 20 mL kg(-1) min(-1)) and 9 at hig her flow rates (324 +/- 93 mL kg(-1) min(-1)). At the lower flow rate, mean aortic pressure, arterial pH, and oxygen tension decreased where as carbon dioxide tension and lactate levels increased when compared w ith prebypass levels. At the higher how rate mean aortic pressure, pH, oxygen tension, carbon dioxide tension, and lactate levels remained s imilar to prebypass levels during the 30-minute study period. When the animals were weaned from the bypass circuit after studies at high flo w rates, arterial oxygen tension and pH decreased whereas carbon dioxi de tension increased to levels similar to those in the low-flow group. We conclude that low fetal cardiac bypass now rates (100 to 125 mL kg (-1) min(-1)) are inadequate to maintain hemodynamics, oxygenation, CO 2 removal, and normal lactate levels when the placenta is used as an i n vivo oxygenator. Higher flow rates (300 to 400 mL kg(-1) min(-1)) ma y limit these changes by improving placental perfusion and function du ring bypass. Despite high now rates, placental dysfunction and fetal b lood gas abnormalities still occur after fetal cardiac bypass.