FETAL CARDIAC BYPASS USING AN IN-LINE AXIAL-FLOW PUMP TO MINIMIZE EXTRACORPOREAL SURFACE AND AVOID PRIMING VOLUME

Background. Progressive metabolic acidosis, hypoxia, and hypercarbia d evelop rapidly after fetal cardiac bypass mainly as a result of an inc rease in placental vascular resistance and a decrease in placental blo od now. A number of factors including fetal stress, priming substances , and extracorporeal surfaces have been identified as possible stimuli causing this placental dysfunction. The purpose of this study was to examine the effects of avoiding priming volume and minimizing extracor poreal surface area on placental hemodynamics and function. Methods. F etal sheep (n = 16) at 118 to 122 days of gestation were subjected to cardiac bypass for 30 minutes using either an in-line axial-now pump ( Hemopump group: n = 8, no prime) or a roller pump with a venous reserv oir (control group: n = 8, priming volume 150 mL). After bypass, the f etuses were observed for 90 minutes. Placental blood now and combined ventricular output were continuously measured with ultrasonic now prob es, and fetal blood gases were measured at specific intervals. Results . Three fetuses in the control group died during the study, whereas al l 8 fetuses in the Hemopump group remained in stable condition through out the study period. During and after bypass, placental blood now was significantly higher (p < 0.0001) and placental vascular resistance w as significantly lower (p < 0.0001) in the Hemopump group than in the control group. Arterial pH and partial pressure of arterial oxygen dec lined significantly less (p < 0.0001), and partial pressure of arteria l carbon dioxide increased significantly less (p = 0.0002) in the Hemo pump group than in the control group. Conclusions. Reducing the extrac orporeal surface area and avoiding external priming substances preserv es placental hemodynamics after fetal cardiac bypass. An inline axial- flow pump is useful in miniaturizing the bypass circuits for potential use in fetal cardiac surgery.