Background The main advantage of pulsatile flow compared with steady flow during cardiopulmonary bypass is to prevent a rise in systemic vascular resistances.We hypothesized that pulsatile flow could overcome the progressive rise in peripheral and placental vascular resistances observed during fetal bypass and leading to progressive irreversible hypoxemia.Methods and Results A study was undertaken in 17 fetal lambs (110 to 140 days of gestation).Fetal bypass was established for a 30-minute period through right atrial and main pulmonary artery cannulation. The circuit had no oxygenator. Flow was delivered by a standard roller pump for the continuous study (group 1, n=9) or by a centrifugal pulsatile pump for the pulsatile study (group 2, n=8). Oxymetric and hemodynamic parameters, along with organ blood flow determined by radiolabeled microspheres counting, were recorded before (T1) and after 10 minutes (T2) and 30 minutes (T3) of bypass. Sao2and Pao2were significantly higher in group 2 than in group 1 at T2 but thereafter deteriorated similarly in both groups, whereas Pco2remained unchanged. Pump flow in group 2 was significantly higher than in group 1 at T2 and T3 (957.6+-49 and 1104+-152 versus 437.6+-23 and 467.8+-43 mL/min, respectively). Systemic vascular resistances during pulsatile bypass were also significantly lower than in group 1 at T2 (402+-12 versus 930+-79 dynes/sec/cm-5) and T3 (374+-60 versus 1017+-192 dynes/sec/cm-5). At T2 and T3, all individual blood flows except the brain but including the placenta were statistically higher in group 2 than in group 1. Placental vascular resistances gradually increased during bypass in group 1 to reach 2.9+-0.2 mm Hg mL-1min-1kg-1at T3 and remained approximately stable in group 2 during 30 minutes of pulsatile bypass, varying from 0.35+-0.02 to 1.26+-0.14 from T2 to T3 (P<.01).Conclusions The data suggest that pulsatile flow for 30 minutes of bypass in a fetal lamb preparation temporarily prevents the progressive hypoxemia observed under steady-flow bypass. Pulsatile flow allows higher pump flow through a significant decrease in systemic vascular resistances. Individual organ blood flow, including placenta, was significantly higher under pulsatile bypass. With technical improvements in the design of pulsatile devices adapted to more physiological beat rates, pulsatility may become a valuable adjunct to overcome placental dysfunction observed during experimental fetal cardiac surgery. (Circulation. 1994;90(part 2):II-47-II-50.)