Venovenous tidal flow perfusion for extracorporeal membrane oxygenation via a single lumen cannula in the right atrium avoids the sacrifice of a carotid artery inherent in venoarterial systems, and eliminates the problems of two cannula venovenous perfusion. Oxygen delivery and hemodynamic effects of a single cannula, single lumen system with tidal flows directed by alternating tubing occluders were studied in six newborn lambs to define optimal system performance and possible adverse hemodynamic effects. The ratio of drainage to infusion time was fixed at 2:1 to avoid excessive reinfusion pressures. Total length of the in/out cycle was varied from 2-6 sec, resulting in a cycling frequency of 30 to 10 cycles/min and a tidal volume of 17-50 ml. Systemic arterial, mixed venous, and pre and post oxygenator partial pressure of oxygen and oxygen saturation were measured. Recirculation, oxygen delivery, and effective bypass flow (total flow--recirculated flow) were calculated. With slower cycling frequency, recirculation progressively fell, and effective flow and oxygen delivery increased (p < 0.001 for each parameter across the cycle length). In these animals, oxygen delivery was limited by low oxygen carrying capacity (mean hemoglobin, 8.1 g/dl). The authors concluded that with longer cycles, the system minimizes recirculation without apparent adverse hemodynamic consequences, achieving sufficient effective bypass flow to assure adequate oxygen delivery when hemoglobin levels are normal.