We studied the distribution of systemic blood flow during venoarterial extracorporeal membrane oxygenation (ECMO) in newborn lambs. We used a three-compartment model that defined partitioning of blood flow to the heart, upper body (brachiocephalic trunk), and lower body (descending aorta). The method used concurrent left ventricular and arterial cannular injections of radiolabeled microspheres to calculate compartment flows and solve the system of equations that defined the partition model. Seven newborn lambs (1-8 d) were studied. A baseline microsphere injection (left ventricle) was performed and the animals were then placed on venoarterial ECMO using right carotid and jugular vein cannulation. The arterial cannula was placed 2-3.5 cm above the aortic valve. After stabilization on ECMO flow rates of 50 and 100 mL/min/kg, differently labeled microspheres were injected simultaneously into the left ventricle and arterial limb of the ECMO circuit. From the flow partition model, distribution of blood flow was calculated. We found that ECMO did not change the overall distribution of blood flow to the three compartments studied. However, blood flow from the ECMO circuit was preferentially directed to the upper body. Coronary arterial and abdominal organ blood flow was predominantly derived from the left ventricle at both ECMO flow rates. Coronary arterial blood flow did not significantly change on ECMO (253 +/- 45 mL/min/100 g at 50 mL/min/kg ECMO flow; 246 +/- 50 mL/min/100 g at 100 mL/min/kg ECMO flow) compared to baseline (186 +/- 31 mL/min/100 g).(ABSTRACT TRUNCATED AT 250 WORDS)