The evaluation of left ventricular systolic performance in infants undergoing extracorporeal membrane oxygenation (ECMO) using traditional ejection-phase indices is hampered by significant alterations in preload and afterload. Therefore, a load-independent index, which relates heart-rate-corrected mean velocity of circumferential fiber shortening (VCFc) to afterload, measured as end-systolic wall stress (ESS), was used to assess left ventricular function in 18 term neonates undergoing ECMO. The mean age at the onset of ECMO was 75.5 h and the duration of therapy was 171 +/- 106 h. Left ventricular performance was highest before the onset of ECMO (VCFc = 1.65 +/- 0.49 circ/s) and decreased toward normal during (1.38 +/- 0.33 circ/s) and following ECMO (1.29 +/- 0.16 circ/s). Initially, nine of 17 (53%) patients had enhanced performance for the degree of afterload but in only 16 of 48 (33%) studies during ECMO and none following ECMO was VCFc elevated beyond the normal range predicted for ESS. These changes in left ventricular performance may be the result of variations in exogenous, as well as endogenous, catecholamines rather than intrinsic alterations in myocardial contractility. It is concluded that the VCFc/ESS relation permits a meaningful assessment of ventricular performance in critically ill neonates undergoing ECMO.