Our aim was to analyze left ventricular fractional shortening during extracorporeal membrane oxygenation under the influence of changing volume loading conditions induced by a ductal left-to-right shunt. In all patients, the fractional shortening was observed using echocardiography before, during, and after bypass, irrespective of the presence or absence of the ductal left-to-right shunt. During membrane oxygenation, there was a significant decrease in fractional shortening (p less than 0.001), with no difference before and after membrane oxygenation. A greater decrease in fractional shortening was observed in the group with a ductal left-to-right shunt when compared to patients lacking the ductal shunt (p less than 0.006). The diastolic diameter of the left ventricle also increased significantly during the membrane oxygenation in those patients with left-to-right ductal shunting. Moreover, the patients with left-to-right shunting showed a very severe decreased fractional shortening, lower than 10 per cent, with significantly greater frequency (p less than 0.05) during the course of membrane oxygenation. Conclusion: An important decrease in left ventricular fractional shortening is observed during veno-arterial extracorporeal membrane oxygenation. Left-to-right shunting during bypass, as seen in the patients with patency of the arterial duct, increases the loading conditions on the left ventricle, and produces a significant increase in left ventricular diastolic dimensions. Despite the effects of volume loading produced by the ductal shunt during bypass, the decrease in fractional shortening is significantly more pronounced for these patients. Therefore, during membrane oxygenation the volume loading produced by the ductal shunt is unable to prevent a decrease in left ventricular fractional shortening.