Twenty minutes after initiating ECMO, PaO2 at flow rates of 1–3 L/min was 28.6 ± 4.5 and 23.7 ± 1.2, 62.6 ± 9.3 and 46.3 ± 3.2, and 127.9 ± 21.5 and 91.9 ± 9.7 mmHg in the appropriate and inappropriate directions, respectively. We reported that sufficient oxygenation cannot be expected when blood is delivered in the inappropriate direction.