Blood which recirculates through the circuit of a veno-venous Extracorporeal Membrane Oxygenation (V-V ECMO) does not contribute to the systemic oxygenation of a patient on V-V ECMO and is called the recirculation fraction (Rf). Theoretically, the optimization of ECMO blood flow is possible using Rf measurements. A prospective, observational study will be performed measuring the Rf of total ECMO blood flow in patients with acute respiratory distress syndrome (ARDS) on V-V ECMO with an ultrasound dilution technique. ECMO blood flow will be optimized by reducing ECMO blood flow in accordance with the measured Rf as long as systemic oxygenation is not compromised.