Extracorporeal Membrane Oxygenation (ECMO) is an invasive and resource intense treatment used to support critically ill patients who have suffered severe cardiac arrest, cardiac failure or respiratory failure (including severe cases of COVID-19). ECMO acts as a mechanical circulatory support temporarily replacing the function of the heart or lungs by oxygenating blood and removing carbon dioxide, allowing time for these organs to recover. Many critically ill patients, including those on ECMO, have an increased risk of bleeding and reduced production/increased destruction of red blood cells (RBCs). This can lead to anaemia (haemoglobin levels \<120 g/l), a condition where the body lacks enough healthy RBCs to carry enough oxygen to the body's tissues. Therefore, patients on ECMO frequently require RBC transfusion, with clinicians having to decide if administering an RBC transfusion (with its associated risks) is higher than tolerating complications of anaemia. ROSETTA is a feasibility study that aims to determine the safety and feasibility of randomizing patients on ECMO to a restrictive RBC transfusion strategy (maintain Hb concentration above 70g/L) or to a more liberal transfusion strategy (maintain Hb concentration above 90g/L). Feasibility is defined as the ability to achieve a mean separation of at least 10g/L between the average lowest daily haemoglobin values in the two study groups.