A 60-year-old man on ICU with severe ARDS and refractory hypoxaemia is commenced on VV-ECMO. Two days later, the ECMO specialist notes dark blood in the drainage (venous) limb and bright red blood in the return (arterial) limb. The membrane oxygenator pre- and post-membrane blood gases show: pre-membrane PaO2 4.5 kPa, post-membrane PaO2 55 kPa. These findings are consistent with: