A 6 year-old boy presented for redo surgery with a sub aortic obstruction. He had previously undergone aortic surgery 4 years prior and the subsequent planned operation was either further relief within the sub-aortic region or possibly a Ross procedure.The patient was 19.5kg and 107cm with a BSA of 0.76m² His calculated flow was 2lpm The chosen circuit consisted of a preconnected Liva Nova D101,3/8” venous line and 1/4” arterial line (departmental protocol describes this up to 2.5lpm). CDI venous and arterial inline monitoring were used.The patient was anaesthetised as per usual protocol. After a re-do sternotomy heparin was administered, and bypass was started initially with a single SVC cannula, reaching 2/3 of full flow. During this time ventilation was maintained. Following IVC cannulation full bypass was reached and the ventilation stopped. Cooling to 34°C was commenced. It was noted that the inline arterial pO₂ was low, FiO₂ was increased but 100%was required to produce an adequate pO₂ i.e. >20kPa. Attempts to reduce the FiO₂ to expected levels were unsuccessful. Venous oxygen saturations were adequate throughout. An arterial blood gas confirmed the inline readings. An inline oxygen gas analyser confirmed the oxygen content. An independent gas flow analyser confirmed the gas supply. The gas line and supply were checked for leaks. The isoflurane vaporiser was temporarily turned off. An emergency oxygen cylinder was attached to the oxygenator, good oxygenation occurred. The blender was attached to emergency oxygen and air cylinders but on reducing the FiO₂ below 100% the pO₂ again dropped to<10kPa. The blender was changed however the issue did not resolve. The aortic cross clamp had not been applied. Circuit pressures and ACTs were acceptable.