A 40-year-old man with severe ARDS is on VV-ECMO with sweep gas flow of 5 L/min and blood flow of 4.5 L/min. FiO2 on the ECMO circuit is 1.0. The native lung ventilator is set to 'rest settings': PCV, Pinsp 10 cmH2O, PEEP 10 cmH2O, FiO2 0.3, RR 10/min. The team wishes to assess native lung recovery. What is the most appropriate way to assess whether native lung function is improving?