A 35-year-old man is admitted to ICU with smoke inhalation injury after a house fire. He has soot in his oropharynx, singed nasal hairs, and hoarse voice. ABG on 15 L/min: pH 7.38, PaO2 28 kPa, PaCO2 4.5 kPa, HCO3 22 mmol/L, COHb 32%, lactate 5.5 mmol/L. What is the most important immediate management priority?