A 32-year-old female is admitted to the emergency department following rescue from a house fire. She is unconscious and her face is covered in soot. There is no evidence of traumatic injury and her blood pressure is normal. Her oxygen saturation on pulse oximetry (SpO2) reads 98% and she is receiving supplementary oxygen via a non-rebreathe oxygen mask. On blood gas analysis, she has a pH of 7.1 with a lactate of 10 mmol l–1, her PO2 is 70 kPa and her PCO2 is 3.5 kPa. Her arterial oxygen saturation (SaO2) is 100%, whilst her fractional oxyhaemoglobin content (FO2Hb) is 90%.