HFNC oxygenation is a novel technique that depends on apnoeic oxygenation. Even though the patient is paralysed and not breathing, oxygenation and carbon dioxide clearance still occurs. This is due to the difference between the alveolar rates of oxygen removal and carbon dioxide excretion which generates a negative pressure gradient. Oxygenation of the patient is therefore due to the high inspiratory fraction of oxygen, apnoeic ventilation and the small continuous positive airway pressure caused by the high flow of gas. This pressure increases lung volume and recruits collapsed alveoli. Carbon dioxide clearance is due to gas mixing and flushing of the airway dead space. The additional humidification of the high flow oxygen prevents mucociliary damage of the airway.