Atelectasis (the lack of gas exchange within alveoli, due to alveolar collapse) affects around 90% of cardiac surgery patients, causing hypoxaemia, and sometimes, acute respiratory failure requiring re-intubation and mechanical ventilation. Positive pressure oxygen systems help reduce atelectasis formation but may be uncomfortable for the patient. Nasal high flow (NHF) oxygen therapy provides low level positive airway pressure and has been demonstrated to be more comfortable than oxygen masks. This research will investigate, using randomised controlled trial methodology, whether administration of NHF prophylactically to cardiac surgical patients post-extubation reduces atelectasis. It will also determine whether NHF is better tolerated in patients when compared to standard oxygen therapy. Given that every year more than one million patients in developed countries have cardiac surgery, such a study - if positive - could impact significantly on how oxygen therapy is provided to patients post-operatively.