At the end of anaesthesia after discontinuation of nitrous oxide and removal of endotracheal tube, 1000/0 oxygen is administered to the patient to prevent:

(Ref : Willer 10th ed) On discontinuation of N2O administration, nitrous oxide gas can diffuse from blood to the alveoli, diluting O2 in the lung. Produce an effect called "Diffusional hypoxia". To avoid hypoxia, 100% O2, rather than air should be administered when N2O discontinued.