Why 100% oxygen has to be given to a patient after recovering from N2O anesthesia?

Diffusion hypoxia may occur at the conclusion of nitrous oxide administration if patients are allowed to inhale room air. The initial high-volume outpouring of nitrous oxide from the blood into the alveoli when inhalation of this gas is discontinued can so dilute the PAo2 that the Pao2 decreases. The occurrence of diffusion hypoxia is prevented by filling the patient's lungs with oxygen at the conclusion of nitrous oxide administration. Basics of Anaesthesia 7e pg: 104