Second gas effect

During induction of general anesthesia when a large volume of nitrous oxide is taken up from alveoli into pulmonary capillary blood, the concentration of gases remaining in the alveoli is increased. This results in effects known as the "concentration effect" and the "second gas effect". These effects occur because of the contraction of alveolar volume associated with the uptake of the nitrous oxide. 

   Second gas effect occurs when the uptake of one gas enhances the rate of rise of alveolar partial pressure of another gas that is administered at the same time. The second gas effect causes an acute increase in the arterial partial pressure of oxygen by 10% when nitrous oxide is initially administered.This is called alveolar hyperoxygenation.