The purpose of this prospective randomized controlled trial is to determine whether using the Oxygen Reserve Index can prevent hyperoxemia in pediatric patients receiving single-lung ventilation.

Participants will have their FiO2 adjusted in a prescribed manner based on the arm to which they are assigned.

The researchers will compare whether blood oxygen levels were lower in the ORI group.

Inclusion Criteria:

• Children under 7 years of age undergoing surgery under unilateral pulmonary ventilation
• Children with American Society of Anesthesiology physical status I, II, III

Exclusion Criteria:

• Patient who have chronic respiratory failure
• Patient who have a history of bronchopulmonary dysplasia, respiratory distress syndrome of neonate, laryngomalacia, tracheomalacia or tracheal stenosis
• Patient whose initial Oxygen Reserve Index value is zero
• Patient who need supplementary oxygen before surgery