Compared with adults, children have higher metabolic needs, and the airway is more likely to collapse. Before tracheal intubation after anesthesia induction, the patient 's spontaneous breathing completely disappears. At this critical stage, the residual oxygen of the lung is consumed, resulting in hypoxemia and atelectasis. Therefore, it is necessary to explore the best oxygenation strategy during intubation. In addition, ultrasound has become a common equipment in the operating room. It has the advantages of portability, repeatability, and no radiation, and can provide strong support for the diagnosis of gastric distension.

At present, hypoxia is still the main cause of complications and death during perioperative period. Compared with adults, children have lower functional residual volume and lower tolerance to hypoxia caused by apnea due to their special physiological and functional characteristics. During anesthesia induction and tracheal intubation, spontaneous breathing completely disappears. At this critical stage, hypoxemia is prone to occur, which in turn causes various serious complications. Mask ventilation may occur mask ventilation difficulties and flatulence ; since the introduction of nasal high-flow oxygen therapy ( HFNC ) into the operating room in 2015, its oxygenation method has been shown to be able to significantly improve blood oxygen when used alone in pre-oxygenation. When tracheal intubation is performed, HFNC can still maintain ventilation in the patient 's nose, so HFNC can combine the advantages of both masks to perform pre-oxygenation. There are few studies on the effect of HFNC on apnea oxygenation in children in the operating room.

Inclusion Criteria:

• Age 2-10 years old;
• American Society of Anesthesiologists (ASA) Level I or II;
• Children with healthy lungs and hearts;
• Clear headed and able to cooperate with anesthesiologists for treatment.

Exclusion Criteria:

• Contraindications for HFNC: (1) Complete obstruction of the upper respiratory tract; (2) Skull base fracture or nasal bone fracture; (3) Patients who refuse to use HFNC;
• The American Society of Anesthesiologists (ASA) rating is greater than Level II;
• Children with upper respiratory tract infections within 2 weeks;
• Pulmonary dysfunction, congenital heart disease in children;