The study propose that the using the high-flow nasl oxygen to provide oxygen for maternal can improve the fetal acidemia and the neonatal outcomes during cesarean section with combined spinal-epidural anesthesia.

Maternal oxygen supplementation is widely used intrauterine resuscitation technique in clinical, which can improve fetal oxygenation and prevent fetal acidemia. The incidence of hypotension is high in cesarean section with combined spinal-epidural anesthesia. Hypotension contributes to insufficient perfusion of uterus and placenta, resulting in decreased fetal oxygen supply and neonatal acidemia. High flow nasal oxygen have a better improvement oxygenation of patients by providing high flow oxygen airflow with heating and humidification through nasal cannula.

Inclusion Criteria:

• An elective cesarean section in Beijing Tongren Hospital
• ≥37 weeks' gestation
• American society of Aneshesiologists（ASA） I-III
• Aged 18 to 45 years old
• The fasting time is 6-8h, and the water restriction time is ≥2h
• Anesthesia: combined spinal-epidural anesthesia
• Agree to the study and sign the informed consent

Exclusion Criteria:

• Fetal or placental abnormalities are known
• Maternal has complications
• Maternal BMI ≥40 kg/m2
• History of difficult intubation or known difficult airway
• Contraindications for HFNO such as nasal lesions or structural changes
• Gastric reflux disease