Spinal anesthesia is the most commonly used anesthetic technique for cesarean section in developed countries, but vasodilation and a decrease in systemic vascular resistance caused by sympathetic blockade result in hypotension in 7-74% of parturients. The fetus receives oxygen from the mother via uteroplacental blood flow, and because uteroplacental circulation during pregnancy has minimal autoregulation, uterine blood flow changes in proportion to maternal blood pressure. Therefore, a reduction in uterine blood flow due to maternal hypotension can lead to fetal hypoxia or acidosis and is associated with low Apgar scores after birth.

Oxygen administration may offer potential benefits, such as improving maternal cerebral perfusion and preventing fetal ischemic injury, and thus low-flow oxygen via a conventional nasal cannula is commonly used. However, there are few studies evaluating the effects of high-flow oxygen administration on fetal well-being during cesarean section under spinal anesthesia. Low-flow oxygen delivery through a conventional nasal cannula, which is commonly used during cesarean section under spinal anesthesia, results in a fraction of inspired oxygen (FiO₂) of less than 40% due to dilution with ambient air. In contrast, OptiFlow THRIVE (Fisher and Paykel Healthcare, Panmure, Auckland, New Zealand) is a device capable of delivering high-flow oxygen through a nasal interface, allowing administration of 100% oxygen to the mother and potentially providing greater protection against fetal ischemic injury. Delivering non-humidified oxygen at flow rates above 10 L/min causes significant discomfort in awake patients, but OptiFlow THRIVE passes the gas through a heated humidification chamber immediately before delivery, enabling the administration of warmed and humidified oxygen even at high flow rates. Therefore, this study aims to compare high-flow nasal cannula oxygen therapy initiated upon operating room admission with conventional low-flow nasal cannula oxygen therapy during cesarean section under spinal anesthesia, assessing their effects on maternal hemodynamic parameters and, ultimately, on fetal acid-base status.

Inclusion Criteria:

• Pregnant women aged 20 years or older with American Society of Anesthesiologists (ASA) physical status class 2-3 who are scheduled to undergo cesarean section under spinal anesthesia.

Exclusion Criteria:

A. Emergency surgery B. Patients in whom massive hemorrhage is anticipated, such as those with placenta previa C. Multiple pregnancy, preterm birth at less than 36 weeks of gestation, intrauterine growth restriction, or fetal anomalies D. History of hospitalization for preterm labor within the past 4 weeks requiring tocolytic therapy E. Respiratory diseases requiring oxygen therapy (e.g., moderate or severe asthma, chronic obstructive pulmonary disease) F. Individuals who are unable to read the consent form (e.g., illiterate individuals or foreigners who cannot read Korean)

As this study evaluates umbilical arterial base deficit measured immediately after birth during cesarean section, only female participants are eligible to this study.