Patients with full stomachs face a high risk of regurgitation and aspiration under general anesthesia. To minimize the time between the loss of airway protective reflexes and successful tracheal intubation, rapid sequence induction intubation is commonly used. However, these patients are particularly vulnerable to hypoxemia during anesthesia induction, especially in emergency cases. Pre-oxygenation before induction is crucial for ensuring patient safety during apnea.

High-flow nasal oxygen (HFNO) therapy, which consists of an air/oxygen blender, an active humidifier, and a single heated circuit, has recently gained widespread use in intensive care units (ICUs) for managing hypoxemic respiratory failure. HFNC can deliver a constant fraction of inspired oxygen (FiO₂) from 0.21 to 1.0 at high flow rates (up to 60 L/min or higher). Its advantages include generating continuous positive airway pressure, reducing anatomical dead space, improving ventilation-perfusion matching, enhancing mucociliary clearance, and decreasing the work of breathing.

Given these benefits, HFNO has the potential to improve pre-oxygenation before and during anesthesia induction in emergency surgery patients with full stomachs.

This is a prospective, single-center, randomized controlled trial designed to evaluate the effects of HFNO on preoxygenation before and during anesthesia induction in emergency surgery patients with full stomachs. Adult patients undergoing emergency surgery with general anesthesia will be enrolled in the study.

After obtaining written informed consent, patients will be randomly assigned to one of the study groups:

- Intervention Group: Patients will undergo HFNO preoxygenation for 3 minutes with a flow rate of 60 L/min of heated and humidified pure oxygen (100% FiO₂, 37°C - Optiflow; Fisher & Paykel Healthcare, Auckland, New Zealand). To minimize air contamination, large or medium nasal cannulae will be selected based on the patient's nostril size. During the intubation process, HFNO will be maintained to facilitate either: Continuous oxygenation while the patient breathes spontaneously, or Apneic oxygenation during laryngoscopy for rapid sequence intubation (RSI).

- Control Group: Patients will undergo preoxygenation for 3 minutes using a face mask (sized appropriately to fit the patient and ensure an airtight seal) connected to an Aisys CS2 ventilation system (General Electric, GE Healthcare, Oy, Finland). In this group, the ventilation system will be set with a fresh gas flow of 10 L/min, FiO₂ = 100%, without inspiratory support or expiratory positive pressure. The face mask (Economy, Intersurgical, Fontenay-sous-Bois, France) will be removed after induction to enable intubation.

Rapid sequence induction and intubation were performed using fentanyl (2 mcg/kg), propofol (2 mg/kg), and rocuronium (1 mg/kg). Intubation was performed 90 seconds after rocuronium administration. Cricoid pressure was applied from the moment the patient lost consciousness until intubation was successfully completed.

The current guidelines advise interrupting intubation to focus on oxygenation (ie, face mask ventilation) for oxygen desaturation ≤94%.

Inclusion Criteria:

• Emergency surgical patients at risk of gastric fullness requiring endotracheal intubation.
• Patients aged 18 years or older.
• Health status classified as ASA I or II.
• Mallampati classification I or II.

Exclusion Criteria:

• Patients with a predicted difficult airway, facial deformities, or an inability to achieve a proper mask seal.
• Patients with respiratory diseases.
• Pregnant patients.
• Patients allergic to anesthesia or resuscitation drugs.
• Patients who do not consent to participate in the study.