Bronchoscopic procedures performed under endobronchial ultrasound (EBUS) guidance are widely used minimally invasive diagnostic methods for the evaluation of mediastinal lymph nodes. Endobronchial Ultrasound-Guided Transbronchial Needle Aspiration (EBUS-TBNA) is an indicated procedure for the diagnosis, staging, and histopathological characterization of primary or metastatic tumors. The procedure is generally performed under moderate or deep sedation to enhance patient comfort and optimize procedural success. Deep sedation has been shown to be a safe and effective approach in terms of respiratory complications in both younger and elderly patients.

However, hypoxemia may develop during EBUS procedures due to mechanisms such as mechanical airway obstruction, sedation-related ventilatory depression, and ventilation-perfusion mismatch. A decrease in oxygen saturation is frequently observed during bronchoscopic procedures, with reported rates of up to 94%. Studies have also reported a hypoxemia rate of approximately 25% during EBUS, even with the use of a nasopharyngeal oxygen cannula. Therefore, maintaining adequate oxygenation throughout the procedure is of critical importance. In patients receiving sedation under spontaneous respiration, oxygen support is typically provided via low-flow systems such as nasal cannulae, which can generally deliver flows of up to 15 L/min.

It has been reported that a fiberoptic bronchoscope occupies approximately 10% of the tracheal cross-sectional area and about 15% at the level of the cricoid ring, leading to partial obstruction. EBUS endoscopes have a larger outer diameter and, in addition, may exert more pronounced negative effects on respiratory mechanics due to the balloon mechanism used. The outer diameter of EBUS bronchoscopes typically ranges between 6.2 and 6.7 mm, mechanically occupying a significant portion of the tracheal lumen. This may restrict tracheal airflow, leading to partial obstruction and consequently predisposing to hypoxemia. Therefore, identifying objective anatomical predictors that can predict the development of hypoxemia is of clinical importance. However, to the best of our knowledge, no study in the literature has statistically investigated the relationship between tracheal measurements and hypoxemia occurring during EBUS.

In this study, the investigators aim to investigate the relationship between pre-procedural tracheal diameter measured by ultrasonography (USG) and the EBUS outer diameter/tracheal diameter ratio (airway occupancy ratio) with the development of hypoxemia during the procedure in patients undergoing EBUS-TBNA. Additionally, it is aimed to determine the optimal tracheal diameter cutoff value that may predict the development of hypoxemia.

Study Design and Patient Population This study will be conducted in accordance with the Declaration of Helsinki and will be carried out at SBÜ Ankara Atatürk Sanatoryum Training and Research Hospital after obtaining approval from the institutional ethics committee. The study is planned as a prospective observational cohort study. Adult patients scheduled to undergo EBUS-TBNA will be included.

The following parameters will be recorded:

• Age
• Sex
• Body mass index (BMI)
• FEV1 value
• Baseline SpO₂
• Lowest recorded SpO₂
• Systolic/diastolic/mean arterial blood pressure and heart rate measured at 10-minute intervals during the procedure
• Sedative drug doses
• Procedure duration
• Tracheal diameter measured by ultrasonography (USG)
• Airway occupancy ratio (EBUS outer diameter / narrowest tracheal diameter measured by USG)
• Duration and total number of hypoxic episodes during the procedure
• Requirement for airway interventions (jaw thrust, mask ventilation, intubation, etc.)
• Whether nasal oxygen flow was increased in case of hypoxemia, and if so, by how many L/min
• Presence of cough, hiccups, secretions requiring aspiration, bronchospasm, and procedure-related bleeding
• Whether the procedure was interrupted

H0: There is no significant association between ultrasonographic tracheal measurements and the development of hypoxemia during Endobronchial Ultrasound-Guided Transbronchial Needle Aspiration (EBUS-TBNA).

H1: There is a significant association between ultrasonographic tracheal measurements and the development of hypoxemia during Endobronchial Ultrasound-Guided Transbronchial Needle Aspiration (EBUS-TBNA).

Ultrasonographic Measurements In this study, tracheal diameter will be measured using a high-frequency linear probe while the patient is in the supine position with the neck in a neutral position. Patients will be instructed to take a deep inspiration, and measurements will be obtained during inspiration at the level just below the cricoid cartilage and at the level of the 2nd-3rd tracheal rings. The narrowest transverse tracheal diameter will be recorded. All ultrasonographic measurements will be performed by the same anesthesiologist.

Sedation Protocol Patients will receive 4 L/min oxygen via nasal cannula along with routine hemodynamic monitoring. Sedation will be administered according to the standard protocol routinely used in our clinic. During the procedure, patients will be maintained under deep sedation corresponding to a Richmond Agitation-Sedation Scale (RASS) score of -4. The total doses of sedative agents used will be recorded.

Inclusion Criteria:

• Patients scheduled for elective EBUS-TBNA
• Patients who provide written informed consent
• Patients with ASA physical status I-III
• Patients aged ≥18 years

Exclusion Criteria:

• Tracheal stenosis or significant airway obstruction
• History of tracheostomy
• Use of home oxygen therapy
• Pre-procedural SpO₂ < 90% on room air
• History of prior thoracic surgery
• Diagnosis of congestive heart failure
• Presence of pleural effusion
• FEV1 < 40%
• COPD patients classified as GOLD stage 3-4
• Restrictive lung disease (FVC < 50%)
• Advanced pulmonary hypertension
• History of surgery affecting neck anatomy
• Tracheal deviation due to large thyroid or mediastinal mass
• ASA physical status IV
• Pregnancy
• EBUS performed under general anesthesia (with endotracheal intubation or laryngeal mask airway)
• Hemodynamic instability

This study will be conducted in accordance with the Declaration of Helsinki and will be carried out at SBÜ Ankara Atatürk Sanatoryum Training and Research Hospital. In patients aged 18 years and older scheduled to undergo EBUS-TBNA, the narrowest pre-procedural tracheal diameter will be determined. The relationship between tracheal diameter and the development of hypoxemia will be analyzed.