To investigate the independent effects and interaction of pre-oxygenation position (left lateral vs left lateral with 30° head-up tilt) and examination position (left lateral vs left lateral with 30° head-up tilt) on hypoxemia during painless gastroscopy and colonoscopy using a 2×2 factorial design.
Aged 18 to 75 years, regardless of gender
Body mass index (BMI): 18.5 - 35 kg/m²
ASA physical status class I, II or III
Scheduled for painless gastroscopy, painless colonoscopy, or combined painless gastroenteroscopy
Severe cardiopulmonary diseases, including severe COPD, heart failure, severe arrhythmia and pulmonary hypertension
Difficult airway identified before anesthesia
Severe obstructive sleep apnea (OSA)
Spinal or thoracic deformity
Parallel
Pre-oxygenation Position （left lateral position）
Procedure/SurgeryAn angle measuring device is placed beside the head of the examination bed to enable precise adjustment of the head position. Pre-oxygenation phase: Administer pure oxygen via a standardized mask with tight fit, at a flow rate of 6 L/min; instruct the patient to take deep breaths while receiving continuous oxygen for 3 minutes, maintaining the same body position throughout. If the position is altered during this phase, discontinue the procedure. Left lateral pre-oxygenation: The patient assumes a left lateral position on a horizontal bed surface with the head elevated at 0°, supported by pillows to maintain a neutral spine position and avoid excessive neck extension or flexion.
Examination Position（left lateral position ）
Procedure/SurgeryExamination Phase: From the initiation of anesthesia and endoscope insertion until completion of the procedure, the patient's position and head-of-bed angle must remain fixed throughout; any adjustments are prohibited during this phase. If the position is altered midway through the examination, the patient will be excluded from the study. Left lateral position examination: The patient assumes a 90-degree left lateral position on a horizontal bed surface with the head of the bed elevated to 0°, supported by pillows to maintain a neutral spinal alignment, avoiding excessive neck extension or flexion.
Pre-oxygenation Position（ left lateral head elevation in a 30-degree position）
Procedure/SurgeryAn angle measuring device is placed beside the head of the examination bed to allow precise adjustment of the head position. Pre-oxygenation phase: The patient receives pure oxygen via a standardized mask with tight fit, at a flow rate of 6 L/min; they are instructed to take deep breaths while receiving continuous oxygen for 3 minutes, maintaining the same body position throughout. If the position is altered during this phase, the procedure is discontinued. Left-sided head elevation (30°): During pre-oxygenation, the patient lies in a left lateral position on a horizontal bed surface with the head elevated by 30°, supported by pillows to maintain a neutral spinal alignment and prevent excessive neck extension or flexion.
Examination Position（left lateral head elevation in a 30-degree position）
Procedure/SurgeryExamination Phase: From the initiation of anesthesia and endoscope insertion until completion of the procedure, the patient's position and head-of-bed angle must remain fixed throughout; any adjustments are prohibited during this phase. If the position is altered midway through the examination, the patient will be excluded from the study. Left lateral decubitus examination: The patient assumes a left lateral position on a horizontal bed surface with the head elevated at 30°, supported by a pillow to maintain a neutral spinal alignment, avoiding excessive neck extension or flexion.
Incidence of hypoxemia
Incidence of SpO₂ ≤ 90% for \>10 seconds during the examination.
Safe duration of pre-oxygenation
Time from cessation of oxygen administration (start of anesthesia induction) to first SpO₂ \<92% for ≥3 seconds. If no desaturation occurs, the value equals the duration from oxygen cessation to end of procedure (right-censored).
Incidence of subclinical respiratory depression
Defined as the proportion of participants experiencing SpO₂ between 90% and 95% (inclusive) at any time during the examination.
Requirement for jaw thrust
Number of participants requiring manual jaw thrust to relieve airway obstruction during the examination.
Requirement for nasal cannula oxygen
Number of participants requiring oxygen supplementation via nasal cannula during the examination.
Safety outcome endpoints：heart rate
Recorded at T0 (before pre-oxygenation), T1 (before induction), T2 (scope insertion), and T3 (scope removal).
Safety outcome endpoints：blood pressure
Recorded at T0 (before pre-oxygenation), T1 (before induction), T2 (scope insertion), and T3 (scope removal)
Safety outcome endpoints：peripheral oxygen saturation (SpO₂)
Recorded at T0 (before pre-oxygenation), T1 (before induction), T2 (scope insertion), and T3 (scope removal).
Safety outcome endpoints：Incidence of hemodynamic instability in recovery
Occurrence of tachycardia (\>100 bpm), bradycardia (\<50 bpm), or hypotension (systolic blood pressure \<80 mmHg) during the recovery period.
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