Ethical approval for this retrospective study (no. 2021-01-008-002) was provided by the institutional review board. Written informed consent for enrollment in the study was renounced because of the retrospective nature of this study.

All patients who underwent operation under general anesthesia and were intubated by the chief resident with the Optiscope^TM at our medical center from October 2020 to February 2021 were eligible for this study. The anesthesiology chief resident who performed all the intubations in this study had performed over 50 successful tracheal intubations using the Optiscope^TM with more than 3 years’ of experience. Patients intubated by other residents and staff anesthesiologists were excluded to minimize inter-intubator variability. Patients with upper airway abnormalities, such as tumor, trauma, inflammation or foreign body, were excluded. Patients with risk of aspiration, history of difficult intubation, or American Society of Anesthesiologists (ASA) physical status classification of ≥ 4 were also excluded.

Patients were assigned to 2 groups depending on the execution status of the BURP maneuver (Group A-BURP maneuver, Group B-conventional maneuver). The staff anesthesiologists assisted the chief resident in intubating patients by performing the BURP maneuver or conventional maneuver. A current staff anesthesiologist at our institute routinely uses the BURP maneuver whenever intubation is attempted regardless of any circumstances including the performing device, and patient characteristics. Thus these patients were assigned to group A. The other staff anesthesiologists do not perform the BURP maneuver, therefore categorizing remaining patients into group B.

In this study, demographic data and the ASA physical status were collected as general data. Airway related variables consisted of the presence of preoperative dental injuries, diagnosis of obstructive sleep apnea history (OSA hx.), thyromental distance (TMD), sternomental distance (SMD), neck circumference (NC), interincisor distance, presence of neck rotation of motion restriction history (ROM hx.), and the Mallampati classification. The sternomental distance was considered the distance from the upper border of the manubrium to the tip of the mandible. It was measured with the head fully extended on the neck with the mouth closed. Intubation data consisting of the degree of manual mask ventilation, the total number of attempts, and intubation time were collected. The degree of manual mask ventilation was recorded by the chief resident who performed the intubations. The degree of mask ventilation was categorized into three groups (easy, moderate, and difficult), and converted into quantitative variables 1, 2, and 3, respectively, for statistical analysis. The primary outcome of this study was the intubation time. The intubation time was defined as the time interval between the removal of the ventilation mask from the patient’s face to just after the connection of the endotracheal tube with the breathing circuit. In each group, the intubation time was limited to 90 seconds per attempt. Whenever the intubation time reached 90 seconds or the SpO₂ < 90%, the attempt was ceased and manual ventilation was performed for 1 min with 100% oxygen [10]. Failed tracheal intubation was defined as failure to secure the airway with three consecutive attempts [4,10]. Upon failing to intubate, alternative devices such as fiberoptic bronchoscope or Pentax-AWS^® were used, and the corresponding suspect was excluded. Postoperatively, sore throat pain (11 Visual analog scale score), oral cavity bleeding, hoarseness, and dental injury were checked by a post-anesthesia care unit nurse who was blinded to the group assignment. The 11 visual analog scale score ranges from 0 to 10, with 0 representing no pain and 10 representing the worst imaginable pain.

All procedures other than the execution of the BURP maneuver were identical between the two groups. Patients entered the operating room without any premedication. Following routine monitoring, including electrocardiography, pulse oximetry, noninvasive blood pressure measurement was performed. After pre-oxygenation with 100% oxygen via facemask for 3 min, general anesthesia was induced with propofol (2 mg/kg) and sevoflurane. Rocuronium 0.6 mg/kg was administered after the loss of consciousness to facilitate intubation with neuromuscular relaxation. Subsequently, 0.2 mg of glycopyrrolate was administered to reduce oral secretion. After a minimum of 120 s from rocuronium administration, intubation was performed with the Optiscope^TM. In group A (BURP group) a staff anesthesiologist performed the BURP maneuver on the thyroid cartilage. The Optiscope^TM was prepared with a tracheal tube (internal diameter 7.5 mm for males and 7.0 mm for females) coated by an anti-fog agent at the end tip of the camera, and the stylet lubricated where the tube was placed. The Optiscope^TM was bent at a 45° angle 5 cm from the distal end. The Optiscope^TM was then inserted into the posterior pharynx using the chin lift with the non-dominant hand. After identifying the epiglottis, the distal tip of the Optiscope^TM was advanced under the epiglottis into the vocal cord and the tube was further advanced until the cuff passed the glottis. Successful intubation was confirmed based on the auscultation of both lung fields and the appearance of continuous end-tidal carbon dioxide monitoring using a ventilator.

Continuous variables were compared using the Student’s t-test, assuming that each variable demonstrated normal distribution following the central limit theorem with a subject number larger than 30. Categorical variables were compared using Fisher’s exact test.

The following variables were each coded, converting them to categorical variables. ASA and Mallampati classifications were coded into their inherent numbers (1, 2, 3 for ASA and 1, 2, 3, 4 for Mallampati). Pre-op dental injury, OSA hx., Neck ROM hx., were coded into 0, or 1, where 0 meant an existent history, while 1 meant no history. Post-op. bleeding, post-op. hoarseness, and post-op. dental injuries were also coded into 0 or 1, where 0 meant no complication and 1 meant the occurrence of complication. Sex was also coded into 0 or 1, where 0 represented male and 1 represented female.

Categorical variables are presented as numbers (percent), and continuous variables are presented as mean ± SD.

All statistical analyses were conducted using IBM SPSS Statistics (version 25.0, IBM Corp., USA).

Multiple linear regression analysis was conducted to identify the effect of each variable on intubation time. The BURP maneuver application status was coded into a categorical variable for this analysis, with group A being coded 0 and group B being 1.

In our study, analysis with a value of P < 0.05 was defined as statistically significant.