The present study was approved by the Ethics Committee of the Affiliated Luan Hospital, Anhui Medical University (NO.2021LL005), and written informed consent forms were signed by all the participants or their relatives before enrollment. We recruited patients from the Affiliated Luan Hospital at Anhui Medical University (Anhui Province, China) between July and September 2022. The trial was retrospectively registered at the China Clinical Trial Registration Center (www.chictr.org.cn, ChiCTR2200061560; Principal investigator: Tao Zhang; Date of registration; 9 June, 2022).
The inclusion criteria were as follows: (1) patients requiring transoral tracheal intubation for general anesthesia for elective surgery in our hospital, (2) ASA class Ⅰ and Ⅱ, (3) 18–64 years of age, and (4) modified Mallampati class Ⅲ and Ⅳ. Patients with any of the following conditions were excluded: (1) a mouth opening < 2 cm, (2) difficulty receiving mask ventilation, (3) anatomical abnormalities of the upper airway (trauma, tumor, and deformity); (4) cervical spine instability, (5) inability to properly understand and cooperate with the experiment. (6) And those who refused to participate in this study.
Anesthesia Protocol
Anesthesia preparation: All patients fasted from food and water before surgery. Upon entering the operating room, the patient's body temperature, heart rate, pulse, oxygen saturation, blood pressure, electrocardiogram and end-tidal carbon dioxide (PETCO2) were recorded. A Narcotrend monitor was used to measure the depth of anesthesia. For females, a reinforced endotracheal tube with an inner diameter of 7.0 mm was used, while, for males, a 7.5 mm reinforced tube was needed. Intravenous access was opened and 1 mg of valacyclovir + 10 mg of dexamethasone were administered. anesthesia was administered by endotracheal general anesthesia. In all patients, dexmedetomidine 0.8 µg/kg was pushed intravenously 10 min before induction of anesthesia.
Induction and maintenance of anesthesia: Induction anesthesia: included sequential injections of 0.05 mg/kg midazolam, 0.5 µg/kg sufentanil, 0.3 mg/kg etomidate, and 0.8 mg/kg rocuronium in all three groups. Tracheal intubation was performed 90 seconds after rocuronium injection. All intubations were performed by anesthesiologists of the same seniority.
Group VL: The video laryngoscope (TDC- K3, UE Medical Equipment Co. Ltd., Zhejiang, China, Fig. 1.A) was fitted with a disposable transparent lens jacket, and the front end of the tracheal tube was shaped to approximately 60° with the same curvature as the anterior part of the visual laryngoscope by applying a metal tube core, with the patient lying in a flat position and the anesthesiologist standing at the head end of the patient. After pushing the jaw open with the right hand, the visual laryngoscope was placed in the patient's mouth with the left hand, along the tongue surface and forward. The lingual-palatal and palatopharyngeal arches are passed to reach the pharyngeal cavity, and when the vocal fissure is exposed in the observation display, the tracheal tube is gently pushed into the trachea with the right hand, and the core is plucked out at the same time.
Group VS: The video stylet (TRS- K2, UE Medical Equipment Co. Ltd., Zhejiang, China, Fig. 1.B) is inserted into the tracheal tube, and the tracheal tube is fixed at the upper end to prevent the lens from sticking out of the tracheal tube and avoid contamination of the lens with secretions. The anesthesiologist opens the patient's mouth with the left hand, lifts the lower jaw, inserts the lens handle into the mouth from the right corner of the mouth with the right hand, following the oral cavity into the lateral wall of the pharynx. After 11–12 cm (at the level of the upper larynx), the lens is aimed to the left and middle of the neck while observing the screen to locate the vocal fold structure. The tracheal tube is fed into the trachea, the core is pulled out and the video stylet is withdrawn, and the capsule is inflated.
FV group: The flexible videoscope (TIC, UE Medical Equipment Co. Ltd., Zhejiang, China, Fig. 1. C) is inserted into the tracheal tube, the tracheal tube is fixed at the upper end of the light-guiding hose, the patient's jaw is lifted to open the mouth and put in the disposable mouth pad. The anesthesiologist holds the handle of the flexible visualization mirror in the left hand. The right hand is used to extend the light-guiding hose through the mouth pad into the patient's mouth and afterward to the pharynx. After the vocal cords are exposed, pass between the vocal cords. The anesthesiologist can adjust the direction of the front end to enter the trachea. At 3 cm from the bulge, the right hand is used to gently push the tracheal tube into the trachea, withdraw the light-guiding hose, and inflate the cuff.
After successful intubation of the tracheal tube, the patient was connected to the anesthesia machine for mechanical ventilation, the tidal volume was adjusted to 10 ml/kg, and the respiratory rate required 12 breaths/min to maintain PETCO2 at 35–45 mmHg. Anesthesia maintenance: Propofol 4–6 mg/kg/h and remifentanil 0.1–0.2 µg/kg/min were pushed in both groups and discontinued at the time of skin closure. This dosage was adjusted according to the measure of anesthetic depth using bispectral index monitoring (BIS) at a target zone of 40–60. Rocuronium and sufentanil were administered as needed during surgery. After surgery, patients were sent to the post-anesthesia care unit (PACU).
Patient Evaluation
The success rate of the first intubation was recorded as the primary observation. The Wilson-C-L classification of the glottic exposure grade (W-C-L 1 = all vocal folds are visible; W-C-L 2 = half of the vocal folds are visible; W-C-L 3 = only the spongy cartilage is visible; W-C-L 4 = only the epiglottis is visible, and not the vocal folds; and W-C-L 5 = no anatomical part of the larynx is visible) and intubation time (time between the end of mask ventilation and confirmation of the waveform by end-tidal carbon dioxide monitoring) were used as secondary observations. According to the consensus of the reviewed literature, successful intubation was defined as the ability to see the true vocal cords and successfully pass the tracheal tube through the true vocal cords within 100 seconds, and intubation was attempted a maximum of three times; failure of all three intubations resulted in withdrawal from the experiment. The intubation device was withdrawn from patients who experienced a failed first intubation and patients were re-oxygenated for three minutes before a second intubation was performed; and the time of intubation was recorded again. Failed intubation was defined as an intubation time longer than 3 minutes or patient oxygen saturation reduced to less than 90%. Patients were followed up in the PACU after the procedure and asked whether they had a sore throat, hoarseness, and other discomfort, to record the occurrence of adverse effects associated with intubation.
Statistical Analyses
To calculate the sample size, we performed a preliminary experiment with 20 patients in each group (60 patients in total), and the first-pass intubation success rate in each group was 75% in Group VL, 95% in Group VS, and 90% in Group FV. With α = 0.05 and test efficacy 1-β = 0.8, a minimum sample size of 155 could be calculated according to PASS15.0.5 software. A total of 189 patients were finally included after supplementing a 20% loss of follow-up rate. Prior to the start of the study, all patient names were randomly placed in opaque envelopes using computer-generated random numbers according to the equipment used for intubation, by someone other than the investigator, and the envelopes containing the random numbers were divided into 3 groups (59 per group). After completion of the procedure, case questionnaires containing only the random numbers were submitted by the researchers, but the followers who conducted the follow-up survey were not aware of the random groupings.
The data were analyzed using SPSS 26.0 statistical software. Continuous variables, such as the basic characteristics of patients, are expressed as the mean ± standard deviation. Comparisons of measurement data were performed using the F test if the conditions of normality and chi-squared were met; otherwise, the rank sum test was used. Comparisons of count data were performed using the chi-square test. All significance tests were two-sided tests with a test level of α = 0.05, and P < 0.05 indicated that the differences were statistically significant.