Study Design and Patient Enrollment
All methods were carried out in accordance with relevant guidelines and regulations. This prospective, randomized, single-blinded, parallel groups study was approved by the Institutional Review Board of the Hokushin General Hospital (approval number: 2017016) and then the protocol was registered at the University Hospital Medical Information Network in Japan (identification number: UMIN000030204; date of registration: 30/11/2017). After obtaining written informed consent at the time of preoperative anesthetic evaluation, consecutive patients aged 20 years or older with American Society of Anesthesiologists (ASA) physical status classification 1 or 2 who were scheduled to undergo elective surgery under general anesthesia and who required oral tracheal intubation were enrolled. Exclusion criteria included the need for rapid sequence induction, known difficult mask ventilation or tracheal intubation, cervical spine instability or cervical myelopathy, symptomatic asthma or reactive airway disease, uncontrolled hypertension, an aneurysm, a history of ischemic heart disease, gastric reflux, stroke or cerebral hemorrhage, and inability to give consent. Eligible patients were randomly allocated to a group for intubation using a malleable stylet without stylet angulation at the holding position (non-angulation group) and to a group for intubation using a malleable stylet with stylet angulation at the holding position (angulation group) in a 1:1 ratio using computer-generated random numbers with a fixed block size of 4. Randomization was stratified according to sex, body mass index (BMI) and airway difficulty score. The allocation was sealed in an opaque envelope, and a study assistant opened the sealed envelope before induction of anesthesia and provided the designated stylet formation according to the group allocation. Because of difficulties with blinding of the stylet formation, intubation operators and study assistants could not be blinded.
Preoperative Evaluation and Anesthetic Technique
During the preoperative anesthetic evaluation, patient characteristics including age, sex, ASA physical status, height, weight, and BMI were recorded. Airway characteristics including thyromental distance, Mallampati score, mouth opening, neck mobility and upper incisors were also evaluated, and the airway difficulty score22 as an indicator of difficulty in airway management was calculated.
In the operating room, patients were monitored by an electrocardiogram, noninvasive blood pressure measurement, pulse oximetry, and capnography. An incompressible headrest with a height of 7 cm was placed beneath the patient’s head to obtain a sniffing position. The entire bed remained horizontal and the patient nose–floor distance was set at 100 cm. All of the patients received preoxygenation with an anesthetic circuit delivering oxygen at a flow of 6 L/min to achieve fractional end-tidal oxygen of at least 0.9. General anesthesia was induced with remifentanil (0.1–0.2 μg/kg/min) and propofol (1.5–2.0 mg/kg). After obtaining loss of consciousness, the patient's lungs were ventilated with a bag and mask, and rocuronium (0.6 mg/kg) was administered to facilitate tracheal intubation. The neuromuscular blockade was monitored using train-of-four stimulation of the ulnar nerve (TOF Watch™; Organon, Dublin, Ireland), and a loss of train-of-four response was confirmed before laryngoscopy.
Tracheal Intubation
A McGRATH™ MAC videolaryngoscope was used for all patients. McGRATH™ MAC 3 and 4 disposable videolaryngoscope blades (Medtronic, Dublin, Ireland) were used for women and for men, respectively. Tracheal tubes (SHERIDAN/CF™; Teleflex, Wayne, PA, USA) with an internal diameter of 7.0 mm for women and with an internal diameter of 8.0 mm for men were used to intubate the trachea. Malleable stylets (PORTEX™ intubation stylet; Smiths Medical, Minneapolis, MN, USA) with an outer diameter of 4.0 mm were used in all patients, and the stylets were lubricated with a water-based lubricant and inserted into the tracheal tube followed by stylet formation as shown in Fig. 2. For both groups, the tracheal tubes were held at 8.0 cm below the proximal tip with the right thumb, index finger, and middle finger in the same way. Tracheal intubation was conducted by one of three anesthetists who were skilled at intubation using a McGRATH™ MAC videolaryngoscope. Prior to the study, operators A, B, and C experienced tracheal intubation with stylet angulation at the holding position for 20, 10, and 10 times, respectively. Videolaryngoscopy was performed with a McGRATH™ MAC videolaryngoscope and the blade tip was placed in the vallecula. When an optimal view of the glottis had been displayed, the Cormack–Lehane grade and percentage of glottic opening (POGO) score23 were recorded and then the tracheal tube was delivered to the glottis. Patients with a Cormack–Lehane grade 3 or 4 were excluded in terms of safety. Unsuccessful intubation was defined as an excess of time from insertion of the distal tip of the videolaryngoscope blade into the oral cavity to the appearance of partial pressure of end-tidal exhaled carbon dioxide trace over 120 s or a reduction of oxygen saturation below 95%. The intervention was terminated after two unsuccessful intubations and then an alternative device was used to intubate the trachea. The operator’s motion was recorded by two cameras. According to the method of a previous study24, camera 1 recorded the side view of the operator. Camera 2 was positioned at a 90-degree angle to camera 1 to record the front view of the operator.
Outcomes
The primary outcome was the time for placement of the tracheal tube, defined as the time from passage of the distal tip of the tracheal tube past the incisors to the appearance of an end-tidal carbon dioxide trace. Secondary outcomes included first-pass success rate for tracheal intubation, operator's perception of difficulty in delivery of the tracheal tube expressed using a 11-point ordinal score with 0 representing easiest and 10 representing most difficult, intubation difficulty scale (IDS) score25, number of attempts for delivery of the tracheal tube, and deflections of the right upper arm, lower arm, and wrist during placement of the tracheal tube. Number of delivering attempts was added if withdrawal and re-advancement of the tracheal tube in the cavity of pharynx occurred. Deflections of the right upper arm, lower arm, and wrist were measured by using video data recording the operator’s motion. The outcomes were evaluated by two assistants who were not involved in the study design.
Statistical Analysis
Categorical data are presented as numbers (proportion), and continuous data are presented as means (standard deviation) if normally distributed and otherwise as medians (interquartile range). Normality of the continuous variables was checked by using the Shapiro-Wilk W-test. Equality of variances was assessed by using Levene’s test.
The primary outcome was the time for placement of the tracheal tube. We compared the time for placement of the tracheal tube between the non-angulation and angulation groups using Welch's t-test. In a secondary analysis, the differences in first-pass success rates for tracheal intubation between the two groups were evaluated using the chi-squared test. The differences in the operator's perception of difficulty in delivery of the tracheal tube, IDS score, and number of attempts for delivery of the tracheal tube between the groups were compared using the Mann-Whitney U-test. The differences in deflections of the right upper arm, lower arm, and wrist during placement of the tracheal tube were evaluated using Student’s t-test, Welch's t-test, or the Mann-Whitney U-test.
All statistical tests were 2-tailed, and a P value of less than 0.05 was considered statistically significant for the primary and secondary outcomes with no adjustment applied. All statistical analyses were performed using GraphPad Prism 7 (GraphPad Software, La Jolla, CA, USA).
Sample Size Estimation
The sample size was determined by using our preliminary data. In our preliminary study, the mean (standard deviation) time for placement of the tracheal tube was 25 (10) s when using a stylet without angulation at the holding position. Assuming that stylet angulation at the holding position would reduce the time for tube placement for 10 s, with type Ⅰ error set at 5% and type Ⅱ error set at 5%, 28 patients were required in each group (ie, 56 patients in total). We decided to recruit 30 patients for each group to compensate for dropouts and missing data. We thus scheduled assessment for eligibility of about 80 patients to accommodate a 25% exclusion rate.