The protocol was approved by the Nagoya City University Graduate School of Medical Sciences and Nagoya City University Hospital Institutional Review Board (Nagoya, Japan, approval number: 60-18-0073). Per our institutional review board’s code of ethics, we used an opt-out method and posted a description of the research protocol on the website of the Nagoya City University Graduate School of Medical Sciences on July 30, 2018, and the patients could withdraw from the study.
Data source and study population
This retrospective observational study included patients who underwent lung resection from April 2015 to March 2018. We included patients who underwent DLT intubation with a Macintosh laryngoscope and a neuromuscular blocking drug, who were ≥15 years of age, and who had an American Society of Anesthesiologists physical status classification (ASA-PS) of 1 or 2. Patients with preoperative hoarseness, those whose tracheas were intubated with a video laryngoscope, those who required emergency surgery, and those with missing data were excluded from this study.
Study variables
The exposure of interest was DLT intubation performed by a trainee or senior anesthesiologist. We divided the patients into two groups: one group comprising patients whose tracheas were intubated by a trainee anesthesiologist and the other comprising those whose tracheas were intubated by a senior anesthesiologist. In Japan, anesthesiologists can be certified as a Japanese Society of Anesthesiologists Qualified Anesthesiologist after completing a 2-year training program. We, therefore, defined trainee anesthesiologists as anesthesiologists with less than two years of anesthesia experience and senior anesthesiologists as those with more than two years of anesthesia experience. These definitions were equivalent to those used in a previous study [7]. We collected the following clinical variables: age, sex, height, weight, body mass index (BMI), ASA-PS, duration of anesthesia, intraoperative fluid balance, DLT size, intubation depth, number of intubation attempts, intracuff pressure of the DLT, Mallampati score, and Cormack–Lehane grade.
Outcome measures
The primary outcome was the incidence of postoperative hoarseness 24 h after surgery. The investigator (YK), who did not perform DLT intubation or manage anesthesia, collected data on postoperative hoarseness using a checklist of postanesthetic adverse events from the electronic medical record. We defined postoperative hoarseness as a patient-assessed change in voice quality. We did not qualitatively or objectively evaluate postoperative hoarseness. We investigated whether the anesthesiologist who assessed postoperative hoarseness was the one who provided anesthesia for the patient and whether he or she was a trainee or senior anesthesiologist.
Perioperative patient treatment
There were no standardized methods of induction or maintenance of anesthesia. After the patient arrived to the operating room, electrocardiography, pulse oximetry, and invasive blood pressure monitoring were performed. Patients received a combination of general and epidural anesthesia. After placement of a thoracic epidural catheter, general anesthesia was induced with propofol (a bolus dose of 1–2 mg/kg or a target-controlled infusion at 3–3.5 μg/ml), fentanyl (1–4 μg/kg) and remifentanil (0–0.3 μg/kg/min). After bolus administration of rocuronium (0.6–1 mg/kg), the attending trainee or senior anesthesiologist performed DLT intubation with a Macintosh laryngoscope. The blade size (3 or 4) was chosen based on anesthesiologist preference and the patient’s physique. Portex® Blue Line® Endobronchial Tubes-left (Smiths Medical, Minneapolis, MN, USA) with a stylet were used for all procedures. A water-soluble lubricant without lidocaine was applied to the tube. We used a 37-Fr DLT for men and a 35-Fr DLT for women, but the tube size was determined by the attending anesthesiologist with consideration of the patient’s height [8]. The attending anesthesiologist guided the DLT into position via a flexible bronchoscope and assessed the tube after moving it to the lateral decubitus position. Anesthesia was maintained with 1%–2.5% sevoflurane or propofol (target-controlled infusion at 2–3.5 μg/mL). The Bispectral Index® value was kept between 40 and 60 throughout the procedure. Postoperatively, residual neuromuscular blockade was reversed with sugammadex (2–4 mg/kg), and the DLT was removed in the operating room.
Statistical analysis
For sample size calculation, we assumed that the incidence of postoperative hoarseness 24 h after surgery in patients who underwent intubation by a trainee or senior anesthesiologist would be 20% and 5%, respectively, based on previous reports [4,5,6]. Thus, 89 patients in each group were required to provide 80% power to detect a statistical difference between the groups using the Fisher’s exact test with a two-sided significance level of 5%.
We conducted propensity score analyses to account for differences in baseline characteristics between the two groups. The c-statistic for evaluating the goodness of fit was calculated. We performed one-to-one propensity score matching by nearest neighbor matching without replacement. The caliper width was set to 25% of the standard deviation of the propensity scores. The confounding factors included in the propensity score model were as follows: age, sex, height, weight, BMI, ASA-PS, duration of anesthesia, intraoperative fluid balance, tube size, tube depth, number of intubation attempts, intracuff pressure, Mallampati score, and Cormack–Lehane grade. We assessed the differences between the two groups before and after propensity score matching with standardized differences. Standardized differences of <10% were considered negligible imbalances in the baseline characteristics between the two groups. We compared the incidence of hoarseness 24 h after surgery between the two groups using the Fisher’s exact test for before matching and the McNemar test for after matching. P values of <0.05 were considered statistically significant. All statistical analyses were conducted using the R software program (version 3.5.0, R Foundation for Statistical Computing, Vienna, Austria).