The aim of this study was to evaluate the association between the airway size measured by CT and the incidence of post-extubation upper airway obstruction symptoms for each sex. We conducted an observational single-center study. We accumulated cases prospectively and analyzed the data retrospectively.
Our hospital is a tertiary emergency medical/severe trauma center with a 12-bed mixed intensive care unit (ICU) located in Yokohama, Japan; a standard urban emergency center. We had 27 full-time physicians (14 board-certified acute care physicians and 9 board-certified critical care physicians) at the start of this study. The average numbers of annual ambulances and patients who received mechanical ventilation in our emergency center were 1261 and 536 per year, respectively.
We enrolled consecutive adult patients (aged ≥ 20 years), who were intubated in the emergency room by an emergency physician or a resident supervised by an emergency physician, from January 2016 to March 2019. Patients who underwent CT scan of the glottic region within three hours before and after intubation were included for analysis. Patients who underwent tracheostomy, were transferred, or died before the first attempt to extubate were excluded. We used oral tracheal tubes with a subglottic drainage lumen (Taper Guard Evac; Medtronic, Minneapolis, MN, USA) or standard oral tracheal tubes with a stylet (Taper Guard with stylet; Medtronic) depending on device availability.
We accumulated the cases prospectively in chronological order and clerks who were not involved in this study distributed the paper database form for all eligible patients to avoid selection bias. An attending physician or a physician in charge recorded the following characteristics at the time of intubation: age, sex, height, body weight, reason for intubation, endotracheal tube size and type, history of tracheostomy and/or prolonged (> 2 weeks) intubation, number of intubation attempts, the intubation doctor’s years of experience (junior resident: 1–2 years, senior resident: 3–5 years, 6 years and more), use of a sedative drugs and neuromuscular blocking agents at intubation. The size of endotracheal tube used, as well as the use of sedative drugs and neuromuscular blocking agents at intubation were decided by an attending physician based on the patient’s condition. The ICU nurses checked the endotracheal cuff at least once every 8 h, and cuff pressure was maintained at 20–24 cm H2O. The timing of extubation was decided by attending physicians. The doctor who performed the extubation assessed and recorded whether the patient had stridor and hoarseness, or both after extubation. To minimize the observer bias, the post-extubation upper airway symptoms was confirmed by multiple doctors including those other than researchers of this study, as much as possible. The doctor who performed the extubation also recorded the use of steroids and the presence of a cuff leak before extubation.
Stridor was defined as a high-pitched inspiratory wheeze with respiratory distress. Hoarseness was defined as changes in voice quality and difficulty in speaking with respiratory distress, regardless of whether medical intervention was required. Cuff-leak test was performed as a qualitative test defined as an audible leak while the endotracheal balloon was deflated.
To minimize observer bias, the transverse diameters of the glottis and cricoid cartilages were measured using CT images by authors who were blinded to post-extubation symptoms. Position of the vocal cords was identified as the area where the thyroid cartilage and vocal cords were visible in the images. The transverse diameters of the glottis were measured as the distance between the vocal cords at widest point in the image. When there were multiple images in which the thyroid cartilage and vocal cords were visible, we selected the one with the narrowest diameter measured. When there was no gap between the endotracheal tube and the vocal cords in the patients that had been already intubated at the time of CT scan, the transverse diameters of the glottis were regarded as same with the outer diameter of the tracheal tube. The value obtained by dividing the transverse diameter of the glottis by the outer diameter (OD) of the endotracheal tube was used as the index of the endotracheal tube size by the airway size. Our primary outcome was post-extubation stridor and hoarseness, or both. At the time of discharge, an attending physician recorded that unplanned reintubation within 48 h, and hospital mortality.
We compared the post-extubation upper airway obstruction symptoms in males and females using a chi-squared test. Then, we performed further analyses on the males and females separately, because airway size differed depending on sex. For each sex, we first divided the patients into two groups: those who had post-extubation stridor and/or hoarseness and those who had no such symptoms. The quantitative variables were expressed as the median [inter-quartile range: IQR] and compared using the Mann–Whitney U test. For the categorical variables, the comparisons were performed using the Fisher’s exact test. Univariate logistic regression analysis was used to evaluate the risk of post-extubation upper airway obstruction symptoms. For logistic regression analysis, we used the transverse diameter of glottis/ endotracheal tube OD ratio < 1 as the variable representative of the ratio of airway size to tube size, because the physical contact of endotracheal tube to tracheal membrane is the most likely mechanism of post-extubation airway edema, and presence of a gap between endotracheal tube and the vocal cords was the most important. A multivariable logistic regression model was applied using intubation attempts, duration of intubation, and the transverse diameter of glottis/tube OD ratio < 1. Regarding the independent variables in multivariable logistic regression analysis, we selected one of the most significant variables from the three areas, such as intubation procedure, patient condition during intubation and airway size, to avoid multicollinearity. We defined multiple intubation attempts as three and more attempts [20, 21]. We performed the multivariable logistic models’ goodness of fit and discrimination ability using the Hosmer–Lemeshow test and the c statistic. We excluded patients with missing data from the analysis. Statistical significance was set at p < 0.05. All statistical analyses were performed using STATA software (Stata/SE 13.0, StataCorp LLC, TX, USA).
This study was approved by an Institutional Review Board, the Ethics Committee of the Yokohama City University Medical Center (D1506007, approval date 17th July 2015). Requirement of informed consent from the patients was waived by the Ethics Committee of the Yokohama City University Medical Center /IRB because of the observational study design.