This prospective, single-center, observational study was approved by the institutional review board of Severance Hospital, Yonsei University Health System (Seoul, Republic of Korea; numbers: 4-2019-0104; April 1, 2019). The study protocol was registered at www.clinicaltrials.gov (NCT03938506, May 9, 2019). Written informed consent was obtained from all patients participating in this study. From January 2020 to November 2020, we enrolled male patients aged 20 years or older scheduled to undergo surgery under general anesthesia using an endotracheal tube of size ID 6.0 or 8.0. Exclusion criteria included the following: patients with a tracheostomy tube; and patients with abnormal tracheal findings such as an endotracheal mass, tracheal deviation, and/or tracheal narrowing.
Standard monitors for pulse oximetry, 3-lead electrocardiography, and non-invasive blood pressure measurement were attached. Anesthesia was induced using 1–2 mg kg-1 propofol (Fresofol 1% MCT; Fresenius Kabi Austria GmbH, Graz, Austria), 0.5–1.0 µg kg-1 remifentanil (Ultian; Hanlim Pharm. Co., Ltd., Seoul, Korea), and 0.6-1.0 mg kg-1 rocuronium (Rocumeron; Ilsung Pharmaceuticals Co., Ltd., Seoul, Korea). Mask ventilation was performed using oxygen at 5 L min-1 and sevoflurane 4.0 vol %. After establishing a complete neuromuscular block, intubation was performed using videolaryngoscope with endotracheal tube of size ID 6.0 or 8.0 (ShileyTM Ta-perGuard oral tracheal tube, Covidien, MA, USA). The cuff of tube was inflated by inserting air into the pilot balloon of tube. The anesthesiologists assessed the adequacy of degree of cuff inflation by palpating pilot balloon with their fingers. Breathing circuit was connected to the tube and mechanical ventilation was started using volume-controlled mode. Target tidal volume was 8 ml kg-1 with a fresh gas flow of 2 L min-1, respiratory rate of 12 breaths min-1, inspiratory-expiratory ratio of 1:2, and positive end-expiratory pressure of 5 cmH2O. After 30 seconds, intracuff pressure was measured using cuff manometer (Cuff Inflator/Pressure Gauge, Smiths Medical Inc, Kent, UK) and recorded. The intracuff pressure was considered inadequate if the pressure was out of the range of 20–30 cmH2O [1, 4]. Then, the intracuff pressure was adjusted to 25 cmH2O and surgery was proceeded. Patient characteristics such as age, height, weight, American Society of Anesthesiologists physical class, and endotracheal tube size were recorded.
The primary outcome was the incidence rate when the intracuff pressure of the endotracheal tube is not within the appropriate range of 20–30 cmH2O. To determine sample size, we estimated the incidence of inadequate intracuff pressure based on previous report [9]. Based on the PASS (version 12, NCSS, LLC, Kaysville, Utah, USA), the required sample size was calculated to be 186 (assuming significant difference of incidence = 10%, Type Ⅰ error = 5%, and power = 80%). In total, 208 participants were needed after accounting for a 10% dropout rate.
The intracuff pressure of endotracheal tube size ID 6.0 and 8.0 was compared using the Student’s t-test. The number of patients who had adequate or inadequate intracuff pressure was compared between the endotracheal tube size ID 6.0 and 8.0 using Chi-square tests or Fisher’s exact test. Clinically significant factors among the pre-operative characteristics, including age, height, weight, and tube size were entered into a multivariable logistic regression model to assess their impact on the incidence of in-adequate intracuff pressure. Values are presented as mean (SD) or numbers (percentages). Analyses were conducted using SAS software version 9.4 (SAS Institute Inc., Cary, NC). A P-value of < 0.05 was considered statistically significant.