Study design and eligible patients
This retrospective study of tracheal intubations was conducted at Asan Medical Centre, a tertiary referral hospital in Korea. Tracheal intubation data between January 2012 and December 2015 were collected. The primary outcome was first-attempt success and secondary outcome was intubation-related complications. All patients aged ≥ 19 years who had been intubated in the ward by medical emergency team (MET) were eligible. Exclusion criteria were as follow: (1) tracheal intubations performed during cardiac arrest because the success of first-attempt intubation might have been affected by cardiopulmonary resuscitation, (2) patients who initially used supraglottic airway devices because the study aimed to compare the direct laryngoscopy and video laryngoscopy, (3) patients whose records were unavailable, 4) duplicated cases.
The institutional review board of the Asan Medical Centre approved this study (approval no. 2016-0599) and granted a waiver of patient consent because of the retrospective nature of the study.
The MET provides airway management to general ward patients who require immediate or cardiopulmonary resuscitation. A MET comprises attending critical care physicians, critical care medicine fellows, internal medicine residents, and critical care qualified nurses and is available 24 h per day, 7 days per week. If the MET is activated via a screening or calling system, the team members proceed to the intubation location with a portable airway bag. This bag contains a video laryngoscope, capnograph, i-gel (Intersurgical Ltd, Wokingham, Berkshire, UK), laryngeal tube (VBM Medizintechnik, Sulz, Germany), gum elastic bougie, tracheal tube exchanger (Cook airway exchange catheter), and percutaneous cricothyroidotomy kit (Melker Emergency Cricothyrotomy Kit, Cook Critical Care, Bloomington, Indiana). The laryngeal tube is one of the supraglottic airway devices that consists of an airway tube with a small cuff attached at the tip and a larger balloon cuff at the middle part of the tube. The i-gel is also a supraglottic airway device that features a non-inflatable cuff and the possibility to introduce a gastric catheter. Direct laryngoscopes were contained in the emergency trolley each general ward. Before starting rotations in ICU, medical residents received airway management programme, consisted with direct laryngoscopy and video laryngoscopy and i-gel, laryngeal tube. Airway management training programme for critical care medicine fellows is provided by attending physician once a month. The curriculum is designed with basic skills with direct laryngoscopy and video laryngoscopy and more difficult scenarios requiring execution with alternative techniques, including i-gel, laryngeal tube, gum elastic bougie, bronchoscopy, and cricothyroidotomy.
In this study, the intubation procedure was conducted in accordance with general guidelines for the airway management of critically ill patients [14, 15]. Although fentanyl (1–3 µg kg−1) and etomidate (0.3 mg kg−1) were the preferred pre-treatment and induction agents, the operators selected the sedatives and dosages after considering each patient’s condition. A sedative was administered 2 min after pre-treatment agent injection, and tracheal intubation was performed 20–30 s later. The operators chose either a curved Macintosh laryngoscope with metal reusable blade or a GlideScope video laryngoscope (Verathon, Bothell, WA, USA) as the initial device. Tracheal intubation was supervised by an experienced operator when performed by an inexperienced operator. If the second intubation attempt was unsuccessful, the experienced operator performed the third attempt. However, for the patient safety, supervisors tended to intubate directly if the patient was expected to have difficult airway or hemodynamically unstable. It is presumed that relatively less severe patients were intubated by inexperienced operators and with direct laryngoscopy for the training purpose. Correct tracheal tube placement was assessed using careful auscultation, end-tidal carbon dioxide measurement, and chest radiography. The end-tidal carbon dioxide level was measured using an EMMA Emergency Capnograph (Masimo Corp., Irvine, CA, USA). After each tracheal intubation, intubation-related information was recorded on a data collection sheet by MET nurses, and all tracheal intubations were reviewed in regular weekly meetings.
Data collection and definition
The following information was recorded on the intubation data collection sheet: patient demographics, operator specialty, time and location of tracheal intubation, reason for intubation, number of intubation attempts, device(s) used, medications (pre-treatment agents, sedatives, and paralytics), complications during tracheal intubation, characteristics of the predicted difficult airway, Cormack–Lehane grade, and vital signs pre- and post-tracheal intubation.
We defined intubation duration as the time interval between infusion of the pre-treatment agent (or sedative if the patient did not receive a pre-treatment agent) and confirmation of tracheal tube placement by capnography. The board-certified physician or surgeon in the ICU (attending physician and fellow) was considered an experienced operator, while a medical or surgical resident-in-training was considered an inexperienced operator. A tracheal intubation attempt was defined as insertion of the laryngoscopy blade into the oral cavity, regardless of whether tracheal tube insertion was attempted. A first-attempt success was defined as the placement of a tracheal tube on the first attempt and difficult intubation was defined as more than two attempts of intubation .
Several factors were investigated before intubation to predict difficult airways. These factors included blood/vomitus/secretion in the airway, cervical immobilisation, neck trauma/mass or vocal cord palsy, the 3-3-2 rule, short neck, obesity, limited mouth opening, small mouth, and large tongue. The 3-3-2 rule was defined as an inter-incisor distance of < 3 fingers, a hyoid-mental distance of < 3 fingers, and a hyoid-thyroid cartilage distance of < 2 fingers.
An event occurring within 30 min after tracheal intubation was considered intubation-related complications. These events included hypotension (systolic arterial pressure < 90 mmHg despite adequate volume loading or inotrope use), severe desaturation (oxygen saturation < 80%), oesophageal intubation, dental injury (tooth extraction), oral bleeding, aspiration of gastric contents, and cardiac arrest/arrhythmia .
Continuous variables are presented as median (interquartile range) or mean (standard deviation). Categorical variables are presented as number (percentage). Differences among categorised groups were compared using either the Chi-square test or Fisher’s exact test, and data for continuous variables were compared using the independent Student’s t test or Mann–Whitney U test. Univariate and multivariate logistic regressions using backward elimination method were performed to identify the factors associated with first-attempt intubation success and intubation-related complications. Calibration of the models was evaluated with the Hosmer–Lemeshow goodness-of-fit test. All statistical comparisons were two-sided, and a p value of < 0.05 was considered statistically significant. Data were analysed using the Statistical Package for the Social Sciences (SPSS), version 22.0 (IBM Corporation, Armonk, NY, USA). To reduce the effect of treatment-selection bias and potential confounding factors in an observational study, we performed an adjustment for differences in baseline characteristics of patients using a propensity-score matching (Additional file 1: Table S1). Compounding factors were age, sex, medical department, pre-intubation blood pressure, pre-intubation heart rate, pre-intubation oxygen saturation, predicted difficult airway, level of operator experience, pre-treatment agent, sedatives, and paralytic agents. Using these methods we could reduce or eliminate confounding by those measured covariates. A power analysis was performed with reference to similar studies conducted in an intensive care unit setting [7, 9, 18]. We determined the power of the study by assuming a first-pass success rate of 65% (direct laryngoscopy) and 80% (video laryngoscopy).