Inclusion criteria

Patients having elective surgery with general anaesthesia and requiring mechanical ventilation via an endotracheal tube are recruited.

Exclusion criteria

Patients are not included in this study if they meet one or more of the following criteria:

• More than one predictor of an anticipated difficult airway (eg, body mass index (BMI) >40 kg/m², unanticipated difficult airway in the medical history (eg, Cormack & Lehane (C&L) ≥III), reduction of the atlanto-occipital joint extension <35°, reduced thyromental distance <6 cm or Mallampati class ≥III)

• Age <18 years

• ASA class IV

• Pregnant or breast feeding

• Participation in other studies

• Unable to provide informed written consent or under guardianship

• Urgent surgical intervention

• At high risk for aspiration.

Concomitant treatments in both groups

First, patients admitted requiring elective tracheal intubation are evaluated for predictors of anticipated difficult intubation (BMI, head extension, thyromental distance, Mallampati class, mouth opening and previous difficult airway (eg, C&L ≥III). The expertise of the participating anaesthesiologists ranges from ‘beginner’ (residents) to ‘expert’ (consultants). All anaesthetists received hands-on training and theoretical introduction to the use of the McGrath video laryngoscopy and DL. Tracheal intubation is performed in both groups following the protocol outlined below (figure 1).

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Figure 1

Study flow chart. ASA, American Society of Anesthesiologists; BMI, body mass index; BURP, backward, upward and rightward pressure; C&L, Cormack & Lehane; DL, direct laryngoscopy; ETT, endotracheal tube; POGO, percentage of glottic opening; SGA, supraglottic airway.

1.%20All patients are monitored for ECG, oxygen saturation (SO₂) and arterial blood pressure (non-invasive or invasive as appropriate). In the McGrath group, a malleable stylet in a ‘hockey-stick’ shape is always used for tube placement.

Preoxygenation is achieved using the device chosen by the provider based on patient characteristics and clinical standard operating procedure (EtO₂ >80%). In the study locations a Pallas/Primus (Dräger Lübeck, Germany) anaesthesia respiratory system is used:

• Tidal volume breathing with normal breaths for at least 3 min or with eight deep breaths over 60 s (8 DB 60 s)28 29

• Anaesthesia ventilator in pressure support (PS) mode (PS eight mbar, Positive end-expiratory pressure 5 mbar and FiO₂ 1.0).29 30

2. After sufficient preoxygenation, anaesthesia is induced with sufentanil (0.2–0.5 µg/kg) and propofol (2–3 mg/kg), and anaesthesia is maintained with either propofol infusion (TIVA) or volatile anaesthetics. After the patient is deeply anaesthetised, the neuromuscular transmission is monitored using acceleromyography of the adductor pollicis. The individual choice of neuromuscular blocking agent depends on the temporal duration of the surgery, necessary of perioperative neurological monitoring, absence of allergies and organ failures. The following agents and specific dosages are used:

• Mivacurium (0.2 mg/kg)

• Atracurium (0.5 mg/kg)

• Rocuronium (0.3–0.6 mg/kg)

• Succinylcholine (1–2 mg/kg).

The train-of-four (TOF) is used for continuous quantitative monitoring of neuromuscular transmission. Complete muscle relaxation is confirmed in the absence of tactile and measured twitches in response to maximal TOF stimulation of the ulnar nerve at the adductor pollicis. The importance of obtaining adequate neuromuscular blockade is emphasised with study personnel.

3. The laryngoscopy attempt begins with a TOF count of 0/4 and is performed using the device indicated by default randomisation:

• Macintosh laryngoscope (DL)

• McGrath MAC video laryngoscope (McGrath) — indirect laryngoscopy is initially performed. However, direct laryngoscopy can also be usedat the discretion of the anaesthetist.

The anaesthetist should achieve the best possible view of the laryngeal structures. External laryngeal manipulations (ELM) could be used to improve the view of the glottis to achieve a C&L I or II. The size of the endotracheal tube and the size of the blade are dependent on the standard operating procedure of the hospital (blade size in both groups: #3 for average patients and #4 for very tall patients (>190 cm height); standard ETT size: 7.0 ID used for female patients and 7.5 ID for male patients). The method of visualisation of the glottis and size of the ETT/ blade is recorded in the case report form (CRF).

4. The laryngoscopy attempt is defined as successful if the tracheal tube is placed (until the black mark on the ET was threaded between the vocal cords) with a single blade insertion within 120 s and without manipulation of the laryngoscope by another provider. The ‘time to intubation’ is defined as the time measured from the opening of the patient’s mouth until the ETT passed the vocal cords. An anaesthesia nurse measures the intubation time using the built-in timer on the anaesthesia respirator. We also analyse three time periods until final placement:

• ‘Time to view’: defined as the time from insertion of the device until visualisation of the glottis

• ‘Time to intubation’: defined as the time from insertion of the device until the ETT passed through the vocal cords

• ‘Time to ventilation’: defined as the time from insertion of the ETT until the time from the insertion of the device into the mouth until confirmation of the first wave of CO₂ of the anaesthesia respirator.

An intubation attempt is defined as an introduction of the laryngoscope blade into the mouth and its removal regardless of whether an ETT was successfully inserted. If this first attempt fails, the provider makes a second laryngoscopy attempt with the same device. Mask ventilation is recommended between the attempts. A total of two laryngoscopy attempts are allowed. If DL fails, the clinician changes to a preferred technique (eg, McGrath, S-Guide, rigid stylet) and records the direct and/or screen view of the McGrath. If McGrath fails after two attempts, the clinician is advised to proceed with a preferred rescue technique (eg, CMAC D-Blade, SGA, iLMA, flexible fibreoptic, rigid stylet). The limitation of two intubation attempts and choice of an alternative technique is recommended by the study protocol and is in accordance with the clinical standard.31 If ELM techniques, such as BURP (specific pressure applied to the cricoid cartilage), are required during laryngoscopy, they are recorded in the CRF. In all cases, an additional individual who is not involved in patient care (either a postgraduate student or a study nurse) is present during induction of anaesthesia to record the study parameters.

Primary outcome measure

The primary outcome measure is successful intubation within 120 s (time to ventilation) with the first-pass attempt.

Secondary outcome measure

• Laryngoscopy technique: whether direct or indirect glottic visualisation was used in the McGrath group is recorded

• Different times for successful tracheal intubation

  • time to view (defined as the time from insertion of the device until glottic view)

  • time to intubation (defined as the time from insertion of the device until the ET passed through the vocal cords)

  • time to ventilation (defined as the time from insertion of the device until the first CO₂ wave on the anaesthesia respirator)

• Number of laryngoscopy attempts

• Failures/crossovers to other rescue techniques (eg, SGA, iLMA, fibreoptic)

• ELM (eg, BURP, cricoid pressure)

• Glottic view with the C&L and percentage of glottic opening score

• Intubation Difficulty Score (IDS)32

• If McGrath is used, occurrence of fogging is recorded

• Comparing the level of training with intubation success

• Complications (eg, desaturation <90% SaO₂, dental or soft tissue trauma)

• Degree of ease or difficulty of tracheal intubation based on the Likert scale (0=easy to 10=difficult)33 34

Subgroup analysis

• Demographics

  • patient (age, gender, BMI, ASA class)

  • airway difficulty score (ADS)35

  • provider analysis (clinical experience, education status, experience in direct and indirect laryngoscopy)

• Type of neuromuscular blocking agent

• TOF count when inserting the laryngoscope

• Type of surgery (eg, thyroidectomy, neck dissection).

Allocation

After eligibility is confirmed and written informed consent is obtained, enrolled participants are randomised 24 hours before the intervention. A web-based service (QuickCalcs, GraphPad Software, La Jolla, California, USA) is used for allocating patients to either DL or McGrath.

Sequence generation

The randomisation sequence was generated by a study nurse in the clinical research unit who is not involved in patient recruitment. The software used to collect the data in the CRF automatically allocated the patients, thereby ensuring concealment and anonymity.

Blinding

Blinding to the type of laryngoscopy is only possible for the patient. The performing anaesthesiologist is informed of treatment group prior to induction of anaesthesia.

Data collection and management

The study data are recorded on a specific paper-based CRF. Prior to measurement, the data from each patient are collected by study personnel. All outcome measurements are recorded during and after the evaluation on the CRF. Any protocol deviations are recorded either on the CRF or in the medical records; a clinical research assistant ensures that all protocol deviations and adverse events are recorded in the database. If adverse events are observed, the ethics committee will be informed in writing.

Every allocated subject will be coded with a specific patient number. After measurement is completed, the study data will be entered into a premade computer-based table (Microsoft Excel, V.14.0, Microsoft, Redmond, Washington, USA). The completed CRF will be secured in the clinical research unit for the next 15 years.

Access to data

Data safety, data quality and statistical analysis will be managed by the two principal investigators, who are responsible for notifying any issues that may arise during the whole prospective study. Data are collected and stored according to good clinical practice (GCP) guidelines and are available to all participating study sites. Any issue occurring during the clinical trial will be reported to the principal investigators.

Description of the patient groups at baseline

The baseline features of the patients will be described using absolute numbers (n) and percentages for categorical variables and the minimum, maximum, mean, SD and quartiles for quantitative variables. We will use the Pearson correlation coefficient to compare patient specifics (eg, BMI, ADS score) between the groups at the baseline.

Analysis of the primary outcome

A χ² test will be used to compare the success rate between the two groups. Multiple regression analysis of subgroup factors will allow the determination of important factors affecting successful first-time intubation comparing DL with McGrath. Relationships between the experience of the provider and the first-attempt intubation rate or time to intubation will be analysed as paired samples with Bonferroni correction for multiple comparisons. The differences will be considered statistically significant if the p value is less than 0.05.

Analysis of the secondary outcomes

Comparison of the view of the glottis, overall intubation time and the Likert scale score will be analysed by the Wilcoxon’s rank-sum test. Comparing the different levels of training with the intubation success will be done with the Spearman's rank correlation coefficient.

Subgroup analysis

We will perform a separate analysis of the specific type of surgery (eg, thyroidectomy, neck dissection), influence of neuromuscular agents and/or patients with difficult intubation, defined as more than two attempts or IDS score >5.

Data monitoring

Prior to the start of patient enrolment, the study physicians and the clinical research assistants were involved in the study protocol and data collection in CRFs. All documents required for the study (eg, informed consent, CRF baseline and perioperative) are available in the operating room, where the study measurement begins. The CRF is prepared and managed by the investigator. Because this is an investigator-initiated trial, the principal investigator meets with clinical research assistants to discuss any problems in data collection and protocol compliance and to evaluate study progress. This study is proposed, managed and will be analysed in accordance with the ICH Guideline for GCP E6 (R2) and following the requirements of German law. All persons (eg, investigator, study assistants) are obliged to follow these rules.

Harms

The study may be temporarily stopped for an individual patient, at the discretion of the attending physician, in case of major serious adverse events suspected to be associated with the type of laryngoscope used. An adverse event or suspected adverse reaction is considered ‘serious’ if, in the view of either the investigator or sponsor, it results in any of the following outcomes: death, a life-threatening adverse event, inpatient hospitalisation or prolongation of existing hospitalisation, and a persistent or significant incapacity or substantial disruption of the ability to conduct normal life functions.

Reporting of severe adverse events (SAE) will be per local research ethics committee (REC) standard operating procedures. SAEs will include the following when occurring as a result of airway manipulation (eg, cardiac arrest, acute circulatory failure, death, vocal cord injury, oesophageal rupture). The principal investigator informs the REC about the SAE. No specific reporting procedure for unexpected serious adverse events is planned.

Auditing

The Clinical Research Unit of the Department of Anaesthesiology, University Medical Centre Mainz reviews the screening form and clinical data at regular intervals.