Description of the condition
Securing the patient’s airway is a critical step during general anaesthesia. Recent data from the Fourth National Audit Project of the Royal College of Anaesthetists and Difficult Airway Society (NAP4) in the UK suggest that tracheal intubation is used for airway management in 38.4% of general anaesthetics, estimated at 1.1 million procedures per year (Woodall 2011). A cuffed tracheal tube, considered to be the most reliable device to secure the airway, is inserted through the mouth and larynx and into the trachea to enable oxygenation and ventilation, and prevent aspiration, during general anaesthesia.
A clear view may be achieved by flexing the lower cervical spine and extending the upper cervical spine (a 'sniffing the morning air' position) enabling the intubator to create 'line of sight' to the larynx in order to pass the tracheal tube. Retractor type laryngoscopes, typically a detachable metal blade with handle (for example the Macintosh curved blade), are used to retract the tongue and soft tissue in the pharynx during this procedure, which is termed 'direct laryngoscopy'. However, whilst these laryngoscopes may be adequate to move soft tissue the intubator still requires line of sight to the larynx, provided by correct head and neck positioning of the patient.
Failed or difficult intubation is associated with complications, such as an increased risk of hypertension, desaturation, unexpected admissions to the intensive care unit (ICU) and death (Caplan 1990; King 1990; Rose 1994). Such difficulties during intubation are estimated to occur in 1% to 6% of cases whereas failed intubation occurs in only 0.1% to 0.3% of cases (Crosby 1998; Shiga 2005).
There are increased airway management difficulties with patients who are obese (Juvin 2003; Lundstrom 2009). In the UK, NAP4 showed that obese patients accounted for 42% of patients who experienced a major airway complication during anaesthesia (Cook 2011). Functional residual capacity (FRC), which is the volume of air left in the lungs at the end of normal expiration, is reduced in obese patients and this, along with other factors, reduces respiratory reserve and makes these patients vulnerable to hypoxia if an airway is lost, making airway management more time critical and increasing the risk of postoperative chest infections and other complications (Adams 2000; Malhotra 2008; Marley 2005).
In addition to obesity there are other reasons why intubation may prove difficult, for example cervical instability, other restrictions on neck flexion, narrow jaw opening, enlarged tongue and poor tissue mobility. Predictive tests are used prior to anaesthesia, for example the Mallampati or Wilson index test (Mallampati 1985; Wilson 1988). The Mallampati score, based on the view of the soft palate when the patient opens their mouth, is the most widely used predictor of difficult intubation; but this and other prediction tests have been shown to have low positive predictive values for difficult intubation (Shiga 2005).
Patients who are admitted to ICU and the emergency department may differ from elective patients scheduled for general anaesthesia. Many patients are admitted to ICU or emergency department because of vulnerable airways, for example due to major trauma requiring cervical spine protection, airway swelling, direct airway trauma or lung injury, major head and neck surgery or infection. In the emergency department airway management may need to be provided at very short notice by critical care teams without the presence of an anaesthetist (Cook 2011).
Descripton of intervention and how it might work
Alternative devices, such as videolaryngoscopes (VLS), rely on fibreoptic or digital technology to transmit an image from the tip of the laryngoscope to an eyepiece or monitor where it is viewed by the intubator. Of these devices, there are both flexible and rigid designs and the aim of their design is to assist in difficult intubations and to reduce difficulty, failure, trauma and other complications. For this review we are interested in the rigid videolaryngoscope which uses a blade to retract the soft tissues and transmits a video image to either a screen attached to the end of the handle or to a monitor. This enables a lighted view of the larynx without direct 'line of sight' and therefore could assist where difficulty is encountered (or predicted) with direct laryngoscopy.
The Cormack and Lehane score describes the intubator’s view of the larynx during laryngoscopy (Cormack 1984), with a score or 4 indicating a poor view and a score of 1 indicating a good view. Studies suggest that the use of videolaryngoscopes improves these visualization scores (for example use of a Storz V-Mac videolaryngoscope compared with a Macintosh laryngoscope in Kaplan 2006). Videolarngoscopes may therefore provide the possibility of more successful intubation for patients in whom direct laryngoscopy may be difficult. They may also be used following unsuccessful attempts to intubate with direct laryngoscopy.
Why it is important to do this review
Whilst the use of videolaryngoscopes may aid visualization, evidence is required to establish if this also equates with increased success of intubation with reduced complications. Recent non-Cochrane reviews of VLS models have concentrated on the impact on process measures, such as the view of the larynx, first time and overall intubation success rates and intubation time, and have concluded that there is limited evidence to support their use in tracheal intubation in unselected populations and in those with known or anticipated difficult direct laryngoscopy (Griesdale 2012; Healy 2012; Niforopoulou 2010). A review and meta-analysis of 17 studies of the Glidescope found advantages for non-expert intubators (Griesdale 2012).
No existing reviews have considered the use of VLS specifically in obese patients. The prevalence of obesity is increasing in both developed and developing countries (current figures: http://www.oecd.org/) and with this there is an increase in the number of obese patients requiring anaesthesia. It is important to establish whether videolaryngoscopy is a more effective technique for this patient group as well as for other selected and unselected groups.
We wish to update the non-Cochrane reviews above by focusing on evidence from randomized controlled trials (RCTs) only and considering, where possible, patient relevant outcomes such as complications. We will aim to consider studies in both unselected and selected populations, and to include studies of obese participants. This will continue the work by the current authors with the published reviews 'Supraglottic airway devices versus tracheal intubation for airway management during general anaesthesia in obese patients' (Nicholson 2013a) and 'Tracheal intubation with a flexible intubation scope for obese patients requiring general anaesthesia' (Nicholson 2013b).
Our primary objective is to assess whether the use of videolaryngoscopes for tracheal intubation in adults requiring general anaesthesia for surgery reduces the risk of complications and failure compared to direct laryngoscopy. Our secondary aim is to assess the benefits and risks of these devices in selected population groups, such as people with obesity.
Criteria for considering studies for this review
Types of studies
We will include randomized controlled trials (RCTs) and quasi-randomized studies, including cluster and cross-over studies. We will not include simulation or manikin studies.
Types of participants
We will include trials of participants aged 16 years and over who require tracheal intubation during general anaesthesia. We will include participants scheduled for surgery as well as participants requiring tracheal intubation in the emergency department or ICU under general anaesthesia. We will include trials with unselected patient populations, those restricted to patients with known or predicted difficult laryngoscopy (for example Mallampati score III or IV or previous Cormack and Lehane score III or IV with direct laryngoscopy) and those restricted to patients with a body mass index (BMI) > 30 kg/m
Types of interventions
We will include studies which compared the use of any model of videolaryngoscope with a direct laryngoscope using a Macintosh blade.
Designs of videolaryngoscopes will include those with and without a conduit as well as those that require the use of a stylet as part of their design. A list of potential models and manufacturers is given in Appendix 1. We will exclude optical stylets.
Types of outcome measures
Our primary outcomes are the potential serious complications arising from difficulties with intubation. We will include failed intubation with the first choice of device as a primary outcome. This is an important indicator of the success of an intubation technique. Failed intubation with the first device may not always result in an adverse consequence for the patient but it increases the risk of serious complications, especially in obese patients (Cook 2012). Other primary outcomes will include hypoxia and serious airway complications. However, we anticipate these outcomes might not be available in many eligible studies. Our secondary outcomes will include mortality as well as surrogate process markers for airway problems, such as the number of attempts. We will aim to also assess the impact on patient-reported measures of sore throat or hoarseness after surgery, and patient experience.
Outcomes will not form part of the study eligibility assessment. Studies that meet the participant, intervention and comparison criteria will be included in the review even if they report no relevant outcomes. We will attempt to contact authors to find out if data were collected on outcomes. If these are not available, the studies will be recorded in a separate category of eligible but no outcome data available.
- Failed intubation or change of device required
- Hypoxia between start of intubation and recovery from anaesthesia, either dichotomous data (episodes of arterial oxygen saturation < 90%) or continuous data (lowest or mean arterial oxygen saturation)
- Mortality within 30 days of anaesthetic
- Serious respiratory complications (including aspiration) within 30 days of anaesthetic
- Laryngeal or airway trauma – including any one of damage to vocal cords, bleeding or dental injury
- Patient-reported sore throat or hoarseness - both early (within two hours of anaesthetic) and late (within 48 hours of anaesthetic)
- Proportion of successful first attempts at tracheal intubation
- Number of attempts for tracheal intubation
- Total time for tracheal intubation and commencement of ventilation
- Difficulty of tracheal intubation - assessed by intubator or observer, using a locally derived or validated difficulty scale
Search methods for identification of studies
We will search for eligible trials in the following databases: Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library, latest issue), MEDLINE via Ovid (1970 to present) and EMBASE via Ovid (1980 to present). We will apply the Cochrane highly sensitive filter for randomized controlled trials in MEDLINE and EMBASE. We will also search the trial registers such as www.clinicaltrials.gov and the World Health Organization (WHO) International Clinical Trials Registry Platfrom (http://www.who.int/ictrp/network/en/) for ongoing trials. Our search strategy for MEDLINE is presented in Appendix 2. We will adapt our MEDLINE search strategy for searching other databases. We will search using both MeSH headings (or equivalent structured vocabulary in other databases) and free text.
We will include any publication that reports study data, including abstracts, letters and articles. We will not use any restriction on language of publication.
Searching other resources
We will undertake forward and backward citation tracing for key review articles and eligible articles identified from the electronic resources. We will contact investigators known to be involved in previous studies to enquire about ongoing or unpublished studies.
Data collection and analysis
Selection of studies
Results of the searches will be collated and duplicates removed. The selection of eligible articles will take place in two stages.
All titles and abstracts will be screened by two authors (Sharon Lewis (SL) and one other) to remove studies that are very unlikely to be eligible. A pilot of 100 titles will be performed before all titles are reviewed in order to clarify criteria for discarding articles at this stage. If no abstract is available but the title is possibly relevant, the full text of the article will be obtained.
When all titles and abstracts have been screened, the full texts of potentially relevant titles will be reviewed by two of the four investigators and recorded on the study eligibility form (a draft is included in Appendix 3). A pilot of 10 papers will be read and then the investigators will meet to compare results and modify the form as required. All potentially relevant papers will then be read and the two investigators will meet to compare results. Differences that cannot be resolved will be referred to Tim Cook (TC) or Andrew Smith (AS). We will record the numbers of papers retrieved and exclusions at each stage, with reasons provided for those reviewed in full text, in a PRISMA flowchart. Details of ineligible papers which are well-known or might appear to be eligible will be summarized in the 'Characteristics of excluded studies' table.
Data extraction and management
Data will be extracted from eligible studies by two investigators using a paper extraction form (Appendix 3). This form will be reviewed after data from the first three papers have been entered, and modified as required. If there are duplicate publications from the same study, we will create a composite dataset from all the eligible publications.
If relevant information or data are not available in the paper, we will contact the lead author to request additional details. We will resolve disagreements by discussion and, if necessary, consultation with TC or AS.
Assessment of risk of bias in included studies
We will use the Cochrane risk of bias tool to assess the quality of study design and extent of potential bias (Higgins 2011). We will consider the following domains:
- sequence generation;
- allocation concealment;
- blinding of participants, personnel and outcomes assessors; incomplete outcome data; and
- selective outcomes reporting.
It will not be possible for the anaesthetist or intubator to be blinded to the intervention in this research question and, similarly, it will be difficult for the assessors of outcomes during intubation to be unaware of the allocation of the patient. Outcomes assessed during or after the operation, such as airway trauma or respiratory complications, could be assessed by staff other than the intubator who were unaware of the laryngoscopy device. It will be feasible for the asleep patient not to know the device used, which may be important for patient-reported outcomes, such as sore throat.
Other sources of bias
We will pay particular attention to sources of funding and the role of manufacturers and document this information in the 'Characteristics of included studies' table. If a study is sponsored or supported by a manufacturer we will attempt to determine the extent of the manufacturer's involvement and whether there is any evidence of selective outcome reporting or other bias. We will review the original protocol of the trial, if this is available, to identify any changes to procedure or missing outcome data that may indicate bias. However, sponsorship by a manufacturer will not be equated necessarily with high risk of bias without other indications. We may undertake sensitivity analyses to assess whether overall results are altered when studies with industry support are omitted.
Cluster designs may be used in this topic, with the anaesthetist, operating theatre or hospital being the unit of randomization. For any cluster-randomized trials that we include, we will pay particular attention to baseline characteristics of the patients and the expertise of the anaesthetist. Skill of the intubator is an important confounder and needs to be addressed by randomization.
Cross-over trials, if we find any, will be included only for certain outcomes related to success rate of intubation. These can be assessed in a cross-over design in which an intubator uses both methods sequentially. Order of insertion must be randomized to prevent familiarity with the airway on the second intubation affecting results. Since many of our outcomes are measured after the operation is completed they cannot be assessed in a cross-over design.
A risk of bias table will be completed for each included study and will be part of the data extraction form. For each outcome, risk of bias assessments will be summarized for each domain in risk of bias graphs and figures, and across all domains in a 'Summary of findings' table.
Measures of treatment effect
The outcomes in this review are mainly dichotomous outcomes (mortality, complications, successful first attempt, failed intubation). For these dichotomous outcomes we will seek to enter totals and numbers of events within each randomization group into RevMan 5.2 and calculate risk ratios with 95% confidence intervals. For time-to-event data, such as mortality, hazard ratios may be the most appropriate effect measure so we may use the generic variance option in RevMan 5.2. We may also need to use odds or risk ratios if we are unable to extract or obtain the raw data of numbers and totals from a study, but we will not combine different outcome measures in the same meta-analysis. For continuous measures, such as time for intubation, mean differences will be calculated. Some outcomes will be recorded in short ordinal scales, such as number of attempts, pain ratings for sore throat, or experience of intubator. These will be converted to dichotomous outcomes where appropriate.
Unit of analysis issues
For any cluster trials included in the review, we will extract data directly from the publication only if the analysis used accounts for the cluster design with a method such as multi-level modelling or generalized estimating equations. If these adjustments are not made within the report, we will undertake approximate analyses by recalculating standard errors or sample sizes based on the design effect. The resulting effect estimates and their standard errors will be analysed using the generic inverse variance method in RevMan.
We may include studies that report more than one comparison, for example groups allocated to two designs of videolaryngoscopes compared to a direct laryngoscopy group. This will not be a problem if we present these results in different comparisons. As we plan to use an amalgamated comparison group, initially at least, we need to address this issue and will divide the control group if data allow or combine the groups into a single pair-wise comparison (section 16.5.4 of Higgins 2011).
Dealing with missing data
We will contact authors to request any missing outcome data. If this is unsuccessful we will perform sensitivity analyses to compare the effects of complete case analysis, worst case scenario and last observation carried forward options on the results of individual studies and any meta-analyses.
Assessment of heterogeneity
We expect that the findings for any given outcome may differ between the studies included in the review. This heterogeneity may be due to:
- BMI > 30 kg/m
2, and degree of obesity;
- anticipated difficulty of airway, with measures such as Mallampti score;
- expertise of intubator, VLS device used e.g. Glidescope or Pentax;
- urgency of intubation: emergency versus elective; or
- site of intubation: in operating theatre, emergency department, ICU.
We will assess heterogeneity using Chi
Assessment of reporting biases
We will examine funnel plots to assess the potential for publication bias if we have 10 or more studies reporting on a particular outcome. We will use visual assessment supplemented by the Egger’s test for asymmetry (Egger 1997). Heterogeneity between studies may lead to asymmetry and we will consider this possibility when reviewing the results.
In addition to studies with no published results, reporting bias may be present within a study with data on some outcomes collected but not reported. Where a report or the study protocol suggests outcomes have not been reported, we will contact the study author to request outcome data.
We will attempt meta-analysis for outcomes for which we have comparable effect measures from more than one study and where measures of heterogeneity indicate that pooling of results is appropriate. An I
We will initially combine all designs of VLS and all population types, where appropriate, before dividing the data by VLS design and unselected and selected groups.
Subgroup analysis and investigation of heterogeneity
We will consider whether the success, speed and complication rate of intubation using a VLS differs for:
- different designs of VLS;
- obese and non-obese patients, anticipated or known difficult laryngoscopy;
- different sites of intubation: in operating theatre, emergency department, ICU; and
- experienced and inexperienced intubator.
We will undertake sensitivity analyses to explore the potential impact of missing data as described in the Dealing with missing data section. We will also carry out analyses stratified by risk of bias and we will model the potential impact of unpublished data on the overall effect estimates we obtain.
Summary of findings
We will use the principles of the GRADE system to give an overall assessment of the evidence relating to each of the following outcomes (Guyatt 2008).
- Hypoxia between one minute before induction to recovery.
- Serious respiratory complications (including aspiration and lower respiratory tract infection) within 30 days of anaesthetic.
- Mortality within 30 days of anaesthetic.
- Failed intubation or change of laryngoscopy device required.
- Number of attempts for tracheal intubation.
- Total time for tracheal intubation and commencement of ventilation.
The GRADE approach incorporates risk of bias, directness of evidence, heterogeneity of the data, precision of effect estimates and risk of publication bias to give an overall measure of how confident we can be that our estimate of effect is correct. SL and one other author will independently use GRADEpro software to create a 'Summary of findings' table for each outcome. Any discrepancies will be discussed and if needed referred to AS or TC for a final decision.
We would like to thank Rodrigo Cavallazz (content editor), Cathal Walsh (Statistical editor), Davide Cattano, Joshua Atkins (peer reviewers) for their help and editorial advice during the preparation of this protocol for the systematic review.
Appendix 1. Example manufacturers of videolaryngoscopes and stylets
- Storz V-MAC, Storz C-MAC and Storz C-Mac D blade (Karl Storz GmbH & Co KG, Tuttlingen, Germany).
- McGrath Series 5 and McGrath Mac (Aircraft Medical Limited, Edinburgh, UK).
- Glidescope Video Laryngoscope (Verathon Medical Inc, Bothell, WA, USA).
- Pentax Airway scope (Pentax_AWS, Ambu A/S, Ballerup, Denmark).
- Airtraq (Prodol Meditec S.A.,Vizcaya, Spain). Bullard (Circon ACMI, Stamford, CT, USA).
- Venner AP Advance (Intervent Direct, Buckinghamshire, UK). King Vision (Kingsystems, IN, USA).
- Vividtrac (Vivid Medical Inc, CA, USA). CoPilot VL (Magaw Medical, TX, USA).
- Disposable videolaryngoscope (Anatech Medical Ltd, New Zealand).
- Ue scope (Taizhou Hanchuang Medical Apparatus Technology Co Ltd, Taizhou, China).
Appendix 2. MEDLINE search strategy - via Ovid
1. (video?laryngoscop* or ((video or indirect) adj3 laryngoscop*) or Airtraq or Bullard or Pentax or Glidescope or McGrath or Storz or Venner or King Vision or Vividtrac or CoPilot VL or UE scope).mp.
2. ((randomized controlled trial or controlled clinical trial).pt. or randomized.ab. or placebo.ab. or drug therapy.fs. or randomly.ab. or trial.ab. or groups.ab.) not (animals not (humans and animals)).sh.
3. 1 and 2
Appendix 3. Study eligibility and data extraction form
1. General Information
2. Study Eligibility
DO NOT PROCEED IF STUDY EXCLUDED FROM REVIEW
3. Population and setting
Provide overall data and, if available, comparative data for each intervention or comparison group.
6. Intervention groups
6.1 Intervention group
6.2 Comparison group - repeated as required
7.1 Dichotomous outcomes such as mortality, complications, successful first attempt, failed intubation
7.2 Continuous outcomes such as time for intubation, pain scales, experience of intubation.
8.1 Dichotomous outcomes
8.2 Continuous outcomes
9. Risk of Bias assessment
11. Other information
Contributions of authors
Sharon R Lewis (SL), Amanda Nicholson (AN), Tim M Cook (TC), Andrew F Smith (AS)
Conceiving the review: AS
Co-ordinating the review: SL
Undertaking manual searches: SL and AN
Screening search results: SL and AN
Organizing retrieval of papers: SL
Screening retrieved papers against inclusion criteria: SL and AN
Appraising quality of papers: SL and AN
Abstracting data from papers: SL and AN
Writing to authors of papers for additional information: SL
Providing additional data about papers: SL and AN
Obtaining and screening data on unpublished studies: SL and AN
Data management for the review: SL
Entering data into Review Manager (RevMan 5.2): SL
RevMan statistical data: AN and SL
Other statistical analysis not using RevMan: AN
Interpretation of data: SL, TC and AN
Statistical inferences: AN, TC and AS
Writing the review: all authors
Securing funding for the review: AS
Performing previous work that was the foundation of the present study: N/A
Guarantor for the review (one author): AS
Person responsible for reading and checking review before submission: SL
Declarations of interest
Amanda Nicholson: from March to August 2011, AN worked for the Cardiff Research Consortium, which provides research and consultancy services to the pharmaceutical industry. Cardiff Research Consortium has no connection with AN's work with The Cochrane Collaboration. AN's husband has small direct holdings in several drug and biotech companies as part of a wider balanced share portfolio. Also see Sources of support.
Tim M Cook: TC has previously been paid by Intavent Orthofix and the LMA company, several years ago, for lecturing and his department has been given free or at cost airway equipment for evaluation or research from numerous airway companies. (This company manufactures and distributes a videolaryngoscope.) He (or his family) has no financial investments or ownership of any such company that he is aware of. No other sources of conflicts of interest.
Sharon R Lewis: see Sources of support.
Andrew F Smith: see Sources of support.
Sources of support
- No sources of support supplied
- NIHR Cochrane Collaboration Programme Grant: Enhancing the safety, quality and productivity of perioperative care. Project Ref: 10/4001/04, UK., UKThis grant funds the work of SRL, AN, AFS and PA performed for this review., Other.