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What do we know about this problem?
palpation of the cricoid membrane is a recommended technique to assess patient airway; however, it may fail. Ultrasonography has been suggested as a superior intervention for its correct identification.
What new contributions does this study make?
This study suggests - with low certainty of the evidence - that ultrasound is superior as compared to palpation for the detection of the CTM, according to clinical trials and observational studies. This association is still present in sensitivity analyses, excluding studies with a high risk of bias, cadaver studies and settings other than the operating room.
INTRODUCTION
Echography represents a highly versatile, safe, non-invasive, free of ionizing radiation, economic and increasingly available tool which may be practically used in any hospital setting. 1
Its value has been widely recognized over the past few years under the term POCUS (point of care ultrasound), based on its convenience to make specific assessments in a short period of time with the purpose of answering precise questions for timely decision-making. With this scheme, the windows assessed are limited, which allows for both ability and reproducibility of the observations 1, hence contributing to making ultrasound a tool to support the advanced management of the airway and the identification of a successful intubation. 2
The unanticipated difficult airway is a scenario with a high risk for morbidity and mortality and thus its prediction has been widely studied. However, it has been challenging to identify high performing clinical tools, in part due to a still incomplete understanding of the mechanisms involved in a difficult airway, the low incidence of this scenario, the morphological variability according to the population studied, the inconsistency of the assessment by different reviewers, and the limited discriminative ability of the predictors; all of these may account for this situation. 3
Over the last few years, a large number of studies have shown that some ultrasound measurements may improve the forecast of the difficult airway 4,5; however, there is no consensus about which are the most appropriate measurements. The "no ventilation - no oxygenation" scenario is one of the most pressing occurrences in the airway because of the high risk of mortality.
Under these circumstances, an emergency open cricothyroidotomy is indicated; nevertheless, in a closed review of claims it has been described that half of the difficult open cricothyroidotomy attempts could be explained by an inadequate identification of the cricothyroid membrane. 6 Considering that ultrasonography could improve the percentage identification of the cricothyroid membrane, a systematic review and a meta-analysis have been suggested to respond to this uncertainty.
OBJECTIVE
To establish whether bedside ultrasonography is superior to palpation in adult and pediatric patients, to identify the cricothyroid membrane at the first attempt.
METHODS
This study adhered to the PRISMA-P 7 recommendations for the development of the protocol and PRISMA for the abstract report; since the protocol referred to a secondary study, it was not submitted to the ethics committee; however, it was registered under code CRD42021223961, prior to the systematic search in the PROSPERO platform.
Primary outcome
The adequate proportion of CTM identification by group. The gold standard to confirm the cricothyroid membrane defined previously was the identification by an expert radiologist or anesthesiologist, with proven experience in ultrasonography and/or computed tomography (CT) or magnetic resonance imaging (MRI).
Planned secondary outcomes
Detection time, degree of ease of the technique, learning curve of each technique, operator satisfaction, adverse events.
Eligibility criteria
Controlled clinical trials, quasi-experimental trials, and diagnostic studies with interventions to identify the CTM using ultrasonography by any health practitioner - regardless of the technique or type of probe, in populations of any age, under any clinical setting: education, emergency department, hospitalization, critical and surgery, were included. The outcome was the identification of the CTM by a radiologist or expert personnel, whether using ultrasound or by any other means. Clinical simulation studies were excluded.
Sources of information
The following databases were reviewed: Medline via PubMed, Embase and Central, in addition to a grey literature search.
Search strategy
An external advisor who is a professional librarian, designed a comprehensive and specific search strategy (Complementary material 1).
Selection process
The titles of the articles found as a result of the searches were filtered to remove duplicates. The selection of eligible articles was carried out in two phases, independently and in duplicate, by two of the authors and an external collaborator.
Data collection process and data list
Data were extracted from the eligible studies by the two investigators independently and in duplicate, according to the designed format. The data list included date, author, population, patient inclusion criteria, exclusion criteria, number of participants, diagnostic criteria used, reference standard used, comparator, confounder control, and observations. This format was reviewed after the first three articles for possible modifications; in case of differences between the two authors, the third author resolved the disagreement. Relevant data not found in the article were requested from the corresponding author by e-mail or through the funding institution.
Risk of bias of individual studies
The assessment of risk of bias of the clinical trial-type studies was conducted using the ROB 2 tool. This is a risk of bias tool reviewed and recommended by the Cochrane collaboration of assessing random trials. It consists of fixed domains on the main types of biases in these studies and a decision-making algorithm on the degree of risk to be rated. 8 For the observational studies, the ROBINS tool 9 was used separately, independently and in duplicate; it was then taken to consensus by the three authors.
Effect measurements
Considering that the outcome assessed is the proportion of adequate identification of MCT, the measure of effect chosen for the meta-analysis was the relative risk with a confidence interval of 95 %.
Synthesis methods
The analysis was planned separating observational studies from clinical trials. After fulfilling assumptions for synthesis including clinical diversity, a quantitative synthesis based on the inverse variance method was proposed, with a random effects model, summarized in a forest plot, the calculation of heterogeneity was performed with the I2 statistic.
Metabias analysis report
If more than 10 studies were found, statistical analysis of publication bias was performed using Egger's test; otherwise, only a funnel plot was used.
Sensitivity and subgroup analysis
A sensitivity analysis by risk of bias was considered and a subgroup analysis by potential effect-modifier variables was planned according to the protocol: the age range, type of operator, type of transducer, setting where the procedure was performed, and the subgroup of patients classified with difficult airway.
Other analyses
Calculation of the aggregate Z-value in a trial sequential analysis (TSA) and the total sample value needed to find differences was performed to identify whether a larger sample size would be required.
Assessment of the certainty of the evidence
It was performed using the GRADE strategy, as recommended by the Cochrane manual for evidence synthesis of interventions. It was planned for each outcome, in the case of clinical trials assuming high certainty and then decreasing the assessment according to five dimensions: risk of bias, inconsistency, indirect evidence, imprecision and publication bias, independent and in duplicate, by means of the GRADEPRO direct access program 10.
The quantitative synthesis, sensitivity analyses and meta-analyses were performed with the Meta package of the R 1.4 statistical software; the calculation of the TSA was performed with the TSA software of the University of Copenhagen 11, both of which are free.
RESULTS
Selection of the studies
The systematic search found 62 specific articles and 362 from other sources. After eliminating duplicates, 362 articles were found, of which 32 passed phase 1 screening by title and abstract, 15 studies were included for narrative synthesis 12-26, of which 6 were controlled clinical trials and 9 observational studies, that were included in the quantitative synthesis. The flow chart of the individual studies is shown in Figure 1.
With regards to the characteristics of the studies included, it should be highlighted that 14 were performed in adult patients or bodies, and only one in pediatric patients; general, obstetric and obese population was included. The most common primary outcome was the proportion of adequate CTM identification, the vast majority under control of a trained anesthesiologist, ultrasound specialist or radiologist; the characteristics of the studies are summarized in Table 1.
Table 1 Characteristics of the studies.
| StudyID and type | Intervention | Control | Number of patients/ tests | Age group | Learning curve | Outcomes | Notes |
|---|---|---|---|---|---|---|---|
| Kristensen 12 2015 ECA | Technical longitudinal ultrasonography | Conventional technical palpation | 35/35 | Adult | Anesthesiologists with a mean experience of 6 years completed a structured training program: an online learning module followed by a 30-minute conference and a 20-minute practical training in live models. | Successful CTM identification. Time to identify the membrane with both techniques. | The interventions were randomized in only one patient |
| Siddiqui 13 2015 ECA | Technical longitudinal ultrasonography | Conventional technical palpation | 24/23 | Adult bodies | 10 minute lecture on cricothyroidotomy with the Portex® cricothyroidotomy kit (Smiths Medical, USA), followed by a 3-minute video of the palpation and ultrasound technique to identify anatomical landmarks and the CTM, followed by practical echography training at least five times using the Portex® device in human bodies. | Establish the results of the cricothyroidotomy performed in human bodies with the use of ultrasound guidance, as compared to digital palpation of anatomic landmarks. | Few details of the bodies assessed. |
| You-Ten 14 2015 ECA | Technical longitudinal ultrasonography | Conventional technical palpation | 56/56 | Adult obstetric | Not reported | Successful CTM identification. | No patients with a known neck deformity, neck surgery and/ or radiation were studied. |
| Forshaw 15 2018 ECA | Technical non explicit ultrasonography | Conventional technical palpation | 10/11 | Pediatric | Not reported | Successful CTM identification. Time to identify the membrane with both techniques. | Only one reviewer |
| Siddiqui 16 2018 ECA | Technical cross-sectional ultrasonography | Technical palpation of laryngeal hands-hake | 114/109 | Adult | Not reported | Successful CTM identification. | Included patients with a history of previous surgery, radiation exposure and/or neck mass. |
| Van Emden 17 2020 ECA | Technical non explicit ultrasonography | Non explicit technical palpation | 60/60 | Adult bodies | Each participant received a short training in ultrasound-assisted identification of the CTM. | To decide whether the participating anesthesiologists will judge body F4L as 'adequate' (assessment of suitable for learning) and 'realistic' (assessment of its appearance, sensation and flexibility as compared with a living human being), as a teaching model for localization of the CTM through palpation or ultrasound. | Bodies of donors with known neck anomalies or surgical procedures were excluded. |
| Aslani 18 2012 EO | Technical non explicit palpation between obese and non-obese | Cross-sectional and longitudinal technical ultrasonography as a control | 56/56 | Adults | Not reported | Successful identification of the CTM. | The recommendation is to assess and secure the airway in 40 seconds or less. |
| Barbe 19 2014 EO | Cross-sectional technical ultrasonography | Conventional technical palpation | 24/24 | Adults | Without a previous anatomic reminder | Successful identification of the CTM. Time to identification of the membrane with both techniques. | At six months, interns obtained better results with ultrasound than through clinical detection. |
| Lamb 20 2015 EO | Cross-sectional and longitudinal technical ultrasonography | Conventional technical palpation | 186/186 | Adults | Not reported | Successful identification of the CTM. | Fail to compare this technique with ultrasound or CT as the gold standard. |
| Yildiz 21 2017 EO | Longitudinal technical ultrasonography | Conventional technical palpation | 120/120 | Adults | Brief surgical anatomy of the airway and ultrasound training to identify the CTM before starting the study | Successful identification of the CTM. | Was only conducted by emergency medical personnel, with the assumption that this is a procedure exclusively of the emergency department. |
| Betul Basaran 22 2018 EO | Cross-sectional and longitudinal technical ultrasonography | Conventional technical palpation | 80/80 | Pediatric | Not reported | Successful identification of the CTM. | The overall success rate (55 %) was higherthan the general success rates in previous studies conducted in adult populations. |
| Oh 23 2018 EO | Conventional vs. laryngeal handshake technical palpation | Longitudinal and cross-sectional technical ultrasonography as a control | 123/123 | Adults | Reading of the DAS 2015 laryngeal handshake method and an illustrative video. | Successful identification of the CTM | Previous training was administered to anesthesia residents and not to ENT residents. |
| Altun 24 2019 EO | Ultrasonografía, técnica transversal y longitudinal | Conventional technical palpation | 110/110 | Adults | 20-minute PowerPoint presentation on the anatomy of the airway in US | Successful identification of the CTM. | The US group was previously trained, but it was not the same with the palpation group. |
| Bowness 25 2020 EO | Ultrasonografía, técnica transversal y longitudinal | Conventional technical palpation and laryngeal handshake technique | 33/66 | Adults | Not reported | Successful identification of the CTM and ensuring that it is still in the same place despite the mobilization. | The times for performing the maneuvers are not specified. |
| Lavelle 26 2021 EO | Ultrasonografía, técnica transversal y longitudinal | Conventional technical palpation | 28/28 | Adult obstetric | Training of a mínimum of 20 echograms and procedure guidelines | Successful identification of the CTM. Time to identification and perception of how easy was the procedure. | The investigators who administered the intervention were blinded to the standard. |
CTM: Cricothyroid membrane ; OS: Observational Study; RCT: Randomized clinical trial.
Source: Authors.
QUANTITATIVE SYNTHESIS
In terms of the outcome "adequate CTM identification", in the opinion of the authors there was not a broad clinical diversity. The validation criteria included the type of operators of the techniques, the populations and the MCT identification technique itself, for the studies included that prevented quantitative synthesis. Therefore, quantitative synthesis of RCTs (Figure 4) and observational studies were performed (Figure 5).
Ultrasound is superior to palpation for the identification of the CTM with a RR 1.88 (95 % CI 1.05; 3.36), risks difference RD of 30 % (95 % CI 8; 53) according to the clinical trials included with high heterogeneity I2 69 %; this result is consistent for observational studies RR 1.76, (95 % CI 1.36; 2.28) RD of 38 % (95 % CI 23; 54) with high heterogeneity I2 94 %.
Not enough information was found for the other suggested outcomes; no adverse events are described and only three studies included the time to identification: Kirskensen (2015) with a mean of 48 seconds SD (20) versus 18 seconds SD (10); Forshaw (2018) with a mean of 35.5 SD (20.69) versus 14.18 SD (7.64) and Barbe (2014) with 27 seconds SD (5) versus 19.5 SD (7.25) for ultrasound and palpation, respectively.
Metabias report
It was not possible to conduct the Egger test due to the small number of studies included; however, a separate plot assessment of the publication bias was performed. The funnel diagram of Complementary material 2 does not give any evidence of publication bias.
Sensitivity analysis
A sensitivity analysis was conducted excluding the studies with high risk of bias and another one excluding the studies in cadavers or in the emergency department setting. (Complementary material 2). The relative risk for the correct identification of the cricothyroid membrane at first attempt was maintained.
Subgroup analysis
The analyses planned in the protocol were not conducted because a sufficient description of the effect-modifying variables was missing.
Certainty of the evidence
Following the assessment using the GRADE tool, the certainty of the evidence found was considered to be low, due to the risk of bias and inaccuracy (Figure 6).
A post hoc sequential analysis of the trials (TSA) was conducted, with conventional analysis in random effects and two-tailed a of 0.05, resulting in an accumulated Z value of -2.02, which favors the intervention; it was also evidenced that the sample needed to identify any differences is of 570 patients, hence confirming the strength of the result (Complementary material 2).
DISCUSSION
Ultrasound has revolutionized the medical practice. This study has shown - with low certainty of the evidence - that ultrasound is superior to palpation for the successful identification of the CTM, with a RR of 1.88 (95 % CI 1.05; 3.36) according to the clinical trials included. This result is consistent with the observational studies RR 1.76 (95 % CI 1.36; 2.28). The association was maintained in the sensitivity analyses, excluding studies with high risk of bias and settings outside the operating room, such as cadaveric models. The association was also maintained in the sequential analysis of the trials indicating that the number of observations was adequate in terms of the sample size needed to identify differences between the two interventions.
It should be highlighted that the CTM ultrasound identification protocols included in most cases the longitudinal technique, whereby after identifying the midline, rotates the probe to find the height of the CTM. The majority of the studies include general adult population; one of them included obstetric population and two of them obese population; hence, the application of the results is particularly focused on these populations in an elective surgical environment.
The study herein described found similar results to those of Hung et al. who in a previous meta-analysis of eight studies-including clinical and observational studies-, compared ultrasound versus palpation, with a reduction of up to 50 % in the number of failed identifications; RR 0-50 (95 % CI 0.33; 0.76). 27
The strength of this study includes the grey literature search which although it didn't add any further studies for the synthesis, it is indeed an indication of the comprehensive nature of the search, the selection, extraction and assessment of biases in an independent manner and in duplicate, in addition to developing the synthesis segregating the clinical trials and the observational studies.
The limitations identified in this review include the statistical heterogeneity; however, clinically the authors consider that there is no diversity in the administration of the interventions, and hence it was deemed appropriate for the authors to do a quantitative synthesis for the primary outcome. Another limitation is the small number of studies which increases the inaccuracy of the results. The meta-analysis of observational studies presents a high risk of bias due to imbalanced confounding factors as a result of the lack of randomization and the inclusion of cadaveric studies that may limit the application of the results in clinical practice.
One additional limitation is the clinical diversity of the populations since the interventions were conducted in obstetric, obese and pediatric patients, and in settings outside the operating room; this could favor the external validity of the use of the results, but adds uncertainty to the magnitude of the effect.
With regards to the gaps in the evidence, because of the low certainty of the studies due to risk biases, inaccuracy and statistical heterogeneity, further studies on the superiority of ultrasound versus palpation are needed, in order to strengthen the certainty of the evidence. Another uncertainty is that no primary studies were found on the combination of the ultrasound and palpation techniques, whether sequentially or in parallel.
CONCLUSION
With a low certainty of the evidence, ultrasound is superior to palpation for the correct identification of the CTM. Further studies are needed, with improved quality of the methodology, to strengthen the certainty and the precision of the evidence.
ETHICAL RESPONSIBILITIES
Endorsement by the ethics committee
The study was not submitted to the ethics committee since this was a secondary study.
Protection of humans and animals
The authors declare that no experiments in humans or in animals were conducted for this research. The authors declare that the procedures followed are consistent with the ethical standards of the committee for responsible human experimentation in accordance with the World Medical Association and the Declaration of Helsinki.
