Transtracheal ultrasound for identifying endotracheal intubation in adults

Objectives

This is a protocol for a Cochrane Review (diagnostic). The objectives are as follows:

To determine the diagnostic accuracy of transtracheal ultrasound for detecting endotracheal intubation in adult patients.

Secondary objectives

Secondary objectives include assessing the diagnostic accuracy of transtracheal ultrasound amongst the following subgroups:

• setting (e.g. prehospital, emergency department, intensive care unit, operating room)

• operator specialty (e.g. emergency medicine, non‐emergency medicine)

• operator experience (e.g. attending physician, resident physician, non‐physician)

• ultrasound technique (e.g. static, dynamic)

• ultrasound transducer (e.g. curvilinear, linear)

Background

Target condition being diagnosed

Endotracheal intubation involves placement of an endotracheal tube into a patient's trachea. It is a procedure commonly performed in multiple settings for airway protection, including the operating room, the prehospital environment, emergency department, and intensive care unit. Of the 313 million surgeries performed worldwide, the majority require endotracheal intubation (Weiser 2015), and approximately 400,000 intubations are performed each year in the USA for emergent airway management (National Center for Health Statistics 2020).

Data suggest that esophageal intubation (incorrect placement of the endotracheal tube into the esophagus instead of the trachea) can occur in 3% to 29% of first pass intubation attempts (Brown 2015; Taboada 2019; Prekker 2023). If this incorrect placement is not quickly identified and corrected, patients can experience significant morbidity and mortality (Crosby 2021; Honardar 2017). Moreover, if a correctly placed endotracheal tube is misdiagnosed as being esophageal, the endotracheal tube will be removed and a repeat intubation procedure unnecessarily performed.

Index test(s)

Ultrasound can be a valuable adjunct for confirming the endotracheal tube location at the bedside (Chou 2015; Das 2015; Gottlieb 2018; Gottlieb 2019; Gottlieb 2020a; Gottlieb 2020b; Gottlieb 2024). Specific benefits include the rapidity of identification and the ability to directly visualize the endotracheal tube without requiring positive pressure ventilations.

The examination is performed by placing a linear or curvilinear ultrasound transducer in a transverse (i.e. horizontal) plane on the anterior neck just superior to the suprasternal notch (Lonchena 2017; Romano 2018). Ultrasound assessment can be performed in real time during the intubation (dynamic technique) or immediately after the intubation has been performed (static technique). In the dynamic technique, the sonographer places the ultrasound transducer on the neck prior to commencing the intubation and assesses for rapid, flutter‐like movement within the trachea as the endotracheal tube passes through the vocal cords, often referred to as the 'snowstorm sign' (Gottlieb 2020a; Gottlieb 2024). In the static technique, the sonographer places the probe on the neck immediately after the intubation has been performed and assesses for the presence of either a thin, hyperechoic membrane immediately deep to the trachea (endotracheal intubation) or a separate air‐mucosal interface lateral to the trachea, referred to as the 'double tract' or 'double barrel' sign (esophageal intubation; Gottlieb 2020a; Gottlieb 2024).

Clinical pathway

After performing an intubation, it is critical to confirm that the endotracheal tube is correctly placed within the trachea. The traditional approach to confirming the endotracheal tube location after intubation involves direct visualization by the intubating provider, followed by adjunctive techniques. These can include auscultation of the lungs for bilateral breath sounds, auscultation of the epigastrum for sounds of gastric insufflation, checking for the appearance of misting within the endotracheal tube, or visualization with transtracheal ultrasound. These are typically followed by qualitative (i.e. colorimetric) or quantitative CO₂ detection.

There are several techniques to determine the endotracheal tube location after intubation. Assessing for endotracheal tube misting (defined as condensation inside the endotracheal tube or catheter mount during exhalation) has a limited role with a false positive rate of up to 69% (Hansel 2023). Lung auscultation, with or without epigastric auscultation, also has insufficient accuracy with a 14% to 18% false positive rate (Hansel 2023). Esophageal detector devices show some promise, with a lower false positive rate of 5% (Hansel 2023), though studies vary considerably with some demonstrating markedly lower accuracy (Takeda 2003; Tanigawa 2001). It also requires a special device, which is not available in many settings.

Measuring the level of exhaled CO₂ is another common indicator of tracheal placement of an endotracheal tube, and is generally considered the gold standard. Exhaled CO₂ can be assessed either qualitatively or quantitatively. Qualitative assessment is performed with a colorimetric device that uses pH‐sensitive paper that changes color in response to the acidic CO₂. Quantitative assessment (i.e. capnometry) is performed with an electronic device using a sidestream analyzer that samples gas from the ventilator circuit and provides a specific number to reflect the concentration of CO₂ exhaled with or without graphic representation (waveform capnography). Despite common usage, CO₂ detection has several distinct disadvantages. First, CO₂ detection requires approximately five rescue breaths for confirmation, which can lead to gastric distension and an increased risk of aspiration if the endotracheal tube has been incorrectly placed in the esophagus (MacLeod 1991; Takeda 2003). Additionally, CO₂ detection can be influenced by the presence of pre‐existing gastric CO₂ contents (e.g. prior bag‐valve‐mask use, recent carbonated beverage ingestion), which can lead to reduced diagnostic accuracy in these groups (MacLeod 1991; Takeda 2003). Moreover, conditions that reduce CO₂ production or delivery to the pulmonary venous system for gas exchange (e.g. cardiac arrest, massive pulmonary embolism) can also reduce accuracy. In fact, studies have reported that quantitative capnography is only 65% to 68% sensitive for detecting tracheal placement of the endotracheal tube during cardiac arrest (Takeda 2003; Tanigawa 2000; Tanigawa 2001). Finally, CO₂ detection cannot determine the depth of the endotracheal tube, limiting the ability to determine either hypopharyngeal placement or single lung intubation (Gottlieb 2022; Gottlieb 2023).

Rationale

While several methods exist for assessing endotracheal tube location after intubation, each of these has distinct limitations. Misidentified endotracheal tube location can lead to significant morbidity and mortality. Ultrasound offers a valuable option for assessing endotracheal tube location, rapidly at the bedside, without requiring positive pressure ventilations.

Objectives

To determine the diagnostic accuracy of transtracheal ultrasound for detecting endotracheal intubation in adult patients.

Secondary objectives

Secondary objectives include assessing the diagnostic accuracy of transtracheal ultrasound amongst the following subgroups:

• setting (e.g. prehospital, emergency department, intensive care unit, operating room)

• operator specialty (e.g. emergency medicine, non‐emergency medicine)

• operator experience (e.g. attending physician, resident physician, non‐physician)

• ultrasound technique (e.g. static, dynamic)

• ultrasound transducer (e.g. curvilinear, linear)

Methods

Criteria for considering studies for this review

Types of studies

We will consider all primary diagnostic test accuracy studies as eligible for inclusion if:

• they evaluate the accuracy of transtracheal ultrasound for endotracheal intubation against a final diagnosis using a reference standard of direct visualization, auscultation, endotracheal tube suction devices, capnography, or capnometry; and

• they provide sufficient data to create 2 x 2 tables to calculate sensitivity, specificity, and negative and positive likelihood ratios.

We will include prospective studies, regardless of whether randomization to ultrasound has occurred. We will exclude retrospective cohort and case‐control studies, case series, and case reports.

Participants

We will include studies of adults (aged ≥ 16 years) who are intubated, and the endotracheal tube location is assessed with transtracheal ultrasound, regardless of setting (e.g. prehospital, emergency department, intensive care unit, operating room). We will only include studies of live patients (i.e. not cadaver studies). If studies provide data on a mixed population of pediatric and adult patients, we will include the adult population if the adult data are presented separately. We will not include studies with mixed populations where adult‐only cohort data are not available.

Index tests

The index test will be transtracheal ultrasound (defined as placement of the ultrasound transducer at the front of the neck) to confirm endotracheal tube location, regardless of whether the static or dynamic technique is used. We will not include studies that use alternate modalities (e.g. diaphragmatic excursion, lung sliding). The index test could be performed by a physician (regardless of specialty) or non‐physician.

Target conditions

The target condition will be correct placement of the endotracheal tube within the trachea (positive test result) or the esophagus (negative test result). We will not include studies that assess endotracheal tube depth in isolation.

Reference standards

The reference standard for endotracheal intubation confirmation will include any combination of direct visualization, auscultation, endotracheal tube suction devices, capnography, or capnometry. All reference standards listed alone or in any combination and performed by any operator are considered of equal validity.

Search methods for identification of studies

Electronic searches

The search strategies for databases included in this review will be designed by our institution’s Library Research Information Specialist. We will search the following electronic databases for all articles pertaining to the review topic.

• Cochrane Central Register of Controlled Trials (CENTRAL) in the Cochrane Library ‐ (latest issue)

• Cochrane Database of Systematic Reviews in the Cochrane Library ‐ (latest issue)

• PubMed (1946 to present)

• Scopus (inception to present)

• Cumulated Index to Nursing and Allied Health Literature (CINAHL) provided by EBSCO (inception to present)

• Latin American and Caribbean Health Sciences Literature (LILACS) provided by LILDBI‐Web application (inception to present)

• Google Scholar (first 200 entries per Bramer 2017)

We will restrict the search to adult participants. There will be no restrictions on language, year of publication, study country, or other such limitations. We will search using controlled vocabularies when provided, and will use a Title/Abstract field code restriction on our keywords. The preliminary search strategy for all sources can be found in Appendix 1; Appendix 2; Appendix 3; Appendix 4; Appendix 5; Appendix 6

Searching other resources

We will search the following clinical trials registries for ongoing trials.

• ClinicalTrials.gov (www.clinicaltrials.gov)

• World Health Organization (WHO) International Clinical Trials Registry Platform (ICTRP; https://trialsearch.who.int/)

In addition, we will check reference lists of reviews and retrieved articles for additional studies. We will contact study authors for additional information where necessary.

Data collection and analysis

Selection of studies

We will transfer search results into Covidence for screening and selection. Articles will be deduplicated before screening for eligibility. Two review authors (MG, AM) will independently screen the titles and abstracts to determine the eligibility of each study identified by the search. Studies that clearly do not satisfy inclusion criteria will be excluded. We will resolve disagreements via consensus, with the addition of a third review author (DJK) to adjudicate if needed. The review authors will obtain full copies of the remaining studies.

Two review authors (MG, AM) will read these studies independently to select relevant studies, and in the event of disagreement, a third review author will adjudicate (DJK). We will not anonymize the studies before assessment. We will collate multiple reports of the same study, so that each study rather than each report is the unit of interest in the review. We will include a PRISMA flow diagram (Page 2021), and 'Characteristics of excluded studies' table in the full review to show the status of identified studies and reasons for exclusion. We will include studies in the review irrespective of whether they report measured outcome data in a 'usable' way (e.g. we will include studies if no 2 x 2 table is provided, but there are sufficient data to derive a 2 x 2 table).

Data extraction and management

Two review authors (MG, DJK) will independently extract data in duplicate using a standard piloted form and check for agreement before entry into Review Manager (RevMan 2025). The form will be piloted on three studies. In the event of disagreement, a third review author will adjudicate (AM). When data are missing or unclear, we will reach out to study authors for additional information. We will collect characteristics of the included studies in sufficient detail to populate a table of 'Characteristics of included studies' in the full review. We will convert data found in studies to a format appropriate for meta‐analysis. We will extract the following information.

• Study characteristics

  • Study date

  • Study design

  • Study setting

  • Study country

  • Study type

  • Details of blinding and allocation concealment

  • Publication type

  • Study funding source

  • Study author conflicts of interest

• Participants

  • Total number of participants

  • Inclusion criteria

  • Exclusion criteria

  • Mean or median age

  • Gender distribution

• Index test

  • Ultrasound technique

  • Ultrasound transducer

  • Operator training

  • Operator specialty

  • Operator experience (level of training)

• Reference standard

  • Reference standard utilized

• Outcomes

  • True positive (TP), true negative (TN), false positive (FP), and false negative (FN) results for transtracheal ultrasound. A positive test result will be correct placement of the endotracheal tube in the trachea, whereas a negative test result will be incorrect placement of the endotracheal tube in the esophagus.

  • TP, TN, FP, FN for transtracheal ultrasound when performed in the prehospital environment versus emergency department versus intensive care unit versus operating room

  • TP, TN, FP, FN for transtracheal ultrasound when performed by an emergency medicine provider versus non‐emergency medicine provider

  • TP, TN, FP, FN for transtracheal ultrasound when performed by an attending physician versus resident physician versus non‐physician

  • TP, TN, FP, FN for transtracheal ultrasound when performed with the static technique versus dynamic technique

  • TP, TN, FP, FN for transtracheal ultrasound when performed with the curvilinear transducer versus linear transducer

Assessment of methodological quality

Two review authors (MG, DJK) will independently assess methodological quality using the Quality Assessment of Diagnostic Accuracy Studies version 2 (QUADAS‐2) tool (Whiting 2011). We will apply the QUADAS‐2 tool in four phases: 1) summarize the review question; 2) tailor the tool to the review and produce review‐specific guidance; 3) construct a flow diagram for the primary study; and 4) assess risk of bias and concerns regarding applicability. The tool will provide evaluation across four domains, including patient selection, index test, reference standard, and flow and timing. All domains provide judgment for risk of bias with ratings of 'low', 'high', or 'unclear'. Additionally, the domains of patient selection, index test, and reference standard will evaluate concerns regarding applicability with ratings of 'low', 'high', or 'unclear'. We will classify studies as overall 'low' risk of bias when judged as 'low' across all four domains or 'low' across three domains with one 'unclear' domain. We will utilize the QUADAS‐2 checklist (Appendix 7). Any disagreements between review authors will be resolved through discussion with a third review author (AM).

Statistical analysis and data synthesis

We will formulate 2 x 2 tables of the number of TP, TN, FP, and FN results from included studies. We will pool all studies, irrespective of the reference standard used. If more than one reference standard is used, we will preferentially use capnography as the reference standard. We will use these data to estimate summary sensitivities and specificities with 95% confidence intervals (95% CI). We will calculate positive and negative likelihood ratios from the summary sensitivity and specificity data. We plan to visually display data using coupled forest plots of sensitivity, specificity, and summary receiver operating characteristic (SROC) plots. We will include a 95% confidence region around the summary points on SROC plots. We will present sensitivities, specificities, and likelihood ratios in a summary table.

We plan to perform meta‐analysis using a bivariate, generalized, linear mixed model (Chu 2006). The components of the bivariate model will be used to create SROC plots (Reitsma 2005). We will calculate summary estimates using the MIDAS, METANDI, or METAN modules (or a combination) for StataMP (Stata). We will transcribe the results from our statistical analyses to Review Manager to formulate our tables and figures (RevMan 2025). We plan to describe study results or perform a univariate random‐effects model analysis when low study inclusion precludes the completion of a bivariate mixed‐effects regression model (Takwoingi 2017).

Investigations of heterogeneity

We will investigate heterogeneity for diagnostic test accuracy when data are sufficient (more than 10 included studies). We will visually examine forest plots and SROC plots in subgroup analyses. We will explore possible sources of heterogeneity by the following subgroup analyses:

• setting (prehospital vs emergency department vs in‐hospital setting vs operating room), as intubating conditions vary by setting, and potential differences in populations may lead to spectrum bias;

• operator specialty (emergency medicine vs anesthesiology vs critical care vs other specialties), as type of training may impact exam performance;

• operator experience, as level of training (resident physician vs ultrasound fellow or attending physician) may impact exam performance;

• ultrasound technique, as the specific technique (dynamic vs static) may impact exam performance; and

• ultrasound transducer, as the specific transducer (curvilinear vs linear) may impact exam performance.

When data are sufficient (more than 10 included studies), we plan to further investigate heterogeneity by meta‐regression of subgroups as covariates using the package MIDAS in StataMP (Stata).

Sensitivity analyses

We will perform a sensitivity analysis to investigate potential sources of bias based on methodological quality. We will include studies classified as overall low risk of bias from the QUADAS‐2 assessment. We will also consider performing sensitivity analysis by:

• excluding studies with outlier results; and

• by studies utilizing a reference standard of capnometry.

Assessment of reporting bias

We will not conduct an assessment of reporting bias because no standard approach is currently available for reviews of diagnostic test accuracy.

Summary of findings and assessment of the certainty of the evidence

We will formulate a summary of findings table and assess the certainty of evidence for the diagnostic test studies according to GRADE criteria (GRADEpro GDT; Schünemann 2020a; Schünemann 2020b). We will analyze TP, TN, FP, and FN results at different prevalences (e.g. TP + FN), which will be the lower quartile, median, and upper quartile of prevalences determined from the included studies. We will state the index test's ability to correctly identify success or failure of intubation as well as cases missed. Two review authors (MG, DJK) will independently rate the certainty of the body of evidence for the outcomes. A third review author (AM) will resolve discrepancies, as needed. GRADE considers the following factors that may decrease the certainty of evidence for systematic reviews of diagnostic studies: risk of bias, indirectness, inconsistency, imprecision, and publication bias. We will rate each domain as not serious, serious, or very serious. We have chosen in advance a confidence interval range of 10% to assist with the judgment of imprecision. The final certainty of evidence will be rated as one of four grades.

• High certainty: we are very confident that the true effect lies close to that of the estimate of the effect.

• Moderate certainty: we are moderately confident in the effect estimate; the true effect is likely to be close to the estimate of the effect, but there is a possibility that it is substantially different.

• Low certainty: our confidence in the effect estimate is limited; the true effect may be substantially different from the estimate of the effect.

• Very low certainty: we have very little confidence in the effect estimate; the true effect is likely to be substantially different from the estimate of effect.

Appendices

Appendix 1. Cochrane Central Register of Controlled Trials (CENTRAL) and Cochrane Database of Systematic Reviews search strategy

#1 MeSH descriptor: [Trachea] explode all trees 465

#2 (trach* OR endotracheal OR intratracheal OR transtracheal OR transcricoid OR suprasternal notch OR anterior neck) 23671

#3 #1 OR #2 23671

#4 MeSH descriptor: [Intubation] explode all trees 6508

#5 MeSH descriptor: [Airway Management] explode all trees 12703

#6 Intubation OR respiration OR tube 51976

#7 #4 OR #5 OR #6 54370

#8 MeSH descriptor: [Ultrasonography] explode all trees 17489

#9 MeSH descriptor: [Ultrasonics] explode all trees 380

#10 ultraso* OR sonograp* OR Imag* 157045

#11 #8 OR #9 OR #10 159730

#12 "Place" OR placed OR placement OR position* OR insert* OR detect* OR guide 229893

#13 #3 AND #7 AND #11 AND #12 734

#14 case report OR case study OR child* OR pediatric OR paediatric OR infant OR animal OR pig OR porcine OR MRI OR computed tomography 367647

#15 #13 NOT #14 350

Appendix 2. PubMed search strategy

(trach*[tiab] OR endotracheal[tiab] OR intratracheal[tiab] OR transtracheal[tiab] OR transcricoid[tiab] OR suprasternal notch[tiab] OR anterior neck[tiab] OR "Trachea"[Mesh])

AND (Intubation[tiab] OR respiration[tiab] OR tube OR "Intubation"[Mesh:NoExp] OR "Airway Management"[Mesh])

AND ( ultraso*[tiab] OR sonograp*[tiab] OR Imag*[tiab] OR "Ultrasonography"[MeSH Terms] OR "Diagnostic Imaging"[Mesh:NoExp] OR "Ultrasonics"[Mesh])

AND ("Place"[tiab] OR placed[tiab] OR placement[tiab] OR position*[tiab] OR insert*[tiab] OR detect*[tiab] OR guide[tiab])

NOT (case report[ti] OR case study[ti] OR child*[ti] OR pediatric[ti] OR paediatric[ti] OR infant[ti] OR animal OR pig OR porcine OR MRI OR computed tomography OR "Child"[Mesh] OR "Infant"[Mesh] OR (Animals[mh] NOT Humans[mh]))

Appendix 3. Scopus search strategy

( TITLE‐ABS ( ( trach* OR endotracheal OR intratracheal OR transtracheal OR transcricoid OR {suprasternal notch} OR {anterior neck} )

AND ( intubation OR respiration OR tube )

AND ( ultraso* OR sonograp* OR imag* )

AND ( "Place" OR placed OR placement OR position* OR insert* OR detect* OR guide ) ) )

AND NOT ( TITLE ( {case report} OR {case study} OR child* OR pediatric OR paediatric OR infant ) OR TITLE‐ABS ( animal OR pig OR porcine OR mri OR {computed tomography} ) )

Appendix 4. CINAHL search strategy

( TI ( trach* OR endotracheal OR intratracheal OR transtracheal OR transcricoid OR suprasternal notch OR anterior neck )

OR AB ( trach* OR endotracheal OR intratracheal OR transtracheal OR transcricoid OR suprasternal notch OR anterior neck )

OR ( (MH "Trachea") OR (MH "Tracheostomy") OR (MH "Intubation") OR (MH "Airway Management") OR (MH "Administration, Intratracheal")

AND ( TI ( Intubation OR respiration OR tube )

OR AB ( Intubation OR respiration OR tube )

OR MH "Endotracheal Tubes")) )

AND ( TI ( ultraso* OR sonograp* OR Imag* )

OR AB ( ultraso* OR sonograp* OR Imag* )

OR (MH "Ultrasonography" OR MH "Ultrasonics") )

AND ( TI ( "Place" OR placed OR placement OR position* OR insert* OR detect* OR guide )

OR AB ( "Place" OR placed OR placement OR position* OR insert* OR detect* OR guide )

OR ( MH "Tube Placement Determination" )

NOT TI ( case report OR case study OR child* OR pediatric OR paediatric OR infant )

OR TI ( animal OR pig OR porcine OR MRI OR computed tomography )

OR AB ( animal OR pig OR porcine OR MRI OR computed tomography )

Appendix 5. LILACS search strategy

((trach* OR endotracheal OR intratracheal OR transtracheal OR transcricoid OR “suprasternal notch” OR “anterior neck”) )

AND ( (Intubation OR respiration OR tube) )

AND (( ultraso* OR sonograp* OR Imag*) )

AND (("Place" OR placed OR placement OR position* OR insert* OR detect* OR guide) )

AND NOT ((case report OR case study OR child* OR pediatric OR paediatric OR infant OR animal OR pig OR porcine OR MRI OR computed tomography ))

Appendix 6. Google Scholar search strategy

(trachea|endotracheal|transtracheal|transcricoid) (Intubation|respiration|tube) ( ultrasound|sonography|Imaging) ("Place"|placed|placement|position|insert|detect|guide)

Appendix 7. Quality Assessment Checklist (QUADAS‐2)

Domain 1: Patient selection

Was a consecutive or random sample of patients enrolled?

Yes ‐ Answer 'yes' if stated a consecutive or random sample was used.

No ‐ Answer 'no' if stated an enrollment method was used other than consecutive or random sampling.

Unclear ‐ Answer 'unclear' if there is no mention of the methods for sampling or enrollment.

Was a case–control design avoided?

Yes ‐ Answer 'yes' if case‐control design was avoided.

No ‐ Answer 'no' if case‐control design was not avoided. Study should be excluded from review.

Unclear ‐ Answer 'unclear' if there is insufficient detail to determine if a case‐control design was avoided.

Did the study avoid inappropriate exclusions?

Yes ‐ Answer 'yes' if there are no concerns regarding inappropriate exclusions.

No ‐ Answer 'no' if patients were excluded because they were difficult to obtain images from.

Unclear ‐ Answer 'unclear' if inappropriate exclusion criteria cannot be determined.

Risk of bias: could the selection of patients have introduced bias?

High ‐ If the answer to any of the signaling questions results in 'no', then rate as 'high'.

Low ‐ If the answer to all of the signaling questions is 'yes', then rate as 'low'. Also, rate as 'low' if only one signaling question is 'unclear' while the remaining are 'yes'.

Unclear ‐ If two or more signaling questions are rated as 'unclear', then rate as 'unclear'.

Applicability: are there concerns that the included patients do not match the review question?

High ‐ If study patients only included those with significant airway abnormalities, then rate as 'high'.

Low ‐ If study patients did not included only those with significant airway abnormalities, then rate as 'low'.

Unclear ‐ If patient inclusion criteria are not specified, then rate as 'unclear'.

Domain 2: Index test

Were the index test results interpreted without knowledge of the results of the reference standard?

Yes ‐ Answer 'yes' if the results of the index test were blinded or completed by an independent party.

No ‐ Answer 'no' if the results were unblinded or the reference standard results were known prior to completion of the index test.

Unclear ‐ Answer 'unclear' if a determination cannot be made.

If a threshold was used, was it prespecified?

Yes ‐ Answer 'yes' if a prespecified threshold (ultrasonographic visualization of the endotracheal tube within the trachea or absence of the double‐barrel sign) was used.

No ‐ Answer 'no' if a prespecified threshold was not used.

Unclear ‐ Answer 'unclear' if a determination cannot be made.

Risk of bias: could the conduct or interpretation of the index test have introduced bias?

High ‐ If the answer to any of the signaling questions results in 'no', then rate as 'high'.

Low ‐ If the answer to all of the signaling questions is 'yes', then rate as 'low'.

Unclear ‐ If the answer to any of the signaling questions results in 'unclear', then rate as 'unclear'.

Applicability: are there concerns that the index test, its conduct, or its interpretation differ from the review question?

High ‐ If the operators performing the examination were not trained on how to perform transtracheal ultrasound, then rate as 'high'.

Low ‐ If the operators received specific training on how to perform transtracheal ultrasound, then rate as 'low'.

Unclear ‐ If the operators training for performing transtracheal ultrasound is not described, then rate as 'unclear'.

Domain 3: Reference standard

Is the reference standard likely to correctly classify the target condition?

Yes ‐ Answer 'yes' if direct visualization, auscultation, endotracheal tube suction devices, capnography, or capnometry was used as the reference standard.

No ‐ Answer 'no' if a reference standard other than the criteria for 'yes' was used. The study should be excluded.

Unclear ‐ Answer 'unclear' if the criteria is not appropriately described as the reference standard.

Were the reference standard results interpreted without knowledge of the results of the index test?

Yes ‐ Answer 'yes' if the clinician completing the reference standard was blinded to the index test result.

No ‐ Answer 'no' if the clinician completing the reference standard was aware of the index test result.

Unclear ‐ Answer 'unclear' if it cannot be determined whether the reference standard result was completed without knowledge of the index test.

Risk of bias: could the reference standard, its conduct, or its interpretation have introduced bias?

High ‐ If the answer to any of the signaling questions results in 'no', then rate as 'high'.

Low ‐ If the answer to all of the signaling questions is 'yes', then rate as 'low'.

Unclear ‐ If the answer to any of the signaling questions results in 'unclear', then rate as 'unclear'.

Applicability: are there concerns that the target conditions defined by the reference standard do not match the question?

High ‐ If patients did not receive an accepted reference standard (defined as direct visualization, auscultation, endotracheal tube suction devices, capnography, or capnometry), then mark as 'high'.

Low ‐ If all patients received the accepted reference standard (defined as direct visualization, auscultation, endotracheal tube suction devices, capnography, or capnometry), then mark as 'low'.

Unclear ‐ If the reference standard is incompletely reported, then mark as 'unclear'.

Domain 4: Flow and timing

Was there an appropriate interval between index tests and reference standard?

Yes ‐ Answer 'yes' if the reference standard was completed at or in close proximity to the time of the index test.

No ‐ Answer 'no' if the reference standard was completed in a delayed fashion following the index test. For example, the index test was completed during the intubation and the reference standard was completed during extubation.

Unclear ‐ Answer 'unclear' if the time frame relation between the index test and the reference standard are not stated.

Did all patients receive a reference standard?

Yes ‐ Answer 'yes' for a reference standard of direct visualization, auscultation, endotracheal tube suction devices, capnography, or capnometry for all patients.

No ‐ Answer 'no' if the reference standard was not applied to all patients.

Unclear ‐ Answer 'unclear' if the study does not mention whether all patients received the reference standard.

Did all patients receive the same reference standard?

Yes ‐ Answer 'yes' if the same reference standard was used for all patients.

No ‐ Answer 'no' if the same reference standard was not used for all patients.

Unclear ‐ Answer 'unclear' if this information cannot be determined.

Were all patients included in the analysis?

Yes ‐ Answer 'yes' if all participants enrolled were included in the accuracy assessment.

No ‐ Answer 'no' if the number of participants enrolled in the study differs from the number included in the accuracy assessment.

Unclear ‐ Answer 'unclear' if there is ambiguity in the number of enrolled and assessed patients.

Risk of bias: could the patient flow have introduced bias?

High ‐ If the answer to any of the signaling questions results in 'no', then rate as 'high'.

Low ‐ If the answer to all of the signaling questions is 'yes', then rate as 'low'. Also, rate as 'low' if only one signaling question is 'unclear' while the remaining are 'yes'.

Unclear ‐ If two or more signaling questions are rated as 'unclear', then rate as 'unclear'.

Contributions of authors

MG: idea synthesis, literature review, protocol design, protocol drafting, and review

DJK: idea synthesis, literature review, protocol design, protocol drafting, and review

GDP: idea synthesis, protocol design, protocol drafting, and review

JW: literature review, protocol design, protocol drafting, and review

AM: idea synthesis, literature review, protocol design, protocol drafting, and review

Sources of support

Internal sources

• No sources of support provided

External sources

• No sources of support provided

Declarations of interest

MG: none

DJK: provides consultant services to Fujifilm Sonosite

GDP: none

JW: none

AM: none

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