Diagnostic Accuracy of Ultrasound for Confirmation of Endotracheal Tube Placement

Diagnostics and Likelihood Ratios, Explained

Positive Findings (Patient Has This)

Finding Increased Disease Probability (Positive Likelihood Ratio)
POCUS for ETT Placement
Confirmation of ETT Placement 34.4 (95% CI 12.7-93.1)

Negative Findings (Patient Doesn't Have This)

Finding Decreased Disease Probability (Negative Likelihood Ratio)
POCUS for ETT Placement
Confirmation of ETT Placement 0.01 (95% CI 0.01-0.02)


Endotracheal intubation is a common intervention in the emergency department (ED) and prehospital setting. Direct visualization of endotracheal tube (ETT) placement through the vocal cords is limited at times, and esophageal intubation can be dangerous if not recognized.1 Therefore, additional methods (e.g., lung auscultation, esophageal detector devices, capnography) are necessary for confirmation of tube placement. However, these methods are not always reliable.2, 3, 4 Point-of-care ultrasonography (POCUS) has increasingly been used as a potential confirmatory tool for ETT confirmation. The 2015 Advanced Cardiac Life Support guidelines state that POCUS may be a useful adjunct for ETT confirmation.5

The meta-analysis discussed here included prospective observational or randomized controlled trials evaluating transtracheal POCUS for ETT placement confirmation in patients older than 18 years.6 All studies included a confirmatory test for comparison (e.g. end-tidal capnography, colorimetric capnography, direct visualization). The primary outcome was diagnostic accuracy of transtracheal POCUS for ETT confirmation, with subgroup analyses including location, provider specialty, provider experience, transducer type, and POCUS technique.

The authors also assessed time to confirmation as a secondary outcome. The authors of the meta-analysis identified 17 studies (n = 1,595 patients) that met their inclusion criteria. Twelve studies were performed in the ED, and five studies were conducted in the operating room. Overall, POCUS was 98.7% sensitive (95% confidence interval [CI] = 97.8% to 99.2%) and 97.1% specific (95% CI = 92.4% to 99.0%), with a positive likelihood ratio (LR+) of 34.4 (95% CI = 12.7 to 93.1) and a negative likelihood ratio (LR–) of 0.01 (95% CI = 0.01 to 0.02). Area under the receiver operating characteristic curve demonstrated a high degree of accuracy (area under the curve = 0.994; 95% CI = 0.982 to 0.998). The mean time to confirmation was 13.0 seconds (95% CI = 12.0 to 14.0 seconds).6 Subgroup analyses demonstrated no statistically significant difference with respect to enrollment location, provider, training, transducer type, or technique.


Although POCUS is a valuable tool for confirmation of ETT placement, it is dependent on the individual provider’s ability to obtain and interpret appropriate images. Thus, it is important that providers receive adequate training before this technique is utilized routinely. Included studies demonstrated significant variation in POCUS training protocols. However, there was no significant difference in accuracy with respect to the training protocol. Of note, a prior study has demonstrated that the learning curve for transtracheal POCUS is relatively rapid.7 POCUS operator experience, specialty, and level of training varied in the included trials, but again, no significant difference was identified on subgroup analysis. Eight studies utilized dynamic technique (using POCUS to guide the intubation), nine used static technique (verifying tube placement after intubation), and one used both. However, there was no statistically significant difference in the diagnostic accuracy between techniques. This was supported by another recent study directly comparing the two techniques, which also demonstrated similar accuracy.8 In addition, there is also significant variability in the POCUS visual findings for guiding or verifying intubation. These findings include the number of air–mucosa interfaces, “snowstorm” flutter in the trachea, and a change in shape of the cricothyroid area, known as the “bullet” sign. Most of the included studies utilized the presence of a single air–mucosa interface to confirm trachea placement and two air–mucosa interfaces to identify esophageal intubation, known as the “double tract” sign. Three studies used the “snowstorm” sign to confirm trachea intubation, which refers to the brief flutter within the trachea as the tube is passed. Further studies are needed to determine the accuracy of these individual findings.

Transducer type varied between studies, but there was no statistically significant difference in accuracy present on subgroup analysis. A recent study comparing transducer types did not demonstrate a difference in accuracy but noted that providers preferred the linear transducer.9 Of note, most studies did not describe the size of the ETT used, although recent literature has suggested that accuracy remains consistent regardless of ETT size.10

The overall statistical heterogeneity was low. The overall risk of bias was also low, but the risk of bias for index testing, patient selection, and flow and timing was unclear, a limitation most likely due to inclusion of observational studies in the meta-analysis. This issue together with presence of publication bias warrants caution in interpreting the findings.

Further studies are needed to directly evaluate whether static versus dynamic technique, linear versus curvilinear transducer, and the selected POCUS finding impact diagnostic accuracy of POCUS for ETT confirmation.

Based on the existing evidence, POCUS appears to be highly sensitive and specific for guiding and verifying ETT placement. POCUS is easily available, rapid, noninvasive, and does not depend on ventilation for confirmation. Therefore, we have assigned a color recommendation of green (benefit > harm) to this technique.

The original manuscript was published in Academic Emergency Medicine as part of the partnership between TheNNT.com and AEM.


Brit Long, MD; Alex Koyfman, MD; Michael Gottlieb, MD, RDMS
Supervising Editor: Shahriar Zehtabchi, MD


June 14, 2019

What are Likelihood Ratios?

LR, pretest probability and posttest (or posterior) probability are daunting terms that describe simple concepts that we all intuitively understand.

Let's start with pretest probability: that's just a fancy term for my initial impression, before we perform whatever test it is that we're going to use.

For example, a patient with prior stents comes in sweating and clutching his chest in agony, I have a pretty high suspicion that he's having an MI – let's say, 60%. That is my pretest probability.

He immediately gets an ECG (known here as the "test") showing an obvious STEMI.

Now, I know there are some STEMI mimics, so I'm not quite 100%, but based on my experience I'm 99.5% sure that he's having an MI right now. This is my posttest probability - the new impression I have that the patient has the disease after we did our test.

And likelihood ration? That's just the name for the statistical tool that converted the pretest probability to the posttest probability - it's just a mathematical description of the strength of that test.

Using an online calculator, that means the LR+ that got me from 60% to 99.5% is 145, which is about as high an LR you can get (and the actual LR for an emergency physician who thinks an ECG shows an obvious STEMI).

(Thank you to Seth Trueger, MD for this explanation!)