In patients undergoing emergent tracheal intubation, there is currently no universally accepted gold-standard test to confirm the location of the endotracheal tube (ETT).1 End-tidal carbon dioxide (CO2) detection is the best of the tests that are routinely utilized to confirm ETT placement, however, it has been shown to have an error rate as high as 1/10 for proper determination of ETT location in emergency intubations.2 As a result, multiple modalities are necessary to confirm ETT location, which can delay mechanical ventilation and other treatments. The lack of a single, reliable test to confirm ETT placement can potentially lead to confusion regarding the location of the tube. This confusion can result in both unrecognized esophageal intubations (“false positive”), as well as successful tracheal intubations that are subsequently removed (“false negative”), subjecting the patient to further unnecessary attempts at airway management. Both scenarios can lead to disastrous consequences.
The advent of point-of-care ultrasound (POCUS) has led to a potential solution to this problem. Over the past decade, emergency physicians, anesthesiologists, and others have studied the ability of POCUS to determine ETT location either during or immediately following laryngoscopy. Multiple approaches have been utilized including transtracheal at the suprasternal notch (Figure 1), transcricoid, and also assessments of lung sliding at the visceral-parietal pleural interface and the diaphragms.
Figure 1. Transverse placement of the high-frequency linear transducer over the trachea just cephalad to the suprasternal notch. Image courtesy of Werner et al.3
Recently, in the Canadian Journal of Anesthesia, Das et al published a systematic review and meta-analysis to definitively answer the question about transtracheal ultrasound’s accuracy in confirming ETT placement.4
Inclusion criteria
- Studies evaluating the diagnostic accuracy of transtracheal ultrasound in confirming ETT location AND
- Results of the index test (POCUS) verified with that of a gold standard (capnography) AND
- Studies that enrolled living adult humans
Findings
Eleven studies encompassing 969 intubations were analyzed. The pooled sensitivity and specificity for the detection of proper ETT placement with US were:
- Sensitivity: 98% (95% C.I. 97-99%)
- Specificity: 98% (95% C.I. 95-99%)
- Positive Predictive Value: 99.5%
- Negative Predictive Value: 93.8%
- + Likelihood Ratio: 46
- – Likelihood Ratio: 0.0157
Of these 11 studies, 3 examined elective intubations and a sensitivity analysis was performed to exclude these trials. This resulted in an aggregate sensitivity and specificity of POCUS in emergency intubations of:
- Sensitivity: 98% (95% C.I. 97-99%)
- Specificity: 94% (95% C.I. 86-98%)
As with all meta-analyses, the robustness and applicability of the results are highly dependent on the quality of the included trials. Eight of the included trials were judged to have a low risk of bias (using the QUADAS-2 tool5) and the aggregate sensitivity and specificity was:
- Sensitivity: 98% (95% C.I. 97-99%)
- Specificity: 98% (95% C.I. 95-100%)
Fine Print
The heterogeneity between the included studies was determined using the inconsistency index (I2) and found to be in the mild-moderate range. However, further inspection into the variability between studies showed 9 different confirmatory findings on POCUS that were used to determine ETT location, including:
- One air-mucosal interface = tracheal intubation.
- Two air-mucosal interfaces = esophageal intubation.
- Two hyperechoic reverberation artifacts inside trachea = tracheal intubation.
- Two hyperechoic reverberation artifacts inside esophagus = esophageal intubation.
- “Bullet sign”= tracheal intubation (Figure 2)
- “Double track sign” = esophageal intubation (Figure 3).
- “Snowstorm sign” = tracheal intubation6
- Dynamic opening of the esophagus by the ETT seen on US performed during laryngoscopy = esophageal intubation. (Video 2)
- Brief flutter deep to the thyroid cartilage = tracheal intubation
Proof is in the pudding
Figure 2. Success! Tracheal intubation visualized on ultrasound (bullet sign). Reverberations are seen in the anterior portion of the lumen of the trachea, just posterior (or deep) to the anterior wall.
Video 1. Dynamic (real-time, concurrent with laryngoscopy and ETT placement) video showing successful tracheal placement of ETT.
Figure 3. The “Double Tract” sign indicating esophageal intubation.
Video 2. Dynamic video showing placement of ETT into the esophagus (located in left paratracheal location in approx. 70% humans).
Transducer Placement Location
While many of these sonographic findings are similar (and some are exactly the same), there is no consistent sonographic finding that is widely accepted amongst investigators. Another source of heterogeneity is in the transducer type and placement. The following locations were used in the included studies:
- Immediately cephalad to suprasternal notch: 8
- Cricothyroid membrane: 1
- Both suprasternal notch and cricothyroid membrane: 2
Six studies used high frequency linear probes, while 5 studies used low frequency curvilinear probes. The precise frequencies used in the various studies were:
- 3.75 MHz: 3
- 3-5 MHz: 1
- 5-10 MHz: 1
- 7-10 MHz: 2
- 9-12 MHz: 1
- 10 MHz: 2
- No frequency noted: 1
At this time, POCUS can (and should) be utilized as an adjunctive method as part of a multimodal approach to verify ETT location. It has an added benefit in cases where end-tidal CO2 detection is less reliable, such as cardiac arrest. Future studies of POCUS verification of ETT location should attempt to standardize the approach and type of equipment (transducer, settings, frequency, etc), and should utilize a technique that will not interfere with attempts at direct laryngoscopy.
References
- American College of Emergency Physicians Board of Directors. Verification of Endotracheal Tube Placement. policy statement. Revised April 2009.
- Li J. Capnography alone is imperfect for endotracheal tube placement confirmation during emergency intubation. J Emerg Med. 2001;20(3):223-229. [PubMed]
- Werner S, Smith C, Goldstein J, Jones R, Cydulka R. Pilot study to evaluate the accuracy of ultrasonography in confirming endotracheal tube placement. Ann Emerg Med. 2007;49(1):75-80. [PubMed]
- Das S, Choupoo N, Haldar R, Lahkar A. Transtracheal ultrasound for verification of endotracheal tube placement: a systematic review and meta-analysis. Can J Anaesth. 2015;62(4):413-423. [PubMed]
- Whiting P, Rutjes A, Westwood M, et al. QUADAS-2: a revised tool for the quality assessment of diagnostic accuracy studies. Ann Intern Med. 2011;155(8):529-536. [PubMed]
- Abbasi S, Farsi D, Zare M, Hajimohammadi M, Rezai M, Hafezimoghadam P. Direct ultrasound methods: a confirmatory technique for proper endotracheal intubation in the emergency department. Eur J Emerg Med. 2015;22(1):10-16. [PubMed]
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