Summary
Point of care ultrasound (POCUS) is increasingly being utilized in airway management. Presented here are some clinical utilities of POCUS, including differentiating endotracheal and esophageal intubation, identification of the cricothyroid membrane in the event a surgical airway is required, and measuring anterior neck soft tissue to predict difficult airway management.
Abstract
With its increasing popularity and accessibility, portable ultrasonography has been rapidly adapted not only to improve the perioperative care of patients, but also to address the potential benefits of employing ultrasound in airway management. The benefits of point of care ultrasound (POCUS) include its portability, the speed at which it can be utilized, and its lack of invasiveness or exposure of the patient to radiation of other imaging modalities.
Two primary indications for airway POCUS include confirmation of endotracheal intubation and identification of the cricothyroid membrane in the event a surgical airway is required. In this article, the technique of using ultrasound to confirm endotracheal intubation and the relevant anatomy is described, along with the associated ultrasonographic images. In addition, identification of the anatomy of the cricothyroid membrane and the ultrasonographic acquisition of appropriate images to perform this procedure are reviewed.
Future advances include utilizing airway POCUS to identify patient characteristics that might indicate difficult airway management. Traditional bedside clinical exams have, at best, fair predictive values. The addition of ultrasonographic airway assessment has the potential to improve this predictive accuracy. This article describes the use of POCUS for airway management, and initial evidence suggests that this has improved the diagnostic accuracy of predicting a difficult airway. Given that one of the limitations of airway POCUS is that it requires a skilled sonographer, and image analysis can be operator dependent, this paper will provide recommendations to standardize the technical aspects of airway ultrasonography and promote further research utilizing sonography in airway management. The goal of this protocol is to educate researchers and medical health professionals and to advance the research in the field of airway POCUS.
Introduction
Portable ultrasonography has evident utility in the perioperative care of patients. Its accessibility and lack of invasiveness are benefits that have led to the rapid incorporation of point of care ultrasound (POCUS) to the clinical care of surgical patients1,2. As POCUS continues to find new indications in the perioperative arena, there are several established indications that have clear benefits over traditional clinical exams. In this methods paper, we review the recent findings and demonstrate how to integrate POCUS into clinical practice or airway management.
Undetected esophageal intubation results in significant morbidity and mortality; therefore, it is critical to identify esophageal intubation immediately and place the tube in an endotracheal location to avoid disastrous respiratory compromise. Traditional confirmation of endotracheal intubation relies on clinical examinations such as auscultation for bilateral breath sounds and chest rise3,4. Even after the American Society of Anesthesiologists (ASA) instituted end-tidal CO2 as a required monitor for identifying endotracheal intubation, there still remained cases of undetected esophageal intubation leading to significant morbidity and mortality5. One main benefit of incorporating tracheal ultrasonography into the intubation procedure is that esophageal intubation can be recognized immediately, and real-time, direct visualization of the tube can be confirmed in the trachea. In a recent meta-analysis, the pooled sensitivity and specificity of endotracheal confirmation were 98% and 94%, respectively, illustrating the superior diagnostic accuracy of this technique6. In this methods paper, a video example will be shown of the tube being placed in the esophagus erroneously, immediate recognition of this complication, and proper placement of the tube in the trachea. This highlights the real-time visual benefits that POCUS allows during an intubation procedure.
Despite advances in supraglottic airways and video laryngoscopy, surgical airway may remain a life-saving necessity in a "cannot intubate, cannot oxygenate" scenario. The updated ASA Difficult Airway Guidelines highlight that in the event of a life-saving invasive airway being required, the procedure must be performed as quickly as possible and by a trained specialist7. In the event a cricothyrotomy is required, the identification of proper anatomy is required to prevent further complications. Utilization of ultrasonography to visualize the anatomy of the cricothyroid membrane (CTM) is a quick and effective technique that is now being suggested preoperatively if there is any concern of a difficult airway8. This technique can be taught in a relatively quick manner, with learners gaining almost complete competency after a brief 2 hour tutorial and 20 expert guided scans9. In this methods paper, two techniques to identify the CTM with POCUS will be demonstrated in the hopes of further educating any healthcare providers who routinely perform airway management.
Preoperative assessment of the patient's airway involves traditional bedside clinical exams (e.g., Mallampati score, mouth opening, cervical range of motion, etc.). There are several problems with these assessments. The first and probably most salient is that they are not very accurate at predicting a difficult airway situation10. In addition, these tests require patient participation, which is not possible in all clinical scenarios (such as in cases of trauma or altered mental status).
Preoperative airway ultrasound measurements have shown improved accuracy in predicting difficult endotracheal tube placement11,12. Anterior neck soft tissue thickness at varying levels has been measured and analyzed as a prediction of difficult intubation. The ultrasonographic measurement of the distance between the skin to epiglottis appears to have the best diagnostic accuracy identified to date13. This measurement has also been shown to improve predictive capability considerably when added to the traditional bedside examinations14. This paper explains how to use POCUS to measure the skin-to-epiglottis distance and incorporate it into the preoperative airway examination, in order to help healthcare providers better predict a difficult airway situation.
In addition, investigators have begun to identify anatomical structures that indicate difficult mask ventilation. One such anatomical structure is the lateral pharyngeal wall, whose thickness (LPWT) has been shown to correspond to the severity of obstructive sleep apnea (OSA) and apnea-hypopnea index15. Preliminary data also suggest that measurement of the LPWT preoperatively provides evidence for the difficulty of mask ventilation16. This methods paper and the associated video will demonstrate how to acquire the LPWT with portable ultrasonography to assess the severity of OSA in a patient and potential for difficulty in mask ventilation.
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Protocol
These studies were approved by the George Washington University Institutional Review Board (IRB # NCR203147). The study subject for all procedures described below (and pictured in figures) was a 32-year-old male who gave full informed consent to the study and publication of de-identified images. Inclusion criteria include any patient undergoing airway management or anesthetic care (especially those who have characteristics of a difficult airway) and exclusion criteria would include any patient who does not consent to this procedure.
1. Differentiating esophageal from endotracheal intubation
- Prior to the induction of general anesthesia, prepare a high frequency, linear ultrasound probe (see Table of Materials) by placing a single layer of ultrasound gel (see Table of Materials) to the probe transducer. Select the linear probe from the transducer menu on the touchscreen and specify MSK (musculoskeletal) from the dropdown menu. Place the ultrasound in scanning mode by pushing the 2D button on the bottom left corner of the touchscreen. Induce general anesthesia as recommended by the attending anesthesiologist.
- Following the induction of general anesthesia,place the probe in the transverse position on the midline of the patient's anterior neck just cephalad to the suprasternal notch (Figure 1A). Ensure the probe marker is on the left of the screen on the ultrasound instrument (see Table of Materials).
- Identify the trachea midline and note the constricted esophagus just lateral to the trachea (Figure 1B). For further anatomic confirmation, scan laterally to identify the carotid artery and internal jugular vein if necessary.
- Check for obvious tracheal and surrounding tissue movement associated with intubation as the endotracheal tube moves into the trachea. In the event that tracheal movement is not observed, slightly twist the endotracheal tube to attempt to generate movement on the ultrasound image.
- Additionally, check that the hyperechoic, posterior aspect of the trachea disappears due to the endotracheal tube, leaving a characteristic acoustic shadowing that is bullet shaped (this is called the "bullet sign", shown in Figure 2). If, instead, there is an esophageal intubation, there will be obvious tissue movement to the left of the trachea, and there will now be two lumens. This is called "double track sign," and there will be two air/mucosal interfaces (Figure 3).
NOTE: Use this ultrasound technique in real-time intubations to obtain immediate feedback as to whether the tube is being placed in the trachea or the esophagus. In addition, consider using this technique during emergency airway management, where end tidal carbon dioxide confirmation may not be reliable due to poor pulmonary blood flow17.
- Additionally, check that the hyperechoic, posterior aspect of the trachea disappears due to the endotracheal tube, leaving a characteristic acoustic shadowing that is bullet shaped (this is called the "bullet sign", shown in Figure 2). If, instead, there is an esophageal intubation, there will be obvious tissue movement to the left of the trachea, and there will now be two lumens. This is called "double track sign," and there will be two air/mucosal interfaces (Figure 3).
2. Identifying the cricothyroid membrane in preparation for a cricothyrotomy
NOTE: For emergency airway management, a cricothyrotomy might be a necessary step if the provider encounters a "cannot intubate, cannot oxygenate" scenario. In the event a difficult airway situation is suspected, the provider may opt to identify the CTM prior to the induction of anesthesia, in case it might be required to perform a cricothyrotomy.
- Perform CTM identification with the patient lying in the supine position and the neck extended. Prepare the ultrasound probe as described in step 1.1. As the CTM is shallow in the neck, place the probe to a depth of approximately 1.5-2 cm based on an average-sized patient.
NOTE: There are two methods for utilizing ultrasound to locate the CTM. - Perform the first method to locate the CTM as described below.
- Place a linear, high frequency probe in the sagittal plane of the patient's neck just caudal to the thyroid cartilage (Figure 4A). The thyroid cartilage appears as the superficial, hypoechoic structure at the cranial side of the scan and casts an acoustic shadow (Figure 4B).
- Next, locate the cricoid cartilage, which is in a caudal location and appears hypoechoic. Identify the CTM lying between these two structures using the underlying air-mucosal interface, which appears as a hyperechoic line that runs the length of the trachea.
- For further confirmation, scan caudal to locate the tracheal rings, which will appear as a hyperechoic "string of beads"18.
NOTE: The second technique for identifying the CTM (step 2.5 to step 2.8) involves using a transverse scanning orientation on the anterior neck. This technique is sometimes referred to as the thyroid-airline-cricoid-airline (TACA) approach19.
- Perform the second technique to locate the CTM as described below.
- Begin by placing a linear high frequency probe in the transverse plane at the level of the thyroid cartilage, which appears as hyperechoic and casts an acoustic shadow-a black triangle with the tip being most superficial (Figure 5).
- Scan in a caudal direction until the black triangle disappears as the thyroid cartilage ends and the CTM begins. Identify this as the air-mucosal interface that appears as a bright white line with reverberation effects (Figure 5).
- Continue scanning in a caudal direction until the CTM ends and the cricoid cartilage appears. The cricoid cartilage will appear as a hypoechoic band surrounding the trachea (Figure 5). Once the cricoid is identified, the sonographer will have located the inferior border of the CTM.
- To ensure that the proper anatomy has been identified, reverse these steps and scan in a cephalad direction, again identifying the CTM and the thyroid cartilage. Once these landmarks have been identified, mark the CTM location on the patient. Once the CTM has been marked, proceed to the induction of anesthesia and airway management as planned, knowing the CTM is properly identified in the rare event a surgical airway is required.
3. Acquisition of parameters for the prediction of difficult airway management
NOTE: For the prediction of difficult airway management, the skin to epiglottis distance and LPWT are measured. These steps should be performed prior to the induction of anesthesia.
- To measure the skin to epiglottis distance, place the patient in the supine position with the neck in a neutral position and prepare the probe and ultrasound as described in step 1.1.
- Place a high frequency, linear probe in the transverse position on the anterior neck at the level of the thyrohyoid membrane (Figure 6A).
- Identify the epiglottis, which appears as the hypoechoic structure midway between the hyoid bone and thyroid cartilage (Figure 6B). The laryngeal surface of the epiglottis forms a hyperechoic line, which represents the air-mucosal interface. Tilt the probe in either direction if the anterior border of the epiglottis is not clearly defined.
- Note an echogenic (fat-filled) pre-epiglottic space20.
- To measure the skin to epiglottis distance, freeze the image by touching the large Freeze button at the bottom of the touchscreen. Next, select the blue Distance button on the right side of the screen. Use a finger to drag one cursor to the superficial surface of the epiglottis, and move the other cursor to the anterior surface of the neck (skin). The skin to epiglottis distance will be displayed in the gray box on the upper left side of the screen.
NOTE: Based on this measurement, it is possible to predict difficult intubation. A skin to epiglottis distance greater than 2.7 cm indicates that a Cormacke-Lehane score of 3 or 4 may be encountered on direct laryngoscopy21.
- To measure the LPWT, place the patient in the supine position with the neck in neutral orientation.
- Place a curvilinear, low-frequency probe in the coronal orientation below the mastoid process and in-line with the carotid artery (Figure 7A).
- Use doppler flow to identify the carotid artery. To accomplish this, press the C button on the bottom left of the screen. Using a finger on the touch screen, move the yellow box over the carotid vasculature. Identify the carotid artery by noting the pulsatile vascular flow.
- To measure the LPWT, freeze the image (Figure 7B) by pushing the Freeze button on the bottom of the screen. Then press the blue Distance button on the right side of the screen. Place one cursor on the inferior border of the carotid artery and the second cursor on the anterior aspect of the airway. The LPWT will then be displayed in the gray box on the upper left side of the screen.
NOTE: In the event of an emergency airway scenario requiring rapid sequence induction, step 3.2 may be skipped, as mask ventilation is not likely to be necessary, and in the interest of time.
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Representative Results
By utilizing real-time ultrasound probe visualization of the trachea, the directions in step 1 of the protocol enable the airway manager to secure the airway expeditiously and safely. The endotracheal tube is quickly recognized and removed from the esophagus by following the steps for placement in the proper endotracheal position under ultrasound visualization (Figure 1, Figure 2, and Figure 3). The advantage of this technique is seeing the placement of the endotracheal tube in the trachea in real time using ultrasound.
Prior to endotracheal tube placement using ultrasound, the CTM can be marked using the directions in step 2 by visualizing the thyroid and cricoid cartilages directly and locating the CTM in longitudinal and cross-sectional views (Figure 4 and Figure 5), so that time is not wasted locating the CTM should it become necessary to create a surgical airway.
The subject in the above-described protocol had a skin to epiglottis distance measurement of 1.9 cm (Figure 6) and LPWT measurement of 2.3 cm (Figure 7). These measurements are not consistent with characteristics of values that seem to predict difficult airway management13, and therefore the induction of anesthesia could occur without further airway management planning and advanced airway equipment. Furthermore, it is unlikely that this patient will have any symptoms of OSA given these measurements (Figure 8).
Figure 1: Ultrasonography of suprasternal trachea and esophagus. (A) As the provider is preparing to intubate the patient, place a linear probe in a transverse orientation on the midline just above the suprasternal notch. (B) The resultant image will reveal the hypoechoic trachea (Tr) with the collapsed esophagus (Eso) just lateral to the trachea. Please click here to view a larger version of this figure.
Figure 2: Confirmation of endotracheal intubation. When the endotracheal tube is properly placed in the trachea, an acoustic shadow is cast from the endotracheal tube and covers the posterior aspect of the trachea. The acoustic shadow resembles the shape of a bullet and therefore is referred to as the "bullet sign". Note that the esophagus (Eso) is in its collapsed state without the endotracheal tube. Please click here to view a larger version of this figure.
Figure 3: "Double tract" sign. The "double tract" sign is an indication of esophageal intubation. The esophagus appears dilated with the tube (small circle) and the trachea appears normal with a notable posterior wall (large circle). Please click here to view a larger version of this figure.
Figure 4: Sagittal scan to identify the cricothyroid membrane (CTM). (A) Place the high frequency probe in a sagittal plane. (B) The thyroid cartilage (blue shading) appears as the hypoechoic structure at the cranial side of the scan and casts an acoustic shadow. The cricoid cartilage (red shading) is the next caudal hypoechoic structure, and the cricothyroid membrane (CTM) lies between the two. The CTM is just superior to the linear hyperechoic air-mucosal interface (AMI). The small, hypoechoic structure caudal to the cricoid cartilage is the first tracheal ring (green shading). Please click here to view a larger version of this figure.
Figure 5: Transverse scan to identify the CTM. This procedure involves scanning in multiple directions (top left). Initially use a linear probe to identify the thyroid (T) cartilage (top right). It appears as a hyperechoic triangle (arrows) and casts a hypoechoic shadow (red triangle). Scan in a caudal direction until the CTM (three arrows) appears as a hyperechoic AMI (A) with reverberations (bottom left). Continue scanning in a caudal direction until the CTM ends and the cricoid cartilage (C; red horseshoe) appears (bottom right). This is referred to as the TACA method19. Please click here to view a larger version of this figure.
Figure 6: Anterior neck scan for skin-to-epiglottis distance. (A) Place a linear probe in a transverse direction at the level of the thyrohyoid ligament. (B) Identify the epiglottis (Epi) as an oblong, hypoechoic structure. Identify the echogenic, pre-epiglottic space (PES) and the air-mucosal interface just deep to the epiglottis. Please click here to view a larger version of this figure.
Figure 7: Coronal scan to measure the lateral pharyngeal wall thickness (LPWT). (A) Place the patient supine with the neck in a neutral position. Lay a curvilinear probe in a coronal orientation on the lateral neck as shown. (B) Measure the LPWT (white line) from the inferior border of the carotid artery (green box) to the anterior aspect of the airway (arrows). Add doppler flow to confirm the carotid artery. Please click here to view a larger version of this figure.
Figure 8: Lateral pharyngeal wall thickness and obstructive sleep apnea (OSA). The LPWT has been correlated with severity of OSA and AHI. This figure has been modified from Bilici et al.22 with permission. Please click here to view a larger version of this figure.
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Discussion
In 2018, a call to action was made by the leadership of the Society of Cardiovascular Anesthesiologists for "Perioperative ultrasound training in anesthesiology"23. Notably, these leaders highlighted that POCUS education should become an essential component of anesthesiology training programs. More recently, experts in anesthesiology further explained the utility and necessity of POCUS in all aspects of perioperative patient care, including airway management24. Experts emphasize that the leaders of the anesthesiology community must champion the education of POCUS and support its incorporation into more regular practice through guidelines and a specific credentialing process. This article and instructional video aim to be a part of those directives in educating anesthesiologists and trainees while promoting future research in the field of airway ultrasound.
Utilization of POCUS to confirm endotracheal intubation has been established as an effective and accurate technique11 and is particularly helpful in unique clinical situations such as the trauma bay and medical emergencies on the wards25,26. Using ultrasound for confirmation is specifically important in patients with little to no pulmonary blood flow, as most other techniques rely on the identification of carbon dioxide in the exhaled breath17.Therefore, this procedure is reliable and preferred for patients in cardiac arrest27. This procedure is limited by the requirement for two individuals skilled in airway management and ultrasonography28. With increasing awareness of airway POCUS and the incorporation into airway management training, it is likely that providers will have the skillset to be proficient in this technique as a part of standard-of-care practice.
Ultrasound identification of the CTM has been conclusively proven to be quicker and more accurate than the traditional palpation technique29. This technique is particularly helpful in patients who are obese19, have a neck pathology30, or are pregnant31. Current recommendations suggest that the CTM should be identified using ultrasound (should time allow) prior to the initiation of airway management if a difficult airway is anticipated8.
Nevertheless, despite its higher effectiveness than the palpation technique, ultrasonographic identification of the CTM is dependent on the availability of the ultrasound equipment. In addition, these studies do not account for the time of transfer of the equipment to the operating room32. Likewise, although a practitioner can be taught to identify the CTM in a relatively short amount of time, this does not guarantee success of the procedure, and therefore should only be performed by an experienced clinician33. Therefore, critical steps for this protocol include having a readily available ultrasound and a practitioner competent and skilled in this technique.
Although it is recommended that the patient be supine when using ultrasound to identify the CTM, this is not essential. The CTM can be identified with the head elevated; however, it is crucial that the patient position is the same between when the CTM was marked and when the surgical airway is performed, as the anatomy can change when the patient's head is raised and lowered34. The CTM is very small and moves in a cephalad direction as the head of the bed is raised from a neutral position; therefore, it is critical that the patient be in the same position if the cricothyroidotomy is performed in order to prevent procedural complications34.
Although bedside clinical examinations have been long used to judge the potential difficulty of airway management, POCUS assessment of the airway has better predictive accuracy and even more superior accuracy when used in combination with traditional airway exams11. The requirement of a skilled sonographer to accurately acquire images and interpret the findings is a current limitation to the use of POCUS for airway management. The critical step in this procedure, if time allows, is to perform this procedure prior to administering any anesthetic agent that may affect the airway or decrease the patient's ventilatory drive35. Ultimately, predicting difficult airway management is a screening tool that may not be possible in settings where time and resources are limited36.
Several recent meta-analyses have concluded that the skin to epiglottis measurement consistently has strong diagnostic accuracy for predicting difficult intubation, as defined by a Cormacke-Lehane score of 3 or greater13,37. However, the studies included in these meta-analyses have high levels of heterogeneity and therefore have not verified that the skin to epiglottis measurement can be definitively used to diagnosis a difficult airway preoperatively. This measurement does have a high negative predictive value (95%-98%); therefore, if this measurement is below the cut-off value of 2.0-2.5 cm, the intubation likely will not be difficult13. Therefore, a measurement greater than 2.0-2.5 cm should be treated as a potential difficult airway, and airway management should be planned accordingly.
Ultrasonographic measurement of the LPWT has good inter-operator reliability, and is highly reproducible. Multiple studies have shown that the thickness of the LPW (as measured by ultrasound or MRI) correlates with the severity of OSA15,38,39. One such study used ultrasonographic measurements of the LPW and showed that LPWT correlated with the severity of OSA based on apnea-hypopnea index as measured by sleep polysomnography (Figure 8)22. An LPWT > 3.5 cm indicates that the patient will probably require more than one provider to mask ventilate or not be able to ventilate at all16. In this case, more sophisticated airway management, including awake fiberoptic intubation, which maintains spontaneous ventilation, may be necessary.
One aim of this paper is to further educate those healthcare providers who regularly provide such care in hopes that it can be an additional skill to implement into their practice. Furthermore, although the data is promising, there have yet to be large, multicenter studies that would lead experts to recommend incorporating airway POCUS into routine daily practice.
As the availability of portable ultrasonography continues to increase, prospects for further innovation and incorporation of POCUS into airway management are promising. The portability, speed, and lack of invasiveness, all benefits of POCUS, will likely further enhance advances and patient safety during routine and emergent airway management.
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Disclosures
None of the authors have any conflicts of interest to disclose.
Acknowledgments
None. No funding was received for this project.
Materials
| Name | Company | Catalog Number | Comments |
| High Frequency Ultrasound Probe (HFL38xp) | SonoSite (FujiFilm) | P16038 | |
| Low Frequency Ultrasound Probe (C35xp) | SonoSite (FujiFilm) | P19617 | |
| SonoSite X-porte Ultrasound | SonoSite (FujiFilm) | P19220 | |
| Ultrasound Gel | AquaSonic | PLI 01-08 |
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