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.
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.
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.
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
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.
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.
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.
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.
The authors have nothing to disclose.
None. No funding was received for this project.
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 |