Image Acquisition using Portable Sonography for Emergency Airway Management

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 patients^1,2. 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 rise^3,4. 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 placement^11,12. 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.

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

1. 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.
2. 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).
3. 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.
4. 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.
   1. 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.

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.

1. 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.
2. Perform the first method to locate the CTM as described below.
   1. 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).
   2. 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.
   3. 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.
3. Perform the second technique to locate the CTM as described below.
   1. 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).
   2. 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).
   3. 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.
   4. 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.

1. 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.
   1. Place a high frequency, linear probe in the transverse position on the anterior neck at the level of the thyrohyoid membrane (Figure 6A).
   2. 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.
   3. Note an echogenic (fat-filled) pre-epiglottic space²⁰.
   4. 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.
2. To measure the LPWT, place the patient in the supine position with the neck in neutral orientation.
   1. Place a curvilinear, low-frequency probe in the coronal orientation below the mastoid process and in-line with the carotid artery (Figure 7A).
   2. 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.
   3. 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.

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 thi...

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 emph...

Materials

List of materials used in this article

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