a b s t r a c t

Background: Prior to the anticipated difficult airway, the emergency physician commonly palpates the neck to identify the presumed location of the Cricothyroid membrane (CTM). In the event of a “cannot-intubate, cannot-oxygenate” airway, precise CTM localization is vital to the success of a cricothyrotomy and hence, the patient’s survival.

Objective: This narrative review will summarize the prospective studies evaluating CTM identification. It will demonstrate the inaccuracy of the classically used landmark palpation technique. It will then describe the use of ultrasound (US) as a superior tool for CTM identification, illustrate the technique itself, and propose its imple- mentation in the pre-intubation checklist for the anticipated difficult airway.

Discussion: Evidence demonstrates that physicians are not sufficiently accurate in palpating the CTM in both ca- davers and volunteers in a stable non-emergent setting. In preparing for a real-time intubation, this fine motor task would be more difficult to achieve. Moreover, this particular patient group may often exhibit difficult airway features. US has been demonstrated to be superior for locating the CTM than landmark palpation across body habitus, gender, and failed airway simulations. This technique carries a short learning curve even without prior airway US experience, along with a high retention rate. An illustrative tutorial is provided.

Conclusion: Emergency physicians may apply this US technique for CTM localization in preparation for an antic- ipated difficult airway. However, since this technique takes longer than landmark palpation, using US is not rec- ommended once already in the midst of a failed airway situation.

(C) 2018

Introduction

Prior to an intubation, the emergency medicine mindset consists of preparing for a “cannot-intubate, cannot oxygenate” airway emergency. Many emergency departments (ED) incorporate a pre-intubation checklist that, among other things, calls for identification of the patient’s cricothyroid membrane (CTM) in case of need for cricothyrotomy. This is classically performed using the landmark palpation technique, in which the skin overlying the CTM is localized between the cricoid and thyroid cartilages along the neck’s midline. Misidentification of the CTM is the leading cause of airway device misplacement [1-3], which may lead to hypoxia, injuries to neck structures, and even death.

The purpose of this narrative review is to summarize the findings of prospective studies evaluating CTM identification and consider their applicability to emergency medicine. The review introduces the use of ultrasound (US) as a safer and more accurate method of identifying the CTM compared to the landmark palpation technique, especially in

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those patients expected to have a difficult airway. It will also illustrate a step-by-step pictorial depiction of how to perform this technique.

Methods

This is a narrative review evaluating the landmark palpation and Point-of-care ultrasound techniques for identifying the CTM, with a discussion of their implementation prior to the anticipated diffi- cult airway. A literature review of the PubMed and Google Scholar data- bases was performed with search date from 1980 to December 2017. Search terms included “cricothyroid membrane + ultrasound OR point-of-care ultrasound”, “airway + ultrasound OR point-of-care ultra- sound”, and “cricothyrotomy + ultrasound OR point-of-care ultra- sound.” The author included studies assessing the accuracy of the landmark and US techniques for CTM identification, along with protocol description of the latter. Case reports, case controls, cohort studies, ran- domized clinical trials, and reviews were included, as determined by relevance to the narrative review. Commentaries and letters were excluded.

https://doi.org/10.1016/j.ajem.2018.07.027

0735-6757/(C) 2018

Discussion

Landmark palpation technique is inaccurate for identifying the CTM

A prospective observational study by Bair et al. evaluated emergency physician volunteers using three different landmark-based techniques for localizing the CTM on a convenience sample of adult patients: 1) pal- pation of either the thyroid cartilage or thyroid notch as a starting point

2) palpating four finger breadths cephalad from the sternal notch 3) de- tection of the horizontal skin crease overlying the CTM. Neither tech- nique was found to correctly palpate the CTM with sufficient accuracy. In 50 patients, 52% of which were female and with an average body mass index (BMI) of 28 kg/m², the best technique (landmark palpation) achieved a success rate of only 62% (95% CI = 48%-76%) [4].

Elliot et al. similarly assessed general palpation by 18 anesthesiolo- gists in six healthy volunteers. Only 32 (30%) of attempts to palpate the CTM were accurate [5]. Lamb et al. found that in 12 adult volunteers, half of whom were female and half obese (BMI cut-off 30 kg/m²), 61 an- esthesiologists successfully palpated the CTM in 72% (95% CI 59-85%) of non-obese men, 39% (95% CI 26-54%) in obese men, 24% (95% CI

12-36%) of non-obese women, and 35% (95% CI 21-49%) in obese women [6]. Of note, the reference standard for CTM identification in these three studies was obtained using US.

Accuracy in localizing the CTM is likely even lower under time- sensitive conditions in the ED. These patients also frequently exhibit difficult-airway anatomical features such as obesity, short neck, subcu- taneous emphysema, and prior surgeries or radiation to the neck. More- over, the cricothyrotomy is a rare procedure, and many emergency medicine residents graduate from residency without having performed one [7]. Emergency physicians may thus overestimate their ability to lo- calize the CTM.

CTM identification is more difficult in obese patients

You-Ten et al. found that digital palpation by anesthesiologists was accurate for localizing the CTM in only 39% (11/28) of obese versus 71% (20/28) of non-obese patients (p = 0.03) [8]. The distance between the reference mark and anesthesiologist skin marker sites was also sig- nificantly greater (5 [2-9.5] mm vs. 1.8 [0.1-6] mm, respectively; p = 0.02). In addition, Aslani et al. found that in the neutral neck position of female patients, the CTM was identified in 24% (10/41) of non- obese versus 0% (0/15) in obese patients (p = 0.048) [9]. Similar results were obtained in the extended neck position. Campbell et al. found that in female subjects, the vertical distance of anesthesiologists’ skin mark- ings from the CTM correlated positively with neck circumference (p = 0.005) [10]. This data correlates with difficult airway predictor tools that incorporate anterior neck thickness. Once again, US identification of the CTM was used as the reference standard for these three studies. In contrast, Elliot et al. found that delineation of a cricothyrotomy entry point was not significantly affected by subjects’ weight, height, BMI, neck circumference, or CTM dimensions [5]. However, their study sample consisted of only six patients examined sequentially, which may have led to a learning effect. Participants were also instructed to find their mark within just 10 s, so the time limit may have served as

a confounder.

CTM identification is more difficult in females

The vertical height and width of the CTM are typically greater in males [11,12]. Moreover, females typically have a less prominent thy- roid cartilage than that of males [8,13]. Campbell et al. found that incor- rect localization of the CTM was more common in females (p b 0.001). The presumed reason was the more acute external angulation of the la- ryngeal cartilage in males [10].

In contrast, Lamb et al. found that the CTM of obese men was not more easily palpated than in obese women (p = 1.0) [6]. This suggests

that depth of soft tissue between neck and CTM conveys a larger effect than any difference in thyroid cartilage angulation between the sexes.

Ultrasound can accurately localize the CTM

US provides a visual representation of the precise structures for which the physician would palpate if performing a cricothyrotomy. Additionally, Soft tissue depth from neck to CTM no longer serves as an obstacle as with the landmark technique. Instead, the thicker subcu- taneous tissue allows for greater transducer contact with skin, a broader visual field, optimization of the focal zone, and reduced transducer pres- sure with less distortion of the superficial visual landmarks [8].

Prior studies have evaluated the use of the high-frequency

(5.0-10.0 MHz) linear US transducer to delineate the CTM in different types of subjects. A randomized control trial assessed on cadavers by Siddiqui et al. randomized 47 anesthesia residents into a digital palpa- tion group and an US group. Two investigators graded the cadavers in advance based on anticipated difficulty of localizing the CTM. It was found that in cadavers with “difficult” and “impossible” landmark pal- pation, US guidance significantly increased the probability by 5.6 times of correct airway device insertion (p = 0.043) while decreasing the incidence of laryngeal and tracheal injuries (p = 0.001) [14]. This is precisely the patient population for which CTM localization prior to intubation would be most important.

Nicholls et al. found that physician perception of a difficult CTM pal- pation may not correlate with the ease of identification using US. For two emergency physicians using US on a convenience sample of 50 adult ED patients, investigators found a significant relationship between the physician perception of palpation difficulty and the patient BMI (p

= 0.009) [15]. This perception was similar in the study by Aslani et al. [9]. However, Sonographic localization was performed in a shorter amount of time in the two patients whose CTM was deemed “difficult” to palpate, likely for the reasons mentioned above.

This finding was supported by a prospective observational study by Barbe et al., in which US improved residents’ ability to locate the CTM in two overweight volunteers compared to using landmarks [16]. Kristensen et al. compared the landmark technique to US on a single morbidly obese female (BMI 45.3). After being taught a structured US technique, 13 of 35 anesthesiologists (37%; 95% CI 21-55%) were suc- cessful within 2 min using the landmark technique, versus 29 of 35 an- esthesiologists (83% (95% CI 66-93) using US (p = 0.0008) [17].

Mallin et al. evaluated the utility of US for CTM localization in simulated crash airway situations consisting of Cricoid pressure, flexion, extension, and neck rotation. The CTM landmark delineated by skin marker after the difficult airway simulation was measured as similar–in both sagittal and axial planes–to that delineated by invisible marker beforehand [12]. This finding supports the reliabil- ity and consistency of CTM localization prior to difficult airway maneuvers.

One study by Yildiz et al., in contrast, did not show an improvement with US [18]. The investigators theorized that landmark palpation suc- cess was higher than in other studies because the cricothyrotomy may be performed and simulated more often by emergency physicians in the emergency room than by anesthesiologists in the operating room. Moreover, of the 24 models, 17 of them were of normal weight. As de- scribed previously, necks with a greater depth to reach the CTM are best visualized using US.

CTM localization takes longer with US than with landmark palpation

Multiple studies show that the US technique takes longer than land- mark palpation for localizing the CTM [Table 1]. These findings do not support use of the US transducer for CTM localization once already in the midst of a failed airway crisis. Taking the time to grab the US ma- chine, waiting for it to turn on, applying the gel, and placing the trans- ducer upon the patient’s neck would add extra time and cognitive

Table 1

CTM localization takes longer with ultrasound than with landmark palpation.

burden to an already stressful situation. Randomized trials would be nearly impossible to conduct due to both the rarity of the cricothyrotomy procedure and ethical considerations [4,14,15,17,18].

Fig. 1. a) High-frequency linear transducer placed in the transverse plane over the trachea in a model of the relevant neck structures b) Corresponding ultrasound image

c) Corresponding ultrasound image with the structures labeled.

How to use ultrasound to localize the CTM [19]

As with any three-dimensional structure of the body being visualized with US, it is important to evaluate the structure in at least two orthogonal planes. For this application, the high-frequency (5.0-10.0 MHz) linear transducer offers the best resolution.

Fig. 2. a) High-frequency linear transducer placed in the transverse plane over the cricoid cartilage in a model of the relevant neck structures b) Corresponding ultrasound image

c) Corresponding ultrasound image with the structures labeled.

Transverse plane

Place the transducer transversely (with the marker toward the patient’s right) on the anterior neck just superior to the sternal notch. The trachea will appear as a dark horseshoe-shaped

Fig. 3. a) High-frequency linear transducer placed in the transverse plane over the cricothyroid membrane in a model of the relevant neck structures b) Corresponding ultrasound image c) Corresponding ultrasound image with the structures labeled.

structure with reverberation artifact extending posteriorly (Fig. 1a, b, and c).

• In sliding cephalad, the cricoid cartilage will appear as a hypoechoic

“arch-shaped” structure [Fig. 2a, b, and c].

• Slide the transducer cephalad to reach the CTM. It will appear as a hyperechoic white line representing the soft tissue-air interface of the CTM’s inner mucosal lining. Reverberation artifact will appear posterior to this white line [Fig. 3a, b, and c].

• Sliding further cephalad, the thyroid cartilage will appear as a hyperechoic triangular (or inverted “V-shaped”) structure [Fig. 4a, b, and c].
• Once all structures are identified, go back and slide caudally to the center of the CTM and mark this area using a skin marker.

  Longitudinal plane

  Place the transducer transversely (with the marker toward the patient’s right) on the anterior neck just superior to the sternal notch [Fig. 1a, b, and c].

  Fig. 4. a) High-frequency linear transducer placed in the transverse plane over the thyroid cartilage in a model of the relevant neck structures b) Corresponding ultrasound image

  c) Corresponding ultrasound image with the structures labeled.

  • Slide the transducer to the patient’s right, such that the right border of the transducer lies over the midline of the trachea [Fig. 5a, b, and c].

  • Rotate the left side of the transducer 90 degrees clockwise into the sagittal plane. The hypoechoic tracheal cartilage rings will appear like a “string of pearls” anterior to the white hyperechoic anterior tra- chea [Fig. 6a, b, and c].
  • Slide the transducer cephalad until fully visualizing the cricoid carti- lage, which appears as a hypoechoic ovoid structure anterior to the trachea. The distal aspect of the thyroid cartilage can also be seen [Fig. 7a, b, and c].
  • Safely slide a needle or paper clip perpendicularly between the trans- ducer and skin until the posterior shadow of the needle can be seen midway between the cricoid and thyroid cartilage. Use a skin marker to mark this area that is the CTM [Fig. 8a, b, and c].

    Comparison of the transverse and longitudinal US orientations

    The transverse plane is more advantageous in patients who have short or flexed necks where longitudinal space is limited. This may often be the case for patients requiring intubation in the ED. Alterna- tively, the longitudinal plane provides more anatomical information. This may be useful for tracheal access such as in small children or in the case of subglottic obstruction.

    In a prospective observational study examining morbidly obese fe- males (BMI 39.0-43.9 kg/m²), Kristensen et al. demonstrated a shorter mean time to CTM identification by anesthesiologists when using the transverse versus the longitudinal technique (24.0 +- 12.4 s versus 37.6 +- 17.9 s; p = 0.0003) [20]. The emergency physician should learn to visualize the CTM in both planes in case anatomy precludes enough space for the longitudinal orientation.

    Fig. 5. a) High-frequency linear transducer placed in the transverse plane over the right half of the trachea in a model of the relevant neck structures b) Corresponding ultrasound image c) Corresponding ultrasound image with the structures labeled.

    Fig. 6. a) High-frequency linear transducer placed in the longitudinal (sagittal) plane over the trachea and cricoid cartilage in a model of the relevant neck structures

    b) Corresponding ultrasound image c) Corresponding ultrasound image with the structures labeled.

    Fig. 7. a) High-frequency linear transducer placed in the longitudinal (sagittal) plane over the cricothyroid membrane in a model of the relevant neck structures b) Corresponding ultrasound image c) Corresponding ultrasound image with the structures labeled.

    Feasibility of learning the US localization technique

    In a prospective non-randomized cohort study by Oliveira et al., six anesthesia trainees (four residents and two fellows) without prior air- way US experience underwent a 2-h training session on neck US. They subsequently performed consecutive US on 20 volunteers with a mean BMI of 25 +- 4 kg/m². The overall success rate was 88.3% with a mean time to localization of 36.9 +- 9.0 s [21]. In assessing 5 consecutive necks three months later, mean success was 86.7%, while the mean time was 47.7 +- 16.0 s. These results suggest that the US technique for identifying the CTM can be learned fairly easily, will not take long to perform in preparing for an intubation, and will be retained over time.

    Conclusion

    Evidence demonstrates that physicians are not sufficiently accurate in using landmark palpation to locate the CTM. Accuracy is likely wors- ened in the time-sensitive ED setting where difficult airway features are also more common. Evidence also reveals the superiority of US com- pared to landmark palpation, with both a short learning curve and high long-term retention. Thus, emergency physicians may incorporate

    Fig. 8. a) High-frequency linear transducer placed in the longitudinal (sagittal) plane over the cricothyroid membrane in a model of the relevant neck structures, with the dominant hand lying a blunt needle (or paper clip) transversely over the skin beneath the transducer

    b) Corresponding ultrasound image c) Corresponding ultrasound image with the structures labeled.

    this technique into their pre-intubation checklist for the anticipated dif- ficult airway in which cricothyrotomy may be needed. Due to the longer duration this may take, however, using US for this purpose is not recom- mended once already in the midst of a failed airway situation.

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