Applied Airway Ultrasound

Bhupinder Natt

Adrienne Yarnish

INTRODUCTION

Ultrasonography offers an alternative to endoscopic evaluation to assess airway anatomy prior to intubation in patients with suspected upper airway pathology or obstruction. In this chapter, we describe the techniques for image acquisition and interpretation of upper airway ultrasound. Specifically, we will describe ultrasonographic identification of the upper airway anatomy, identification of the cricothyroid membrane (CTM) for an emergent cricothyrotomy, and evaluation of laryngeal edema in preparation for extubation.

UPPER AIRWAY ANATOMY

Image Acquisition and Structure Identification

The patient can be seated upright, or semirecumbent, but the ideal position is supine with an extended neck with the operator positioned on either side facing the patient and the screen. A high-frequency linear probe (5.0 to 12.0 MHz) is best utilized for this exam since it gives better resolution for the relatively shallow soft tissue structures. The transverse and sagittal views are both used to identify the relevant structures.

Scanning typically progresses in the cranial to caudal direction. First, the probe is placed in the transverse orientation in the submandibular space and tilted upward, so that the base of the tongue and its musculature can be identified. The tongue, owing to its air-mucosal interface appears as a curvilinear hyperechoic structure. The mylohyoid, geniohyoid, paired hyoglossus, and genioglossus muscles are arranged in a proximal to distal manner relative to the probe in this orientation (Fig. 10.1).

Figure 10.1: Probe position and the view of the base of the tongue (red arrows).

The hyoid bone becomes visible caudally as a thin hyperechoic inverted U-shaped linear structure with acoustic shadowing (Fig. 10.2).

Figure 10.2: Probe position and the view of the hyoid bone (red arrows).

Further down, the epiglottis is a hypoechoic structure in the far field. This view has been likened to a “sloth face” with the mouth of the sloth being the epiglottis (Fig. 10.3). Its posterior border will appear bright due to the air interface. Tongue protrusion can be used to mobilize the epiglottis for further identification. Typically, it should measure less than 2.5 mm in the anterior-posterior diameter. In some individuals, it can be difficult to visualize the epiglottis, in which case, it is likely to be of normal size.

Figure 10.3: Probe position (left) and epiglottis (red arrow), likened to a sloth face (right).

As scanning continues further inferiorly, the thyroid cartilage is easily identified as bilateral symmetrical hyperechoic structures joining in the midline. With fine movements through this plane, the vocal cords can be identified around the central tracheal shadow. Phonation will cause movement of the vocal cords making them more apparent. True cords are hypoechoic, while the vocal ligaments are hyperechoic (Fig. 10.4).

Figure 10.4: Probe position and view of the vocal cords.

Application:

Epiglottic inflammation can be identified with ultrasonography. A normal epiglottic thickness is 2.5 ± 0.15 mm. Increased anterior-posterior diameter at the edges is suspicious for epiglottitis and can make endotracheal intubation difficult.¹
Vocal cord structure and movements can be identified during ultrasonography. Inadequate lateral movement, small tracheal shadow, distortion of normal cord structure due to lesions should raise the suspicion for an anatomically difficult airway and tube placement ( Video 10.1).

View Quicktime Video

Video 10.1. Vocal Cord Tumor and Computed Tomography Correlation

IDENTIFICATION OF THE CRICOTHYROID MEMBRANE

Identifying the CTM by palpation is only variably successful. In nonobese men, it can be palpated up to 72% of the time, but success decreases to 39% in obese men and women.³^,⁴^,⁵ The skin crease method only identifies the position of the CTM in half of patients. Similarly, the “four-finger” technique is successful less than half the time.²^,³^,⁴ Ultrasonography is superior to landmark techniques for locating the CTM.⁵ Once the CTM is identified and marked, manipulation of the neck, repositioning, and patient movement does not tend to significantly alter the location of the CTM relative to the marking.⁶^,⁷

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