1. Correct Answer: C

Rationale: When performing sonographic assessment of the airway, it is extremely important to follow a systematic step-by-step approach. Two techniques have been described for identification of the cricothyroid membrane: (a) the longitudinal “String of Pearls” (SOP) and (b) the transverse “Thyroid-Airline-Cricoid-Airline” (TACA). The SOP technique is the most commonly used and it also allows identification of the interspaces between tracheal cartilages. The transverse TACA technique can be very useful in patients with short neck or severe neck flexion limitation. In the SOP technique, the first step requires identification of the trachea in a transverse approach at the level of the suprasternal notch (Figure 57.3A, top row). At this point, the transducer is slid to the patient’s right side until only half of the tracheal cartilage is displayed on the screen (Figure 57.3B, second row). Then, the transducer is rotated 90° clockwise, carefully maintaining it over the tracheal midline; this rotation allows visualization of the trachea in a longitudinal plane, with several hypoechoic rounded structures (cricoid and tracheal cartilages) aligned anteriorly to a hyperechoic line (air-mucosa interface), similar to a “string of pearls” (Figure 57.3C, third row). The cricoid cartilage appears as a larger and more anterior structure compared to the tracheal rings, with the first ring usually appearing as the longest one (Figure 57.1, structure C). The distal part of the thyroid cartilage is then seen cephalad to the cricoid cartilage. The cricothyroid membrane can be identified between the cricoid and thyroid cartilages (Figure 57.3D, last row).

For the transverse TACA technique, the first step requires identification of thyroid cartilage (seen as a triangular structure) (Figure 57.4A, top row), followed by a caudal movement of the transducer until the cricothyroid membrane can be identified (Figure 57.4B, second row). Further caudal movement will reveal the cricoid cartilage, appearing as a hypoechoic horse-shoe structure (Figure 57.4C, third row). The final step is a cranial movement of the transducer, aimed at reidentification of the cricothyroid membrane (Figure 57.4D, last row) and marking of the skin for possible cricothyroidotomy.

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