The inverted‐V shape during transverse laryngeal ultrasonography for cricothyroid membrane localisation

Inaccurate cricothyroid membrane localisation can result in cricothyroidotomy failure. Ultrasonography improves membrane localisation accuracy vs. palpation 1, but sensitivity often is considerably below 1.0 2, 3. The ‘thyroid‐airline‐cricoid‐airline’ localisation technique is quicker and easier than, and as sensitive as, the ‘string‐of‐pearls’ technique, both described by Kristensen et al. 3, 4. This technique identifies the inverted V shape of the thyroid cartilage, and the vocal cords contained within. However, other structures can mimic a V shape 5, and scanning from the upper to lower ends of the cartilage does not reliably identify the cords 6.

We conducted a study of transverse ultrasonographic laryngeal anatomy, and hypothesised that an inverted‐V shape would reliably (≥ 95%) identify the thyroid cartilage and hence allow cricothyroid membrane localisation.

With ethics committee approval and informed consent, an anaesthetic consultant experienced in airway ultrasonography performed transverse laryngeal ultrasonography in 80 mixed elective surgical patients (38 women, median (IQR [range]) age 59 years (46–70 [18–84]) and body mass index 30 kg.m⁻² (26–34 [17–54]). This anaesthetist, and a second trained anaesthetist, assessed ultrasonographic images from the upper and lower ends of the thyroid cartilage for the presence of inverted‐V shapes, also the cricothyroid membrane and cricoid cartilage. Combined inverted‐V detection rates were 96% (95%CI 92–99%, p = 0.30, agreement 93%) and 93% (95%CI 88–97%, p = 0.78, agreement 86%) for identifying the upper and lower ends of the thyroid cartilage, respectively, and 73% (95%CI 66–80%, p =1.00, agreement 61%) and 3% (95%CI 1–7%, p = 1.00, agreement 94%) in the cricothyroid membrane and cricoid cartilage, respectively. We found that the sensitivity and specificity of an inverted V shape for reliably identifying the thyroid cartilage were 0.80 and 0.15, respectively.

Ultrasonography identified the inverted‐V shapes of the thyroid cartilage images with high sensitivity and interindividual agreement, but with low specificity (i.e. inverted‐V shapes were seen in cricothyroid membrane images), potentially confusing membrane localisation. We noted that the membrane's air‐tissue border has a similar appearance to that of the cricoid cartilage air‐tissue border, with which it is seamlessly continuous, further confusing localisation (Fig. 1). Further echolocalisation knowledge is required to improve the reliability of ultrasound‐guided cricothyroidotomy 7.