The height of the cricothyroid membrane on CT

The height of the cricothyroid membrane on CT scans in trauma patients – Anaesthesia 2017 A journal club presentation by Gregg Baxter

Background Emergency surgical airway is the final step in all difficult airway algorithms. One option for surgical airway is the cricothyrotomy. DAS recommends a size 6. 0 internal diameter tube be used (Ext. diameter up to 8. 0). Emergecny cric. has complication rate of 30%. Use of over-sized tube can lead to fracture of the laryngeal cartilage.

Cricothyroid membrane

Cricothyroid membrane Previous reports suggest size of membrane as: 7. 5 -10. 5 mm in female cadavers 10. 0 -13. 0 mm in male cadavers Population variation. Size of tube for emergency cric should be smaller than membrane size when measured in neutral position.

Study Uk based study – Plymouth Hospitals NHS Trust Patients who received a trauma CT as part of initial imaging strategy were eligible for inclusion. January 2014 -June 2015 Cross-sectional Study

How it was carried out… 2 Radiology trainees studied reconstructed midline sagittal images. Electronic callipers were placed from most superior aspect to most inferior aspect of cricothyroid membrane. Readings taken from this measurement.

Exclusion criteria Any image where measurement could not be taken: Trauma scans that did not include the neck Scans with movement artefact that prevented accurate measurement Scans in which patients were found not to have sufficient calcification of the thyroid and cricoid cartilages. Young, female patients were therefore most likely to be excluded.

Data Sample size of 478 required to give a 5% level of significance and 95%CI for a precision of 0. 2 mm in measurement of height. Age, sex, c-spine immobilisation and tracheal intubation were also noted. Krippendorf’s alpha was used to assess inter-rater agreement between the 2 radiologists. Shapiro-Wilks test to assess normailty of data.

Results 745 scans assessed of which 482 were suitable. 78. 2% of scans male patients, 21. 8% female Mean age of patients: Men – 51, Women – 62. 8 Inter-rater test showed an excellent agreement between the 2 radiologists, such that data analysis was based on the measurements by 1 of them only. Mean cric. membrane height in men 7. 89 mm, in women 6. 00 mm

Results

Results

Results Ages were regrouped into 2 cohorts for males 16 -35 years: 6. 59 mm 36 -93 years: 8. 25 mm No significant differences were observed between age and height of membrane in female patients. Presence of tracheal intubation and/or c-spine immobilisation had no significant effect on membrane height.

Discussion

Pros Adult, UK population - reproducible Statistically significant results Relatively large sample size Accepts its own limitations

Cons First study of its kind, CT measurement of cric membrane has not been validated (though this difficult to achieve). Measurement is ‘static’ height and may not reflect capacity of the space (affected by patient positioning and pliability of surounding tissues). Poor numbers for female and especially young female patients.

Recommendations Selection of tube sizes available including 3. 0, 4. 0 and 5. 0 mm internal diameter tubes. Information on likely size of cric membrane should be included in theoretical and practical training sessions. Do not be surprised when your first choice of tube does not pass during cricothyrotomy.

Scope for future work Collecting data on longitudinal membrane height. Investigating relationship between patient height and membrane height. External validation across a variety of patient populations. A role for Ultrasound in assessing membrane height?