Cricothyroid membrane identification with ultrasonography and palpation in cadavers with a novel fixation technique (Fix for Life)

Editor,

Emergency front-of-neck access (eFONA) to the airway by performing a cricothyroidotomy is the last resort in a ‘can’t intubate can’t oxygenate’ situation.¹ The crucial initial step in this procedure is the correct identification of the cricothyroid membrane (CTM). Particularly in female patients, the traditional palpation method has proved challenging.² The use of ultrasonography in locating the CTM is promising and may increase the success rate, but evidence is yet conflicting.³ Using ultrasonography preprocedurally in elective anaesthesia cases has been propagated as a standard operational procedure to identify and mark the CTM when inspection or palpation of landmarks is difficult.⁴

The training of cricothyroidotomy in the clinical setting is difficult because the incidence is low, and the acute setting is not appropriate to train this procedure. Mannequins and animals serve as training models, but they do not closely reflect human anatomy nor its variance. Using recently deceased patients is not always ethical and fresh frozen human cadavers of body donors to science have time constraints, due to ongoing putrefaction. Formalin-fixed cadavers become very firm and inflexible.⁵

Recently, Fix for Life (F4L) embalmed cadavers have been described to be suitable and realistic in the training of basic airway management techniques, without the above disadvantages.⁵ The F4L cadaver could also be appropriate to train in identification of the CTM. In the current study, the primary aim was to determine if anaesthesiology participants would judge the F4L cadaver ‘suitable’ (assessment of suitability for learning) and ‘realistic’ (assessment of look, feel and flexibility compared with a living human) as a teaching model in locating the CTM by palpation or ultrasonography. Second, we compared success rates and time required to identify the CTM.

Ethical approval was provided by the Medical Ethics Review Committee of VU University Medical Center (Amsterdam, the Netherlands. Reference 2018.429) on 13 August 2018 (Chairperson Prof C. Boer). Because of the more challenging anatomy concerning the identification of the CTM, three female cadavers were used.² Age at death, weight, length, BMI and neck circumference were recorded. Cadavers of body donors with known neck abnormalities (e.g. tumour masses, goitre) or surgical procedures (including eFONA) were excluded. In each cadaver, the head was placed in the extended position and the correct CTM location was marked with a dot of invisible ink, becoming visible only under ultraviolet (UV) light.

Forty anaesthesiologists and trainees participated after informed consent. Each participant received a brief training in ultrasonography-aided identification of the CTM.⁴ The participants were then allowed to practise the ultrasonography techniques on a F4L cadaver which was not included in the study. Each participant was randomly allocated to either the palpation group or the ultrasonography group using a sealed envelope technique. After randomisation, the participants were asked to identify the CTM in the three cadavers and to mark their localisation with a blunt anatomical probe.

The time required to identify the CTM was recorded. Successful identification of the CTM was defined as ‘positive’ if the participants’ probe mark lay within 5 mm of the predefined location after cross-checking using UV light. The participants were asked to classify the difficulty of identifying the CTM using an established four-grade system (1, easy/visible landmarks; 2, moderate/requires light palpation of landmarks; 3, difficult/requires deep palpation of landmarks; or 4, impossible/landmarks are not palpable).⁶ This procedure was repeated for each of the three cadavers in the same order. After completion of the procedures on all three cadavers, the participants of both groups were asked to give one overall verbal rating score (VRS) (1, worst score; 10, best score) for ‘realism’ and one for ‘suitability’ of the F4L cadaver model.

We based the sample size estimation on the obtainable width of the two-sided 95% confidence interval (CI) for the estimation of ‘realism’ and ‘suitability’ (primary outcomes), and considered a margin of error of no more than 1 (total width of the CI no larger than 2) as acceptable precision. Assuming normal distribution of the data and an expected SD of 2, we therefore required 18 participants in each group. Accounting for possible dropouts, we aimed at 20 participants in each group. The calculation was performed with PASS 16 (NCSS Statistical Software, Kaysville, Utah). Generalised estimating equations were used to compare the groups for the primary and secondary outcomes, with professional level and experience as covariates. Significance was set at a P value of 0.05.

All participants completed the experimental tasks in all three cadaver models, resulting in a total of 120 attempts, that is 60 attempts per group. Characteristics of the cadavers are shown in Table 1. VRS for realism and suitability, success percentages in locating the CTM, time to identify the CTM, difficulty scores and characteristics of the participants are shown in Table 2.

Table 1.

Characteristics of the three female Fix for Life cadavers

	Cadaver 1	Cadaver 2	Cadaver 3
Age at death (years)	68	90	75
Weight (kg)	52	66	61
Length (m)	1.70	1.67	1.66
BMI (kg m−2)	18.0	23.7	22.1
Neck circumference (cm)	42	52	38

Table 2.

Verbal rating scores for realism and suitability of the Fix for Life cadaver model, success rates of locating the cricothyroid membrane, mean time to identify the cricothyroid membrane, difficulty scores and grade and years of professional experience of the participants in the ultrasonography and palpation groups

	US group	PAL group	P
VRS realism	8 (8 to 9)	8 (7 to 8)	0.001
VRS suitability	8.5 (8 to 10)	8 (8 to 8)	0.030
Success in locating CTM, n	55 (91.7%)	42 (70.0%)	0.011
Time to identify CTM (s)	34.3 (21.4 to 47.2)	12.0 (8.8 to 15.2)	<0.001
Difficulty score, median [IQR]
Cadaver 1	2 [1 to 3]	2.5 [2 to 3]	0.014
Cadaver 2	2 [2 to 2]	3 [2 to 3]	<0.001
Cadaver 3	2 [1.25 to 2.75]	2 [2 to 2.75]	0.315
Consultant/trainee	16/4	10/10	0.663*
Professional experience (years)	12.3 ± 7.1	11.1 ± 8.9	0.149*

Values are median (95% CI), median [IQR], number (n), number (%), mean time (95% CI) or mean ± SD. CTM, cricothyroid membrane; PAL, palpation; US, ultrasonography; VRS, verbal rating scores.

^*For the comparison of baseline covariates, the absolute standardised mean difference rather than the P value is reported.

The F4L cadaver model received high VRS scores for both ‘realism’ and ‘suitability’ as a teaching model for the localisation of the CTM by palpation and these scores were even higher for the ultrasonography technique. The identification of the CTM using ultrasonography in F4L cadavers was more successful compared with digital palpation (success rate 91.7 versus 70.0%, P = 0.011). However, the time to identify the CTM was markedly longer in the ultrasonography group. The identification of the CTM in cadavers with a larger neck circumference were rated as easier by participants in the ultrasonography group, presumably as more pretracheal soft tissue helps focus the ultrasonographic beam.²

A limitation of the study was the imbalance in baseline characteristics of the participants; however, this had no significant effect on the outcomes. Moreover, sample sizes were not calculated for between-group comparisons and therefore these data should be regarded as preliminary. Complications in eFONA, such as excess time, incision errors, tube misplacements, haemorrhage and cartilage injury, warrant adequate training and the upkeep of skill for this procedure.⁷ Since the correct identification of the CTM is recognised as the critical initial step, and since gaining confidence in one's ability to correctly identify the CTM may decrease the barrier to perform cricothyroidotomy when indicated, we suggest that training with the F4L cadaver model contributes to a timely, correct application of cricothyroidotomy in the future.