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The accuracy of locating the cricothyroid membrane by palpation – an intergender study
© Campbell et al.; licensee BioMed Central Ltd. 2014
Received: 10 June 2014
Accepted: 27 October 2014
Published: 22 November 2014
The cricothyroid membrane (CTM) is the recommended site of access to the airway during cricothyrotomy to provide emergency oxygenation. We sought to compare the ability of physicians to correctly identify the CTM in male and female patients.
In a prospective observational study, anaesthetists were asked to locate the CTM by palpation which was then identified using ultrasound and the distance between the actual and estimated margin of the CTM was measured. Participants assessed the ease of CTM palpation using a visual analog scale. In a second series, the angulation of the posterior junction of the thyroid laminae was measured using ultrasound.
23 anaesthetists and 44 subjects participated. A total of 36 assessments were carried out in each gender. Incorrect identification of the CTM was more common in females (29/36 vs. 11/36, P < 0.001) and the distance from the CTM in the vertical plane was greater (11.0 [6.5–20.0] vs. 0.0 [0.0–10.0] mm, P < 0.001). In females distance from the CTM correlated positively with neck circumference (P = 0.005) and BMI (P = 0.00005) and negatively with subject height (P = 0.01). Posterior thyroid cartilage angulation was greater in females (118.6 ± 9.4° vs. 95.9 ± 12.9°, P = 0.02) and was lower in patients with correctly identified CTMs (100.0 ± 14.9° vs. 115.6 ± 15.9°, P = 0.02). VRS palpation correlated with decreased posterior thyroid cartilage angulation (P = 0.04).
CTM localisation is more difficult in female subjects irrespective of body habitus. It may be prudent to localize this structure by additional means (e.g. ultrasound) in advance of any airway manoeuvres or to modify the cricothyrotomy technique in the event that it is necessary in an emergency.
KeywordsAirway Anatomy Anaesthesia – Emergency Complications- Airway Obstruction Complications – Hypoxia Training
Emergency access to the trachea via the cricothyroid membrane (CTM) is a core skill for anaesthetists and constitutes the ultimate rescue manoeuvre to restore oxygenation in failed intubation algorithms. It may also be the primary airway choice in circumstances such as facial trauma [1–4]. Recently published figures from the NAP 4 project in the United Kingdom indicate that this procedure had a failure rate of 64% when performed by anaesthetists . The failure rate was highest for narrow bore cannula (63%) and somewhat lower for wide bore cannula attempts (43%) suggesting that equipment performance may contribute to the outcome. The overall failure rate of 64% is considerably greater than that reported in older published series where the procedure was frequently performed by pre-hospital paramedical personnel [5–8]. The reasons for this discrepancy are unclear. Equipment related issues aside, the success or failure of the procedure is obviously also dependent on the ability of the clinician to correctly identify the CTM in the first instance. Failure to identify this structure may result in serious complications including failure to oxygenate or trauma to adjacent structures in the airway and vasculature [6–10]. The more obvious sources of difficulty in identifying the CTM include obesity or altered neck anatomy, but in many of the patients in whom the procedure failed, risk factors were absent.
A previous study that was conducted by our research group found that misidentification of the cricothyroid membrane is common in female subjects but did not include a direct comparison with males . In the current study which is a follow on from the previously referenced work, we sought to determine if it is more difficult to locate the CTM by palpation in females than males. We used ultrasound to identify the CTM in a group of male and female subjects and compared the ability of Anaesthetists to identify this structure using palpation. On the basis that the female larynx is generally smaller and less prominent than in males, we hypothesised that misidentification of the CTM would be more common in female than male subjects.
The study was conducted following approval by the ethics and research committees of the Rotunda and Mater Misericordiae University Hospitals, Dublin. Written informed consent was obtained from patients and volunteer subjects. The Rotunda Hospital is a University-affiliated tertiary level centre for Obstetrics and Gynaecology. The Mater Misericordiae University Hospital is a tertiary level general Hospital. Recruitment took place from January to July 2013. Each trainee in the department of Anaesthesia undergoes teaching in invasive airway access using a variety of airway access devices on a manikin model at the commencement of their training rotation in this hospital. No additional training was provided prior to this study.
Series 1: subject characteristics and results
Female N = 24
Male N = 20
71.0 (61.9 – 78.2)
82.1 (72.3 – 91.1)
1.65 (1.65 – 1.67)
26.3 (24.8 – 31.9)
27.0 (23.0 – 27.8)
BMI >25 N (%)
BMI >30 N (%)
Neck Circumference (cm)
39.5 (36.5 – 40.0)
41.0 (40.5 – 42.0)
Participant experience (years)
10.0 (2.0 – 18.75)
5.0 (2.0 – 10.0)
Distance from CTM (mm)
15.3 (10.0 – 20.4)
2.0 (0.0 – 10.8)
Time to identification of CTM (sec)
14.0 (12.0 – 21.0)
22.0 (15.0 -28.0)
Time to identify CTM by BMI
BMI <25 (sec)
BMI 25-30 (sec)
BMI >30 (sec)
Series 2: subject characteristics and results
Female N = 10
Male N = 15
VRS palpation (cm)
Thyroid Cartilage Angle (Degree)
BMI >25 N (%)
BMI >30 N (%)
Neck Circumference (cm)
Data were analyzed using Sigma Stat (Version 2.0; Jandel Corporation, San Rafael, CA). Categorical data are presented as numbers and percentages and were analyzed by Chi-square or the Fisher exact test as appropriate. Continuous data were analyzed by the Student-t Test and/or ANOVA, as appropriate. Associations were determined by regression analyses. Continuous data are presented as mean (standard deviation) and median (interquartile range). In series 1, a total sample size of 72 was projected on the basis of correct identification of 60% in the male and 25% in the female subjects (α = 0.05, β = 0.8).
Angulation of the posterior aspect of the thyroid cartilage was greater in females than males (118.6 ± 9.4° vs. 95.9 ± 12.9°, P < 0.001). This angle was lower in patients with correctly vs. incorrectly identified CTMs (100.0 ± 14.9° vs. 115.6 ± 15.9°, p = 0.02). VRS palpation was associated with increased neck circumference (P = 0.006), increased height (P = 0.002) and with decreased thyroid cartilage angle (P = 0.04). Ease of palpation was greater in correctly vs. incorrectly identified CTMs (VRS 7.9 ± 1.6 vs. 6.4 ± 1.7) and was greater in males than females (8.3 ± 1.0 vs. 6.2 ± 1.9, P = 0.02).
The data from this study show that practicing anaesthetists frequently misidentify the cricothyroid membrane and that misidentification is more frequent in female than male subjects. Furthermore, the magnitude of the error, i.e. distance from the CTM, is also greater in females than males. Within the female group the extent of error worsened with increasing BMI and neck circumference suggesting that, as might be expected, increased fat or other soft tissue obscures the anatomy and interferes with the ability of clinicians to accurately locate the CTM. This finding was not replicated in male subjects and suggests that in men increasing adiposity has a lesser effect on the ability to locate the CTM. The likely mechanism for this intergender difference is the shape of the adjacent laryngeal cartilage which is more readily identifiable because of its more acute external angulation as supported by our ultrasonic findings. The relevance of these findings is in emergency surgical airway management where misidentification of the CTM and subsequent cricothyrotomy could lead to either failure to oxygenate through malplacement or damage to surrounding vascular or airway structures .
Previous studies by our group and others have shown that misidentification of the CTM is common and particularly so in the female population [9, 11]. Speculative reasons for this include the similarity on palpation of the thyrohyoid space superiorly and paired tracheal rings inferiorly (more common in females), both of which may be mistaken for the CTM [10, 12]. Our finding that misidentification was more commonly below the actual CTM in females may suggest that the cricoid cartilage may be mistaken for the thyroid cartilage. Despite an obviously greater number of accurate assessments in males, the CTM was not identified correctly in 11/36 (28%) of male assessment attempts. The inability to identify the CTM in a large proportion of patients from both genders may explain at least in part the frequent failure of cricothyroid access in cadaveric laboratory experiments, prospective clinical series and meta-analysed data [5, 10, 13–15].
Our study has several weaknesses. First, we infer that if a cricothyroidotomy were performed at a misidentified site the failure rate would be equivalent to the misidentification rate. Realistically the failure rate may be higher as there is additional capacity for error due to equipment misuse or malfunction which is reflected in the high failure rate of cannula cricothyroidotomy in the NAP 4 study. Second, the study used a convenience sample and no attempt was made to randomize subjects to participants or vice versa. The number of assessments by each participating anaesthetist was not limited, and it is possible that one individual’s performance could have exerted a significant effect on the outcome of the study. No participant did more than one assessment on a single day, in an attempt to reduce any learning effect from repeated palpation. The data obtained are consistent with that obtained in a previously reported study by Aslani in females and with the cricothyroidotomy failure rate of 33% in military medicine. Third, no time limit was imposed, which does not realistically reflect the type of clinical scenario that could be anticipated. Fourth, the participants were not blinded to the gender of the subject. Fifth, all subjects were examined in the neutral anatomic position as opposed to the neck extended position which is recommended when performing a surgical airway. Last, we used the same definition of a correct marking as in Aslani’s study, which included a marking within 5 mm of the midline . This is an arbitrary marking which as Aslani states may be generous. We used this measurement both for consistency of comparison and because it is likely that incisions beyond these borders might be outside the airway or may damage adjacent structures.
This clearly has implications for any clinician involved in airway management and in particular in obstetric anaesthesia. The recently reported incidence of failed intubation was 1/100-1/224 in this field and these studies include a number of patients who had attempted cricothyroid access [26, 27]. Increasing BMI with pregnancy and the well documented worsening of Mallampati scores in labour may contribute to difficulty both with intubation and rescue surgical airway in this group of patients . Consideration should be given to the use of ultrasound technology in CTM identification in patients who have predictably difficult anatomy [29–32]. It may be preferable to assess and mark the anatomy prior to embarking on induction of anaesthesia rather than struggling to locate the CTM as the final manoeuvre in a failed intubation algorithm when a patient is already hypoxic. Modifications of the technique of cricothyroidotomy may also be required where the anatomy is difficult. This could include an extended longitudinal incision with dissection as opposed to the ‘stab’ approach which is more appropriate when the anatomy is easily identifiable .
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