
==== Front
Medicine (Baltimore)
Medicine (Baltimore)
MD
Medicine
0025-7974
1536-5964
Lippincott Williams & Wilkins Hagerstown, MD

39029084
MD-D-24-00545
00086
10.1097/MD.0000000000038591
3
3300
Research Article
Observational Study
Evaluation of neck ultrasound measurements as a difficult airway predictor: A prospective observational study
https://orcid.org/0000-0002-7254-554X
Çinar Köse Elif Özge MD a*
Tekin Murat MD murattekin71@hotmail.com
a
Cesur Sevim MD svmcsr@gmail.com
a
Çam İsa MD dr.isa.cam@gmail.com
b
Baykara Zehra Nur MD nur.baykara@yahoo.com
a
a Department of Anesthesiology and Reanimation, Kocaeli University Faculty of Medicine, Kocaeli, Turkey
b Department of Radiology, Kocaeli University Faculty of Medicine, Kocaeli, Turkey.
* Correspondence: Elif Özge Çinar Köse, Department of Anesthesiology and Reanimation, Kocaeli University Faculty of Medicine, Kocaeli, Turkey (e-mail: elifozgee@hotmail.com).
19 7 2024
19 7 2024
103 29 e3859114 1 2024
23 5 2024
24 5 2024
Copyright © 2024 the Author(s). Published by Wolters Kluwer Health, Inc.
2024
https://creativecommons.org/licenses/by-nc/4.0/ This is an open-access article distributed under the terms of the Creative Commons Attribution-Non Commercial License 4.0 (CCBY-NC), where it is permissible to download, share, remix, transform, and buildup the work provided it is properly cited. The work cannot be used commercially without permission from the journal.

This study aimed to investigate the relationship between cervical measurements and difficult airways using ultrasonographic measurements. American Society of Anesthesiologists grade I to III, male or female, 120 adult patients, undergoing elective surgery were enrolled in the study. The study involved measuring the distance of the trachea, cricoid cartilage, thyroid cartilage, vocal cord anterior commissure, and hyoid bone to the skin using a 10 to 13 MHz linear ultrasound probe in the transverse plane. Additionally, the length of the cricothyroid and thyrohyoid membranes, along with their distance from the skin, were measured using the probe in the sagittal plane. Subsequently, another experienced anesthesiologist conducted mask ventilation and intubation after the patient’s induction of general anesthesia. Throughout this process, the patient was assessed for difficulties in mask ventilation, laryngoscopy, and intubation. 28 (23.3%) patients had a difficult airway. Analyzing the measurements associated with difficult airways, the most reliable predictor was the epiglottis midline-skin distance [AUC (area under the curve): 0.847, P < .001, cutoff: >19.9, sensitivity: 78.6%, specificity: 79.4%]. Additionally, other factors such as hyoid bone to skin distance, thyroid cartilage to skin distance, thyrohyoid membrane to skin distance, and vocal cord anterior commissure-skin distance were also identified as predictors for a difficult airway. The increase in the distance of the epiglottis midline, vocal cord anterior commissure, hyoid bone, thyrohyoid membrane, and thyroid cartilage to the skin at the level of the isthmus measured by ultrasonography is predictive of difficult airways. Based on our study outcomes, we assert that ultrasonographic evaluation can be used in the prediction of difficult airways.

airway management
airway ultrasonography
anesthesia complications
difficult airway
difficult intubation
difficult laryngoscopy
difficult ventilation
OPEN-ACCESSTRUE
SDCT
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pmc1. Introduction

Ensuring and securing airway patency is a primary responsibility of anesthesiologists. Delayed airway management can lead to hypoxia, irreversible brain damage, or death. Therefore, anesthetists must assess the airway, take necessary precautions, and maintain patency safely.[1]

Difficulty in providing airway patency, termed “difficult airway,” occurs less frequently but has severe consequences. Its incidence ranges from 1.5% to 13.5% in the general population and 10.5% to 20.5% in obese patients.[2] There is no standard definition for a difficult airway. The ASA (American Society of Anesthesiologists) defines a difficult airway as “A clinical situation in which anticipated or unanticipated difficulty or failure is experienced by a physician trained in anesthesia care, including but not limited to one or more of the following: facemask ventilation, laryngoscopy, ventilation using a supraglottic airway, tracheal intubation, extubation, or invasive airway.”[3]

In the United States, 30% to 40% of anesthesia-related deaths are associated with poor airway management. Twenty-seven percent of closed cases against anesthesiologists involve difficult/impossible intubation with no reports of preoperative airway evaluation.[4]

Difficult airways can be classified as predictable and unpredictable, requiring a detailed assessment by an experienced anesthetist. Airway assessment begins with the patient’s history and continues with the physical examination. There are some tests and criteria that can be used during the physical examination. These can be listed as Mallampati classification, atlantooccipital joint extension, thyromental length, sternomental length, hyomental length, and distance between incisors. It is also possible to benefit from radiological evaluations such as direct radiography, tomography, magnetic resonance imaging, fluoroscopy, and ultrasonography.[1]

It is an important requirement to evaluate beforehand whether the patient can maintain spontaneous or artificial respiration under anesthesia. To show the expected benefits, evaluation methods must have high sensitivity, selectivity, and positive predictive values. Thus, necessary preparations will be made for intubations that are expected to be difficult, while unnecessary preparation can be avoided in cases where intubation is not expected to be difficult.

This study is aims to evaluate the preoperative neck ultrasonography measurements as a predictor of difficult airways in adult patients who will undergo elective surgery under general anesthesia.

2. Materials and methods

This prospective, observational, and single-blind (evaluative-blind) study was done at Kocaeli University Faculty of Medicine Hospital after obtaining the approval of the Kocaeli University Faculty of Medicine Ethics Committee (KOU KAEK/15.bI.03-02.09.2021) and informed consent of the patients. The study complies with the Declaration of Helsinki and the Good Clinical Practices guidelines. The study was completed within 2 months.

When power analysis was performed with the G*Power 3.1.9.2 program, taking effect size = 0.5, α = 0.05 and Power (1‐β) = 0.80, it was determined that a total of 120 patients should be studied.[5]

The study included 120 ASA I-III adult patients requiring intubation for elective surgery under general anesthesia. Uncooperative patients, pregnant women, and those with head/neck trauma or surgery were excluded (see Diagram, Supplemental Digital Content, http://links.lww.com/MD/N194, which shows the plan of the study).

Preoperative records and ultrasonography measurements were taken by a 5-year experienced anesthesiologist, and airway difficulty during mask ventilation, laryngoscopy, and intubation was evaluated by another 5-year experienced anesthesiologist, who was blind of the recordings and measurements. The anesthesiologist who performed the ultrasonography measurements received training on this subject from the radiology department.

In the preoperative waiting room, the patient was placed in the supine position on the stretcher, and measurements were made in the upper airway using an ultrasound device (ESAOTE Mylab X6, Genoa, Italy) and a 10 to 13 Hz linear ultrasound probe. The probe was first placed transversely at the level of the jugular notch and the trachea-skin distance was measured. The probe was slid cranially and the cricoid cartilage–skin distance, the thyroid cartilage–skin distance, the anterior commissure–skin distance of the vocal cords, and the epiglottis midline–the skin distance were measured. Then, the probe was placed sagittally at the level of the cricothyroid membrane and the membrane length and the distance of the membrane midline to the skin were measured. The thyrohyoid membrane was also seen by sliding the probe cranially, and the membrane length and the distance from the midline of the membrane to the skin were measured (Fig. 1).

Figure 1. Ultrasonographic measurements. (A) Thyroid cartilage–skin distance. (B) Vocal cord anterior commissure–skin distance. (C) Epiglottis–skin distance. (D) Hyoid bone–skin distance. (E) Thyrohyoid membrane length.

The patients were taken to the operating room after their measurements were completed and placed in the supine position. For induction of anesthesia, 2 mg/kg of propofol (Propofol®, Fresenius Kabi, Istanbul, Turkey), 1 mcg/kg fentanyl (Talinat®, Vem Ilac, Istanbul, Turkey), 0.6 mg/kg rocuronium (Esmeron®, MSD, Istanbul, Turkey) intravenously. Mask ventilation was performed with a transparent, disposable plastic mask after induction, and its difficulty was evaluated with the Han Scale classification (Table 1). After adequate muscle relaxation was achieved, intubation was started. Intubation tube ID: 8.0 for male patients and ID: 7.5 for female patients (Beybi Plastik Sanayi A.Ş., Istanbul, Turkey) was used. The patient’s Cormark-Lehane Scale was determined. The instrument used for intubation (direct laryngoscope with Macintosh blade, video laryngoscope, fiberoptic, FastTrack Laryngeal Mask Airway) was specified. It was recorded whether the intubation was successful or not, and if so, the number of attempts.

Table 1 Han scale difficult mask ventilation classification.

1	Easy mask ventilation	
2	Need for an supporting device such as an oral airway (with/without use of neuromuscular blocking agent)	
3	Difficult-inadequate or unsustainable ventilation/requirement of 2 practitioners (with/without use of neuromuscular blocking agent)	
4	Impossible – failure to provide mask ventilation (with/without use of neuromuscular blocking agent)	

While patients with Han Scale classes 1 were considered “easy ventilation,” patients with 3 and 4 were considered “difficult ventilation”; Cormark-Lehane classes 1 and 2 were evaluated as “easy laryngoscopy,” and those with 3 and 4 as “difficult laryngoscopy.” Intubation was considered a “difficult intubation” if it was performed with a standard laryngoscope with 3 or more attempts.

Statistical analysis was performed with IBM SPSS 20.0 (IBM Corp., Armonk, NY) and MedCalc programs. Normal distribution was evaluated by Kolmogorov–Smirnov and Shapiro–Wilk tests. Normally distributed variables were given as mean ± standard deviation, and non-normally distributed variables were given as median (25th–75th percentile). Categorical variables were shown as frequency (percentage). Differences between groups were determined by the independent sample t-test for normally distributed numerical variables and by the Mann–Whitney U test for non-normally distributed numerical variables. Relationships between categorical variables were evaluated with Chi-square analysis. cutoff, sensitivity, and specificity values were calculated by ROC (receiver operating characteristics) analysis. In the testing of two-sided hypotheses, P < .05 was considered sufficient for statistical significance.

3. Results

The study was included 120 patients aged 18 to 88 years, 66 of these patients were female and 54 were male. The mean BMI (body mass index) was 27.2 ± 5.1 (Table 2).

Table 2 Patients demographic datas.

Age	49.9 ± 16.1	
Gender
Female
Male	
66 (55%)
54 (45%)	
ASA score
1
2
3	
58 (48.3%)
44 (36.7%)
18 (15.0%)	
Mallampati score
1
2
3
4	
36 (30.0%)
61 (50.8%)
22 (18.3%)
1 (0.8%)	
BMI (kg m‐2)	27.2 ± 5.1	
To BMI by classification
Weak
Normal weight
More fat
Obese	
2 (1.7%)
41 (34.2%)
44 (36.7%)
33 (27.5 %)	
Night snore story
Yes
No	
48 (40.0%)
72 (60.0)	
CPAP need diagnosis of OSAS.
Yes
No	
2 (1.7%)
118 (98.3%)	
ASA = American Society of Anesthesiologists, BMI = body mass index, CPAP = continuous positive airway pressure, OSAS = obstructive sleep apnea syndrome.

While the mean age was 49.83 ± 1.66 years in patients with easy airways, it was 50 ± 1.47 years in patients with difficult airways, and there was no significant difference between them. There was no statistical difference between the airway groups in gender and ASA score. While the mean BMI was 26.48 ± 4.82 in patients with easy airways, it was 29.74 ± 5.52 in patients with difficult airways, and there was a significant difference between them (P < .05) (Table 3).

Table 3 Age, gender, BMI, neck circumference ASA score, mallampati score, mandibulsr protrusion, AO (Atlanto-occipital) joint extension, HM (hyomental distance), TM (thyromental distance), SM (Sternomental distance), II (Interincisor) distance, beard presence, condition of teeth, diagnosis of OSAS and presence of night snoring in patients with easy and difficult airway.

	Easy airway (n = 92)	Hard airway (n = 28)	P	
Age	49.83 ± 1.66	50.50 ± 1.47	0.850	
Female/male	49/43	17/11	0.492	
BMI	26.48 ± 4.82	29.74 ± 5.52	0.003	
BMI groups 1/2/3/4	2/36/32/22	0/5/12/11	0.128	
Neck circumference	38.38 ± 4.74	39.26 ± 4.23	0.377	
ASA 1/2/3	47/31/14	11/13/4	0.454	
Mallampati 1/2/3/4	34/53/5/0	2/8/17/1	< 0.001	
Mandibular protrusion A/B/C	62/28/2	11/15/2	0.006	
AO joint extension
(1/2/3/4)	32/38/19/3	7/16/3/2	0.288	
HM distance	5.13 ± 1.05	5.28 ± 1.23	0.505	
TM distance	7.47 ± 1.11	7.57 ± 1.29	0.733	
SM distance	14.62 ± 2.00	14.65 ± 1.84	0.623	
II distance	4.37 ± 0.88	4.41 ± 0.84	0.853	
Beard yes/ no	16/76	5/23	0.955	
Teeth 1/2/3/4/5	5/2/14/21/50	1/2/1/9/15	0.307	
Snore yes/no	32/60	16/12	0.035	
OSAS diagnosis yes/no	1/91	1/27	0.369	
*BMI groups: 1: underweight, 2: normal weight, 3: excess overweight, 4: obese.

†AO joint extension: 1: >35°, 2: =22–34°, 3: =12–21°, 4: <12°.

‡Teeth status: 1: upper denture, 2: lower denture, 3: missing, broken tooth, 4: toothless, 5: full.

Mask ventilation was classified according to the Han Scale,[6] the “easy ventilation” group consisted of 99 (82.5%) patients in the 1st and 2nd classes, and the “difficult ventilation” group consisted of 21 (17.5%) patients in the 3rd and 4th classes.

Laryngoscopy was classified according to the Cormark-Lehane scale. The “easy laryngoscopy” group consisted of 94 (78.3%) patients in the 1st and 2nd classes, while the “difficult laryngoscopy” group consisted of 26 (21.6%) patients in the 3rd and 4th classes.[3]

Intubation was classified according to whether more than 3 attempts were required with the standard laryngoscope.[3] The “difficult intubation” group consisted of 16 (13.3%) patients who required more than 3 attempts for successful intubation. Only 1 (0.8%) of these patients could not be intubated and the operation was completed by ventilating with FastTrack Laryngeal Mask Airway.

Patients with at least one of the difficult ventilation, difficult laryngoscopy, or difficult intubations were grouped as “difficult airway.”[1] There were 28 (23.3%) patients with difficult airways.

The relationship between ultrasound measurements and difficult mask ventilation, difficult laryngoscopy, difficult intubation, and difficult airway was evaluated with the ROC graph.

The best predictor for difficult mask ventilation was the epiglottis midline to skin distance measurement [AUC (area under the curve): 0.860, P < .001, cutoff: >20.7, sensitivity: 71.4%, specificity: 86.9%]. Trachea-skin distance and vocal cord anterior commissure–skin distance were also found to be associated with difficult mask ventilation (respectively AUC: 0.638, P = .04, cutoff: >9.7, sensitivity:57.1%, specificity: 72.7%; AUC: 0.664, P = .01, cutoff: >7.5, sensitivity: 85.7%, specificity: 43.4%). No correlation was observed with cricoid cartilage-skin distance, thyroid cartilage–skin distance, hyoid bone–skin distance, cricothyroid membrane length, cricothyroid membrane-skin distance, thyrohyoid membrane length, and thyrohyoid membrane–skin distance.

The best predictor for difficult laryngoscopy was again the epiglottis–skin distance measurement (AUC: 0.848, P < .001, cutoff: >20, sensitivity: 76.9%, specificity: 78.7%). Hyoid bone and vocal cord anterior commissure–skin distance were also associated with difficult laryngoscopy (respectively AUC: 0.628 P = .03, cutoff: >8.8, sensitivity: 96.1%, specificity: 26.6%; AUC: 0.693, P < .005, cutoff: >7.8, sensitivity: 84.6%, specificity: 50.0%). No correlation was observed with cricoid cartilage–skin distance, thyroid cartilage–skin distance, trachea–skin distance, cricothyroid membrane length, cricothyroid membrane–skin distance, thyrohyoid membrane length, and thyrohyoid membrane–skin distance.

The best predictor for difficult intubation was the epiglottis to skin distance (AUC: 0.819, P < .001, cutoff: >19.6, sensitivity: 93.8%, specificity: 66.4%). The vocal cord anterior commissure–skin distance was also associated with difficult intubation (AUC: 0.671, P = .01, cutoff: >6.5, sensitivity: 100%, specificity:27.9%). No relationship was observed with cricoid cartilage–skin distance, thyroid cartilage–skin distance, trachea–skin distance, hyoid bone–skin distance, cricothyroid membrane length, cricothyroid membrane–skin distance, thyrohyoid membrane length, and thyrohyoid membrane–skin distance.

When looking at the correlation for difficult airways in general, the best predictor was the epiglottis midline–skin distance with high sensitivity and specificity (AUC: 0.847, P < .001, cutoff: >19.9, sensitivity: 78.6%, specificity: 79%, 4). Hyoid bone to skin distance (AUC: 0.625, P = .04, cutoff: >12.5, sensitivity: 42.9%, specificity: 79.4%), vocal cord anterior commissure to skin distance (AUC: 0.672, P = .002, cutoff: >7.4, sensitivity: 85.7%, specificity: 42.4%), thyroid cartilage to skin distance (AUC: 0.623, P = .03, cutoff: >4.2, sensitivity: 85.7%, specificity: 34.8%), and the thyrohyoid membrane–skin distance (AUC: 0.628, P = .03, cutoff: >8.6, sensitivity: 65.4%, specificity: 56.4%) were found to be associated with difficult airway (Fig. 2).

Figure 2. Evaluation of airway measurements made by ultrasonography using ROC (receiver operating characteristic) curves in terms of difficult airway. (A) Thyroid cartilage–skin distance. (B) Vocal cord anterior commissure–skin distance. (C) Epiglottis–skin distance. (D) Hyoid bone-skin distance. (E) Thyrohyoid membrane–skin distance.

4. Discussion

Airway management is a critical skill for anesthetists, with life-threatening complications being rare but severe. For example, “inability to ventilate” occurs in <1 in 5000 in routine general anesthesia and requires an emergency surgical airway in 1 in 50,000. However, deaths due to difficult airways rise to 25%.[7]

The probability of anesthetists encountering difficult intubation has been reported as 1% to 18%. Accordingly, unsuccessful intubation rates vary between 0.05% and 0.35%.[8] In our study, difficult intubation was observed with a rate of 13.3%, and unsuccessful intubation with a rate of 0.8%.

Safe testing and careful examination of the airway are needed to predict a difficult airway. Screening tests are expected to have high sensitivity and specificity. Standard bedside airway examination tests do not appear to be good screening tests. Given this, future research is needed to develop high-sensitivity tests and evaluate their use in difficult airway screening.[9]

Safe to use, fast and reproducible, ultrasound has become an important tool for various diagnostic or therapeutic purposes in the operating room and intensive care units in recent years. The use of ultrasound in airway management is relatively new. It can be used to determine the correct placement of the tracheal tube or the correct size of pediatric and dual-lumen tubes, diagnosis of upper airway obstruction, and guidance for percutaneous tracheostomy/cricothyrotomy.[10–14] It can be used to perform laryngeal nerve blocks to facilitate awake intubation, to diose post-extubation stridor, and to confirm proper Laryngeal Mask Airway position.[14–16]

We did this study to evaluate the effectiveness of cervical ultrasonography measurements as a predictor of difficult airways during the preoperative period. In our study, we examined the diagnostic value of ten ultrasonography measurements performed in the anterior cervical region for difficult airways increase in the distances of the epiglottis midline, thyrohyoid membrane, hyoid bone, vocal cord anterior commissure, and, thyroid cartilage to the skin at the level of the isthmus at the level of the thyrohyoid membrane was found to be predictive of the difficult airway (respectively P < .0001, P = .035, P = .040, P = .002, P = .034).

In prospective, observational studies, conducted by Falcetta et al[5] with 301 patients and Pinto et al[17] with 74 patients, a high correlation was found between the ultrasonographic measurement of anterior cervical soft tissue thickness at the level of the thyrohyoid membrane (epiglottis midline–skin distance) and the Cormack-Lehane score, like our study, on direct laryngoscopy. In our study, the epiglottis-skin distance measurements made at the level of the thyrohyoid membrane were the measurement with the highest predictive value, sensitivity, and specificity of the difficult airways (P < .0001). Increased thickness at the level of the anterior epiglottic space may affect the ability to visualize the glottis with a Macintosh blade on direct laryngoscopy. This can be explained by the anatomical model recently created by Greenland.[18] This model proposes that the upper airways are shaped by 2 curves, a primary and a secondary. Both curves must be aligned with the visual axis for an adequate laryngoscopic view. The increased distance between the skin and the epiglottis may be the result of a deepening upward concavity of the primary curve, which leads to a poor glottic image.

In our study, the vocal cord anterior commissure–skin distance was found to be predictive of the difficult airway. However, it has high sensitivity and low specificity (Table 4). Like this result, Ezri et al[19] and Alessandri et al[20] found a relationship between soft tissue thickness at the vocal cord level and difficult mask ventilation and difficult laryngoscopy; Komatsu et al[21] and Falcetta et al[5] did not show a relationship. However, the increase in soft tissue thickness at the level of the vocal cord will impair the direct laryngoscopic view by increasing the upward concavity of the secondary curve in the Greenland anatomical model,[18] and this increase would be expected to be associated with a difficult airway.

Table 4 Ultrasonographic airway measurement analyses.

	Mean ± standard deflection	Airway-ROC curve analysis	
Trachea–skin distance	9.07 ± 3.20	AUC: 0.609
P: .065	
Cricoid cartilage–skin distance	6.70 ± 2.11	AUC: 0.545
P: .4752	
Thyroid cartilage–skin distance	5.72 ± 2.38	AUC: 0.623
P: .034
Cutoff: >4.2 (mm)
Sensitivity: 85.7%
Specificity: 34.8%	
Vocal cord anterior commissure–skin distance	8.46 ± 2.44	AUC: 0.672
P: .0023
Cutoff: >7.4 (mm)
Sensitivity: 85.7%
Specificity: 42.4%	
Epiglottis–skin distance	18.58 ± 3.20	AUC: 0.847
P: <.001
Cutoff: >19.9 (mm)
Sensitivity: 78.6%
Specificity: 79.4%	
Hyoid bone–skin distance	10.74 ± 2.92	AUC: 0.625
P: .0406
Cutoff: shold value: > 12.5 (mm)
Sensitivity: 42.9%
Specificity: 79.4%	
Thyrohyoid membrane length	12.19 ± 4.61	AUC: 0.550
P: .385	
Thyrohyoid membrane–skin distance	8.09 ± 2.61	AUC: 0.658
P: .0357
Cutoff: >8.6 (mm)
Sensitivity: 65.4%
Specificity: 56.4%	
Cricothyroid membrane–length	12.73 ± 3.98	AUC: 0.532
P: .601	
Cricothyroid membrane–skin distance	9.60 ± 2.48	AUC: 0.550
P: .430	
Significant P value is shown in bold.

ROC = receiver operating characteristic, AUC = area under curve.

In our study, a relationship was shown between an increase in the distance of the hyoid bone to the skin and a difficult airway. The sensitivity of this relationship was found to be low and the specificity high (Table 4). Adhikari et al,[22] Wu et al,[23] Alessandri et al,[20] and Fulkerson et al[24] found a similar relationship in their studies. This is since the soft tissue thickness in the anterior neck at the level of the hyoid bone has a greater effect on the concavity of the primary curve in the Greenland anatomical model.[18]

In our study, a relationship was found between the distance of the thyroid cartilage and thyrohyoid membrane to the skin from the isthmus level and the difficult airway. While the skin distance of the thyroid cartilage shows high sensitivity and low specificity; the distance of the thyrohyoid membrane to the skin showed moderate sensitivity and specificity (Table 4). This result is consistent with the relationship between difficult mask ventilation and difficult laryngoscopy in the increase in soft tissue thickness at the level of the thyroid cartilage and thyrohyoid membrane in the study of Allesandri et al[20] and Wu et al.[23] This is due to the increase in the concavity of the secondary curve in the Greenland anatomical model,[18] again like the soft tissue thickness at the level of the vocal cord.

In our study, there was no relationship between the distances of the cricoid cartilage and cricothyroid membrane to the skin and the difficult airway. This expected result is due to the small effect of these measurements on the secondary curves in the Greenland anatomical model.[18] Likewise, no correlation was found between the length of the cricothyroid membrane and the thyrohyoid membrane, and the difficult airway. These measurements were not included in other studies on difficult airways.

According to the results of our study, a distance between the epiglottis midline and skin above 20.7 mm can predict difficulty in mask ventilation, above 20 mm in direct laryngoscopy, and above 19.6 mm in intubation. In general, an epiglottis–skin distance above 19.9 mm can predict airway difficulty. In the study of Falcetta et al[16] the cutoff of the epiglottis–skin distance for difficult laryngoscopy was determined as 25.4 mm. In our study, the vocal cord anterior commissure–skin distance was above 7.5 mm in difficult mask ventilation, above 7.8 mm in difficult direct laryngoscopy, and above 6.5 mm in difficult intubation. In general, vocal cord anterior commissure–skin distance above 7.3 mm can predict airway difficulty. In the study of Allesandri et al[20] the mean was 8.1 mm in the group with difficult laryngoscopy, it was 7.5 mm in the easy laryngoscopy group; 8.3 mm in the difficult mask ventilation group, and 7.6 mm in the easy mask ventilation group.

In our study, difficult laryngoscopy was recorded in 26 (21.6%) patients. Intubation with a Macintosh blade was unsuccessful at a rate of 10.8% and intubation was achieved with a video laryngoscope. Obtaining an adequate laryngoscopic view is essential for successful intubation using the direct laryngoscope, but difficult laryngoscopy and difficult intubation are not synonymous. Successful intubation also depends on the practitioner’s expertise and the patient’s characteristics and circumstances Asai et al[25] found that only 1 of 3 Cormack-Lehane class 4 patients was difficult to intubate; one is easy, and one is moderately difficult to intubate; Cormack-Lehane showed that out of 68 patients with class 3, intubation was difficult for 5, moderate for 50, and easy for 13. Therefore, “limited appearance” in direct laryngoscopy can become a critical factor in some situations, such as in an emergency, when a double lumen tube has to be inserted, or if the laryngoscopy performer is inexperienced.

In our study, predicted and unpredictable difficult airways were evaluated together, and patients with test findings suggestive of difficult airways in preoperative difficult airway screening tests were also included. The mean BMI of the patient population in the study was 27.2 ± 5.1 and a significant difference was found between the easy and difficult airway patient groups in terms of BMI (P < .05). We think that this may be the reason why we found a difficult intubation rate of 13.3% and a difficult airway rate of 23.3% in the patient population in our study, whereas these are 1% to 18% and 1.5% to 13.5% in the literature.[2,8] This constitutes a limitation of our study.

Supraglottic airway equipment and extubation are not included in the definition of difficult airways in the current ASA guideline during our study.[26] For this reason, difficult supraglottic airway device placement and extubation were not evaluated in our study. In the updated ASA 2022 guideline, the placement of supraglottic airway devices and extubation have also been added to the definition of the difficult airway.[3]

Our data analyses support those ultrasound measurements at these levels can be obtained in <2 minutes and support the use of these measurements not only in the normal clinical setting, but also in the uncooperative patient where standard clinical screening tests cannot be easily performed, or in critical environments where the head must be kept in a neutral position for cervical spine protection. However, further studies are required to test the predictive ability of ultrasound for airway management in an emergency setting.

5. Conclusion

In our study, it was found that the increase in the distance of the epiglottis midline at the level of the thyrohyoid membrane, the anterior commissure of the vocal cord, the hyoid bone, the thyroid cartilage at the level of the isthmus, and the thyrohyoid membrane to the skin correlated with the difficult airway. There was no correlation between the other measurements (cricoid bone–skin distance, trachea–skin distance, thyrohyoid membrane length, cricothyroid membrane length, and skin distance) and difficult airway.

Ultrasound is a high-frequency sound wave, radiation-free, bedside, inexpensive, fast, and easy to apply. The use of ultrasonography in airway evaluation is very recent. Measurements have not yet been standardized. Therefore, new studies are needed. According to our results, we believe that ultrasonographic evaluation can be used in the prediction of predictable and unpredictable difficult airways.

Author contributions

Conceptualization: Elif Özge Çinar Köse.

Data curation: Elif Özge Çinar Köse, İsa Çam.

Methodology: Murat Tekin.

Project administration: Elif Özge Çinar Köse, Murat Tekin.

Resources: Elif Özge Çinar Köse.

Supervision: Murat Tekin.

Validation: Murat Tekin, Sevim Cesur.

Visualization: Elif Özge Çinar Köse.

Writing – original draft: Elif Özge Çinar Köse.

Writing – review & editing: Murat Tekin, Sevim Cesur, Zehra Nur Baykara.

Supplementary Material

Abbreviations:

ASA American Society of Anesthesiologists

AUC area under the curve

BMI body mass index

ROC receiver operating characteristics.

Written informed consent was obtained from the parents.

This study was performed in line with the principles of the Declaration of Helsinki. Approval was granted by the Ethics Committee of Kocaeli University Faculty of Medicine (KOU KAEK/15.bI.03-02.09.2021).

ClinicalTrials.gov ID and date: NCT05347121/21.04.2022.

The authors have no funding and conflicts of interest to disclose.

The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.

Supplemental Digital Content is available for this article.

This manuscript was previously posted to Research square: doi: https://doi.org/10.21203/rs.3.rs-3088145/v1.

How to cite this article: Çinar Köse EÖ, Tekin M, Cesur S, Çam İ, Baykara ZN. Evaluation of neck ultrasound measurements as a difficult airway predictor: A prospective observational study. Medicine 2024;103:29(e38591).
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