: 2019  |  Volume : 2  |  Issue : 2  |  Page : 71--76

Correlation between ultrasonographic evaluation of the airway and Cormack-Lehane view by direct laryngoscopy in the Indian subpopulation

Gagan Kumar Narula1, Abdul Nasser2,  
1 Department of Anaesthesiology, Base Hospital, Delhi Cantt., New Delhi, India
2 Department of Anaesthesiology, INHS Jeevanti, Vasco-da-Gama, Goa, India

Correspondence Address:
Dr. Abdul Nasser
Department of Anaesthesiology, INHS Jeevanti, Vasco-da-Gama - 403 802, Goa


Background: An important aspect of airway management is assessment of the airway to predict difficult intubation. Ultrasonography has recently emerged as a noninvasive tool that is helpful for preoperative airway assessment and management. Patients and Methods: In this study, four sonographic measurements, namely (a) distance of the pre-epiglottic space (PES), (b) distance from epiglottis to the vestibular ligaments' midpoint (EVL), (c) skin to hyoid bone distance (DSHB) and (d) distance from the skin to epiglottis midway between hyoid bone and thyroid cartilage (DSEM) were correlated with the Cormack-Lehane grade obtained by direct laryngoscopy. Results: Of the 77 patients included for data analysis, 19 patients (24% of total) had difficult intubation. Of those with difficult intubation, 63% were female. Pearson's analysis revealed a correlation of Cormack-Lehane grade as weak negative with PES, weak positive with EVL, very weak positive with DSHB and very weak negative with DSEM. To predict difficult airway, the cutoff values for PES, EVL, DSHB and DSEM as determined by the Youden index were found to be 2.8 mm (sensitivity 21.1% and specificity 84.5%), 9.3 mm (sensitivity 36.8% and specificity 86.2%), 12.7 mm (sensitivity 42.1% and specificity 79.3%) and 6.8 mm (sensitivity 21.1% and specificity 87.9%), respectively. The area under the curves of PES, EVL, DSHB and DSEM were all below 0.6. Conclusion: PES, EVL, DSHB and DSEM are poor in predicting difficult intubation. These parameters can, however, be used to rule out difficult intubation because they are all highly specific.

How to cite this article:
Narula GK, Nasser A. Correlation between ultrasonographic evaluation of the airway and Cormack-Lehane view by direct laryngoscopy in the Indian subpopulation.Airway 2019;2:71-76

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Narula GK, Nasser A. Correlation between ultrasonographic evaluation of the airway and Cormack-Lehane view by direct laryngoscopy in the Indian subpopulation. Airway [serial online] 2019 [cited 2020 Aug 14 ];2:71-76
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Full Text


An important aspect of airway management in clinical anaesthesia is assessment of the airway to predict the likelihood of ease or difficulty with bag-mask ventilation, laryngoscopy and intubation. Prediction of the difficult airway enables the anaesthesiologist to prepare for this challenging clinical scenario. However, despite careful airway assessment, direct laryngoscopy sometimes results in unanticipated poor laryngeal views, leading to morbidity and mortality due to failure in airway management.

In the Indian population, the incidence of difficult laryngoscopy and intubation is 9.7% and 4.5%, respectively.[1] Various bedside screening tests used for predicting difficult airway have poor-to-moderate sensitivity.[2] Few studies worldwide have been conducted to assess the efficacy of ultrasonographic measurements of the soft tissue in front of the neck and the airway in predicting difficult laryngoscopy.[3],[4],[5],[6],[7]

The purpose of this study was to determine the correlation between Cormack-Lehane grade of direct laryngoscopy with the son ographic measurements of: (a) distance of the pre-epiglottic space (PES), (b) the distance from the epiglottis to the midpoint of the vestibular ligaments (EVL), (c) skin to hyoid bone distance (DSHB) and (d) the distance from the skin to epiglottis midway between hyoid bone and thyroid cartilage (DSEM); and whether based on the correlation, difficult intubation can be predicted.

Study design

This study was conducted in the preoperative holding area and operation theatre complex of a tertiary care Armed Forces Hospital. This was a prospective observational study approved by the Hospital Ethics Committee. Informed consent was obtained from all patients who participated in this study.

 Patients and Methods

This study included patients in American Society of Anesthesiologists Physical Status I-III belonging to either gender, aged 18-60 years and undergoing elective surgery requiring general anaesthesia with direct laryngoscopy and endotracheal intubation. Patients with prior facial, cervical, pharyngeal and epiglottic surgery, or with a history of trauma and difficult intubation, were not included in this study. Patients with head-and-neck pathologies, those with mouth opening <3 cm, those with arthritis or diabetes mellitus of ≥10 years standing and those who were unable to extend their neck by ≥30° were also excluded from the study. All edentulous patients, those with a tracheostomy tube in situ and those whose body mass index was >30 kg/m2 were also excluded.

Based on available data, it was anticipated that approximately 10% of patients would have difficult laryngoscopy.[1] With an estimated 5% risk, minimum power of 80% and at 95% confidence level, using the CDC software tool Epi Info 7 [developed by Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia (USA)], a sample size of 58 was calculated.

A high-frequency linear probe (Probe HFL38x, 13-6 MHz) of Sonosite M-turbo Ultrasound System with a scan depth of 6 cm was used. In the preoperative room, the ultrasound view of the airway of all the study patients was obtained. The patients were asked to lie down supine with active maximal head-tilt/chin-lift [Figure 1]. The probe was placed in the submandibular area in the midline. Without changing the position of the probe, the linear array of the ultrasound probe was rotated in the transverse planes from cephalad (coronal) to caudal (oblique transverse). The following study measurements were obtained with the oblique-transverse view of the airway: distance of the PES and the distance from the epiglottis to the midpoint of the vestibular ligaments (EVL) [Figure 2].{Figure 1}{Figure 2}

With the patient in supine position and the head and neck in neutral position [Figure 3], the thickness of the anterior neck soft tissue was obtained with the transverse view at the level of the hyoid bone. The DSHB was measured as shown in [Figure 4]. With the neck still in the neutral position, the probe was moved to the level of the thyrohyoid membrane [Figure 5]. DSEM was measured as shown in [Figure 6].{Figure 3}{Figure 4}{Figure 5}{Figure 6}

In addition, the Mallampati class, interincisor gap and thyromental distance were also recorded. The patients were subsequently taken to the operating room, and anaesthesia was induced using propofol 2 mg/kg, fentanyl 2 μg/kg and vecuronium 0.1 mg/kg. Endotracheal intubation was performed by an anaesthesiologist with at least 2 years of experience in performing tracheal intubation. Macintosh blade size 3 and size 4 were used for female and male patients, respectively. During the intraoperative direct laryngoscopy, the Cormack-Lehane grading was recorded as noted below:

Grade I - Visualisation of the entire laryngeal apertureGrade II - Visualisation of parts of the laryngeal aperture or the arytenoidsGrade III - Visualisation of only the epiglottisGrade IV - Visualisation of only the soft palate.

Cormack-Lehane Grades I and II were recorded as easy intubation and Grades III and IV as difficult intubations. The first view of the glottis was recorded and used to grade Cormack-Lehane grade. Number of attempts at intubation, external laryngeal pressure and use of any airway adjuncts were also recorded. If three or more attempts were required for successful intubation, they were included in difficult intubation group regardless of the Cormack-Lehane grade.

A Pearson correlation was computed to assess the relationship between the four sonographic measurements (PES, EVL, DSHB and DSEM) and the Cormack-Lehane grade. Alpha value of <0.05 was considered significant. Finally, receiver operating characteristic (ROC) curves were drawn to determine the cut-off value of ultrasound parameters using the Youden index for the prediction of difficult intubation.


A total of 97 patients were enrolled in this study. The first 20 patients were excluded from the final analysis as these were considered to be on the ultrasonic assessment learning curve of the investigators. The subsequent 77 patients were included for data analysis. The baseline demographic characteristics of the patients are listed in [Table 1].{Table 1}

Out of the 19 patients with difficult intubation (24% of total), 63% were female. Laryngoscopy Grades I, II, III and IV were obtained in 27, 32, 17 and 1 patient, respectively. External pressure during intubation to improve the glottic view was required for three patients with easy intubation and for 15 patients with difficult intubation. One patient with Cormack-Lehane Grade I required the use of bougie and three attempts for successful intubation. For six patients, an airway adjunct in the form of bougie or stylet was used to achieve successful endotracheal intubation. Only one patient had a Cormack-Lehane Grade IV.

A Pearson correlation was computed to assess the relationship between the ultrasonographic measurements and the Cormack-Lehane grade. The mean values of the four ultrasonographic parameters of PES, EVL, DSHB and DSEM stratified by level of intubation and gender are shown in [Table 2]. Correlation against Cormack-Lehane grade showed a weak negative correlation with PES (r = -0.225; P = 0.05); a weak positive correlation with EVL (r = 0.232; P = 0.04); a very weak positive correlation with DSHB (r = 0.163; P = 0.156) and a very weak negative correlation with DSEM (r = -0.157; P = 0.174).{Table 2}

PES, EVL, DSHB and DSEM were further assessed by drawing the ROC curves, with the Cormack-Lehane Grades III or IV being considered as a difficult airway. The cut-off values for PES, EVL, DSHB and DSEM to predict difficult airway as determined by the Youden index were 2.8 mm (sensitivity 21.1% and specificity 84.5%), 9.3 mm (sensitivity 36.8% and specificity 86.2%), 12.7 mm (sensitivity 42.1% and specificity 79.3%) and 6.8 cm (sensitivity 21.1% and specificity 87.9%) respectively.

No significant difference from the area under the reference line (area = 0.5) was seen with all the four measurements of PES, EVL, DSHB and DSEM. [Table 3]{Table 3}


Preoperative assessment for predictors of difficult intubation continues to generate interest amongst anaesthesiologists, and is likely to do so in the future as well. The methods available which include (but are not limited to) modified Mallampati classification, thyromental distance, neck extension and interincisor distance have low sensitivity and specificity and are associated with considerable observer bias. Apart from these noninvasive methods that are available, the experience of the anaesthesiologist will always play a significant role in the prediction of difficult intubation, contributing to the predictive value of these tests.

Ultrasonography of the airway may be combined with other parameters for airway assessment because no single parameter has been shown to be conclusively accurate until now. The use of noninvasively obtained visuals of the airway using ultrasonography can be recommended as a supplementary tool to the busy anaesthesiologists' armamentarium. Fulkerson et al. who did an extensive review of studies that evaluated the utility of ultrasound in the preoperative setting found only one retrospective and nine prospective observational studies. In all these studies, airway locations scanned were variable using different protocols and patient positioning. The outcome variable for comparison was uniformly the Cormack-Lehane grade. Significance for sonographic prediction of difficult laryngoscopy occurred at the following three locations: hyomental distance (P < 0.01), anterior tissue at the hyoid bone (P < 0.0001) and the thyrohyoid membrane (P < 0.0001). The vocal cords and sternal notch levels had conflicting significance. Limitations in these studies included lack of standard scanning protocols.[8] However, ultrasonography is a portable, easy-to-use tool that is freely available in the present scenario.[9],[10],[11],[12]

In the present study, we have attempted to demonstrate the feasibility of using ultrasonography in predicting difficult intubation in the Indian subpopulation. We found that the correlation between the ultrasonographic parameters and Cormack-Lehane grade were weak or very weak, and can be assumed to have limited clinical significance. The area under the curves of PES, EVL, DSHB and DSEM were all below 0.6, suggesting that they are poor parameters in predicting difficult intubation. However, all the parameters were highly specific, suggesting that they can all be used to rule out difficult intubation.


The measurements in the Indian subpopulation may not be very sensitive in predicting difficult intubation, unlike what is seen in studies conducted in Caucasian, African-American and Middle Eastern populations.[3],[4] Our small study population may have contributed to this finding. The cut-off values of all the parameters cannot be validated because only 19 patients were in the difficult intubation group and only one of them had a Cormack-Lehane Grade IV.

In this study, variables such as dentition of the patients, experience of the anaesthesia providers, laryngoscopy equipment used, number of intubation attempts, external laryngeal pressure, use of adjunct airway equipment and view of the glottis (first view, best view or view immediately before passing the endotracheal tube) were controlled. Our small study population of 77 patients may limit the conclusions that can be reached and may be the reason for low sensitivity. The easy and difficult laryngoscopy groups were uneven, with only one patient presenting with Cormack-Lehane Grade IV. The investigators were not blinded to the study hypothesis, which could have introduced some observer bias.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patients have given their consent for their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.


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