Introduction

Examination of  the airway is an essential component of the anesthesiologist ‘s pre-operative assessment. It enables to predict the ease of visualizing the glottis and to perform intubation.⁽¹⁾. The preintubation clinical screening tests (mallampatti classification, interincisorgap,thyromental distance, testing neck mobility)to assess for difficult laryngoscopy have poor to moderate sensitivity⁽⁴⁾. Despite the availability of multiple airway assessment methods, unexpected difficult intubations occur with a frequency of 1-8%⁽²⁾.

The Cormack lehane classification is frequently used to describe the best view of the larynx seen during laryngoscopy. Although it is one of the most used classification, one major drawback is that it cannot be applied for predicting difficult tracheal intubation in patients undergoing intubation for the first time.⁽³⁾

Ultrasound imaging is a safe, simple, painless and non invasive modality through which soft tissues can be visualized and identified.⁽³⁾

There is limited literature available that compares the ultrasound parameters of the airway to the CL grade and physical parameters. This study will be undertaken to compare the utility of the  physical with ultrasound parametersof the airway in  predicting difficult airway.

Review of literature

1.     An observational study was done by Preethi et al in 2016 on various parameters measured by ultrasonography of neck: anterior neck soft tissue thickness at the level of the hyoid (ANS-Hyoid), anterior neck soft tissue thickness at the level of the vocal cords (ANS-VC) and ratio of the depth of the pre-epiglottic space (Pre-E) to the  distance from the epiglottis to the mid-point of the distance between the vocal cords (E-VC)in  comparison with Mallampati (MP) class, thyromental distance (TMD) and sternomental distance (SMD)   in 100 patients undergoing general endotracheal anaesthesia to predict CL grade  during intubation.  The incidence of difficult intubation was 14%. An ANS-VC >0.23 cm had a sensitivity of 85.7% in predicting a CL Grade of 3 or 4, which was higher than that of MP class, TMD and SMD. However, the specificity, PPV and accuracy were lower than the physical parameters. The NPV was comparable. They concluded that ultrasound is a useful tool in airway assessment. ANS‑VC >0.23 cm is a potential predictor of difficult intubation. ANS‑Hyoid is not indicative of difficult intubation. The ratio Pre‑E/E‑VC has a low to moderate predictive value.

2.     C M hui et al hypothesised that sublingual ultrasound provides additional information to predict a difficult airway with greater success than traditional methods. They recruited 110 patients to perform sublingual ultrasound on themselves following brief instructions in 2014. Ability to view the hyoid bone on sublingual ultrasound, mouth opening distance, thyromental distance, neck mobility, size of mandible and modified Mallampati classification were recorded and assessed for ability to predict a difficult intubation based on the grade of laryngoscope. Visibility of the hyoid using ultrasound was associated with a laryngoscopic grade of 1–2 (p< 0.0001), and (p < 0.0001) had a positive likelihood ratio of 21.6 and a negative likelihood ratio of 0.28. Each of the other methods had considerably lower positive likelihood ratios and lower sensitivity. Their results suggest that sublingual ultrasound is a potential tool for predicting a difficult airway in addition to conventional methods.

3.     An observational study was done on 72 ASA I -III patients undergoing general anaesthesia requiring endotracheal intubation by Gupta et al  in 2012 who obtained the following measurements  with the oblique – transverse view  of the ultrasound : (a)the distance from the epiglottis to midpoint of the distance between the vocal folds,(b)the depth of the pre-epiglottic space, (c) the total time taken by the operator to achieve the ultrasound images. The data was then compared with the Cormack – Lehane classification on direct laryngoscopy and the ultrasound modification of the CL grading was developed .They found that the prediction of CL grades can be adequately made by the ratio of Pre -E and E-VC distances. The average time to complete the ultrasound examination of the airway was 31.7+_12.4 seconds.

4.     A observational study on 51 patients undergoing elective surgery was done by  Srikar Adhikari et al in 2011. The ultrasound (US) measurements of tongue and anterior neck soft tissue were obtained. The laryngoscopic view was graded using Cormack and Lehane classification by anesthesia providers on the day of surgery. The sonographic measurements of anterior neck soft tissue were greater in the difficult laryngoscopy group compared to the easy laryngoscopy group at the level of the hyoid bone and thyrohyoid membrane . No significant correlation was found between sonographic measurements and clinical screening tests.  They concluded  that sonographic measurements of anterior neck soft tissue thickness at the level of hyoid bone and thyrohyoid membrane can be used to distinguish difficult and easy laryngoscopies. Clinical screening tests did not correlate with US measurements, and US was able to detect difficult laryngoscopy, indicating the limitations of the conventional screening tests for predicting difficult laryngoscopy.

5.     A randomised study was done on 50 morbidly obese patients undergoing bariatric surgery by T. Ezri et al in 2003. They   quantified the neck soft tissue from skin to anterior aspect of trachea at the vocal cords using ultrasound . Thyromental distance less than 6cm,mouth opening less than 4cm,limited neck mobility,mallampatti score more than 2, abnormal upper teeth, neck circumference >45 cm, and sleep apnoea were considered predictors of difficult laryngoscopy. Of the nine (18%) difficult laryngoscopy cases, seven had obstructive sleep apnoea history; whereas, only 2 of the 41 easy laryngoscopy patients did have. Difficult laryngoscopy patients had larger neck circumference [50 (3.8) vs. 43.5 (2.2) cm; and more pre-tracheal soft tissue. . Soft tissue values completely separated difficult and easy laryngoscopies. None of the other predictors correlated with difficult laryngoscopy. Thus, an abundance of pretracheal soft tissue at the level of vocal cords is a good predictor of difficult laryngoscopy in obese patients.⁽⁵⁾

Though of late, ultrasound is used as a new modality for airway assessment, there are very few studies with large patient numberdescribing the utility of ultrasound scan of the airway routinely. Hence we would like to do the study.

Justification and need for the study

Ours is a trauma centre, a tertiary referral teaching hospital and we deal with lot of  patientswith different build and sizes who can present with unanticipated difficult airways. Since the e presently available physical parameters have high interobserver variability, an objective measurement of the airway using a non invasive simple bedside instrument would help in being prepared for any unanticipated difficult scenarios and reduce morbidity and mortality considerably.

Outcome measure

a.    Anterior soft tissue thickness at the level of the hyoid  -ANS – hyoid

b.    Anterior soft tissue thickness at the level of the vocal cords – ANS VC

c.     Ratio of depth of the preepiglottic space (Pre-E )to the distance from the epiglottis to the mid point of the distance between the vocal cords.(E- VC)

d.    The total time taken by the operator to achieve the final ultrasonic image

The above parameters will be measured and compared with the physical parameters such as

a.  malampatti class,

b. thyromental distance

c. sternomental distance

d.  neck circumference

 The incidence of difficult intubation will be identified by direct laryngoscopy by an experienced anesthesiologist (>5yrs )and graded according to the Cormack lehane grading. Both physical and ultrasound parameters will be compared with incidence of difficult intubations and plotted.