European Journal of Cardiovascular Medicine

Airway assessment of patients for pre-operative laryngoscopy and intubation is very essential for detecting difficult airways. Several airway assessment tests are used to predict difficult airways. These include Mallampati score, Wilson`s score, Rocke`s risk probability, Arne`s simplified model and the Lemon score^1.

 Obesity is an important cause of difficult airways. The incidence of unanticipated difficult airways is 14.3-17.5% in obese patients as compared to 5.8% in the general population^2. . Conventional airway assessment tools have shown moderate sensitivity and inter-observer variability, limiting their reliability especially in obese patients³ . They are also difficult to perform in emergency and critical care settings.

Plain radiography can play a part in airway assessment. A reduction in space (<5mm) between the occiput and C1 spinous process is an indicator of difficult airways^4. In cases with soft tissue neck tumours and bone deformities and bone fractures, CT plays an essential role in assessment of difficult airways.

Ultrasonography has impacted anaesthesiology in many aspects especially in vessel localisation and regional blocks. Recently, there has been a rising interest in its use in difficult airway. USG is used to measure soft tissue thickness in anterior neck which is used to assess the extent of difficult airways⁴ . Studies suggest that increased soft tissue thickness may impede the displacement of Pharyngeal structures⁵ . It is suggested that a larger pre-tracheal soft tissue reduces the mobility of the pharyngeal soft tissue which can be measured by USG thereby helping in the prediction of difficult airways⁶.

Our study is based on the assessment of the accuracy of pre-epiglottic soft tissue thickness on USG to predict difficult airways.

We conducted our study in Government Medical College Nanded after approval from ethical committee.  Our study was a prospective observational study where the doctor performing a particular procedure was blinded about other parameters. A total of 180 patients were included in our study.

Inclusion Criteria:

Patients aged between  18-70 years of either gender.

willing to give consent.

patients posted for elective surgery requiring general anaesthesia.

Exclusion Criteria:

Patients with maxillofacial fractures and neck instability .

 Patients who required emergency and rapid intubation.

 Patients refusal to participate.

Patients were pre-operatively assessed for intubation by performing BMI, ASI score and Mallampati score. Patients age and sex was also documented along with other parameters. Written consent for the study was done.

Point of care USG was done before surgery to assess anterior neck soft tissue thickness. Patient was positioned in neutral, supine position and anterior neck soft tissue thickness was measured. We used two parameters : Skin to epiglottis thickness (DSE) and skin to vocal cord distance (DSVC). 

Patients were taken for surgery after clearance from anaesthesia department and Anaesthetist conducting the procedure was kept unaware of the pre-operative findings on USG.

On the day of surgery, patients were kept  nil per mouth for 8 hrs for heavy solid meals, 6 hrs for semisolids and 2 hrs for plain water. On receiving patients in pre-operative area, baseline parameters (heart rate , BP, spo2 and ECG were recorded. Intravenous line was secured. Anaesthetic agents were given as per protocol. Direct laryngoscopy was performed using a Mc Intosh blade. Laryngoscopy findings were documented using Cormac Lehane (CL) grading system. Trachea was intubated using a stylated endotracheal tube . CL Grade I is full visualisation of glottis with CL Grade 2 as partial visualisation of glottis. CL Grade 3 is partial visualisation of epiglottis and CL Grade 4 is non-visualisation of glottis or epiglottis. CL Grade 1 and 2 were considered as easy laryngoscopy and CL Grade 3 and 4 were considered as difficult laryngoscopy. Laryngoscopy findings were correlated with BMI, Mallampati scoring system and anterior neck soft tissue thickness at the level of epiglottis and glottis

A total of 180 patients were included in the study. 102 were females and 78 were males. BMI of 64 patients was less than 25 .36 patients had BMI between 25-30 and 52 patients had BMI between 30-35. 28 patients had BMI between 35-40.

On airway examination, 50 patients had Mallampati score of 1. Mallampati score of 2, 3 and 4 was found in 68, 54 and 8 patients respectively.  ASI Grade I was seen in 72 patients, Grade II in 84 patients, Grade III in 24 patients.

There is a total of 58 patients in CL Grade 1, 48 patients in CL Grade II, 60 patients in CL Grade III and 14 patients in CL Grade IV. The distribution of CL Grade was compared with distance from skin to epiglottis (DSE) and distance from skin to vocal cord (DSVC).

TABLE 1

TABLE 2

TABLE 3

From table 3, True positives were 39, True negatives were 86, False positives were 23 and false negatives 35 with sensitivity and specificity of Mallampati score in detecting difficult airways of 52.7% and 78.9% respectively.                                                                             

TABLE 4

TABLE 5

 We also calculated sensitivity, specificity, PPV and NPV for above parameters. We kept DSVC of 0.75 as cut-off for difficult laryngoscopy and DSE of 1.75 as cut-off for difficult laryngoscopy. Findings were tabulated in table 5 and 6 as below.

                                                                 TABLE 6

TABLE 7

We conducted our prospective study over a period of 6 months in Government Medical College Nanded. There were  total of 180 patients in our study: 102 were females and 78 were males.

We fond BMI moderately sensitive in predicting difficult airways; 61.5% of patients with BMI from 30-35 had difficult airways and 78.6% patients with BMI greater than 35 had difficult airways. Significant association with BMI was found in a study conducted by Saumya Jain et al where 83.33% patients with BMI > 46.9 had difficult airways(2,7)

Mallampati score had moderate sensitivity in predicting difficult airways in our study with sensitivity of 52% and specificity of 79%. Our findings were comparable to a study conducted by Arun Ahirwar et al where sensitivity and specificity of Mallampati score was 50% and 80% respectively(8,9)

Our study was primarily based on USG measurements of anterior neck soft tissue thickness in predicting difficult airways. We considered CL grade 1 and CL grade 2  as easy airways and CL grade 3 and CL grade 4 as difficult airways. Ultrasound has gained significant importance in anaesthesia ranging from vascular access and nerve block to predicting difficult airways and size and position of endotracheal tubes. High frequency probe is used to assess soft tissue thickness at the level of epiglottis (DSE) and at the level of vocal cords (DSVC). We found that CL3 and CL 4 patients have increased DSE with average from 1.85-1.90mm as compared to CL 1 and CL 2 (Mean DSE ranging from 1.4-1.45mm) with P value less than 0.001. we assumed DSE of 1.75mm at lower limit for difficult airways and found that  DSE  cut-off of 1.75 in predicting difficult airways was excellent with sensitivity and specificity of 89% and 85.8% respectively. PPV and NPV were 81.5% and 91.9% respectively. Our findings were supported by studies conducted by Suran Mohammad et al and Gupta et al.  Gupta et al found sensitivity of 100% and specificity of 95% using DSE of 1.64 as cut-off measurement.(10,11)

Contrary results were found by a study conducted by Alessandri et al with accuracy of 64.4% in 194 patients undergoing ENT surgeries but the study was limited by taking only ENT patients and cutoff values were not reported(12).

Our study found DSVC measurements as accurate method in predicting difficult airways. In our study, DSVC of ≥ 0.75 had sensitivity, specificity, PPV and NPV of about 90.7%, 86.8%, 82.9% and 93.9% respectively in predicting difficult airways. Our findings were supported by a study conducted by   Himandarshi et al who conducted a study on 155 patients  and found that increased DVSC  had a sensitivity of 95.1% and specificity of 85.7% (13). Allesandri et al found that a cutoff of DSVC of 0.75cm which is comparable to our study (14).

We found statistically significant relationship between increased anterior neck soft tissue thickness on high resolution USG and difficult airways which was in accordance with majority of studies. However, this is no single cut-off measurement parameter which can be used universally in routine pre-procedural assessment of difficult airways. Also, the procedure of USG assessment has n been universally standardised and ethnic and other factors haven’t been extensively studied (15,16).

Well-defined and conducted studies are needed with standardisation of USG protocols with univocal and common definitions of difficult airways to discriminate between easy and difficult laryngoscopy. Current guidelines don’t support the use of USG of anterior neck  soft tissues as a single predictive tool to detect difficult airways and hence integration of USG with other routine used tests will help in better airway assessment(17,18). USG assessment of airways may help in detecting difficult airways when other clinical tests are equivocal.

Our study is limited by small sample size and lack of heterogeneity of study group. Also, emergency patients were not studied in our group.

Ultrasound assessment of anterior neck soft tissue thickness has shown excellent results in predicting difficult airways and it is gaining widespread usage for assessment of upper airways. Ultrasound has proven superior to clinical methods in predicting difficult airways. USG measurement of DSE and DSVC have shown excellent sensitivity and specificity in predicting difficult airways. USG is simple, reliable and non-invasive bedside procedure. Extensive studies are needed to set up protocol parameters and proper techniques of USG assessment of airways, so that the USG assessment becomes a valuable tool in airway management.

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