European Journal of Cardiovascular Medicine

Airway management is of prime importance to the anaesthesiologist. For securing airway, tracheal intubation using direct laryngoscopy remains the method of choice in majority of the cases. No anaesthetic drug is safe unless diligent efforts are made to secure and maintain an intact airway. The reported incidence of difficult laryngoscopy and tracheal intubation occurs in 1.5% to 8% of patients in general anaesthesia¹. Difficult laryngoscopy and intubation cause increased risk of complications to the patient ranging from sore throat to airway trauma. In some cases, inability to maintain a patent airway may lead to serious complications like hypoxic brain damage or death. Of all the deaths occurring in operating room that are anaesthesia related ,30% to 40% are attributed to the inability to manage a difficult airway². Of the overall analysis in closed claims project for adverse respiratory events, 17% were attributed to difficult or impossible intubation³.

Although prediction and forecasting are tenacious, in light of the complications, considerable attention has been given to predict difficult intubation in patients. Many preoperative airway assessment tests such as inter-incisor gap (IIG), head and neck movements (HNM), horizontal length of mandible (HLM), Modified Mallampati test (MMT), Sternomental distance (SMD), Thyromental distance (TMD) and Ratio of Height to Thyromental distance (RHTMD) may be used as anatomical parameters to predict difficult intubations but sensitivity and positive predictive value for these individual tests are low while false positive results are high⁴

So, predicting a difficult intubation employing a myriad of measurements and observations has not demonstrated itself to be practical approach or even reliable in clinical practise. Several studies have evaluated the predictive criteria individually or in arbitrary combinations, there has been no sufficiently powered systematic multivariate analysis of the clinical variables like upper lip bite test (ULBT), ratio of height to thyromental distance (RHTMD), Horizontal length of the mandible (HLM) Head and neck movements (HNM), Sternomental distance (SMD) and Thyromental distance (TMD) studied simultaneously. Hence this study was undertaken to compare the various airway assessment predictors for difficult endotracheal intubation.

This Prospective observational study was conducted among patients undergoing elective surgery under general anaesthesia in Department of Anaesthesiology, Bangalore Medical College and research institute. Duration of study was  November 2018 to March 2020

Inclusion Criteria:

Patients who gave informed written consent

Male and female patients aged 18-65 years

Patients scheduled for elective surgeries under general anesthesia

Patients under ASA grade 1 and 2.

Exclusion Criteria:

Uncooperative and unwilling patients

History of burns/trauma involving head and neck

History of airway surgeries

Tumor/mass in the neck or airway

Restricted mobility at neck and mandible

Patients with inability to sit

Edentulous

Patients in the need of awake intubation

Pregnant females

BMI >35

In the study majority, 143 subjects (35.75 %) belonged to the age group 31 to 40 years followed by 90 subjects (22.5 %) in the age group of 21 to 30 years. 73 subjects (18.25 %) belonged to the age group of 41-50 years, 56 subjects (14 %) belonged to the age group of 51-60 years, 28 subjects (7 %) belonged to the age group of 10-20 years, and 10 subjects (2.5 %) belonged to >60 years of age.

In this study, 217 subjects (54.25%) were males and 183 subjects (45.75%) were females.

In the study , 210 subjects (52.5%) belong to ASA grade 1 and 190 subjects (47.5%) belong to ASA grade 2.

155 subjects (38.75%) had normal range of BMI in the range of 18.5-24.9 and 245 subjects (61.25%) had BMI in the range of 25-30 and were overweight (pre obese). We did not encounter any patients with class 1 obesity of BMI 30.0-34.9. We did not include patients >35 BMI falling in class 2 and 3 obese categories in the study.

Thus, in our study there is no role of and BMI in prediction of difficult airway.

Table 1: Investigations distribution of patients studied

Upper Lip Bite Test-ULBT.

Head and Neck Movements-HNM. Horizontal Length of Mandible-HLM.

Based on ULBT, 308 subjects (77%) belong to Grade I, 84 subjects (21%) belong to Grade II and 8 subjects (2%) belong to Grade III.

Based on HNM, 393 subjects (98.25%) belong to Grade I and 7 subjects (1.75%) belong to Grade II.

Based on HLM, 384 subjects (96%) belong to Grade I and 16 subjects (4%) belong to Grade

Sternomental Distance-SMD Thyromental Distance-TMD

Ratio of Height to Thyromental Distance-RHTMD

Based on SMD, 346 subjects (86.5%) belong to Grade I and 54 subjects (13.5%) belong to Grade II.

Based on TMD, 373 subjects (93.25%) belong to Grade I and 27 subjects (6.75%) belong to Grade II.

Based on RHTMD, 316 subjects (79%) belong to Grade I and 84 subjects (21%) belong to Grade II.

Table 2: Cormack Lehane Score

Based on Cormack Lehane scoring, 236 subjects (59%) belong to Score 1, 92 subjects (23%)

belong to Score 2, 64 subjects (16%) belong to Score 3 and 8 subjects (2%) belong to Score 4.

 Table 3: ULBT/HNM/HLM/SMD/TMD/RH TMD of patients studied in relation to CL score

There was significant association between HLM and CL grade, SMD and CL grade, TMD and CL grade

 and RHTMD and CL grade. There was no significant association between ULBT and CL grade, and HNM CLgrade

Table 4: Validity of Measurements in predicting difficult intubation (CL grade gold standard)

In the study out of 72 subjects with difficult grade in CL: 27 subjects (37.5%) had difficult TMD grading 54 subjects (75%) had difficult SMD grading 2 subjects (2.8%) had difficult ULBT grading 4 subjects (5.6%) had difficult HLM grading 3 subjects (4.2%) had difficult HNM grading

65 subjects (90.3%) had difficult RHTMD grading.There was significant association between CL grade and MMT, IIG, TMD, SMD, RHTMD scoring

Table 5: Validity of TMD grade in predicting difficult intubation compared to CL Grading

TMD had sensitivity of 37.5%, Specificity of 100%, PPV of 100%, NPV of 87.94%, Diagnostic accuracy of 88.75% and Kappa agreement was 0.496 (Moderate agreement) in comparison with CL grade.

 Table 6: Validity of ULBT grade in predicting difficult intubation compared to CL Grading

ULBT had sensitivity of 2.778%, Specificity of 98.17%, PPV of 25%, NPV of 82.14%, Diagnostic accuracy of 81% and Kappa agreement was 0.01452 (Slight agreement) in comparison with CL grade.

HLM had sensitivity of 5.556%, Specificity of 96.34%, PPV of 25%, NPV of 82.29%, Diagnostic accuracy of 80% and Kappa agreement was 0.02724 (Slight agreement) in comparison with CL grade.

HNM had sensitivity of 4.167%, Specificity of 98.78%, PPV of 42.86%, NPV of 82.44%, Diagnostic accuracy of 81.75% and Kappa agreement was 0.0455 (Slight agreement) in comparison with CL grade

RHTMD had sensitivity of 90.28%, Specificity of 94.21%, PPV of 77.38%, NPV of 97.78%, Diagnostic accuracy of 93.5% and Kappa agreement was 0.7933 (Substantial agreement) in comparison with CL grade.

Although there are many preoperative tests to predict difficult airway, they are far from being ideal i.e, one which is easy to perform, highly sensitive, highly specific and which possess high predictive value with few false positive predictions. Allahyary et al⁹ stated that any preoperative assessment test of difficult tracheal intubation should have a high sensitivity and specificity to result in minimal false positive or false negative.

The incidence of airway and haemodynamic complications increases beyond two Laryngoscopic attempts especially during emergency airway management⁵³. Thus, this study becomes clinically significant as repeated tracheal intubation attempts during difficult intubation may contribute to patient morbidity (including permanent brain damage) and mortality^,10 Prediction of a difficult airway to prevent unanticipated difficult tracheal intubation and consequent events and development of plan to convert a difficult intubation into an easy one is an important concern for anaesthesiologists¹¹.

The demographic characteristics in the present study was comparable to studies done by  Tadese Tamire et al¹² and Philip S et al¹³ that is , majority of subjects were middle aged, males constituting higher percentage and majority with ASA grade I. In this study, the incidence of difficult intubation based on CL grade was found to be 18% which was within the 0.5-18% stated by Merah et al ¹⁴ reported incidence of 1-4%. It was also comparable to other studies that stated the incidence to be between 1% and 15% ¹⁵.

In the present study sensitivity, specificity for thyromental distance (TMD) was 37.5% and 100% respectively. In a study Salimi et al¹⁶ reported a sensitivity of 55% and specificity of 88%, Khan et al¹⁷ reported sensitivity of 73% and specificity of 82.2%. Also, from the studies in the above table shows that TMD has low sensitivity and higher specificity. This wide variation in reported sensitivity in various studies may be because of incorrect evaluation of the measurement from inner or outer mentum and anthropometric peculiarities.

We found a 75% sensitivity and 100% specificity for sternomental distance (SMD) in predicting easy intubation. Srinivasa S et al, D. Shobha et al⁵ and Tadese Tamire et al¹² observed higher specificity compared to sensitivity. This is in concordance with studies made by Ramadhani et al¹⁸ where they found a specificity of 87.1%, Savva et al¹⁹ where they found a specificity of 88.6% and specificity for SMD in Naithani et al²⁰ was 90.2%.

In the present study, the sensitivity of Upper lip bite test (ULBT) is only 2.778% that means in about 97% could not identify possibility of difficult intubation. This is in contrast to the results obtained by D. Shobha et al,⁵ Khan et al¹⁷, Azmat ali et al²¹ wherein they found a sensitivity of 96.64%, 45%, 76.5%, respectively. Our study is in concordance with the study done by Karci et al²², wherein they found sensitivity of 13%. The lower sensitivity of ULBT in our study can be explained due to low incidence of ULBT Class III in our study. Repeated demonstrations are required for the patients to perform ULBT and few fails to understand the procedure. Also, in few patients due to reflex movement of upper lip in the reverse direction over the upper teeth which may alter the point of meeting of vermilion line with lower incisors. In the same individual measured, the ULBT may vary according to the effort applied by the patient. The specificity of ULBT in our study was 98.17%, which correlates with the studies done by Khan et al¹⁷ (88.7%), Eberhart et al²³ (92.5%), Hester et al ⁶³(97%).

HLM is a metric parameter and if used on its own, it does not have much predictive value. Moreover, assessment using a measuring tape is usually influenced by the rater’s error. Nevertheless, an HLM of at least 9 cm should guarantee easy intubation¹⁴. Our findings were in concordance with D Shobha et al⁵.

In the present study HNM had Sensitivity of 4.167%, Specificity of 98.78%, PPV of 42.86%, NPV of 82.44% and Diagnostic Accuracy of 81.75%. Our findings were similar to the studies by D. Shobha et al⁵.

The RHTMD is based on precise measurement of patient’s TMD and height, so making inter observer variations highly unlikely (on the contrary to significant inter observer variations found with the MMT). The RHTMD has some limitations, it depends on accurate measurement of patient’s TMD and height. Also, the cutoff point of RHTMD for prediction of difficult laryngoscopy is race dependent. So, cut off points should be calculated separately for each population²².

In the study out of 72 subjects with difficult grade in CL, 37.5% were difficult in TMD, 75% were difficult in SMD, 2.8% were difficult in ULBT, 5.6% were difficult in HLM, 4.2% were difficult in HNM and 90.3% were difficult in RHTMD. There was significant association between CL grade and TMD, SMD, RHTMD scoring.

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