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THE VALIDITY OF PREOPERATIVE ASSESSMENT OF NECK

CIRCUMFERENCE TO THYROMENTAL HEIGHT RATIO (NC/TMH) AS A RELIABLE PREDICTOR OF DIFFICULT LARYNGOSCOPY AND INTUBATION IN OBESE ADULT PATIENTS UNDERGOING ELECTIVE SURGERY UNDER GENERAL ANAESTHESIA IN A

TERTIARY CARE HOSPITAL - A PROSPECTIVE OBSERVATIONAL STUDY.

DISSERTATION SUBMITTED IN PARTIAL FULFILLMENT OF M.D BRANCH X (ANAESTHESIOLOGY) DEGREE EXAMINATION OF THE TAMILNADU DR. M.G.R MEDICAL UNIVERSITY, CHENNAI, TO BE HELD IN APRIL 2020

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THE VALIDITY OF PREOPERATIVE ASSESSMENT OF NECK

CIRCUMFERENCE TO THYROMENTAL HEIGHT RATIO (NC/TMH) AS A RELIABLE PREDICTOR OF DIFFICULT LARYNGOSCOPY AND INTUBATION IN OBESE ADULT PATIENTS UNDERGOING ELECTIVE SURGERY UNDER GENERAL ANAESTHESIA IN A

TERTIARY CARE HOSPITAL - A PROSPECTIVE OBSERVATIONAL STUDY.

Dissertation submitted to the

THE TAMILNADU DR. M.G.R MEDICAL UNIVERSITY, CHENNAI In partial fulfilment of the requirements for the degree of

MASTER OF MEDICINE IN

ANAESTHESIOLOGY By

ANDREW SOLOMON Reg. No: 201720351

DEPARTMENT OF ANAESTHESIA CHRISTIAN MEDICAL COLLEGE VELLORE

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CERTIFICATE

This is to certify that the dissertation entitled “The validity of preoperative assessment of neck circumference to

thyromental height ratio (NC/TMH) as a reliable predictor of difficult laryngoscopy and intubation in obese adult patients undergoing elective surgery under general anaesthesia in a tertiary care hospital - a prospective observational study” is an authentic record of research work carried out by Dr.

Andrew Solomon under my supervision and guidance in the department of Anaesthesia, Christian Medical College, Vellore in partial fulfilment of the requirements for the M.D Anaesthesiology Examination Branch X of the Tamilnadu Dr. M.G.R Medical University to be held in April 2020 and no part thereof has been submitted for any other degree.

Vellore Dr. Balaji Kuppuswamy, M.D 30/09/2019 Professor

Department of Anaesthesiology Christian Medical College Vellore - 632004

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CERTIFICATE BY THE HEAD OF THE DEPARTMENT/PRINCIPAL

This is to certify that the dissertation entitled “The validity of preoperative assessment of neck circumference to thyromental height ratio (NC/TMH) as a reliable predictor of difficult laryngoscopy and intubation in obese adult patients undergoing elective surgery under

general anaesthesia in a tertiary care hospital - a prospective observational study” is a bonafide work done by Dr. Andrew Solomon under the direct supervision and guidance of Dr. Balaji Kuppuswamy, M.D, Professor, Department of Anaesthesiology, Christian Medical College, Vellore.

Dr. Raj Sahajanandan, M.D, FRCA Dr. Anna Pulimood, M.D Professor and Head Principal

Department of Anaesthesiology Christian Medical College Christian Medical College Vellore

Vellore

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DECLARATION

I, Andrew Solomon, do hereby declare that the dissertation entitled “The validity of preoperative assessment of neck circumference to thyromental height ratio (NC/TMH) as a reliable predictor of difficult laryngoscopy and intubation in obese adult patients undergoing elective surgery under

general anaesthesia in a tertiary care hospital - a prospective observational study” is a genuine record of research work done by me under the supervision of Dr. Balaji Kuppuswamy, Professor, Department of Anaesthesia, Christian Medical College, Vellore and has not been previously formed the basis for the award of any degree, diploma, fellowship or other similar title of any University or institution.

Vellore Dr. Andrew Solomon 30/09/2019

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This is to certify that this dissertation work titled “The validity of preoperative assessment of neck circumference to Thyromental height ratio (NC/TMH) as a reliable predictor of difficult laryngoscopy and intubation in obese adult patients undergoing elective surgery under general anaesthesia in a tertiary care hospital - A prospective observational study” of the candidate Dr. Andrew Solomon with registration number 201720351 in the branch of anaesthesiology has been

submitted for verification. The website urkund.com has been personally verified for the purpose of plagiarism check. I found that the uploaded thesis file

contains pages from introduction to conclusion and the result shows 7% of plagiarism in the dissertation.

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CHRISTIAN MEDICAL COLLEGE Office of the Treasurer

Date: 28-05-2018

Dear Dr. Andrew Solomon

As requested by the Vice-Principal (Research) with the IRB Minute No. 11042 a new Fluid research account opened for your Project, the account no are as follows:-

22 Z 442

This is for your information

Thanking you.

Yours sincerely P. BASKARAN Sr. Manager (F&A) Accounts

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ACKNOWLEDGEMENTS

Initially, I would like to thank “God Almighty” for giving me the energy and the drive to do this work.

I wish to express my sincere thanks and deepest sense of gratitude to my guide, Dr. Balaji Kuppuswamy, Professor of Anaesthesia, Christian Medical College, Vellore for his unfailing guidance, invaluable suggestions, critical assessment and constant encouragement throughout my work. I am also greatful to Dr. Raj Sahajanandan, Professor and Head of Department of Anaesthesia, Christian Medical College, Vellore for providing necessary facilities to carry out this work successfully. I am deeply indebted to all consultants and post graduate residents of the department of Anaesthesia, CMC Vellore for being a part of my study by filling up intubation difficulty scale for the study patients.

I am also greatful to the Heads of the various surgical units for allowing me to include their patients as part of my study. I express my sincere thanks to research scholars of the Department of Biostatistics for their valuable advice during the statistical analysis of the dissertation. I would also like to place on record my obligation to staff nurses of all the wards and the secretary of the pre anaesthesia clinic for enabling me to recruit patients for my study. To the

patients who consented to my study, I am deeply thankful, for without them this

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study would not have been possible. Finally, but not the least I wish to thank my parents and family who have always prayed for me and gave their

unconditional support.

Andrew Solomon

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ABSTRACT

Objectives

This study was done to assess the validity of neck circumference to thyromental height ratio (NC/TMH) for predicting a difficult laryngoscopy and intubation among obese adults undergoing elective surgeries under general anaesthesia. It also enabled us to compare NC/TMH ratio with other parameters such as thyromental distance, sternomental distance and modified Mallampati test in predicting difficult laryngoscopy and intubation among these individuals.

Patients and Methods

The calculated sample size for our prospective observational study was 100 obese patients. We were able to recruit 120 patients for airway assessment. But only 101 patients were subjected to statistical analysis, the remaining being excluded due to incomplete data.

Patients were recruited during the time period from January 2018 and July 2018. We assessed the predictability of neck circumference / thyromental height ratio as a reliable test for difficult laryngoscopy and intubation and also

compared the same with other routine tests namely thyromental and

sternomental distance and modified Mallampati test. An IDS score of five and

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above was considered as difficult laryngoscopy and intubation.

Results

The predictors of difficult intubation were analysed by univariate logistic regression analysis using measures of central dispersion for continuous variables and proportions for categorical variables. This showed that a decreased thyromental distance, increased age, decreased thyromental height, increased neck circumference, a modified Mallampati grade of III or IV, an NC/TMH ratio ≥ 5 were statistically significant variables correlating with difficult intubation. Multivariate logistic linear regression analysis done using chi square test showed that NC/TMH ratio and neck circumference were statistically significant. In comparison with neck circumference, the NC/TMH ratio had a higher positive predictive value and a comparable area under the ROC curve. A difficult intubation among obese patients was calculated with an incidence of 20.5%

Interpretation and conclusions

In conclusion, a difficulty laryngoscopy and intubation in obese patients was associated with a modified Mallampati grade of III and IV, decreased thyromental distance and NC/TMH ratio ≥ 5. Even though in our study, the NC/TMH ratio provided lower sensitivity and specificity compared to similar studies, it showed a

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high positive predictive value. Hence we can consider NC/TMH ratio as a useful predictor for a difficult laryngoscopy and intubation.

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Table of Contents

INTRODUCTION ... 19

AIM OF THE STUDY ... 22

OBJECTIVES OF THE STUDY ... 22

HYPOTHESIS ... 23

BACKGROUND ... 24

ANATOMY OF AIRWAY ... 24

MOUTH ... 25

THE PALATE ... 26

PHARYNX ... 27

LARYNX ... 28

IMPLICATIONS OF OBESITY IN ANAESTHESIA ... 37

RESPIRATORY PATHOPHYSIOLOGY IN OBESITY ... 37

ANAESTHETIC IMPLICATIONS OF OBESITY ... 38

DIFFICULT LARYNGOSCOPY / INTUBATION ... 40

PROBLEM STATEMENT ... 41

JUSTIFICATION FOR THE STUDY ... 43

REVIEW OF LITERATURE... 43

GLOBAL EPIDEMIOLOGY: ... 45

PREVALENCE AND INCIDENCE: ... 47

COMPARISONS OF VARIOUS AIRWAY ASSESSMENT TESTS: ... 52

USE OF IDS SCORE FOR INDIRECT LARYNGOSCOPY ... 65

INDIAN EPIDEMIOLOGY: ... 66

SUMMARY OF THE REVIEW OF LITERATURE ... 75

RESEARCH METHODOLOGY ... 78

STUDY DESIGN ... 78

STUDY LOCATION ... 78

STUDY SETTING ... 78

STUDY PERIOD ... 79

STUDY POPULATION ... 79

SELECTION OF PARTICIPANTS FOR THE STUDY ... 79

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INCLUSION CRITERIA... 79

EXCLUSION CRITERIA ... 80

SAMPLE SIZE AND SAMPLING PROCEDURE ... 81

SAMPLE SIZE CALCULATION ... 81

METHODOLOGY ... 82

DATA COLLECTION ... 84

PREOPERATIVE ASSESSMENT ... 84

INTRAOPERATIVE ASSESSMENT ... 89

STATISTICAL METHODS ... 93

Calculation of Sensitivity and Specificity of NC/TMH ratio ... 95

Other calculations ... 96

Definitions: ... 97

ETHICAL CONSIDERATION ... 100

RESULTS ... 100

DEMOGRAPHIC DATA ... 101

PRIMARY OBJECTIVE ... 107

SECONDARY OBJECTIVE ... 115

DISCUSSION ... 118

DEMOGRAPHIC DATA ... 119

PRIMARY OBJECTIVE ... 120

SECONDARY OBJECTIVES ... 122

SUBANALYSIS ... 125

CONCLUSION ... 126

LIMITATIONS - EFFORTS TO OVERCOME THEM ... 127

SUMMARY ... 128

BIBLIOGRAPHY ... 131

APPENDIX ... 139

LIST OF ABBREVIATIONS USED ... 139

LIST OF TABLES ... 141

LIST OF FIGURES ... 143

CLINICAL RESEARCH FORMS ... 144

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INTRODUCTION

The term “Obesity” is defined as an excess fat deposition and accumulation in the body causing major health problems. The World Health Organisation (WHO) defines an obese individual as a person who has a body mass index (BMI) greater than or equal to 30 kg per square meter of body

surface. Since the above mentioned range of BMI characterizes the definition of obesity in our target population, we are considering patients with a BMI greater than 30kg.m2 for assessment of an anticipated difficult airway in this study.

The assessment of a difficult airway is multimodal and it is based on various anatomical parameters involving the upper airway mainly the oral

cavity and the structures of the pharynx. A clear understanding of the anatomy of the neck and upper airway is essential for an accurate assessment of an anticipated difficult airway. A difficult intubation is the second most common damaging event leading to anaesthetic complications due to inablilty of early recognition of an anticipated difficult airway. Most complications have occurred when a possible difficult airway was not recognised early. Over the years of evolution of anaesthetic practice, it has become clear that the importance of

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prior assessment of a difficult airway has proven to be a means to decrease complications in anaesthesia. In view of all these factors, in the past decades, there have been numerous developments in the form of various devices and techniques for assessment of the same, namely the modified Mallampati test and the Cormack and Lehane scoring system for predicting a

difficult intubation.

Among the individuals who were not obese versus the individuals who were obese, the incidence of a difficult laryngoscopy is more or less similar (about 10%). But there have been more reports of a difficult laryngoscopy and intubation among the obese patients. This can be due to the changes in the upper airway present among them. There are a few clinical predictors which increase the risk of a difficult airway in the patients with obesity. Some of these factors are an increased neck circumference, Mallampatti’s grade III or IV and a diagnosis of an obstructed sleep apnoea syndrome (OSAS) etc to name a few.

A measurement of a neck circumference alone may not be sufficient to quantify the soft tissue located within the neck. Using a magnetic resonance imaging (MRI), a proposal was made by Horner that in the areas around the collapsible regions of the pharynx, there was more fat present among the

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patients with obesity suffering from OSA. This study was done by Ezri et al(1), using ultrasonography, which suggested that among the patients with obesity.

A measurement of the volume of soft tissue at the suprasternal notch and vocal cord level can be useful in prediction of a difficult airway. They also proved that the volume of pretracheal soft tissue measured sonologically was able to distinguish an easy from a difficult laryngoscopy and intubation. These findings mentioned above clearly point out that why it is relatively easy to perform laryngoscopy and intubation in some obese patients while it is not so in others.

As per Etezedi et al.(2) a difficult laryngoscopy has accounted for nearly 1.5 – 20% of all intubations and that there is a clear relationship that is present between a difficult laryngoscopy and the height between the two borders of the thyroid cartilage and the mentum when the patient is in supine position with the head in neutral with his/her mouth closed. This has been termed as the

thyromental height test.

We aimed to utilize this test and measure the ratio of

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neck circumference to thyromental height and estimate its use in assessment of difficult laryngoscopy in patients with obesity coming for surgery under general anaesthesia and also to compare the same with other modalities of upper airway assessment such as thyromental distance(TMD), Sternomental

distance(SMD) and modified Mallampati test(MMT). Hence, by the end of our analysis we were able to arrive at a concensus with regard to the incidence of difficult intubation in patients with obesity coming for elective surgery under general anaesthesia.

AIM OF THE STUDY

To assess the validity of neck circumference to thyromental height ratio (NC/TMH) as an important parameter for difficult laryngoscopy and intubation amongst adult patients with obesity undergoing elective surgery under general anaesthesia.

OBJECTIVES OF THE STUDY

Primary Objective:

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(a)The assessment of the relationship between the neck circumference to thyromental height ratio(NC/TMH) with intubation difficulty in adult patients with obesity undergoing elective surgery under general anaesthesia.

Secondary Objective:

(a)To compare the neck circumference to thyromental height ratio (Nc/TMH) with modified Malampatti test(MMT), neck circumference(NC), thyromental distance(TMD) and sternomental distance(SMD) as reliable tests for predicting difficult laryngoscopy in obese adult patients

(b)To find out the incidence of difficult laryngoscopy in obese adult patients undergoing elective surgery under general anaesthesia

HYPOTHESIS

The ratio of Neck circumference to thyromental height (Nc/TMH ratio) of more than or equal to five will predict a difficult laryngoscopy and intubation and will have a better statistical and clinical significance as compared to other standard indices of airway assessment among obese patients.

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BACKGROUND

ANATOMY OF AIRWAY

The term “Airway” refers to the extra pulmonary air passage and it is composed of the nasopharynx, oropharynx and laryngopharynx, larynx, trachea and bronchi. The functions of the airway in an awake person includes those of filtration of inspired air, conditioning of the ambient air, humidification and conduction of air to and from the lungs for the process of gaseous exchange.

During the process of induction and maintenance of anaesthesia, the airway is in a passive state of function due to the suppression of the nervous system which controls the vital respiratory functions. The anaesthetist should possess the ability of ventilating the patient either by bag and mask method or intubate if necessary when the patient is at this state. In order to assess and anticipate a situation of difficult airway and to formulate a plan of safety for the patient, he or she should be well versed with the anatomy of the airway, its applications and the various different methods of airway assessment.

Anatomically, the airway can be classified into an upper airway and lower airway. The upper airway is composed of the nose and nasal passages, the

nasopharynx, mouth and the oropharynx. The lower airway is composed of the larynx,

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trachea, bronchi, bronchial tree and the alveoli.

MOUTH

The mouth (Fig 1) consists of the oral cavity and the vestibule. The cavity communicates with the vestibule through the angle of the mouth. The vestibule is composed of the gums and teeth within and of the cheeks and lips on the outside of it.

The cavity of the mouth is bounded above by the soft and hard palate, anteriorly by the teeth and alveolar arch, posteriorly by the oropharyngeal isthmus and inferiorly by the anterior two thirds of the tongue below (Fig 2).

Fig 1: Mouth

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Fig 2: Anatomy of Oral cavity

THE PALATE

The palate is composed of two parts, namely the hard palate and the soft palate.

Hard palate:

This is made up of the palatine process of the maxilla and the horizontal plates of the palatine bones.

Soft palate:

This hangs like a curtain from the posterior edge of the hard palate.

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PHARYNX

The pharynx (Fig 3) is a hollow muscular tube and it serves as a common upper pathway for both the alimentary and respiratory tracts. It is connected anteriorly with the nasal cavity, the mouth and the larynx. This is divided into the nasopharynx, oropharynx and laryngopharynx (Fig 4). Posteriorly, it leans over the cervical

vertebrae and the prevertebral fascia.

Fig 3: Lateral view of pharynx

Fig 4: Anatomy of pharynx

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LARYNX

The larynx is the part of the respiratory tract which is located beneath the pharynx and it contains the vocal cords. It is a tubular shaped structure which is about two inches in length, and it opens into the laryngeal part of the pharynx above and is continuous with the trachea below.

The functions of the larynx are as follows:

 Deglutition

 Respiration

 Phonation

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STRUCTURE OF LARYNX

The larynx (Fig 5) is made up of four components:

 A skeletal framework made up of cartilage

 Ligaments and membranes

 Laryngeal muscles - extrinsic and intrinsic

 Mucosal lining

The cartilaginous skeleton is made up of:

 Single Unpaired cartilages

a. Thyroid b. Cricoid c. Epiglottic

 Paired cartilages

a. Arytenoid b. Corniculate c. Cuneiform

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Fig 5: Larynx - anterior and lateral view

The laryngeal ligaments (Table 1) are divided into two types namely

 Extrinsic ligaments

 Intrinsic ligaments

Extrinsic ligaments Intrinsic ligaments Thyrohyoid membrane and ligaments Elastic membrane

Cricothyroid membrane and ligaments Quadrangular membrane Cricotracheal ligament Median cricothyroid ligament Epiglottis Thyroepiglottic ligament

Vocal ligament Table 1: Ligaments of larynx

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The muscular skeleton of larynx consists of

 The Suprahyoid muscles a. Geniohyoid

b. Mylohyoid c. Stylohyoid d. Digastric

 The longitudinal muscles a. Palatopharyngeus b. Salphingopharyngeus c. Stylopharyngeus

 The Infrahyoid muscles a. Sternohyoid b. Sternothyroid c. Omohyoid

THE INLET OF LARYNX

The inlet of the larynx faces backward and upward and opens into the laryngeal part of the pharynx. The boundaries for the laryngeal inlet (Fig 6) are as follows:

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 Anteriorly: The upper margin of the epiglottis

 Posteriorly: Arytenoid cartilages

 Laterally: Aryepiglottic folds

 Inferiorly: Arytenoid cartilages

Fig 6: Inlet of Larynx

THE CAVITY OF LARYNX

The cavity of the larynx (Fig 7) begins at the laryngeal inlet and ends at the cricoid cartilage at its lower border. It gradually narrows

downwards upto the region of the vestibular folds (Rima vestibuli) and it becomes the narrowest at the region of the vocal folds (Rima glottidis).

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Fig 7: Laryngeal cavity

The blood supply of the larynx is as follows

 Arteries:

a. Upper half of larynx: Superior laryngeal artery, branch of superior thyroid artery

b. Lower half of larynx: Inferior laryngeal artery, branch of inferior thyroid artery

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 Veins:

They accompany the corresponding arteries.

 Lymphatics:

The lymphatic vessels drain into the deep cervical lymph nodes.

 Nerve supply:

The nerve supply of the larynx is as follows

a. Sensory:

Above the vocal cords: Internal laryngeal nerve, branch of the superior laryngeal branch of the vagus nerve

Below the vocal cords: Recurrent laryngeal nerve, branch of the vagus nerve

b. Motor:

All intrinsic muscles of the larynx are supplied by the recurrent laryngeal nerve except for cricothyroid muscle.

The cricothyroid muscle is supplied by the external laryngeal nerve

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which is a branch of the superior laryngeal branch of vagus nerve.

LARYNGOSCOPIC ANATOMY

It is essential to get the mouth, the oropharynx and the larynx into one plane in order to view the vocal cords during direct laryngoscopy and to

proceed with intubation. Elevation of the head for about 10 cms with a pillow under the occiput, making sure that the shoulders are on the table will cause the laryngeal and the pharyngeal axis to fall in a straight line. A straight line is created from the incisors upto the glottis along with the shortest distance when the patient’s neck is flexed at the lower cervical spine and extended at the atlanto - occipital joint. This position is referred to as the sniffing position (Fig 8 and 9).

Fig 8: Sniffing position

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Fig 9: Sniffing position for intubation

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IMPLICATIONS OF OBESITY IN ANAESTHESIA

DEFINITION

Obesity can be defined by an excessive increase of body fat in the body causing major health impairment. A person is termed obese if his body mass index (BMI) is greater than or equal to 30 kg per square meter of body surface as per World Health Organisation. Since the above mentioned range of BMI characterizes the definition of obesity in our target population, we are considering patients with a BMI greater than 30kg.m2 for assessment of an anticipated difficult airway in this study.

RESPIRATORY PATHOPHYSIOLOGY IN OBESITY

There are various pulmonary disorders that are associated with obesity which are of major concern to the anaesthesiologist. Amongst them, the most common ones are obesity hypoventilation syndrome / obstructive sleep apnoea and cor pulmonale. In addition to the above, a decreased pulmonary reserve is seen in most of the patients suffering from obesity even in the absence of a specific

pulmonary disorder. There is an increased incidence of a restrictive pulmonary disease in these patients. The patients belonging to the morbid obesity group are found to have

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a reduced forced vital capacity (FVC), functional residual capacity (FRC) and total lung capacity (TLC) along with a decreased expiratory reserve volume and an increased resistance to work of breathing.

CLASSIFICATION OF OBESITY

The International classification of adult obesity according to BMI is described below (Table 2):

Terminology Body mass index (kg/m²)

Overweight ≥ 25.00

Pre obese 25.00 - 29.99

Obese ≥ 30.00

Obesity class I 30.00 - 34.99

Obesity class II 35.99 - 39.99

Obesity class III ≥ 40.00

Table 2: Classification of obesity according to BMI

ANAESTHETIC IMPLICATIONS OF OBESITY

The physiological changes occurring in obesity are the mainstay for the major anaesthetic challenges in obese patients coming for surgery under general anaesthesia. These changes include those that take place in the airway, respiratory system, cardiovascular system, gastrointestinal system and

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pharmacological changes in drug metabolism and elimination.

The various pathophysiological changes in obesity are given as follows (Table 3):

System involved Changes

Cardiovascular Increase in cardiac output and blood volume causing

cardiomegaly and left ventricular hypertrophy

Decreased venous return

Hypertension and Ischemic heart disease

High risk of thromboembolism, deep vein thrombosis,

Pulmonary embolism and arrhythmias

Respiratory Decreased lung compliance

Reduced functional residual capacity

Increased work of breathing

V/Q mismatch may lead to hypoxemia post induction

Airway Difficult mask ventilation and intubation

Decreased mobility of the head and neck

Short neck and large tongue

Anterior position of larynx

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Obstructive sleep apnoea

Gastrointestinal Increased gastroesophageal reflux

Increased risk of aspiration

Pharmacological considerations

Increased volume of distribution

Increased requirement and clearance for fat soluble anaesthetic agents

Table 3: Pathophysiological changes in obesity.

DIFFICULT LARYNGOSCOPY / INTUBATION

The definition of a difficult airway by the ASA task force is as follows

“The clinical situation in which a conventionally trained anaesthesiologist experiences difficulty with mask ventilation or difficulty with tracheal intubation or both”. It also involves a complex association between patient related factors, skills and the

preference of the anaesthesiologist and the clinical scenario.

DEFINITION OF DIFFICULT MASK VENTILATION

The definition of difficult mask ventilation as quoted by the ASA task force:

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“A situation in which an unassisted anaesthesiologist is unable to maintain the oxygen saturation above 90% using 100% oxygen and positive pressure mask ventilation in a patient whose oxygen saturation was greater than 90% before anaesthetic intervention”

“A situation in which an unassisted anaesthesiologist is unable to prevent or reverse the signs of inadequate ventilation during positive pressure mask ventilation”

DEFINITION OF A DIFFICULT INTUBATION

The definition of a difficult intubation as given by the ASA task force is as follows

“A situation in which a proper insertion of the tracheal tube with conventional laryngoscopy requires more than three attempts or more than ten minutes”

PROBLEM STATEMENT

Obesity is associated with diabetes mellitus, hypertension, ischemic heart disease as well as a difficult airway, difficult laryngoscopy and intubation. A difficult airway and intubation can increase the mortality and morbidity and it is often associated with obesity. As per a recent study done by Unni et al(3)., the prevalence of obesity among the Indian

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population is around 8%. A pilot study done by the primary investigator among patients who had a preoperative anaesthesia check up in Christian Medical College, Vellore showed a prevalence rate of obesity as 13%.

As per various literature sources which have been published, the incidence of difficulty for intubation among obese

individuals ranges from 11-22 percent. In a meta analysis done by Shige et al., and Jovin et al., the incidence of difficulty for intubation amongst a population of obese patients was reported to be around 15.7% (95% CI, 13.8 – 18.4%) and 15.5% respectively. Another study by Voyager et al., examined 1800

obese patients for endotracheal intubations and showed that the incidence of a difficult intubation among them was 20%. As per Rita et al., the incidence of a difficult intubation was 18%. The other studies done by Gonzales et al., Kim et al., showed a slightly lesser incidence of a difficult laryngoscopy and

intubation.

As a significant number of obese patients undergo surgeries on a daily basis for a wide variety of reasons and the literature highlighting the increased incidence of a difficult intubation, we

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decided to look for a predictor to anticipate a difficult airway and plan the management for the same.

JUSTIFICATION FOR THE STUDY

Assessment of an airway is a basic skill for an anesthesiologist and the inability to maintain an airway under anaesthesia is a major cause of morbidity and mortality. Among the various tests which are recommended for assessment of a difficult airway and prediction of difficult laryngoscopy, neither of them are the best nor do any of them have a high value of accuracy. Most of the data available is from the west. There is a lacunae in our knowledge of the incidence of difficult airway in Indian population. Generating this data will help us plan preoperatively and encourage us to be prepared for an anticipated difficult airway and thereby help us to reduce the burden of mortality and morbidity caused by difficult laryngoscopy in anaesthesia. In a resource poor country like India, it will also help us to gauge the economic impact of the same.

REVIEW OF LITERATURE

One of the major primary responsibilities of the

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anaesthesiologist is to maintain the airway. If there is an interruption of gaseous exchange due to inability to maintain the airway, then major adverse outcomes can arise. It is the duty of the practitioner to identify those individuals with obesity in whom there are risk factors for difficulty for laryngoscopy and intubation. In order for safe intubation and ventilation to occur, one must be able to identify a difficult airway. The ability to predict a potentially difficult laryngoscopy and intubation has received great recognition and importance as it plays a very vital role in bringing down the mortality and morbidity.

The present knowledge of the same has been discussed in this dissertation as follows:

 Global epidemiology

 Prevalence and Incidence

 Comparison of various airway assessment tests

 Use of Intubation difficulty score (IDS) for Indirect Laryngoscopy

 Indian epidemiology

 Summary of the review of literature

In the field of anaesthesiology, airway management is one of the greatest concerns of the anaesthesiologist and one of the areas of prime importance to ensure a successful safe anaesthetic experience for a patient during surgery(4).

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Difficulties involved with the airway and tracheal intubation significantly contribute to morbidity and mortality associated with anaesthesia. Moreover, in recent years, a difficult airway has received more importance and focus as its prevalence in

anaesthesia has been increasing. A difficult intubation is an event which is not easy to predict before the induction of anaesthesia. As per the Practice guidelines, a standard definition for a difficult airway cannot be identified in literature. It is defined as when there is a difficulty with face mask ventilation of the upper airway, difficulty with endotracheal intubation or both, encountered by a conventionally trained

experienced anaesthetist. It shows the interaction between the clinical setting, patient factors and the skills of the anaesthetist. An accurate analysis of this interaction

requires a precise collection and communication of data. Hence in this study we would like to analyse and assess the importance of management of difficult airway in

anaesthesia by evaluation with various anatomical measurements pertaining to the upper airway, mainly the oral cavity and pharynx for the same.

GLOBAL EPIDEMIOLOGY:

The incidence of a difficult airway in anaesthesia has been rising globally and is one of the most potent life threatening events for a patient under anaesthesia as well as a major intraoperative challenge for the anaesthesiologist.

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Hence an understanding into the global worldwide occurance of a difficult airway with relation to various demographic and population related factors is commendable for emphasis on early anticipation and evaluation of the same. It also ensures prompt intervention to prevent life threatening events and catastrophical outcomes as a result of poor recognition of a difficult airway.

In the past century, it has become clear that the importance of a good preoperative assessment of the airway is a mean to decrease intraoperative complications in

anaesthesia. One of the major causes of intraoperative airway complications in anaesthesia has been found to occur in patients with obesity. With various lifestyle modifications, the prevalence of obesity has increased three-folds worldwide. As per WHO(2), the term Obesity is defined as an excess fat accumulation in the body. A person who has a body mass index (BMI) greater than or equal to 30kg per square meter of body surface area is said to be obese. Maria et al.(5) in her study stated that in a population of asian predominance, in management of metabolic problems, the definition of obesity has been changed to that with a BMI more than equal to 25kg per square meter of body surface area. Since airway management is not affected by the same, we have taken a BMI ≥ 30kg/m² for airway assessment in obese patients.

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PREVALENCE AND INCIDENCE:

The prevalence of a difficult airway in anaesthesia has been on the rise with changes in lifestyle and demographic factors. Suboptimal management of a

difficult airway is one of the major causes of mortality and morbidity. It is one of the most frequently occuring events leading to claims of anaesthetic malpractice. Most problems occur when a difficult airway hasn’t been recognized earlier beforehand.

It has become clear over the past decades that a good assessment of the airway prior to surgery is of prime importance in reducing anaesthetic complications with regard to the airway. As a result of these events, different techniques and devices were found many years ago for assessing a patient with difficult airway.

According to Pollard et al.(6) a study done to assess the incidence of difficult intubation and failed intubation amongst obstetric patients showed that out of 2800 obstetric general anesthetics in the database were studied of which 1090 (39%) were deemed as emergencies. There were two cases of failed intubation and seven cases of unanticipated difficult intubations (1:410 cases, 0.25%

of all obstetric cases, 95% confidence interval 0.1- 0.5%), six of which

occurred during emergency surgery. There was an increase in obesity (p = .004) and ASA PS (p = .03) over the period of the study. The incidence of difficult intubation

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was not found to be significantly changed (p = .70). Despite an increase in ASA PS score and obesity, there was no increase in the incidence of difficult intubation in obstetric patients. Limitations of the study include its retrospective design, and the small number of difficult intubation cases identified.

In a study done by Akhlaghi et al,(7) airway assessment is a basic skill for an anaesthesiologist and inability to maintain the

airway is the leading cause of mortality and morbidity related to anaesthesia. Among the tests recommended for prediction of a difficult laryngoscopy, none stand out to be the best nor do they have high diagnostic value. The aim of their study was to assess the usefulness of a new "laryngoscopic exam test (LET)" to predict a difficult

laryngoscopy. Three hundred and ten patients aged 16-60 years took part in the study.

Airway was assessed with upper lip bite test, LET test and modified mallampati tests preoperatively, and as a gold standard, during intubation, Cormack and Lehane's laryngoscopic grading was done. A Cormack and Lehane's laryngoscopy grade of ΙΙΙ or ΙV was considered a difficult intubation. A difficult intubation was calculated with an incidence of 6.0 %. A higher specificity, accuracy (P<0.04) and sensitivity was observed in the LET test. Among these three teste there were no significant

differences observed(P=0.400). Hence, the LET is a simple alternative bedside test to

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predicting difficult intubation.

A study done by Nasa et al.,(8) studied 400 patients and gave a comparison between modified MP test, Thyromental test and Neck extension and concluded as follows (Table 4).

PARAMETERS Modified MP test Thyromental test Neck extension

Sensitivity (%) 33 79 42

Specificity (%) 97 97 98

PPV (%) 32 57 77

Accuracy (%) 77 85 83

Table 4: Accuracy of modified MP test, thyromental test and Neck extension in prediction of a difficult airway as per Nasa et al.

As per a study by Ahangari et al(2), a case of a difficult laryngoscopy has an incidence of 2% to 20%. They stated that a close relationship exists between a difficult laryngoscopy and the

height from the border of the mentum to the thyroid cartilage, with the

patient lying in supine position with a closed mouth. This has been named the

"thyromental height test" (TMHT). The aim in our study was to assess its ability in the prediction of a difficult laryngoscoṁpy. Three hundred fifteen consecutive patients of

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both sexes above the age of 16 years scheduled for general anesthesia took part in the study. Assessment of airway was done with thyromental distance, sternomental distance modified Mallampati test, and TMHT preoperatively. Thereafter, during intubation assessment of Cormack and Lehane laryngoscopy grade was done.

The anaesthetist performing the laryngoscopy was not aware of the preoperative airway assessment values. The validity and prediction indexes for the other tests and TMHT test were calculated and this was taken as the primary end point of the study.

The values of sensitivity and specificity were in the range of 47.45 to 51.05 mm.

To apply these values clinically, a value of 50 mm was chosen as a cutoff.

Compared to the other tests (all P < 0.0001), TMHT was more accurate implying that the TMHT as a predictor, proved to be more accurate than the other tests in predicting difficult laryngoscopy.

Abdel Naim et al.(10), performed a trial to find a suitable bedside examination tool which helps in prediction of difficult intubation.

They found that body mass index, neck circumference, Mallampati score > 3 and thyromental distance were the only helpful tests for bedside assessment for prediction of a difficult airway. They used magnetic resonance imaging and found that around the collapsible areas of the pharynx, there was more fat present in patients with OSA.

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Hence, body fat distribution may provide a better insight to difficult intubation when compared to neck circumference. They assessed the ratio of neck circumference to thyromental distance as a predictor of difficult intubation in patients with OSA and compared it with Mallampati scoring system. Fifty OSA patients of both sexes

between the ages of 18 – 60 years scheduled for general anesthesia with endotracheal intubation participated in the study. The Mallampati classification, thyrometal

distance, neck circumference, Body mass index and NC/TM ratio was calculated.

Intubation Difficulty Scale (IDS) was used to assess intubation difficulty. All the variables were correlated with IDS and comparison between Mallampati score and NC/TM ratio for predicting difficult intubation was done. The values of sensitivity and specificity were also measured.

Among the 50 patients, difficult intubation as per the IDS scale was observed in 11 patients. The statistically significant variables associated with a difficult intubation were NC/TM ratio and Mallampati score. There was a strong correlation of NC/TM ratio to difficult intubation(P value=0.01 odds ratio 37.5), but that of Mallampati score was lesser (P value=0.05 and odds ratio was 14.5). A higher sensitivity of 100%

and specificity of 82% was observed with NC/TM ratio, while Mallampati score had a sensitivity of 90% with a specificity of 61%. There was an independent association of difficult intubation seen in obese patients with OSA with an NC/TM ratio of more

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than 5.15 and Mallampati score of III or IV.

Parameters MP III/IV TMD MHD < 45 SMD < 12 II < 35

Sensitivity 33 18 20.5 14 22.9

Specificity 90 93 92 96 93.25

PPV 07 04 5.4 07 07

NPV 97 97 97 99 97

Odds Ratio 4.6 2.5 2.8 4.0 4.4

p value <0.001 0.017 0.004 <0.001 <0.001

Table 5: Tests for difficult intubation as per Cattano et al.

COMPARISONS OF VARIOUS AIRWAY ASSESSMENT TESTS:

In a study by Jain et al.(11) In a comparison among patients scheduled for general surgery versus those undergoing coronary artery bypass grafting it was seen that there was a higher incidence of difficult laryngoscopy and intubation among the patients who underwent CABG. It is very important that the prediction of a difficult laryngoscopy is done accurately in CABG patients so that hemodynamic responses and requirements of myocardial perfusion and oxygenation is reduced. In recent studies, the thyromental height test has proven to be one of the highly sensitive and specific tests for bedside assessment in the prediction of a difficult airway. In our

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prospective observational study, we evaluated the validity of the TMHT to predict a difficult laryngoscopy. In the study, 345 patients of both sexes between the ages of 35 to 80 years undergoing CABG belonging to American Society of Anaesthesiology class III were recruited in the study. Airway was assessed using thyrometal distance, sternomental distance, modified Mallampati test and TMHT. Direct laryngoscopy was done intraoperatively according to Cormack and Lehane laryngoscopic grading. The accuracy of TMHT was assessed with the findings of laryngoscopy grading and preoperative data. They calculated the positive predictive value, negative predictive value, sensitivity and specificity of the other tests. Among the 345 patients who were studied, the mean age of study population was 56.7 years (SD 9.1). It was seen that the specificity was good in all the tests but, they had poor sensitivity. The TMHT test had a sensitivity of 75% with 95% accuracy. An ROC curve analysis of TMHT showed a cutoff of 52.17. This increased sensitivity to 81.25% and specificity to 92.3%. This shows that a higher sensitivity and specificity with good negative and positive predictive values were seen with TMHT in comparison with other tests.

Noorizaad et al.,(12) compared the mallampatti test and thyromental distance and the following are the results (Table 6).

Table 6: Comparison of modified MP test and thyromental test (Noorizaad et al)

Parameters Modified MP test Thyromental test

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Sensitivity (%) 38.0 17.3

Specificity (%) 77 86.9

PPV (%) 12.2 9.9

NPV (%) 93.8 92.8

False Negative (%) 62.2 82.8

False Positive (%) 23.2 13.3

Accuracy (%) 73.80 81.51

As per Ander et al,(13) The laryngeal view of the inlet is greatly improved with the use of a videolaryngoscope. It has been shown that the time taken for intubation of obese patients is much lesser with the use of the Storz C-MAC videolaryngoscope in comparison to others. They wanted to assess whether the time taken for intubation of an obese patient was reduced with the use of the Storz C-MAC videolaryngoscope when compared to direct laryngoscopy (Macintosh blade). A total of eighty patients with BMI >35 kg/m2 participated in the study. They were

randomised into two groups for endotracheal intubation where, one group were

intubated with the standard Macintosh blade and the other group were intubated using either Macintosh laryngoscope, or the Storz C-MAC with Macintosh blade. There was no previous history of a difficult airway in these patients. The time taken from taking the laryngoscope by the anaesthetist until detection of end tidal carbon dioxide was

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calculated as Time-to-intubation (TTI). There was no significant difference in TTI among these two devices. The mean difference was 1.7 s (95% CI:6.9 to 3.5 s). In the videolaryngoscopy group, all the patients were intubated successfully whereas in the direct laryngoscopy group, five patients were able to be intubated successfully with the help of an alternative device.

According to Ahmadi et al,(14) It will be more difficult to intubate the trachea of obese patients when compared to patients of normal weight. In a total of 100 patients of morbid obesity with BMI more than 40 kg/m² a study was done to identify the factors which contribute to a complicated direct laryngoscopy and tracheal intubation. Measurements of weight, height, mouth opening, neck

circumference, thyrometal distance, sternomental distance and Mallampati score were measured preoperatively. A grading of the direct laryngoscopic view was along with a recording of the number of attempts at tracheal intubation was done. It was seen that there was no association of absolute obesity or elevated body mass index with

difficulty in intubation. A high Mallampati score and large neck circumference were the only variables that predicted problems with intubation difficulty. It is not known whether the neck circumference required an intervention such as fiberoptic

bronchoscopy for airway establishment. At the end of the study, neither BMI nor

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obesity were found useful to predict problems with tracheal intubation. But, the potential for difficult intubation may be increased in patients with a large neck circumference and a high Mallampati score ≥ 3. It was concluded that tracheal intubation difficulties cannot be predicted by obesity alone, but with the addition of other related factors.

In a study done by Hirmanpour et al,(15) A good assessment of clinical factors and landmarks of airway anatomy help in preoperative detection of laryngoscopic difficulties. The ability to predict a difficulty in laryngeal visualisation using various difficult airway predictors in combination and isolation was done in this study. The variables used were modified Mallampati test, upper lip bite test, the ratio of height to thyromental distance, neck circumference and neck circumference to thyromental distance ratio. A difficult laryngoscopy with CL grades ≥ III was reported in 53 patients (8.10%). Significant variations were seen in

RHTMD, NC, TMD and NC/TMD between easy and difficult laryngeal visualisations (P < 0.05). In the receiver operating characteristics curve analysis, the main end-point area was lower for MMT with AUC = 0.499, (95% CI, 0.050 - 0.535) and ULBT with AUC = 0.501, (95% CI, 0.460 - 0.541) when compared to RHTMD with AUC = 0.628, (95% CI, 0.590 – 0.665), NC with AUC = 0.692, (95% CI, 0.655 – 0.725),

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TMD with AUC = 0.607, (95% CI, 0.565 – 0.645) and NC/TMD with AUC = 0.688, (95% CI, 0.625 – 0.725). The six ROC curves had differences of statistical

significance (P < 0.05). This implies that in the predictive analysis of difficult intubation, the NC/TM is not comparable with NC, RHTMD and ULBT tests.

A study by Honarmand et al,(16) In a similar manner, he compared the predictability to visualise a difficult laryngeal view using the following variables for assessment of difficult airway, in both isolation and

combination. These were hyomental distance ratio, modified Mallampati test, the ratio of height to thyromental distance and upper lip bite test. In isolation, ULBT proved to be the most sensitive test with a sensitivity of 91% and the least sensitive was

hyomental distance with a 56.9% sensitivity. The sensitivity of HDMR was 86.3%.

The highest negative predictive value and area under the ROC curve in isolation was seen in the ULBT test. The area under the ROC curve was significantly lesser for MMT when compared with ULBT, RHTMD and HMDR (p < 0.04). In the area under the ROC curves for ULBT, HMDR, and RHTMD (P > 0.04) there were no differences of significance. This shows that in a general population, HMDR can be compared to RHTMD and ULBT in the prediction of laryngoscopic difficulty.

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Kim et al.,(17) made an analysis on 260 patients, obese and non obese and difficult intubation was analysed using Intubation

difficulty scale. The conclusion made was that the difficult intubations were more prevalent in the obese patients group. The intubation difficulty was associated

independently with a Wilson score of greater than or equal to two, NC/TMD ratio of greater than or equal to five and a Mallampati score of III or IV. An NC/TMD ratio of greater than or equal to five provided a moderate to fair sensitivity, specificity and a negative predictive value. It was also recommended that the Cormack and Lehane score alone was compared to the Intubation difficulty score for recording a difficult intubation because the IDS reflects all the moments of an endotracheal intubation whereas the Cormack and Lehane grading only considers the laryngoscopic view. A poor laryngoscopic view did not always correlate with a difficult intubation.

The other variables such as neck circumference, the NC/BMI and NC/SM were also analysed. These variables failed to show a positive correlation with difficult intubation. (Table 7).

MP III / IV TMD SMD NC/TM ≥ 5 NC/SM ≥ 2 Wilson Score

History of DA

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Sensitivity 60 58 48 89 82 48 36

Specificity 90 92 82 82 57 90 98

PPV 47.52 49.50 29 45 23 47 72

NPV 92.85 93 90 97 95 91 92

Table 7: Tests for difficult intubation as per Kim et al.,

In a study done by Safavi M et al,(18) he utilized the modified Mallampati test, thyromental distance, ratio of height to

thyrometal distance and upper lib bite test to predict difficult intubation in paediatric patients. The Cormack and Lehane classification was used to evaluate difficult laryngoscopy. Sensitivity, specificity, positive predictive value and AUC values for each test were measured. The sensitivity and specificity of modified Mallampati test were 76.92% and 95.54%, while those for ULBT were 69.23% and 97.32%. The optimal cutoff point for the ratio of height to thyromental distance and thyromental distance for predicting difficult laryngoscopy was 23.5 (sensitivity, 57.70%;

specificity, 86.64%) and 5.5cm (sensitivity, 61.56%; specificity, 99.05%). The modified Mallampati was the most sensitive of the tests. The ratio of height to thyromental distance was the least sensitive test in paediatric population

An analysis of logistic regression by De Leon et al

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(19) on height to thyromental distance ratio and difficult airway risk factors was done based on variables calculated preoperatively. These were thyromental distance, height, sternomental distance, inter incisor distance, neck movement range and modified Mallampati class. Difficult laryngoscopy was identified using IDS score. A patient with difficult mask ventilation or laryngoscopy was considered a difficult airway. The analysis showed that there were eight airway characteristics to contribute to a difficult airway, which are the BMI 3.08 (1.65-6.70), height-to-TMD ratio 3.56(1.91-8.40), history of a previous difficult airway 2.80 (1.25-5.20), modified Mallampati class 3.35 (1.80-7.15), a thick neck 2.10 (1.05-4.40), sternomental distance 1.45 (0.65- 3.10), neck movement range 1.99 (0.98-3.90) and inter-incisor distance 1.03 (0.51- 2.55). To predict a difficult airway, the height-to-TMD ratio cutoff was 22.8.

A study done by Anahita et al.,(20) on 657 obstetric patients concluded that the area under the receiver operating curve was lower for Mallampatti score with AUC = 0.497 (95% CI, 0.045 – 0.536) and ULBT with AUC = 0.500 (95% CI, 0.461 - 0.539) compared to RHTMD with AUC = 0.625 (95% CI, 0.590 – 0.665), NC with AUC = 0.690 (95% CI, 0.655 – 0.725), TMD with AUC = 0.605 (95% CI, 0.565 – 0.645) and NC/TMD scores with AUC = 0.689 (95% CI, 0.625 - 0.724) respectively.

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It also shows that 70% sensitivity and specificity with an NPV of 97% for NC/TM which other indices were not able to give. Most of the other indices provided isolated high sensitivity but extremely low specificity and PPV while the other two indices namely RHTMD and NC were giving low sensitivity and high specificity (Table 8).

The advantage of the Neck circumference / Thyromental height ratio is its increased sensitivity than the other two tests, thus decreasing the false negative (3.5%) prediction. This study supports the use of NC/TMH ratio

preoperatively to predict a potentially difficult airway.

Parameters MP III / IV ULBT HT/TM NC/TM

Sensitivity (%) 83 100 27 71.8

Specificity (%) 27 0.33 96 70

PPV (%) 10 8.2 32 17

NPV (%) 95 100 95 97

Accuracy 58 35 90.5 70

Likelihood Ratio 1.15 1.00 5.60 2.42

Odds ratio/relative risk 0.67/1.50 1.01/0.99 6.41/0.21 5.96/0.19

AUC of ROC curve 0.498 0.500 0.627 0.685

Table 8: Importance of NC/TM ratio as per Anahita et al.,

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As per Krobbuaban et al(21),

Evaluation of landmarks of airway anatomy and clinical factors prior to surgery can help in prediction of laryngoscopic difficulty, but reliability of the above is not clear.

In this study a comparison between odds ratio and predictive value of mouth

opening versus RHTMD, neck movement, TMD and Mallampati class was done. The results revealed that laryngoscopic difficulty (CL grade ≥ 3) was seen in 70 cases.

(12.6%). Fewer false negatives with a higher predictive value and sensitivity were seen with RHTMD in comparison with the other variables. For a difficult laryngoscopy as per multivariate analysis, there were three criteria which were independent for the same. These were Mallampati class ≥3, RHTMD ≥ 24 and neck movement ≤ 80 degrees. RHTMD had the highest odds ratio of 6.72 when compared with

Mallampati class (2.95) and neck movement (2.75) hence proving to be useful in the screening for difficult laryngoscopy.

According to Leoni et al(22), analysis of predictive factors for difficult mask ventilation in obese patients was done using the measurements of modified Mallampati test, patient's Height/Thyromental distance ratio, Inter-Incisor Distance, Protruding Mandible (PM), history of Obstructive Sleep Apnoea and Neck Circumference (NC). DMV was defined as Grade III mask

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ventilation by the Han's scale (MV inadequate, unstable or requiring two practitioners).

Independent DMV predictors were identified by multivariate analysis. BMI averaged 43.5±8.5 kg/m2. DMV was reported in 25 out of 300 patients (8.5%; 95%CI 5.6 -11.9%). The multivariate analysis retained NC (1.17; P<0.001), limited PM (1.96; P = 0.046) and Mallampati test (OR 2.10; P = 0.009) as risk predictors for DMV. Male gender was also included in the final model (1.87; P = 0.06) as

biologically important variable. Obese patients show increased incidence of DMV with respect to the undifferentiated surgical population.

In a study by Liaskou et al(23), Upper airway anatomy and its paramaters with more emphasis on oropharyngeal structures were assessed for diagnostic value in the prediction of laryngoscopic difficulty. They found that laryngoscopy was difficult in 12.6% of the study population. The values of TMD ≤7 cm, STMD ≤15 cm, RHTMD >18 and NC >37.5 cm were taken as cutoff in this study. The highest sensitivity (88.4%) and NPV (95.2%) was seen in RHTMD and the highest specificity (83.9%) in TMD. The area under curve (AUC) were as follows for the TMD 0.65, STMD 0.66, RHTMD 0.63 and NC 0.55 hence showing that these proved to be poor predictors in isolation for difficult laryngoscopy. A model which includes all the variables was found to have a significant accuracy for prediction of the

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According to Riad et al,(24) a study was done to assess whether a difficult intubation could be predicted in surgical patients with morbid obesity using the variables of BMI and neck circumference. They found that male sex, increased neck and waist circumference and BMI ≥ 50kg/m² were

associated with intubation difficulty and an increased thyromental distance in addition to the above was associated with difficult mask ventilation. An analysis with multiple logistic regression revealed that BMI ≥ 50 kg/m² and neck circumference ≥ 42cm were independently able to predict difficult intubation and Male sex and BMI ≥ 50 kg/m² in isolation were able to predict difficult mask ventilation.

According to a study by Wajekar et al(25), difficult intubation has an estimated incidence of 1.5 – 18% in patients scheduled to undergo general anaesthesia. Intubation failure has been reported at an incidence of 0.05 - 0.35% in the above population. All the patients had three airway assessment tests done preoperatively and the Cormack - Lehane classification was used during intubation to quantify the exposure of the laryngeal inlet. These tests were compared with one another using predictive values namely, the positive and negative predictive

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values, sensitivity and specificity. They found that laryngoscopic difficulty was calculated at 11.5% and intubation failure was measured at 0.50%. This showed that in isolation, none of these tests are reliable for assessment of a difficult airway. It was also seen that these tests when used in combination with each other showed an

increase in their diagnostic value. Hence they concluded that these tests when used in isolation have poor to moderate power of discrimination and when used in

combination are of diagnostic value.

USE OF IDS SCORE FOR INDIRECT LARYNGOSCOPY

Intubation difficulty indices

Macintosh Laryngoscope

IDL Z – test For

difference

Permutation test for difference

Interpretation

IDS score vs VAS difficulty

0.860 0.697 0.0001 0.0003 Correlation

significantly Increased for Mac vs IDL IDS score

vs duration of intubation

0.752 0.580 0.003 0.038 Correlation

significantly elevated for Mac vs IDL

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0.820 0.748 0.078 0.125 Correlation

not

different for Mac vs IDL

Table 9: Use of IDS score for Indirect Laryngoscopy

INDIAN EPIDEMIOLOGY:

As per Selvi et al(26), A study was done in an Indian population to estimate the incidence of intubation and laryngoscopic difficulty and also to assess the anatomy of the airway and risk factors contributing to them in the above study population. The study revealed that difficult laryngoscopy and intubation was

calculated with an incidence of 9.5% and 4.5%. By using univariate logistic regression analysis it was seen that male gender, Mallampati class ≥ 3, short neck, limited neck mobility, history of snoring and cervical spondylosis, thyromental distance, ratio of height and TMD, inter incisor distance ≤ 3.5 and limited protrusion of mandible were factors associated with laryngoscopic difficulty. When analysed further using

multivariate analysis with logistic regression to identify independent factors, it was seen that there were four variables which had independent association with difficult

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laryngoscopy. These were snoring, IID ≤ 3.5cm, limited neck mobility and

Mallampati class ≥ 3. During assessment of patients from India with a normal airway they found the incidence of difficult laryngoscopy and intubation was 9.7% and 4.5%.

Minor intubation difficulty was seen in 49% of the cases in the study population.

According to Dhanger et al(27), a study was done to

determine the incidence of difficult intubation and assess the validity of bedside tests in prediction of the same in an Indian population. Among the 200 patients that took part in the study, 25 patients had difficult intubation with an incidence of 13%.

Among different variables, the Mallampati score and NC/TMD were associated in isolation with difficult intubation. Area under ROC curve showed a cut-off point of 3 or 4 for Mallampati score and 5.62 for NC/TMD to predict difficult intubation. The diagnostic accuracy of NC/TM ratio and Mallampatti score were better compared to other bedside tests to predict difficult intubation in Indian population.

As per literature, it has been recorded way back in the past that there have been several instances of history of maintenance of difficult airway, especially a difficult laryngoscopy and intubation. In a study done by

Luckhaupt et al., The first tracheal intubation in a patient with dyspnoea was recorded

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by Avicenna in the year 1037 BC, an arabian doctor. Per oral endotracheal intubations were practiced as early as the 18th century. The first endotracheal intubation was

performed by William McEwen in the year 1880. Bannister et al emphasised that the position of the head and neck for direct laryngoscopy is of sole importance to align the oro - pharyngeal and laryngeal axes correctly for a successful endotracheal intubation.

A study done by Gillespie et al., showed that the flexion of the neck at

the lower cervical spine and extension at the atlanto-occipital joint will enable the alignment of the axes and thereby facilitate a correct position for endotracheal intubation. A study done by Cass et al., mentions about five main causes of

encountering a difficulty in laryngoscopy and endotracheal intubation and the factors contributing to difficulty in visualization of the glottis.

These were as follows:

 A short muscular neck

 A receding mandible with obtuse angles

 A outward protruding maxillary incisor teeth

 Immobility of the mandible caused by temporo-mandibular joint arthritis and ankylosis

 A high arched palate and a narrow long mouth.

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In the year 1997, obesity was recognised by the World Health Organisation (WHO) as a global health epidemic. The Lancet in 2013 published a study on the global burden of disease in obesity and it stated that there has been a steady increase in the rates of both obesity and overweight patients in adults, amounting to a 28% increase, with the number of overweight and obese patients rising from 857 million in the year 1980 to 2.1 billion in 2013. The estimation made by this report is that by the year 2015, 36% of the males and 28% of females will be obese and that these figures will have risen to 47% and 36% by the year 2025. In a study done by Unnikrishnan et al(28) ., the overall prevalence of overweight patents in India was 33% and that of obesity was 6.8% among men and women.

In a prospective observational study done by Majigoudar et al.,(29) the thyromental height test (TMHT) was compared with modified Mallampati test (MMT) and Thyrometal distance (TMD) for prediction of a difficult laryngoscopy. In this study, they hypothesised that TMHT was more accurate in predicting a difficult laryngoscopy when compared with TMD and MMT. Sixty consecutive patients between the ages of 18 - 65 years, belonging to ASA grade I and II, requiring GA for elective surgery were assessed and underwent airway evaluation with TMH, TMD and MMT tests. Following anaesthetic induction, the ease of laryngoscopic view was

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graded based on the Cormack and Lehane laryngoscopic grading. Laryngoscopic difficulty was defined as a CL grade of ≥ 3. Among the 60 patients recruited in the study, 48 patients belonged to ASA grade I and 12 patients were ASA grade II. The incidence of difficult laryngoscopy was 6.6%. TMH was less accurate when compared to MMT and TMD. TMH is a simple inexpensive test that can be done for assessment of difficult laryngoscopy which is comparable in terms of specificity and NPV but less accurate when compared to MMT and TMD.

Riad W et al.,(30) conducted a prospective observational study on obstetric patients, the aim of which was to calculate a cut off for neck circumference to predict laryngoscopic difficulty. In this study they recruited ninety four patients posted for elective caesarean section under general anaesthesia.

Airway assessment including neck circumference measurement was done for the patients before surgery. The primary outcome was considered as an intubation difficulty with IDS > 5. Analysis by univariate logistic regression showed that Neck circumference (P < 0.004) and Mallampati score (P < 0.010) were difficult intubation predictors. When multivariate analysis was done to assess independent predictors for intubation difficulty, it showed that both the variables were able to predict intubation difficulty in isolation. A sensitivity analysis with 100% sensitivity and 95% specificity

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