THE IMPORTANCE OF NECK CIRCUMFERENCE TO THYROMENTAL DISTANCE RATIO (NC/TM DISTANCE RATIO) AS A PREDICTOR OF DIFFICULT INTUBATION IN OBESE PATIENTS COMING FOR ELECTIVE SURGERY UNDER GENERAL ANAESTHESIA IN A TERITATRY CARE HOSPITAL – A PROSPECTIVE OBSERVATIONAL STUDY. DISSERTATION SUBMITTED IN PARTIAL

Background and objectives This study was done to assess the ability of neck circumference to thyromental distance ratio (NC/TM distance ratio) for predicting difficult intubation among obese patients coming for surgery under general anaesthesia. It enabled us to compare NC/TM distance ratio to routinely used Mallampati score and neck circumference as reliable tests for predicting difficult intubation. This study also identified incidence of difficult intubation among obese individuals. Patients and methods. After approval of institutional review board and ethical committee of Christian Medical College Vellore, 250 obese patients (body mass index greater than or equal to 30) within time frame of September 2014 and March 2015 was assessed preoperatively with the help of performa after obtaining informed consent. Neck circumference / thyromental distance ratio (NC/TM distance ratio) was calculated from the performa. Validated Intubation difficulty score (IDS score) for each obese patient was assessed intra operatively by the anaesthetist who performed intubation . The entire study population were divided into easy and difficult intubation groups based on the IDS score. IDS score greater than or equal to five was considered as difficult intubation. NC/TM distance ratio greater than or equal to five was correlated with IDS score greater than or equal to five. The study assessed the statistical significance of NC/TM distance ratio and difficult intubation by univariate and multivariate logistic regression analysis and its comparison with Mallampati score and neck circumference with respect to sensitivity / specificity/ positive predictive value and negative predictive value. The study also calculated the incidence of difficult intubation among obese patients Results Binary univariate logistic regression analysis of predictors of difficult intubation showed age greater than sixty, increased neck circumference, decreased thyromental distance, modified Mallampati test, NC/TMD ratio ≥ 5 as statistically significant variables that were associated with a difficult intubation (p ≤ 0.05). Binary multivariate logistic regression analysis showed only neck circumference (p=0.030 [odd ratio 2.519(1.0945.802)] and NC/TMD ratio (p <0.001 [odd ratio 23.680(10.638-52.713)] independently predicted difficult intubation. However NC/TMD ratio had higher specificity / PPV and larger AUC on an ROC curve compared to neck circumference. The incidence of difficult intubation among obese patients was 20.8 %. Interpretation and Conclusions. Among obese patients, NC/TMD ratio can be considered as a better preoperative predictor of difficult intubation and incidence of difficult intubation among them was as high as 20.8 percent.


INTRODUCTION
Obesity may be defined as a health condition in which excess of fat deposition occurs and has become a major health challenge. As per World Health Organisation (WHO), individual's whose body mass index (BMI) greater than or equal to 30 kg per square meter of body surface is termed as obese 1 . The study done by Misra et al., 73 among Asians, the definition of obesity has been changed to BMI ≥ 25 kg.m-2 for metabolic managements, but it doesn't effect the acute management of the airway, so we are considering BMI ≥ 30 kg.m-2 for airway assessment of obese patients. Inability to maintain oxygenation among the obese population leads to complications which can account for the 30% of the deaths. 2 The ASA (American society of anaesthesiologists) closed claim data analysis of adverse respiratory events had found out that one third of death was attributed solely to anaesthesia due to inability to maintain airway. 3 When anaesthesia malpractice claims were considered, difficult intubation was the second most frequent damaging event. 4

Most catastrophes
have occurred when possible difficult airway was not recognized early. 5 The importance of preoperative assessment of airway to reduce anaesthesia related complications has been evaluated over the last century. In view of all above mentioned findings several methods and techniques were developed, Cormack and However, measurement of neck circumference alone may not attribute to the amount of soft tissue at various topographic regions within the neck. Using magnetic resonance imaging (MRI), Horner 6 proposed that among obese patients with OSA'S, more fat was present in areas surrounding the collapsible segments of the pharynx. The study done by Ezri et al 7 using ultrasonography suggested that difficult airway among obese patients can be predicted by quantifying the neck soft tissue at the level of the vocal cords and suprasternal notch. They further noted that the only measurement that fully distinguishes easy and difficult intubation was the amount of pretracheal soft tissue as quantified by ultrasonography .The above findings point out that why some obese patients are easy to intubate , while others not.
Moreover by review of literature, we found that increased neck circumference had good sensitivity and relatively low specificity as well as decreased thyromental distance had high specificity and low sensitivity for predicting difficult intubation preoperatively. So the hypothesis was that by taking the ratio between these two above indices a new predictor of difficult intubation with better statistical and clinical outcome can be generated.
So, in this dissertation we aspire to explore a preoperative predictor of difficult intubation, named ratio of neck circumference to thyromental distance which needs no special equipment, minimal time for performance and is not uncomfortable to patient. It is a non invasive test which has got better statistical significance compared to other indices.

AIM OF THE STUDY
To assess the importance of neck circumference to thyromental distance ratio (NC/TM distance ratio) as a predictor of difficult intubation in obese patients coming for elective surgery under general anaesthesia.

OBJECTIVES OF THE STUDY
PRIMARY OBJECTIVE -To assess the correlation between the ratio of neck circumference to thyromental distance (NC/TM distance ratio) and validated intubation difficulty score 8 (IDS) in obese patients coming for elective surgery under general anaesthesia.
SECONDARY OBJECTIVES-1.To compare neck circumference / thyromental distance ratio (NC/TM distance ratio) with Mallampati score and neck circumference as reliable tests for predicting intubation difficulty in obese patients.
2. To find out the incidence of intubation difficulty among obese individuals coming for elective surgery under general anaesthesia.

HYPOTHESIS
The ratio of neck circumference and thyromental distance greater than or equal to five will predict difficult intubation and will have better statistical and clinical significance as compared to other standard indices of airway assessment among obese patients.

ANATOMY OF AIRWAY
The terminology "airway" means extra pulmonary air passage and it consists of nasal and oral cavities, pharynx, larynx, trachea and bronchi. The major functions of the airway in an awake state include filtration and conditioning of ambient air, humidification, and conduction of air to and from the lungs for gaseous exchange.
Due to suppression of nervous system which controls the vital respiratory function, the airway is converted to passive state during induction and maintenance of anaesthesia. The ability to ventilate the patient by either bag mask or to intubate is essential for the anaesthetist at this state. In order to anticipate difficult airway and to formulate a plan of safety for the patient, he/she should be well versed with airway anatomy, its application, and various methods of airway assessment.

Laryngeal inlet
It faces backward and upward and opens into the laryngeal part of the pharynx. Its opening is bounded by: • Anteriorly : by the upper margin of epiglottis  • Motor  All intrinsic muscles, except cricothyroid is supplied by the recurrent laryngeal nerve.
 External laryngeal nerve, a branch of the superior laryngeal branch of vagus nerve supplies cricothyroid muscle.

Laryngoscopic anatomy
Getting the mouth, the oropharynx and the larynx into one plane is essential to view the vocal cord at direct laryngoscopy and to proceed with intubation. Elevation of the head about 10 cms with pads under the occiput with shoulders remaining on the table aligns the laryngeal and pharyngeal axis. Flexion of the neck and extension at the atlantooccipital joint creates the shortest distance and most nearly straight line from the incisor teeth to glottic opening. This position is termed the sniffing position.

Definition of obesity
As per World Health Organisation (WHO), individual's whose body mass index (BMI) greater than or equal to 30 kg per square meter of body surface is termed as obese. 1 As per Misra et al., 73 for Asians, the definition of obesity has been changed to BMI ≥ 25 kg.m-2 for metabolic managements, but it does not effect the acute management of the airway so we are considering BMI ≥ 30 kg.m-2 for airway assessment of obese patients

Obesity respiratory pathophysiology
Associated with obesity, various pulmonary disorders are of major concern to anaesthetists. Most amongst these are obesity hypoventilation syndrome /obstructive sleep apnoea and cor-pulmonale. In addition to above, patients with morbid obesity usually have decreased pulmonary reserve even if they do not have specific pulmonary disorder. These patients also have an increased incidence of restrictive pulmonary disorder. Morbidly obese patients have reduced forced vital capacity (FVC) functional residual capacity (FRC) and total lung capacity (TLC) with decreased expiratory reserve volume and increased respiratory resistance.

Classification of obesity 1, 67
The International Classification of adult obesity according to BMI  Obese class 3 ≥ 40 Table 2: Obesity classification according to body mass index (BMI).

Obesity and its anaesthetic implications 9
The major anaesthetic challenges in obesity are due to the pathophysiological changes of obesity. They include changes in airway, respiratory system, cardiovascular system, gastrointestinal system and pharmacological variations.

Definition of difficult airway
The definition of difficult airway by ASA task force is "the clinical situation in which a conventionally trained anaesthesiologist experiences difficulty with mask ventilation, difficulty with tracheal intubation, or both." It also involves complex association between patient factors, skills and preference of the practitioner and the clinical scenario.

Definition of difficult mask ventilation
They defined difficult mask ventilation as: 1. Situation in which 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.
2. Situation in which unassisted anaesthesiologist is unable to prevent or reverse signs of inadequate ventilation during positive pressure mask ventilation.

Definition of difficult intubation.
They also defined difficult intubation when "proper insertion of the tracheal tube with conventional laryngoscopy requires more than three attempts or more than 10 minutes".

PROBLEM STATEMENT
Obesity is associated with hypertension, ischemic heart disease, diabetes mellitus as well as difficult airway and intubation. Difficult intubation can increase the morbidity and mortality and is often associated with obesity. As per recent study by Unnikrishnan et al., 11 prevalence of obesity among Indian population is around 7%. The pilot study done by primary investigator among patients who had preoperative anaesthesia check up in Christian medical college, Vellore showed prevalence of obesity as 11 %.
The incidence of intubation difficulty among obese individuals ranges from 11-22 percent as per various literatures published. The incidence of intubation difficulty among obese population as reported in a meta-analysis by Shiga et al., 12 and Juvin et al., 13  In view of significant number of obese patients undergoing surgery daily for various reasons and the literature highlighting the increased incidence of difficult intubation, we decided to look for the predictor to anticipate the difficulty and been able to plan the management.

JUSTIFICATION FOR THE STUDY
The preoperative identification of difficult airway decreases anaesthesia related morbidity significantly. But there is no single bed side screening tool which provides accurate identification of difficult airway preoperatively. In our pre anaesthesia clinic we use Mallampati score as a routine screening test for assessment of airway. As per meta analysis done by Lundstrom et al., 16 Mallampati score III or IV were found in only 35 15 Anahita et al., 49 NC/TM distance ratio will enable us to consider all the above mentioned features for predicting difficult intubation in obese patients.
There are no Indian studies which showed significance of the same.
Moreover as per Ezri et al., 7 and Horner et al., 6 difficulty in intubating an obese patient depends upon, 1) Amount of neck soft tissue at the level of suprasternal notch and vocal cords.
 The measurement of neck circumference will clinically quantify amount of neck soft tissue at the level of vocal cords and suprasternal notch.
 The measurement of thyromental distance will clinically quantify amount of pretracheal soft tissue and also provides distribution of the fat in anterior neck.
The above two factors responsible for difficult intubation among obese patients as mentioned by Ezri et al., 7 and Horner et al., 6  Vander Linde et al., 33 suggested that no single anatomical factor determined the ease of direct laryngoscopy, but rather a combination of them.
Syker et al., 34 in their report on confidential enquiries into maternal death in the The study done by Unnikrishnan et al., 11 showed that the overall prevalence of overweight in India was 33.5% (35.0 vs 32.0) and of obesity was 6.8 % (7.8 vs 6.2 ) among women and men respectively.

POPULATION
The incidence of intubation difficulty among obese individuals ranges from 11-22 percent. The incidence of intubation difficulty among obese population has been reported in a meta-analysis by Shiga et al., 12 and Juvin et al., 13 70 showed 17 % of difficult intubation There are not many validated Asian / Indian studies which shows incidence of intubation difficulty among obese population.

PREDICTOR ESPECIALLY FOR OBESE POPULATION?
As per Juvin et al., 13 which studied 134 non obese and 129 obese population , intubation difficulty was noted in 3 non obese patients and 20 obese patients ( p=0.00001) which accounts for 2.2% and 15.5 % respectively. The mean minimal value of oxygen saturation during the intubation was 88% ± 10% (range of 50%-99%) among the 20 obese patients where intubation difficulty was noted, whereas it was 96% ±7% (range of 64%-100%) in the obese patients for where easy intubation was noted (P = 0.0006). The oxygen saturation values noted during intubation were 99% ± 1% and 95% ± 8% in lean and obese patients, respectively. The high risk of desaturation as well as more difficulty in intubating obese patients warrants research to identify difficult intubation predictors among them preoperatively.
As per Lavi et al., 38  When comparing obese and non obese population, as per Shailaga et al., 37 incidence of intubation difficulty among obese patients was slightly high.
The metaanalysis done by Shiga et al., 12  However, Gaszynski et al., 65 used ASA definition of intubation difficulty (more than 3 attempts or duration ˃ 10 minutes) among 87 obese patients and reported that the incidence was similar among obese and lean patients (4.6%). But here they used ASA definition of difficult intubation, which is no longer practically recommended.
In a prospective, Canadian study 47 among general surgical patients, tracheal intubation was recorded as difficult, as well as there was need of multiple larygoscopies when the patient population was obese. (P <0.01). A higher incidence of difficult intubation (17%) was noted by Rita et al., 70 in a study of 210 obese patients .
From the literature search it can be concluded that there is increased incidence of difficult intuabtion among obese population as well as increased risk of desaturation while intubation, which warrants preoperative identification of difficult airway especially for them so that morbidity and mortality can be decreased. Therefore, analyzing the individual factors that are closely associated with intubation difficulty is important and is further mentioned below.

PREDICTORS OF DIFFICULT AIRWAY
The metaanalysis done by Shiga et al., 12   As per Juvin et al., 13 who studied obese and lean patients concluded that only independent risk factor for difficult intubation among obese patients was Mallampati score of III or IV.(Odds ratio 12.51; 95% CI, 2.01-77.81), but it has low specificity and positive predictive value. They pointed out the inability of the classic risk factors to predict intubation difficulty in obese patients. The high risk of desaturation as well as more difficulty in intubating obese patients warrants research to identify difficult intubation predictors among them preoperatively The modified mallampati score was described as a moderately good (60%) predictor of intubation difficulty among obese patients as per Lavi et al., 38 .
The study failed to establish a single preoperative predictor for difficult intubation and hence warrant new predictors.

PREDICTOR OF DIFFICULT AIRWAY
(High specificity and low sensitivity) The study done by Abdel et al., 19 showed a high specificity (99) percent with NPV OF 92 percent and low sensitivity for decreased thyromental distance to predict difficult airway.
As per Hiremath et al., 46 who analyzed 15 OSA and Non OSA patients found that intubation difficulty was associated with decreased thyromental distance , mandibular length and greater soft palate length ( p< 0.05) . However sample size was too small to give a comment.
El Ganzouri et al., 25 illustrated decreased thyromental distance of < 6 cm, as a risk factor for intubation difficulty with sensitivity of 16.8% and specificity of 99%, PPV of 15 % and NPV of 99% As per Rose et al., 47  Tse et al., 26 concluded that thyromental distance less than 7 had a very low sensitivity 32 % and a low positive predictive value (PPV) of 20% and specificity of 80 and NPV of 89 when used alone.
As per Cattano et al., 27    As per Brodsky et al., 22 who studied on hundred morbidly obese patients, neither obesity nor increased body mass index predicted problems with tracheal intubation. However, a high Mallampati score (III or IV) and large neck circumference may increase the potential for difficult intubation. Increased neck circumference was the only patient risk factor that did have a significant effect on the probability of intubation difficulty (P =0.02). The logistic regression model predicted that the odds of a problematic intubation in a particular patient with a neck circumference 1 cm larger than that of another patient are 1.13 (95% CI, 1.02 to 1.25) times the odds of the patient with a 1-cm smaller neck circumference.
With a neck circumference of 40 cm and 60 cm, the probability of a problematic intubation was approximately 5% and 35 % respectively. This study strongly recommended neck circumference as a predictor of intubation difficulty among obese population.
Hekiert et al., 23 studied among obese patients who underwent tracheal intubation under general anaesthesia found out that Mallampati score was of limited utility to the anaesthetists as a difficult airway predictor. Increased neck circumference in female patients was correlated with increase in Cormack-Lehane score (p = .02).
Iyer et al., 48 analyzed patients who had undergone gastric banding concluded that severe obstructive sleep apnoea and neck circumference more than fourty four centimeter were factors associated with intubation difficulty.
As per San Lee et al., 24 for patients with intubation difficulty, their neck circumferences were significantly increased (P = 0.014). Moreover, 70% of the patients with difficult intubations had neck circumferences ≥ 40 cm and 35% of the patients with easy intubation had a neck circumference ≥ 40 cm. Thus, the factor that maximally influenced the intubation difficulty was the thickness of the neck. They concluded that if the Mallampati score is III or IV and the neck circumference is greater than or equal to 40 cm, then it can be predicted that intubation will be difficult, so proper plan for intubation should be made.
Most of the studies recommended increased neck circumference as a good sensitive indicator with good NPV, but lacks better specificity for predicting difficult intubation.

PREDICTOR.
(High sensitivity, specificity and NPV) There is no single gold standard bed side scoring system for predicting difficult intubation among obese individuals. The most commonly used bed side screening tool for detecting difficult airway is Mallampati score (in preanaesthesia clinic of our hospital) or a combination of multiple scoring system.
There are very few studies which analysed NC/TM distance ratio as bedside tool for predicting difficult airway. Following are the studies.
Kim et al., 18 analysed 260 patients (obese and nonobese) and intubation difficulty was analysed by using intubation difficulty score scale. The conclusion was difficult intubations were more common in the obese patients group.
Intubation difficulty was independently associated with a Wilson score greater than or equal to two, NC/TMD ratio greater than equal to five and Mallampati score III or IV. A NC/TMD ratio greater than or equal to five provided a moderate-to-fair sensitivity, specificity, and a negative predictive value.  The study itself has many limitations like not blinded adequately, lack of use of ramp position which is supposed to be the initial position for the obese patients and use of standardized scope for the first attempt instead of use of blade size depending on the need..
In our study we have given the operator to decide the technique to be used for the patient and record the score as per the used technique. There was no strict rule that intubation should be done only with one particular technique.
The study done by Anahita et al., 49 on 657 obstetric patients has come to a conclusion that AUC of the receiver operating curve, was lower for The advantage of neck circumference/ thyromental ratio is its increased sensitivity than the other two tests, thus decreasing false-negative (3.4%) predictions. This study strongly supported the use of assessing NC/TMD ratio preoperatively to predict a potentially intubation difficulty as it is an easy and simple test.
But this study was done on obstetric patients and anatomical airway variation in obstetric population is being described also. The following

INTUBATION. WHY DID WE SELECT INTUBATION DIFFICULTY SCALE (IDS) SCORE?
There are multiple methods of scoring intubation is being described.     As per the pilot study mentioned above, eleven percent of patients seen in pre anaesthesia clinic were obese and difficult intubations were recorded for twenty two percent among obese patients. By looking at these numbers, the need and feasibility of our study was arrived.

DATA COLLECTION
Data collection was done in two steps. 2. Thyromental distance (cm) -measured using a measuring tape and is termed as the distance from thyroid notch to mentum with neck fully extended.
3. The ratio of neck circumference to thyromental distance.

Intra operative assessment
Difficulty of intubation was assessed by anesthetist by filing up intubation difficulty score (validated IDS score) 8 after intubation. .Intubation difficulty score consist of seven variables from N1 to N7. The sum of N1 to N7 gives total IDS score. Any score of greater than or equal to five was considered to be difficult intubation and score less than five considered to be easy intubation.  Any extra technique other than standard direct larngoscopy will get additional points (For example, use of Bougie / Glide scope / Fibre optic intubation / Video assisted intubation.)  First attempt of intubation to be done by anesthetist of at least three years of experience in anaesthesia and airway management

DATA MANAGEMENT AND STATISTICAL METHODS
All the study related records were kept in the sole custody of the principal investigator. Completed consent forms and questionnaires were locked in cupboards. All digital data were remained in password-protected computer file or tablet PCs. To further protect the identity of the participants, each person participating in a study was assigned a subject code number at the top of consent form, and was used whenever possible instead of that person's name. This code number was used in the request to anesthetist who was intubating the patient. This way the principal investigators were unaware of the patient details.

Statistical methods
The study obese populations were divided into easy intubation group (IDS score < 5) and difficult intubation group (IDS score ≥ 5). The study variables were expressed as mean (standard deviation). The differences between both sexes were analyzed using Chi square test and differences between the difficult intubation and easy intubation groups was analyzed using a univariate binary logistic regression model to find out the significant risk factors for intubation difficulty.
The different variables compared were the following: age, height, weight, gender, experience of the anaesthetist, previous h/o difficult intubation, body mass index, Mallampati score, NC, TMD and NC/TM distance ratio. In the second step, in order to find out independent risk factors of intubation difficulty, all the significant variables from the previous step were analyzed using binary multivariate logistic regression (forward-Wald model) Receiver operating characteristic (ROC) curve were used to identify the diagnostic performance of the significant risk factors. After identifying the adequate cut-off points by selecting the maximum specificity while sensitivity ≥ 80%, the continuous variables will be transformed into binary variables to compare the accuracy of the tests. A value of P < 0.05 was considered to be as significant.

DATA ANALYSIS ACCORDING TO OBJECTIVES OF THE STUDY
1. Primary objective -To assess the correlation between the NC/TM distance ratio and validated intubation difficulty score.

2.
Secondary objectives-To compare neck circumference / thyromental distance ratio with Mallampati score and neck circumference as reliable tests for predicting difficult intubation.

Analysis of primary objective
The study population as per sample size was divided into easy intubation group (IDS score < 5) and difficult intubation (IDS score ≥ 5)

FALSE POSITIVE
= An easy intubation that had been predicted to be difficult.
TRUE NEGATIVE = An easy intubation that had been predicted to be easy.
FALSE NEGATIVE = A difficult intubation that had been predicted to be easy.

OR
Percentage of correctly predicted easy intubations as a proportion of all predicted easy intubations, i.e.: True negatives / (true negatives + false negatives)

Analysis of secondary objective
Classified IDS scores into three categories for the purpose of calculation of incidence of difficult intubation.
 Easy intubation-zero score  Slight difficult intubation -score between zero and five.
 Moderate to major difficult intubation -score greater than or equal to five.

ETHICAL CONSIDERATION
There was no potential risks/harm to the patients in our study since it was a cross sectional observational study with noninvasive techniques. The study involved preoperative detection of difficulty in proving general anaesthesia in obese population using a simple tool which did not have any harmful effect on the subject.

RESULTS
A total of 328 obese patients were assessed for our study between September 2014 and March 2015 and among them, 250 patients who underwent endotracheal intubation were recruited for the study after obtaining informed consent. The patients excluded were those who underwent only regional anaesthesia, those who had regional blocks alone, those who had surgery using laryngeal mask airway and not willing to participate for the study. The SPSS software (version 16.0) was used to analyze the data. A total of 250 obese patients who underwent tracheal intubation were divided into two arms, namely easy intubation group (IDS score less than 5) and difficult intubation group (IDS score greater than or equal to 5). There were 52 and 198 patients among easy and difficult intubation groups respectively. The following are the results.  Comorbidities: Among the ASA 2 and 3 patients, the specific comorbidities are elaborated in the pie chart below. The category "others" includes cerebrovascular accident, bronchial asthma, intracranial mass, chronic obstructive pulmonary disease.  Table 13: Binary multivariate logistic regression analysis to determine the independent risk factors of intubation difficulty (forward-Wald analysis) * Significant correlation as P value ≤ 0.05. ** Highly significant correlation as P value ≤ 0.01.  The following are the ROC Curve for various predictors of DI

RESULTS OF SECONDARY OBJECTIVES
The second secondary objective was to find out incidence of intubation difficulty among obese population. The difficulty of intubation was determined using IDS scale. A score of zero indicates easy intubation, score between zero to five denotes slight difficulty, score greater than or equal to five shows moderate to major difficulty and score infinity shows impossible intubation. The following pie diagram illustrates the incidence of DI among obese patients.
This pie chart clearly states that the incidence of mild to major difficulty in intubation among the obese patients is around 92%. Further categorizing the intensity of difficulty of intubation , 20.8% of the patients had the major difficult intubation whereas rest are slightly difficult .

SUBANALYSIS
The following are the sub analysis of the study which is illustrated in the  -85 - The following pie diagram depicts various techniques/ instruments used for assisting intubation among obese patients. The term "others" denoted use of fibreoptic intubation, Macoy, small size endotracheal tube and C-MAC.

DISCUSSION
This study was done among obese patients to identify the significance of NC/TMD ratio as a difficult intubation predictor, its comparison with standard DI predictors and also to calculate incidence of difficult intubation among them.
Following are the discussion of the analyzed data.

A) DEMOGRAPHIC DATA
Analysis of demographic data revealed that difficult intubation was more common among females (59.6 %). Most of the patients of difficult intubation group belongs to ASA classification 2 and 3 (73.1 %). The co-morbid condition common among obese patients were in order of essential hypertension, diabetes mellitus and hypothyroidism. More than fifty percent of obese patients were diagnosed to have essential hypertension. The significance of the previous history of difficult intubation was not able to assess since, it was not well documented previously or patient was unaware of it.

SUMMARY
Difficult intubation can increase morbidity and mortality related to anaesthesia and is often associated with obesity. One of the methods to achieve successful airway management during surgery is to have a preoperative anaesthetic evaluation with emphasis on difficult airway predictors. Using magnetic resonance imaging (MRI), Horner 6 proposed that among obese patients with OSA'S, more fat was present in areas surrounding the collapsible segments of the pharynx. So distribution of fat in anterior neck may give a better suggestion of intubation difficulty than measuring circumference of neck alone.
We assumed that obese patients have a large amount of neck soft tissue that can be presented by the ratio of neck circumference to thyromental distance .Recently a new index was proposed for evaluation of difficult intubation in obese patients -the neck circumference to thyromental distance ratio. It was proposed that a NC/TM distance ratio ≥ five was a better method than other established indices in western population. In this study we analyzed the ability of NC/TM distance ratio greater than or equal to five to predict difficult intubation in obese Indian population. We measured the correlation between difficult intubation determined by Intubation Difficulty Scale and NC/TM ratio and its importance in predicting difficult intubation .
The methodology involved explaining the patient through an information leaflet, obtained informed consent and recruited them into study. Then a preoperative interview was conducted based on a performa covering demography and airway assessment. Intraoperative assessment of intubation was done using validated IDS scale by the anaesthetist who intubated the case.
Following that univariate and multivariate analysis were done and sensitivity and specificity of each variable was calculated.
The study concluded with the following results as, NC/TMD ratio can be considered as better bedside screening tool for predicting difficult airway               -115 -

CLINICAL RESEARCH FORMS
Preoperative assessment of obese patients.
Intubation difficulty score sheet

1) WHAT IS OBESITY?
Obesity is a medical condition in which excess body fat has accumulated to the extent that it may have a negative effect on health, leading to reduced life expectancy and/or increased health problems. People are considered obese when their body mass index (BMI), a measurement obtained by dividing a person's weight by the square of the person's height, exceeds 30 kg /m 2 . (WEIGHT/ HEIGHT ) 2

2) WHAT IS GENERAL ANAESTHESIA?
General anesthesia is a medically induced coma and loss of protective reflexes resulting from the administration of one or more general anesthetic agents. A variety of medications may be administered, with the overall aim of ensuring sleep, amnesia, analgesia, relaxation of skeletal muscles, and loss of control of reflexes of the autonomic nervous system.

3) WHAT IS ENDOTRACHEAL INTUBATION AND DIFFICULT INTUBATION?
Tracheal intubation, is the placement of a flexible plastic tube into the trachea (windpipe) to maintain an open airway or to serve as a conduit through which to administer certain drugs.
An intubation is called difficult if a formally trained anesthesiologist needs more than 3 attempts or more than 10 min for a successful endotracheal intubation.

4) WHAT ARE THE CONSEQUENCES OF OBESITY?
 Coronary heart disease ( Heart attack)

1) WHAT IS THIS STUDY ABOUT?
Obese patients can have above mentioned consequences as well as experience difficulty in intubation (difficulty to provide general anesthesia) This study is about preoperative prediction of difficult intubation by measuring the ratio of neck circumference / distance between thyroid cartilage and chin thereby reducing incidence of difficulty in providing general anesthesia by taking extra precautions/ additional equipment to maintain airway. Neck circumference and thyromental distance are measured using a measuring tape as part of evaluation of airway preoperatively .These measurements are considered for assessing probability of intra operative difficult intubation..

2) WHAT IS NECK CIRCUMFERENCE/ THYROMENTAL DISTANCE? HOW IS IT MEASURED?
Neck circumference is measured using a measuring tape at the middle portion of neck and thyromental distance is measured as the distance from thyroid notch to chin when neck is fully extended.

3) WHAT ARE THE RISKS AND BENEFITS TO ME IF I TAKE PART?
There are no risks involved in this study. The benefits are it will detect difficulty in providing general anesthesia among obese patients preoperatively and will provide a new bed side screening test . .

4) CAN I WITHDRAW FROM THE STUDY AFTER SIGNING CONSENT FORM?
You can always withdraw from the study at any point of time

5) WILL MY NAME AND PERSONAL DETAILS BE PUBLISHED/GIVEN TO A THIRD PARTY?
Your name and personal details will be kept confidential. There will not be a passage of your personal information to third party.