After obtaining approval from the Institutional Ethics Committee and written informed consent will be taken from the eligible patients with American Society of Anesthesiologists’ physical status (ASA PS) I and II who are scheduled to undergo elective surgery requiring endotracheal intubation will be enrolled in this prospective comparative observational single-center study. Each patient will be assessed before the surgery for a detailed pre-anaesthetic evaluation and clinical assessment of the airway parameters will be done. Airway assessment included are mouth opening, assessment of receding mandible or protruding upper teeth and subjective assessment of head and neck extension.

The following predictive test measurements will be obtained and recorded:

1. Acromioaxillo Suprasternal notch index (AASI) will be measured with the patients lying in supine position and the upper limbs resting by the side of the body. Using a ruler, a vertical line drawn from the top of the acromion process to the superior border of the axilla at the pectoralis muscle (line A). Then a second line drawn perpendicular to line A from the suprasternal notch [line B]. The portion of line A that lay above the intersection by line B on line A will be termed as line C. AASI is calculated by dividing the length of line C by that of line A (AASI = C/A). AASI value of >0.49 is considered as a good predictor for difficult visualization of larynx (DVL).

2. Thyromental height test (TMHT): The height between the anterior border of the thyroid cartilage (on the thyroid notch just between the two thyroid laminae) and the anterior border of the mentum (on the mental protuberance of the mandible) will be measured, with the patient lying supine using a pillow with mouth closed. This height will be measured with a digital depth gauge (INSIZE® Electronic Depth Gage, INSIZE). A measurement of < 50 mm is considered to be a good predictor of difficult visualization of larynx (DVL).

3. Modified Mallampati Classification (MMP): Mallampati classification with patient in the sitting position, mouth maximally opened, and tongue protruded, while the observer looking from the patient’s eye level will inspect the pharyngeal structures with a pen torch, without the patient phonating. The oropharyngeal view is graded as Class I where soft palate, fauces, uvula, pillars visible; Class II where soft palate, fauces, uvula visible; Class III where soft palate, base of uvula visible; and Class IV where soft palate not visible at all. Class I and II are classified as easy visualization of larynx (EVL), whereas Class III and IV are classified as difficult visualization of larynx (DVL).

4. Thyromental distance (TMD): It is measured as straight distance between the thyroid notch and the lower border of mental prominence, with the head fully extended and the mouth closed, using a rigid ruler. A measurement of ≤ 6.5 cm is considered to be a good predictor of difficult laryngoscopy.

5. Sternomental distance (SMD): It is measured as straight distance between the superior border of the manubrium sterni and the lower border of mental prominence, with the head in full extension and the mouth closed, using a rigid ruler. An SMD ≤ 13.5 cm is considered to be a good predictor of difficult laryngoscopy.

6. Neck circumference (NC): Circumference of neck measured using a measuring tape, at the level of thyroid cartilage. NC of > 40 cm is considered to be a good predictor of difficult laryngoscopy.

During endotracheal intubation the best laryngoscopy view will be noted according to the Cormack – Lehane (CL) grading system.

Grade I = full glottis view;

Grade IIa = glottis partly visible, anterior commissure not visualized;

Grade IIb = Arytenoids or posterior part of the vocal cord just visible.

Grade III = only epiglottis is visible;

Grade IV = no epiglottis visualized.

CL grades I and IIa are inferred as an easy visualization of the larynx (EVL) and CL grades IIb, III and IV as difficult visualization of the larynx.

‘Difficult Intubation’ is defined as the placement of the endotracheal tube by using conventional laryngoscopy that required > 2 attempts, lasted > 10 min, or required alternate methods to intubate.

The ‘time taken for intubation’ is defined as the time point from initiation of the first direct laryngoscopy attempt to confirmation of successful endotracheal intubation by continuous waveform capnography.

The study subjects would be advised about the fasting guidelines. Premedication in the form of tab. Pantoprazole 40mg will be given the day before and on the day of surgery. On the day of surgery, after confirming the duration of preoperative fasting, in the preoperative room, all enrolled patients will undergo a duly explained bedside Acromioaxillo Suprasternal Notch Index (AASI), Thyromental height test (TMHT), modified Mallampati test (MMP), Thyromental distance (TMD, Sternomental distance (SMD) & neck circumference (NC) tests.

Thereafter, the patients will be shifted to the operation theatre and standard monitors will be connected. After adequate pre-oxygenation, general anesthesia will be induced with inj. Propofol 2 mg/kg IV, inj. Midazolam 0.05 mg/kg IV, inj. Fentanyl 2mcg/kg IV, and inj. Succinyl choline 2 mg/kg IV. After 30-60 seconds of mask ventilation, a single experienced investigator (≥ 2 years of experience) will perform the laryngoscopy using Macintosh blade of appropriate size (3 or 4) in the sniffing position.  The glottic view obtained on first attempt of direct laryngoscopy without any external laryngeal maneuvers will be graded according to the Cormack–Lehane classification.

All laryngoscopies will be performed by a single experienced anesthesiologist who will not be involved in preoperative airway examination.