The Airway "An opening must be attempted in the trunk of the trachea, into which a tube of reed or cane should be put; you will then blow into this, so that the lung may rise again and the heart becomes strong." Vesalius 1555 The anaesthetist is responsible for maintenance of the airway and gas exchange. 28% of deaths due to anaesthesia are due to inability to intubate or ventilate (ASA). Normal airway patency is maintained by tension in the muscles connecting the larynx to the sternum, skull and mandible. Contraction of the pharyngeal dilator and tongue muscles opens the airway in a phasic manner during inspiration. Loss of muscle tension due to deep sleep, alcohol, sedation or disease may lead to pharyngeal obstruction, specially in the presence of anatomical abnormalities. The oropharynx is most prone to obstruction (posterior border of the tongue). As it is also part of the gastro-intestinal tract the protective airway reflexes are essential to prevent aspiration. Management of the difficult airway depends on the experience of the anaesthetist, the equipment available, and the degree of preparation for the event. Therefore pre-operative examination and assessment of the airway are essential. Airway Assessment The purpose of airway assessment is to assess the degree of ease I difficulty of: l. mask ventilation 2. laryngoscopic access and tracheal intubation The difficulty may be easily recognized or the potential difficulty may be subtle and require careful examination. Five predictors for difficult mask ventilation is given in the word OBESE. Obese (BM1>26), Bearded, Elderly (> 55years), Snoring, Edentulous 1. HISTORY • Age (limited head extension), pregnancy, diabetes • Dentition (missing, capped, damaged or loose teeth) • Hoarseness, snoring : exclude sleep apnoea (see page 4.32 ) • Rheumatoid arthritis, cervical spondylosis, radiation, scarring of the neck • Previous surgery of the neck e.g. tracheostomy, thyroidectomy • Hemi-mandibulectomy, tempero mandibular joint dysfunction • Cranio-facial abnormality syndromes (Down's, acromegaly) • Previous anaesthetics with airway problems 

EXAMINATION Face Teeth The profile highlights deformity e.g. maxillary protrusion and receding chin where the sub-mandibular space is inadequate for the tongue. Incisor distance on mouth opening <2 fingers (blade cannot be inserted) Missing left incisors and loose teeth : blade will damage teeth or slip into the gap, interfering with the line of vision (LOY) Maxillary teeth protruding anterior to the mandibular teeth ("buck teeth") where the blade will enter in a cephalad direction. Jaw : Inability to protrude the jaw anterior to the maxillary teeth (small jaw, restricted submandibular space, tongue not displaced easily e.g. Down's syndrome). Mouth : Narrow high arched palate, large tongue Neck : Obesity, short I thick neck (>45cm) collar size >17, buffalo hump Inadequate range of motion of the head (flexion <35°, extension <300) 3. TESTS No one test is fail safe and all checks (A,B,C) should be done. Together they improve predictability. A. MALLAMPATI TEST modified by Samsoon and Young (Fig. 4.1) This test estimates mouth opening as well as the tongue I pharyngeal relative size by assessing how much of the pharynx is obscured by the tongue. Seat patient with the head in neutral position, and ask him to open the mouth wide, and protrude the tongue as far out as possible, without phonation. Keeping the observer's eyes in line with the patient's mouth, classify the airway, according to the structures visible in the pharynx. Higher the class the more difficult the intubation is likely to be. Hard pala1e Soft pala1e Pilars ~eo--II III IV Figure. 4.1 View of the pharyngeal structures in the Mallampati test Class I : Soft palate, fauces, uvula tip, back of pharynx, posterior tonsillar pillars Class II : Soft palate, fauces, uvula, back of pharynx Class III: Only soft palate and base of uvula Class IV: Only hard palate 

Limitations of the Mallampati test There are significant false negative and false positive rates, as this test fails to consider neck mobility and the size of the mandibular space. There is also significant inter observer variability in classification. It is a specially useful test in patients with sleep apnoea (see page 4.32). B. ATLANTO-OCCIPITAL HEAD EXTENSION Head extension at the atlanto-occiptaljoint of30-35° with flexion at the atlanto-axial joint brings the laryngeal and pharyngeal axes nearly into a straight line (Fig. 4.5c). There/ ore assessing the adequacy of head extension is mandatory. Measurement of head extension must not be done in the supine position as apparent head extension can occur due to bowing of the cervical spine even if the atlanto occipital joint is fixed. Such head extension during intubation displaces the larynx anteriorly interfering with the line of vision (Fig. 4.6). Tests for head extension (normal 35°) • Estimate the degree of head extension by the angle traversed Ask the patient to sit with head held erect facing directly to the front and then extend the head to look at the ceiling. • Delilkan's sign Place the index finger of the left hand under the tip of the jaw and the index finger of the right hand on the occipital tuberosity with the patient's head in the neutral position. Then ask the patient to look at the ceiling. (Fig. 4.2) Extension normal : L finger higher than R Difficult : L and R same level Extremely difficult : L finger lower than R Figure 4.2 Delilkan 's sign 

Measure the sterno-mental distance (Sava distance) Sternum v A Thyromental distance (Patel distance) B Sterno-mental distance (Sava distance) Figure 4.3 Patel & Sava distances In the extended position, measure the distance from the supra-sternal notch to the tip of the chin. The critical distance is 12.5 cm. (Fig 4.3 B) C. THE SUB-MANDIBULAR SPACE Thyro-mental distance (Patel) measures the sub-mandibular space (Fig. 4.3A). Extend the patient's head and measure from the tip of the chin to the thyroid with a ruler or finger breadths. Easy if >6.5cm or> 3 finger breadths Very difficult if <6cm or <3 finger breadths (adults) Easy if > 2 adult finger breadths for a child, and > 1 for an infant The sub-mandibular space (anterior to larynx) determines how easily the laryngeal axis will align with the pharyngeal axis when the atlanto-occiptal joint is extended. It is difficult to displace the tongue when the submandibular space is small. The larynx is also more anterior in these patients. Both factors interfere with the LOV. (Fig 4.6) THE RULE OF 3, 3, 2 gives good predictability. Mouth opening 3 finger breadths, Thyro-mental distance 3 finger breadths, Thyroid-to floor of mouth 2 finger breadths Investigations • X-ray of head and neck (neutral and extended positions) for "Bamboo spine" or atlanto-occipital restriction (occiput -C1-C2 gaps) • CT scan for tracheal obstruction, deviation, stenosis, retro-sternal tumours, pharyngeal tumours, abscess, oedema, etc. • Indirect laryngoscopy 

DIFFICULT AIRWAY PREDICTION IN DIABETES 1. Palm print (with ink) : Grade 1 Grade 2 Grade 3 all digits clear inter-phalangeal areas of 4'h and 5'h not seen inter-phalangeal areas of 2"d to 5'h not seen Grade 4 only tips of digits seen 2. Prayer sign : bring palms together and look for gaps in between. POSITIONING FOR DIFFICULT AIRWAY IN OBESITY AND PREGNANCY Obese patients need a higher elevation of the head to maximize upper airway patency. The Head Elevated Laryngoscopy Position (HELP) is obtained by placing a ramp of pillows or towels to ensure a 30° elevation of the upper back and shoulders. The head is elevated, but flexed relative to the chest, reproducing the position patients in respiratory distress use (except being supine). Maximal upper airway patency occurs when the tragus is horizontally aligned with the sternal notch. When intubating, the face plane of the patient should be parallel to the ceiling (not overextended). The level of the head of the patient should be low compared to the laryngoscopist, who may have to stand on a platform. HELP or RAMP position has many beneficial effects : 1. Increases the space behind the tongue and epiglottis, the thyro-mental distance. 2. Allows maximum jaw distraction and mouth opening. 3. Cricoid cartilage is more easily accessible. 4. Increases the FRC 5. Improves mechanics of ventilation, both with SR and mask ventilation. 6. In massive obesity, increases the apnoeic time to hypoxia, and shortens the time of mask ventilation to normalize Sp02• 7. Helps dynamic lifting during laryngoscopy (near impossible in large patients